《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_fibrous molithic-56

Materials Science and Engineering A 500(2009)130-149 Contents lists available at Science Direct Materials Science and Engineering A ELSEVIER journalhomepagewww.elsevier.com/locate/msea Dense and near-net-shape fabrication of Si3 N4 ceramics M.H. Bocanegra-Bernala, *, B Matovic b to de Fisica de materials, Miguel de Cervantes 120 Complejo industrial Chihuahua, 31109 Chihuahua, Chihuahua, Mexico b Vinca Institute of Nuclear Sciences, Materials Science Laboratory, Belgrado, Serbia ARTICLE INFO A BSTRACT With silicon nitride significant progress has been made in order to search for fully dense, strong, reli- 1 July 2008 able structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more September 2008 ceramic components based on Si3N4 are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect Keywords: ilicon nitride free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs On the other side, the high hardness of Si3 Na ceramics is almost always cost prohibitive to shape component jection molding by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication boasting Aqueous slurries imize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si3N4 ceramics is given in detail. All kinds of these techniques(injection molding, gel- casting, robocasting, mold shape deposition, rapid prototyping)and their advantages and disadvantages re explained. O 2008 Elsevier B.V. All rights reserved 1. Introduction the high temperature properties of the ceramics such as creep and high temperature strength[17, 32, 33]. Considering this, it is very Structural ceramics based Si3 Na have been explored since the important to stress that the recent advances in improving prop- late 1960s [1]emphasizing Si3 NA based materials primarily for use erties are mainly attributed to improved processing techniques. in high temperature, structural applications such as heat engines. purer raw materials and the use of gas pressure sintering or HIP Taking into account their unique combination of properties, silicon techniques in order to reduce critical flaw size [33 nitride and related materials have probably become the most thor- It is a common practice to densify Si3N4 by alternative tech- oughly characterized non-oxide ceramics with wide applications niques and or supplementary means such as nitridation of silicor cluding heat exchangers, turbine and automotive engine com- powder or with the application of pressure in order to assist the ponents, valves and cam roller followers for gasoline and diesel sintering process. These techniques can be summarized: i) Reac ngines and radomes on missiles as well as insulators, electronic tion Bonding Silicon Nitride( rBsn), ii Hot Pressing Silicon substrates, high Tc superconductors, tool bits, wear surfaces, to(HPSN), iii) Sintering Silicon Nitride(ssn), iv) Sintering Re ame a few [2, 3-17 The market for these applications is very high; Bonding Silicon Nitride(SrBSn), v) Hot Isostatic Pressing there are still several difficulties that must be overcome before the Nitride(hipSn), vi) Hot isostatic Pressing Reaction Bonding Sili full potential of structural ceramics based silicon nitride is real- con Nitride(HIPrBSn). vii) Hot Isostatic Pressing Sintered Silicon ized The sintering of silicon nitride is very difficult because of the Nitride(hipssn)and viii) Hot Isostatic Pressing Sintered Reaction low self-diffusivity of this covalent material [13-20, 21-30 Doping Bonded Silicon Nitride(HiPsrBsn)[20, 22-34. It is very difficult ure Si3 Na with of some oxides provides the formation ofintergran- to produce pure dense silicon nitride ceramics by means of con- lar liquid phase which aids the further densification of the silicon ventional sintering(simple heating of powder compacts) due to nitride during different sintering routes [3-7, 13-31. These oxides, the high degree of covalent bonding between silicon and nitroger however, remain as grain boundary glassy phase, which deteriorate [35]. The principal reason for this is that the diffusion of sil on(at1400°CDs≈05×10-19m2s-l) and nitrogen(at1400° DN≈6.8×10-10m2s-l) in the volu 526144394801;fax+526144394823. tremely slow [36]. Taking into account that the den- sification by sintering requires mass transport via volume or grain 0921-5093/s-see front matter o 2008 Elsevier B.V. All rights reserved. doi:10.016/msea2008

Materials Science and Engineering A 500 (2009) 130–149 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Dense and near-net-shape fabrication of Si3N4 ceramics M.H. Bocanegra-Bernal a,∗, B. Matovic b a Centro de Investigación en Materiales Avanzados, CIMAV S.C., Departamento de Física de Materiales, Miguel de Cervantes # 120 Complejo Industrial Chihuahua, 31109 Chihuahua, Chihuahua, Mexico b Vinca Institute of Nuclear Sciences, Materials Science Laboratory, Belgrado, Serbia article info Article history: Received 11 July 2008 Received in revised form 4 September 2008 Accepted 8 September 2008 Keywords: Silicon nitride Gelcasting Injection molding Robocasting Aqueous slurries abstract With silicon nitride significant progress has been made in order to search for fully dense, strong, reli￾able structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more ceramic components based on Si3N4 are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect￾free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs. On the other side, the high hardness of Si3N4 ceramics is almost always cost prohibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication processes that can produce complex-shaped components with a minimum of machining as well as to min￾imize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si3N4 ceramics is given in detail. All kinds of these techniques (injection molding, gel￾casting, robocasting, mold shape deposition, rapid prototyping) and their advantages and disadvantages are explained. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Structural ceramics based Si3N4 have been explored since the late 1960s [1] emphasizing Si3N4 based materials primarily for use in high temperature, structural applications such as heat engines. Taking into account their unique combination of properties, silicon nitride and related materials have probably become the most thor￾oughly characterized non-oxide ceramics with wide applications including heat exchangers, turbine and automotive engine com￾ponents, valves and cam roller followers for gasoline and diesel engines and radomes on missiles as well as insulators, electronic substrates, high Tc superconductors, tool bits, wear surfaces, to name a few [2,3–17]. The market for these applications is very high; there are still several difficulties that must be overcome before the full potential of structural ceramics based silicon nitride is real￾ized. The sintering of silicon nitride is very difficult because of the low self-diffusivity of this covalent material [13–20,21–30]. Doping pure Si3N4 with of some oxides provides the formation of intergran￾ular liquid phase which aids the further densification of the silicon nitride during different sintering routes [3–7,13–31]. These oxides, however, remain as grain boundary glassy phase, which deteriorate ∗ Corresponding author. Tel.: +52 614 4394801; fax: +52 614 439 4823. E-mail addresses: miguel.bocanegra@cimav.edu.mx (M.H. Bocanegra-Bernal), mato@vin.bg.ac.yu (B. Matovic). the high temperature properties of the ceramics such as creep and high temperature strength [17,32,33]. Considering this, it is very important to stress that the recent advances in improving prop￾erties are mainly attributed to improved processing techniques, purer raw materials and the use of gas pressure sintering or HIP techniques in order to reduce critical flaw size [33]. It is a common practice to densify Si3N4 by alternative tech￾niques and/or supplementary means such as nitridation of silicon powder or with the application of pressure in order to assist the sintering process. These techniques can be summarized: i) Reac￾tion Bonding Silicon Nitride (RBSN), ii) Hot Pressing Silicon Nitride (HPSN), iii) Sintering Silicon Nitride (SSN), iv) Sintering Reaction Bonding Silicon Nitride (SRBSN), v) Hot Isostatic Pressing Silicon Nitride (HIPSN), vi) Hot Isostatic Pressing Reaction Bonding Sili￾con Nitride (HIPRBSN), vii) Hot Isostatic Pressing Sintered Silicon Nitride (HIPSSN) and viii) Hot Isostatic Pressing Sintered Reaction Bonded Silicon Nitride (HIPSRBSN) [20,22–34]. It is very difficult to produce pure dense silicon nitride ceramics by means of con￾ventional sintering (simple heating of powder compacts) due to the high degree of covalent bonding between silicon and nitrogen [35]. The principal reason for this is that the diffusion of sili￾con (at 1400 ◦C DSi ≈ 0.5 × 10−19 m2 s−1) and nitrogen (at 1400 ◦C DN ≈ 6.8 × 10−10 m2 s−1) in the volume or at the grain boundaries of Si3N4 is extremely slow [36]. Taking into account that the den￾sification by sintering requires mass transport via volume or grain boundary diffusion and since such diffusion is a thermally activated 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.09.015

M.H. Bocanegra-Bermal B Matovic/ Materials Science and Enginee process, a higher sintering temperature would result in a highly sintering times and temperatures, reduction of grain size and dis- lense material but at high temperatures Si3 N4 starts to dissociate tribution, higher sintered densities with the use of low pressures and reduction or sintering aids, etc, all of which lead to obtain Many different sintering techniques have been developed since excellent properties [47-51. There has been considerable interest the material properties strongly depend on the fabrication method in developing ceramic materials for use in advanced heat engines and the silicon nitride cannot be considered as a single material Therefore, possessing low density and excellent thermomechanical [38]. As was previously outlined, the most common sintering meth- properties, ceramics materials provide a means for producing heat ods used to consolidate Si3 N4 based ceramics [20]are: i) Reaction engines with very efficiency ceiling far above what is presently pos- bonding(RBSN), ii) Hot pressing(HPSN), iii) Hot isostatic press- sible with today's super alloys. Silicon nitride(si3 N4), has received considerable attention due to its high decomposition tempera and vi) Sintering reaction bonding(SRBSN) On the other hand, ture(approximately 1880 C)as well as excellent creep properties in order to manufacture Si3 N4 ceramics for application in heat 3, 52, 53. Today, commercially available Si3 N4 powders are pre- engines, development of near-net-shape fabrication methods is pared by means of various routes, already in use for production critical [39]. Recognizing this need, several companies in the United on a technical scale because these powders are the starting point States, Europe and japan have invested significant resources to for dense materials, namely develop injection molding, gelcasting of silicon metal, and slip cast- [40]. Among the shapemaking techniques, slip casting is belie parts 2.1. Silicon nitridation be a method appropriate for prototypes, whereas injection mold- Chemically pure Si powder (particle diameter <10 um)is ing is ideally suited for high-volume, cost effective production of nitrided in an atmosphere of NH3, N2/H2 or N 2 as follows: complex parts [28, 41 Nowadays, robocasting is a new freeform brication technique for dense silicon nitride ceramics [42). This 3Si+ 2N2 colloidal method has shown its potential to improve the strength nd reliability of high-performance ceramics. Ceramic components under controlled furnace conditions such as bed-depth[54].The with simple or complex shapes can be rapidly produced from nitriding process results in Si3 N4 lumps which are crushed and a computer aided-design(CAD) drawing directly to a finished milled. The reaction(1)is the high temperature reaction of silicon omponent that requires little or no machining after fabrication powder with a nitrogen and it is a reaction strongly exothermic with 4243] AH-733 k] mol-I. It is important to stress that the density of sil- More complex techniques for manufacturing silicon nitride icon is 2329 m- and that of silicon nitride is -3185kgm-3,so ceramics have been used to produce reliable parts Injection mold- that a volume expansion of 21.7% occurs during nitride formation. ing slip casting, robocasting aqueous, gelcasting, are some of them At normal nitriding temperatures (1200-1450 C) silicon nitride [42, 44, 45. The fabrication techniques to obtain near-net-shape shows no noticeable plasticity, and as the overall compact volume Si3Na ceramics as well as their advantages and disadvantages are change during nitridation of silicon powders is essentially zero, it given in detail. is clear that considerable internal rearrangement of product mate- rial must occur within the pre-existing void spaces of the compact 2. Manufacture of silicon nitride powders The nitridation method has proven to be flexible for Some studies on the sintering of silicon nitride powders to duction of very different powder qualities. The raw silicon nitride dense bodies have shown the great importance of surface com- formed in the nitridation process already consists of morphology position: the nature and amount of sintering aids(such as Al2O3. such as whiskers, elongated particles, and equiaxed particles [6] a Y203, Yb2O3. ZrO2 etc. )at high temperatures(1750-2000 C)[3-9] well as spherical after the milling process [56]. Additionally, with depended on the surface oxygen content of the powders 6]. How- wet milling, very fine powders of up to 25m2g-l or more with ever, the sintering of silicon nitride ceramics without additives is narrow particle size distributions can be produced. n important approach to reducing impurity phases in the densi Important attention is drawn to quality-determining steps of ed bodies achieving Si3 N4 ceramics with the intrinsic properties of powder production, which are assumed to be responsible for the the materials. Ceramics free of aids sintered under high pressures sinter-active behavior of the finest powders. A high a-phase(95%) exhibited improved mechanical properties at high temperatures content is desirable in order to ensure beneficial transformation mpared to those sintered with additives [8, 9 nto the B-form during sintering, leading to densification and the These materials tend to be expensive due to the high cost of formation of an interlocked needle structure with high strength. In the silicon nitride powder used to produce them. Therefore, the silicon powder, unless the silica layer is disrupted either physically reduction of cost has been recognized as a major factor for the or chemically, the nitridation reaction does not start [ 57.Afterinit introduction of silicon nitride ceramics into the marketplace with ation, the nitridation reaction proceeds and is controlled by factors as mean particle size and size distribution, the nature and distri- alloys, tungsten carbides and some ceramics such as Al2O3 and zro ution of impurities in the starting silicon powder, size and size in automotive aerospace, metal processing and forming, mineral distribution of open porosity in the silicon compacts, dimensions processing, machining, oil field services, petrochemical, semicon- of the silicon compacts, and nitriding conditions ductor processing industries, etc. [10-12 As indicated a number years ago [3], there is a great advan- 22. Vapor phase reaction tage to processing ceramics from powders with an idealized set of physical and chemical characteristics [46 as follows: i)small By means of this method, a fine amorphous silicon nitride pow- size less than 1 um, i) narrow size distribution, ii equiaxed mor- der is obtained from the gas phase reaction of silicon tetrachloride, phology tending towards spherical, no agglomeration, or very weak SiCl4, and ammonia at temperature of 1546 C according to the agglomerate bonds which can be broken during processing and reaction: iv) high degree of chemical and crystal purity. With these char- acteristics, it is possible to obtain advantages such as reduction of 3SiCla (g)+ 4NH3()- Si3N4+ 12HCI(g)

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 131 process, a higher sintering temperature would result in a highly dense material but at high temperatures Si3N4 starts to dissociate [35,37]. Many different sintering techniques have been developed since the material properties strongly depend on the fabrication method and the silicon nitride cannot be considered as a single material [38]. As was previously outlined, the most common sintering meth￾ods used to consolidate Si3N4 based ceramics [20] are: i) Reaction bonding (RBSN), ii) Hot pressing (HPSN), iii) Hot isostatic press￾ing (HIPSN), iv) Sintering (SSN), v) Gas pressure sintering (GPSN) and vi) Sintering reaction bonding (SRBSN). On the other hand, in order to manufacture Si3N4 ceramics for application in heat engines, development of near-net-shape fabrication methods is critical[39]. Recognizing this need, several companies in the United States, Europe and Japan have invested significant resources to develop injection molding, gelcasting of silicon metal, and slip cast￾ing for fabrication of complex cross-section structural ceramic parts [40]. Among the shapemaking techniques, slip casting is believed to be a method appropriate for prototypes, whereas injection mold￾ing is ideally suited for high-volume, cost effective production of complex parts [28,41]. Nowadays, robocasting is a new freeform fabrication technique for dense silicon nitride ceramics [42]. This colloidal method has shown its potential to improve the strength and reliability of high-performance ceramics. Ceramic components with simple or complex shapes can be rapidly produced from a computer aided-design (CAD) drawing directly to a finished component that requires little or no machining after fabrication [42,43]. More complex techniques for manufacturing silicon nitride ceramics have been used to produce reliable parts. Injection mold￾ing, slip casting, robocasting aqueous, gelcasting, are some of them [42,44,45]. The fabrication techniques to obtain near-net-shape Si3N4 ceramics as well as their advantages and disadvantages are given in detail. 2. Manufacture of silicon nitride powders Some studies on the sintering of silicon nitride powders to dense bodies have shown the great importance of surface com￾position: the nature and amount of sintering aids (such as Al2O3, Y2O3, Yb2O3, ZrO2, etc.) at high temperatures (1750–2000 ◦C) [3–9] depended on the surface oxygen content of the powders [6]. How￾ever, the sintering of silicon nitride ceramics without additives is an important approach to reducing impurity phases in the densi- fied bodies achieving Si3N4 ceramics with the intrinsic properties of the materials. Ceramics free of aids sintered under high pressures exhibited improved mechanical properties at high temperatures compared to those sintered with additives [8,9]. These materials tend to be expensive due to the high cost of the silicon nitride powder used to produce them. Therefore, the reduction of cost has been recognized as a major factor for the introduction of silicon nitride ceramics into the marketplace with a broad range of properties to replace the stainless steels, super alloys, tungsten carbides and some ceramics such as Al2O3 and ZrO2 in automotive, aerospace, metal processing and forming, mineral processing, machining, oil field services, petrochemical, semicon￾ductor processing industries, etc. [10–12]. As indicated a number years ago [3], there is a great advan￾tage to processing ceramics from powders with an idealized set of physical and chemical characteristics [46] as follows: i) small size less than 1 m, ii) narrow size distribution, iii) equiaxed mor￾phology tending towards spherical, no agglomeration, or very weak agglomerate bonds which can be broken during processing and iv) high degree of chemical and crystal purity. With these char￾acteristics, it is possible to obtain advantages such as reduction of sintering times and temperatures, reduction of grain size and dis￾tribution, higher sintered densities with the use of low pressures and reduction or sintering aids, etc., all of which lead to obtain excellent properties [47–51]. There has been considerable interest in developing ceramic materials for use in advanced heat engines. Therefore, possessing low density and excellent thermomechanical properties, ceramics materials provide a means for producing heat engines with very efficiency ceiling far above what is presently pos￾sible with today’s super alloys. Silicon nitride (Si3N4), has received considerable attention due to its high decomposition tempera￾ture (approximately 1880 ◦C) as well as excellent creep properties [3,52,53]. Today, commercially available Si3N4 powders are pre￾pared by means of various routes, already in use for production on a technical scale because these powders are the starting point for dense materials, namely: 2.1. Silicon nitridation Chemically pure Si powder (particle diameter 95%) content is desirable in order to ensure beneficial transformation into the -form during sintering, leading to densification and the formation of an interlocked needle structure with high strength. In silicon powder, unless the silica layer is disrupted either physically or chemically, the nitridation reaction does not start[57]. After initi￾ation, the nitridation reaction proceeds and is controlled by factors as mean particle size and size distribution, the nature and distri￾bution of impurities in the starting silicon powder, size and size distribution of open porosity in the silicon compacts, dimensions of the silicon compacts, and nitriding conditions. 2.2. Vapor phase reaction By means of this method, a fine amorphous silicon nitride pow￾der is obtained from the gas phase reaction of silicon tetrachloride, SiCl4, and ammonia at temperature of 1546 ◦C according to the reaction: 3SiCl4(g) + 4NH3(g) → Si3N4 + 12HCl(g) (2)

M.H. Bocanegra-BemaL B Matovic/ Materials Science and Engineering A 500(2009)130-149 Although a-Si3N4 crystallizes between 1673 and 2046C, these or, on the other hand, silazanes compounds containing Si-N-Si silicon nitride powders have interesting properties including high bonds as follows: hemical purity; amorphous microstructure and me in the manometers scale [18, 58] as well as are suitable raw materi- 2(CH3 )3 SiCl 3NH3-[(CH3)3SiJ2NH +2NH4CI als of advanced silicon nitride ceramics. However, it is interesting It is very important to note that the stability of sily to note that silicon nitride powders with particle size nanomet- with respect to silazane formation increases with functional group ric range, can be densified and sintered without additives under size and structures of representative silazanes [54 Si3 NA pow ultrahigh pressure(1.0-5.0 GPa) between room temperature and ders by means of conversion of silazanes have been achieved 1600°C[59] in various physical forms. Very thin films(<1 um) have been Although these powders are generally present in amorphous deposited from gas mixtures of hexamethyldisilazane/ NH3 and form, obtaining crystallized ceramics requires sintering at least hexamethylclotrisilizane/NH3 using chemical vapor depositi oC). On the(CVD)technology, as well as fiber bundles of a-Si3 Na with diam- other hand, sintering of amorphous powders bellow the crystal- eter of approximately 1. um at 1400 C by means of pyrolisis of lization temperature may generate bulk amorphous ceramics. LiLi hexaphenylcyclotrisilizane in nitrogen [64, 65. et al.[59 reported two typical pressing results, together with nose sintered at high temperature, where by means of XRD was 3. Fabrication of near-net-shape Si3 Na ceramics identified that sintered specimens obtained below 1000-1100C emained amorphous. The high relative density obtained indicates The articles manufactured by near-net-shape forming tech- that the amorphous nano-size powders can be almost fully den- niques involve generally little, if any post-densification machining, sified below the crystallization temperature under sufficient high surface preparation or cleaning prior to use. There is a great pressure. Therefore, bulk Si3 N4 amorphous ceramics can be formed challenge to fabricate complex-shaped components with high reli- at sintering temperatures slightly below that the onset of crystal- ability and with defect-free microstructures at acceptable costs. zation. Moreover, the sintering of amorphous nano-size powder The high hardness of Si3 NA ceramics is almost always cost pro- vithout additives is an important approach to reducing impurities hibitive to shape components by hard machining. It is therefore phases in the sintered bodies and hence achieving Si3N4 ceram- great effort exhibited in the development of near-net-shape fab- ics with the intrinsic properties of the materials and improved rication processes that can produce complex-shaped components mechanical and high temperature compared to those sintered with with a minimum of machining as well as to minimize the number dditives [60 and size of microstructural defects within design limits. Injection molding, gelcasting, robocasting, mold shape deposition, rapid pro- 3. Imide dec totting of them It is considered as a liquid phase reaction method [54 It is 3.1. Injection molding of SiaN4 ceramics interesting to note that reactions attracting attention in the 1980s vere first investigated as 1830. In that year a white precipitate was Silicon nitride, when properly prepared, is a superlatively tough obtained from the interaction of Sicl and ammonia gas 61) in ceramic whose high temperature stability; low weight; and wear, an inert solvent(benzene)at approximately 273 K In later stud- erosion and corrosion resistance have put it high up on the wish ies[55] the product of this reaction was considered to be silicon list of turbine engine designers. Injection molding of ceramics tetramide, Si(NH2)4. However the precipitate was unstable and lost was initially demonstrated over 50 years ago [66-68] and it is an NH3(g) at ambient temperature to give silicon diimide, Si(NH)2 attractive method among the processes for near-net-shape produc hich is heated at high temperature in N2 or NH3 atmosphere after tion of ceramic parts, requiring little subsequent grinding and no eparating ammonium halide[ 62]. From the different methods for need of machining 169). The use of polycrystalline high tempera- manufacturing silicon nitride, the thermal decomposition method ture ceramics in different applications such as turbochargers and of Si(NH)2 is considered to be very suitable for use in the mass pro- gas turbine vanes, blades and rotors [70-72], reciprocating [73, 74] uction of Si3N4 powder with high quality. because the starting and turbine engines [75, 76] has been possible by considerable materials can be easily and highly purified and the productivity is developments in the fabrication of fine powders [77, 78]. Success high[55]. However, a-Si3 N4 powders synthesized by diimide route of the injection molding process of Si3N4 is critically depending produce powders with a high area and fine particle size(10-30 nm), on starting powder, binder, and the process parameters such as but they are prohibitively expensive [63] molding and binder removal conditions and subsequent densifi- It is very important to control the crystallization a to B ratio cation 39]. The development of injection molding technology for and grain morphology of the product, because th e better control sintered silicon nitride was initiated at gte labs under a sub- of these characteristics of Si3N4 powder is considered to be the contract to the Detroit Diesel Allison Division( DDA)of General portant key point in the production of high-performance Si3N4 Motors as a part of the Ceramic Applications in Turbine Engines ceramics [61] (CATE)[28 Successful development of a injection molding process fornet-shape thick-cross-section(1 cm) 2.4. Silazanes as precursor of Si3N4 components is expected to have a strong impact on the commer- cial development of automotive gas turbines and other related It is known that the chlorosilanes react with NH. primary or heat engines applications. The aim of the injection molding tech- econdary amines to form silymines as follows [63] nology is therefore to produce an unsintered pai which will shrink isotropically to yield a shape slightly over (C2H5)3SiCl 2NH3-(C2H5 B3SINH2+NH4CI (3) size for final machining. Distortion of the ceramic body during molding, binder removal or sintering may render the component useless (CH3)3SiCl 2NH(C2H5 )2-(CH3)3SiN(C2H5)2+(C2H5)2NH2CI The injection molding of Si3 N4 ceramics normally consists of five steps as follows: i) powder processing, ii)powder binder (4) compounding, iii) injection molding, iv) binder burnout and v)

132 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 Although -Si3N4 crystallizes between 1673 and 2046 ◦C, these silicon nitride powders have interesting properties including high chemical purity; amorphous microstructure and mean particle size in the manometers scale [18,58] as well as are suitable raw materi￾als of advanced silicon nitride ceramics. However, it is interesting to note that silicon nitride powders with particle size nanomet￾ric range, can be densified and sintered without additives under ultrahigh pressure (1.0–5.0 GPa) between room temperature and 1600 ◦C [59]. Although these powders are generally present in amorphous form, obtaining crystallized ceramics requires sintering at least above the crystallization temperature (>1000–1300 ◦C). On the other hand, sintering of amorphous powders bellow the crystal￾lization temperature may generate bulk amorphous ceramics. LiLi et al. [59] reported two typical pressing results, together with those sintered at high temperature, where by means of XRD was identified that sintered specimens obtained below 1000–1100 ◦C remained amorphous. The high relative density obtained indicates that the amorphous nano-size powders can be almost fully den￾sified below the crystallization temperature under sufficient high pressure. Therefore, bulk Si3N4 amorphous ceramics can be formed at sintering temperatures slightly below that the onset of crystal￾lization. Moreover, the sintering of amorphous nano-size powder without additives is an important approach to reducing impurities phases in the sintered bodies and hence achieving Si3N4 ceram￾ics with the intrinsic properties of the materials and improved mechanical and high temperature compared to those sintered with additives [60]. 2.3. Imide decomposition method It is considered as a liquid phase reaction method [54]. It is interesting to note that reactions attracting attention in the 1980s were first investigated as 1830. In that year a white precipitate was obtained from the interaction of SiCl4 and ammonia gas [61] in an inert solvent (benzene) at approximately 273 K. In later stud￾ies [55], the product of this reaction was considered to be silicon tetramide, Si(NH2)4. However the precipitate was unstable and lost NH3 (g) at ambient temperature to give silicon diimide, Si(NH)2 which is heated at high temperature in N2 or NH3 atmosphere after separating ammonium halide [62]. From the different methods for manufacturing silicon nitride, the thermal decomposition method of Si(NH)2 is considered to be very suitable for use in the mass pro￾duction of Si3N4 powder with high quality, because the starting materials can be easily and highly purified and the productivity is high [55]. However, -Si3N4 powders synthesized by diimide route produce powders with a high area and fine particle size (10–30 nm), but they are prohibitively expensive [63]. It is very important to control the crystallization  to  ratio and grain morphology of the product, because the better control of these characteristics of Si3N4 powder is considered to be the important key point in the production of high-performance Si3N4 ceramics [61]. 2.4. Silazanes as precursor of Si3N4 It is known that the chlorosilanes react with NH3, primary or secondary amines to form silymines as follows [63]: (C2H5)3SiCl + 2NH3 → (C2H5)3SiNH2 + NH4Cl (3) (CH3)3SiCl + 2NH(C2H5)2 → (CH3)3SiN(C2H5)2 + (C2H5)2NH2Cl (4) or, on the other hand, silazanes compounds containing Si–N–Si bonds as follows: 2(CH3)3SiCl + 3NH3 → [(CH3)3Si]2NH + 2NH4Cl (5) It is very important to note that the stability of silylamines with respect to silazane formation increases with functional group size and structures of representative silazanes [54]. Si3N4 pow￾ders by means of conversion of silazanes have been achieved in various physical forms. Very thin films (1 cm) components is expected to have a strong impact on the commer￾cial development of automotive gas turbines and other related heat engines applications. The aim of the injection molding tech￾nology is therefore to produce an unsintered particle assembly which will shrink isotropically to yield a shape slightly over￾size for final machining. Distortion of the ceramic body during molding, binder removal or sintering may render the component useless. The injection molding of Si3N4 ceramics normally consists of five steps as follows: i) powder processing, ii) powder binder compounding, iii) injection molding, iv) binder burnout and v)

M.H. Bocanegra-Bermal B Matovic/ Materials Science and Engineering A 500(2009)130-149 densification by sintering and or Hot Isostatic Pressing(HIPing) duced by injection molding with sufficient dimensional control for 28.79-81 The control of each of these process steps and appropri- turbine engine applications. ate selection of the starting material (powder and organic/ aqueou In the fabrication of Si3Na radial power turbine wheel, three binder)are critically important for the overall process success 70 forming methods have been used: i) injection molding, ii slip cast The optimum selection of a binder system is one of the most crit- ing and iii) cold isostatic pressing. Considering that one of the ical factors in silicon nitride part fabrication by injection molding must difficult components to form by conventional ceramic form- [39]. The binder system used for injection molding of Si N4 parts ing processing has been the radial turbine wheel, the development contains a paraffin wax(90 w/), a liquid epoxy (5 w/o), and a sur- of the forming technique for a large, complex-shaped compo- factant(5 w/o)[79). Very care must be taken during binder removal nent as well as the development of a high strength and refractory in order to avoid delamination and cracks as well as part bloat- material to achieve a turbine inlet temperature(Tit)of 1350.Cis ing. During this stage the organic binder filling the spaces between required [1, 25, 39, 82-84, 86]. Takatori et al. [1 used for fabrication ceramic grains must be removed without disrupting the part. The of radial turbine wheel Si3N4 with additions of 5 wt% Y203 and binder removal is accomplished by a thermal cycle which results 5 wt% MgAl2 O4(spinel) as additives. Using this composition, sev- in controlled binder distillation and degradation. When molding eral small components for reciprocating engines were fabricated a submicron ceramic powder such as silicon nitride it was also by injection molding. However, the ceramics obtained had good discovered that an extremely long thermal cycle can be equally mechanical properties at moderate temperatures, but the hightem- detrimental. As the liquid binder is depleted in long cycles the fine perature strength of the ceramics was no satisfactory for gas turbine N4 particles tend to rearrange into denser packing configurations components which would operate at 1350 C In order to overcome under capillary forces, leading to shrinkage. The part exterior of the this problem, two methods for injection molding the wheel were ceramic body is at a more advance stage of binder removal where attempted: one body forming(the whole body of the wheel is injec- no rearrangement of particles can occur and the internal shrinkage tion molded at one time)and two-piece forming(the wheel was produces interior crack formation observable only by radiography divided in two pieces that are injection molded separately. In the 288283 case of one body forming, the sintered bodies presented internal In the works concerning to injection molding of Si3 N4 and car- cracks as well as others were damaged by surface crack genera ried out by different authors, the major objective was to identify tion and broke down during removal binder Takatori et al. [1 and a Si3 N4 powder-based formulation and binder combination with Shimizu et al. [25] concluded that one body forming of the large proved resistance to the stresses generated during solidification radial wheel was not practical using the current injection mold and cooling of injection molded components in the mold Specif- ing method. Therefore the processes of fabrication of radial wheel ically, several key variables on the properties of injection molded were attempted dividing the hub of the wheel the thickest sec- parts have been studied such as particle size distribution of the tion )in two axially symmetrical parts to reduce the thickness about starting powder, binder composition, powder solid loading, and in half [ 87]. The resulting parts were injection molded separately compounding shear level [1, 39, 82-84. and the survival probability of the wheel after binder removal step A variety of turbine components and related parts have been increased markedly. These parts were joined by CIP treatment after produced, supporting several turbine development programs. binder removal. It is noteworthy that even for the divided two-piece Bandyopadhyay and Neil [70 reported two compositions used process; prolonged heat schedule was required with the purpose for fabrication of components: Si3 Na containing 6 w/o Y2O3 and to obtain a sound binderless body labeled as PY6 and Si3 N4 with additions of 6 w/o y203 and 1.5 to Althou injection molding technique is highly viable for fab- 2.0 w/o Al2O3 and labeled as AY6 [85 which in turn is the mate- rication of complex-shaped ceramic components and numerous rial with superior strength properties from room temperature to prototype silicon nitride components have been fabricated by 1200.C meanwhile, the PY6 composition was designed to provide different laboratories in the world being tested successfully in heat- strength maintenance to temperatures at or above 1200C due to engine environments, specific challenges are required [88 such a grain boundary phase wh nore refractory than that of AY6. as: i) achieving raw and palletized feedstocks with consistent and Glass encapsulated HIPing process was carried out at 1750.C or stable flow characteristics, ii) developing mold designs that allow higher in order to obtain full density. These compositions have also feedstock to fill completely while minimizing defects during han- een reported by Neil et al. [40 and although the compositions dling and subsequent processing, iii) attaining enough control over have remained the same, substantial improvements in the material the entire process to ensure high, reliable yields of uniform parts properties of these systems have been realized through improve- and iv)reducing the manufacture cycle time so that the result- nents in process control and microstructure engineering. However, ing silicon nitride parts are competitive with all-metal components Neil et al. [40] identified that for powder/ binder formulations with that they are intended to replace. The application of powder injec- similar binder systems, powders with higher ages of coarser tion moulding enables the production of intricate features and particle agglomerates measured higher material viscosity at the unusual geometries, offering and economic solution to difficult injection molding temperature. Increases in the powder solids production problems when part complexity goes beyond of more lations also increased the material viscosity for all powder formu- basic forming technologies such as dry pressing. This technique offers excellent batch to batch repeatability and process capabil Neil et al.(28]reported difficulties with the large cross-section ities achieving tolerances of +<0.3% for applications in markets rotors in both the molding and binder removal steps within the as aerospace communications, automotive, electronic, chemical, rogram of Ceramics Applications in Turbine Engines. The slight medical, etc. rinkage in the binder system during the solidification was magni- fied due to the much larger cross-section of the rotor hub. with this 3. 2. Gelcasting of si3 Na ceramics problem at hand, a new binder system was developed which com bined low shrinkage during the solidification in the die with greatly Gelcasting is a molding technique for ceramic and metallic mate- improved binder removal characteristics. Therefore, radial turbine rials [89-93] which offers distinct res as an alternative tors were molded using this binder system which was visually to the more conventional ceramic nethods such as dry flaw free after molding and binder removal. The same authors have pressing, slip casting and injection [89, 94-97 Principal reported that liquid phase sintering of Si3 N4 ceramics can be pro- advantages include near-net-shape forming, high green density

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 133 densification by sintering and/or Hot Isostatic Pressing (HIPing) [28,79–81]. The control of each of these process steps and appropri￾ate selection of the starting material (powder and organic/aqueous binder) are critically important for the overall process success [70]. The optimum selection of a binder system is one of the most crit￾ical factors in silicon nitride part fabrication by injection molding [39]. The binder system used for injection molding of Si3N4 parts contains a paraffin wax (90 w/o), a liquid epoxy (5 w/o), and a sur￾factant (5 w/o)[79]. Very care must be taken during binder removal in order to avoid delamination and cracks as well as part bloat￾ing. During this stage, the organic binder filling the spaces between ceramic grains must be removed without disrupting the part. The binder removal is accomplished by a thermal cycle which results in controlled binder distillation and degradation. When molding a submicron ceramic powder such as silicon nitride it was also discovered that an extremely long thermal cycle can be equally detrimental. As the liquid binder is depleted in long cycles the fine Si3N4 particles tend to rearrange into denser packing configurations under capillary forces, leading to shrinkage. The part exterior of the ceramic body is at a more advance stage of binder removal where no rearrangement of particles can occur and the internal shrinkage produces interior crack formation observable only by radiography [1,28,82,83]. In the works concerning to injection molding of Si3N4 and car￾ried out by different authors, the major objective was to identify a Si3N4 powder-based formulation and binder combination with improved resistance to the stresses generated during solidification and cooling of injection molded components in the mold. Specif￾ically, several key variables on the properties of injection molded parts have been studied such as particle size distribution of the starting powder, binder composition, powder solid loading, and compounding shear level [1,39,82–84]. A variety of turbine components and related parts have been produced, supporting several turbine development programs. Bandyopadhyay and Neil [70] reported two compositions used for fabrication of components: Si3N4 containing 6 w/o Y2O3 and labeled as PY6 and Si3N4 with additions of 6 w/o Y2O3 and 1.5 to 2.0 w/o Al2O3 and labeled as AY6 [85] which in turn is the mate￾rial with superior strength properties from room temperature to 1200 ◦C meanwhile, the PY6 composition was designed to provide strength maintenance to temperatures at or above 1200 ◦C due to a grain boundary phase which is more refractory than that of AY6. Glass encapsulated HIPing process was carried out at 1750 ◦C or higher in order to obtain full density. These compositions have also been reported by Neil et al. [40] and although the compositions have remained the same, substantial improvements in the material properties of these systems have been realized through improve￾ments in process control and microstructure engineering. However, Neil et al. [40] identified that for powder/binder formulations with similar binder systems, powders with higher percentages of coarser particle agglomerates measured higher material viscosity at the injection molding temperature. Increases in the powder solids loading also increased the material viscosity for all powder formu￾lations. Neil et al. [28] reported difficulties with the large cross-section rotors in both the molding and binder removal steps within the program of Ceramics Applications in Turbine Engines. The slight shrinkage in the binder system during the solidification was magni- fied due to the much larger cross-section of the rotor hub. With this problem at hand, a new binder system was developed which com￾bined low shrinkage during the solidification in the die with greatly improved binder removal characteristics. Therefore, radial turbine rotors were molded using this binder system which was visually flaw free after molding and binder removal. The same authors have reported that liquid phase sintering of Si3N4 ceramics can be pro￾duced by injection molding with sufficient dimensional control for turbine engine applications. In the fabrication of Si3N4 radial power turbine wheel, three forming methods have been used: i) injection molding, ii) slip cast￾ing and iii) cold isostatic pressing. Considering that one of the must difficult components to form by conventional ceramic form￾ing processing has been the radial turbine wheel, the development of the forming technique for a large, complex-shaped compo￾nent as well as the development of a high strength and refractory material to achieve a turbine inlet temperature (TIT) of 1350 ◦C is required [1,25,39,82–84,86]. Takatori et al. [1] used for fabrication of radial turbine wheel Si3N4 with additions of 5 wt% Y2O3 and 5 wt% MgAl2O4 (spinel) as additives. Using this composition, sev￾eral small components for reciprocating engines were fabricated by injection molding. However, the ceramics obtained had good mechanical properties atmoderate temperatures, but the high tem￾perature strength of the ceramics was no satisfactory for gas turbine components which would operate at 1350 ◦C. In order to overcome this problem, two methods for injection molding the wheel were attempted: one body forming (the whole body of the wheel is injec￾tion molded at one time) and two-piece forming (the wheel was divided in two pieces that are injection molded separately. In the case of one body forming, the sintered bodies presented internal cracks as well as others were damaged by surface crack genera￾tion and broke down during removal binder. Takatori et al. [1] and Shimizu et al. [25] concluded that one body forming of the large radial wheel was not practical using the current injection mold￾ing method. Therefore, the processes of fabrication of radial wheel were attempted dividing the hub of the wheel (the thickest sec￾tion) in two axially symmetrical parts to reduce the thickness about in half [87]. The resulting parts were injection molded separately and the survival probability of the wheel after binder removal step increased markedly. These parts were joined by CIP treatment after binder removal. It is noteworthy that even for the divided two-piece process; prolonged heat schedule was required with the purpose to obtain a sound binderless body. Although injection molding technique is highly viable for fab￾rication of complex-shaped ceramic components and numerous prototype silicon nitride components have been fabricated by different laboratories in the world being tested successfully in heat￾engine environments, specific challenges are required [88] such as: i) achieving raw and palletized feedstocks with consistent and stable flow characteristics, ii) developing mold designs that allow feedstock to fill completely while minimizing defects during han￾dling and subsequent processing, iii) attaining enough control over the entire process to ensure high, reliable yields of uniform parts and iv) reducing the manufacture cycle time so that the result￾ing silicon nitride parts are competitive with all-metal components that they are intended to replace. The application of powder injec￾tion moulding enables the production of intricate features and unusual geometries, offering and economic solution to difficult production problems when part complexity goes beyond of more basic forming technologies such as dry pressing. This technique offers excellent batch to batch repeatability and process capabil￾ities achieving tolerances of ±<0.3% for applications in markets as aerospace, communications, automotive, electronic, chemical, medical, etc. 3.2. Gelcasting of Si3N4 ceramics Gelcasting is amolding technique for ceramic andmetallicmate￾rials [89–93] which offers distinct advantages as an alternative to the more conventional ceramic forming methods such as dry pressing, slip casting and injection molding [89,94–97]. Principal advantages include near-net-shape forming, high green density

and low organic levels in the dried green ceramics. Therefore, a final mechanical properties of Si3 N4 parts: i)the surface quality wide variety of ceramic materials have been prepared using gel of the mold that is transferred into the final part. Therefore, the casting process including Si3 N4, SiC, AlO3 and zro,, among others. higher this surface roughness, leads to lower the final mechanical In gel casting, slurry made from ceramic powder and a water-based strength. The defects in the mold surface can cause notches which monomer solution is poured into a mold, polymerized in situ to lead to stress concentration in the final part and ii)with perfectly mobilize the particles in a gelled part, removed from the mold smooth mold surface, the difference between bulk microstructure while still wet, then dried and fired. If the solvent for the monomers and surface microstructure has to be considered when describing is organic, it is no aqueous gel casting: if is water, it is aqueous gel- the mechanical properties of silicon nitride parts. The maximum casting [94, 96, 98 The development of an aqueous process using strength is achieved with polished samples. Similarly, Stampfl et acrylamide as monomer was completed in 1988[95, 99]. However, al. [93 obtained strength values of 414, 950, and 983 MPa in ncerns regarding health, safety and disposal of acrylamide caused Si3N4 unpolished, Si3N4 polished and Si3 N4 GPS and polished industrial rejection of the process because the acrylamide is a neu- respectively, where all samples were sintered at 1750 C in nitro- toxin. Therefore, the development of a low toxicity process was gen atmosphere. On the other hand, materials such as aluminum, initiated to deal with the lack of acceptance and it was fully demon anodized aluminum, brass, glass, graphite indium alloys, neoprene trated in 1990 90 rubber, plaster and polyethylene are commonly used in construct- eramic parts from different ceramics such as aluminum oxide ing gelcasting molds. Al203, and high-performance silicon nitride Si3 N4, have been pro- injection molding, before a ceramic body can be sintered, luced by gelcasting ranging in size from 6 kg with thin the binder added must be removed. One advantage of gelcasting sections as small as 0. 2 mm and solids loading as high as 55-60 vol% is the small amount of polymer that remains in the green bod in alumina slurries and 45-57 vol% in silicon nitride suspensions after drying [101]. The dried gelcast ceramic contains only about [99 Although gelcast bodies typically shrink -23% during densifi- 2-6wt% polymer, which depends on the solids loading of the slurry, tion and resulting sometimes in distortion in the densified parts, the concentration of monomers in the premix, and the density this technique starts to be used not only for manufacturing of com- of powder. For comparison, a 45 vol% solids silicon nitride with plicated shaped dense products such as Si3N4 parts of turbines but the composition Si3N4+5 wt% Al203 +5 wt% Y203 made using a also for manufacturing of porous ceramic[ 90, 100). The most 15 wt% 4: 1 Methacrylamide-N, N'-Methylene bisacrylamide(MAM- dvanced works in this field are already in the phase or commer- MBAM) premix contains about 5.5 wto polymer in the dried part, alization and, Allied Signal Ceramics Components (torrance CA. meanwhile an injection molded silicon nitride with the same com- USA)working together with ORNL (Oak Ridge, TN, USA)has devel- position and solid loading would contain about 27 wt% polymer, ped and automated gelcasting fabrication process of production nearly five times as much. During burn out binder process, a lower Si3 N4 ceramic turbine rotors [100 temperature is required to remove the polymer carefully or else To optimize gelcasting of silicon nitride, Omatete[98 reported the final product may have defects and cracks. Heating rates on the he optimal gelcasting condition for the AlliedSignal Ceramic com- order to 0.5-1oCmin- to temperatures as high as 650C have been onents GN-10 silicon nitride formulation in a near-production used successfully for both silicon nitride and alumina gelcast parts. nvironment. The principal criterion used to determine optimum On the other side the AlliedSignal engineers [98, 99, 103, 104] noted design was the green strength. The investigation predicted 80% that gelcasting did not have two problems that plague injection increase in green strength(4.3 MPa versus 2.4 MPa )but the results molding such as the binder that can be as high as 20% of the weight howed only 60%increase(3.8 MPa). It is preferred that gelcasting of the ceramic versus to 4% in gelcasting and the other problem is slurries be at least 50 vol% solids: higher solids loadings are desir- related to the burning out the binder in injection molding that may ables. In most of the cases, solids loadings above 501 ompared to less than a day for a gelcast ceramic Addi- achieved and some cases, solids loading above 60 vol% are attain- tionally, defects and cracks can also develop in other stages of the ble. Notwithstanding, sometimes these requirements cannot be injection molding process such as drying. Such problems are rarely and Si3N4(Ube E10)is an example of a ceramic powder in which it seen in gelcast ceramics if they are properly dried [101] is nearly impossible to achieve a solids loading above 45 vol% in a In the manufacture of Si3 Na gelcast gelcast slurry due to that this powder is very difficult to disperse for turbine engines, the reproducibility of the parts is a critical any application inasmuch as possess a high surface area. In this con- tor. Janney et al. [101]reported the fabrication of a series of silicon text and with the purpose to obtain Si3 N4 ceramics with excellent nitride batches prepared under identical conditions and gelcast properties, a series of commercial dispersants was evaluated for Eleven batches were prepared for the repeatability study and each heir efficacy in dispersing silicon nitride in water[ 90]. The follow- batch was prepared on a different day over a period of 18 days. The ng conditions should be obtained for a good dispersant in silicon results were favorable and the uniformity of the castings is shown nitride: i)The 24-h sedimentation height should be high: i. e the clearly by the standard deviation values which indicate a varia- powder should not have settled out of a suspension in a short time tion of only 0. 1-0.3% about the average value for the dimensions should pack very well when is does finally settle out of suspension, bodies show that gelcasting process is reproducible. A standard and iii) the 3-week cloudy /clear interface height should be high; deviation of only 0.02 mm or about 0. 1% was obtained i.e. the finest of the particles should stay in suspension for a very As outlined above, gelcasting provides an excellent alternative long time and should not reagglomerate and settle. Therefore, the to manufacturing large, complex-shaped components such as tur- evelopment of excellent dispersant system is critical. As example, bine rotors, valves and cam followers for gasoline and dieseleng the Table 3 of [101 summarizes the dispersants cor In reason of this, the high degree of homogeneity required to for gelcasting ceramics including Si3N produce excellent parts can be retained. Janney et al. [101 taking The mold selection. mold fabrication and mold use are the crit- data from Pollinger [105 ]illustrated this point wherein after drying. ical aspects of successful gelcasting 93, 101. Proper selection of the green density of several sections of the rotor was determined mold material, fabrication method, filling method and mold release by the Archimedes immersion method. They observed that the In make the difference between producing excellent parts and variation in green density was extremely low and all the sections producing"also rans"Stampfl et al. [93, 102] have concluded that measured except one were within 0. 2% of the average green density two parameters in addition to the bulk properties determine the of 57.77% of the theoretical density. This is an especially significant

134 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 and low organic levels in the dried green ceramics. Therefore, a wide variety of ceramic materials have been prepared using gel casting process including Si3N4, SiC, Al2O3 and ZrO2, among others. In gel casting, slurry made from ceramic powder and a water-based monomer solution is poured into a mold, polymerized in situ to immobilize the particles in a gelled part, removed from the mold while still wet, then dried and fired. If the solvent for the monomers is organic, it is no aqueous gel casting; if is water, it is aqueous gel￾casting [94,96,98]. The development of an aqueous process using acrylamide as monomer was completed in 1988 [95,99]. However, concerns regarding health, safety and disposal of acrylamide caused industrial rejection of the process because the acrylamide is a neu￾rotoxin. Therefore, the development of a low toxicity process was initiated to deal with the lack of acceptance, and it was fully demon￾strated in 1990 [90]. Ceramic parts from different ceramics such as aluminum oxide Al2O3, and high-performance silicon nitride Si3N4, have been pro￾duced by gelcasting ranging in size from 6 kg with thin sections as small as 0.2 mm and solids loading as high as 55–60 vol% in alumina slurries and 45–57 vol% in silicon nitride suspensions [99]. Although gelcast bodies typically shrink ∼23% during densifi- cation and resulting sometimes in distortion in the densified parts, this technique starts to be used not only for manufacturing of com￾plicated shaped dense products such as Si3N4 parts of turbines but also for manufacturing of porous ceramic objects [90,100]. The most advanced works in this field are already in the phase or commer￾cialization and, Allied Signal Ceramics Components (Torrance, CA, USA) working together with ORNL (Oak Ridge, TN, USA) has devel￾oped and automated gelcasting fabrication process of production Si3N4 ceramic turbine rotors [100]. To optimize gelcasting of silicon nitride, Omatete [98] reported the optimal gelcasting condition for the AlliedSignal Ceramic com￾ponents’ GN-10 silicon nitride formulation in a near-production environment. The principal criterion used to determine optimum design was the green strength. The investigation predicted 80% increase in green strength (4.3 MPa versus 2.4 MPa) but the results showed only 60% increase (∼3.8 MPa). It is preferred that gelcasting slurries be at least 50 vol% solids; higher solids loadings are desir￾ables. In most of the cases, solids loadings above 50 vol% can be achieved and some cases, solids loading above 60 vol% are attain￾able. Notwithstanding, sometimes these requirements cannot be and Si3N4 (Ube E10) is an example of a ceramic powder in which it is nearly impossible to achieve a solids loading above 45 vol% in a gelcast slurry due to that this powder is very difficult to disperse for any application inasmuch as possess a high surface area. In this con￾text and with the purpose to obtain Si3N4 ceramics with excellent properties, a series of commercial dispersants was evaluated for their efficacy in dispersing silicon nitride in water [90]. The follow￾ing conditions should be obtained for a good dispersant in silicon nitride: i) The 24-h sedimentation height should be high: i.e. the powder should not have settled out of a suspension in a short time, ii) the 3-week sedimentation height should be low; i.e. the powder should pack very well when is does finally settle out of suspension, and iii) the 3-week cloudy/clear interface height should be high; i.e. the finest of the particles should stay in suspension for a very long time and should not reagglomerate and settle. Therefore, the development of excellent dispersant system is critical. As example, the Table 3 of [101] summarizes the dispersants commonly used for gelcasting ceramics including Si3N4. The mold selection, mold fabrication and mold use are the crit￾ical aspects of successful gelcasting [93,101]. Proper selection of mold material, fabrication method, filling method and mold release can make the difference between producing excellent parts and producing “also rans”. Stampfl et al. [93,102] have concluded that two parameters in addition to the bulk properties determine the final mechanical properties of Si3N4 parts: i) the surface quality of the mold that is transferred into the final part. Therefore, the higher this surface roughness, leads to lower the final mechanical strength. The defects in the mold surface can cause notches which lead to stress concentration in the final part and ii) with perfectly smooth mold surface, the difference between bulk microstructure and surface microstructure has to be considered when describing the mechanical properties of silicon nitride parts. The maximum strength is achieved with polished samples. Similarly, Stampfl et al. [93] obtained strength values of 414, 950, and 983 MPa in Si3N4 unpolished, Si3N4 polished and Si3N4 GPS and polished, respectively, where all samples were sintered at 1750 ◦C in nitro￾gen atmosphere. On the other hand, materials such as aluminum, anodized aluminum, brass, glass, graphite, indium alloys, neoprene rubber, plaster and polyethylene are commonly used in construct￾ing gelcasting molds. As injection molding, before a ceramic body can be sintered, the binder added must be removed. One advantage of gelcasting is the small amount of polymer that remains in the green body after drying [101]. The dried gelcast ceramic contains only about 2–6 wt% polymer, which depends on the solids loading of the slurry, the concentration of monomers in the premix, and the density of powder. For comparison, a 45 vol% solids silicon nitride with the composition Si3N4 + 5 wt% Al2O3 + 5 wt% Y2O3 made using a 15 wt% 4:1 Methacrylamide-N,N’-Methylene bisacrylamide (MAM￾MBAM) premix contains about 5.5 wt% polymer in the dried part, meanwhile an injection molded silicon nitride with the same com￾position and solid loading would contain about 27 wt% polymer, nearly five times as much. During burn out binder process, a lower temperature is required to remove the polymer carefully or else the final product may have defects and cracks. Heating rates on the order to 0.5–1 ◦C min−1 to temperatures as high as 650 ◦C have been used successfully for both silicon nitride and alumina gelcast parts. On the other side, the AlliedSignal engineers [98,99,103,104] noted that gelcasting did not have two problems that plague injection molding such as the binder that can be as high as 20% of the weight of the ceramic versus to 4% in gelcasting and the other problem is related to the burning out the binder in injection molding that may take a week compared to less than a day for a gelcast ceramic. Addi￾tionally, defects and cracks can also develop in other stages of the injection molding process such as drying. Such problems are rarely seen in gelcast ceramics if they are properly dried [101]. In the manufacture of Si3N4 gelcast ceramic components for turbine engines, the reproducibility of the parts is a critical fac￾tor. Janney et al. [101] reported the fabrication of a series of silicon nitride batches prepared under identical conditions and gelcast. Eleven batches were prepared for the repeatability study and each batch was prepared on a different day over a period of 18 days. The results were favorable and the uniformity of the castings is shown clearly by the standard deviation values which indicate a varia￾tion of only 0.1–0.3% about the average value for the dimensions measured. The measurement of diameter both green and sintered bodies show that gelcasting process is reproducible. A standard deviation of only 0.02 mm or about 0.1% was obtained. As outlined above, gelcasting provides an excellent alternative to manufacturing large, complex-shaped components such as tur￾bine rotors, valves and cam followers for gasoline and diesel engines [104]. In reason of this, the high degree of homogeneity required to produce excellent parts can be retained. Janney et al. [101] taking data from Pollinger [105]illustrated this point wherein after drying, the green density of several sections of the rotor was determined by the Archimedes immersion method. They observed that the variation in green density was extremely low and all the sections measured except one were within 0.2% of the average green density of 57.77% of the theoretical density. This is an especially significant

135 accomplishment given the large vari e purpose ness in the rotor about 50 m due to der form silico of low cost 20% of the c preforms silicon nitride a series of tests of the complex shapes mus or [119]. Assembly Mold SDM Shape

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 135 accomplishment given the large variation in cross-sectional thick￾ness in the rotor taking in to account that the hub of the rotor is about 50 mm in diameter and the blade tips are only 1.5 mm tick. Variations in green density >2.5 th which caused cracking of the parts during consolidation at high temperature, were obtained with slipcast rotor of the same ceramic composition used with gelcast￾ing. Although gelcasting was developed as a near-net-shape forming process, green machining of gelcast parts can be particularly useful for producing prototypes, for custom manufacturing or for adding features to a cast part that would be too difficult or too costly to include in the mold [106]. The high strength of the green body is of great advantage for handling of the parts before sintering and for being able to produce large castings. This is achieved with a uniform distribution of the binder throughout the casting and with inherent strength of the crosslinked polymer [90,97,98,101,103,104]. Some studies have shown that adding a plasticizer such as glycerine or poly (ethylene glycol) to the gelcasting formulation markedly improves the machinability of green gelcast parts. In Addition, with the proper preparation gelcasting formulation (binder, plas￾ticizer, and dispersant, for example) and the addition of 3–8% sintering aids (usually a combination of Y2O3, La2O3, and Al2O3) allows densification and predictable shrinkage in near-net-shape moderate-pressure gas sintering (3 h at 1800–1900 ◦C in N2 at 1–2 MPa) [107]. Especially in the case of silicon nitride and advanced ceramics, the control of the microstructure, grain growth and acceptable final properties is crucial for the quality of final part. In order to achieve this, dispersed submicron powders are used in the suspensions [93,103]. With high quality silicon nitride powders (oxygen below 0.2%), the parts routinely achieve bending rupture strength over 750 MPa at temperatures up to 827 ◦C, very high fracture tough￾ness and Weibull modulus above 15 [108]. The highest Weibull modulus has been obtained using La2O3 as the primary sinter￾ing aid in the formulations [109]. With the proper sintering cycle, a bimodal microstructure is obtained which enhances toughness and high temperature creep strength [110]. Using of Lu2O3 (which forms a Lu2Si2O7 phase) for most or all of the sintering aids results in slightly reduced RT strength, Weibull modulus, and shrinkage accuracy. Since for applications in gelcasting process most surfaces of the sintered part cannot be further machined or ground to obtain the near-shape, the surface microstructure has to be considered in addition to the bulk microstructure. Therefore, the roughness of the sintered part is determined by two factors as follows: i) the original particle size and ii) the amount of grain growth of the -needless which build up during the --Si3N4 phase transfor￾mation. Stampfl et al. [93] have reported the typical -needles in a glassy matrix in the bulk material of sintered silicon nitride, while due to the inhibited grain growth at the surface, the individual pow￾der particles sinter together to form a fairly smooth surface with surface roughness of the final part between 0.5 and 1.8 m. It is well known that Si3N4 based ceramics tend to be pro￾hibitively expensive due to the high cost or silicon nitride powders used to produce them. Therefore, nowadays the reduction of cost has been recognized as a major need for the successful introduction of silicon nitride ceramics in the wide marketplace [111,112]. With this expectative at hand, Sintered Reaction Bonded Silicon Nitride (SRBSN) is an attractive alternative to sintered silicon nitride which is formed by reacting silicon powder with nitrogen gas in order to form silicon nitride taking into account that silicon is a raw material of low cost [92,97,109,113] (high purity silicon powder is only about 20% of the cost of silicon nitride powder). On the other hand, silicon preforms undergo less sintering shrinkage than performs made of silicon nitride powders. However, silicon metal performs of very complex shapes must be made by expensive cumbersome forming process such as injection molding. Nonetheless, with the purpose to make these silicon performs, gelcasting is a simple, inexpensive process which has been developed as a method for forming ceramic greenware. In gelcasting of silicon metal compositions wherein the typical slurry is aqueous and basic (having pH of about 8.4), two difficulties have been observed: i) the poor dispersion characteristics of the sil￾icon powder in the slurries and ii) the generation of gas bubbles in the slurry caused by reaction of the siliconmetal with water. Several experimental investigations showed that these processing difficul￾ties could be overcome by reducing the pH of the aqueous slurry or by using isopropyl alcohol as the solvent system in place of water. Therefore, both an acidic aqueous system (with 35–40 vol% solids) and an alcohol-based system (with 50 vol% solids) were developed and successfully adapted for SRBSN gelcasting. With these param￾eters, Nunn et al. [97] reported that the green density of the as-cast samples showed a distinct difference between the two gelcastings systems, obtaining green densities of about 43% in aqueous slurries while the alcohol-based bodies had green densities of 51–55%. High green density can be detrimental conventional nitriding processes due to the tendency of the nitriding reaction to start at the surface of the sample and progress inward. Likewise, the volume expan￾sion can close off the pore structure and prevent the nitrogen gas from reaching the unreacted silicon in the interior of the sample, especially in thick bodies. On the other hand, Kiggans et al. [114] and Kiggans and Tiegs [115] have shown that microwave heating results in improved mechanical properties in the final SRBSN product and their study was based in the composition 67 wt% Si-metal (Elkem Metallurgical grade) + 13 wt% Y2O3 (Molycorp-5600) + 4 wt% Al2O3 (RCHP-DBM) + 14 wt% Si3N4 (Stark LCION) + 2 wt% SiO2 (U.S. Silica-5 micron) which was turbomilled for ∼2 h with 4 mm Si3N4 media in isopropanol and Darvan-C and PVP as dispersants. With this com￾position the weight gain obtained (after the single-step nitridation and sintering treatment) was about 60% and is considered as near complete nitridation since weight losses occur during the sintering step [92,116]. The authors in their experimental study found that appears to be no relationship between the final sintered densities and the green densities of the materials calculating as theoretical density 3.3 g cm−3. For the fabrication of Si3N4 ceramic components for micro gas turbine engines, Assembly Mold SDM Shape Deposition Manufac￾turing (SDM) has been used in combination with gelcasting [117]. Assembly Mold Shape Deposition Manufacturing is a derivation of Mold SDM, an additive-subtractive layered manufacturing process developed at Stanford University [118]. With this technique, once a fugitive assembly mold has been made, the gelcasting process is applied to build a monolithic ceramic part. However, the major drawback of the SDM process is the possibility of geometrical inaccuracy during the mold assembly process. In order to combine both Assembly Mold SDM and gelcasting processes, the Rapid Prototyping Laboratory (RPL) at Stanford University developed a miniature Si3N4 ceramic gas turbine with its industrial partners [117]. These two techniques have allowed the fabrication of the rotor group as well as inlet nozzle to spin at 800,000 rpm to generate 100W where due to the complexity of the geometry, the casting mold is decomposed into five parts: a cap, a turbine, an interconnect, a compressor and a shaft. Nonetheless, Liu et al. [117] reported that after the rotor group was sintered, it is found that the geometry features have shifted from the concentric center (an eccentricity of 0.5 mm). However, after removing the errors from the fabrication and the mold assembly and provide better geometric support, the functionality of the rotor group using these processes has been demonstrated by a series of tests of the turbine and the compressor [119]. Assembly Mold SDM Shape

M.H. Bocanegra-BemaL B Matovic/ Materials Science and Engineering A 500(2009)130-149 Deposition Manufacturing process will be analyzed in more detail imentations, the optimal conditions for robocast found by He et al. later on. Compared to other colloidal processes(for example, slip [120 were 52 vol% Si3 N4 with 1 wt% Darvan 821A as dispersant casting and tape casting) the outstanding advantage of gelcasting and 0. 4 wt% aluminum nitrate at pH 7.8-8.5 based on rheologi is that its slurry can be in situ consolidated which in turn results cal studies of Si3 Na slurries. Likewise, no meshing, warping and in near-net-shape forming. This promising technique has been cracking were observed during forming and drying and the green employed with submicrometer or micrometer ceramic materials and sintered densities were about 56 and 99.3% of theoretical den- such as alumina, zirconia, silicon nitride, etc For Si3N4, however, sity, respectively. On the other hand, the sintered ceramics showed it is difficult to form a dense structure because of gas-discharging regular hexagonal shapes of cross-section of B-Si3N4 fibers which reactions. In this process, parts shrink and also experience warpage are characteristics of typical dense silicon nitride ceramics and are as a direct result of drying of the part. There are several factors believed to be responsible for the superior mechanical properties of which contribute to this as humidity, temperature and geometry. Si3N4 ceramics. Robocasting technology can also fabricate parts of large thicknesses that are unobtainable using slip casting. Likewise, (HA)that show promise as load-bearing scaffolds for bonxyapatite this technique has been used to develop lattices of hy he repal The Robocasting is a relatively new freeform fabrication tech- The advantages of robocasting technique are many. The aqueous nique for dense ceramics wherein to control deposition of ceramic systems are binderless and have very low toxicity. a densified part urries through an orifice the use of robotics are required [43, 120). can be made in less of 24 h. This technique is also amenable to Ceramic components with simple or complex shapes can be rapidly multi-material fabrication[121, 122). roduced form a computer aided-design drawing directly to a finished component that requires little or no machining after fab- 3. 4. Si3 Na ceramic components by mold shape deposition rication. The highlighted of this novel technique is moldless and manufacturing(MOLD SDM) inderless, and ceramics parts can be formed, dried, and sin ered within 24 h. This procedure uses the deposition of highly As outlined above gelcasting is a versatile method for making concentrated colloidal slurries with low organic content(<1 wt%) quality ceramic parts, however due to the poor interlayer bonding to construct complex, three-dimensional (3-D)components in it was not possible to build parts incrementally in layers. However, layer-by-layer built sequence [121, 122]. Robocasting has great by using the waxes and soldermasks, as part and support materi- romise for the rapid manufacture of complex, multiphase assem- als, respectively, it would be possible to build complex fugitive wax plage devices, such as piezoelectric ceramic-polymer composites, molds using SDM. These molds could then be used for gelcasting. ohotonic-band-gap lattices as well as Si3N4 ceramics in advanced This combination of process and materials was the starting point engines and gas turbines. for Mold SDM [117, 118. Based on the limitations of the existing pro- It is well known that Si3 Na is a non-oxide material and gener- cesses, the purpose for SDM and then mold SDM of Si is quite difficult to process via colloidal procedures. Hence, it is to improve the green part fabrication process and to develop is important to understand aspects about the dispersion and rheol- method to produce high quality green parts with high shape com ogy of Si3 N4 in order to obtain the optimal robocasting conditions. plexity and low cost Mold SDM makes molded parts using fugitive tion formed by a-Si3N4 with particle size of 0.77 um, surface area tion of this interesting technique is described elsewhere 18/ As example, He et al. [120]used in their investigations a composi- wax molds built using SDM techniques. A more complete descrip of 7.7mg. As dispersant was used Darvan 821A(with 40 wt% of Since mold SDM is based on SDM it shares many of the same ammonium polyacrylate of molecular weight about 3500). Finally, advantages and disadvantages over ot the ph was adjusted nalytical grade nitric acid(IN)and processes. The principal advantage of Mold SDM over other layered ammonium hydroxide solutions(40%). Robocast bars were formed manufacturing processes is that the final part is cast monolithi- and dried overnight at room temperature and sintered by pressure cally. This is beneficial for two reasons: i) the finished part will sintering in a N2 atmosphere from 1600 to 1800C for 1-2 h. The not contain any layer boundaries which in turn can be a source of final results indicated that the 1 wt% dispersant added is the most weakness due to incomplete bonding or the presence of foreig efficient concentration for dispersing Si3 N4 powder in aqueous sus- particles, ii) the finished part will not contain any of the residual pensions. This result implies that further addition of the dispersant stresses that typically result from layered manufacturing. The mold beyond a certain level had no more measurable contribution to may contain residual stresses, but these will not be transferred to he interfacial charge of the powder because the adsorbent dis- the cast part. Therefore, the lack of residual stresses in the finished persant layers on powder surfaces are saturated. Other authors, part will reduce the tendency for distortion. The elimination of the for example Albano and garrido [ 123] and Liu and Malghan [124 need for interlayer bonding in the part material allows Mold SDM who used other experimental methods obtained similar results to to use materials which cannot be used in SDM. The ACR(Advanced the obtained by He et al. [120] who concluded that the minimum Ceramics Research)gelcasting formulations are one example[ 126]. viscosity (greatest degree of dispersion)occurred at 1 wt% Darvan Mold SDM has three disadvantages over SDM. i)a third compatible lize a solids content of less than 47 vol%. When the volume percent of materials that can be used, i) there are extra casting an i material, the mold material, is required. The additional materials increases, the viscosity also increases and at low solids loading, removal steps which increase process time and iii)mold filling Newtonian. At 35 vol% solids, the slurries begin to show pseudo- vents can be added to ensure complete mold fillin e es sprues and dispersed slurries exhibit very low viscosity and are rheologically issues may limit part geometry, although in many cas plastic behavior and the viscosity is even relatively low while solids The materials selection process for Mold SDM is an important content around 50 vol%, interparticle interactions and interparticle factor in order to obtain a successful implementation of the process collisions become dominant, the viscosity begins to increase appre There are two categories of materials used in mold SDM: those that ciably and the rheological behavior becomes highly shear-thinning. are used to build molds and those that are cast into the finished With these conditions to the hand, for optimal robocasting, it is molds to produce parts. The two categories have different property desirable to robocast with slurries that have solids loading close to requirements because of their different uses. Property require- he dilatant transition (about 47 vol%)[43, 125 After several exper- ments can also be roughly divided into two groups: those that are

136 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 Deposition Manufacturing process will be analyzed in more detail later on. Compared to other colloidal processes (for example, slip casting and tape casting) the outstanding advantage of gelcasting is that its slurry can be in situ consolidated, which in turn results in near-net-shape forming. This promising technique has been employed with submicrometer or micrometer ceramic materials such as alumina, zirconia, silicon nitride, etc. For Si3N4, however, it is difficult to form a dense structure because of gas-discharging reactions. In this process, parts shrink and also experience warpage as a direct result of drying of the part. There are several factors which contribute to this as humidity, temperature, and geometry. 3.3. Robocasting of Si3N4 aqueous slurries The Robocasting is a relatively new freeform fabrication tech￾nique for dense ceramics wherein to control deposition of ceramic slurries through an orifice the use of robotics are required [43,120]. Ceramic components with simple or complex shapes can be rapidly produced form a computer aided-design drawing directly to a finished component that requires little or no machining after fab￾rication. The highlighted of this novel technique is moldless and binderless, and ceramics parts can be formed, dried, and sin￾tered within 24 h. This procedure uses the deposition of highly concentrated colloidal slurries with low organic content (<1 wt%) to construct complex, three-dimensional (3-D) components in layer-by-layer built sequence [121,122]. Robocasting has great promise for the rapid manufacture of complex, multiphase assem￾blage devices, such as piezoelectric ceramic–polymer composites, photonic-band-gap lattices as well as Si3N4 ceramics in advanced engines and gas turbines. It is well known that Si3N4 is a non-oxide material and gener￾ally is quite difficult to process via colloidal procedures. Hence, it is important to understand aspects about the dispersion and rheol￾ogy of Si3N4 in order to obtain the optimal robocasting conditions. As example, He et al. [120] used in their investigations a composi￾tion formed by -Si3N4 with particle size of 0.77 m, surface area of 7.7 m2 g−1. As dispersant was used Darvan 821A (with 40 wt% of ammonium polyacrylate of molecular weight about 3500). Finally, the pH was adjusted with analytical grade nitric acid (1N) and ammonium hydroxide solutions (40%). Robocast bars were formed and dried overnight at room temperature and sintered by pressure sintering in a N2 atmosphere from 1600 to 1800 ◦C for 1–2 h. The final results indicated that the 1 wt% dispersant added is the most efficient concentration for dispersing Si3N4 powder in aqueous sus￾pensions. This result implies that further addition of the dispersant beyond a certain level had no more measurable contribution to the interfacial charge of the powder because the adsorbent dis￾persant layers on powder surfaces are saturated. Other authors, for example Albano and Garrido [123] and Liu and Malghan [124] who used other experimental methods obtained similar results to the obtained by He et al. [120] who concluded that the minimum viscosity (greatest degree of dispersion) occurred at 1 wt% Darvan dirpersant. Conventional silicon nitride slurries for slip casting typically uti￾lize a solids content of less than 47 vol%. When the volume percent increases, the viscosity also increases and at low solids loading, dispersed slurries exhibit very low viscosity and are rheologically Newtonian. At 35 vol% solids, the slurries begin to show pseudo￾plastic behavior and the viscosity is even relatively low while solids content around 50 vol%, interparticle interactions and interparticle collisions become dominant, the viscosity begins to increase appre￾ciably and the rheological behavior becomes highly shear-thinning. With these conditions to the hand, for optimal robocasting, it is desirable to robocast with slurries that have solids loading close to the dilatant transition (about 47 vol%) [43,125]. After several exper￾imentations, the optimal conditions for robocast found by He et al. [120] were 52 vol% Si3N4 with 1 wt% Darvan 821A as dispersant and 0.4 wt% aluminum nitrate at pH 7.8–8.5 based on rheologi￾cal studies of Si3N4 slurries. Likewise, no meshing, warping and cracking were observed during forming and drying and the green and sintered densities were about 56 and 99.3% of theoretical den￾sity, respectively. On the other hand, the sintered ceramics showed regular hexagonal shapes of cross-section of -Si3N4 fibers which are characteristics of typical dense silicon nitride ceramics and are believed to be responsible for the superior mechanical properties of Si3N4 ceramics. Robocasting technology can also fabricate parts of large thicknesses that are unobtainable using slip casting. Likewise, this technique has been used to develop lattices of hydrixyapatite (HA) that show promise as load-bearing scaffolds for bone repair. The advantages of robocasting technique are many. The aqueous systems are binderless and have very low toxicity. A densified part can be made in less of 24 h. This technique is also amenable to multi-material fabrication [121,122]. 3.4. Si3N4 ceramic components by mold shape deposition manufacturing (MOLD SDM) As outlined above, gelcasting is a versatile method for making quality ceramic parts, however due to the poor interlayer bonding it was not possible to build parts incrementally in layers. However, by using the waxes and soldermasks, as part and support materi￾als, respectively, it would be possible to build complex fugitive wax molds using SDM. These molds could then be used for gelcasting. This combination of process and materials was the starting point for Mold SDM [117,118]. Based on the limitations of the existing pro￾cesses, the purpose for SDM and then Mold SDM of Si3N4 ceramics is to improve the green part fabrication process and to develop a method to produce high quality green parts with high shape com￾plexity and low cost. Mold SDM makes molded parts using fugitive wax molds built using SDM techniques. A more complete descrip￾tion of this interesting technique is described elsewhere [118]. Since Mold SDM is based on SDM it shares many of the same advantages and disadvantages over other Rapid Prototyping (RP) processes. The principal advantage of Mold SDM over other layered manufacturing processes is that the final part is cast monolithi￾cally. This is beneficial for two reasons: i) the finished part will not contain any layer boundaries which in turn can be a source of weakness due to incomplete bonding or the presence of foreign particles, ii) the finished part will not contain any of the residual stresses that typically result from layered manufacturing. The mold may contain residual stresses, but these will not be transferred to the cast part. Therefore, the lack of residual stresses in the finished part will reduce the tendency for distortion. The elimination of the need for interlayer bonding in the part material allows Mold SDM to use materials which cannot be used in SDM. The ACR (Advanced Ceramics Research) gelcasting formulations are one example [126]. Mold SDM has three disadvantages over SDM. i) a third compatible material, the mold material, is required. The additional materials compatibility and processing requirements may restrict the range of materials that can be used, ii) there are extra casting and mold removal steps which increase process time and iii) mold filling issues may limit part geometry, although in many cases sprues and vents can be added to ensure complete mold filling. The materials selection process for Mold SDM is an important factor in order to obtain a successful implementation of the process. There are two categories of materials used in Mold SDM: those that are used to build molds and those that are cast into the finished molds to produce parts. The two categories have different property requirements because of their different uses. Property require￾ments can also be roughly divided into two groups: those that are

M.H. Bocanegra-Bermal B Matovic/ Materials Science and Engineering A 500(2009)130-14 137 necessaries to make the Mold SDM process work, and those that is ceramic components for use in gas turbine engines. Ideally Mold are desirable because they improve the process by making it better, SDM can be used as a production process, rather than a proto- faster or cheaper. The material properties related to deposition are: typing process. Prototyping processes are useful during design but low shrinkage, low viscosity, good wetting for the replication of fine there is always the issue of how the production parts will be made details, strong interlayer bonding curing temperatures compatible [119 In large turbines, both vanes and blades are large enough to vith other materials On the other side the properties related to be manufactures with internal and film cooling which allows high casting and curing are: i)Viscosity: lower viscosity materials are temperature metals to be used in most cases. However, in the case easier to cast because they flow through the small passages in the of micro-scale turbines(diameter <100 mm), the limits of scale pre- molds more easily. Lower viscosity materials are also easier to deair clude active blade cooling, and thus the blades must be solid which because bubbles can move through the liquid and up to the surface makes ceramics desirable, if they can be produced [ 128. more easily, ii) Wetting: mold filling is also dependent on how well On the other hand. the reliability and the lack of the shap- the part material wets the mold material. If wetting is poor the ing techniques have been the major issues for the application of part material will have difficulty flowing into fine features in the Si3 N4 ceramics to gas turbines and rotors in particular. However nold Applying pressure during casting will help but this compli- the second issue has been overcome by using Mold SDM process cates the process, iii) Working time: longer working times make in combination with gelcasting. Taking into account the high rotat ting easier because there is more time to fill the mold and deair ing speeds(designed to rotate at 800.000 rpm with turbine inlet it Longer working times usually correlate with longer cure time temperature above 1000C), the straightness of the shaft and the but this is not as much of an issue with part materials because there shape accuracy is crucial for a functioning device. Therefore, with is only one casting operation per part, instead of one per mold layer the proper development of high-speed bearing technology and the as there is with mold and support materials, iv)Cure conditions: application of Mold SDM process to manufacture of the hot rotating whatever conditions are required to cure the part material must be elements in silicon nitride, micro-scale gas turbine engine appears compatible with the mold material. Materials are often cured by to be a possibility. In addition to ceramic parts, Mold SDM pro- heating them, in which case the cure temperature must be suffi- cess can be used to make parts from a variety of castable materials ciently low that it does not cause the mold to soften or melt. The including polymers such as polyurethane, epoxy and silicone. The cure exotherm must also be taken into account, v )Cure exotherm: Mold SdM process has been automated by the addition of depo- any materials cure exothermically and since the molds are made sition and curing hardware to a commercially available milling of wax this can be an issue if enough heat is generated that the mold machine Parts have been made without manual intervention using softens or melts. Faster curing materials generally exhibit higher this machine. Two main disadvantage of this process are i) the cure exotherms, vi) Chemical compatibility: part materials must be imperfect deposition of wax generating micro bubbles and dis- chemically compatible with the mold material and the mold mate- tortion, and ii) to preserve the straightness during the sintering rial must not chemically inhibit the curing of the part material or [118, 128]. affect the properties of the materials, and vii). Shrinkage on cur part materials must shrink as little as possible on cure for two rea-.5. Rapid prototyping of si3 Na ceramic parts sons. First, if there is significant shrinkage then the cured part may not accurately duplicate the geometry of the mold cavity Section a wide variety of commercially available systems for rapid pro- of the part might shrink away from the mold surface and cause sink totyping enables the user to fabricate prototypes with almost any holes on the part surface. Second, if the part shrinks it may break shape in a large range of different sizes 93. Most RP techniques put itself or the mold due to the stress created [118 less emphasis on materials issues, and if they do 129, 130 it is not All ceramic parts made using Mold SDM have been made by easy to switch between different materials. Rapid Prototyping pro- elcasting. The initial material was proprietary non-aqueous alu- cesses, also known as Solid Freeform Fabrication(SFF) processes. mina gelcasting slurry developed by ACr Silicon nitride slurry has build parts in a layerwise fashion. There are a number of reasons also been developed, based on the alumina formulation. However, for adopting this approach: i) shape complexity, ii)elimination of due to differences in the surface chemistries of alumina and silicon tooling, iii) simplified process planning, iv)automated fabrication, nitride the organic components are slightly different in each slurry. v)material limitations, vi) surface quality and vii) material quality. The silicon nitride slurries are more difficult to make and do not Some RP processes have been used in smaller scale in order urrently have as high solids loadings as the alumina slurries[ 127. to manufacturing Si3N4 ceramic components, such as: a)Stere- For example, the solids loading for Al2O3 slurry is about 50% and lithography( SLA)[131 For this process, the sintered Si3Na was cure conditions of 30 min at 55C compared to solids loading of only 90% dense which account for the low strength of 412 MPa 52% and cure of 30 min at 55C required for Si3 N4 slurries. reported by Zimbeck et al. [132], b)Fused Deposition Model- Cooper [118] reported the processing conditions for the silicon ing(FDM)[133-135. The strength obtained with silicon nitride nitride gelcasting slurries used in Mold SDM as follows: 1)Cur- ceramic components by using this technique was 824+ 110 MPa ing at 50-60 C for 2 h where the parts are hermetically sealed [135. c) Three-Dimensional Printing(3DP)[136 Si3 N4 ceramics and evacuated to exclude air, ii) Drying performed in air 4 h at produced by this technique have obtained strength values about 6°c.1.5hat96°,2hat155°,15hat165°,2hat186°,i)570MPa137] Burnout performed in air using the following time-temperature Most of these techniques were developed to prepare plastic, profile:80°hlto160°c,10hlto300°c2hat300°c6°ch-1wax, or paper parts[138] However, although the individual pro to 400C, 2 h at 400 C, 12Ch-I to 500 C, 18Ch- to 600 C, 1h cess differs, they all produce a solid part directly from a 3D CAD at 600C, iv) Sintering carried out in nitrogen atmosphere at tem- drawing, without the need of dies or molds. The initial steps for eratures between 1700 and 1750 C. It is very important to control each of the different flexible manufacturing techniques are simi- the atmosphere to obtain the best results. lar. An integration of rapid prototyping technologies into standard The goal in developing Mold SDM is to develop a production ceramic shaping processes has been studied by Loschau [139)and manufacturing process to enable the fabrication of complex func- Knitter et al. [140]. After debinding and sintering, there are complex tional ceramic parts, particularly in the hot sections of the engines functional ceramic components that can be immediately used. It where highly stressed turbines and other components must sur- also possible than after the correction of the shrinkage which was vive in contact with very hot gasses[128 ]. The primary application estimated during an interactive step, must be started in order to

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 137 necessaries to make the Mold SDM process work, and those that are desirable because they improve the process by making it better, faster or cheaper. The material properties related to deposition are: low shrinkage, low viscosity, good wetting for the replication of fine details, strong interlayer bonding curing temperatures compatible with other materials. On the other side, the properties related to casting and curing are: i) Viscosity: lower viscosity materials are easier to cast because they flow through the small passages in the molds more easily. Lower viscosity materials are also easier to deair because bubbles can move through the liquid and up to the surface more easily, ii) Wetting: mold filling is also dependent on how well the part material wets the mold material. If wetting is poor the part material will have difficulty flowing into fine features in the mold. Applying pressure during casting will help but this compli￾cates the process, iii) Working time: longer working times make casting easier because there is more time to fill the mold and deair it. Longer working times usually correlate with longer cure times, but this is not as much of an issue with part materials because there is only one casting operation per part, instead of one per mold layer as there is with mold and support materials, iv) Cure conditions: whatever conditions are required to cure the part material must be compatible with the mold material. Materials are often cured by heating them, in which case the cure temperature must be suffi- ciently low that it does not cause the mold to soften or melt. The cure exotherm must also be taken into account, v) Cure exotherm: many materials cure exothermically and since the molds are made of wax this can be an issue if enough heat is generated that the mold softens or melts. Faster curing materials generally exhibit higher cure exotherms, vi) Chemical compatibility: part materials must be chemically compatible with the mold material and the mold mate￾rial must not chemically inhibit the curing of the part material or affect the properties of the materials, and vii). Shrinkage on cure: part materials must shrink as little as possible on cure for two rea￾sons. First, if there is significant shrinkage then the cured part may not accurately duplicate the geometry of the mold cavity. Sections of the part might shrink away from the mold surface and cause sink holes on the part surface. Second, if the part shrinks it may break itself or the mold due to the stress created [118]. All ceramic parts made using Mold SDM have been made by gelcasting. The initial material was proprietary non-aqueous alu￾mina gelcasting slurry developed by ACR. Silicon nitride slurry has also been developed, based on the alumina formulation. However, due to differences in the surface chemistries of alumina and silicon nitride the organic components are slightly different in each slurry. The silicon nitride slurries are more difficult to make and do not currently have as high solids loadings as the alumina slurries [127]. For example, the solids loading for Al2O3 slurry is about 50% and cure conditions of 30 min at 55 ◦C compared to solids loading of 52% and cure of 30 min at 55 ◦C required for Si3N4 slurries. Cooper [118] reported the processing conditions for the silicon nitride gelcasting slurries used in Mold SDM as follows: i) Cur￾ing at 50–60 ◦C for 2 h where the parts are hermetically sealed and evacuated to exclude air, ii) Drying performed in air 4 h at 86 ◦C, 1.5 h at 96 ◦C, 2 h at 155 ◦C, 15 h at 165 ◦C, 2 h at 186 ◦C, iii) Burnout performed in air using the following time-temperature profile: 80 ◦C h−1 to 160 ◦C, 10 ◦C h−1 to 300 ◦C, 2 h at 300 ◦C, 6 ◦C h−1 to 400 ◦C, 2 h at 400 ◦C, 12 ◦C h−1 to 500 ◦C, 18 ◦C h−1 to 600 ◦C, 1 h at 600 ◦C, iv) Sintering carried out in nitrogen atmosphere at tem￾peratures between 1700 and 1750 ◦C. It is very important to control the atmosphere to obtain the best results. The goal in developing Mold SDM is to develop a production manufacturing process to enable the fabrication of complex func￾tional ceramic parts, particularly in the hot sections of the engines where highly stressed turbines and other components must sur￾vive in contact with very hot gasses [128]. The primary application is ceramic components for use in gas turbine engines. Ideally Mold SDM can be used as a production process, rather than a proto￾typing process. Prototyping processes are useful during design but there is always the issue of how the production parts will be made [119]. In large turbines, both vanes and blades are large enough to be manufactures with internal and film cooling which allows high temperature metals to be used in most cases. However, in the case of micro-scale turbines (diameter <100 mm), the limits of scale pre￾clude active blade cooling, and thus the blades must be solid which makes ceramics desirable, if they can be produced [128]. On the other hand, the reliability and the lack of the shap￾ing techniques have been the major issues for the application of Si3N4 ceramics to gas turbines and rotors in particular. However, the second issue has been overcome by using Mold SDM process in combination with gelcasting. Taking into account the high rotat￾ing speeds (designed to rotate at 800.000 rpm with turbine inlet temperature above 1000 ◦C), the straightness of the shaft and the shape accuracy is crucial for a functioning device. Therefore, with the proper development of high-speed bearing technology and the application of Mold SDM process to manufacture of the hot rotating elements in silicon nitride, micro-scale gas turbine engine appears to be a possibility. In addition to ceramic parts, Mold SDM pro￾cess can be used to make parts from a variety of castable materials including polymers such as polyurethane, epoxy and silicone. The Mold SDM process has been automated by the addition of depo￾sition and curing hardware to a commercially available milling machine. Parts have been made without manual intervention using this machine. Two main disadvantage of this process are i) the imperfect deposition of wax generating micro bubbles and dis￾tortion, and ii) to preserve the straightness during the sintering [118,128]. 3.5. Rapid prototyping of Si3N4 ceramic parts A wide variety of commercially available systems for rapid pro￾totyping enables the user to fabricate prototypes with almost any shape in a large range of different sizes [93]. Most RP techniques put less emphasis on materials issues, and if they do [129,130] it is not easy to switch between different materials. Rapid Prototyping pro￾cesses, also known as Solid Freeform Fabrication (SFF) processes, build parts in a layerwise fashion. There are a number of reasons for adopting this approach: i) shape complexity, ii) elimination of tooling, iii) simplified process planning, iv) automated fabrication, v) material limitations, vi) surface quality and vii) material quality. Some RP processes have been used in smaller scale in order to manufacturing Si3N4 ceramic components, such as: a) Stere￾olithography (SLA) [131]. For this process, the sintered Si3N4 was only 90% dense which account for the low strength of 412 MPa reported by Zimbeck et al. [132], b) Fused Deposition Model￾ing (FDM) [133–135]. The strength obtained with silicon nitride ceramic components by using this technique was 824 ± 110 MPa [135], c) Three-Dimensional Printing (3DP) [136]. Si3N4 ceramics produced by this technique have obtained strength values about 570 MPa [137]. Most of these techniques were developed to prepare plastic, wax, or paper parts [138]. However, although the individual pro￾cess differs, they all produce a solid part directly from a 3D CAD drawing, without the need of dies or molds. The initial steps for each of the different flexible manufacturing techniques are simi￾lar. An integration of rapid prototyping technologies into standard ceramic shaping processes has been studied by Loschau [139] and Knitter et al.[140]. After debinding and sintering, there are complex functional ceramic components that can be immediately used. It is also possible than after the correction of the shrinkage, which was estimated during an interactive step, must be started in order to

M.H. Bocanegra-BemaL B Matovic/ Materials Science and Engineering A 500(2009)130-149 obtain the final dimension of the ceramic components. Rapid pro- between Si3 N4 and metal oxides around the sintering temperatures typing technology can shorten the product development time (-1727C)are an important factor in elucidating the role of sinter- ach beyond the limit of conventional machine tools, and lower the ing aids. On the other side metal oxides such as Mn3 O4, Moo, Feo, costs for making a prototype RP combined with microwave sinter- Li,Oz and Cr3 O3 are expected to decompose Si3 NA. Experimental ing can provide a fast manufacturing route, from virtual to finished results have shown that lanthanide oxide such as Nd2 O3, Pr203 and part in less than one day. These techniques allow great flexibility in Sm2O3 as well as actinide oxide ac2O3 also work well as sintering the design of the part without incurring large tooling costs[80, 81. aids for Si N4. All these additives produce a liquid phase during fi This flexibility translates into time-and-cost savings, which can be a ing at high temperatures by reaction with the Sioz always present significant advantage by reducing the time-to-market. The process on Si3N4 powder particles [152, 153]. This process, however, deter- was extremely reproducible and is being used for material qualifi- minates high temperature mechanical properties because they are tion in an aerospace application. Microstructural and mechanical dependent on the characteristics of the secondary grain boundary properties are currently under investigation. phase [154. such as composition, amount, and state of the phase [155-157. Numerous research groups have intensively studied the 4. Processing techniques for dense Si3N4 ceramics sintering of silicon nitride with a vast variety of additives [158. Despite all these efforts the quest for the optimum Si3 N4 sinte For an optimum utilization of the inherent good properties of ing aid continues. Beside the choice of the proper additive, also N4. the powders must be fully densified to compacts. However, the subsequent crystallization of the glassy intergranular phase due to the high degree of covalent bonding, it is very difficult to has been studied as a means to further increase strength since sinter these materials to full density. As a consequence, sever this one limits the high temperature properties because it softens alternative techniques have been developed to improve the den- and results in the easy slide of SiNa grains at high temperature sification of Si3 N4 based ceramics. It is explained that fully dense [159]. Si3 NA can be produced by the addition of densification additives Considering the mentioned above, after the liquid phase sinter- hich allow liquid phase sintering. there is a wide range of sin- ing or cooling, the microstructure of dense SiaNa consists mainly tering additives for Si3 N4 that have been explored to date. Since of B-Si3N4 and the liquid solidifies to amorphous or partially the densification rate is strongly related to the type and amount crystalline secondary phases, with are located either of the grain of sintering additives, as are the microstructure development and boundaries in the form of thin layers or at triple junctions as can the thermal and mechanical properties of sintered bodies, there be seen in Fig. 1 [35]. A typical feature of sintered Si3 Na ceramics is is still considerable effort to find and optimize sintering aids for the morphology of the Si3N4 grains. Residual a-grains are equiaxed high-performance Si3 N4 materials. and the B-phase exhibits an elongated grain structure with an aspect ratio(ratio of length to thickness)usually en the range of 5 4. 1. Pressureless sintering to 10 [160, 161]. The obtained microstructure is controlled by the SinA starting powders as well as the additives used and the pre This is a conventional technique that has been used to man- sureless sintering parameters. The type and amount of sintering ufacture ceramics for many centuries [141 and it is a process additives determine the liquid forming temperature, the onset of which a part of the material being sintered is in liquid state(sim- densification and its rate during sintering 162). Similarly, they ple heating of powder compacts). LPS is important for systems also define the morphology of the B-grains and the characteristics which are difficult to densify by solid state sintering [35]. i.e. ceram- of the grain boundary phase, which in turn, as outlined previously cs that possess a high degree of covalent bonding such as Si3N4 ontrols the high-temperature propertie can express the role of the additive (Si3N4+ SiO2+ impurities)+ Additive Sintering Temperature B-Si3N4+ Liquid(SiOz+ Additive Si3 N4) [Sintering Cooling B-Si3N4+ Amorphous/ Crystalline Phases(SiO2+ Additives Final Product Various kinds of Si3N4 ceramics have been developed by this The eutectic temperatures of the more commonly used oxide method [142]. So far, sintering of silicon nitride to high densities systems for the liquid phase of silicon nitride are listed in Table 1 has demanded the addition of metal oxides [21, 143-145]. nitrides [35,163, 164]. other oxide additives used in manufacturing of Si3N4 as aIN [16, 146, 147]and rare earth oxides [143, 148-150 According and their melting temperatures are also included in this table. The to Negita [21], an important condition for effective metal oxide sin- alkali and alkaline-earth oxides have a low melting point and the tering aids is that they do not decompose Si3N4 to N2 and SiO2 or Sio viscosity of the resulting liquid is also low. A good example for dif- gas in the sintering process, i. e reactions between metal oxides and ferent behavior of sintering additives is Magnesia and Yttria. The i3N4 do not proceed. Metal oxides that satisfies this conditions are temperature of liquid formation for the Mgo-Sio2-Si3Na system MgO, Al203, Sc203, ZrO2, CeO2, Ce203, LiO2, Y203, La203, CaO, Beo, is lower by nearly 100C in comparison with the Y2O3-Sio2-Si3N4 HfO2, and SrO. Almost all of these metal oxides are reported to be (according to Table 1). A summary of magnesia and yttria additives uccessful sintering aids for Si3 N4[151]. This suggests that reactions and their compounds is shown in Table 2

138 M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 obtain the final dimension of the ceramic components. Rapid pro￾totyping technology can shorten the product development time, reach beyond the limit of conventionalmachine tools, and lower the costs for making a prototype. RP combined with microwave sinter￾ing can provide a fast manufacturing route, from virtual to finished part in less than one day. These techniques allow great flexibility in the design of the part without incurring large tooling costs [80,81]. This flexibility translates into time-and-cost savings, which can be a significant advantage by reducing the time-to-market. The process was extremely reproducible and is being used for material qualifi- cation in an aerospace application. Microstructural and mechanical properties are currently under investigation. 4. Processing techniques for dense Si3N4 ceramics For an optimum utilization of the inherent good properties of Si3N4, the powders must be fully densified to compacts. However, due to the high degree of covalent bonding, it is very difficult to sinter these materials to full density. As a consequence, several alternative techniques have been developed to improve the den￾sification of Si3N4 based ceramics. It is explained that fully dense Si3N4 can be produced by the addition of densification additives, which allow liquid phase sintering. There is a wide range of sin￾tering additives for Si3N4 that have been explored to date. Since the densification rate is strongly related to the type and amount of sintering additives, as are the microstructure development and the thermal and mechanical properties of sintered bodies, there is still considerable effort to find and optimize sintering aids for high-performance Si3N4 materials. 4.1. Pressureless sintering This is a conventional technique that has been used to man￾ufacture ceramics for many centuries [141] and it is a process in which a part of the material being sintered is in liquid state (sim￾ple heating of powder compacts). LPS is important for systems which are difficult to densify by solid state sintering [35], i.e. ceram￾ics that possess a high degree of covalent bonding such as Si3N4. Various kinds of Si3N4 ceramics have been developed by this method [142]. So far, sintering of silicon nitride to high densities has demanded the addition of metal oxides [21,143–145], nitrides as AlN [16,146,147] and rare earth oxides [143,148–150]. According to Negita [21], an important condition for effective metal oxide sin￾tering aids is that they do not decompose Si3N4 to N2 and SiO2 or SiO gas in the sintering process, i.e. reactions between metal oxides and Si3N4 do not proceed. Metal oxides that satisfies this conditions are MgO, Al2O3, Sc2O3, ZrO2, CeO2, Ce2O3, LiO2, Y2O3, La2O3, CaO, BeO, HfO2, and SrO. Almost all of these metal oxides are reported to be successful sintering aids for Si3N4 [151]. This suggests that reactions between Si3N4 and metal oxides around the sintering temperatures (∼1727 ◦C) are an important factor in elucidating the role of sinter￾ing aids. On the other side, metal oxides such as Mn3O4, MoO, FeO, Li2O2 and Cr3O3 are expected to decompose Si3N4. Experimental results have shown that lanthanide oxide such as Nd2O3, Pr2O3 and Sm2O3 as well as actinide oxide Ac2O3 also work well as sintering aids for Si3N4. All these additives produce a liquid phase during fir￾ing at high temperatures by reaction with the SiO2 always present on Si3N4 powder particles [152,153]. This process, however, deter￾minates high temperature mechanical properties because they are dependent on the characteristics of the secondary grain boundary phase [154], such as composition, amount, and state of the phase [155–157]. Numerous research groups have intensively studied the sintering of silicon nitride with a vast variety of additives [158]. Despite all these efforts the quest for the optimum Si3N4 sinter￾ing aid continues. Beside the choice of the proper additive, also the subsequent crystallization of the glassy intergranular phase has been studied as a means to further increase strength since this one limits the high temperature properties because it softens and results in the easy slide of Si3N4 grains at high temperature [159]. Considering the mentioned above, after the liquid phase sinter￾ing or cooling, the microstructure of dense Si3N4 consists mainly of -Si3N4 and the liquid solidifies to amorphous or partially crystalline secondary phases, with are located either of the grain boundaries in the form of thin layers or at triple junctions as can be seen in Fig. 1 [35]. A typical feature of sintered Si3N4 ceramics is the morphology of the Si3N4 grains. Residual -grains are equiaxed and the -phase exhibits an elongated grain structure with an aspect ratio (ratio of length to thickness) usually en the range of 5 to 10 [160,161]. The obtained microstructure is controlled by the Si3N4 starting powders as well as the additives used and the pres￾sureless sintering parameters. The type and amount of sintering additives determine the liquid forming temperature, the onset of densification and its rate during sintering [162]. Similarly, they also define the morphology of the -grains and the characteristics of the grain boundary phase, which in turn, as outlined previously, controls the high- temperature properties. The following reactions can express the role of the additive: The eutectic temperatures of the more commonly used oxide systems for the liquid phase of silicon nitride are listed in Table 1 [35,163,164]. Other oxide additives used in manufacturing of Si3N4 and their melting temperatures are also included in this table. The alkali and alkaline-earth oxides have a low melting point and the viscosity of the resulting liquid is also low. A good example for dif￾ferent behavior of sintering additives is Magnesia and Yttria. The temperature of liquid formation for the MgO–SiO2–Si3N4 system is lower by nearly 100 ◦C in comparison with the Y2O3–SiO2–Si3N4 (according to Table 1). A summary of magnesia and yttria additives and their compounds is shown in Table 2

M.H. Bocanegra-Bermal B. Matovic/ Materials Science and Engineering A 500(2009) 2 Fig 1. Typical microstructure of a liquid phase sintered SiaNa ceramics schematic(a)and SEM micrograph(b)-(1)Si3N4 matrix grains: (2)crystalline secondary phase: and ()amorphous residue at triple junctions and grain boundaries [From Ref [3511- After pressureless sintering of silicon nitride ceramics, the grain growth resulting in the highest aspect ratio grains compared formed intergranular phase could be crystallized by slow cool with other rare earth oxides[174. However, Luo et al. [175 used ing or alternately by heat treatment. The devitrification increases high oxynitride glass with y+ and La+ in order to control the high temperature strength of sintered body substantially, for microstructure development and B-Si3N4 grain growth forming example, the strength at 1000C can be increased about 50% glassy phase with high softening temperature and high viscosity ver that of as-sintered one[12]. Various authors have reported at grain boundaries during sintering increasing the elevated tem- different techniques in order to improve the high temperature perature properties of Si3N4 ceramics. strength: i)decreasing of impurities such as Ca and Fe contained he commercializing of silicon nitride ceramics for structural in silicon nitride powders[165). ii)devitrification of intergranu- applications as bearings, requires an extensive effort to obtain an lar glassy phase [11, 155, 166]. iii)addition of sintering aid which advanced silicon nitride bearing and it is very important to correlate does not leave any residual glassy phase [167]. iv) adoption of the physical/mechanical properties to the cture which dvanced techniques such as Hot isostatic Pressing(HIP), to con- develops during synthesis and processing [16, 142, 176-179 Liu and solidate silicon nitride with or without a small amount of additives Nemat-Nasser[16 obtained Si3 Na ceramics exceeding 99% theoret 13,44,168-170 ical density with pressureless sintering and 6 wt% Y2 03, 6wt% AIN Rice[171 ]reported the preparation of Si3 N4 ceramic composites and 1 wt% TiO as sintering aids where the sintering parameters reinforced with whiskers, for example with Sic whiskers, because (temperature and time)were optimized in an effort to control, i ) the lese prevent viscous sliding between grains at high temperatures percentage of a-phase to B-phase transformation, ii) the stabiliza- and improve the strength at high temperatures practically. Mit- tion of the a-Si3N4 structure and iii) the acicular B-grain growth. omo et al. [172 reported to dissolve the ingredients of the glass Takatori et al. [12 examined the devitrification of the intergran- phase, which act as the liquid phase in the early and middle stages ular glassy phase in silicon nitride ceramics sintered with 5 wt% of sintering, into a-and B-Si3 Na grains at the final stage of sintering. Y203 and 5 wt% MgAl2O4. Mg-spinel in one of the most effective It is important to point out the study of lanthanum oxide additives for the densification of silicon nitride at relatively low (La2O3)doped B-Si3Ng sintered sample carried out by Shibata et sintering temperature al. [173 where they observed rare earth segregation in silicon herefore, the simultaneous addition of Y203 and Mg-spinel nitride ceramics at subnanometre dimensions, taking into account is effective to produce a high strength silicon nitride alloy. the that Laz O3 additions are known to strongly promote anisotropic temperature dependence of the bending strength for as-sintered and heat-treated bodies where each specimen had about the same able 1 strength of 700 MPa at room temperature was revealed by Takatori Oxide additives used for the densification of Si3 N4 [from Ref [3511. et al. [12]. Similarly, the strength of the as-sintered sample dropped Additive(M,Oy) Temperature of liquid formation( to 450 MPa at 1000.C, which was improved 50% with the heat treatment at approximately 1250 and 1350 C On the other hand Silicate(MrOy-SiO2) Oxynitride(MxOy-SiOz-Si3 N4) 480122 Summary of sintering studies for Si3 Na at atmospheric pressure with magnesia and CeO2 yttria additives and their compounds [from Ref [351. Additive Sintered density 1435 435121l mperature(c) Al203 1595 1470121 00-1700 1650-1900 Y,03+2 wtX Al,03 10 wt% MgO Al203 600-1750 10wt%Y203+3 wt% Al 1600-1750 3.5-20wt%Y2O3+20wt 1750-1825 Pr,03 10 mol% Y203+20 mol% SiO2 1750 4-17wt%Y2O3+2-4wt%A2O3 500-1750

M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 139 Fig. 1. Typical microstructure of a liquid phase sintered Si3N4 ceramics [schematic (a) and SEM micrograph (b)]. (1) Si3N4 matrix grains; (2) crystalline secondary phase; and (3) amorphous residue at triple junctions and grain boundaries [From Ref. [35]]. After pressureless sintering of silicon nitride ceramics, the formed intergranular phase could be crystallized by slow cool￾ing or alternately by heat treatment. The devitrification increases high temperature strength of sintered body substantially, for example, the strength at 1000 ◦C can be increased about 50% over that of as-sintered one [12]. Various authors have reported different techniques in order to improve the high temperature strength; i) decreasing of impurities such as Ca and Fe contained in silicon nitride powders [165], ii) devitrification of intergranu￾lar glassy phase [11,155,166], iii) addition of sintering aid which does not leave any residual glassy phase [167], iv) adoption of advanced techniques such as Hot Isostatic Pressing (HIP), to con￾solidate silicon nitride with or without a small amount of additives [13,44,168–170]. Rice [171] reported the preparation of Si3N4 ceramic composites reinforced with whiskers, for example with SiC whiskers, because these prevent viscous sliding between grains at high temperatures and improve the strength at high temperatures practically. Mit￾omo et al. [172] reported to dissolve the ingredients of the glass phase, which act as the liquid phase in the early and middle stages of sintering, into -and -Si3N4 grains at the final stage of sintering. It is important to point out the study of lanthanum oxide (La2O3) doped -Si3N4 sintered sample carried out by Shibata et al. [173] where they observed rare earth segregation in silicon nitride ceramics at subnanometre dimensions, taking into account that La2O3 additions are known to strongly promote anisotropic Table 1 Oxide additives used for the densification of Si3N4 [from Ref. [35]]. Additive (MxOy) Temperature of liquid formation (◦C) Silicate (MxOy–SiO2) Oxynitride (MxOy–SiO2–Si3N4) Li2O 1030 1030 [121] MgO 1543 1390 [121] Y2O3 1650 1480 [122] CeO2 1560 1460 [121] ZrO2 1640 1590 [121] CaO 1435 1435 [121] Al2O3 1595 1470 [122] Additive (MxOy) Melting temperature (◦C) Sc2O3 2300 Ce2O3 2776 La2O3 2315 BeO 2530 HfO2 2758 SrO 2430 Nd2O3 2272 Pr2O3 2200 Sm2O3 2300 Ac2O3 1596 grain growth resulting in the highest aspect ratio grains compared with other rare earth oxides [174]. However, Luo et al. [175] used high oxynitride glass with Y3+ and La3+ in order to control the microstructure development and -Si3N4 grain growth forming glassy phase with high softening temperature and high viscosity at grain boundaries during sintering increasing the elevated tem￾perature properties of Si3N4 ceramics. The commercializing of silicon nitride ceramics for structural applications as bearings, requires an extensive effort to obtain an advanced silicon nitride bearing and it is very important to correlate the physical/mechanical properties to the microstructure which develops during synthesis and processing [16,142,176–179]. Liu and Nemat-Nasser [16] obtained Si3N4 ceramics exceeding 99% theoret￾ical density with pressureless sintering and 6 wt% Y2O3, 6 wt% AlN and 1 wt% TiO2 as sintering aids where the sintering parameters (temperature and time) were optimized in an effort to control, i) the percentage of -phase to -phase transformation, ii) the stabiliza￾tion of the -Si3N4 structure and iii) the acicular -grain growth. Takatori et al. [12] examined the devitrification of the intergran￾ular glassy phase in silicon nitride ceramics sintered with 5 wt% Y2O3 and 5 wt% MgAl2O4. Mg-spinel in one of the most effective additives for the densification of silicon nitride at relatively low sintering temperature. Therefore, the simultaneous addition of Y2O3 and Mg-spinel is effective to produce a high strength silicon nitride alloy. The temperature dependence of the bending strength for as-sintered and heat-treated bodies where each specimen had about the same strength of 700 MPa at room temperature was revealed by Takatori et al. [12]. Similarly, the strength of the as-sintered sample dropped to 450 MPa at 1000 ◦C, which was improved ≈50% with the heat treatment at approximately 1250 and 1350 ◦C. On the other hand, Table 2 Summary of sintering studies for Si3N4 at atmospheric pressure with magnesia and yttria additives and their compounds [from Ref. [35]]. Additive Sintering temperature (◦C) Sintered density (%Th. D.) 5 mol% MgO 1500–1700 86 10 mol% spinel (MgO.Al2O3) 1650–1900 96 5 wt% MgO + 0.15 wt% CaO + 0.8 wt% FeO + 4 wt% Y2O3 + 2 wt% Al2O3 1750 95 10 wt% MgO.Al2O3 1600–1750 97 5 wt% MgO + BeO + CeO2 1800 97 4 mol% Y2O3 + 2 mol% Al2O3 1725 Not mentioned 10 wt% Y2O3 + 3 wt% Al2O3 1600–1750 98 3.5–20 wt% Y2O3 + 20 wt% Al2O3 1750–1825 100 10 mol% Y2O3 + 20 mol% SiO2 1750 90 4–17 wt% Y2O3 + 2–4 wt% Al2O3 1500–1750 95