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 m) have been deposited from gas mixtures of hexamethyldisilazane/NH3 and hexamethylclotrisilizane/NH3 using chemical vapor deposition (CVD) technology, as well as fiber bundles of -Si3N4 with diam￾eter of approximately 1.3 m at 1400 ◦C by means of pyrolisis of hexaphenylcyclotrisilizane in nitrogen [64,65]. 3. Fabrication of near-net-shape Si3N4 ceramics The articles manufactured by near-net-shape forming tech￾niques involve generally little, if any, post-densification machining, surface preparation or cleaning prior to use. There is a great challenge to fabricate complex-shaped components with high reli￾ability and with defect-free microstructures at acceptable costs. The high hardness of Si3N4 ceramics is almost always cost pro￾hibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fab￾rication processes that can produce complex-shaped components with a minimum of machining as well as to minimize the number and size of microstructural defects within design limits. Injection molding, gelcasting, robocasting, mold shape deposition, rapid pro￾totyping are some of them. 3.1. Injection molding of Si3N4 ceramics Silicon nitride, when properly prepared, is a superlatively tough ceramic whose high temperature stability; low weight; and wear, erosion and corrosion resistance have put it high up on the wish list of turbine engine designers. Injection molding of ceramics was initially demonstrated over 50 years ago [66–68] and it is an attractive method among the processes for near-net-shape produc￾tion of ceramic parts, requiring little subsequent grinding and no need of machining [69]. The use of polycrystalline high tempera￾ture ceramics in different applications such as turbochargers and gas turbine vanes, blades and rotors [70–72], reciprocating [73,74] and turbine engines [75,76] has been possible by considerable developments in the fabrication of fine powders [77,78]. Success of the injection molding process of Si3N4 is critically depending on starting powder, binder, and the process parameters such as molding and binder removal conditions and subsequent densifi- cation [39]. The development of injection molding technology for sintered silicon nitride was initiated at GTE Labs under a sub￾contract to the Detroit Diesel Allison Division (DDA) of General Motors as a part of the Ceramic Applications in Turbine Engines (CATE) [28]. Successful development of a cost-effective ceramic injectionmolding process for net-shape thick-cross-section (>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)