北京化工大学：《材料导论》课程阅读材料（无极非金属）Progress_in_SiAlON_ceramics.pdf_大学文库

Progress in SiAlON ceramics V.A. Izhevskiy a,1, L.A. Genova a , J.C. Bressiani a , F. Aldinger b,* a Instituto de Pesquisas EnergeÂticas e Nucleares, IPEN/CNEN-SP, Trav. R, 400, Cidade UniversitaÂria, SaÄo Paulo, SP-05508-900, Brazil bMax-Planck-Institut fuÈr Metallforschung und UniversitaÈt Stuttgart, Institut fuÈr Nichtmetallische Anorganische Materialien, Heisenbergstrasse 5, D-70569, Stuttgart, Germany Received 9 June 1999; received in revised form 21 December 1999; accepted 30 January 2000 Abstract In view of the considerable progress that has been made over the last several years on the fundamental understanding of phase relationships, microstructural design, and tailoring of properties for speci®c applications of rare-earth doped SiAlONs, a clear review of current understanding of the basic regularities lying behind the processes that take place during sintering of SiAlONs is timely. Alternative secondary phase development, mechanism and full reversibility of the a0 to b0 transformation in relation with the phase assemblage evolution are elucidated. Reaction sintering of multicomponent SiAlONs is considered with regard of wetting behavior of silicate liquid phases formed on heating. Regularities of SiAlON's behavior and stability are tentatively explained in terms of RE element ionic radii and acid/base reaction principle. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Phase equilibria; Phase transformations; SiAlONs; Sintering 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00039-X Journal of the European Ceramic Society 20 (2000) 2275±2295 Contents 1. Introduction......................................................................................................................................................... 2276 2. Occurrence of nitrogen melilite and nitrogen woehlerite solid solutions containing aluminum ........................... 2277 3. Subsolidus phase relationships in R2O3±Si3N4±AlN±Al2O3 systems ................................................................... 2282 4. Thermal stability of RE-a-SiAlONs and the reversibility of a0 to b0 transformation.......................................... 2285 5. Reaction densi®cation of a and (a+b) SiAlONs ................................................................................................2290 6. Conclusions..........................................................................................................................................................2293 Acknowledgements................................................................................................................................................... 2293 References ................................................................................................................................................................ 2293 * Corresponding author. E-mail address: aldinger@aldix.mpi- stuttgart.mpg.de (F. Aldinger). 1 Invited scientist, on leave from the Institute for Problems of Material Science, National Academy of Sciences of Ukraine, Kiev, Ukraine

2276 V.A.Izhevskiy et al.Journal of the European Ceran ic Societ2012000)2275-225 1.Introduction with 0.08 0.17 and SiAlONs is a neral name for a large family of the so-called ceramic alloys based on silicon nitride.Initially of 0.095 to 0.1 nm,and significantly higher,x=0.25,for they were discov ered in the early 1970s and have beer small cations such as ytterbium with a develope nm. More recent inve d that the proc ssing parameters and the system has been shown ndye the SilON phases were investigated and characterized.The latter applies to B-SiAION,SiAION. ions present has not been determined but is anyhow AIN polytypoid pha -SiAION,as well as to a variety crysta secon The n value thoroughly investigated.The vast amount of data accu- and m to decrease,implying that the most oxygen-rich SiAION phase will be found in the Yb-a-SiAION system tine rall composio n in the Y-S any intero SiAlON-A-SiAION sient liquid phase sintering was not achieved,SiAlON phase normally appears as equiaxed became one of the commercially produced microstructure of the ma that rest a H ms elongated are solid solutions ure as and B. equivalent substitution of Al-O for Si-N and has glassy grain-boundary phase by orating con the formula Si-Al stituents of sintering aids into the crystal structure T can be varied continuous vise wil Ibe present as subst mount of the the phase diagram.The unit cell of B'contains two SiAION composites,which may become in the plane SiN A1,0 -MeN-3AIN o for high emperature structura AIN) the Me-Si The the -SAlON:B-SiAlON phase ratio by slightly changing the overall composition are the systems Y-SiAION possibilities to lithium.magpe sium.calcium.vttrium and the rare arth in con ctionwith tailorin the propertie for applications. Hardnes marke can Me Si isms may act.the first is simi the material and the thermal shock resistance.+Other ways of changing properties of SiA by n(Al an A13 means replac being retained by incorporation of mMeinto the arch lead to discovery of the full reversibility of the phase structure to Btransformation for certain phase assemblies region was most extensively stu- der conditions of post ering trea tment field exte

1. Introduction SiAlONs is a general name for a large family of the so-called ceramic alloys based on silicon nitride. Initially, they were discovered in the early 1970s1ÿ3 and have been developed actively since. In the following years, fully dense polycrystalline bodies were prepared by pressureless sintering techniques. Profound understanding of basic regularities of the interrelation between the starting powder's properties, processing parameters and the properties of consolidated materials was achieved. The majority of the SiAlON phases were investigated and characterized. The latter applies to b-SiAlON, a-SiAlON, AlN polytypoid phases, O0 -SiAlON, as well as to a variety of crystalline secondary phases, mostly silicates, alumosilicates, and oxynitrides. Nitrogen-rich glasses were also thoroughly investigated. The vast amount of data accumulated was critically evaluated and summarized in several reviews.4ÿ6 Although the initial goal of creating a single-phase silicon nitride-based ceramics without any intergranular amorphous phases due to the transient liquid phase sintering was not achieved, SiAlON ceramics became one of the commercially produced high-tech ceramic materials. There are two SiAlON phases that are of interest as engineering ceramics, a-SiAlON and b-SiAlON, which are solid solutions based on a and b-Si3N4 structural modi®cations, respectively, and designated respectively as a0 and b0 . b-SiAlON is formed by simultaneous equivalent substitution of Al±O for Si±N and has most commonly been described by the formula Si6ÿzAlzOzN8ÿz. In this formula z can be varied continuously from zero to about 4.2.7ÿ9 The homogeneity region of bSiAlON extends along the Si3N4±(Al2O3AlN) tie line of the phase diagram. The unit cell of b0 contains two Si3N4 units. The a-SiAlON has a unit cell comprising four Si3N4 units and forms a limited two-dimensional phase region in the plane Si3N4 4 3 Al2O3 AlN ÿ MeN3AlN of the Me±Si±Al±O±N phase diagram. In the latter diagram the b-SiAlON falls on one border of the plane mentioned. Of special interest are the Me±Si±Al±O±N systems where the a-SiAlON phase is stabilized by Me ions, such as lithium, magnesium, calcium, yttrium, and the rare-earth (RE) metals except lanthanum, cerium, praseodymium, and europium.10ÿ12 The homogeneity range composition of a-SiAlON can be described by the general formula MemSi12ÿ(pm+n)Al(pm+n)OnN16ÿn for a metal ion Mep+. Two substitution mechanisms may act; the ®rst is similar to that of b-SiAlON with n (Si+N) being replaced by n (Al+O). The second mechanism is further replacement of pmSi4+ by pmAl3+, the electron balance being retained by incorporation of mMep+ into the phase structure. The a-SiAlON phase region was most extensively studied in the Y±Si±Al±O±N system, where the two-dimensional phase ®eld extension can be expressed as YxSi3ÿ(3x+n)-Al(3x+n)OnN4ÿn, with 0.08<x<0.17 and 0.13<n< 0.31.11,13ÿ16 The lower solubility limit is located at x 0.08 and 0.20 for cations with the radius of 0.095 to 0.1 nm, and signi®cantly higher, x=0.25, for small cations such as ytterbium with a radius of 0.085 nm.17 More recent investigations,18,19 showed that the a phase area becomes larger with the decreasing size of the Me ion, i.e. in the order Nd (0.99A )<Sm (0.96A )<Dy (0.91A )< Y(0.89A )<Yb(0.87A ). Moreover, the Yb system has been shown to have the largest a-SiAlON phase area because both Yb3+ and Yb2+ ions are present in the a-SiAlON phase. The exact amount of Yb2+ ions present has not been determined but is anyhow expected to vary with the temperature and preparation conditions used.The n value of the Yb-a-SiAlON phase has been shown to increase with increasing Yb2+ content, and m to decrease, implying that the most oxygen-rich aSiAlON phase will be found in the Yb-a-SiAlON system. By changing the overall composition in the Y-SiAlON system, (a+b)-SiAlON ceramics can be prepared with a varying a-SiAlON:b-SiAlON phase ratio.20, 21 a-SiAlON phase normally appears as equiaxed grains in the microstructure of the material, while b-SiAlON phase forms elongated grains with an aspect ratio of 4 to 10. However, some recent research of, in particular, Smand Y-doped a-SiAlONs,22, 23 which provides even better possibilities of microstructure tailoring. It is evident from its chemical composition that a-SiAlON phase oers possibility of reducing the amount of residual glassy grain-boundary phase by incorporating constituents of sintering aids into the crystal structure, which otherwise will be present as substantial amounts of residual glassy phase and deteriorate the high temperature performance of the material. Thus, in principle, a-SiAlON enables the preparation of the practically glass-free SiAlON composites, which may become interesting candidates for high-temperature structural and engineering applications. The possibility of varying the a-SiAlON:b-SiAlON phase ratio by slightly changing the overall composition was shown to open many possibilities to prepare Y-SiAlON ceramics with desired properties. This is of the utmost importance in connection with tailoring the properties for speci®c applications. Hardness increases markedly with increasing a-SiAlON phase content, whereas the fracture toughness decreases. High a-SiAlON content also improves oxidation resistance of the material and the thermal shock resistance.4 Other ways of changing properties of SiAlON ceramics by means of replacing Y2O3 by other RE-oxide additives were also investigated.24 Precisely this direction of research lead to discovery of the full reversibility of the a0 to b0 transformation for certain phase assemblies under conditions of post-sintering heat treatment.25ÿ27 It has been observed that the a-SiAlON phase is less stable at low temperatures and decomposes into rare- 2276 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295

5 ociery202000275-229 227 The e of the transformation is more pronounced with Both types of phases are very stable refractory com- heat-eatment temperature.The mechanism rever to P2s This pounds with high melting points (0 limit of the liquid region.these phases ideal requirements for a grain boundary phase in SiA merely by tre ON mics.Ho ver,the most important distinct ness can be achieved from a single starting composition the traditional oxide-base secondary phases form one which opens new perspectives for their successful unconditionally. I his becomes sabso utely obvious if one is t rehensive overall view of the new devel these diagrams,separating the SiN corner and the opments in SiAlONs that will facilitate advances of this plane of additive oxides and their compounds.In con :f for further study to be mor iofpoTK region t issues as thermal stability of different RE-doped (B SiAlON composites,phase relationships in this systems during initial heating because of the ternary eutectic of nisms reve intermediate temperature for lack of liquid,but it ogen melilite and nitrogen woehlerite ons con Nitrogen-containing (or N-)melilites and nitrogen- ration and massive containing (or N-)woehlerites (N-woehlerites are also powder relocation. Additionally, both melilite and known asJ crystalliz the Al devitrification process.The latter occurs in solid state and cours at i tures mentioned above can be avoided by formation in 900and1100C.3 Under such conditions they oxidize the material of melilite and woehlerite based solid solu rapidly to crystobalite and RSi(where R um instead of pure M(R)an volume.and this results in eatastrophic eracking and and RSi total failure of material.N-melilites have the genera The solid solutions will be further referred to as formula R2Si3O3N and e ph onds are pr fth repla t which in turn to th the oxidation resistance s th only se melilite [M(R) 110 he majority linkage of corner-sharing Si(O.N)units in tetrahedra d in SiAlON shown that the solid solubility range of M(R)phases ir which hold the ets are rare earth Lr on ect of alur RE im are detem d by iom 34 ng ic f Nu tain either SiON ditetrahedral units containing with large ionic radii.i.e..R-Nd and Sm,up to one Si

earth-rich intergranular phase(s) and b-SiAlON with a remarkably elongated crystal morphology. The extent of the transformation is more pronounced with increased heat-treatment temperature. The mechanism of the fully reversible a0 to b0 transformation was investigated by the same authors.28 This transformation provides an excellent possibility for optimizing phase content and microstructure without further additions of oxides and nitrides merely by heat treatment at appropriately chosen temperatures. In this way, premeditated values of hardness, strength and toughness can be achieved from a single starting composition, which opens new perspectives for their successful application. The objective of the present review, therefore, is to provide a comprehensive overall view of the new developments in SiAlONs that will facilitate advances of this subject, and allow areas for further study to be more clearly identi®ed. The review will cover such important issues as thermal stability of dierent RE-doped (a+b) SiAlON composites, phase relationships in this systems, alternative secondary phases for SiAlON ceramics, and mechanisms of reversible a0 to b0 transformation. 2. Occurrence of nitrogen melilite and nitrogen woehlerite solid solutions containing aluminum Nitrogen-containing (or N-)melilites and nitrogencontaining (or N-) woehlerites (N-woehlerites are also known as J-phases) have been recently closely re-evaluated as the candidates for alternative secondary phases in Si3N4-based ceramics after the Al-containing solid solutions of this phases were discovered and investigated.29ÿ32 Both phases were discovered relatively early in the course of silicon nitride development and at the time were considered as undesirable secondary phases due to their poor oxidation resistance at temperature between 900 and 1100C.33 Under such conditions they oxidize rapidly to crystobalite and y-R2Si2O7 (where R=yttrium or rare earth metal) with a 30% increase in speci®c volume, and this results in catastrophic cracking and total failure of material. N-melilites have the general formula R2Si3O3N4 and occur as an intermediate phase during sintering of a number of Si3N4-based ceramics. In particular, N-melilite is located in the phase compatibility regions of aÿ(a+b)-SiAlONs, which in theory can produce a good microstructure of SiAlONs Nmelilite as the only secondary phase. N-melilite [M(R)], has a tetragonal structure that is built up of an in®nite linkage of corner-sharing Si(O,N)4 units in tetrahedral sheets perpendicular to the [001] direction. Sandwiched between these sheets are Y3+ or rare earth Ln3+ ions which hold the silicon oxynitride layers together.34 The structure of N-woehlerite is monoclinic and contains either Si2O5N2 or Si2O6N ditetrahedral units containing N in the bridging position and arranged lengthwise along the a-axis. These units are linked by RE polyhedra. Both types of phases are very stable refractory compounds and occur in almost all rare-earth SiAlON systems.34 With high melting points (1900C for yttrium N-melilite35) and a composition close to a nitrogen-rich limit of the liquid region, these phases ful®ll some of the ideal requirements for a grain boundary phase in SiAlON ceramics. However, the most important distinction of melilite±woehlerite type of secondary phases is that they do not form a eutectic with the matrix Si3N4 while the traditional oxide-base secondary phases form one unconditionally. This becomes absolutely obvious if one considers phase diagrams of R±Si±(Al/O)±N systems. A liquid phase region exists at high temperature on all these diagrams, separating the Si3N4 corner and the plane of additive oxides and their compounds. In contrast, there is no liquid phase region between M(R)/J(R) and Si3N4 corner on any of the phase diagrams of R±Si± (Al/O)±N systems. Although some liquid does form during initial heating because of the ternary eutectic of SiO2, Al2O3, and R2O3, the liquid should be consumed largely by Si3N4 to form M(R) or J(R). Thus, sintering of the latter compositions is relatively dicult at the intermediate temperature for lack of liquid, but it improves drastically once the melting point of M(R) and J(R) is exceeded. Indeed, the lack of liquid at the intermediate temperature could be advantageous because it reduces the possibility of melt evaporation and massive powder relocation. Additionally, both melilite and woehlerite type phases crystallize readily on cooling directly from the liquid, which leads to quicker and much more complete crystallization as compared to any devitri®cation process. The latter occurs in solid state, and consequently all diusion processes are by several orders of magnitude slower than in the liquid. The catastrophic oxidation at intermediate temperatures mentioned above can be avoided by formation in the material of melilite and woehlerite based solid solutions containing aluminum instead of pure M(R) and J(R). The general formulas of these solid solutions are R2Si3ÿxAlxO3xN4ÿx and R4Si2ÿxAlxO7xN2ÿx, respectively. The solid solutions will be further referred to as M0 (R) and J0 (R). In these phases, Si±N bonds are progressively being replaced by Al±O bonds, leading to the increase of the oxygen content, which in turn improves the oxidation resistance. Until now, the regularities of solid solubility of aluminum in M(R) and J(R) were studied for the majority of RE elements (Nd, Sm, Gd, Dy, Yb, and Y) that are used in SiAlON production. It was unequivocally shown that the solid solubility range of M0 (R) phases in respect of aluminum are determined by ionic radius of the RE element. With decreasing ionic radius the solubility limits in melilite solid solution decrease. For elements with large ionic radii, i.e., R=Nd and Sm, up to one Si V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295 2277

can be replaced by Al. Consequently, yttrium melilite has the lowest observed aluminum solubility (x=0.6) in rare earth elements melilite solid solutions.36 Both ytterbium and lanthanum do not stabilize N-melilite.37ÿ39 It was, however, shown that ytterbium forms J0 (R) solid solution that can serve as an alternative secondary phase in this system. On the other hand, lanthanum forms only the M0 phase with the highest substitution level (x=1), which is a point phase in La±Si±Al±O±N system, and the M0 (La) homogeneity range does not exist. Here it must be speci®cally emphasized that phase pure M(R) or M0 (R) were never formed neither in the form of a secondary phase in any RE-doped SiAlON system, nor if synthesized separately according to stoichiometry. Appreciable amounts of J0 (R), REAG (RE3Al5O12) or other phases were present depending on the RE oxide used, the amount of J0 (R) increasing with the decrease of the RE ionic radius. The results of the M0 (R) solubility range determination for dierent systems are summarized in Table 1.40 The data on the correlation between the x value in M0 (R) solid solution formula and the lattice parameters of M0 (R) were determined for the majority of RE stabilizing elements.29,36,41 Linear increase of the lattice parameters with increasing of the x values was observed for all systems investigated up to a certain x value for respective M0 (R) phases. However, these x values do not necessarily represent true solubility limits as all samples contained additional phases, as it was mentioned earlier, some of which were rare earth rich. It can, however, be noted that the c/a ratios are almost constant with increasing x-value for each M0 (R) phase which indicates that the structural changes introduced when Si±N pairs are replaced by Al±O ones are isotropic in all M0 (R) phases studied. Since the nominal x value, i.e., the one determined by the powder mixture composition, was not relevant due to the multiphase nature of the synthesized materials, SEM±EDS studies have been used to estimate solubility limits of Al in various M0 (R) phases. The maximum aluminum content, as determined in Ref. 36, were x=0.98, 0.9, 0.78, 0.71, and 0.57 for R= Nd, Sm, Gd, Dy, and Y, respectively. Thus, in M0 (Nd) and M0 (Sm) systems one Si±N pair per formula unit can be replaced by one Al±O pair in accordance with previous ®ndings29,41,42 while the true solubility limits in the M0 (Gd) and M0 (Dy) systems seem to be somewhat less (0.75) and even smaller in the M0 (Y) system (0.60). These ®ndings also con®rm that the limit of aluminum solubility decreases with the ionic radii of the RE element.43 It was also shown, that the substitution of Si±N by Al±O in the M(R) structure produces a lattice expansion.36 The fact that the maximum value of lattice expansion was found in the M0 (Nd) system (1.66%) and the minimum one in the M0 (Y) system (0.70%) further supports the conclusion that the investigated systems exhibit dierent solid solubility ranges. A tentative attempt to rationalize the interrelation between the ionic radii of RE element and the solubility limits of dierent M0 (R) phases was made in Ref. 40 and is based on the simple considerations regarding the Table 1 Compilation of experimental data on the formation of N-melilite in dierent systems40 Cation Ionic radius r 3 R A a r 3 R :r2ÿ O Al-free N-melilite formation Formation of M0 (R) Maximum aluminum solubility in M0 (R) La3+ 1.016 0.77 Does not form No study Experimental data currently unavailable Nd3+ 0.995 0.75 Observed Observed x = 1.0 Sm3+ 0.964 0.73 Observed Observed x = 1.0 Dy3+ 0.908 0.69 Observed Observed x = 0.7 Y3+ 0.893 0.68 Observed (not at 1500C) Observed x = 0.7 Yb3+ 0.858 0.65 Does not form No study Experimental data currently unavailable Fig. 1. Schematic representation of the structure of yttrium N-melilite showing separation of the negatively charged silicon oxynitride layers by Y3+ cations.40 2278 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295

structure of melilite-type phases. The projection of the yttrium melilite on (100) may be schematically represented as shown in Fig. 1. With non-bridging anions taking up positions on the apex of the tetrahedra each silicon oxynitride layer of the N-melilite structure possesses an overall negative charge that has to be balanced by the RE cations. These cations separate the silicon oxynitride layers by a ®nite distance. In systems with small cation sizes such of as yttrium or dysprosium, the negatively charged layers are allowed to come relatively closely together and possibly create a repulsive force which can destabilize the melilite structure. A structurally unstable M0 (R) phase may result in a low aluminum accommodation capability. However, the exact mechanism involved in this process is currently unknown. One of the main issues of interest in regard of the properties of the M0 (R) phases is their behavior at high temperatures primarily in relation with densi®cation of respective SiAlON compositions. Al-free melilites tend to become unstable at temperatures exceeding 1800C. Melting of melilites is thought to be impossible because the edge of the liquid region in the four component R± Si±O±N plane does not reach as far as the melilite composition (i.e., 67 eq.% nitrogen). Instead, the melilite phases are believed to decompose with signi®cant loss of nitrogen, resulting in the formation of J phase. For M0 (R), stability at high temperature is determined by two factors. With increasing aluminum content, the limiting composition of M0 (R) gradually moves towards the liquid region, and at the same time the liquid region can accommodate a higher nitrogen concentration with increasing temperature, approaching the terminal composition of M0 (R). Current preparative evidence suggests that the liquid region eventually reaches the M0 (R) composition, and M0 (R) melts.29 This generates large amounts of liquid which, in combination with silicon, nitrogen and other components, promotes the precipitation of a-SiAlON phase and accelerates the ®nal densi®cation. This eect has been widely observed in several SiAlON systems.41,44 It is clear that the melting temperature of M0 (R) varies in dierent systems according to the densifying cation, but a low melting temperature for M0 (R) is a key factor in the preparation of dense a and ab-SiAlON ceramics. According to Ref. 29 the M0 (Sm) melts at about 1700C, and on cooling at normal furnace cooling rate M0 (Sm) reprecipitates, suggesting that any liquid with M0 (R) composition does not form a glass. Although it has not yet been con®rmed that dissolution of aluminum in M(R) actually lowers the melting temperature of these phases, but the closeness of the M0 (R) composition to the liquid-forming region certainly dominates the behavior of M0 (R) phases at high temperatures. As it will be discussed further, the high temperature behavior of M0 (R) phases plays an important role in the reversible a0 to b0 transformation in SiAlON ceramics. Another important factor in designing and processing of various silicon nitride based materials, (a+b)-SiAlONs in particular, are the phase relations of M0 (R) with the neighboring phases in dierent M±Si±Al±O±N systems. The most thorough and systematic research of this subject was accomplished in Refs. 45 and 46. This research covered the majority of RE oxides (R=La, Gd, Dy, Er, Y, Nd, Sm, and Yb) that are used as sintering aids in complex SiAlON systems. Firstly, it was shown that the nominal compositions of N-melilite do not form a single-phase material for neither of the RE Table 2 Reactions and phase formation in R2Si3O3N4 compositions46 a Rare-earth element, R Reaction temperature (C)/time (h) Appearance Phase analysis by XRD M(R) lattice parameter (A ) a c Lanthanum 1650/2 (Sintering) White K, 1:2 1550/1.5 (HP) Gray K, 1:2 1600/1.5 (HP) Gray K, 1:2 1650/1.5 (HP) Black 1:2, K Neodymium 1550/1.5 (HP) Blue M. K (vw) 7.721 5.036 1700/1.5 (HP) Blue M, K (w) Samarium 1700/2 (Sintering) White M, K (vw) 1700/1.5 (HP) Brown M, K (vw) 7.695 4.991 Gadolinium 1700/2 (Sintering) White M, K (vw) 7.650 4.961 Dysprosium 1700/2 (Sintering) White M, J (w) 7.618 4.925 Europium 1700/2 (Sintering) Pink M, J, 2:1 1700/1.5 (HP) Pink M, J (mw) 7.585 4.896 Ytterbium 1700/2 (Sintering) Black M, J, 2:1 1700/1.5 (HP) Black M, J (mw) 7.563 4.876 a mw, medium weak, w, weak, vw, very weak, HP, hot pressing. V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295 2279

elements investigated. Moreover, it was unequivocally shown that M(R) is not formed with lanthanum, which, depending on the synthesis conditions, forms a mixture of K-phase (La2O3 .Si2N2O) and 1:2 phase (La2O32- Si3N4) in this or that proportion. The results of the attempts of M(R) synthesis with dierent RE elements are listed in Table 2. The tendency for forming dierent components in high-temperature synthesis can be rationalized using the concept of acid-base reactions.47 Silicon nitride may be considered as more acidic than Si2N2O, which enters the compositions of J (2R2O3Si2N2O) and K (R2O3Si2N2O) phases, formed by dierent RE elements as ``impurity'' or additional phases (see Table 2), and light RE oxides as more basic than heavy RE oxides. The various minor phases formed in the presence of melilite then can be viewed as the product of the following reactions: 1:2 phase La2O3 2Si3N4 strong base=strong acid K phase La2O3 Si2N2O strong base=weak acid J phase 2Y2O3 Si2N2O weak base=weak acid For example, as the basicity of R2O3 decreases with decreasing ionic radius, it prefers to react with a weak acid to form J phase rather than K phase or 1:2 phase. It is also interesting to note that the molar ratio of base to acid in K(R) and J(R) increases from 1:1 to 2:1; this re¯ects the need for a larger amount of the weaker base to react with the same acid. Interesting results and trends were discovered for M0 (R) phases when an attempt was made to synthesize the nominal composition R2Si2AlO4N3. M0 (R) solid solution was found in all cases except ytterbium (Table 3). Overall, the systematic trend of phase distribution discovered was very similar with the one for M(R), with the minor phases favoring K-phase, in the case of lanthanum, and J-phase, in the case of heavy rare-earth elements. However, some dierences were also discovered. First, in the case of lanthanum, melilite solid solution was formed. This is in contrast with Al-free composition, which did not form La-melilite. The lattice parameters of M0 (La) were larger than all of the other M(R) and M0 (R) found so far (a=7.855A and c=5.120A ). The composition of M0 (La) was found to be La1.98Si1.82 Al0.96O4.21N3.08, i.e., essentially an x=1 compound in the R2Si3ÿxAlxO3+xN4ÿx series. Similar compositional investigations revealed that the x value decreases monotonically with the atomic number of the RE element. This can be interpreted as a size eect. From the interrelation between the ionic radii of RE elements and both x value and (cubic root of) unit cell volume, (a2 c)1/3, from M(R) and M0 (R) (Fig. 2) it is evident that the unit cell has expanded in M0 (R) relative to M(R). This is accompanied by the substitution of Si±O bond (bond length 1.62A ) and Si±N bond (1.74AÊ ) by Al-O bond (1.75AÊ ). Apparently, in the case of lanthanum, which has a very large radius, only after maximal substitution, x=1, is it possible to incorporate lanthanum into the Table 3 Reactions and phase formation in R2Si2AlO4N3 compositions46 a Rare-earth element, R Reaction temperature ( C)/time (h) Appearance Phase analysis by XRD M(R) lattice parameter (A ) a c Lanthanum 1650/2 Partly melted 1:2, K 1600/2 White, not dense M0 , K, 1:2 7.855 5.120 1550/1.5 (HP) Gray K, 1:2, M0 Neodymium 1650/2 Purple M0 7.766 5.055 Samarium 1650/2 Brown M0 7.730 5.014 Gadolinium 1650/2 Gray M0 7.689 4.993 Dysprosium 1650/2 Yellow M0 , J0 (vw) 7.655 4.955 Yttrium 1650/2 White M0 , J0 (w) 7.629 4.929 Europium 1650/2 Pink M0 , J0 (mw) 7.608 4.920 Ytterbium 1650/2 Black J0 a mw, medium weak, w, weak, vw, very weak, HP, hot pressing. Fig. 2. X(Al) atomic fraction in M0 (R) and cell size as a function of ionic radii of rare-earth elements.46 2280 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295

melilite structure. In case of ytterbium, which has a very small radius in contrast, it is apparently stable only in the limit of smallest volume realizable at x=0. The lattice parameters of melilite and its solid solution obtained for dierent RE elements at dierent x values can be correlated to each other, according to Ref. 46, by the following formula: aA 6:795 0:892r 0:045x cA 4:121 0:874r 0:033x where r is the ionic radius (in angstroms) of the RE ion using Ahrens scale. These formulas were obtained by a regression analysis of the experimental data on XRDdetermined lattice parameters of M(R) and M'(R) formed by dierent RE elements and can be used for identifying the composition of R2Si3ÿxAlxO3+xN4ÿx once the identity of the rare-earth element is known. As to the two other phases, K and J, that were present in the synthesized samples the mechanism of formation and the trends of stability were also formulated in Ref. 46. Formation of K phase was attributed to the reaction of R2O3 with the surface-oxidized Si3N4, forming Si2N2O. In the case of the Al-containing compositions, the oxidized nitride was supposed to be incorporated into M0 (R) solid solution thus preventing the formation of the K phase. For J phase, formation of J0 (R) solid solutions was con®rmed for a number of RE elements. Moreover, it was shown that the stability of both J(R) and J0 (R) increases with the decrease of ionic radii of RE elements. The x value for J0 (R) was shown to increase with the ionic radius of the RE element, which is similar to the trend found for M0 (R). Recognizing the systematic trends in M0 (R), J0 (R), a0 , and b0 compositions, which vary with dierent RE elements, a series of tentative diagrams were suggested in Ref. 46 to delineate the phase relationships between melilite and neighboring phases. The RE aluminoxynitride, R2AlO3N, denoted as 1:1, was included into consideration, based on the ®ndings of Ref. 48. Four prototypical diagrams, presented in Figs. 3±6, illustrate these relationships: For lanthanum, there is no a0 solid solution; the x=1 compound of M0 (R) forms. In addition, La2Si6O3N8 (1:2 phase) exists as a neighboring phase (see Fig. 3); Fig. 3. Phase relations of M0 (R) with neighboring phases in R±Si±Al± O±N system.46 Fig. 4. Phase relations of M0 (R) with neighboring phases in R±Si±Al± O±N (R = Gd, Dy, Er, Y) system.46 Fig. 5. Phase relations of M0 (R) with neighboring phases in R±Si±Al± O±N (R = Nd, Sm) system.46 Fig. 6. Phase relations of M0 (R) with neighboring phases in Yb±Si± Al±O±N system.46 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295 2281

For neodymium and samarium, there is an extensive solid solution of both a0 and M0 (R), but only J phase, not J0 (R) solid solution, is compatible with M0 (R). In addition, R2AlO3N forms and is compatible with M0 (R) and J (see Fig. 4). The compatibility of RAlO3 (R=Nd, Sm) with M0 (R) were not resolved. According to Ref. 46, M0 (R) is compatible with RAlO3, while according to Ref. 49, SmAlO3 may be just a product that forms on cooling at some speci®c conditions and, in fact, is not compatible with M0 (R). In Ref. 42 only coexistence of M0 (R), R2AlO3N and J phase at 1550C was found. For gadolinium, dysprosium, europium, and yttrium, there was found an extensive solid solution of a0 , but the solid solution range of M0 (R) was shown to be compatible with M0 (R), while the compound R2AlO3N failed to form in this case (see Fig. 5). Ytterbium was shown to enter a0 solid solution and J0 (R) solid solution. However, only M(R), not M0 (R), was proven to form in this case (see Fig. 6). 3. Subsolidus phase relationships in R2O3±Si3N4±AlN± Al2O3 systems As already mentioned, the importance of using RE oxides for the densi®cation of silicon nitride ceramics was recognized in recent years. Not only are they very eective along with alumina for densi®cation, either singly or in combination with yttria, but they can also be accommodated in the aSi3N4 lattice forming a-SiAlON, thus providing the opportunity for decreasing the transient liquid phase content after sintering and hence reducing the amount of residual grain boundary glass. Therefore, phase relationships in Ln±Si±Al±O±N systems are of particular interest. Y±Si±Al±O±N system was investigated most thoroughly. Initially, research in this system has been restricted to the region bounded by Si3N4, b-SiAlON, Al2O3, SiO2 and Y2O3, which does not include the solid solution a-SiAlON. Huang et al. reported the a-SiAlON formation in the system Si3N4±AlN±Y2O3 50 and Si3N4± AlN±RE.17 The systems Si3N4±AlN±Y2O3 and Si3N4± (AlNAl2O3)±(YN3AlN) have also been studied by the same authors.51 Later, with the emergence of a-SiAlON, information on the nitrogen-rich part of the Y±Si±Al±N± O system became necessary. Thirty-nine four-phase equilibria, also named compatibility tetrahedra, had Table 4 Subsolidus compatibility tetrahedra in Si3N4±AlN±Al2O3±Y2O3 52 a Al2O3±b60±15R-YAG Al2O3±15R±15R0 ±YAG Al2O315R0 ±12H0 ±YAG Al2O3±12H0 ±21R0 ±YAG Al2O3±21R0 ±AlN±YAG 15R±15R0 ±12H±12H0 ±YAG 12H±12H0 ±21R±21R0 ±YAG 21R±21R0 ±27R±27R0 ±YAG 27R±27R0 ±2Hd ±2Hd0 ±YAG 2H±2Hd ±AlN±YAG 21R0 ±27R0 ±AlN±YAG 27R0 ±2Hd0 ±AlN±YAG 21R±27R±YAG±J0 (R) 27R±2Hd ±YAG±J0 (R) 2Hd ±AlN±YAG±J0 (R) AlN±YAG±J0 (R)±YAM AlN±YAM±J±Y2O3 b60±b25±15R±YAG b25±15R±12H±YAG b25±b10±12H±YAG b10±a0 ±12H±YAG a0 ±12H±21R±b10 a0 ±21R±b10±b8 a0 ±21R±b8±27R a0 ±b8±27R±b2 a0 ±27R±b5±2Hd a0 ±b5±2Hd ±b2 a0 ±2Hd ±b2±AlN a0 ±b2±AlN±Si3N4 a0 ±12H±21R±YAG a0 ±21R±YAG± M a0 ±21R±27R±M a0 ±27R±2Hd ±M a0 ±2H±AlN±M M±21R±YAG±J0 (R)M±21R±27R±J0 (R) M±21R±27R±J0 (R) M±27R±2Hd ±J0 (R) M±2Hd ±AlN±J0 (R) M±AlN±J0 (R)±J a YAM, 2Y2O3 .Al2O3; J, 2Y2O3 .Si2N2O; J0 (R) = 2Y2O3 .Al2O3± Y2O3 .Si2N2O; M=Si3N4 .Y2O3; 15R, 12H, 21R, 27R, 2Hd are Si±rich terminals of AlN polytypoids; 15R0 , 12H0 , 21R0 , 27R0 , 2Hd0 are Al± rich terminals of AlN polytypoids. Fig. 7. Representation of Y±Si±Al±O±N system showing phases occurring in the region bound by Si3N4, Y2O3, Al2O3, and AlN, and Si±Al±O±N behavior diagram at 1700C.52 Fig. 8. Representation of compatibility of the YAG with polytypoid phases, AlN, and Al2O3, 12 compatibility tetrahedra are formed.52 2282 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295

been established in the region Si3N4±AlN±Al2O3±Y2O3 (Table 4).52 The subsolidus phase relationships in the region Si3N4±AlN±YN±Y2O3 were also determined. Only one compound, 2YN:Si3N4, was con®rmed in the binary system Si3N4±YN contrary to the early results of Thompson53 who reported the existence of three compounds: 6YN.Si3N4, 2YN.Si3N4, and YN.Si3N4. This contradiction could be explained by the purity of the YN used as a starting material, and in that case the Xray diraction lines of 2YN.Si3N4 and YN.Si3N4 reported by Thompson may be regarded as a mixture of melilite (Y2Si3O3N4), J phase (2Y2O3 .Si2N2O), and other oxygen-containing phases. The solubility limits of the aSiAlON on the Si3N4±YN3AlN join were determined to range from m=1.3 to 2.4 in the formula Ym/3Si12ÿm AlmN16. No quinary compounds were found. Seven compatibility tetrahedra were established in the region Si3N4±AlN±YN±Y2O3 (Table 4). Sixty-eight compatibility tetrahedra were established in the system Y±Si±Al±N±O: 39 in the region bounded by Si3N4, SiO2, AlN, Al2O3, and Y2O3, seven in the region bounded by Si3N4, AlN, YN, and Y2O3, and 22 in the region Si3N4, b60, Al2O3, SiO2, and Y2O3. (here and further bn with n=0±60 is the denomination of bSiAlON solid solution proposed in Ref. 7; n value indicates the degree of Si±N for Al±O substitution). All this data are graphically presented in Figs. 7±11. Subsolidus phase relationship in R2O3±Si3N4±AlNAl2O3 systems until now were thoroughly investigated for most of RE elements, although not in such a detailed and profound way as the Y-doped one. The determination of the whole system, represents a very large amount of work and most initial studies have restricted to the sub-systems R±Si±O±N, R±Al±O±N (R=Ce, Pr, Nd and Sm) and some planes involving a and b-SiAlON.17,41 It is known that elements in the lanthanide series are similar to yttrium in compound formation and phase relationships. Previous research on R±Si±O±N subsystems,54 indicates that the phase relationships are nearly the same as in the Y±Si±O±N system, but in R±Al±O±N systems55 the phase relationships were shown to vary with atomic number of the rare earth element. The phase relationships in the systems with high Z-value of the rare earth element are similar to those in Y±Al±O±N.56 In the systems R±Al±O±N (where R=Ce to Sm) to nitrogen containing compounds exist Ð R2AlO3N and R12Al12O18N2 (magneto-plumbite, MP compound) Ð and no garnet phase occurs; instead the 1:1 aluminate phase becomes stable.55 The R2AlO3N and MP compounds do not occur in those systems containing rare earth oxides with cations smaller than that of Gd.17,55 Therefore, in the ®ve-component systems with the low Z-value rare earth elements, the phase Fig. 9. Representation of compatibility of the a-SiAlON with polytypoid phases (from 2Hd to 12H), AlN, and b0 , eight compatibility tetrahedra are formed.52 Fig. 10. Compatibility terahedron a0 -b10-12H-YAG.52 Fig. 11. Subsolidus phase relationships in the region bound by Si3N4, AlN, YN, and Y2O3. 52 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295 2283

relationships become slightly dierent from those in Y± Si±Al±O±N. In the oxygen-rich region where two ®vecomponent phases57,58 Ð U phase (R3Si3ÿxAl3+x O12+xN2ÿx) and W phase (R4Si9 Al5O10N) Ð exist, the former is stable for rare earth cations between La and dysprosium the latter stable only in lanthanum, cerium and neodymium systems. None of the phases above occurs in the yttrium system. The stability of N-melilite (R2Si3ÿxAlxO3+xN4ÿx) in dierent RE systems was discussed in the previous section of this paper. Most closely and most recently the subsolidus phase relationships were studied in neodymium and samarium,59 and dysprosium60 containing R2O3±Si3N4±AlN± Al2O3 systems. The attention paid to the former two elements is caused by the eorts to substitute yttrium by some cheaper RE elements in Si3N4based ceramics without hindering the properties. Dysprosium containing system was chosen because dysprosium is one of the central elements in the RE series: in this region, phase relationships in R±Si±Al±O±N systems are changing from those observed for the low-Z elements (La, Nd, Sm) to those observed for high-Z elements (Er, Yb... and Y). It was therefore interesting to ®nd out whether these changes occur simultaneously or whether there is overlap in this region between the dierent phase relationships observed for the high-Z and low-Z rare earths. According to Ref. 59, samarium has the same behavior as neodymium in regard of phase formation, but it was noted that melting temperatures were lower in the samarium-containing system and consequently, at a given temperature, the size of the liquid-forming region is larger. For both samarium and neodymium containing systems, 44 compatibility tetrahedra were established (Table 5). Some speci®c features of the systems investigated as compared to the Y±Si±Al±O±N system were discovered. Unlike the yttrium containing system where J phase, or woehlerite (Y4Si2O7N2), forms a continuous solid solution with YAM (Y4Al2O9) in neither the Smnor the Nd- containing system was the formation of such solid solutions observed, although the J phase itself Table 5 Subsolidus compatibility tetrahedra in the systems Si3N4±AlN±Al2O3± R2O3 (R = Nd and Sm)59 a Al2O3±b60±15R±MP Al2O3±15R±15R0 ±MP Al2O315R0 ±12H0 ±MP Al2O3±12H0 ±21R0 ±MP Al2O3±21R0 ±AlN±MP 15R±15R0 ±12H±12H0 ±MP 12H±12H0 ±21R±21R0 ±MP 21R±21R0 ±27R±27R0 ±MP 27R±27R0 ±2Hd ±2Hd0 ±MP 2Hd ±2Hd0 ±AlN±MP 21R0 ±27R0 ±AlN±MP 27R0 ±2Hd0 ±AlN±MP AlN±2Hd ±MP±LnAlO3 2Hd ±27R±MP±LnAlO3 27R±21R±MP±LnAlO3 21R±12H±MP±LnAlO3 12H±15R±MP±LnAlO3 b60±15R±MP±LnAlO3 b60±Al2O3±MP±LnAlO3 b60±b25±15R±LnAlO3 b25±15R±12H±LnAlO3 b25±b10±12H±LnAlO3 b10±12H±LnAlO3±M0 (R) b10±12H±21R±M0 a0 ±b0±b10±M0 a0 ±b10±21R±M0 a0 ±b10±b8±21R a0 ±b8±21R±27R a0 ±b8±b5±27R a0 ±b5±27R±2Hd a0 ±b5±b2±2Hd a0 ±b2±2Hd ±AlN a0 ±b2±b0 AlN a0 ±AlN±2Hd ±M0 a0 ±2Hd ±27R±M0 a0 ±27R±21R±M0 M0 ±12H±21R±LnAlO3 M0 ±21R±27R±LnAlO3 M0 ±27R±2Hd ±LnAlO3 M0 ±2Hd ±AlN±LnAlO3 M0 ±AlN±LnAlO3±J(R) M0 ±AlN±J(R)±M AlN±LnAlO3±J(R)±Ln2AlO3N LnAlO3±J(R)±Ln2AlO3N±Ln2O3 a MP; LnAl12O18N; M0 , Ln2Si3ÿxAlxO3+xN4ÿx; J(R), Ln4Si2O 7N2; 15R, 12H, 21R, 27R, 2Hd are Si±rich terminals of AlN polytypoids; 15R0 , 12H0 , 21R0 , 27R0 , 2Hd0 are Al±rich terminals of AlN polytypoids. Fig. 12. Representation of Sm(Nd)±Si±Al±O±N system showing phases occurring in the region bound by Si3N4, Sm(Nd)2O3, Al2O3, and AlN, and Si±Al±O±N behavior diagram at 1700C.59 Fig. 13. Sm(Nd)2O3 is compatible with all polytypoid phases (Si-rich terminal), AlN, and Sm(Nd)Al12O18N (MP compounds) forming ®ve Fig. 14. M0 (melilite solid solution) is compatible with b-SiAlON b0-b10 and a±SIAlON forming a0 -b0 -M0 compatibility tetrahedron.59 2284 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295