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 exte1. 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, alumo￾silicates, and oxynitrides. Nitrogen-rich glasses were also thoroughly investigated. The vast amount of data accu￾mulated 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 tran￾sient 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 Si6ÿzAlz￾OzN8ÿz. In this formula z can be varied continuously from zero to about 4.2.7ÿ9 The homogeneity region of b￾SiAlON 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 MeN
3AlN 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 simi￾lar to that of b-SiAlON with n (Si+N) being replaced by n (Al+O). The second mechanism is further repla￾cement 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 stu￾died in the Y±Si±Al±O±N system, where the two-dimen￾sional 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 pre￾sent 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 a￾SiAlON 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, Sm￾and Y-doped a-SiAlONs,22, 23 which provides even bet￾ter possibilities of microstructure tailoring. It is evident from its chemical composition that a-SiAlON phase o€ers possibility of reducing the amount of residual glassy grain-boundary phase by incorporating con￾stituents of sintering aids into the crystal structure, which otherwise will be present as substantial amounts of residual glassy phase and deteriorate the high tem￾perature 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