236 V.A.I-hevskiy et al Journal of the European Ceramic Society 2(2)2275-2295 AIO 2m+N O.N the plane es,while the cha amount and microstructure of the resulting crystaline oxynitride phase.However,in the course of the study of additives (Yb2O.Dy2O Sm2O either singly or in combination with Y)Mandal et al reported tha he tra RE(n from urred i ted between 1000 and that extent of the which degrades the mechanical and chemical properties transformation was more pronounced with increasing ve the temperature. temperatures structure of both are ne of the boundary phase in nitrogen ceramics by converting it emae恢 rrangement between the two The transfor mation between p pha has usually achieved via a conventional glass-ceramic pro. and substantial atomic diffusion.and hence requires significant amounts of thermal energy.As a result of the above its T point bu without being melte ng covalen t nature of the bounding asso ated willSiAlON formers, some amount of additives remains as a residual M±Si±Al±O±N intergranular glassy phase, which degrades the mechanical and chemical properties of the material above the glass-softening temperature, which normally is about 900±1100C. Post-sintering heat treatment is one of the accepted methods to eliminate or minimize the glassy grain boundary phase in nitrogen ceramics by converting it into refractory crystalline phase(s), thus improving the materials performance at elevated temperatures. This is usually achieved via a conventional glass-ceramic pro￾cess during which the glass devitri®es at a temperature above its Tg point but without being melted.58,62 There has been much work in understanding of the grain boundary crystallization, but very little attention has been paid to the e€ects of the post-sintering heat treat￾ment on the stability of SiAlON phases, while the char￾acterization of the ®nished product has always focused on the account of the residual glass and the type, amount and microstructure of the resulting crystalline oxynitride phase. However, in the course of the study of densi®cation and post sintering heat treatment of (a+b)-SiAlON compositions containing selected RE additives (Yb2O3, Dy2O3, Sm2O3 either singly or in combination with Y2O3). Mandal et al.25 reported that the transformation from a0 to b0 readily occurred in some RE-(a0 +b0 ) composite materials when heat trea￾ted between 1000 and 1600C and that extent of the transformation was more pronounced with increasing temperatures. Although the structure of both a0 and b0 phases are basically built up of corner sharing (Si, Al)(O, N)4 tetra￾hedra, there is a distinct di€erence in the atomic arrangement between the two phases.63,64 The transfor￾mation between a0 and b0 phases has a reconstructive nature, which involves the breaking of chemical bounds and substantial atomic di€usion, and hence requires signi®cant amounts of thermal energy. As a result of the strong covalent nature of the bounding associated with both the a0 and b0 structures, the atomic di€usivity of the species making up the lattices is inherently low. It is, Fig. 17. Graphic representation of compatibility of the DyAG with polytypoid phases, AlN, and Al2O3, 12 compatibility tetrahedra are formed.60 Fig. 18. DyAG is compatible with b-SiAlON b10±b60 and a±SiAlON (oxygen-rich) forming a0 -b10-12H-DyAG compatibility tetrahedron.60 Fig. 19. M0 (melilite solid solution) is compatible with b-SiAlON b0- b10 and a±SiAlON forming a0 -b0 -M0 compatibility tetrahedron.60 Fig. 20. Representation of Dy±Si±Al±O±N system showing phases occurring in the region bound by Si3N4, Dy2O3, Al2O3, and AlN, and Si±Al±O±N behavior diagram at 1700C (a-SiAlON plane = the plane with a-SiAlON composition DyxSi12ÿ(m+n)Alm+nOnN16-n).60 2286 V.A. Izhevskiy et al. / Journal of the European Ceramic Society 20 (2000) 2275±2295