PHASE REACTION AND SINTERING BEHAVIOR IN THE PSEUDOTERNARY SYSTEM ALN-Y2O3-AL2O3

Phase reaction and sintering behavior in the ternary system AlN-Y2O3(0 -10 wt%)-Al2O3(0-40 wt%) at temperatures ranging from 1600 degrees to 2000 degrees C were studied. The density of the sintered 100 wt% AlN s pecimens increased monotonically with firing temperature, but the maxi mum value was limited to 95%, even at 2000 degrees C firing. All the s pecimens containing Y2O3, regardless of their Al2O3 content, achieved full density by sintering at 1800 degrees C. However, at and above 190 0 degrees C, the densities decreased with increased Al2O3 content. Thi s densification behavior appeared to be based on the grain morphology and decomposability of the existing compounds in the products. The pha ses present in the sintered AlN-Al2O3 mere composed of AlN, Al2O3, and gamma-AlON below 1900 degrees C, and 27R, 20H, and 16H at and above 1 950 degrees C. In the AlN-Y2O3-Al2O3 specimen, yttrium aluminum garnet (YAG) phase formed in the first stage at a temperature < 1600 degrees C, after which aluminum oxynitrides appeared. In the specimens sinter ed at 1950 degrees C or higher, gamma-AlON phase, with a higher Al2O3: AlN ratio, and 27R phase: with a lower Al2O3:AlN ratio, coexisted, and the contents were quantitatively low in comparison with the Y2O3-free specimens, although Y2O3 trapped Al2O3 to produce YAG. This differed from the AlN-Al2O3 phase diagram proposed by McCauley et al. The grain morphologies depended on the presence of the above phases. The grains based on AlN and gamma-AlON were granular, and those based on 27R, 20 H, and 16H polytypoids were platelike.