Phase Reaction and Sintering Behavior in the Pseudoternary System AlN‐Y 2 O 3 ‐AI 2 O 3

Phase reaction and sintering behavior in the ternary system AlN‐Y 2 O 3 (0‐10 wt%)‐Al 2 O 3 (0‐40 wt%) at temperatures ranging from 1600° to 2000°C were studied. The density of the sintered 100 wt% A1N specimens increased monotoni‐cally with firing temperature, but the maximum value was limited to 95%, even at 2000°C firing. All the specimens containing Y 2 O 3 , regardless of their Al 2 O 3 content, achieved full density by sintering at 1800°C. However, at and above 1900°C, the densities decreased with increased Al 2 O 3 content. This densification behavior appeared to be based on the grain morphology and decomposability of the existing compounds in the products. The phases present in the sintered A1N‐Al 2 O 3 were composed of AlN, Al 2 O 3 , and γ‐AION below 1900°C, and 27 R , 20 H , and 16 H at and above 1950°C. In the A1N‐Y 2 O 3 ‐Al 2 O 3 specimen, yttrium aluminum garnet (YAG) phase formed in the first stage at a temperature <1600°C, after which aluminum oxynitrides appeared. In the specimens sintered at 1950°C or higher, γ‐AION phase, with a higher Al 2 O 3 :A1N ratio, and 27 R phase, with a lower Al 2 O 3 :A1N ratio, coexisted, and the contents were quantitatively low in comparison with the Y 2 O 3 ‐free specimens, although Y 2 O 3 trapped Al 2 O 3 to produce YAG. This differed from the A1N‐Al 2 O 3 phase diagram proposed by McCauley et al. The grain morphologies depended on the presence of the above phases. The grains based on A1N and γ‐AION were granular, and those based on 27 R, 20 H, and 16 H poly‐typoids were platelike.