Inhomogeneous grain growth and elongation of Dy-alpha sialon ceramics at temperatures above 1800 degrees C

A series of samples with dysprosium cc-sialon compositions and different am ounts of sintering additives has been fabricated from alpha-Si3N4, AlN, Al2 O3 and Dy2O3 starting powders, using pressureless sintering (PLS) at 1800 d egrees C plus gas pressure sintering (GPS) at the same and higher temperatu res under a relatively low gas pressure of 0.9 MPa N-2. The resultant alpha -sialon grains show significantly different features, such as regularly fin e and equiaxed, elongated, and even a few extraordinarily large with high a spect ratio grains in the fine matrix, which has rarely been observed in al pha' ceramics. It is suggested that the temperature strongly influences the grain morphologies of alpha-sialon, playing an important role particularly in the latter stage of the "nucleation-growth'' process. Such a kind of mi crostructural morphology as mixed with equiaxed and elongated grains toughe ns the alpha-sialon ceramics and leaves them still hard as usual. Overly hi gh temperature treatment leads to the preferential growth of a few grains w hich become excessively large and long, and do not significantly improve th e materials toughness. (C) 2000 Elsevier Science Ltd. All rights reserved.