O′-sialon ceramics prepared by hot isostatic pressing

Abstract

Abstract O′-sialon ceramics, Si2 − xAlxO1 + xN2 − x, with x = 0−0·2 have been prepared from Si3N4, SiO2 and Al2O3 by glass-encapsulated hot isostatic pressing at 1600, 1750 and 1900°C and are compared with materials prepared by pressureless sintering at 1600 and 1775°C. Additional series were prepared with 1,5 or 10 wt% Y2O3 as a sintering aid. Formation of the O′-sialon phase during sintering was dependent on the presence of a liquid phase and was promoted by alumina or yttria additions, especially when added together. Single-phase O′-sialon ceramics were never found, as small amounts of Si3N4 were always present. The microstructure consisted mainly of large elongated plate-like O′-sialon grains. Inclusions of β-Si3N4 were found in the small grains, which caused a significant amount of defects. Dense O′-sialon materials were hard (HV 10 about 1500) and brittle (K1C about 3·5 MPa m 1 2 ) at room temperature. The generally low toughness might be caused by the numerous defects in the Si2N2O structure in combination with a strong bond between the grains and the intergranular phase.