O'-SIALON CERAMICS PREPARED BY HOT ISOSTATIC PRESSING

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-degrees-C and are compared with materials prepared by pressureless sintering at 1600 and 1775-degrees-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 beta-Si3N4 were.found in the small grains, which caused a significant amount of defects. Dense O'-sialon materials were hard (HV10 about 1500) and brittle (K1C about 3.5 M Pa m1/2) 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.