HIP-SINTERED BETA-SIALONS AND MIXED ALPHA-BETA-SIALONS DENSIFIED WITH Y2O3 AND LA2O3 ADDITIONS

Various fully dense sialon materials sintered by the glass-encapsulated hot isostatic pressing technique were synthesized using Y2O3 and/or La2O3 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/La2O3:100/0, 75/25, 50/50, 25/75, 0/100. The samples were sintered at two different temperatures, 1550 and 1825° C. At the lower temperature, unreacted α-Si3N4 was present in the samples in addition to β-sialon and secondary phases. The samples sintered at 1825° C showed that yttrium but not lanthanum favoured α-sialon formation. The amount of intergranular phase increased by about 50% when Y2O3 was replaced by La2O3. The La-sialon ceramics have as good an indentation fracture toughness as the Y-sialon ceramics, about 5 MPam-1/2, but the Vicker's hardness is slightly lower, being 1400 kg mm-2 at a 98 N load. © 1990 Chapman and Hall Ltd.