Hardness degradation in liquid-phase-sintered SiC with prolonged sintering

The effect of the sintering time on the Vickers hardness of liquid-phase-sintered SiC with 10 wt% YAG additives was studied for materials fabricated in Ar and N2 atmospheres. The hardness of the Ar-sintered materials was found to decrease markedly with increasing sintering time, whereas, in contrast, the decrease was only marginal for the N2-sintered materials. Berkovich nanoindentation tests showed that the softening could not be attributed in either case to degradation of the SiC grains, thereby pointing to the intergranular YAG phase as the responsible. That the cause was degradation of the YAG phase was confirmed by X-ray energy-dispersive spectrometry and Hertzian indentation tests. The far more pronounced softening observed for the Ar-sintered materials reflects the more severe degradation that the YAG phase undergoes during sintering in this atmosphere.