POROSITY DEPENDENCE OF MECHANICAL STRENGTH AND FRACTURE-TOUGHNESS IN SIC-AL2O3-Y2O3 CERAMICS

Influence of residual porosity on the flexural strength and fracture t oughness of pressureless-sintered alpha-SiC ceramic with controlled mi crostructure, characterized by equiaxed-grain and a mixture of equiaxe d/plate grains were investigated together with the trend of sigma-K-IC behavior. Both the flexural strength and the fracture toughness of th e SiC ceramics showed an exponential decrease with porosity, but the e xtent of the porosity dependence was dependent on the microstructure; a higher porosity-dependent strength was seen for the equiaxed-grain m icrostructure, while the dependence became weak for the mixed one. The trend of flexural strength-toughness dependence appeared to be positi ve and became more pronounced for the SiC ceramics with mixed-grain mo rphology, suggesting a strengthening effect due to the presence of rei nforced platelike phase. Crack deflection appeared to be the principal mechanism for toughness improvement in the SiC-Al2O3-Y2O3 ceramics.