Effects of initial α‐phase content on properties of pressureless solid‐state sintered SiC ceramics

Abstract α‐ and β‐SiC starting powders of similar particle sizes were used to investigate the effect of initial α‐phase content on the electrical, thermal, and mechanical properties of pressureless solid‐state sintered (PSS) SiC ceramics with B 4 C and C additives. For β‐SiC starting powders, a coarse‐grained microstructure with elongated platelet grains was formed by the transformation of 3C to 6H and finally to 4H‐SiC phase. In contrast, materials prepared from α‐SiC powders exhibited a fine‐grained microstructure with platelet grains. This study revealed the beneficial effect of α‐SiC starting powders in achieving low electrical resistivity and high thermal conductivity in PSS SiC ceramics, which was attributable to their higher sinterability, lower impurity content, and lower 6H to 4H‐SiC phase transformation rate compared with β‐SiC powders. The electrical resistivity decreased by an order of magnitude and the thermal conductivity increased by 32% with an increase in initial α‐phase content from 0 to 100%. The flexural strength increased by approximately 16% with increasing initial α‐phase content due to a decreased flaw size with decreasing grain size. However, the fracture toughness and hardness were insensitive to the change in initial α‐phase content.