β→α Phase Transformation and Properties of Solid-State-Sintered SiC Ceramics with TaC Addition

Dense SiC-based composite ceramics were fabricated by means of the ex situ addition of TaC using solid-state spark plasma sintering (SPS). Commercially available β-SiC and TaC powders were chosen as raw materials. Electron backscattered diffraction (EBSD) analysis was conducted to investigate the grain boundary mapping of SiC-TaC composite ceramics. With the increase in TaC, the misorientation angles of the α-SiC phase shifted to a relatively small range. It was deduced that the ex situ pinning stress from TaC greatly suppressed the growth of α-SiC grains. The low β→α transformability of the specimen with the composition of SiC-20 vol.% TaC (ST-4) implied that a possible microstructure of newly nucleated α-SiC embedded within metastable β-SiC grains, which could have been responsible for the improvement in strength and fracture toughness. The as-sintered SiC-20 vol.% TaC (ST-4) composite ceramic had a relative density of 98.0%, a bending strength of 708.8 ± 28.7 MPa, a fracture toughness of 8.3 ± 0.8 MPa·m(1/2), an elastic modulus of 384.9 ± 28.3 GPa and a Vickers hardness of 17.5 ± 0.4 GPa.