Phase formation, microstructure and mechanical properties of TaB2–SiC composites synthesized by reaction hot pressing: Effects of SiC contents and oxygen impurity

Abstract

TaB₂–SiC composites with 0–20 wt% SiC contents were prepared by reaction hot pressing using Ta, B and SiC powders as starting materials. Densification, constituent phases, microstructure and mechanical properties, namely Vickers hardness, fracture toughness and flexure strength, were evaluated by means of inspection of sintering shrinkage, density measurement, XRD, SEM, TEM, HRTEM, EDS, SAED, indentation and three-point bending tests. Phase formation, including dominant (TaB₂, SiC) and minor (TaC, glassy SiO₂) phases, was checked by thermodynamic assessment relevant with Ta₂O₅ and B₂O₃ impurities introduced into the reaction system by the starting powders. The results showed that mixtures of Ta, B and SiC powders would initiate self-propagating combustion synthesis (SPCS) at about 900 °C; a transient hold at 800 °C for 1 h could suppress this reaction. Volatilization of B₂O₃ impurity in association with local high temperature due to SPCS helped reduce the TaC and glassy SiO₂ minor phase formation. TaB₂–SiC composites could reach 99 % relative densities with 3 wt% SiC addition and above. Compositions with <7.5 wt% SiC additions showed uniform dispersion of SiC particles at multi-TaB₂ grain junctions while compositions with higher SiC content revealed SiC segregation. Bending strength of 503 MPa and fracture toughness of 6.69 MPa⋅m^1/2 could be reached with 7.5–20 wt% SiC addition.