Microstructure and mechanical and thermal properties of ternary carbides in Hf–Al–C system

A Hf–Al–C composite composed of Hf3Al3C5, Hf2Al4C5 and Hf3Al4C6 has been successfully synthesized by a hot pressing method; its microstructure and mechanical and thermal properties were systematically characterized. Hf–Al–C composite conserves the high hardness and stiffness similar to HfC. Interestingly, the composite exhibits much higher strength and fracture toughness than HfC due to its fine and anisotropic grains. Diffusion-accommodated grain-boundary sliding of Hf–Al–C ceramics at high temperature is inhibited by glass-free grain boundaries and tight interlocking of grains at grain-edge triple junctions, resulting in high remaining stiffness up to 1600 °C. Dislocations on the basal planes of Hf–Al–C ceramics with a Burgers vector of 13〈112¯0〉 can be activated at high temperature. Hf–Al–C composite shows higher coefficient of thermal expansion and specific heat capacity as well as lower thermal conductivity than HfC. The superior mechanical and thermal properties make Hf–Al–C compounds good high-temperature structural materials.