Low-temperature synthesis and oxidation resistance of random combination of Hf, Nb, and Ta carbides microcuboids

Based on the dissolution-precipitation mechanism, this article successfully synthesized binary and ternary transition metal carbide microcuboids with random combinations of Hf, Nb, and Ta by annealing monocarbides/cobalt powders. Accelerated mass transport rate through the flow of molten alloys (Co-Hf-Nb-Ta) instead of slow solid diffusion made the low-temperature pressureless sintering technique (1500°C) a reality. Furthermore, the equilibrium morphology was driven by the gradient Gibbs potential of carbides induced by the different local curvature of powders and anisotropic interfacial energy. (Hf_0.5Ta_0.5)C possessed the optimal oxidation resistance among all mentioned carbides, even competed with (Hf_1/3Nb_1/3Ta_1/3)C. During the isothermal oxidation at 800∼1200°C, the doping of Nb and Ta in carbides assisted the monoclinic-orthorhombic HfO₂ transition at ambient pressure, besides, TaC can also restrain the orthorhombic-monoclinic transition of Nb₂O₅. Moreover, oxidation kinetics parameters concluded that the addition of HfC and TaC contributed to the decreasing reaction order and the increasing activation energy, respectively.