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

Abstract

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. (Hf0.5Ta0.5)C possessed the optimal oxidation resistance among all mentioned carbides, even competed with (Hf1/3Nb1/3Ta1/3)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.