The Oxidation Mechanism of TaC/Ni Composites

Abstract

In this paper, the in situ TaC/Ni composites were prepared by reactive sintering method using Ta, Ni and graphite as raw materials, and their oxidation behavior at 873, 973 and 1073 K in air is investigated by static cyclic oxidation method. The results present that the oxidation behavior of composites conforms to the linear kinetic law. At 873 K, the oxidation of TaC and Ni–Ta matrix generate NiO, Ta₂O₅, TaO₂ and NiTa₂O₆. The oxide scale is consisted by double continuous layers, including the outer NiO and inner Ta₂O₅ layer, due to the diffusion of Ni ion through the oxide ion vacancies in Ta₂O₅. The Oxygen inward diffuse along the interface between TaC and Ni–Ta matrix, and then dissolves in TaC and replaces C sites to generate Ta oxides. At 973 K, more Ta oxides occupy the oxide scale, forming the alternative distribution of NiO and Ta oxides, resulted from the accelerated diffusion of Ta ions. At 1073 K, the oxide scale is mainly taken up by NiTa₂O₆ with slight NiO. The formation reaction of Ta₂O₅ and NiTa₂O₆ shows high Pilling Bedworth ratio near to 2, resulting in the expansion and compressive stress in oxide scale. The oxidation of composites is primarily controlled by the inward diffusion of Oxygen, leading the formation of non-protective oxide scale with pores and cracks on surface. One effective method to improve the oxidation resistance of TaC/Ni composites is to restrict the formation of Ta₂O₅ and NiTa₂O₆, to inhibit the appearance of cracks in oxide scale.