Development of silicide coatings to ensure the protection of Nb and silicide composites against high temperature oxidation

In this work, we describe the fabrication of coatings containing (TiNbXCr)7Si6 silicide (with X = Fe, Co and Ni) by halide-activated pack-cementation on Nb and silicide composites. The coating oxidation performance is evaluated isothermally at temperatures ranging from 1100 °C to 1300 °C and cyclically at 1100 °C. In all the coating systems examined, the M7Si6 silicide formed in the protective coating and constituted the outmost layer for coatings containing Co, whereas M6Si5 and a Cr-rich silicide constituted the outmost compounds for Fe and Ni coatings, respectively. The results of isothermal oxidation tests show that all these compounds form protective oxide layers leading to very low mass gains, both at 1100 °C and 1200 °C. Under cyclic conditions at 1100 °C, coatings containing Co and Ni exhibited very long lifetimes, exceeding 2600 1-hour cycles. The beneficial effect of Ni and Co diminished at 1300 °C, due to rapid interdiffusion between substrate and coating in comparison to that found in the Fe containing coating.