Structure and Oxidation Behavior of Zr–Y Modified Silicide Coatings Prepared on an Nb–Ti–Si–Cr Based Ultrahigh Temperature Alloy

Zr–Y modified silicide coatings have been prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy by a pack cementation process. The effects of amount of Zr powder in the pack mixture and co-deposition temperature on the coating structure were assessed. The coatings prepared at 1,250 °C with different amounts of Zr powders in the pack mixture have similar structures, mainly consisting of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer, a thin (Ti,Nb)₅Si₄ middle layer and a 1–2 µm thick (Nb,X)₅Si₃ inner layer. The increased amount of Zr powders in the pack mixtures led to a significant decrease in the coating thickness. The coatings prepared at 1,150 and 1,200 °C have similar structures to that prepared at 1,250 °C. However, when the co-deposition temperature increases to 1,300 and 1,350 °C, a (Ti,Nb)₅Si₃ outermost layer formed in addition to the three layers mentioned above. The Zr–Y modified silicide coating can protect the base alloy from oxidation at least for 100 h at 1,250 °C in air. The good oxidation resistance of the Zr–Y modified silicide coating is attributed to the formation of a dense scale consisting of SiO₂ and TiO₂.