Research on Oxidation Resistant ZrSi₂–NbSi₂ Bilayer Coatings for an Nb–Ti–Si–Cr Based Ultrahigh Temperature Alloy

Oxidation-resistant ZrSi₂–NbSi₂ bilayer coatings were prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy using a two-step (first sputtering Zr film, and then Si–Y co-deposition) process. The coatings prepared with different Zr film thicknesses possess a similar structure, consisting of a ZrSi₂ outer layer, a (Nb,X)Si₂ (X represents Ti, Cr and Hf) middle layer and a (Ti,Nb)₅Si₄ inner layer. The higher co-deposition temperature (Si–Y co-deposition at 1350 °C) would cause cracks at the interfaces between the constituent layers of the coatings. The coated specimens possess much lower mass gains than the base alloy after exposure of same static oxidation times at 1250 °C. After oxidation, a dense scale consisting of a mixture of SiO₂, ZrSiO₄, ZrO₂, Al₂O₃, TiO₂ and Cr₂O₃ formed, which could protect the specimen from oxidation at least for 100 h at 1250 °C in air.