Microstructure and oxidation behavior of ZrSi₂-NbSi₂ multilayer coatings on an Nb-Ti-Si-Cr base ultrahigh temperature alloy

The rather poor oxidation resistance of Nb-Si base ultrahigh temperature alloys has seriously limited their practical applications at high temperatures. Niobium disilicide coatings, especially those modified by reactive elements (RE) such as Zr and Y, have been shown to possess good anti-oxidation properties at high temperatures due to the formation of a protective RE-containing SiO₂ scale. Halide activated pack cementation (HAPC) is one of the most widely used techniques for preparing protective coatings on Nb-Si base ultrahigh temperature alloys, because compact coatings and metallurgical substrate/coating bonds can be obtained with using this technique. However, only a very limited amount of Zr and Y can be diffused into the coatings by a single co-deposition pack cementation process as a result of their large atomic radii and high melting points. To solve this problem, a method such as magnetron sputtering, which can be used for producing overlay coatings with different composition ratios of coating elements, seems to be feasible. In the present study, ZrSi₂-NbSi₂ multilayer coatings were prepared on an Nb-Ti-Si-Cr base ultrahigh temperature alloy by first magnetron sputtering 2 μm thick Zr-film, and then Si-Y co-deposition at respectively 1150, 1250 and 1350℃ by HAPC process. The structures and formation processes, as well as the static oxidation behavior of the coatings were investigated. The results show that the coating prepared at respectively 1150, 1250 and 1350℃ had similar structures, consisting of a ZrSi₂ outer layer, a (Nb, X)Si₂ (X=Ti, Cr, Zr and Hf) middle layer and a (Ti, Nb)₅Si₄ inner layer. However, the higher co-deposition temperature (1350℃) could cause cracks at the interfaces between the constituent layers of the coatings. The formation of the coating was dominated by inward diffusion of Si, accompanied with a certain degree of outward diffusion of Nb, Ti and Cr from the base alloy during the second Si-Y co-deposition process. The oxidation tests demonstrated that the ZrSi₂-NbSi₂ multilayer coating possessed excellent oxidation resistance. After oxidation, a dense scale consisting of SiO₂, TiO₂, ZrSiO₄ and Cr₂O₃ formed on the coating, which can protect the base alloy from oxidation at least for 100 h at 1250℃ in air.