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 mu m thick Zr-film, and then Si-Y co-deposition at respectively 1150, 1250 and 1350 degrees C 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 degrees C had similar structures, consisting of a ZrSi2 outer layer, a (Nb, X)Si-2 (X=Ti, Cr, Zr and Hf) middle layer and a (Ti, Nb)(5)Si-4 inner layer. However, the higher co-deposition temperature (1350 degrees C) 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 degrees C in air.