Microstructures of Si-Cr co-depositing coatings prepared on Ti-Nb-Si based ultrahigh temperature alloy

The Cr-modified silicide coatings were prepared on Ti-Nb-Si based ultrahigh temperature alloy by Si-Cr co-deposition processes at 1250, 1350 and 1400°C for 10 h, respectively. The results show that the Si-Cr co-deposition coatings have multi layer structure. The phase constituents of outer layers and intermediate layers of the coatings change with increasing co-deposition temperatures. The outer layer and intermediate layer of the coating prepared at 1250°C for 10 h consist of (Ti, X)₅Si₃ (X represents Nb, Hf and Cr elements) and (Ti, X)₅Si₄, respectively. The transitional layer of this coating is very thin and composed of (Ti, X)₅Si₃. The content of Cr increases significantly in the outer layer of the coating prepared at 1350°C for 10 h, and (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase forms in this layer besides (Ti, X)₅Si₃ phase. The intermediate layer is composed of (Ti, X)₅Si₄ and (Ti, X)₅Si₃ phases, and the transitional layer composed of (Ti, X)₅Si₃ phase is thickened obviously. (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase is the single phase constituent in the outer layer of the coating prepared at 1400°C for 10 h, and the intermediate layer is composed of (Ti, X)₅Si₃ thoroughly. Discontinuous (Nb, Ti)₃Si blocks are also found between the transitional layer and substrate. The influence of temperature on the diffusion ability of Cr atoms is greater than that of Si atoms in the Si-Cr co-deposition processes investigated.