Microstructure of Si-Cr co-deposition coatings on a Ti-Nb-Si based high temperature alloy by pack cementation process

Si and Cr were co-deposited on a Ti-Nb-Si based high temperature alloy by pack cementation processes at 1250 and 1300 °C for 10 h, respectively. XRD, SEM and EDS were used to identify the structure, phases and composition distribution of the coatings. The results show that the Si-Cr co-deposition coatings prepared with different pack mixtures at 1250 °C exhibit different structure and phase constituents. Both the reaction and diffusion speeds of Si and Cr are reduced with the decrease of the content of activator NaF in the pack mixtures. The coating fabricated at 1250 °C with activator NaF of 8 wt% in the pack mixture is composed of (Nb, Ti)Si₂ outer layer with a small quantity of (Ti, X)₅Si₃ (X denotes Nb, Cr and Hf) phase, (Ti, X)₅Si₄ intermediate layer and (Nb, Ti)5Si₃ transitional layer. In the coating prepared with 5 wt% NaF in the pack mixture at 1250 °C, single (Ti, X)₅Si₃ phase is found in its outer layer and many micro-pores are observed, and the intermediate layer is composed of (Ti, X)₅Si₄ and the transition layer is very thin. Compared with the Si deposition coating, the amount of cracks decrease evidently, but more micro-pores are detected in the outer layer for the Si-Cr co-deposition coatings, and thickness of the Si-Cr co-deposition coatings decreases obviously. (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase and (Ti, X)₅Si₃ phase form in outer layer of the coating when the co-deposition temperature is raised up to 1300 °C due to the increase of reaction and diffusion speed of Cr atoms.