Microstructure of Si-Y co-deposition coating prepared on a Nb-based ultrahigh temperature alloy by pack cementation process

Si-Y co-deposition coatings on an advanced Nb-based ultrahigh temperature alloy were prepared by pack cementation process at 1050 and 1150°C for 5-20 h, respectively. The coating structure, phase constituent and structure formation of the coatings have been analyzed. It has been found that the coating is mainly composed of compact upper layer and loose lower layer (Nb, X)Si₂ (X represents Ti, Cr and Hf), a (Nb, X)₅Si₃ transitional layer and an Al-rich diffusion zone just above the substrate. The coating growth kinetics accord with the parabolic laws. The Al-rich zone has a suppressive effect on the growth of the coating. EDS analyses revealed that the content of Y in the coating increased evidently with rising co-deposition temperature, but increased slowly with prolonging co-deposition time.