Y and Al modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy prepared by pack cementation process

Y and Al modified silicide coatings were prepared on an Nb-Ti-Si based ultrahigh temperature alloy by co-depositing Si, Al and Y at 1150°C for up to 10h, respectively. The deposition of Al and Si occurred in a sequential manner during the pack cementation process. At the initial stage, the element deposited was primarily Al with very little Si and an Al₃(Nb,X) (X represents Ti, Cr and Hf elements) layer formed preferentially. After a short period of holding time, Si started depositing and Si-Al co-deposition took place. However, this Si-Al co-deposition period was not long. When the holding time was longer than 1h at 1150°C, Si deposition dominated the coating growth process. The coating growth kinetics at 1150°C followed a parabolic law. The coating prepared at 1150°C for 10h had a multi-layer structure, with a thick (Nb,X)Si₂ outer layer, a thin (Ti,Nb)₅Si₄ middle layer and an Al, Cr-rich inner layer. The coating could protect the Nb-Ti-Si based alloy from oxidation at 1250°C in air for at least 100h. The excellent oxidation resistance of the coating was attributed to the formation of a dense scale mainly consisted of TiO₂, SiO₂ and Al₂O₃.