Structure formation of co-deposition of Si and Al coatings on advanced Nb-based ultrahigh temperature alloy by pack cementation process

Al-modified silicide coatings on an advanced Nb-based ultrahigh temperature alloy were prepared by co-depositing Si and Al at 1000, 1050, 1100 and 1150°C for 8 h, respectively. The results show that the coatings obtained by these processes have a multiple layer structure. The outer layers of the coatings have the different phase constituents at different coating temperature, but all the innermost layers of the coatings are constitutive of (Nb, Ti) Al₃ and (Cr, Al) ₂ (Nb, Ti). Al-modified (Nb, X) Si₂ (X represents Ti, Cr and Hf) coatings with an Nb₃Si₅Al₂ outermost layer are obtained by co-depositing Si and Al at 1050°C for 8 h. The coatings prepared at 1000°C for 8 h are mainly constitutive of (Nb, Ti) Al₃, and the coatings prepared at 1100°C for 8 h and 1150°C for 8 h are mainly constitutive of (Nb, X)-(Si, Al)₂ with 2.35%Al (mole fraction).