A comparative study of two kinds of Y and Al modified silicide coatings on an Nb-Ti-Si based alloy prepared by pack cementation technique

Two kinds of Y and Al modified silicide coatings on an Nb-Ti-Si based alloy were prepared by pack cementation technique. The microstructure and oxidation behavior of both coatings were studied. Both coatings had a multiple layer structure, but the outer layers were composed of either Y- and Al-doped (Nb,X)Si₂ or Y-doped (Nb,X)₃Si₅Al₂+(Nb,X)Si₂ phases, respectively. The former coating could protect the substrate alloy from oxidation at 1250°C for 100h, but the latter coating could only endure for less than 20h. The scale formation mechanisms and microstructural changes of both coatings upon oxidation have been illustrated.