FORMATION OF Ge-Y MODIFIED SILICIDE COATINGS ON Nb-Ti-Si BASE ULTRAHIGH TEMPERATURE ALLOY

Nb-Si base ultrahigh temperature alloy is a promising candidate for high temperature applications due to its high melting point, high strength and relatively low density at elevated temperatures. However, the poor oxidation resistance of this alloy has still be a major barrier to its high temperature applications. Ge and Y modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy have been prepared successfully by co-depositing Si, Ge and Y in order to improve its high temperature oxidation resistance. The structure, constituent phases and formation process of the coatings have been revealed using XRD, SEM and EDS. The results showed that the coatings had obvious layered structure which was composed of a (Nb, X) (Si, Ge)(2) (X represents Ti, Cr and Hf elements) outer layer and a (Ti, Nb)(5)(Si, Ge)(4) inner layer. The concentration of Ge in the coating increased with increase in the distance away from the coating surface. Compared with the single Y modified silicide coating, the addition of Ge did not change the coating structrue, but reduced the absorption and deposition of Si and retarded the coating growth. The coatings prepared at 1250 degrees C for different time (0-8 h) had similar structures and the coating growth kinetics followed a parabolic law.