Preparation and structural formation of oxidation-resistant silicide coatings on Nb-based alloy by pack cementation technique

Oxidation-resistant silicide coatings on an Nb-based alloy were prepared by pack cementation technique. The holding temperatures were 1050, 1150 and 1250°C and the holding time were 2, 5, 10, 15 and 20 h, respectively. The structure, phase constituents and compositional distribution in the coatings were analyzed by SEM, EDS and XRD techniques. The formation mechanism and growth kinetics of the coatings were discussed and measured. The results show that the phase constituent of the coatings is (Nb, X)Si₂ (X represents Ti, Cr, Hf and Al elements). Double layers exist in the coatings, and the upper layer is denser. A transitional layer and a diffusion zone form between the coating and base material. The coating growth kinetics at 1050, 1150 and 1 250°C were all fitted well to parabolic laws.