Cyclic oxidation of diffusion aluminide coatings on cobalt base super alloys

The oxidation resistance and failure mechanism of four diffusion coatings developed for use on cobalt based super alloys were investigated. Metallography and 1450 K cyclic oxidation were used to evaluate the coatings on air cast FSX-414. The results indicate none of the ‘modified’ diffusion coatings exhibited oxidation resistance significantly better than the appropriate baseline, and, in several cases, the oxidation resistance of the ‘modified’ coatings was inferior to baseline. Electron microprobe composition profiles across the coatings indicate that the solubility of chromium and tungsten in cobalt aluminide (CoAl), with or without platinum, is lower than in the substrate. Therefore, these elements are concentrated in the substrate ahead of the coating–alloy interface during inward coating growth. The concentration of Cr and W ahead of the coating front is sufficient to form a continuous layer between the cobalt aluminide and the substrate. The results suggest that cracking of the Cr and W enriched layer coupled with oxidation inside the crack network undercuts the aluminide coating, resulting in massive coating delamination.