Microstructure and formation mechanism of a Cr-modified Platinum Aluminide coating on In-738 Lc

Chromium-modified platinum aluminide coating was applied on a nickel base superalloy In-738 Lc in this research. Conventional chromium diffusion by pack cementation process was replaced by chromium electroplating and coating microstructure and its formation mechanism was investigated. Microstructural and microanalytical characterization were carried out, using scanning electron microscopy and EDX microanalyzer. In addition, X-ray diffraction (XRD) method was used for phase analysis of the surface layer. Microstructure of the coating consisted of several layers which includes, a surface layer of (Pt,Ni) Al+PtAl2; two intermediate layers of (Pt,Ni)Al layer with Cr-enriched precipitates, and a precipitate-free Ni Al layer; and an innermost interdiffusion layer. Analysis of the results showed that this type of coating microstructure are formed by inward diffusion of aluminum in the initial stage of aluminising following by outward diffusion of nickel at the later stages of animalizing. The results also showed that formation of a chromium enriched surface layer prior to aluminising had no significant effect on the sequences of coating formation, but it could promote formation of AtAl2 phase instead of the formation of solid solution of pt in Ni Al phase in the surface layer