OXIDES FORMED ON SUPER ALLOY COATINGS

After service it was found that the surface colouration of turbine blades within an engine coated with MCrAlY overlay coatings had colourations ranging from light grey to black. Since the samples were exposed to identical service conditions, it is suspected that a variation in coating quality may be causing this oxidation difference. The engine manufacturer wants to understand the reason for this variation and has supplied two blades possessing each extreme of colouration. Suggest a work package to assess the following; What are the oxides that have formed on the surface of each of the superalloy coatings? How uniform is the thickness of these oxides? What is the surface roughness of the coated blades? What are the phase distributions within the coatings on each blade? Is one coating more suseptable to cracking/spallation? Is the composition of the coatings the same in both samples (consider both bulk and local compositions)? Do the two coatings on each blade have grains with the same size and shape? (NOTE: You are working in a lab that provides characterisation services to industry – you are given the following information about a potential project. For one of the following briefs provide a detailed experimental plan describing the analysis that you would do to answer the questions posed. Suggest the techniques (taught in this module) that you are going to use for the selected case study. State any assumptions that you make and include details of how you would prepare the samples. The report should consider mainly the techniques introduced in this course but, where relevant, other techniques can be discussed.)