Coating Developments to Restrict Strategic Materials Usage,

Hot section gas turbine engine components have relied on the use of superalloys which contain high percentages of strategic elements to attain an optimum compromise of high temperature strength, corrosion and oxidation resistance. Due to limited availability and price fluctuations of certain key alloying elements, such as cobalt and tantalum, selection and substitution of superalloys with lower strategic element content is being accomplished. The change in alloy often leads to the use of materials with less corrosion and oxidation resistance. This paper describes the development of protective coatings that have enabled the implementation of less environmentally stable alloys with no loss, and sometimes an increase, in component capability and life. Development of tailored MCrA1Y overlay coatings, improved diffusion aluminides, thermal barriers, and alternate wear resistant coatings has enabled the coating corrosion and oxidation life to be more independent of the base superalloy composition and hence allow more latitude in alloy selection. The resulting coating developments and their effects on reducing strategic element use are presented.