Protective coatings – purpose,role,and design

Abstract

A wide range of coatings and coating processes is now available for protecting components in gas turbines operating in a variety of conditions. Coated aerofoils are less likely to fragment, and they keep their original shape – and hence their aerodynamic efficiency – for longer. Aluminide diffusion coatings perform well in relatively clean environments, especially on alloy bases which have a moderate resistance to the many forms of hot corrosion, and with additions of chromium or platinum they provide resistance to sulphate–induced hot corrosion. In hotter or more aggressive corrosive environments, overlay coatings based on MCrAlY (M = Fe,Co, Ni), CoCr, and MCrSi(Al), applied by electron beam evaporation or plasma spraying, are suitable for a wide variety of applications. It is not yet possible to design coatings from first principles for particular applications. Development still proceeds by repeated selection and testing, but modern technology has made this approach more effective than it was in the 1960s. The future is likely to see the rate of progress slow down; advances are expected to take the form of more efficient and flexible processes rather than a broadening of the range of applications.

MST/280