Innovative coating for aggressive environments: land-based gas turbine applications.

Severe demands on coatings for gas turbine engines that must operate in significantly more aggressive environments than previously required lead to the development of a new resistant system. Both conventional aluminide and MCrAlY coatings must maintain a reservoir of aluminium that forms a protective alumina scale on the surface to extend their life. The loss of aluminium occurs by a spallation of the alumina scale due to thermal cycling and by interdiffusion into the substrate. To have a high durability life, innovative coatings must ensure the aluminium concentration and activity in the coating is as high as possible, along with a high chromium concentration. Such coatings are usually of MCrAlY-type where M is equal to nickel, cobalt or a nickel-cobalt alloy. ONERA has developed a new MCrAlY coating via an innovative electroless-like process. The aim of this work was to improve it in order to reach above coatings specifications. The aluminium concentration of the electroless-like coating has been modified by using a modified alurninisation and/or by interposing a diffusion barrier. The aluminium. activity has also been changed by using a platinum coating. The solubility of chromium has been altered by using a modified aluminide such as platinum modified aluminide. Processes including electrolytic, electroless-like and vapour pack cementation are described. The structures of these layers and of the whole system are discussed. Corrosion test results and microscopic investigations are also presented.