State of the art on high-temperature corrosion-resistant coatings

Abstract

A high-temperature protective coating must meet several criteria: provide adequate environmental resistance, be chemically and mechanically compatible with the substrate, be applicable. Comprehensive reviews on high-temperature coatings have appeared regularly since the early 1970s. Our purpose is not to recapitulate the material covered therein but rather to focus on recent trends, and point out some research perspectives. Historically the development of high-temperature protective coatings has been linked with the evolution of demanding applications such as super-alloy components in gas turbine engines; the searches for better performance (higher inlet temperatures, longer lifetimes, etc.) and for cost-saving solutions (use of contaminated low-grade fuels) have been the main incentives for developing the different coatings now available in production: simple and “modified” diffusion coatings, overlay coatings, thermal barriers. In recent years research and development activity has been concentrating on the following points. 1. (a) Degradation mechanisms in high-temperature corrosion of metallic coatings; basic studies on the growth mechanisms of oxide scales, for example are still required, in particular to understand the role of addition elements such as platnum and palladium. 2. (b) Alternative techniques for depositing MCrAlY coatings; electrolytic codeposition and electrophoresis, for example, have been developed at the laboratory stage and these permit the deposition of MCrAlY coatings with claimed economic and technical advantages over processes already in production. 3. (c) Thermal barrier coatings; ceramic coatings have been applied to sheet metal combustor components for about 15 years; only recently have they been used in the turbine section. Two challenges remain though to exploit these coatings on turbine blades: improve their reliability and, in the case of stationary gas turbines, their hot corrosion resistance. Both structural and mechanical approaches are required to determine, in particular, the role of microstructure, microcracking, porosity, residual stresses, oxidation of the bond layer in the degradation mechanisms of these coatings. 4. (d) Mechanical properties of coated systems; the intrinsic mechanical properties of coating materials are still poorly described and the lack of information hinders the adequate modelling of the behaviour of coating-substrate composite systems. In parallel, an increasing activity is noted concerning the design and development of high-temperature coatings for protecting materials other than superalloys, for instance ceramic composites and titanium-based alloys.