Coating service experience with industrial gas turbines

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Abstract
Ield experience with high–temperature coatings on land–based gas turbine blades and vanes is reviewed. The article covers all relevant coatings such as the chromium type, precious metal aluminides, and overlay coatings of the NiCrSi and MCrAlY (M = Fe, Co, Ni) types. Long term service behaviour, allows, various guidelines to be drawn for the selection of a coating appropriate to particular operating conditions. In the low–temperature hot–corrosion regime, high chromium containing coatings appear to be the most effective and the aluminides less so. At higher temperatures, aluminides are superior and offer protection comparable to that given by MCrAlY overlays. The advantage of these overlays lies in a higher compositional flexibility, especially when deposited by low–pressure (vacuum) plasma spraying. Service performance can be significantly affected by the mechanical integrity of the coatings; some ductility is required to accommodate thermal stresses. A general lifetime prediction for coatings is extremely difficult, because of the wide variety of turbine operating parameters. However, if specific operating conditions are known, prediction is possible. MST/294