Manufacturing Optimization for Bondcoat/Thermal Barrier Coating Systems

Abstract

A reliable lifetime prediction rule for bondcoat/thermal barrier coating (BC/TBC) coated parts in gas turbine operation is necessary to determine remnant service life. The specimens investigated were coated with MCrAlY plus yttria partially stabilized zirconia applied by vacuum plasma spraying and atmospheric plasma spraying processes, respectively. The performances of these laboratory specimens were statistically assessed, combining long term oxidation testing with thermal cycling, thus superimposing thermomechanical loading on the laboratory specimens to more accurately represent engine conditions. A design of experiment (DOE) approach was used for manufacturing optimization of the BC/TBC system. The life of the coating system is influenced by several manufacturing parameters such as BC thickness, BC roughness, TBC thickness, TBC porosity, and TBC stiffness. Specimens with a suitable variation in these parameters were produced to ensure a balanced test matrix of fractional factorial DOE. Based on results derived from laboratory testing the specifically tailored parts, first and second order effects of manufacturing parameters on lifetime were quantified. The findings revealed that the second order effects (the interaction of manufacturing parameters) were more important on the lifetime of the BC/TBC system than the corresponding first order effect (single parameter). For instance, the variation in BC thickness or BC roughness led to a scatter of lifetimes of 10% and 60%, respectively, whereas their interaction resulted in a scatter of lifetime of 150% for the same range of coating parameters. Further examples of such pairings are also demonstrated. Finally, a lifetime prediction for three quality classes (high, medium, and low qualities) has been demonstrated. The difference in achievable lifetime highlights the importance of manufacturing parameters in determining the life of the BC/TBC system.