Evolution of a PtAl2 coating on the nickel-base alloy CMSX-6 subjected to thermo-mechanical fatigue

A detailed analysis is presented which examines the degradation caused by thermo-mechanical fatigue of a platinum aluminide (PtAl2) coating on a nickel superalloy substrate. A series of samples of the commercial single crystal nickel-base superalloy CMSX-6 were diffusion coated with PtAl2. These test-pieces were then been subjected to thermo-mechanical testing under strain controlled mechanical cycles designed to reproduce the stress history of the critical regions in turbine blades manufactured from such a material when used in service in aero gas turbine engines. The set of strain ranges chosen was designed to produce a wide spread in sample lives from less than 20 h to almost 500 h. The coating composition is characterised both for tested and untested material by X-ray diffractometry of incrementally polished samples for phase identification at different levels throughout the depth of the coating, and by electron probe micro-analysis of perpendicular sections of the coating. Preliminary measurements of the residual stresses within the coating are also reported.