Thermal-mechanical fatigue of MAR-M 509 with a thermal barrier coating

An experimental set-up utilizing indirect induction heating suitable for thermal-mechanical fatigue (TMF) tests of high-temperature materials with thermal barrier coatings (TBC) is described. This set-up employed for TMF testing of the Co-based superalloy MAR-M 509 with a NiCoCrAlY bond coat and a ZrO2 topcoat was able to generate temperature gradients across the TBC similar to the in-service conditions inside a gas turbine. The tests show the development of an extensive inelastic strain during the high temperature compressive part of each cycle usually counterbalanced during the tensile part, a fast saturation of the stress response within the first loading cycles, and an exponential decrease in lifetime with increasing applied mechanical strain range. Microstructural analysis demonstrates that life of the samples was not limited by TBC-related processes but by cracks initiated on oxidized carbides on the uncoated surface of the samples.