Morphological instabilities of metallic oxidation protection coatings for gas turbine blades

Abstract

Turbine blades in aircraft engines are usually protected with a metallic oxidation protective coating. Morphological instabilities such as rumpling and wrinkling on the surfaces of the metallic coatings have recently received significant attention since it can be directly linked to failures of the coated component. The evolution of such morphological instabilities is governed by material parameters, service conditions, and time dependent changes of the material system, such as oxidation. With a newly developed test facility, which can simulate the low cycle fatigue load of turbine blades very closely to the service conditions, we have performed thermal and thermo-mechanical fatigue tests on coated tubular specimens resulting in different features of surface rumpling: (i) Thermal cycling with no thermal gradient over the wall thickness of the coated specimen gives no surface rumpling; (ii) Thermal cycling with such a thermal gradient results in random distribution of rumples; and (iii) Thermal cycling with a thermal gradient and superposed tensile mechanical loading results in rumples aligned with the axis of the cylindrical test specimen. The experiments have been modeled and simulated by finite element methods. The numerical results capture the experimentally observed behavior and elucidate the thermo-mechanical response of the coated system.