Mechanical testing of thermally stressed materials with rough interfaces: Mechanically induced delamination cracking in thermal barrier composites

Interfacial fracture of thermally stressed layered composites is often characterized by application of a mechanical load. Here a novel mechanical testing method was applied to study the delamination of thermal barrier composites. With a geometry similar to the double cantilever beam compressive loading was transferred into tensile stress normal to the average location of the interfaces. As-sprayed and annealed composites were tested. Fracture relevant microstructural changes and crack growth in the top coat and at the interface between bond coat and top coat were monitored using scanning electron microscopy. The local strain situation was determined via the analysis of the image contrast. Finite element modeling was used to illustrate the effect of the interface roughness on the failure origin. Similar as reported in literature for thermal loading, mechanical loading of a rough interface causes high stresses in interfacial roughness peaks and valleys. However, contrary to thermal loading, the position of the maximum in tensile and compressive stress is independent of the existence of a thermally grown oxide. Implications for the mechanical and thermo-mechanical testing of layered composites are discussed.