TBC-Adherence in Indentation Testing

Kurzfassung

A critical measure of the adherence of a Thermal Barrier Coating (TBC) to the substrate is its interfacial fracture toughness. Indentation testing perpendicular to the surface has been proposed by several authors as a method for estimating the interfacial fracture toughness of TBC systems. Indenting the surface with a Rockwell brale C indenter generates local coating debonding, which size can be used for calculating the interfacial fracture toughness. This test has several striking advantages: it is easy to perform, small quantity of material is necessary, elaborate sample preparation is not required, and the test can be applied on components. The drawback is the difficult interpretation of the test results, which are sometimes misleading. In this study, instrumented Rockwell indentation tests have been performed systematically on TBC systems with the ceramic top coat deposited by electron beam physical vapor deposition. Specimens in “as coated” condition and after different heat treatments, which impair the interfacial fracture toughness, have been investigated. The damages induced in the TBC system during the indentation process have been analyzed by microscopic investigation of cross sections after testing with different maximum loads. After heat treatment, smaller delamination crack sizes were measured than in ‘as coated’ condition, indicating an increase of interfacial fracture toughness after heat treatment. It was found that microstructural changes during the heat treatment led to a shielding of the weakest interface in the TBC system in a way that the indentation induced crack did not reach this weakest interface.