Evaluation of the Rockwell indentation test for interfacial fracture toughness measurement of Thermal Barrier Coating systems

A critical measure of the adherence of a Thermal Barrier Coating (TBC) to the substrate is its interfacial fracture toughness. Indentation testing has been proposed by several authors as a method for estimating the interfacial fracture toughness of TBC systems: indentation perpendicular to the surface 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 systematically been performed 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. It was found that crushing and compaction of the brittle TBC, crack initiation within the TBC, and deformation of the substrate are controlling the formation of debonding cracks. The experimental results are discussed in terms of energy consumption for the observed damage processes and driving force for the evolution of debonding cracks