Thermal barrier coatings (TBC) are complex multi-layer material systems, used in gas turbines for power generation and propulsion. One basic requirement for a TBC is sufficient adhesion to its substrate. A measure of the adhesive properties of a coating is the interfacial fracture toughness, which may be determined by means of indentation testing. In this work, the Rockwell brale C indentation test has been evaluated for the estimation of the interfacial fracture toughness between the ceramic topcoat and the underlying metal for an industrially used TBC-system. The investigated TBC comprised a substrate of a single crystal super alloy, a plasma sprayed 180μm thick metallic oxidation protection layer, and a 280μm thick ceramic topcoat, applied by electron beam-physical vapour deposition. The Rockwell indentation test generated delamination cracks of a butterfly shape while the delaminated coating did not buckle. This fracture behaviour was in contrast to earlier reported results, which show - more or less - circular delamination cracks and buckling (Vasinonta). In order to understand the specific fracture behaviour, displacement controlled indentation tests were conducted on the TBC-system and on the metallic substrate, with and without metallic protective coating. The imprints of the indenter and the fractured coatings were investigated by means of optical and scanning electron microscopy. It was found, that anisotropy in the deformation behaviour of the single crystal substrate has caused the observed butterfly shape of the delamination. Since, the coating was thick with respect to the indentation depth, the interaction between indenter and coating affected the available energy for propagating delamination cracks. For example, after heat treatment, the energy for achieving a certain indentation depth has increased, and shorter delamination cracks were observed.
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