MECHANICALLY INDUCED DELAMINATION CRACKING IN THERMAL BARRIER COMPOSITES

摘要

Structural components of advanced gas turbines utilize ceramic thermal barrier coatings (TBCs) with low thermal conductivity. The thermal barrier function is further increased by the large number of processing defects typically originating from the deposition process of the ceramic coating. In particular, plasma sprayed TBCs are known to reduce the thermal conductivity by inter-lamellar defects (cracks). However, these defects also have to be considered as sources of delamination which ultimately yields failure of the TBC. To study the initial stages of delamination, e.g. the extension of the inter-lamellar cracks, a novel mechanical testing method was applied. With a geometry similar to the double cantilever beam compressive loading was transferred into tensile stress normal to the interfaces of the thermal barrier composite. Thus a mode I opening is given to the inter-lamellar cracks in the TBC and to the flaws along the interface with the bond coat (BC). As sprayed and annealed TBCs were tested. Fracture relevant microstructural changes in the TBC and at the interface between BC and TBC were monitored with the resolution of optical and scanning electron microscopy. The growth of favorably oriented cracks was measured as a function of applied load. Also the local strain situation was determined via the analysis of the image contrast. The obtained strain and crack growth results are analyzed and discussed with respect to failure mechanism, critical strain and strength.