1999 Volume 107 Issue 1242 Pages 159-165
Main components in gas turbine such as combustor, transition duct, first-stage nozzles and blades are coated with ceramic materials to prevent attacks by high-temperature and burnt gas environments. These coated components in service are subjected to frequent thermo-mechanical loading due to start-stop and steady operations. Recently, many damages, such as microcrack and/or delamination, have also been reported in these components. In order to comprehend mechanical properties of ceramic coating materials, we had measured Young's modulus and hardness changing with temperature and carried out 4-point bending tests. The aim of this study is to comprehend the damage process in ceramic coating layers and to propose a new strength evaluation method for crack initiation in the layer which is based on the Weibull statistical theory. As a result, it was found from continuous observations that coating damages proceeded as follows. In the elastic region of a stress-strain curve, a crack initiated at the coating surface and then propagated along the bonding interface with increasing load. An evaluation method of crack initiation in coating layer was proposed by considering a residual stress caused by the coating process. It was indicated that the proposed method could estimate with a good accuracy the crack initiation strength of coating layers from strength data of coating material.
