Statistical evaluation of crack initiation strength in ceramic coating layer

Main components in gas turbine such as combustor, transition duct, first-st age 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-s top and steady operations. Recently, many damages, such as microcrack and/o r delamination, have also been reported in these components. In order to co mprehend mechanical properties of ceramic coating materials, we had measure d 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 proc ess in ceramic coating layers and to propose a new strength evaluation meth od for crack initiation in the layer which is based on the Weibull statisti cal theory. As a result, it was found from continuous observations that coa ting 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 i nitiation in coating layer was proposed by considering a residual stress ca used by the coating process. It was indicated that the proposed method coul d estimate with a good accuracy the crack initiation strength of coating la yers from strength data of coating material.