ガスタービン翼用耐熱セラミックコーティングのレーザ熱衝撃/疲労評価法の開発

抄録

In this paper, an attempt to develop a thermal shock and fatigue test of ceramic thermal barrier coating (TBC) for gas turbine blades is presented, using a laser heating technique. This method uses a cylindrical specimen which simulates the curvature of the leading edge of a gas turbine blade. A plasma-sprayed ZrO₂-Y₂O₃/NiCrAlY TBC system was tested. The cylindrical specimens used consisted of ZrO₂-8wt%Y₂O₃ thermal barrier layers (0.35 mm thick) deposited on a stainless steel substrate, with a NiCrAlY bond coat (0.15 mm thick). A CO₂ laser (maximum output=50 W) was used for the thermal shock and fatigue tests, where a laser beam with a diameter of 4 mm was irradiated onto the coating surfaces. The specimen inner surfaces were air-cooled or water-cooled to achieve various temperature drops within the ZrO₂ coating. Concurrently, laser experiments on coating surface temperature (T_s) and back-surface temperature (T_b) were measured, and acoustic emission (AE) monitoring was carried out to detect the onset of the coating damage. Using the experimental results, a thermal shock fracture mechanism map was constructed for the ZrO₂ TBC as a T_s-T_b diagram. The T_s-T_b diagram indicated that the most probable damage of the TBC system under the advanced gas turbine environments was delamination growth at the ZrO₂/NiCrAlY interface which leads to the spalling of the ZrO₂ layer. In order to improve the thermal shock/fatigue resistance of the ZrO₂ coating, vertical cracks were induced within the ZrO₂ coating by laser heating, and the effect of the laser heat processing on the thermal resistance was examined. It is demonstrated that the thermal resistance of the TBC is significantly enhanced by the laser precrack processing.