Damage evolution of TBC by rapid thermal cycling test based on a laser irradiation

Abstract

Thermal barrier coating (TBC) is deposited onto the surface of gas turbine blade in order to prevent from a high-temperature combustion gas flow. Crack and delamination of ceramic top coating (TC), which come from a high heat flux loading, are serious problem in TBC. In this study, the rapid thermal cycling device based on a laser irradiation was developed. It was then investigated how those damages progress in TC subjected to rapid thermal cyclic loading based on the cross-sectional observation and the monitoring of acoustic emission (AE). As a result, a sintered layer was formed beneath the surface of the TC. It was also found that both vertical and horizontal cracks were initiated and propagated in TC. The monitoring of AE revealed that these cracks are initiated in cooling periods and vertical cracks are also formed in TC at early stage of thermal cycles. In addition, AE counts were strongly related to maximum temperature condition. Finally, mechanical factors of those damages were discussed based upon theoretical evaluation using the TBC model including the sintering phenomenon. It was identified that the damage was initiated with temperature gradient in the TC for early stage of thermal cycles and was progressed with a tensile stress accumulated due to sintered layer formation for later stage.