Study of the TBC delamination in nanosecond laser percussion drilling of inclined film cooling holes

To protect superalloys under extremely high temperatures in aero-engine core components, thermal barrier coatings (TBC) are sprayed on the material surface and thousands of film cooling holes are drilled which produce a cooling air film. A large proportion of these holes must be drilled at inclined angles relative to the surface in order to provide optimal cooling effects. Laser drilling has been widely applied in the industrial fabrication of film cooling holes due to its high precision, high efficiency, flexibility in drilling angle, and ability to drill all kinds of materials. In particular, nanosecond lasers have attracted lots of interest because they can achieve a good balance between drilling quality and efficiency. However, TBC is prone to delaminate during laser drilling, which severely reduces the lifetime of aero-engine components. In this study, we investigate TBC delamination in nanosecond laser percussion drilling of inclined film cooling holes in TBC-coated cobalt-based superalloy. It was found that the leading edges of the inclined holes suffer from more severe TBC delamination than the trailing edges. By studying the effects of laser focus position, pulse width, pulse energy, repetition rate and average power, we found that faster hole breakthrough and less heat accumulation were essential for the reduction of TBC delamination. Through multi-physical numerical simulations, it was demonstrated that the asymmetric heat transfer effect in laser inclined drilling causes much larger interfacial thermal stress on the leading edge than the trailing edge, qualitatively explaining our experimental observations. The results of this study contribute to the understanding of TBC delamination in laser drilling of inclined film cooling holes.