Reduction of TBC delamination through a two-step strategy in nanosecond laser drilling of inclined film cooling holes

The pursuit of extreme thrust and efficiency in aero engines demands unprecedentedly high working temperatures in combustion chamber liners and turbine blades. Thermal barrier coatings (TBC) and film cooling holes (FCH) are two indispensable components which work together to protect the high-temperature superalloys from thermal damage. Today, laser drilling has been widely accepted as the most favorable technique for fabricating FCH into TBC-coated superalloys. However, one major drawback of laser drilling is its thermal nature, which can induce severe TBC delamination, particularly for inclined FCH. In this paper, we propose a two-step laser drilling strategy which can effectively reduce TBC delamination: a small preliminary through-hole is drilled in the 1 st step, followed by subsequent full drilling in the 2 nd step. The preliminary hole creates an efficient pathway for material removal in the 2 nd drilling step, preventing excessive melt pool formation and heat accumulation, which significantly reduces TBC delamination. To validate our proposition, a theoretical model based on the level-set method was built to simulate the melt hydrodynamics, and laser drilling experiments were performed using a home-built nanosecond laser trepan drilling system. Both theoretical and experimental results demonstrate the important role of the preliminary hole in accelerating melt removal and reducing TBC delamination. Moreover, the effects of the preliminary hole position and size were also investigated and optimized. Our study provides a new solution plan for high-quality laser drilling of FCH.