Study of delamination induced by laser drilling of thermally sprayed TBC interfaces

The efficiency of the aero-engines combustion chambers with thermal barrier coating (TBC) is improved when highly numerous cooling holes are laser drilled with inclined angles. However, during the laser drilling process, especially in the percussion mode, a detrimental crack can be generated at the TBC interface. Thus, each hole could be edged with a nonvisible delaminated area underneath the ceramic top-coat. The present work is focused on the thorough study of the delamination induced by laser percussion drilling when interrupted drilling conditions are achieved. Shallow angles were applied on hole each having one different and increasing number of laser pulses (from 1 to 4). Crack length was assessed by conventional metallographic preparation. A special experimental method was carried out in order to inspect the delaminated interface and the lateral edge of a semi-hole. This non-destructive assessment of the delamination of laser drilled TBC was complemented by a 3D imaging of a semi-hole using X-Ray microtomography. Results are presented paying attention on both crack initiation and propagation during the laser percussion drilling of plasmasprayed TBC.