Delamination-free millisecond laser drilling of thermal barrier coated aerospace alloys

Millisecond (ms) pulsed laser drilling is currently state-of-the-art in producing acute angle film cooling holes over aero-engine and gas-turbine components made from uncoated nickel superalloys. After laser drilling, most of these components are coated with a high-temperature thermal barrier coating (TBC) to maintain the temperature of the component at a level appropriate for its application. It is desirable to produce holes over the TBC coated components; however, the current state-of-the-art ms laser drilling of acute angle holes over TBC coated materials results in a high level of coating delamination and hence is not used in manufacturing industries. The recent introduction of ms quasi-continuous wave (QCW) fiber lasers has had a significant impact on industrial laser drilling. This paper reports the results of a fundamental investigation carried out on ms QCW laser drilling of angular holes over TBC coated superalloys and provides a pragmatic solution to the issue of TBC delamination. In addition to the investigation on traditional percussion and trepanning laser drilling processes, a new method of drilling called “laser drilling post-laser TBC decoating” is evaluated with the aim of achieving delamination-free laser drilling of TBC coated aerospace alloy.