An experimental investigation into the defects of laser-drilled holes in thermal barrier coated Inconel 718 superalloys

Abstract

Delamination and microcracks as well as spatters are known to be inherently associated with laser drilling thermal barrier coated materials. In this study, response surface method (RSM) and single-variable method were employed to correlate the laser parameters including pulse length, peak power, pulse rate, and pulse numbers as well as the interactions among them with such defects in laser percussion drilling. Detailed observation and analysis reveals that narrow pulse length, lower peak power, and low pulse rate can produce smaller cracks area on the side wall, among which the effect of pulse length is more outstanding. As for spatters around hole entrance, the interactions among these factors were identified and a combination of higher peak power, proper pulse length (in this model not exceed 1 ms), higher pulse rate, and small number of pulses are conducive to minimize the spatter volume. Furthermore, the thermal effect near the thermal barrier coating/bond coat (TBC/BC) is especially prominent in laser drilling with wide pulse length and low peak power. The removal mechanism occurred near TBC/BC also determines the delamination crack size. And higher peak power with narrow pulse length should be a good strategy in order to break through the TBC as soon as possible and shelter the TBC or TBC/BC from thermal damage.