Polarization effect on hole evolution and periodic microstructures in femtosecond laser drilling of thermal barrier coated superalloys

A high-pulse-energy femtosecond pulsed Ti:sapphire laser was employed to perform laser drilling of 2.4-mm-thick thermal barrier coated In718. The influences of polarization on hole profile and morphology of periodic structures on the sidewall of drilled holes were explored. Based on Fresnel formula, combined with the effects of multiple reflections and energy-trapping, a comprehensive analysis of the effect of polarization on polarization removal was proposed. The microstructures on the sidewall of drilled holes for different layers under different polarizations were compared. The fabricated structures on the thermal barrier coating (TBC) layer showed a strong dependence on the polarization state of the laser beam. The structures with ball-like profile were generated under circular polarization while pine needle-like periodic structures were produced under linear polarization. However, for both the bond coat and substrate layers, the microstructures of the sidewall were independent of the polarization state of the laser beam. For TBC layer the interaction between the resonance of charge carriers and the second harmonic of the surface contributed to a corrugated structure in harmony with the polarization state. Thereinto, the charge carriers may remember the polarization state of the light beam and maintain this information because of the low mobility.