Experimental study on nanosecond-millisecond combined pulse laser drilling of alumina ceramic with different spot sizes

Ceramic with outstanding mechanical and physical properties has been widely used in various engineering applications. The low processing efficiency and quality of the conventional millisecond laser prevents the broad applications of laser drilling technology in ceramic. In this paper, a novel combined pulse laser (CPL) that consisting of an initial nanosecond pulse train followed by a millisecond pulse train was proposed for drilling the alumina ceramic. Experimental results showed that the pre-treatment of the initial nanosecond pulses could introduce a keyhole with a depth about 21.5 mu m, which improved the performance of the subsequence millisecond pulses to reduce the diameter and cracks of the hole. In addition, the effect of the CPL drilling of alumina ceramic under different spot sizes was systematically analyzed. It was shown that the keyhole ablated by the initial nanosecond pulses could be used to limit the hole diameter even the millisecond laser had a bigger spot size. Ultimately, it was demonstrated that the diameter, defects and taper angle of the hole could all be decreased by increasing the repetition rate of the subsequent millisecond pulses. The millisecond pulses with a single pulse energy density about 57 J/cm(2) could drill through the alumina ceramic, which was two orders of magnitude lower than conventional millisecond laser. The research results have an important guiding for improving the processing efficiency and quality of millisecond laser, especially for the laser processing of ceramic materials.