A study of thermal and mechanical effects on materials induced by pulsed laser drilling

Holes drilled in metals and silicon using different short-pulse lasers (copper vapour, Nd : YLF and titanium: sapphire) were characterized under optical and electron microscopy. The aim was to analyze and compare the thermal and mechanical effects on materials induced by laser drilling with a wide range of pulse widths (50 ns to 200 fs) and power densities (108 W/cm2 to 1015 W/cm2). Heat affected structural zones around micro-holes drilled in cold-rolled copper were revealed by analyzing ion-polished hole sections in the scanning electron microscope using electron channelling contrast. The heat affected zones were found to have a maximum width of 5 μm to 10 μm, independent of the duration of the pulses. Mechanically and thermally induced deformations and slip phenomena, including “prismatic punching”, were observed in laser-drilled molybdenum monocrystals. The dislocation arrangement which developed during laser drilling of silicon monocrystals was visualized by transmission electron microscopy. The microscopic studies showed that for percussion drilling in the high fluence range characterized by high ablation rates of a few micrometres per pulse the use of shorter pulses (τH < 50 ns) did not lead to any appreciable reduction in the melt component, material re-deposition or thermal load on the wall of the hole. In addition, the increasing mechanical loads on the material due to the higher pressure in the drill channel becomes a limiting factor for the precision of the processing.