Formation mechanism of micro-spikes on various metals with femtosecond laser pulses

Formation of conical micro-spikes on various metal surfaces (316L stainless steel, Ti-6Al-4V, and Al5754) under femtosecond irradiation at high repetition rate is reported. Two types of micro-cone morphologies formed at these high repetition rates under high and low-fluence conditions were clearly distinguished. At low fluence (near the ablation thresholds), conical spikes with low aspect ratio and non-uniform distribution forms through random evolution. At high fluence, semi-uniform conical spikes with higher aspect ratios are formed through simultaneous progressive evolution procedure with increasing the number of scans. Experimental results are presented showing the progression of random micro-spike formation to uniform micro-spikes as fluence increases and how scan-speed affects the size of the spikes. For both formation processes, the scan speed is found to be an effective parameter to control the micro-cone size (or micro-cone number density).