Nanosecond ablation of alumina with an ytterbium fibre-laser: experimental study, topography and damage evaluation

Abstract

Abstract : The use of advanced ceramics has been limited by the high machining costs, with grinding and polishing accounting for 50-80% of the final product's total cost; and the inability of achieving satisfactory dimensional control and surface quality. Laser ablative processes are an alternative, considering the absence of wear-subjected tools and the possibility of very fine material removal. Although the research on laser ablation is a current topic of interest, with growing number of publications year after year, it's been focusing mainly on ultra-short pulses (in the order of pico- and femtoseconds), while the research on the low-cost and low-energy-consuming nanosecond regime (> 100 ns) is relatively scarce, specially for ceramics. This process outcomes are still difficult to predict, due to the many simultaneous and self-interacting physical processes that take place in a relatively short time. In this study, an experimental analysis has been carried out for a three-dimensional machining process with a 120 ns pulsed ytterbium fibre-laser on dense Al2O3 samples, initially through design of experiments, and later trough a step-by-step optimization procedure, evaluating ablation rate and resulting roughness. Then, surface morphology and fracture strength were evaluated for two of the "optimized" parameters sets. A very well-defined difference in removal rate and resulting surface topographies was observed, suggesting a threshold point between distinct ablation mechanisms. One of the combinations gave rise to interesting features of straight, angled shock-waves around melt pits. The fracture strength for both regimes also differed significantly, with a clear increase (121,6% e 163,5%) of the Weibull modulus combined with a decrease of the characteristic stress, compared to the non-ablated samples.