Ultrafast laser ablation of tungsten carbide: Quantification of threshold range and interpretation of feature transition

Tungsten carbide was manufactured by picosecond laser in this study. Shapes of the ablated craters evolved from parabolic-like (less than 10 pulses) to Gaussian-like (more than 500 pulses) as the pulse number increased. The shape changes were closely associated with the discontinuous diameter expansion of ablated crater. To explain these phenomena, two thresholds were identified: an upper threshold of 0.129 J/cm2 and a lower threshold of 0.099 J/cm2. When the laser energy exceeded the upper threshold, ablation occurred under the laser-energy-dominated mode. When the laser energy fell between the upper and lower thresholds, ablation occurred under the cumulative-effect-dominated mode. The transition of ablation mode contributed to the diameter expansion and shape change. In addition, elemental composition varied significantly at the ablated crater and heat-affected zone (HAZ), which were related to the degrees of reactions that occurred at different distances from the laser. Finally, surface hardness decreased from base material (32.52 GPa) to edge of crater (11.59 GPa) due to the escape of unpaired interstitial C atoms from the grain boundaries.