Pulsed laser ablation of nickel: Phase transformation mechanism and gas dynamic process

To study the phase transformation mechanism and gas dynamic process for the laser-ablated plume, direct and indirect evidences of potential laser ablation mechanisms were sought experimentally and analytically. Two phase transformation mechanisms, normal vaporization and phase explosion, were introduced as the laser ablation mechanisms for the interaction between a KrF excimer laser (λ = 248 nm) and a nickel specimen in ambient air. For each experimental condition the flow processes of the plume were modeled based on gas dynamic theories. Measuring the velocity of the plume front, the transmissivity of the plume, and laser energy loss to the ambient, the laser-ablated plume was diagnosed experimentally. Employing the measured velocity of the plume front as an input variable for the gas dynamic calculations of the normal vaporization model and performing the calculations, the laser ablation mechanism was determined analytically. As the laser fluence increases, both experimental and analytical studies show evidence of a transition of the laser ablation mechanism from normal vaporization to phase explosion at a laser fluence of 5.0 J/cm 2. To verify this transition, the recoil pressure exerted on the target surface was also calculated from the gas dynamic calculations, and compared to the measured value obtained by using a PVDF transducer and the impedance mismatch method. As a result, the use of gas dynamic calculations for the normal vaporization model was validated when the laser fluence was less than 5.0 J/cm2, and the transition of the laser ablation mechanism was confirmed. Morphological studies of ablation craters were performed to obtain direct evidence of phase explosion, and showed re-solidified droplets deposited on the periphery of the ablation crater when the laser fluence was greater than 5.0 J/cm2. Finally, the ablation rate was measured and compared to the calculated value obtained based on equilibrium evaporation kinetics and the Clausius-Clapeyron equation. The comparison showed that the Clausius-Clapeyron equation was inappropriate for the temperature-pressure relation at the target surface during pulsed laser ablation