Thermal model for nanosecond laser ablation of alumina

A thermal model based on heat-conduction equation and Hertz–Knudsen equation for vaporization has been employed to simulate nanosecond pulsed laser based ablation of alumina. Heat transfer in the laser irradiated target with allowance for phase transitions, provides estimates for temperature distribution within the target and material ejection rate via ablation. Good agreement between calculated and experimentally measured data on mass ablation rate per pulse and its dependence on incident laser fluence from 5 to 22 J/cm2, validated our theoretical model. Observed deviation between calculated and experimentally measured ablation rates at high average laser fluence levels was explained by ablation induced progressive degradation of target surface. Absence of abrupt increase in ablation rate with increased laser fluence suggested material ejection largely via normal boiling rather than explosive boiling mechanism. Calculated maximum surface temperature of the target was found to lie well below empirically estimated thermodynamic critical temperature for alumina, corroborating our observations on absence of onset of explosive boiling in alumina target on laser irradiation. Our simulation study enables proper selection of laser fluence, successfully minimizing laser induced target damage, as well as, degradation of micro-structural and mechanical properties of alumina films deposited via pulsed laser ablation.