Heat transfer and phase explosion during nanosecond laser ablation

Numerical calculations were carried out to investigate the theoretical impact of the dielectric transition upon short pulse laser ablation of metals. The results showed that the optical properties of a metal surface after a transition to the dielectric state are critical for understanding laser ablation. The study found that phase explosion would not occur when the dielectric layer is assumed transparent. However, a dielectric layer absorption coefficient with a value of only 1% of the room temperature value results in significant absorption in the dielectric layer, allowing the surface to reach the upper limit of superheating.; Experiments were carried out to study phase change phenomena due to high power pulsed irradiation. A parametric study of the ablation crater depth was performed to demonstrate the threshold nature of phase explosion. A distinctive change in the ablation crater reflected in a significant increase of the crater depth was observed when the laser fluence was above specific (material dependent) values. The jump in the crater depth was used to identify the threshold fluence for phase explosion for the two metals considered in this study.; Ablation was imaged using a shadowgraph technique. Images captured weak vaporization below the threshold while images above the threshold revealed emission of large, low velocity particles resulting from phase explosion. The dynamics of the ablation process indicated that the phase explosion began near the end of the laser pulse, without a significant time delay. An energetic study of the shock wave showed that the transition from normal vaporization to phase explosion is also reflected in an increase of the energy content of the shock waves. This energy represents a measure of the energy liberation due to phase explosion.