Time-resolved imaging of explosive phase change in metals

Abstract

ABSTRACT Phase explosion is a non-equilibrium boiling process resulting from homogeneous vapor nucleation in a superheated liquid near the critical point. Phase explosion may occur during nanosecond laser ablation since heterogeneous nuclei responsible for normal boiling do not have sufficient time to grow. Understanding the explosive phase change process is critical for developing models of material removal, and requires time-resolved diagnostics. A time-resolved shadowgraph technique was developed which was capable of probing ablation with nanosecond time exposures and nanosecond time delay resolution. Experiments were performed to investigate the transition from normal surface vaporization to phase explosion during nanosecond laser ablati on of aluminum and nickel. The threshold nature of phase explosion was observed by a discontinuous jump in the ablation depth at fluences of approximately 5.2 J/cm 2 and 6.9 J/cm 2 for aluminum and nickel, respectively. Shadowgraph images captured weak vaporization and shock waves below the threshold. At higher fluences, large droplets and vapor were observed as a result of phase explosion. The phase explosion process began shortly after the end of the laser pulse, consistent with existing estimates of homogeneous nucleation time lags in the research literature. Shock wave propagation was consistent with Taylor scaling below and above the phase explosion threshold. Keywords: phase explosion, nanosecond, laser ablation, micromachining, homogeneous nucleation