Tuesday, 12 October 2004 - 2:40 PM

This presentation is part of : Advanced modeling for laser-material processing

Numerical simulation of dielectric transition during laser ablation

Cristian Porneala and David A. Willis. Southern Methodist University, Department of Mechanical Engineering, 5990 Airline Rd, Dallas, TX 75205

Phase explosion occurs during high-fluence laser ablation as a result of homogenous vapor nucleation from the superheated liquid phase. Phase explosion is expected to occur near 0.9Tcr, the boundary of thermodynamic stability of a liquid. Prior to reaching this limit, large fluctuations in material properties are expected to occur, transforming the electrically conductive liquid into a dielectric. This work investigates the dielectric transition during Nd:YAG laser ablation of aluminum using a one-dimensional numerical heat transfer model. The dielectric transition was modeled by assuming that for temperatures above 0.8Tcr, the superheated aluminum liquid is semi-transparent to the laser irradiation. This resulted in the formation of a dielectric layer that propagated into the aluminum. When the absorption coefficient of the dielectric layer was set to zero, the dielectric layer thickness ranged from 200 nm at low fluence to 500 nm or more at larger fluences. However, the surface temperature never increased above 0.8Tcr. A non-zero absorption coefficient found from the literature was also used, resulting in an increase in the thickness of the dielectric layer with little increase in surface temperature. Since the surface temperature did not increase above 0.8Tcr when the dielectric transition was considered, and since phase explosion is expected to occur at 0.9Tcr, the question of how phase explosion can occur becomes difficult to answer. The results demonstrate that improved knowledge of the dielectric layer optical properties is required for accurately predicting the threshold fluence for phase explosion.

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