Ablation and plasma effects during nanosecond laser matter interaction in air and water

Yunfeng Cao, Purdue University

Abstract

Despite extensive research work, a clear understanding of laser matter interaction in the high energy nanosecond laser ablation process is still lacking, which may differ significantly depending on surrounding medium, laser parameters, and target material characteristics. The mechanical and thermal effects of the confined plasma and water breakdown plasma during laser ablation in water have not been fully investigated as well. ^ In this work, nanosecond laser ablation of metal targets in air and water is investigated through a self-contained hydrodynamic model under different laser fluences with the consideration of phase explosion. In case of nanosecond laser ablation of aluminum in water, deeper crater depths are found in all the conditions studied in this work. The analysis of the shock compression in air and water indicates that the shock compression is mainly responsible for this enhancement of ablation in water. ^ The mechanical effects of confined plasma is also investigated, including the target surface integrity change and induced residual stresses in the Laser shock peening (LSP) process and shock wave propagation and spallation behavior in LSP. By combining a 3-D finite element model with a previously developed confined plasma model, the residual stresses induced in the substrate material as well as the indentation profile on the substrate surface are predicted for both single shot and overlapping LSP. The spallation induced by shock wave propagation in targets during the laser shock peening process is also investigated in this work. The spallation zone location is calculated for various materials with different thickness of foils and various laser shock peening parameters and validated against with previously reported experimental results. ^ The melt ejection behavior during nanosecond laser ablation with phase explosion is successfully predicted by combined molecular dynamics (MD) and smoothed particle hydrodynamics (SPH) simulations and validated against the experiments. The commonly adopted 0.9 Tc (critical temperature) criterion for phase explosion boundary is also assessed with the prediction of the ablation depth for both aluminum and copper, and it is found that the 0.9 Tc criterion does not always work. ^ Laser induced water breakdown plasma, which is generated by the strong interaction between nanosecond laser and water, is used in this work to etch the surface layer of a carbon fiber reinforced composite sample. It is found that the polymer layer can be effectively removed by the plasma while the carbon fiber remains almost intact.^

Degree

Ph.D.

Advisors

Yung Shin, Purdue University.

Subject Area

Engineering|Mechanical engineering

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