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Laser surface structuring of fine features

Marcus Ardron, Frank Albri, Krystian Wlodarczyk, StÚphanie Giet, Duncan Hand

Overview

In a large number of applications concerned with creating sub-micron features on materials such as metals, there is a preference to use a femto-second laser in the ablative regime. Femtosecond laser-based processes exhibit negligible or even the complete absence of heat affected zones (HAZ) and highly controllable removal rates. In contrast nanosecond lasers normally induce a larger HAZ which is detrimental to the formation of small surface features.

In this project, we investigate a novel laser based texturing technique which results in highly controllable surface deformations induced by melting of material with a nanosecond laser. In conventional nanosecond laser-based processes material is removed by ablation, in a fast sublimation process. In contrast, at lower laser powers, the material is only heated to the melting regime and the melt pools exists for a few nanoseconds before the melt pool re-solidifies. This allows a number of phenomena to occur in the melt pool and various melt fluctuations to arise as the hot liquid phase is in contact with the machining atmosphere.

Figure 1 : Diffraction gratings produced by surface melting of a metal sample with a UV nanosecond laser.

Key to this process is the harnessing of the dynamics of deformations of the local melt pool. We have identified the key parameters qualified their impact on the process. Literature and experimental observations demonstrate that the local deformations are functions of two main phenomena; in particular the chemical compositions of both the material and the atmosphere in contact with the surface of the melt have a strong impact on the shape and depth of the features as well as on the process repeatability. Collectively, the following interact to form the geometry on the surface:

  • thermal gradient across the radiated area;

  • chemical composition of material;

  • composition of the atmosphere above the target;

  • type, amount and distribution of chemical reaction across the melt pool;

  • surface tension gradients.

Knowledge of the material behaviour as a function of its composition allows the selection of specific laser parameters to generate the required texturing. Conversely, it also allows the selection of the ideal material for a specific laser process aimed at high precision, high production speed or a combination of both.

Figure 2 : White interferometer 3D image of a surface grating machined with a nanosecond UV laser on a metal sample. The period (peak-to-peak longitudinal distance) is 8Ám, the peak-to-valley depth is ~200▒20nm.

 

 

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