Suppression of melt flows in laser ablation: application to clean laser processing

• It is shown that in laser ablation of materials with large Prandtl numbers (mainly ceramics and polymers) a motion of the melt along the surface caused by the vapour plume pressure is essentially retarded for thin enough melt layers due to the onset of viscous friction. For polymers in nanosecond laser ablation this melt displacement can be obtained to be less than the ablation depth per pulse (which is typically 0.2-1 µm for nanosecond irradiation), when the absorption coefficient, α, and the kinematic viscosity, v, satisfy the condition α2v > 108 s-1. Thus, clean precise laser ablation for such polymers can be explained simply in terms of the thermal mechanism, without invoking the concept of photochemical decomposition, in terms of absorption coefficient, melt viscosity and pressure of the ablation plume on the irradiated surface. From this point of view, several factors facilitating clean laser ablation in multipulse irradiation are discussed. However, for metals (usually having a very small Prandtl number) the viscous friction has no significant effect on the retardation lateral melt flow to the periphery. The quality of the laser spot border can still be improved by using laser pulses shorter than 1 ps. In this case, in a shallow spot, the alternative (explosive) melt expulsion mechanism becomes predominant, producing material removal mainly transverse to the spot surface. Thus, the lateral (along the surface) component of melt expulsion appears to be strongly suppressed, having no chance to spoil the border of the spot.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Bearbetnings-, yt- och fogningsteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Manufacturing, Surface and Joining Technology (hsv//eng)

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Manufacturing Systems Engineering

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