In the previous articles, Laser Cleaning Background and Laser Coating Removal by Different Laser Methods, the methodologies used are based on an ablation removal method, being subject to thermal diffusion as the whole volume of the material removed needs to be vaporised.
In the semiconductor fabrication industry, high energy pulses have been used since the late 80s to remove metal films with minimum damage to the substrate. Subsequently, the technology evolved to provide a tool for transparent coating detachment in the optics industry and semi-transparent paint removal.
Detachment significantly increases coating removal efficiency as compared to pure ablation. Near-infrared radiation, with a wavelength near 1 μm and invisible to the naked eye, is adequately transmitted by most polymer and organic based materials, like paints. They are semi-transparent even when scattering additives or other pigments are introduced in the polymer matrix of the paint. Polymers transmission in the NIR region is higher than the visible radiation, mainly due to the larger wavelength, and limited molecular vibration interaction. Consequently, by the simplified Beer-Lambert equation, the beam intensity transmitted reduces in negative exponential relationship to the coating depth, governed by the absorption coefficient at the laser wavelength. Absorption at the coating to substrate interface is typically boosted, due to the 0 practical optical transmission of the metal, and due to surface roughness acting as absorbing discontinuities. Moreover, the vaporised material is trapped by the two layers contributing to huge interfacial pressure. It is this pressure that powers coating removal via cracking and ejection of large areas of the coatings, flaking the paint off instead of burning it off.
Consequently, it is only necessary to vaporise a very small volume of material in order to achieve detachment of material with thickness orders of magnitude greater than the vaporised layer. Additionally, laser beam intensity equal to the detachment threshold needs to reach the interface via the Beer-Lambert equation. This ablation threshold is typically lower than a smooth and clean semi-transparent coating surface. High pulse energy q-switched lasers inherently offer high enough intensity I0, achieving single pulse detachment of most paints up to 100 μm thick. Process efficiency is hence improved in a non-linear fashion when the detachment threshold is reached.
Increasing pulse energy further and distributing over larger surfaces to maintain irradiation just above the detachment threshold only reduces further the detachment threshold. This phenomenon occurs due to the increase of evaporated species at the interface and increasing the area over which the detachment pressure is applied. Hence, less pressure is necessary to overcome the shear forces on the perimeter of the irradiated area that withholds the coating (Figure 1a). The coating removal rate and efficiency consequently increase at a much steeper gradient than the volume ablation process.
Figure 1: a) Interfacial pressure P and b) hybrid detachment with P1, P2, P3 … Pulses To Detachment (PTD), induced by small (B1) and large (B2) pulse energy beams delivering irradiance It1 and It2 respectively
In part four (Hybrid Detachment in Laser Coating Removal) of this five part series, we’ll explore how the ablation-detachment hybrid process is used to achieve high removal efficiency.
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