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With the advancement of science and technology and the pursuit of extreme precision, the concept of cleaning is no longer limited to simple cleaning such as “washing dishes”. People’s scope of cleaning objects is constantly expanding, and the standards for cleaning requirements are constantly improving. As early as 1965, Nobel Prize winner Schawlow used a pulsed laser to irradiate a piece of paper with ink printed on it. The ink font on the paper quickly vaporized, and the paper itself was not damaged. The ink on the paper was successfully “erased”. Since then, the door to pulsed laser cleaning technology has been opened. In 1973, the Asmus team first reported the use of laser to clean cultural relics; in 1974, Fox used Q-switched neodymium glass laser to effectively remove the paint layer on resin glass and metal substrates; in 1982, Zapka and others from IBM’s German Manufacturing Technology Center used focused lasers to irradiate the mask and successfully cleaned the particulate contaminants attached to the mask. After more than 40 years of development, laser cleaning technology has made great progress and progress.
Principle and mechanism of laser cleaning
Laser cleaning is an advanced cleaning technology that uses high-energy laser beams to irradiate the surface of an object, and quickly evaporates or peels off impurities, contaminants or coatings through optical and thermal effects.
The core component of laser cleaning technology is a pulsed laser with large pulse energy, high average power and high peak power. As we all know, laser is a light source with high brightness, high consistency and high directionality. Pulsed lasers release high-energy laser beams in a very short time, with high peak power and instantaneous power density. Compared with continuous lasers, high-power pulsed lasers can generate high temperatures in an instant, but due to the extremely short time, the heat has no time to be transmitted to the surrounding materials, thereby greatly reducing the thermal impact of the laser on the substrate material. High-power pulsed lasers can also achieve precise control of the laser cleaning process by adjusting the pulse energy and frequency. This controllability can be customized according to different cleaning needs to ensure that it is suitable for different materials and application scenarios. When the laser beam is irradiated on the surface to be cleaned, the laser energy is absorbed and produces a strong thermal effect on the contaminant in a very short time. This thermal effect causes the surface temperature of the contaminant or coating to rise, causing it to evaporate, decompose or peel off. At the same time, the high energy density of pulsed laser allows it to directly penetrate certain materials without damaging the substrate surface, making the cleaning process more efficient.
Due to the complex and diverse composition and structure of the cleaning object, the mechanism of laser action on it is diverse. Therefore, laser cleaning is not just a simple high-energy ablation, but also involves decomposition, ionization, degradation, melting, combustion, gasification, vibration, splashing, expansion, contraction, explosion, peeling, and shedding. Therefore, the process of pulsed laser cleaning is a complex optical, thermal, mechanical and other comprehensive physical and chemical change process. Laser cleaning is a non-mechanical contact surface pretreatment method. The laser beam can act on the surface of the sample in a set scanning mode, so that the laser can fully interact with the surface dirt, rust layer or coating. After the surface material absorbs the energy of the laser, the laser energy is converted into the required thermal energy, chemical energy and mechanical energy for cleaning. At present, there are two main theories about the mechanism of pulsed laser cleaning: laser ablation mechanism and thermoelastic expansion peeling mechanism.
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(1) Laser ablation mechanism
The thermal action ablation mechanism in the pulsed laser cleaning process is closely related to the laser power density. In the ablation mechanism, since high-power pulsed lasers can release a large amount of energy in a very short time, a high-energy-density laser beam is generated. This allows the laser beam to be concentrated in a small area in a short time, and can quickly heat and evaporate the contaminants or coatings on the target surface. When the energy of the laser is sufficient to destroy the chemical bonds of the surface material, the chemical bonds vibrate, bend, or even break, causing the molecules to decompose and the surface contaminants to be decomposed by light. When the power density of laser cleaning is greater than 10^8 W/cm^2, the contamination layer on the surface of the material may undergo plastic deformation after absorbing the energy of the laser and produce explosive rebound stress;