Laser Cleaning is an effective method to remove dirty particles and film layers of different materials and sizes on solid surfaces. Through the high brightness and good directional continuous or pulsed laser, through optical focusing and spot shaping to form a specific spot shape and energy distribution of the laser beam, irradiated to the surface of the contaminated material that needs to be cleaned, the attached pollutant material absorbs the laser energy, will produce vibration, melting, combustion, and even gasification and a series of complex physical and chemical processes. And finally make the pollutants out of the surface of the material, even if the laser acts on the cleaned surface, most of them are reflected off, causing no damage to the substrate, so as to achieve the effect of cleaning.
Laser cleaning can be classified according to different classification standards. For example, whether the substrate surface is covered with liquid film in the laser cleaning process is divided into dry laser cleaning and wet laser cleaning. The former is the direct irradiation of the laser on the surface of the pollutant, and the latter requires the application of moisture or liquid film on the laser cleaning surface. The efficiency of wet laser cleaning is high, but the wet laser cleaning requires manual coating of liquid film, so the composition of liquid film can not change the properties of the matrix material itself. Therefore, compared with dry laser cleaning technology, the application range of wet laser cleaning has certain limitations. Dry laser cleaning is the most widely used laser cleaning method, which uses a laser beam to directly irradiate the surface of the workpiece to remove particles and films.
1. laser dry cleaning
The basic principle of laser dry cleaning is that after the particles and the material substrate are irradiated by the laser, the absorbed light energy is converted into heat energy in an instant, causing the instant thermal expansion of the particles or the substrate or both at the same time, and acceleration is generated instantly between the particles and the substrate. The force generated by the acceleration overcomes the adsorption force between the particle and the substrate and causes the particle to escape from the substrate surface.
According to the different absorption methods of laser dry cleaning, laser dry cleaning can be mainly divided into the following two forms:
1. For dust particles with a melting point greater than the base material (or with a large difference in laser absorption rate) : The particle absorption laser irradiation is stronger than the substrate absorption (a) or the opposite (b), at this time the particle absorption laser light energy converted into heat energy, causing the thermal expansion of the particle, although the amount of thermal expansion is very small, but the thermal expansion is in a very short period of time, so it will produce a huge instantaneous acceleration on the substrate, while the substrate reacts on the particle. The force overcomes the binding force on each other and detach the particles from the substrate.The schematic diagram is as follows:
2. For dirt with a low boiling point: surface dirt directly absorbs laser energy, instant high-temperature boiling evaporates, and direct vaporization removes dirt, as shown in the following figure.
2. laser wet cleaning
Laser wet cleaning is also known as laser steam cleaning, compared with dry cleaning, wet cleaning is on the surface of the cleaning part has a thin layer of a few microns thick liquid film or medium film, liquid film by laser irradiation liquid film temperature rises instantly and produce a large number of bubbles gasification reaction. The impact force generated by the gasification explosion overcomes the adsorption force between the particles and the substrate. According to the particle, liquid film and substrate on the laser wavelength absorption coefficient is different, laser wet cleaning can be divided into three types.
1. The substrate absorbs laser energy strongly
When the laser is irradiated to the substrate and the liquid film, the absorption of the laser by the substrate is far greater than that of the liquid film, so an explosive gasification phenomenon occurs at the interface between the substrate and the liquid film, as shown in the figure below. In theory, the narrower the pulse time, the easier it is to overheat at the junction, resulting in a greater explosive impact.
2. Liquid film absorption laser energy strongly
The principle of this cleaning is that the liquid film absorbs most of the laser energy, and explosive gasification occurs on the surface of the liquid film, as shown in the figure below. At this time, the efficiency of laser cleaning is not as good as the substrate absorption, because the explosion impact force is on the surface of the liquid film at this time. When the substrate absorbs, the bubble and explosion occur at the intersection of the substrate and the liquid film, and the explosion impact force is easier to push the particles away from the substrate surface, so the substrate absorption cleaning effect is better.
3. The substrate and liquid film both absorb laser energy
At this time, the cleaning efficiency is very low, after the laser irradiation to the liquid film, a part of the laser energy is absorbed, the energy is dispersed in the entire liquid film, the liquid film boils and produces bubbles, and the remaining laser energy is absorbed by the substrate after passing through the liquid film, as shown in the figure. This method requires more laser energy to produce boiling bubbles, which can cause an explosion. So this method is very inefficient.
When using the method of substrate absorption for laser wet cleaning, because most of the laser energy is absorbed by the substrate, the junction between the liquid film and the substrate will be overheated, and bubbles will be generated at the interface. Compared with dry cleaning, the wet type uses the impact force generated by the junction bubble explosion to achieve laser cleaning. At the same time, a certain chemical substance can be added to the liquid film to chemically react with pollutant particles to reduce the adsorption force between the particles and the substrate material, so as to reduce the threshold of laser cleaning. Therefore, wet cleaning can improve the efficiency of cleaning to a certain extent, but there are certain difficulties at the same time, the introduction of liquid film may lead to new pollution, and the thickness of liquid film is difficult to control.
3. Factors affecting the quality of laser cleaning
4. the effect of laser wavelength
The premise of laser cleaning is laser absorption, therefore, when choosing a laser light source, it is necessary to first combine the light absorption characteristics of the cleaning workpiece and select a laser suitable for the band as a laser light source. In addition, the experimental research of foreign scientists shows that the cleaning of pollutant particles with the same characteristics, the shorter the wavelength, the stronger the cleaning ability of the laser, and the lower the cleaning threshold. It can be seen that under the premise of meeting the light absorption characteristics of the material, in order to improve the effect and efficiency of cleaning, a laser with a shorter wavelength should be selected as a cleaning light source.
5. influence of power density
During laser cleaning, there is an upper damage threshold and a lower cleaning threshold for laser power density. In this range, the greater the laser power density of laser cleaning, the greater the cleaning capacity, and the more obvious the cleaning effect. Therefore, the power density of the laser should be improved as much as possible without damaging the base material.
6. effect of pulse width
The laser cleaning light source can be continuous light or pulsed light, and the pulsed laser can provide a high peak power, so it can easily meet the threshold requirements. Moreover, the study found that in terms of the thermal effect on the substrate caused by the cleaning process, the impact of the pulsed laser was smaller, and the heat affected area caused by the continuous laser was larger.
7. the effect of scanning speed and frequency
Obviously, in the process of laser cleaning, the faster the speed of laser scanning, the lower the frequency and the higher the efficiency of cleaning, but this may cause a decrease in the cleaning effect. Therefore, in the actual cleaning application process, the appropriate scanning speed and scanning frequency should be selected according to the material characteristics of the cleaning workpiece and the contamination situation. The overlap rate during scanning, etc., will also affect the cleaning effect.
8. the effect of defocusing amount
Before laser cleaning, the laser mostly converges through a certain focusing lens combination, and the actual laser cleaning process is generally carried out in the case of defocus, the larger the defocus, the larger the spot on the material, the larger the scanning area, the higher the efficiency. When the total power is constant, the smaller the defocusing amount, the greater the power density of the laser and the stronger the cleaning ability.
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