Laser cleaning is an effective method for removing dirty particles and films of different materials and sizes from a solid surface.
When using a high-brightness and well-directed continuous or pulsed laser, a laser beam with a specific spot shape and energy distribution is formed after optical focusing and spot shaping. The laser beam is then beamed onto the surface of the contaminated material that needs to be cleaned.
After the contaminating material fixed to the surface absorbs the laser energy, it will go through a series of complex physical and chemical processes, such as vibration, fusion, combustion and even vaporization, causing the pollutant to detach from the surface of the material.
Even if the laser is applied to the clean surface, most of it will be reflected without causing damage to the substrate, thus achieving the cleaning effect.
See the following figure for an example of cleaning aluminum alloy with red paint on the surface.
Laser cleaning can be classified according to different criteria. For example, it can be divided into dry laser cleaning and wet laser cleaning based on whether a liquid film is applied to the surface of the substrate during the laser cleaning process.
The former involves directly irradiating the polluting surface with laser beams, while the latter requires applying moisture or liquid film to the surface to be cleaned. Wet laser cleaning is efficient but requires manual coating of the liquid film, which should not alter the properties of the substrate material.
Therefore, compared with dry laser cleaning technology, the application scope of wet laser cleaning is somewhat limited.
Dry laser cleaning is currently the most widely used laser cleaning method, in which laser beams are used to directly irradiate the surface of the workpiece to remove particles and films.
Dry laser cleaning
The basic principle of dry laser cleaning is that when particles and substrate are irradiated by laser beams, the absorbed light energy is converted into thermal energy instantly. This causes instantaneous thermal expansion of the particles, substrate or both, generating an acceleration between them.
The force produced by this acceleration overcomes the adhesion force between the particles and the substrate, causing the particles to detach from the surface of the substrate.
(1) Dry laser cleaning can be divided into two main forms according to different absorption mechanisms:
For dust particles with melting points higher than those of the matrix (or with significantly different laser absorption rates):
The absorption of particles under laser irradiation is stronger (a) or weaker (b) than that of the substrate. In this case, the energy absorbed from the laser is converted into thermal energy, causing thermal expansion of the particles.
Although the amount of thermal expansion is very small, it occurs within an extremely short period of time, resulting in a huge instantaneous acceleration acting on the substrate between the particles and the substrate.
Meanwhile, the substrate also acts on the particles, overcoming the adhesion force between them and causing the particles to detach from the substrate, as shown in Figure 1.
(2) For dirt with lower boiling points:
Surface dirt directly absorbs laser energy, causing instant high-temperature boiling and evaporation, which removes dirt by direct vaporization. The principle is shown in Figure 2.
Laser wet cleaning principle
Laser wet cleaning, also known as laser vapor cleaning, is a laser cleaning method in which there is a thin or medium liquid film of several micrometers on the surface of the cleaned object.
Compared to dry cleaning, wet cleaning involves the presence of such a film, which, when irradiated by the laser beam, undergoes an instantaneous increase in temperature and generates a large number of bubbles that produce gasification reactions.
The impact force generated by the gasification explosion overcomes the adhesion force between the particles and the substrate.
Depending on the different absorption coefficients of laser wavelength by particles, liquid films and substrates, laser wet cleaning can be divided into three types.
(1) When the substrate strongly absorbs laser energy:
When the laser is irradiated on the substrate and the liquid film, the absorption of the substrate by the laser is much greater than that of the liquid film.
Therefore, an explosive vaporization phenomenon occurs at the junction 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 produce superheat at the junction, resulting in greater explosive impact force.
(2) When the liquid film strongly absorbs laser energy:
This cleaning principle involves the liquid film absorbing most of the laser energy and undergoing explosive vaporization on its surface, as shown in the figure below.
In this case, the efficiency of laser cleaning is not as good as when the substrate absorbs the laser, because the explosive impact force only occurs on the surface of the liquid film.
When the substrate absorbs the laser, bubbles and explosions occur at the junction between the substrate and the liquid film, and the explosive impact force is more likely to push the particles away from the surface of the substrate. Therefore, the cleaning effect of substrate absorption is better.
(3) When the substrate and liquid film absorb laser energy:
In this case, cleaning efficiency is low. After the laser is irradiated on the liquid film, part of the laser energy is absorbed and the energy is dispersed throughout the liquid film.
The liquid film boils to produce 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 generate boiling bubbles and explosions. Therefore, the efficiency of this method is very low.
When using substrate absorption for laser wet cleaning, most of the laser energy is absorbed by the substrate, causing overheating at the junction between the liquid film and the substrate, producing bubbles at the interface.
Compared with dry cleaning, wet cleaning uses the explosive impact force generated by the bubble explosion at the interface to achieve laser cleaning.
At the same time, a certain amount of chemical substances can be added to the liquid film to react with the polluting particles and reduce the adhesion force between the particles and substrate materials, thereby lowering the limit for laser cleaning.
Therefore, wet cleaning can improve cleaning efficiency to a certain extent, but it also has some difficulties. The introduction of a liquid film can lead to new pollution, and the thickness of the liquid film is difficult to control.
Factors affecting the quality of laser cleaning
Influence of laser wavelength:
The premise of laser cleaning is laser absorption, therefore, when selecting a laser source, it is necessary to match the light absorption characteristics of the cleaned object to choose a laser that is suitable for the band as the laser source.
Furthermore, experimental research carried out by foreign scientists has shown that to clean particles with the same characteristics, the shorter the wavelength, the stronger the laser's cleaning ability and the lower the cleaning threshold.
Therefore, under the premise of meeting the light absorption characteristics of the material, in order to improve the cleaning effect and efficiency, lasers with shorter wavelengths should be selected as cleaning light sources.
Influence of power density:
When performing laser cleaning, there is an upper damage limit and a lower cleaning limit for the laser power density. Within this range, the higher the laser power density during laser cleaning, the greater the cleaning capacity and the more significant the cleaning effect.
Therefore, the laser power density should be increased as much as possible without damaging the substrate material.
Influence of pulse width:
The laser source for laser cleaning can be continuous or pulsed light. Pulsed lasers can deliver very high peak power, making it easier to meet threshold requirements.
Additionally, research has found that pulsed lasers have a smaller impact on the thermal effects of substrates during cleaning, while continuous lasers have a larger thermal effect area.
Influence of scanning speed and frequency:
Obviously, in the laser cleaning process, the faster the laser scanning speed and the fewer times, the higher the cleaning efficiency, but this may result in a decrease in cleaning effectiveness.
Therefore, in practical cleaning applications, the appropriate scanning speed and frequency should be selected according to the material characteristics of the cleaned object and the degree of contamination. The overlap rate during scanning will also affect the cleaning effect.
Influence of the amount of blur:
Before laser cleaning, the laser is usually focused using a certain combination of focusing lenses. During the actual laser cleaning process, it is usually performed in a blurred state.
The greater the amount of blur, the larger the spot of light on the material, the larger the scanning area, and the greater the efficiency. When the total power is constant, the smaller the amount of defocus, the higher the laser power density and the stronger the cleaning ability.
