The principle of laser cleaning machine is more complicated, and may include both physical and chemical processes. In many cases, physical processes are the main process, accompanied by some chemical reactions. The main processes can be classified into three categories, including gasification process, shock process and oscillation process.
Gasification Process
When the high-energy laser is irradiated on the surface of the material, the surface absorbs the laser energy and converts it into internal energy, so that the surface temperature rises rapidly and reaches above the vaporization temperature of the material, so that the pollutants are separated from the surface of the material in the form of steam. Selective vaporization usually occurs when the absorptivity of the surface contaminants to the laser is significantly higher than that of the substrate. As shown in the figure below, the pollutants on the surface of the stone have a strong absorption of the laser and are quickly vaporized. When the pollutants are removed and the laser is irradiated on the stone surface, the absorption is weak, more laser energy is scattered by the stone surface, the temperature change of the stone surface is small, and the stone surface is protected from damage.
A typical chemical-based process occurs when a laser in the ultraviolet band is used to clean organic contaminants, which is called laser ablation. Ultraviolet lasers have short wavelengths and high photon energy, such as KrF excimer lasers, with a wavelength of 248 nm and photon energy as high as 5 eV, which is 40 times higher than CO2 laser photon energy (0.12 eV). Such a high photon energy is enough to destroy the molecular bonds of organic matter, so that C-C, C-H, C-O, etc. in organic pollutants are broken after absorbing the photon energy of the laser, resulting in pyrolysis gasification and removal from the surface.
Impact Process
The shock process is a series of reactions that occur during the interaction between the laser and the material, and then a shock wave is formed on the surface of the material. Under the action of the shock wave, the surface contaminants are broken up and become dust or debris peeled off the surface. There are many mechanisms that cause shock waves, including plasma, steam, and rapid thermal expansion and contraction. Using plasma shock waves as an example, it is possible to briefly understand how the shock process in laser cleaning removes surface contaminants. With the application of ultra-short pulse width (ns) and ultra-high peak power (107 – 1010 W/cm2) laser, the surface temperature will still rise sharply even if the surface absorbs the laser lightly, reaching the vaporization temperature instantly. Above, the vapor formed above the surface of the material, as shown in (a) in the following figure. The temperature of the vapor can reach 104 – 105 K, which can ionize the vapor itself or the surrounding air, forming a plasma. The plasma will block the laser from reaching the surface of the material, and the vaporization of the surface of the material may stop, but the plasma will continue to absorb the laser energy, and the temperature will continue to rise, forming a local state of ultra-high temperature and high pressure, which produces an instantaneous 1-100 kbar on the surface of the material. The impact is gradually transferred to the inside of the material, as shown in Figures (b) and (c) below. Under the action of the shock wave, the surface contaminants are broken up into tiny dust, particles or fragments. When the laser is moved away from the irradiation position, the plasma disappears and a negative pressure is created locally, and the particles or fragments of contaminants are removed from the surface, as shown in Figure (d) below.
In order to avoid damage to the substrate, a laser with a higher peak power (1012 W/cm2) can also be used to ionize the air above the surface of the material to form a plasma shock wave to clean up the contaminants on the surface of the material. Plasma can cause very strong shock waves, but the formation of shock waves does not necessarily require the formation of plasma. The vapor pressure formed by rapid vaporization can also form an impact on the substrate. For example, a liquid film is covered on the surface of the material. The “steam” cleaning method that uses the rapid vaporization process of the liquid to improve the cleaning efficiency is a typical example.
Oscillation Process
Under the action of short pulses, the heating and cooling processes of the material are extremely rapid. Because different materials have different thermal expansion coefficients, under the irradiation of short-pulse laser, the surface contaminants and the substrate will undergo high-frequency thermal expansion and contraction of different degrees, resulting in oscillation, causing the contaminants to peel off from the surface of the material. During this exfoliation process, vaporization of the material may not occur, and plasma may not be generated. Instead, the shear force formed at the interface of the contaminant and the substrate under the action of oscillation destroys the bond between the contaminant and the substrate. . Studies have shown that when the incident angle of the laser is slightly increased, the contact between the laser and the particle contamination and the substrate interface can be increased, the threshold of laser cleaning can be reduced, the oscillation effect is more obvious, and the cleaning efficiency is higher. However, the incident angle should not be too large. Too large an incident angle will reduce the energy density acting on the surface of the material and weaken the cleaning ability of the laser.
