Laser cleaning technology is considered to be the most reliable and effective surface treatment technology due to its cleaning characteristics such as no force, no chemical reaction, no thermal effect and suitable for various materials. However, the laser cleaning process is very complex, involving a wide variety of material removal mechanisms. This is mainly due to the interaction between the laser and the material, including ablation, decomposition, ionization, degradation, melting, burning, vaporization, vibration, splashing and many other physical and chemical changes on the surface of the material. At present, there are mainly three laser cleaning methods: laser ablation cleaning method, liquid assisted laser cleaning method, laser shock wave cleaning method.
The main mechanisms of laser ablation cleaning methods include evaporation, plume, ionization and explosion, as shown in Figure 1.It means that the laser directly acts on the material to be removed from the surface of the substrate, and the environmental conditions can be air, inert gas or vacuum. When the laser is irradiated on the surface of the material, both the substrate and the cleaning materials thermally expand. As the time for the laser to interact with the cleaning material increases, if the temperature is lower than the gasification threshold of the cleaning material, the cleaning material only changes physically. The difference between the thermal expansion coefficient of the cleaning material and the substrate causes pressure at the interface, causing the cleaning material to crack, so that the cleaning material is removed or peeled off the surface of the substrate. If the temperature is higher than the vaporization threshold of the cleaning material, two situations occur at this time: when the ablation threshold of the cleaning material is less than the substrate ablation threshold, the laser energy density needs to be adjusted to be between the cleaning material and the substrate ablation threshold . In this way, the cleaning material can be effectively removed without causing damage to the substrate.
The mechanism of liquid assisted laser cleaning to remove materials mainly includes liquid vibration, boiling and explosion, as shown in Figure 2.It refers to pre-covering the surface of the cleaning material with liquid (water, alcohol or other liquids), and then irradiating it with a laser. The liquid film absorbs laser energy and causes a powerful explosion of the liquid medium. The explosively boiling liquid moves at high speed and transfers energy to the surface cleaning material. The explosive force is sufficient to remove surface dirt to achieve cleaning purposes.
The process method and cleaning mechanism of the laser shock wave cleaning method are very different from the previous two. It does not come into contact with the substrate. As shown in Figure 3, move the workpiece or the laser head to adjust the laser focus to the vicinity of the particles. Then, the laser is emitted in a direction parallel to the surface of the substrate. After the laser is output, the air at the focal point will be ionized, creating a shock wave. The shock wave expands rapidly in a spherical shape and extends to contact with the particles. When the moment of the lateral component of the shock wave acting on the particles exceeds the moment of the longitudinal component + the particle adhesion force, the particles will be removed by rolling.
Among the above three methods, the laser ablation cleaning method has simple operating conditions and is the most widely used. All kinds of coatings, paints, particles or dirt can be removed as long as the appropriate laser type is selected and the optimized process parameters are used. The laser cleaning machine produced by Jinan Senfeng Laser Technology Co., Ltd. adopts this cleaning method, which makes the cleaning efficiency high and the cleaning effect is very good.
When using the liquid film-assisted laser cleaning method, you need to consider selecting the appropriate laser wavelength. The higher the absorption rate of the liquid film to the laser, the more obvious the advantage. The liquid film assisted laser cleaning method can make up for the problem of insufficient impact pressure in the laser ablation cleaning process, and is often used to remove some cleaning objects that are more difficult to remove. However, this method has two disadvantages: one is that the process is troublesome and the process is difficult to control; the other is that the chemical composition of the substrate surface is easy to change after cleaning, and new substances are generated.
The laser shock wave cleaning method is mainly used to remove particles (sub-micron or nano-level). This method has very strict process requirements. It is necessary to ensure that the laser parameters can ionize the air, but also to maintain a proper distance between the laser and the substrate to ensure that the impact force acting on the particles is large enough.
As a laser cleaning machine manufacturer, Jinan Senfeng Laser Technology Co., Ltd. supplies the best laser cleaning machine. If you want to know more about laser cleaning machine, please contact SENFENG LEIMING.
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