Application of Laser in Stone Cleaning

Laser cleaning principle Laser has the characteristics of high brightness, high directionality, high monochromaticity and high coherence, which is unmatched by ordinary light sources. With the high brightness of the laser, after the lens is focused, it can generate tens of thousands of degrees or even tens of thousands of degrees of temperature near the focal point. The high directionality of the laser allows the laser to efficiently transmit over long distances. The monochromaticity of the laser is extremely high and the wavelength is single, which is advantageous for focusing and wavelength selection.

Laser cleaning can be divided into two major categories in terms of its cleaning mechanism: one is the use of clean substrates (also known as precursors) and surface attachments (sewage) for a certain wavelength of laser energy, with very different absorption coefficient. Most of the laser energy radiated to the surface is absorbed by the surface attachments, causing it to be heated or vaporized, evaporated, or instantaneously expanded, and is driven by the vapor stream formed on the surface to escape the surface of the object, achieving the purpose of cleaning. On the other hand, the substrate does not suffer damage due to its minimal laser absorption energy at this wavelength. For such laser cleaning, choosing the right wavelength and controlling the laser energy is the key to achieving safe and efficient cleaning. The other type is a cleaning method that is suitable for cleaning the substrate and the surface attachment with little difference in the laser energy absorption coefficient, or the substrate is sensitive to the acidic vapor formed by the coating heating, or the coating may generate toxic substances when heated. . This type of method usually uses a high-power, high-repetition-rate pulsed laser to strike the surface being cleaned, converting part of the beam into sound waves. After the sound wave hits the hard surface of the lower layer, the near-back part and the incident sound wave generated by the laser light explode slightly, the coating is crushed, pressed into a powder, and then removed by the vacuum pump, and the underlying substrate is not damaged.

Compared with traditional cleaning methods such as high-frequency ultrasonic cleaning and laser cleaning and mechanical friction cleaning, chemical corrosion cleaning, liquid solid strong impact cleaning, there are obvious advantages. It is efficient, fast, low cost, small thermal load and mechanical load on the substrate, cleaning is non-injury; waste can be recycled, no environmental pollution; safe and reliable, does not harm the health of the operator; multi-function, can clear all kinds of differences Thickness, coating of different components; easy to realize automatic control of the cleaning process, remote cleaning and other remote control.

Laser cleaning methods from the method analysis, laser cleaning methods are four kinds: 1, laser dry-cleaning method, which uses pulsed laser direct radiation decontamination; 2, laser + liquid film method, that is deposited first layer of a liquid film on the substrate surface, Then use laser radiation to decontaminate; 3, laser + inert gas method, that is, when the laser radiation, with inert gas blown to the surface of the substrate, when the dirt peeled from the surface will be immediately blown away from the surface of the gas to avoid the surface is Recontamination and oxidation again; 4. Use a laser to loosen the dirt and then clean it with a non-corrosive chemical method. At present, the first three methods are commonly used. The fourth method is only seen in the cleaning of stone artifacts.

The application of laser cleaning The old, high-grade stone works such as stone carvings and stone carvings have become the earliest areas of application of laser cleaning technology due to their extremely delicate and delicate surface structure. It has been found that the use of laser to remove the dirt on the surface of stone artifacts has its unique advantages. It can very well control the movement of the light beam on a complex surface, remove the dirt without damaging the artifact stone. For example, in September 1992, the UNESCO World Cultural Heritage Organization organized a 20th anniversary to commemorate the establishment of the organization and repaired the famous Amiens Cathedral in the United Kingdom. Engraving is the key to engineering. During the one-year maintenance of the Virgin Mary, the maintenance staff used a laser beam to remove a few millimeters thick layer of black dirt covered by the marble carving pattern. The original color of the marble surface is reflected and the beautiful sculpture is reproduced. Brilliance. Another example is that the collection of stonework by Insbrentier, one of the most important collections of stone sculptures in the UK, has also been cleaned by lasers. Figure 1 shows the Austrian heritage conservationists cleaning the stone sculptures on St. Stephen's Cathedral in the middle of the 14th century with YAG lasers with articulated arms.

People observed the laser-cleaned stone carving surface with an electron microscope and found that the structure of the stone after the laser cleaning did not change, and the cleaned surface was smooth and flat with no damage. This is completely different from the surface cleaned by the micro-particle spray method (blasting method). The damage of the marble surface structure after particle jet cleaning is unavoidable, especially for marble surfaces with existing sulfate scale. Electron microscopy also found that after laser irradiation, the properties of the underlying rock material were neither degraded nor changed. At present, after the lime is cleaned with laser, the surface dirt of marble and other high-grade stone materials has become a new and promising business Project. In addition to the cleaning of stone materials, laser cleaning has a good effect in the cleaning of glass, quartz, metal, molds, teeth, chips, electrodes, magnetic heads, disks, and various microelectronic products. application.