Laser cleaning technology has become an indispensable tool in various industrial processes, ranging from industrial production to cultural relic preservation. In industrial settings, ensuring the quality of workpieces during processes like electroplating, phosphating, welding, and assembly of integrated circuits requires the removal of surface contaminants such as oil, dust, rust, and adhesives. Similarly, in the realm of art restoration, laser cleaning plays a pivotal role in reviving cultural relics and artworks by eradicating dirt and rust from their surfaces.
Traditional cleaning methods, such as mechanical cleaning, chemical cleaning, and ultrasonic cleaning, have long been the go-to techniques in China. However, they face limitations concerning environmental impact and precision requirements. Mechanical cleaning lacks the precision necessary for high cleanliness standards and often leads to surface damage. Chemical cleaning poses environmental pollution risks and is limited in achieving the desired level of cleanliness when dealing with complex dirt compositions. Additionally, ultrasonic cleaning, while effective, struggles to clean sub-micron-sized particles, and the size of the cleaning tank restricts its usability for intricate components. Furthermore, the drying process after cleaning presents its own set of challenges.
To meet the growing demand for environmentally friendly cleaning technologies, the world cleaning industry has witnessed the emergence of various eco-conscious solutions, with laser cleaning technology being one of the most promising.
Principle and Method of Laser Cleaning
Laser cleaning technology harnesses the interaction effect between high-energy laser beams and matter to achieve a precise and efficient cleaning process. The laser beam’s unique characteristics, including high directionality, monochromaticity, coherence, and brightness, play a crucial role in its effectiveness. The following features of laser beams are utilized in the cleaning process:
- High Concentration of Energy: The focused laser beam generates extremely high temperatures near the point of focus, causing surface contaminants to evaporate, gasify, or decompose instantly.
- Small Divergence Angle: The laser beam’s ability to be focused into various spot diameters allows control over the energy density, causing dirt to expand and detach from the object’s surface.
- Generation of Ultrasonic Waves: Laser beams create ultrasonic waves on solid surfaces, leading to mechanical resonance and the dislodging of dirt.
Laser cleaning technology operates through two main mechanisms, depending on the optical properties of the substrate material and the dirt being removed:
Different Absorption Coefficients: One method uses a laser wavelength with a large difference in absorption coefficient between the substrate and surface attachments. Most of the laser energy is absorbed by the contaminants, leading to their rapid heating, vaporization, or expansion, effectively detaching them from the object’s surface without damaging the substrate.
High-Power and High-Repetition-Rate Pulses: The other method applies when the absorption coefficients of the substrate and contaminants are similar, or when the substrate is sensitive to acidic vapors produced by heating coatings. High-power, high-repetition-rate pulse lasers cause small explosions in coatings, converting part of the light beam into sound waves. These sound waves generate high-energy waves that crush the coatings into powder, allowing for their removal without damaging the substrate.
Methodologically, laser cleaning can be classified into four approaches:
- Laser Dry Cleaning Method: Directly employing pulsed laser radiation to remove dirt.
- Laser + Liquid Film Method: Depositing a layer of liquid film on the substrate’s surface before using laser radiation to remove dirt. The rapid heating of the liquid film causes explosive vaporization, loosening and blowing away the dirt.
- Laser + Inert Gas Method: Utilizing inert gas blown towards the substrate’s surface while applying laser radiation. The dirt is peeled off and immediately blown away by the gas, preventing re-pollution and oxidation.
- Laser-First, Chemical Cleaning Later: First using laser to loosen the dirt and then cleaning with non-corrosive chemical methods.
The first three methods are most commonly used, with the fourth method reserved primarily for cleaning stone cultural relics.
Application Status of Laser Cleaning Technology
Laser cleaning technology has found widespread application in preserving high-end stone artworks, including stone carvings and sculptures that require delicate handling. Due to their fragile surface structures, traditional cleaning methods often pose risks to these valuable cultural relics. Laser cleaning, however, offers unique advantages by precisely controlling the laser beam’s movement on complex surfaces, ensuring thorough dirt removal without harming the stone material.
As laser cleaning technology continues to evolve and gain recognition for its efficacy and environmental friendliness, it is expected to find increasing use in various other industries, catering to diverse cleaning needs. As a new era of precision and eco-conscious cleaning dawns, laser cleaning stands as a beacon of hope, driving innovation and sustainability in industrial and cultural preservation practices alike.