Laser cleaning technology

Introduction

In recent years, laser cleaning has become one of the research hotspots in the industrial manufacturing field, covering topics such as processes, theory, equipment, and applications. In industrial applications, laser cleaning technology has reliably cleaned a wide range of different substrate surfaces, including steel, aluminum alloys, titanium alloys, glass, and composite materials. The industries utilizing laser cleaning span aerospace, aviation, shipbuilding, high-speed rail, automotive, molds, nuclear power, and marine sectors.

The origins of laser cleaning technology can be traced back to the 1960s, offering advantages such as excellent cleaning effects, wide application range, high precision, non-contact operation, and good accessibility. It holds extensive potential for industrial manufacturing, production, and maintenance, and is expected to partially or completely replace traditional cleaning methods, emerging as one of the most promising green cleaning technologies of the 21st century.

Laser Cleaning Methods

The laser cleaning process is highly complex, involving numerous material removal mechanisms. For a specific laser cleaning method, various mechanisms may simultaneously occur during the cleaning process. This is primarily due to the interaction between the laser and the material, which can lead to various physical and chemical changes on the material surface, including ablation, decomposition, ionization, degradation, melting, combustion, vaporization, vibration, spattering, expansion, contraction, explosion, stripping, and detachment.

Currently, the typical laser cleaning methods include three main types: laser ablation cleaning, liquid film-assisted laser cleaning, and laser shockwave cleaning.

(1) Laser Ablation Cleaning Method

The mechanism of this method primarily involves thermal expansion, vaporization, ablation, and phase explosion. The laser directly acts on the material to be removed on the surface of the substrate, with environmental conditions that can include air, rare gases, or a vacuum. The operation conditions are simple, and it is the most widely used method. It can remove various coatings, paints, particles, or contaminants.

Figure 1: Process Schematic of Laser Ablation Cleaning Method

When the laser irradiates the material surface, both the substrate and the contaminant first undergo thermal expansion. As the interaction time between the laser and the contaminant increases, if the temperature is below the vaporization threshold of the contaminant, only physical changes occur. The difference in the thermal expansion coefficients between the contaminant and the substrate generates pressure at the interface, causing the contaminant to bend, tear away from the substrate surface, and generate cracks, mechanical fractures, and vibrational breakage. The contaminant is then removed or stripped from the substrate surface in an ejected manner.

If the temperature exceeds the vaporization threshold of the contaminant, two situations can occur: 1) The ablation threshold of the contaminant is lower than that of the substrate; 2) The ablation threshold of the contaminant is higher than that of the substrate.

In both cases, the contaminant undergoes physical and chemical changes such as melting, vaporization, and ablation. The cleaning mechanism becomes more complex, involving not only thermal effects but also possible molecular bond breakage between the contaminant and substrate, decomposition or degradation of the contaminant, phase explosion, ionization of the vaporized contaminant, and plasma generation.

(2) Liquid Film-Assisted Laser Cleaning

The primary mechanism of this method includes liquid film boiling, vaporization, and vibration. It requires the selection of an appropriate laser wavelength, which can compensate for the lack of shock pressure in the laser ablation cleaning process, and is useful for removing more difficult contaminants.

As shown in Figure 2, a liquid film (water, ethanol, or other liquids) is first applied to the surface of the contaminant, and then the laser is directed at it. The liquid film absorbs the laser energy, causing the liquid medium to undergo a strong explosion. The boiling liquid moves at high speed, transferring energy to the surface contaminant. The high transient explosive force is sufficient to remove the surface contaminants, achieving the cleaning goal.

Figure 2: Process Schematic of Liquid Film-Assisted Laser Cleaning

The liquid film-assisted laser cleaning method has two main drawbacks:

1. The process is complicated and difficult to control.
2. Due to the use of a liquid film, the chemical composition of the substrate surface can be easily altered after cleaning, leading to the formation of new substances.

(3) Laser Shockwave Cleaning Method

The process and mechanism of this method differ significantly from the previous two. The mechanism primarily involves shockwave force for removal, and the target cleaning objects are mainly particles, particularly submicron or nanoscale particles. The process requires strict conditions, as it must not only ensure the ionization of air but also maintain an appropriate distance between the laser and the substrate to ensure that the shockwave acting on the particles is sufficiently strong.

The process schematic of laser shockwave cleaning is shown in Figure 3. The laser is emitted parallel to the surface of the substrate without making contact. The workpiece or laser head moves to adjust the laser focus near the particles. After laser output, the air at the focus point undergoes ionization, generating a shockwave that expands rapidly in a spherical shape, extending to the particles. When the moment generated by the lateral component of the shockwave force on the particles is greater than the moment from the longitudinal component and the adhesive force of the particles, the particles begin to roll and are removed.

Figure 3: Process Schematic of Laser Shockwave Cleaning

Laser Cleaning Technology

In the 1980s, the rapid development of the semiconductor industry led to higher demands for cleaning techniques to remove contamination particles from silicon wafer masks. The key challenge was overcoming the significant adhesive force between the contaminant particles and the substrate. Traditional chemical cleaning, mechanical cleaning, and ultrasonic cleaning methods were unable to meet these needs, whereas laser cleaning technology proved effective in addressing these contamination problems, leading to rapid research and application development in this area.

In 1987, the first patent application for laser cleaning was filed. In the 1990s, Zapka and others successfully applied laser cleaning technology to semiconductor manufacturing processes, removing micro-particles from mask surfaces, marking the early industrial applications of laser cleaning technology. In 1995, researchers used a 2 kW TEA-CO2 laser to successfully clean aircraft fuselages by removing paint.

Entering the 21st century, with the rapid development of ultrashort pulse lasers, both domestic and international research and applications of laser cleaning technology gradually increased, focusing on the surface cleaning of metal materials. Notable foreign applications include aircraft fuselage paint removal, mold surface degreasing, engine carbon deposit removal, and surface cleaning of weld joints before welding. The Edison Welding Institute in the U.S. used laser cleaning on the FG16 fighter jet, achieving a cleaning volume of 2.36 cm³ per minute at a laser power of 1 kW.

It is also worth noting that the laser paint removal of advanced composite material components has become a major research hotspot. Composite surfaces such as propeller blades of the U.S. Navy’s HG53 and HG56 helicopters, as well as the horizontal stabilizer of the F16 fighter jet, have already been treated with laser paint removal. However, the application of composite materials in Chinese aircraft is relatively recent, so this research is still in its early stages.

Moreover, surface treatment of CFRP composite material joints before bonding to improve joint strength using laser cleaning technology is another current research focus. Adapt Laser Company has provided fiber laser cleaning equipment for Audi TT car production lines to clean oxidation films from lightweight aluminum alloy door frames. Rolls-Royce in the UK used laser cleaning to remove oxide films from titanium alloy aircraft engine parts.

Domestic scholars began research in the field of laser cleaning later than their international counterparts. The high cost of short-pulse lasers and the need for higher cleaning efficiency to achieve higher practical value in applications have led to very few related applications at present.

In recent years, several universities, research institutes, and enterprises in China have gradually carried out research on the industrial applications of laser cleaning technology and have developed laser cleaning equipment.

Harbin Institute of Technology has conducted research on removing Al-Si coatings from automotive hot-formed steel surfaces, rust removal from steel surfaces, oxide film removal from aerospace aluminum and titanium alloys before welding, contaminant removal from aluminum matrix composite materials, and cleaning of ceramic materials.

The China Academy of Engineering Physics has conducted research on tire mold release agent removal, paint stripping from aircraft wings and radar covers (composite materials), tank armor de-painting, and rust removal from marine components.

Shanghai Linshi Laser Technology Co., Ltd. has carried out research on surface cleaning processes for turbine blades, aerospace intake ducts (titanium alloy), and lightweight alloys before welding, and has developed laser cleaning equipment.

The Shenyang Institute of Automation, Chinese Academy of Sciences, has explored cleaning processes for nuclear power pipeline internal contaminants.

Dazhu Laser Technology Co., Ltd. has conducted research on the removal of oxides from planetary gear housings (cast aluminum) and driven bevel gears, oxides from copper parts, graphite from piston surfaces, and paint from saw blades.

Suzhou University has carried out research on rust removal from automotive housings, contaminant removal from track maintenance, and scaling removal from insulating porcelain bottles.