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Review

The Fundamental Mechanisms of Laser Cleaning Technology and Its Typical Applications in Industry

by
Zhihu Zhou
1,
Weipeng Sun
2,
Jiajun Wu
1,*,
Hongwei Chen
2,
Fei Zhang
1 and
Shuangxi Wang
1,*
1
Key Laboratory of Intelligent Manufacturing Technology, Shantou University, Ministry of Education, Shantou 515063, China
2
Shantou Plant of Huaneng Power International Inc., Shantou 515000, China
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(5), 1445; https://doi.org/10.3390/pr11051445
Submission received: 31 March 2023 / Revised: 28 April 2023 / Accepted: 30 April 2023 / Published: 10 May 2023
(This article belongs to the Special Issue Developments in Laser-Assisted Manufacturing and Processing)
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Abstract

:
Laser cleaning is an advanced surface-cleaning technology that can lead to the instant evaporation and stripping of the attachments found on a substrate’s surface, such as contaminants, rust, and coatings; it uses a high-energy laser beam to irradiate the components’ surface. Compared with common surface-cleaning technologies, laser cleaning has the advantages of precision, efficiency, and controllability. In this paper, the fundamental mechanisms of laser cleaning technology are summarized in detail; these include the laser thermal ablation mechanism, the laser thermal stress mechanism, and the plasma shock wave mechanism. The operational principles, characteristics, and application range of each mechanism are discussed. Their typical applications in industry are outlined according to the differences in the substrate materials used, including metallic materials, nonmetallic materials, and semiconductor elements. This study provides a significant reference and guiding basis for researchers to further explore the fundamental mechanisms of laser cleaning, as well as various aspects of the typical industrial applications of laser cleaning.

1. Introduction

Surface quality is one of the most important criteria when evaluating the overall performance of components. Surface attachments, such as contaminants, rust, and coatings, have adverse effects on a component’s performance [1,2]. Surface-cleaning technologies have been widely applied in many fields to improve surface quality and maintain exact performance standards. The most common surface-cleaning technologies include mechanical cleaning, chemical cleaning, and ultrasonic cleaning. These surface-cleaning technologies can effectively improve surface quality and prolong the service life of components. With the rapid development of modern industries, especially the advanced equipment manufacturing industry, the requirements for surface quality and component performance have gradually increased. Therefore, due to their existing limitations, common surface-cleaning technologies have gradually become incapable of meeting the demand for components with excellent performance and high-quality surfaces [3]. For instance, mechanical cleaning has disadvantages including longer cleaning times, being labor-intense, and providing poor security. Additionally, this method cannot clean unique parts [4]. Chemical cleaning has negative impacts on the operators and the environment due to the chemical organic solvents used during the cleaning process. Additionally, the substrate materials may be corroded by the chemical agent if it is used for long periods [5]. It is difficult for practitioners of ultrasonic cleaning to clean large-scale components, and they cannot thoroughly clean the submicron particles attached to these components’ surfaces [6].
The laser was one of the most significant natural science inventions of the 20th century. Since the first laser equipment was produced in 1960 [7], lasers have rapidly been applied to various fields due to their valuable and unique properties. Lasers have now been widely applied in almost all fields, such as industry, agriculture, medicine, national defense, and so on [8]. Based on the interaction mechanism between a laser beam and materials, laser cleaning technology has been proposed for surface-cleaning purposes. Compared with common surface-cleaning technologies, such as mechanical cleaning, chemical cleaning, and ultrasonic cleaning, laser cleaning has the advantages of high precision and high efficiency, being environmentally friendly, producing low amounts of damage, exhibiting controllability, and having the potential to be used in a wide range of applications [9]. Therefore, laser cleaning is regarded as an advanced surface-cleaning technology and is expected to replace other surface-cleaning technologies. In 1969, Bedair et al. [10] first proposed the concept of laser cleaning and used laser cleaning to remove contaminants from silicon surfaces without causing any surface damage. In 1973, laser cleaning was used in the field of cultural relic protection [11]. In 1995, laser cleaning was used to remove the coatings attached to aircraft surfaces [12]. The coatings attached to aircraft surfaces could be selectively removed, which indicated that laser cleaning is highly adaptable and can be used to clean sensitive aircraft surfaces. In 2001, Lee et al. [13] used lasers to induce the ionization of air and discovered that a laser-induced plasma shock wave could effectively remove the tiny particles on wafers’ surfaces. This research heralded the discovery of the plasma shock wave, an important mechanism in laser cleaning. In 2012, Chen et al. [14] used laser cleaning to remove the rust from the surfaces of ship hulls. The applications of laser cleaning have become a hot topic in laser processing technology research in recent decades, which have gradually expanded from laboratory research to actual industrial application fields.
With the rapid development of laser cleaning, research related to laser cleaning has attained new heights [15]. However, this research mainly focuses on the effect of laser cleaning on the surface quality of substrate materials; in contrast, studies on the mechanisms of laser cleaning and its applications in various industries remain relatively scarce. In this work, the three fundamental mechanisms of laser cleaning technology are summarized, including the laser thermal ablation mechanism, the laser thermal stress mechanism, and the plasma shock wave mechanism. The principles and characteristics of each mechanism are discussed. The differences between these mechanisms and the application range of different mechanisms are explored. Then, current application cases are introduced, according to the nature of the substrate material, and are classified into cases for metallic materials, nonmetallic materials, semiconductor elements, and other applications. In addition, some of the existing problems of laser cleaning are discussed. This work provides a reference for clarifying the mechanism of action between the laser and the substrate, being of guiding significance for researchers further studying the primary mechanisms of laser cleaning and exploring its typical applications in industry.

2. Laser Cleaning Mechanism

Because the interaction between a laser beam and a material involves a series of physical and chemical processes, such as decomposition, ionization, vibration, expansion, stripping, shedding, vaporization, and explosion, laser cleaning mechanisms are highly complex [16,17]. At present, laser cleaning mechanisms mainly consist of the laser thermal ablation mechanism [18], the laser thermal stress mechanism [19], and the plasma shock wave mechanism [20]. While attachments are removed from the substrates’ surface, there are various competing laser cleaning mechanisms. The dominance of each mechanism is dependent on the optical penetration depths and the thermal diffusion lengths. In addition, the relative importance of each mechanism depends on the physical parameters, such as the laser cleaning medium, substrate materials, and contaminants [9,21]. Therefore, it is necessary to summarize the fundamental mechanisms of laser cleaning technology in detail.

2.1. Laser Thermal Ablation Mechanism

When a pulsed laser beam directly irradiates the attachments on substrates’ surfaces, the temperature of the attachments and substrate material rapidly rises. When the temperature exceeds the gasification threshold, the attachments are vaporized; this is accompanied by the phenomena of combustion, decomposition, ablation, exfoliation, etc. The evaporation process is known as a laser thermal ablation mechanism, as shown in Figure 1 [17]. When the substrate’s surface is exposed to high-energy ultraviolet laser light, it may lead to the appearance of photochemical effects [22,23]. The photochemical effect occurs only when the photon energy is greater than the molecular binding energy. At this time, the molecular bonds are broken from the attachments, and the material transforms into a loose state, which is helpful in promoting the evaporation of the dirty layer.
When a laser beam vertically irradiates a material’s surface, the temperature increase ΔT can be expressed as follows [24]:
Δ T = P π ω 0 K
where P is the laser absorption rate, ω0 is the radius of laser beam spot, and K is the thermal conductivity.
Within the solo pulse action time, the laser energy W can be expressed by the following equation:
W = ρ h [ C s ( T m − T 0 ) + C p ( T b − T m ) + L m + L r ]
where ρ is the mass density; h is the depth of the attached material; T0 is the initial temperature; Tm is the melting temperature; Tb is the boiling temperature; Cs is the specific heat capacity of the solid; Cp is the specific heat capacity of the liquid; and Lm and Lr are the fusion heat and vaporization heat, respectively.
When the ablation threshold of the attachment is lower than the substrate’s, the laser energy density should be adjusted and its value be maintained above the ablation threshold of the attachment but below that of the substrate. At this time, the attachment can be effectively removed without damaging the substrate. When the ablation threshold of the attachment is greater than the substrate’s, the laser parameters need to be controlled to minimize the negative impact on the substrate. For instance, Tang et al. [25] applied laser cleaning technology to remove sulfide from martensitic stainless steel. In this work, a fiber laser with a wavelength of 1064 nm was selected. When the laser energy density was less than 0.41 J/cm2, the laser cleaning effect was not evident as the laser energy density was lower than the laser cleaning threshold. When the laser energy density was over 0.41 J/cm2 but less than 8.25 J/cm2, the attachment was successfully removed from the substrates’ surface without any damage. However, when the laser energy was over 8.25 J/cm2, the substrates’ surface was damaged by the high-energy laser.

2.2. Laser Thermal Stress Mechanism

The laser thermal stress mechanism is another mechanism discussed in the fields of laser cleaning. A schematic of the laser thermal stress mechanism is shown in Figure 2 [9]. Unlike the laser thermal ablation mechanism, the laser thermal stress mechanism utilizes the stress effect induced by the laser beam rather than the laser thermal effect. When the pulsed laser beam directly irradiates the substrate’s surface, the attachment and substrate material absorb the laser pulse energy, causing the temperature to rise. Because the laser pulse width is very short, the process of heating and cooling the materials is completed in a very short time, resulting in quick thermal expansion and a high-pressure solid lifting force. The attachment is sprayed from the substrate’s surface once the solid lifting force surpasses the van der Waals force [26]. Fox et al. [27] first discovered the laser thermal stress effect. They found that oscillation was exhibited in the thin plate when the laser beam irradiated the surface and was accompanied by a burst effect, resulting in the attachment being removed from the substrate’s surface.
The following example considers the removal of coatings by laser cleaning. The space’s rectangular coordinate system is established first, and the normal direction of coatings is taken as the z-axis direction. The following conditions are assumed to be true:
  • The laser’s intensity in the z-axis direction is assumed to follow the laser absorption law.
  • The heat conduction depth in the z-axis direction is much smaller than the diameter of the laser beam spot.
  • Both the coating and substrate are regarded as adiabatic.
  • The laser beam profile is thought to have a flat-topped distribution on the plane.
Therefore, the one-dimensional heat conduction equation in the z-axis direction can be established as follows [28]:
ρ c ∂ T ( z , t ) ∂ t = λ ∂ 2 T ( z , t ) ∂ z 2 + α I 0 A e − A z , ( 0 ≤ z ≤ l , 0 ≤ t ≤ τ )
where T(z, t) is the temperature; and ρ, c, λ, α, A, I0, l, and τ are the mass density, specific heat capacity, thermal conductivity, laser absorption rate, laser absorption coefficient, initial laser light intensity, thickness, and pulse width, respectively.
The thermal expansion induced by the laser leads to strain and stress. Moreover, the coatings can be assumed to be an isotropic elastomer, so the thermal stress σ can be expressed by the following equation:
σ = Y ε = Y Δ l l = Y γ Δ T z , t
where Y is the elastic modulus, Δl is the length of thermal expansion in the z-axis direction, ε is the strain, and γ is the linear expansion coefficient of the material.
Therefore, the thermal stress that causes the coating to be sprayed from the substrate’s surface (z = l) can be expressed as follows [29]:
σ p = Y p ε = Y p γ p Δ T p l , t
where σp is the thermal stress that causes the coating sprayed from the substrate’s surface.
The thermal stress at substrate’s surface (σs) can be expressed as follows [29]:
σ s = Y s ε = Y s γ s Δ T s 0 , t
The cleaning force of thermal stress Δσ for the coating can be expressed as follows:
Δ σ = σ s − σ p = Y s γ s Δ T s − Y p γ p Δ T p
Because the adhesion force between the coating and the substrate is affected by many factors, the adhesion force can be simplified as the force between two parallel planes. The adhesion force fA can be expressed as follows:
f A = h 12 8 π 2 Z 3
where Z is the distance between the two contact parallel planes, and h12 is the Lifshitz–van der Waals constant, which is related to the contact materials and can be expressed by the following equation:
h 12 = 4 3 A 12
where A12 is the Hamaker constant of the mutual contact between the coating and the substrate, which can be calculated using the following equation:
A 12 ≈ A 11 A 22
where A11 is the Hamaker constant of contact between identical coatings, and A22 is the Hamaker constant of contact between identical substrates. As such, the adhesion force can be expressed as follows:
f A = A 11 A 22 6 π z 3
When Δσ > fA, the coating is sprayed from the substrate’s surface.

2.3. Plasma Shock Wave Mechanism

The plasma shock wave mechanism is very different from the previously introduced laser cleaning mechanisms. Rather than directly irradiating the substrate’s surface, the laser beam is parallel to the substrate’s surface [30]. The pulsed laser beam passes through the focusing lens and is focused on the superjacent of the attachments. It should be noted that the laser beam does not directly come into contact with the attachment, and their distance is very short (within several millimeters, in general). When the laser energy density is higher than the breakdown threshold of ambient air, the air is broken down, and the ionization phenomenon is produced at the same time, which leads to the introduction of the plasma shock waves. This mechanism should be distinguished by the ionization that is created by a perpendicular laser beam. The perpendicular laser beam incites the vaporization of the surface material. Then vapor becomes partially ionized and efficiently absorbs the laser energy, which leads to high pressures and a shock wave. Although the process of the latter includes the production of plasma, the cleaning method and relative medium are different from those of the plasma shock wave method.
Plasma shock waves lead to the stress effect, which can help to remove the attachments from the substrate’s surface. The plasma shock wave mechanism is well suited to removing the molecules from the substrate’s surface. Figure 3 displays a schematic of the plasma shock wave mechanism [31].
The adhesion force between attachments and substrates mainly consists of the van der Waals force, capillary force, and electrostatic force. When the attached particle is small enough, the adhesion force is mainly reflected as van der Waals force. In the plasma shock wave mechanism, the stress effect induced by plasma shock waves can be applied to overcome the van der Waals force. As a result, the attached particles can be removed from the substrate’s surface. The van der Waals force F can be expressed as follows:
F = h r 8 π z 2 + h δ 2 8 π z 3
where r is the radius of the attached particle, h is the van der Waals force constant, δ is the radius of the adhesion area, and z is the atomic distance between the attached particle and the substrate’s surface.
The plasma shock wave pressure ps can be expressed as follows [32]:
p s = p 1 γ + 1 × 2 γ − ( γ − 1 ) M s − 2 M s − 2
where Ms is the Mach number of the plasma shock wave, γ is the adiabatic index of air (γ = 1.4), and p1 is the atmospheric pressure.
The plasma shock wave mechanism is mainly determined by the relative position of the attached particles and the laser focal point. When the position of the attached particles is vertically below the laser focal point, the stress effect induced by the plasma shock wave causes the elastic deformation of the attached particles. When the elastic potential energy induced by the plasma shock wave is greater than the adsorption energy generated by the van der Waals force, the attached particles are removed from the substrate’s surface. When the crest surface of the plasma shock wave obliquely reaches the attached particle, the laser cleaning torque exceeds the resistance torque at the contact point between the attached particles and the substrate, and the attached particles are removed from the substrate’s surface by way of rolling. The critical pressure Pc that leads the attached particles to separate from the substrate’s surface can be expressed as follows:
P c = 2 α ( F + m g ) A s ( D cos θ − 2 α sin θ )
where As is the effective affect area of the plasma shock wave, m is the mass of the attached particle, g is the acceleration of gravity, θ is the angle between the applied force and the plane of the substrate’s surface, D is the diameter of the attached particle, and a is the radius of the contact area between the attached particles and the substrate. When Ps > Pc, attached particles are removed from the substrate’s surface. One drawback of the plasma shock wave mechanism is that there is a cleaning blind area. To improve the cleaning efficiency and eliminate the cleaning blind points, the irradiation angle of the laser beam needs to be adjusted according to the actual situation, or a double beam must be selected to induce plasma shock waves.
In the fields of laser cleaning, the laser thermal ablation mechanism and the laser thermal stress mechanism are the most common laser cleaning mechanisms. Their common feature is that the laser beam directly irradiates the substrate’s surface. However, the laser beam in the plasma shock wave mechanism is parallel to the substrate’s surface. The stress effect induced by the plasma shock wave can help to remove attachments from the substrate’s surface. Therefore, the plasma shock wave mechanism is mainly used to remove submicron or nanometer particles from the substrate’s surface. Generally, the laser cleaning mechanism is closely related to the laser wavelength. If the medium has a low-efficiency absorption of the laser wavelength, the laser thermal stress mechanism is dominant. As for highly absorbent mediums, the laser thermal ablation mechanism is dominant. When using specific laser equipment to clean materials with weak laser absorption, the possibility of increasing the laser power or decreasing the scanning speed can be taken into consideration.
Some actual laser cleaning methods, such as dry or steam cleaning, selective vaporization, ablation, spallation, evaporative pressure, etc., may use more than one mechanism. The differences in materials’ physical properties lead to substantial variation in the values of the absorbed laser intensity and the interaction time, thereby having an influence on laser cleaning methods [30]. Figure 4 provides an overview of the various methods and the range of their absorbed laser intensities; the interaction time and the boundaries of each method are not defined. Under certain conditions, there may be an overlap between these methods’ domains. As such, we should select appropriate laser equipment and suitable laser process parameters to remove the various attachments, such as contaminants, rust, and coatings, from the substrate’s surface. The laser thermal ablation process and the laser thermal decomposition process are irreversible. To protect the substrate material from laser thermal damage, suitable laser parameters and the correct operations should be selected [33]. Additionally, as surface micro-melting cannot be avoided in some cases, it must be emphasized that a change in the surface roughness during laser cleaning is incidental. Moreover, the effect of the roughness change may be positive and negative at the same time. This is different from laser polishing.

3. Typical Applications of Laser Cleaning in Industrial Fields

Industrial equipment involves different kinds of substrate materials, and the characteristics of laser cleaning are different for different materials [34,35]. In this section, the typical applications of laser cleaning in industry are introduced for metallic materials, nonmetallic materials, semiconductor elements, and other applications; some existing cleaning problems are also described.

3.1. Metallic Materials

Metallic materials are widely applied in industrial fields such as automobile assembly, shipbuilding, aerospace, and so on. Because attachments such as oil films, coatings, paints, and oxide layers can be successfully removed with laser cleaning, metallic corrosion can be prevented and the surface defects can be repaired, resulting in the improvement in the service life of components.

3.1.1. Oil Film

In industrial fields, using oil films to cover a substrate’s surface is one of the most effective methods to protect metallic materials from oxidation and corrosion. This technique plays an important protective role in the production, storage, and transportation of metallic components. However, contaminants such as oil stains and dust have an adverse impact on the processes of welding, painting, and packaging metallic components. Therefore, it is necessary to remove the oil stains and contaminants from metallic substrates’ surfaces at regular intervals. Laser cleaning has been proven to be an efficient means of removing oil stains from substrates’ surfaces. For instance, Ahn et al. [36] found that both an ultraviolet laser (KrF excimer laser with wavelength of 248 nm and pulse width of 25 ns) and an infrared laser (Nd:YAG laser with wavelength of 1064 nm and pulse width of 6 ns) could effectively remove the lubricating oil from the surfaces of carbon steel and stainless steel. It was found that the damage threshold of the substrate material was always greater than the cleaning threshold of the oil films. Therefore, the laser beam could not cause thermal damage to the substrate material.
Alshaer et al. [37] used laser cleaning technology to remove oil films from the surface of aluminum alloy. A cross-sectional view of the interface between the laser-cleaned and uncleaned zones is displayed in Figure 5 [37]. In this work, a Nd:YAG laser with a wavelength of 1064 nm and a pulse width of 100 ns was selected. Figure 5 shows that a surface layer of about 19 μm was removed and the laser-cleaned area appeared to be smoother. This indicated that laser cleaning is an effective method for removing oil films from metallic substrates’ surfaces.

3.1.2. Coating and Paint

Coatings and paints are other useful methods to protect metallic materials from oxidation and corrosion. However, with the increasing service time required for coatings and paints, their protective abilities are gradually lost. In engineering fields, the old invalid coatings and paints need to be removed first and then replaced with new ones. As an advanced surface-cleaning technology, laser cleaning shows a good cleaning effect when removing waste coatings or paints from metallic substrates’ surfaces. For instance, Gao et al. [38] used laser cleaning technology to remove the epoxy adhesive coating layers from a 2024 aluminum alloy experimental sample. The experimental results showed that the obtained residual primer paint layer was very smooth. In addition, the tiny holes on the substrate’s surface induced by laser thermal ablation led the contact area to increase, which helped to improve the adhesion force for the new coatings. Liu et al. [39] used a pulsed fiber laser with a wavelength of 1064 nm to remove the 35 μm thick epoxy zinc yellow paint layer coated on the surface of the TC4 titanium alloy plate; they achieved this by adjusting the laser’s cleaning speed and energy density, and the surface roughness of the titanium alloy after laser cleaning was similar to that of the uncleaned one.

3.1.3. Oxide Layer

In addition to oil films and coatings, the removal of oxide layers by laser cleaning is also an important application. Unlike other surface layers such as oil films, coatings, and paints, the corrosion layer of metallic materials is mainly composed of its own oxide. As such, laser cleaning treatments differ for different metallic materials. For instance, Wan et al. [40] used laser cleaning to remove the oxide layer from the surface of Q235 carbon steel. A fiber laser with a wavelength of 1064 nm, a pulse width of 340 ns, a repeat frequency of 250 kHz, and a laser power of 90 W was used in this work. They found that laser cleaning effectively removed the oxide layer and reduced the surface roughness. In addition, when the surface corrosion conditions were not severe, the cleaning effect of laser cleaning was better than that produced by the mechanical polishing method [41]. Zhu et al. [42] used a traditional pickling process and laser cleaning to remove the zinc oxide layer from a steel plate; a metallograph of the zinc oxide layers’ surface after laser cleaning and pickling is shown in Figure 6 [42]. As shown in Figure 5, the zinc coating of a steel plate subjected to laser cleaning pretreatment was more uniform, brighter, and had fewer defects than that subjected to the traditional pickling process.
However, the cleaning effect is not always ideal. For instance, Li et al. [43] used a laser to clean pure TA2 titanium. The oxide layer was effectively removed, but, due to the instantaneous high temperature caused by the laser, the surface exhibited brittle fractures. Atanassova et al. [44] used laser cleaning to remove corrosion from copper and brass samples. Under direct observation, the corrosion seemed to be effectively removed, but the cleaning area was not uniform under an optical microscope. It is also necessary to optimize the laser process parameters and select suitable laser equipment. Prokuratov et al. [45] used several kinds of laser to remove the ferric oxide layer and found that nearly every kind of laser could cause the oxide layer to dehydrate into an Fe3O4 layer, which caused the cleaning surface to become dark. Only a Ti:Sa laser with a pulse width of 800 nm and a pulse frequency of 150 Hz could remove the oxide layer without changing surface color, but the cleaning efficiency was very low.

3.2. Nonmetallic Materials

When nonmetallic substrates are exposed to various environments, attachments such as aging coatings, acid corrosion, encrustation, and biofilms appear on the surface. So, it is necessary to remove the surface attachments to improve the surface quality and prolong the material’s service life.
Due to their good insulation properties, glass, ceramics, and resin are widely applied in many fields. Laser cleaning performs well in terms of the surface cleaning of these materials. For instance, Ren et al. [46] used laser cleaning technology to remove contamination from electrical insulation parts such as glass, ceramics, and silicone rubber. They found that lasers can achieve nondestructive cleaning when the laser process parameters are adjusted, without influencing the performance of the insulation. Laser cleaning can also be used to conduct selective cleaning, such as by separating combinations of these materials. Barnier et al. [47] used two kinds of pulsed ultraviolet laser (an ArF laser with a wavelength of 193 nm and a pulse width of 18 ns; a Krf laser with a wavelength of 248 nm and a pulse width of 33 ns) to successfully remove the acrylic protective sleeve from the outer layer of a single-mode fiber. The removal of the outer resin layer did not cause any damage to the inner fiber. As for nonmetallic substrates, it is very difficult to remove attachments from glass substrates due to their transparency. Because the absorption rate of glass is relatively low and its laser cleaning threshold is close to the damage threshold, it is difficult to achieve nondestructive cleaning. To solve this problem, Weng et al. [48] proposed laser-induced backside wet cleaning technology. In this experiment, a Nd:YAG laser with a pulse width of 150 ns and a repeat frequency of 2 kHz was used to clean a metallic plate placed at the back of a glass substrate. Because the metallic plate absorbed most of the laser energy, turbulent bubbles were generated on the surface of the metallic plate. Under the impact of the bubbles, alumina particles with a size of 0.5 μm could be removed from the back surface of the glass. The cleaning results for lasers of different powers are shown in Figure 7 [48], which shows that the laser cleaning area is free of any alumina particles.
For stone materials, Pozo-Antonio et al. [49] analyzed the composition of a layer of contaminated granite and summarized various laser cleaning experiments with granite. The results showed that the laser had a good cleaning ability, removing the pigment graffiti and biofilm layers on the granite’s surface, but the effect of cleaning on the sulfate crust was not ideal.
Hybrid structures manufactured from different materials are being more widely used with the growing demand for light-weight structures, especially in the aircraft manufacturing industry. In this case, the use of multimaterial assemblies constitutes a possible solution. For example, with carbon-fiber-reinforced plastics (CFRPs) coupled with polycarbonate (PC), it is possible to obtain hybrid structures with high strength and good overall performance. To improve the materials’ adhesive performance, the covering epoxy layer of CFRPs needs to be removed before the multimaterial joining process is undertaken. Genna et al. [50] used laser cleaning technology to remove the epoxy layer from CFRPs. A macrograph of the CFRP laminate surface prior to and after laser cleaning is shown in Figure 8 [50]. In this work, a Q-switched Yb:YAG fiber laser with a laser power of 30 W, a wavelength of 1064 nm, a pulse frequency of 30 kHz, and a pulse width of 50 ns was selected. It is clear that the first matrix layer up to the fiber’s exposition was removed without any apparent damage. The laser cleaning of multiple materials has two main aims. One is the removal of surface coatings. For example, laser cleaning has been used to remove the paint layer from aircraft skins for decades [51]. The other aim is to clean the joints of the composite material and improve the strength of the joints. Yokozeki et al. [52] used laser cleaning to remove the CFRP before bonding. A pulsed CO2 laser with a wavelength of 10.6 μm, a pulse frequency of 50 Hz, and a laser power of 250 W was selected. The experimental results showed that the shear strength of the bonded joint was increased after laser cleaning. Compared with mechanical grinding with sandpaper, laser cleaning can enhance the material’s strength and prevent the fracturing of surface fibers that occurs with sandpaper treatment.

3.3. Semiconductor Element

With the rapid development of science and technology, electronic components are becoming smaller and smaller, while the level of integration for electronic components is becoming higher and higher. However, the quality of electronic components is seriously affected by the micro-/nano-impurity particles during the manufacturing process. Therefore, removing micro-/nano-impurity particles from the surfaces of semiconductors has become an urgent problem [53]. In the 1990s, researchers from IBM used the liquid-film-assisted laser cleaning method to remove particles from the surfaces of photomasks, which promoted the industrial application of the laser cleaning of semiconductor components [54]. Compared with traditional semiconductor cleaning technologies, such as chemical reagents combined with physical contact, laser cleaning has the advantages of not requiring contact, being highly precise, and generating no pollution. Therefore, laser cleaning is one of the most promising methods for the removal of polluted particles in the semiconductor industry.
Among the semiconductor components, silicon wafers constitute the core of integrated circuit manufacturing. In the manufacturing process, pollution inevitably appears on the surface of silicon wafers, so it is necessary to remove pollution particles from the affected surfaces. Around the year 2000, some scholars discovered that laser cleaning technology could effectively remove various particles (such as gold, molybdenum, and silicon) from wafers and thin films [55,56]. Lee et al. [57] used laser cleaning to remove the alumina particles from silicon wafers; for this experiment, a Q-switched Nd:YAG laser with a wavelength of 1064 nm and a pulse energy of 1.2 J was selected. Images of the wafer surfaces before and after laser cleaning are shown in Figure 9 [57]. Many particles that were uniformly deposited on the wafer surface were successfully removed by laser cleaning.
Due to the high-accuracy requirements and vulnerability of silicon wafers, many researchers have focused on the damage mechanisms and thresholds of substrates during laser cleaning. For instance, Kim et al. [58] used laser cleaning (the plasma shock wave mechanism) to remove SiO2 particles with a diameter size of 50 nm from the EUVL mask layer. In further research, it was found that when the distance between the laser focus and the substrate was 1.5 mm, the surface of the experimental sample was damaged. Moreover, when the distance was over 3 mm, the particles with a diameter size below 100 nm could be removed without any damage. Lai et al. [59] used laser cleaning with the plasma shock wave mechanism to remove nanoparticles from the silicon wafer substrate. In this work, a Nd:YAG laser with a wavelength of 1064 nm, a pulsed width of 12 ns, and a repeat frequency of 1 Hz was applied. The researchers found that the existence of nanoparticles could reduce the damage threshold of the substrate. When the size of the particle was increased to 140 nm, the damage threshold of the silicon substrate could be reduced to approximately 40% of that of the clean substrate. In addition, some researchers investigated the force and the movement behavior of particles during the laser cleaning process. Liu et al. [60] investigated the dynamic process of nano copper particles on the surface of silicon wafers in laser irradiation; they obtained the initial velocity of the particles after laser irradiation and the average acceleration of the particles during the laser irradiation period, which provided a theoretical reference for researchers to investigate the mechanisms of particle removal from the surface of silicon wafers.

3.4. Other Applications

In addition to the fields of metallic materials, nonmetallic materials, and semiconductor elements, laser cleaning technology has been widely applied in other fields.
In the nuclear industry, laser cleaning is the most effective way to deal with radioactive pollutants, such as the removal of radioactive particles from the surface of glass and nuclear pollution from nuclear power plant reactor pipes [61]. Roberts et al. [62] used laser cleaning technology to remove uranium compound pollution from a metallic substrate’s surface, as shown in Figure 10 [62]. The contamination was highly variable in terms of thickness and coverage, and the overall appearance exhibited rust. After laser cleaning, the uranyl nitrate pollution mixed in the rust layer was almost completely removed from the surface of the experimental sample.
In the field of optics, optical materials with pyroelectric and piezoelectric symmetry can show a bulk photovoltaic effect; applying laser cleaning technology can inhibit optical damage. For instance, Kösters et al. [63] used laser cleaning to clean lithium niobate crystals, which successfully reduced the number of electrons lost and prevented a refractive index change in the crystals.
Laser cleaning can remove ink or paint from the surfaces of natural fibers including wood and paper. For instance, Scholten et al. [64] used laser cleaning to remove ink smudges from paper samples. An ink smudge located on the paper’s surface c be removed successfully, but it is difficult to remove the entire ink smudge if it has deeply penetrated between the fibers.
Laser cleaning technology can also be used to remove biofilms in the fields of biomedicine [65]. Compared with traditional surface-cleaning technologies, laser cleaning has the advantages of better cleaning and disinfection abilities. In almost every field of surface cleaning, laser cleaning exhibits the ability to replace common surface-cleaning technologies or even to create a new surface-cleaning method.
Apart from these separate surface-cleaning applications, the precision processing technology that combines laser cleaning with laser polishing and laser etching has gradually become a mainstream production technology. This new precision processing technology can achieve nondestructive microprocessing and improve a material’s surface properties. For example, Miguel et al. [66] used laser cleaning to improve the self-cleaning properties of painted surfaces on a galvanized stainless-steel substrate. A femtosecond laser with pulse width of 350 fs, a wavelength of 1032 nm, a pulse frequency of 300 kHz, and a pulse energy of 16 μJ was selected in this work. After the laser cleaning, particles of a controlled size were applied to the surface and then successfully removed from the surface by adjusting the roll-off angle of the cleaning sample. The surface hydrophobic process at a tilt angle of 5 degrees is shown in Figure 11 [66]. It can be regarded as an indirect cleaning method that changes the surface structure of material and reduces the adhesion force of attachments. Polymer substrates possess desirable properties such as high oil resistance, good temperature resistance, low dielectric loss, etc.; they are widely used. Precision processing technology has broader applications for polymer substrates because it can more easily change the surface structure of polymer substrates. Features such as discrete spots, grooves, conical features, or hierarchical structures can be produced in the polymer, as shown in Figure 12 [67]. These features can not only retain the material’s original properties but also change the surface characteristics, including the adhesion, friction, self-cleaning properties, hydrophobicity, and so on [68,69]. For instance, some researchers used UV lasers to clean aged polyethylene and polyimide films, improving the hydrophobicity of the surface and effectively reducing the degree of organic dirt adhesion at the same time [67].

4. Conclusions

Laser cleaning can remove different attachments from various substrates’ surfaces and is regarded as an advanced surface-cleaning technology. We summarized the fundamental mechanisms of laser cleaning technology and outlined the typical industrial applications of laser cleaning. The main inferences are presented below.
(1)
The laser thermal ablation mechanism, the laser thermal stress mechanism, and the plasma shock wave mechanism can be expressed as evaporation processes, vibration processes, and impact processes, respectively. The laser thermal ablation mechanism and the laser thermal stress mechanism are the most common laser cleaning mechanisms. The common feature of the two mechanisms is that the pulse laser beam directly irradiates the surface. In the plasma shock wave mechanism, the laser beam is parallel to the surface of the substrate.
(2)
The laser cleaning mechanism is closely related to the laser wavelength; with a highly absorbent medium, the laser thermal ablation mechanism is dominant. With a less absorbent medium, the laser thermal stress mechanism is dominant.
(3)
Laser cleaning has unique advantages in many industrial fields. It can be used to remove most attachments from different substrate materials, such as metallic materials, nonmetallic materials, and semiconductor elements, and in other applications. By selecting suitable laser process parameters and the appropriate laser equipment, surface damage can be completely avoided.
(4)
In some scenarios, laser cleaning does not show an ideal cleaning effect. Complex or deep corrosion can be pretreated using mechanical and chemical cleaning methods. This can help to improve the cleaning result of the laser cleaning.

Author Contributions

Conceptualization, Z.Z. and J.W.; methodology, Z.Z.; validation, W.S., J.W. and S.W.; formal analysis, H.C.; investigation, Z.Z. and F.Z.; resources, W.S. and S.W.; data curation, Z.Z. and F.Z.; writing—original draft preparation, Z.Z., J.W. and F.Z.; writing—review and editing, Z.Z., W.S., J.W., H.C., F.Z. and S.W.; visualization, Z.Z.; supervision, W.S. and J.W.; project administration, S.W.; funding acquisition, H.C. and S.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the R&D Plan of Huaneng Power Group, China (HNKJ22-HF38) and the Scientific Research Foundation of Shantou University (NTF22001).

Data Availability Statement

All data supporting the conclusions of this study are included within the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of laser thermal ablation mechanism [17].
Figure 1. Schematic of laser thermal ablation mechanism [17].
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Figure 2. Schematic of laser thermal stress mechanism [9].
Figure 2. Schematic of laser thermal stress mechanism [9].
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Figure 3. Schematic of plasma shock wave mechanism [31].
Figure 3. Schematic of plasma shock wave mechanism [31].
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Figure 4. Absorbed intensity versus interaction time, showing regimes of various laser cleaning methods [30].
Figure 4. Absorbed intensity versus interaction time, showing regimes of various laser cleaning methods [30].
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Figure 5. A cross-sectional view of the laser-cleaned/uncleaned interface, showing the material removal depth from laser cleaning [37].
Figure 5. A cross-sectional view of the laser-cleaned/uncleaned interface, showing the material removal depth from laser cleaning [37].
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Figure 6. Metallograph of the zinc oxide layer surface: (a) laser cleaning; (b) pickling [42].
Figure 6. Metallograph of the zinc oxide layer surface: (a) laser cleaning; (b) pickling [42].
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Figure 7. Optical microscope images of the areas of alumina particle removal: (a) at a laser power of 13 W; (b) at a laser power of 39 W [48].
Figure 7. Optical microscope images of the areas of alumina particle removal: (a) at a laser power of 13 W; (b) at a laser power of 39 W [48].
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Figure 8. Macrograph of the CFRP laminate surface prior to and after laser cleaning [50].
Figure 8. Macrograph of the CFRP laminate surface prior to and after laser cleaning [50].
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Figure 9. Images of wafer surfaces before and after the laser cleaning process: (a) before laser cleaning; (b) after laser cleaning [57].
Figure 9. Images of wafer surfaces before and after the laser cleaning process: (a) before laser cleaning; (b) after laser cleaning [57].
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Figure 10. Laser cleaning removal of a uranium mixture from a steel surface [62].
Figure 10. Laser cleaning removal of a uranium mixture from a steel surface [62].
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Figure 11. Time series of water droplets rolling off of a hydrophobic painted surface created by laser irradiation [66].
Figure 11. Time series of water droplets rolling off of a hydrophobic painted surface created by laser irradiation [66].
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Figure 12. SEM images of various polymer layers after laser irradiation: (a) line-like dots on carbon-doped polycarbonate; (b) discrete spots on polyetheretherketone; (c) conical structures on PI [67].
Figure 12. SEM images of various polymer layers after laser irradiation: (a) line-like dots on carbon-doped polycarbonate; (b) discrete spots on polyetheretherketone; (c) conical structures on PI [67].
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Zhou, Z.; Sun, W.; Wu, J.; Chen, H.; Zhang, F.; Wang, S. The Fundamental Mechanisms of Laser Cleaning Technology and Its Typical Applications in Industry. Processes 2023, 11, 1445. https://doi.org/10.3390/pr11051445

AMA Style

Zhou Z, Sun W, Wu J, Chen H, Zhang F, Wang S. The Fundamental Mechanisms of Laser Cleaning Technology and Its Typical Applications in Industry. Processes. 2023; 11(5):1445. https://doi.org/10.3390/pr11051445

Chicago/Turabian Style

Zhou, Zhihu, Weipeng Sun, Jiajun Wu, Hongwei Chen, Fei Zhang, and Shuangxi Wang. 2023. "The Fundamental Mechanisms of Laser Cleaning Technology and Its Typical Applications in Industry" Processes 11, no. 5: 1445. https://doi.org/10.3390/pr11051445

APA Style

Zhou, Z., Sun, W., Wu, J., Chen, H., Zhang, F., & Wang, S. (2023). The Fundamental Mechanisms of Laser Cleaning Technology and Its Typical Applications in Industry. Processes, 11(5), 1445. https://doi.org/10.3390/pr11051445

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