In-process monitoring techniques for laser cleaning

Diverse in-process monitoring techniques based on acoustic, chromatic and intelligent approaches have been developed for laser cleaning not only to achieve the sound-cleaned surface but also to control the process in an automatic manner. The cleaning of various materials such as copper, marble, paper have also been carried out by using Q-switched Nd:YAG radiation. The process was successfully monitored by detecting the acoustic emission induced by laser–surface interactions during laser cleaning. Novel surface monitoring was achieved by chromatic modulation technique. The monitoring of laser fluence based on neural network logic was carried out by means of the recognition of acoustic spectrum patterns. The prediction system of surface damage has been also developed using fuzzy rule base in the same way as a human expert. These techniques may provide unique information for characterising the process as well as a promise of successful applications for laser cleaning techniques in real practical fields.     

Removal mechanisms of micro-scale particles by surface wave in laser cleaning

This paper is to investigate the mechanisms of micro-scale particle removal by surface wave, which was induced by a short pulse laser in a cleaning process. The authors analyzed the adhesive forces of particles on substrate surface and the clearance force produced by surface wave in laser cleaning. The physical model of particle removal by laser-induced surface wave was established to predict the removal area and the processing conditions of laser cleaning. In this research, a KrF excimer laser was applied to irradiate 304 stainless steel specimen distributed with copper particles to generate surface wave for copper particle removal. Considering that a time-varying and uniformly distributed heat source irradiates on material surface with thermao-elastic behavior, the displacement and acceleration of substrate induced by a pulsed laser were solved by an uncoupled thermal–mechanical analysis based on the finite element method. The processing parameters such as laser energy, laser spot size are discussed, respectively. A series of laser cleaning experiments were designed to compare with computation results. The results show that the removal area by surface wave beyond the laser spot increases with the laser energy and that, the surface acceleration decreases with the increase of the laser spot size.     

Surface cleaning of metals by pulsed-laser irradiation in air

The surface-cleaning effect of metals was investigated using KrF-excimer-laser irradiation of metal surfaces in air. The laser-induced cleaning of copper, stainless steel and aluminum surfaces was studied. It is found that laser cleaning is an effective cleaning process for metals even if the metal surfaces are heavily contaminated. It is also found that short wavelength and pulse duration are necessary for laser surface-cleaning. The energy density of the laser pulse is an important parameter in the cleaning process. Low energy density results in a cleaner surface but a larger pulse number is required, whereas high energy density can achieve higher cleaning efficiency but the temperature rise can cause surface oxidation and secondary contamination. In contrast to the KrF-excimer-laser, the pulsed CO₂ laser is not effective in surface-cleaning. The mechanisms of laser cleaning may include laser photodecomposition, laser ablation and surface vibration due to the impact of the laser pulse. Laser cleaning provides a new dry process to clean different substrate surfaces and can replace the conventional wet cleaning processes such as ultrasonic cleaning with CFC and other organic solvents.     

Experimental study on laser cleaning of paint layer on white marble surface北大核心CSCD

In order to effectively remove the surface paint of the cultural relic of white marble, the area extrapolation method and laser-induced plasma spectroscopy (LIPS) method were used to obtain the ablation threshold power of the gold, silver paint layer and white marble surface. On the basis of this, the optimal laser power for removing paint without damaging the white marble substrate was determined. The image processing method was used to study the cleaning degree and variation trend of cleaning rate of the gold and silver paint on the surface of 10 mm×10 mm white marble by laser cleaning, and the optimal laser spot overlap rate and the optimal cleaning times were obtained. Finally, the image processing method was used to evaluate the cleaning effect of laser cleaning of paint layer on the white marble surface. More than 93% of the cleaning degree shows that the synergistic use of the area extrapolation method, LIPS method and image processing method can effectively improve the laser cleaning efficiency of the paint layer on white marble surface. Copyright ©2022 Journal of Applied Optics. All rights reserved.     

Evaluating the use of laser radiation in cleaning of copper embroidery threads on archaeological Egyptian textiles

Cleaning of copper embroidery threads on archaeological textiles is still a complicated conservation process, as most textile conservators believe that the advantages of using traditional cleaning techniques are less than their disadvantages. In this study, the uses of laser cleaning method and two modified recipes of wet cleaning methods were evaluated for cleaning of the corroded archaeological Egyptian copper embroidery threads on an archaeological Egyptian textile fabric. Some corroded copper thread samples were cleaned using modified recipes of wet cleaning method; other corroded copper thread samples were cleaned with Q-switched Nd:YAG laser radiation of wavelength 532 nm. All tested metal thread samples before and after cleaning were investigated using a light microscope and a scanning electron microscope with an energy dispersive X-ray analysis unit. Also the laser-induced breakdown spectroscopy (LIBS) technique was used for the elemental analysis of laser-cleaned samples to follow up the laser cleaning procedure. The results show that laser cleaning is the most effective method among all tested methods in the cleaning of corroded copper threads. It can be used safely in removing the corrosion products without any damage to both metal strips and fibrous core. The tested laser cleaning technique has solved the problems caused by other traditional cleaning techniques that are commonly used in the cleaning of metal threads on museum textiles.     

Approximation of transformation media-based reshaping action by genetic optimization

The aim of this paper is to study the laser-induced backside wet cleaning techniques for glass substrates. Two kinds of laser cleaning techniques are proposed in this study. The first involves applying an Nd:YAG laser to the backside of the substrate which is submerged in water. A metal plate is placed below the glass substrate. Most of the laser energy will be absorbed by the metal plate. The metal then vaporizes the water and generates a turbulent bubble flow. The bubble flow removes the alumina particles from the surface of the glass substrate. The second involves using a CO₂ laser to generate turbulent bubble flow to remove the particles. Both methods were successfully demonstrated for the removal of submicron particles of 0.5 μm in size. The phenomena of bubble generation and diffusion are presented in the paper. Because the laser is applied to the backside of the substrate, the damage due to the laser heat can be significantly reduced. The quality and efficient of the backside processing is better than those of the front side processing. The proposed techniques have great potential to provide an improved solution for glass cleaning.     

Matrix laser cleaning: a new technique for the removal of nanometer sized particles from semiconductors

In this paper a new laser-based technique for the removal of nanoparticles from silicon wafers, called matrix laser cleaning, is introduced. In contrast to the already existing technique dry laser cleaning damages of the substrate can be avoided. Furthermore no liquids are used, avoiding problems that occur, e.g. in steam laser cleaning and other wet cleaning techniques. We show that damage free particle removal of polystyrene particles with diameters of at least down to 50 nm is possible with a cleaning efficiency very close to 100% within a single shot experiment. Furthermore the cleaning threshold is independent of the particle size. PACS 64.70.Hz; 68.43.Vx; 81.65.Cf     

The potential use of plume imaging for real-time monitoring of laser ablation cleaning of stonework

In this work, the potential of plume imaging was investigated as a real-time monitoring technique for the control of the laser cleaning process in cultural heritage objects with emphasis on encrusted stonework. Optical pulses generated by Nd:YAG laser were used to remove a high absorbing layer of crust from the surface of a lower absorbing marble substrate. Two regimes of operation have been employed for laser cleaning test, Q-switched ns mode, and short free running μs mode. Laser induced breakdown spectroscopy (LIBS) was also employed to study the stratigraphy of the cleaned surfaces, and thus to confirm the validity of the suggested monitoring technique. The tests proved the prospective of introducing plume imaging techniques as a straightforward and inexpensive tool for the real-time monitoring of laser ablation and the determination of the critical point for a safe surface cleaning intervention.     

碳纤维复合材料的激光清洗等离子体光谱在线检测研究

近年来碳纤维复合材料（CFRP）由于性能优异,受到工业领域广泛关注。采用激光清洗技术预处理碳纤维复合材料表面的污染物和环氧树脂等杂质,有利于改善碳纤维复合材料表面性能,提高碳纤维复合材料胶接界面的结合强度。在线检测激光清洗过程,实时判断碳纤维复合材料的表面清洗质量,是保证激光清洗效果的关键环节,也是激光清洗装置自动化、集成化的核心技术。激光诱导等离子体光谱技术可以快速分析材料表面元素变化,实现在线检测激光清洗表面状态,在激光清洗领域有很广的应用前景。采用Nd∶YAG高能量脉冲激光器产生的1 064 nm激光在空气环境中诱导产生等离子体,利用改进型光栅光谱仪(ME5000)获取等离子体光谱,在线检测激光清洗碳纤维复合材料。研究外界空气环境对等离子体光谱检测结果的影响,发现350~700 nm波段的元素谱线可用于碳纤维复合材料表面物质成分分析;采用电子扫描显微镜观测的激光清洗表面形貌和X射线电子能谱仪测得的元素变化共同表征等离子体光谱检测的有效性,通过采集不同激光能量以及不同作用次数的等离子体光谱图,获得碳纤维复合材料表层树脂物质通过激光单次清洗干净的阈值,研究激光清洗质量与激光诱导等离子体谱线成分及其强度变化的关系。结果表明：在获取的激光诱导等离子体光谱中,光谱图中谱线波长在393.3 nm的S(Ⅱ)和589.5 nm的S(Ⅱ)谱线可有效在线表征碳纤维复合材料表面清洗质量;激光单次去除干净表面环氧树脂的阈值为10.68 mJ;低激光能量时需要清洗多次可以去除干净表面树脂;高激光能量时清洗单次可使表面树脂去除干净,多次清洗易造成基体损伤。实验结果为激光清洗碳纤维复合材料的智能集成化应用提供工艺依据和技术支持。     

Laser applications in nuclear power plants

This paper reports the state of the art of using a solid-state Nd:YAG laser for material processing applications such as cutting, welding and drilling of several components of operational nuclear reactors in radioactive environment. We have demonstrated several advantages of laser-based material processing over conventional methods, and these are discussed briefly. At NPCIL, we have used laser techniques to cut stainless steel sheets up to 14 mm thickness and stainless steel weld up to a depth of 3 mm. This remotely operable laser system has been engineered for its robustness with proper fixtures and tooling for various material processing operations on industrial scale.     

Surface preparation of Ti–3Al–2.5V alloy tubes for welding using a fiber laser

Ti–3Al–2.5V tubes are widely used in aircraft hydraulic systems. Meticulous surface preparation before welding is necessary to obtain a sound weld involving these alloy tubes. Conventionally this is done by cleaning with environmentally malign toxic chemicals, such as, hydrofluoric acid and nitric acid. This paper describes the laser-cleaning process of the surface of these tubes with a fiber laser as a preparation for pulsed gas tungsten arc welding and results obtained. A simple one-dimensional heat equation has been solved to evaluate the temperature profile of the irradiated surface. It is shown that surface preparation by laser cleaning can be an environmentally friendly alternative process by producing acceptable welds with laser-processed tubes.     

Development of the new IC decapsulation technology

An IC is generally decapsulated with wet etching technology. The influences of environment and worker's health in usage and waste of the chemicals are seen as problems. Improvement of processing time is also needed. In order to overcome them, the laser processing technology is applied to IC decapsulation. In this paper, the relationship between processing condition and the processed surface quality is reported to select a suitable laser source. In addition, the new IC decapsulation technology, which is the combination of laser and wet etching, is proposed. To achieve the complex machining, firstly, the method of measuring the reflection light intensity of the laser is proposed to estimate the residual molding thickness. Secondly, the relationship between intensity of reflected laser and mold thickness to IC is shown to construct a mold thickness monitoring system. Finally, processing time and the volume of chemical solution used a process are compared with a conventional wet etching process.     

深紫外光刻光学薄膜

深紫外波段是目前常规光学技术的短波极限,随着波长的缩短,深紫外光学薄膜开发面临一系列特殊的问题;而对于深紫外光刻系统这样的典型超精密光学系统来说,对薄膜光学元件提出的要求则更加苛刻。本文主要介绍了适用于深紫外光刻系统的薄膜材料及膜系设计;对薄膜沉积工艺、元件面形保障、大口径曲面均匀性等超精密光学元件的指标保障关键问题进行了讨论;对环境污染与激光辐照特性等光刻系统中薄膜元件环境适应性的重要因素进行了深入分析。以上分析为突破高性能深紫外光刻光学薄膜开发瓶颈,更好地满足深紫外光刻等极高精度光学系统的应用需求指明了方向。     

Application of laser speckle analysis for the assessment of cementitious surfaces subjected to laser cleaning

This paper focuses on the application of laser speckle technique for the assessment of the effectiveness of laser cleaning of cementitious surfaces. Laser speckle-based methods are non-contact, highly resolving techniques for the measurement of displacement, rotation, and strain of an illuminated area on a rough surface. Since the intensity of reflected light depends on the geometrical microstructure and colour of the samples, any alterations of the surface result in different speckle images. Analysis of speckle images presented here is based on the analysis of the distribution of intensity of reflected light obtained in a selected plane, and analysis of statistical parameters describing such distribution (skewness and kurtosis). A wide range of laser-cleaned mortar samples with different geometrical microstructure and moisture content has been subjected to the assessment by He-Ne laser. Laser speckle method has been successfully used to identify the effectiveness of the laser cleaning process with respect to different surface conditions. It appears that the changes in kurtosis and skewness should be mainly associated with the alterations of geometrical microstructure. Whereas, mean light intensity seemed to depend predominantly on the mortar's absorption characteristics (colour).     

激光分离脆性材料的研究

传统分离脆性材料的技术由于易产生残余应力、显微裂纹与边部碎屑等缺陷，越来越不能满足半导体工业高精度与高清洁度的要求。激光微细加工技术以无污染、无接触及加工精度高、操作柔性好等优势，正成为一种很有潜力的脆性材料精密加工技术。介绍了用于分离脆性材料的几种典型激光微细加工技术，包括激光烧蚀切割技术、激光诱导张应力控制微裂纹扩展技术与激光剥离技术的工艺原理、特点及研究现状，指出了其存在的主要问题并探讨了其改进措施。最后预测了激光分离技术的发展前景。     

大芯径空气包层微结构光纤实现kW级激光传输

kW级高功率激光柔性传输是激光清洗、激光焊接、激光刻蚀等高功率激光加工领域中必备的环节,而实现高功率激光低损耗传输的光纤是其关键器件。目前高功率激光传输光纤采用大芯径传能光纤,仍然存在着弯曲损耗大、柔性差等问题,并且使用过程中要经常维护。华南师范大学特种光纤研究中心提出一种大芯径空气包层微结构光纤,利用包层的空气孔可以极大降低激光泄露的风险,降低光纤制备过程中对耐高温涂敷层的严苛要求,实验结果证实该光纤在室温无制冷条件下可实现kW级激光传输,从而为10 kW级高功率激光柔性传输奠定基础。     

Detergency of stainless steel surface soiled with human brain homogenate: an XPS study

In the detergency field of re-usable medical devices, a special attention is focused on the non conventional transmissible agent called prions which is a proteinaceous infectious agent. Few cleaning procedures are effective against prions and few techniques are available to study cleaning effectiveness with respect to proteins in general. In our study, X-ray photoelectron spectroscopy (XPS) has been used to evaluate the effectiveness of detergent formulations to remove proteins from stainless steel surface soiled with a brain homogenate (BH) from human origin. Our results showed that XPS is a reliable surface analysis technique to study chemical species remaining on surface and substrate properties after cleaning procedures. A semi-quantitative evaluation of the detergency effectiveness could also be performed.     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

Laser cleaning of 19th century Congo rattan mats

There is a growing interest by art conservators for laser cleaning of organic materials, such as wooden artworks, paper and textiles, since traditional cleaning with solvents can be a source of further decay and mechanical cleaning may be too abrasive for sensitive fibers. In this work we present a successful laser cleaning approach for 19th century rattan mats from the Brooklyn Museum collection of African Art, now part of the study collection at the Conservation Center in New York. Tests were carried out using the fundamental (1064 nm) and second harmonic (532 nm) wavelength of a Q-switched Nd:YAG laser to measure threshold values both for surface damage and color changes for different types of rattan samples. The irradiated substrates were investigated by optical microscopy, scanning electron microscopy and by UV-vis spectroscopy in order to determine the efficiency of laser cleaning and to assess possible deterioration effects that may have occurred as a result of laser irradiation. The study showed that by using the laser emission at 532 nm, a wavelength for which photon energy is below the bond dissociation level of the main cellulosic compounds and the water absorption is negligible, it is possible to select a range of laser fluences to remove the black dust layer without damaging the rattan material.     

Influence of Elemental Compositions in Laser Cleaning for Automotive Coating Systems

Laser coating removal is an approach that has the potential to replace conventional chemical-based stripping methods in industry. In addition to the laser parameters, the efficiency of this cleaning technique depends also on the coating system itself. For this reason, we study the influence of the elemental compositions of two Malaysian automotive coated substrates, referred to here as A and B, on laser coating removal mechanisms using the Cynosure Cynergy Nd:YAG laser. The optimum laser coating removal efficiencies for both samples A and B are determined based on the depths of 360 craters obtained using a given formula. Selected crater depths on both of the stipulated samples are then subjected to energy dispersive X-ray analysis to determine their elemental compositions. The results indicate that sample A, which has higher aluminum content, shows greater efficiency in coating system removal, with balanced carbon and oxygen compositions aiding coating reduction during the cleaning process.