Characterization at a local scale of a laser-shock peened aluminum alloy surface

The influence of a laser shock peening mechanical surface treatment on 2050-T8 aluminum alloy has been investigated, mostly using Scanning Kelvin Probe Force Microscopy. Volta potential difference maps around Al(CuFeMn) precipitates were performed before and after laser-shock peening to determine the influence of laser treatment versus galvanic coupling near precipitates, and resulting pit initiations. It has been shown that laser shock peening either preserves or reduces precipitate-matrix Volta potentials gradients, which in this later case, and correlated to recent corrosion electrochemical investigations, could explain corrosion improvement obtained after laser-shock peening treatments of aluminum alloys. The influence of crystal orientation and plastic deformation, and more specifically the effect of laser-induced compressive residual stresses or work-hardening, on the Volta potential values and on the pitting corrosion behavior was also addressed.     

Laser removal of copper particles from silicon wafers using UV, visible and IR radiation

Laser removal of small copper particles from silicon wafer surfaces was carried out using Nd:YAG laser radiation from near-infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm). It has been found that both 266 nm and 532 nm are successful in removing the particles from the surface whereas 1064 nm was shown to be ineffective in the removal of particles. The damage-threshold laser fluence at 266 nm was much higher than other wavelengths which provides a much wider regime for safe cleaning of the surface without causing any substrate damage. The cleaning efficiency was increased with a shorter wavelength. The effect of laser wavelength in the removal process is discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning forces for the three wavelengths. Received: 31 May 2000 / Accepted: 14 July 2000 / Published online: 20 June 2001     

CO2 laser assisted removal of UO2 and ThO2 particulates from metal surface

Pulsed laser assisted removal of uranium dioxide and thorium dioxide particulates from stainless steel surface have been studied using a TEA CO₂ laser. Decontamination efficiency is measured as a function of laser fluence and number of pulses. Threshold fluence for the removal of UO₂ particulates has been found to be lower than that required for the removal ThO₂ particulates. Usage of a ZnSe substrate, that is transparent to the laser wavelength used here, enabled us to decouple the cleaning effect arising out of absorption in the particulates from that in the substrate and has contributed towards understanding the mechanism responsible for cleaning. The experimental observations are also corroborated by simple theoretical calculations.     

Laser assisted decontamination of metal surface: Evidence of increased surface absorptivity due to field enhancement caused by transparent/semi-transparent contaminant particulates

Small signal absorption measurements of the incident coherent radiation by the metal surface have revealed an increase in the absorption by the surface in presence of transparent/semi-transparent particulates on it. This effect, identified as field enhanced surface absorption, has been found to increase with reduction in the average particulate size. Consequently higher laser assisted removal efficiency of contamination from a metal surface has been observed for smaller contaminant particulates. These measurements have been carried out utilizing coherent radiations of two different wavelengths so chosen that for one the particulates are totally transparent while for the other they are partially transparent.     

Characterization and compositional study of a ZrO2 engineering ceramic irradiated with a fibre laser beam

Fibre laser surface treatment (FLST) of a cold isostatic pressed (CIP) ZrO₂ engineering ceramic (ZEC) was performed using various processing gas compositions. This is the first time that a surface treatment of ZEC has been employed hitherto by using the fibre laser (FL) radiation to observe the changes on and within the surface of the engineering ceramic; in particular, material removal, surface topography, chemical composition, changes in the surface hardness and distribution of the heat affected zone (HAZ). Bonding of the grain boundaries was found through surface melting with all FL irradiated samples to some extent, but the effect was more marked on the sample FL irradiated with an Ar assist gas and proved to be the most effective combination for modifying the surface morphology. The surface finish and the material removal were varied with the changes in the gas composition. Maximum material removal was observed when an O₂ assist gas was employed on account of the O₂ generating an exothermic reaction. This in turn, produced excessive heating. The compositional analysis revealed a chemical change occurring within the FL irradiated surfaces, regardless of the assist gas used, with the ZEC transforming to zirconia carbide (ZrC).     

An energy approach to the modelling of particle removal by pulsed laser irradiation

An energy model to explain particle removal mechanism has been developed. This model is based on a detailed investigation of contact deformation of a particle on a solid surface, as well as particle motion during the process of substrate surface expansion under uniform laser irradiation. Calculation results show that small particles mainly gain kinetic energy during pulsed laser irradiation, whereas large particles mainly gain elastic deforming potential energy. The particle removal condition is derived from the viewpoint of energy. The relationship of particle removal efficiency with laser fluence and particle size is discussed. Theoretical results are compared with experimental results. Received: 30 July 1998 / Accepted: 14 December 1998 / Published online: 17 March 1999     

Processing of concretes with a high power CO2 laser

Laser material processing, being a non-contact process, minimizes many of the complexities involved in the decontamination and decommissioning of nuclear facilities. A high power laser beam incident on a concrete surface can produce spalling, glazing or vaporization, depending upon the laser power density and scan speed. This paper presents effect of various laser processing parameters on the efficiency of material removal by surface spalling and glazing. The size of laser beam at constant fluence or energy density had significantly different effect on the spalling process. In thick concrete block cutting the flow or removal of molten material limits the cutting depth. By employing repeated laser glazing followed by mechanical scrubbing process cutting of 150 mm thick concrete block was carried out. Gravitation force was utilized for molten materials to flow out while drilling holes on vertical concrete walls. The dependence of the incident laser beam angle on drilling time was experimentally studied.     

Acoustic substrate expansion in modelling dry laser cleaning of low absorbing substrates

Acoustic expressions have been derived for the thermal expansion of substrate surfaces due to irradiation by an exponential laser pulse. The result of acoustic effects on three substrates (silicon, glass and silica) with different absorptions has been calculated.It has been shown that for substrates having relatively low absorptions, like silica and glass, acoustic considerations substantially reduce thermal expansion of the substrate caused by irradiation by nanosecond laser pulses relative to a quasi-static expansion model. In particular, the expansion of the substrate occurs over a much longer time frame than when the quasi-static approximation holds. Consequently, acceleration of the substrate surface is greatly reduced and laser cleaning threshold fluences for particle removal are increased.The predictions of the model of Arnold et al. when developed for acoustic considerations give reasonable agreement with experimentally found threshold fluences for alumina particles on silica and glass substrates although it underestimates the ratio of the threshold cleaning fluences of silica and glass. This could be due to the model underestimating the contribution of surface expansion to the laser cleaning process. The influence of multiple reflections in the substrate and departure from one dimensionality in the heat conduction on the threshold fluence was found to be insignificant. Thermal contact between the particle and the substrate was also found to have little effect on laser cleaning threshold fluences. Another mechanism that may enhance surface expansion is the 3D focussing of radiation by the particles. PACS 42.62.Cf; 81.65.Cf; 42.55.Lt     

Plasma-mediated ablation of biofilm contamination

Ultra-short pulsed laser removal of thin biofilm contamination on different substrates has been conducted via the use of plasma-mediated ablation. The biofilms were formed using sheep whole blood. The ablation was generated using a 1.2 ps ultra-short pulsed laser with wavelength centered at 1552 nm. The blood contamination was transformed into plasma and collected with a vacuum system. The single line ablation features have been measured. The ablation thresholds of blood contamination and bare substrates were determined. It is found that the ablation threshold of the blood contamination is lower than those of the beneath substrates including the glass slide, PDMS, and human dermal tissues. The ablation effects of different laser parameters (pulse overlap rate and pulse energy) were studied and ablation efficiency was measured. Proper ablation parameters were found to efficiently remove contamination with maximum efficiency and without damage to the substrate surface for the current laser system. Complete removal of blood contaminant from the glass substrate surface and freeze-dried dermis tissue surface was demonstrated by the USP laser ablation with repeated area scanning. No obvious thermal damage was found in the decontaminated glass and tissue samples.     

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.     

Optodynamic analysis of direct laser writing of graduation lines

The direct laser writing of graduation lines in Cr thin films on glass substrates has been investigated. The Nd-YAG laser and the astigmatic optical system have been used to write rectangular holes in the Cr film. The optimal writing parameters: the laser pulse energy, the Cr layer thickness and the substrate-objective distance were determined using the optoacoustic probe beam deflection method to detect the evaporation of the material. The evaporation is concluded to be the essential process in the laser writing of graduation lines. The rims formed by the surface tension gradient at the hole edges indicate that the laser writing of graduation lines is a typical two-phase removal process.     

Kinetics of laser-induced surface melting and oxide removal in single-crystalline Ge

The process of oxide removal in crystalline Ge using a pulsed ultraviolet laser has been studied by means of real-time reflectivity measurements with nanosecond resolution. The interaction of laser radiation with a clean, oxide-free surface has been characterized and the inhomogeneous and homogeneous energy density melting thresholds of c-Ge for 193 nm radiation have been determined. The values are 180 and 370 mJ/cm², respectively. We have demonstrated that it is possible to remove an oxide overlayer by irradiation in vacuum and to produce a surface that shows the same response to laser radiation as a smooth, oxide-free, chemically cleaned surface. Under certain specific irradiation conditions it is even possible, after removing the oxide overlayer, to produce an enhanced crystalline quality in the near-surface region compared to that obtained upon chemical cleaning as evidenced by Rutherford backscattering/channeling measurements.     

基于LIBS光谱与成分分析的飞机蒙皮激光除漆可控性研究

激光除漆作为激光清洗技术的分支,有望替代传统打磨及化学除漆工艺,实现飞机蒙皮表面漆层的可控清除,但除漆过程及质量的可控性依赖于有效的原位、在线监测技术。针对飞机铝合金蒙皮表面多漆层结构,采用LIBS技术对不同漆层、不同厚度时漆层特征元素进行光谱与成分分析,在信号解译基础上建立漆层去除层数、去除厚度与LIBS光谱变化的内在关联,实现除漆过程质量的实时监测与反馈控制。结果表明,分层除漆过程中面漆、底漆完全清除后,漆层特征元素（Fe,Ti）的光谱峰消失。LIBS监测到面漆的特征元素Fe在501.494 1和521.517 9 nm处Fe Ⅰ 的光谱特征峰消失时,判定面漆完全清除。监测到底漆特征元素Ti在498.173 0,499.107 0和521.039 0 nm处Ti Ⅰ 的光谱特征峰消失时,判定底漆完全清除。厚度除漆时,随漆层厚度降低或激光脉冲作用次数增加,漆层特征元素（Ca）的光谱峰强相应降低,至漆层厚度为0时（完全去除）,漆层特征元素光谱峰消失,同时基体特征元素（Al）光谱峰出现。LIBS监测616.217 0,643.907 0和422.673 0 nm处Ca Ⅰ 的光谱信号强度变化能够监测激光除漆时剩余漆层厚度,实现对激光除漆厚度的可控清除监测。另外,结合EDS与SEM测试分析,验证了LIBS用于飞机蒙皮激光除漆过程与效果监测、分层与厚度控制的可行性,表明在不损伤基体氧化层的前提下,通过监测对应波长位置的面漆、底漆特征元素光谱与成分变化规律能够实现激光分层可控、厚度可控除漆。     

Laser-based modification of wettablility for carbon fiber reinforced plastics

This paper presents a laser-based technique for surface preparation of carbon fiber reinforced plastics (CFRP) for bonded repair. Ablative and non-ablative treatment of the surface is produced by variation of laser power and the resulting surface energy determined by goniometric measurements. Wettability has been directly related to a calculated wetting envelope. The investigations show an additional major influence of the surface topography on the shear strength of the joint. The direction of the applied laser lines in relation to the fiber direction was identified as an essential influence for this roughness. A ns IR-laser demonstrates high potential for surface preparation as well as for selective ply removal. The laser prepared surfaces are examined through optical microscopy, scanning electron microscope (SEM), and contact angle measurements. The wettability studies show a significant increment in surface energy after laser treatment, in relation to non-treated as well as surface grinding samples.     

Laser plasma shockwave cleaning of SiO2 particles on gold film

A Nd:YAG laser (1064 nm) induces optical breakdown of the airborne above the gold-coated K9 glass surface and the created shockwave removes the SiO₂ particles contaminated on the gold films. The laser cleaning efficiency has been characterized by optical microscopy, dark field imaging, ultraviolet-visible-near infrared spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and the Image-pro software. The relationships between removal ratio and particle position and laser gap distance have been studied in the case of single pulse laser cleaning. The results show that the 1064 nm laser induced plasma shockwave can effectively remove the SiO₂ particles. The removal ratio can reach above 90%. The effects of particle position and laser gap distance on the cleaning efficiency are simulated for the single pulse laser cleaning. The simulated results are consistent with the experimental ones.     

离子束清洗在激光薄膜中的应用

介绍了在激光薄膜中End Hall型离子源离子束清洗的应用。通过实验验证了基片的二次污染和离子束的清洗效果,观测了离子束清洗前后基片的表面形貌变化。研究了用离子束清洗基片时对薄膜抗激光损伤阈值的作用。分析了用离子束清洗基片时其基片表面的性质,如清洁度、表面能、接触角、表面形貌的变化机理。指出了杂质微粒的去除和附着力的增加是如何使薄膜抗激光损伤阈值显著提高的。     

纳秒紫外重复脉冲激光烧蚀单晶硅的热力学过程研究

采用波长为355 nm的纳秒紫外重复脉冲激光对单晶硅片进行了盲孔加工实验, 观测了随脉冲增加激光烧蚀硅片的外观形貌和盲孔孔深、孔径的变化规律, 并对紫外激光辐照硅片的热力学过程进行了分析. 研究结果表明:紫外激光加工硅盲孔是基于热、力效应共同作用的结果, 热效应会使得硅材料熔化、气化甚至发生电离产生激光等离子体,为材料的去除提供条件;激光等离子体冲击波以及高温气态物向外膨胀会对熔化材料产生压力致使其向外喷射,为重复脉冲的进一步烧蚀提供了条件;力效应主要沿着激光传输的方向,垂直于硅表面,使得去除部位主要集中在孔的深度方向,达到较高的孔径比,实验观察孔径比可达8:1;此外,激光等离子体的产生也阻止了激光对靶面的作用,加之随孔深的增加激光发生散焦,使得烧蚀深度有一定的限制,实验观察烧蚀脉冲个数在前100个时加工效率较高.     

Characterization and corrosion study of NiTi laser surface alloyed with Nb or Co

The interest in NiTi alloys for medical applications has been steadily growing in recent years because of its biocompatibility, superelasticity and shape memory characteristics. However, the high Ni content in NiTi alloys is still a concern for its long-term applications in the human body. The release of Ni ion into the human body might cause serious problems, as Ni is capable of eliciting toxic and allergic responses. In view of this, surface modification to reduce the surface content of Ni and to improve the corrosion resistance, both of which would reduce Ni release, is an important step in the development of NiTi implants. In the present study, NiTi was surface alloyed with Nb or Co by laser processing. The fine dendritic structure characteristic of laser processing has been described in terms of rapid solidification. The amount of surface elemental Ni was reduced to 10% and 35% for the Nb-alloyed and Co-alloyed layer, respectively. The corrosion resistance in Hanks’ solution (a simulated body fluid) was increased as evidenced by a reduced passive current density and a higher pitting potential for both the Nb- and Co-alloyed specimens. The composition and hardness profiles along the depth of the modified layer were correlated with the distribution of the dendrites. The microhardness of the alloyed layers was around 700-800 Hv, which was about four times that of the untreated NiTi specimens.     

Femtosecond laser patterning of Ta0.1W0.9Ox/ITO thin film stack

Selective laser patterning of thin films in a multilayered structure is an emerging technology for process development and fabrication of optoelectronics and microelectronics devices. In this work, femtosecond laser patterning of electrochromic Ta_0.1W_0.9O_x film coated on ITO glass has been studied to understand the selective removal mechanism and to determine the optimal parameters for patterning process. A 775 nm Ti:sapphire laser with a pulse duration of 150 fs operating at 1 kHz was used to irradiate the thin film stacks with variations in process parameters such as laser fluence, feedrate and numerical aperture of objective lens. The surface morphologies of the laser irradiated regions have been examined using a scanning electron microscopy and an optical surface profiler. Morphological analysis indicates that the mechanism responsible for the removal of Ta_0.1W_0.9O_x thin films from the ITO glass is a combination of blistering and explosive fracture induced by abrupt thermal expansion. Although the pattern quality is divided into partial removal, complete removal, and ITO film damage, the ITO film surface is slightly melted even at the complete removal condition. Optimal process window, which results in complete removal of Ta_0.1W_0.9O_x thin film without ablation damage in the ITO layer, have been established. From this study, it is found that focusing lens with longer focal length is preferable for damage-free pattern generation and shorter machining time.     

Conditions for laser-induced plasma to effectively remove nano-particles on silicon surfaces

Particles can be removed from a silicon surface by means of irradiation and a laser plasma shock wave.The particles and silicon are heated by the irradiation and they will expand differently due to their different expansion coefficients,making the particles easier to be removed.Laser plasma can ionize and even vaporize particles more significantly than an incident laser and,therefore,it can remove the particles more efficiently.The laser plasma shock wave plays a dominant role in removing particles,which is attributed to its strong burst force.The pressure of the laser plasma shock wave is determined by the laser pulse energy and the gap between the focus of laser and substrate surface.In order to obtain the working conditions for particle removal,the removal mechanism,as well as the temporal and spatial characteristics of velocity,propagation distance and pressure of shock wave have been researched.On the basis of our results,the conditions for nano-particle removal are achieved.