Laser Ablated Superconducting and Related Thin Films:A Microscopical Investigation

Laser ablated high temperature superconducting and related thin films are investigated with amicroscopical point of view.The microstructure and microchemistry of three thin films(Y-Ba-Cu-O,Bi-Pb-Sr-Ca-Cu-O and Sr-Ca-Cu-O)are demonstrated as examples of laser ablation products.     

飞秒激光烧蚀铝片的热弛豫效应模拟与实验研究

研究了在不同能量密度的单脉冲飞秒激光下烧蚀铝片的过程。通过分析基于双温模型有限差分法所模拟出的数据图,并结合飞秒激光烧蚀实验的结果,从而研究不同激光能量密度与铝片烧蚀之间的联系。研究表明：飞秒激光能量密度与飞秒激光烧蚀的关联,而实验所测的结果进一步表明了提升飞秒激光能量密度对加工铝材料的加工效率以及加工质量的影响。通过研究和实验结果能够得出随着飞秒激光能量密度的增加,飞秒激光烧蚀期间铝材料的热弛豫过程将加长,烧蚀强度也将有所增加,同时铝材料加工后得出形貌质量也将有所提高,这个结论对于飞秒激光烧蚀金属材料的研究具有一定的意义。     

193nm和248nm波长准分子激光对玻璃及聚合物刻蚀特性的比较

将波长为193 nm的Ar F准分子激光和波长为248 nm的Kr F准分子激光经光学系统分别垂直照射到不同材料的表面,通过改变激光脉冲数目,在大气背景下进行实验,用三维轮廓仪对照射后的样品表面形貌及表面粗糙度进行测试分析。研究了准分子激光与非金属材料相互作用的机理,并对比得到了193、248 nm激光对聚合物材料PMMA(聚甲基丙烯酸甲酯)及HF4光学玻璃的刻蚀特性。结果表明:在激光能量密度为1.5 J/cm2时,波长193 nm激光比波长248 nm激光在刻蚀PMMA时能得到更好的效果,而在刻蚀HF4光学玻璃时却恰好相反。在准分子激光与非金属材料相互作用的过程中,既不是光源的波长越短,刻蚀效果越好,也不是光与物质相互作用中的光化学机理所占的比重越大,刻蚀效果越好;最终刻蚀效果的好坏取决于很多因素。     

Effect of Pulsed Laser Ablation and Continuous Laser Heating on the Adhesion and Cohesion of Cold Sprayed Ti-6Al-4V Coatings

The individual and cumulative effects of in situ pulsed laser ablation and continuous laser pre-heating on adhesion and cohesion strength of cold sprayed Ti-6Al-4V coatings are investigated. Laser beams were coupled to a cold spray gun in order to ablate and pre-heat the substrate surface a few milliseconds prior to the impact of the spray particles. Cohesion and adhesion strength were evaluated by scratch test, standard ASTM C633 pull test and laser shock (LASAT) technique. The effects of laser ablation before and during cold spray operations were investigated. Results demonstrate that laser ablation of the substrate before cold spraying led to a smooth surface which improved adhesion strength. However, when laser ablation was maintained throughout the cold spray process, i.e., in between the coating layers, a reduction of cohesion and adhesion was observed. These negative effects were circumvented when laser ablation and laser pre-heating were combined.     

CrNiMo钢激光淬火处理的抗烧蚀性能研究

通过激光淬火表面强化技术,在CrNiMo钢表面形成马氏体相变的表面强化薄层.采用小电流累积烧蚀方法分解了材料烧蚀过程;对比分析相同烧蚀条件下CrNiMo钢在激光处理与未经激光处理的表面烧蚀形貌及其烧蚀质量差.结果表明:激光淬火确实能提高材料的抗烧蚀性能.     

Laser ablation of titanium alloy under a thin and flowing water layer

Underwater laser ablation has become an alternative machining process that is able to reduce the thermal damage in work materials caused by lasers. However, the disturbance of water to the laser beam is a crucial concern for the ablation performance in water and cut surface quality obtained. In this study, a new laser ablation technique has been proposed, in which a waterjet was applied to impinge the top workpiece surface in order to form a thin and flowing water layer. With the assist of such water layer during the laser ablation, the redeposition and heat-affected zone can be minimized. Titanium alloy (Ti–6Al–4V) selected as a work sample was grooved by using a nanosecond-pulse laser under different machining conditions. The cut geometry and heat-affected zone were observed and analyzed to justify the process performance. The metallurgical change and cracks that occurred on and underneath the groove surface were also investigated in this study. The experimental results revealed that a clean cut with less thermal damage can be obtained when the workpiece was ablated by a laser under the flowing water layer. In addition, a narrower and deeper groove can be fabricated when a higher waterjet flow rate was applied. The laser ablation under the flowing water layer developed in this study could be a potential method for machining titanium alloy or even other thermal-sensitive materials.     

飞秒激光刻蚀对纳米金刚石薄膜表面微观特性的影响

利用飞秒激光对纳米金刚石涂层进行刻蚀试验,通过改变激光的重复频率,输出功率以及焦点扫描速度,研究不同的激光加工参数对金刚石涂层烧蚀结果的影响。利用白光干涉仪器、SEM、拉曼光谱仪研究了飞秒激光刻蚀后涂层表面微观粗糙度、微观形貌以及碳相结构变化。采用面积推算法计算出扫描速度1 mm/s,有效脉冲数为90时的烧蚀阈值。结果表明：金刚石涂层表面飞秒激光诱导的条纹状结构周期（LIPSS）接近飞秒激光波长,改变飞秒激光重复频率对涂层表面形貌修饰影响不大;由于烧蚀饱和作用,飞秒激光功率增加至80 mW过程中涂层表面微观粗糙度持续减小随后维持在325 nm左右;激光扫描速度的增大可使LIPSS特征消失,当扫描速度增加至1.4 mm/s后,涂层表面微观粗糙度不再继续降低而是随着速度的增大而增大。激光诱导的金刚石涂层表面石墨化程度越高,涂层表面微观粗糙度则越低;当有效脉冲数为90时纳米金刚石薄膜的飞秒激光烧蚀阈值为0.138 J/cm²。     

Laser-assisted laser ablation method for high-quality hydroxyapatite coating onto titanium substrate

Hydroxyapatite (HAp) coating on titanium (Ti) or Ti alloy implant materials is one of the important technologies for improving the bioactivity of their surface. We recently developed a new HAp coating method using two laser beams, laser-assisted laser ablation method (LALA method). In this method, two excimer lasers were used. One laser beam from KrF laser, the ablation laser, is used for ablation of a HAp target. The other beam from ArF laser, the assist laser, is used to irradiate a Ti substrate surface during formation of the HAp coating. The assist laser plays an important role in the formation of a crystalline HAp coating and improves the strength of adhesion to the Ti substrate.The coating quality varied with the timing of the assist laser irradiation. A coating deposited with a long assist laser delay contained a large amorphous component. High-quality coatings were obtained with delay time between 2 and 10 μs.Using the present method, we succeeded in fabricating thin (≤1 μm) HAp coatings with high crystallinity and high adhesion strength.     

Effects of femtosecond laser ablation on the surface morphology and microstructure of a bulk TiCuPdZr glass alloy

The effects of femtosecond laser ablation on the surface characteristics and microstructure of a bulk TiCuPdZr glass alloy were investigated. The heat influence zone of femtosecond laser ablated with a laser energy of 100 μJ exhibits a ripple-like feather, while a porous structure appears on the surface of the specimen ablated by a 200 μJ femtosecond laser. The contents of Ti, Zr, and Pd on the ablated surface decrease and that of Cu increases with increasing laser energy. The crystallization process occurs on the glass alloy specimens during femtosecond laser ablation, and the crystallinity of a 100 μJ femtosecond laser-ablated specimen is greater than that of a 200 μJ femtosecond laser-ablated one.     

飞秒激光加工参数对RB-SiC表面形貌的影响

目的 揭示飞秒激光加工参数对反应烧结碳化硅(Reaction-Bounded Silicon Carbide,RB-SiC)表面形貌的影响规律。方法 通过改变激光能量密度和有效脉冲数,研究RB-SiC表面烧蚀槽的形貌变化规律,确定飞秒激光加工RB-SiC的去除机理。采用扫描电镜、共聚焦显微镜、X射线能谱仪和拉曼光谱仪分析RB-SiC烧蚀前后的表面形貌演变行为。结果 激光能量密度在0.62~10.48 J/cm²时,Si富集区域形成凹陷结构,SiC颗粒区域形成周期性结构(Laser-Induced Periodic Surface Structures,LIPSS),周期约为970 nm。随着激光能量密度的增加,凹陷结构扩大加深,表面球形纳米颗粒增多,烧蚀槽宽度呈对数增长。有效脉冲数在69~ 1 379,Si富集区域的去除量高于SiC颗粒区域的去除量。随着有效脉冲数增加,烧蚀槽深度显著加深,凹陷结构扩展成深坑结构,飞溅至烧蚀槽外侧的纳米颗粒聚集成团簇物,由Si、SiC和非晶态SiO2构成的沉积物在烧蚀槽边缘形成堆积层。结论 降低激光能量密度能够减少RB-SiC表面凹陷和纳米颗粒,有助于提升烧蚀形貌的一致性。增加有效脉冲数会促进烧蚀槽底部深坑结构的产生,进而扩大Si与SiC去除量之间的差异。     

A mathematical model of the removal of gold thin film on polymer surface by laser ablation

The removal effects of an Au thin film on polymer sheets by an ultrashort-pulsed Nd:YAG laser ablation was studied. A theoretical model to describe the laser energy required F₀ and optimal scanning velocity ν were presented and afterwards affirmed by experimental data. It is shown that the function of ν to lnF₀ follows a linear relationship; an increase in laser flux F₀ led to an initial rise in ablation rates R_a followed by a decline. Namely there exists a maximum ablation rate or a best removal effect and it corresponds to an ascertainable laser energy and beam scanning velocity. Using the optimized laser processing parameters the Au thin film could be completely removed and satisfactory cleaning was achieved. The analytical method and the results were taken as a guide for accurately determining the laser parameters and proper conditions for the practical utilization of micro processing of the thin film by laser-ablation.     

An investigation of the synthesis of metallic nano-particles by laser ablation

A numerical model is proposed to simulate the deposition process of the metallic particles under laser irradiation, in order to investigate the formation mechanism of the nano-particles for the mass production. The deposition of nano-particles has been effectively made by a Q-switched laser irradiation of nickel substrate. In the analysis of the localized ablation phenomenon, which was induced by a high peak power laser, the deposition of nickel nano-particles with narrow size distribution has been examined. The analysis of fabrication mechanisms has been studied for the process design from the macroscopic to microscopic views and the process characteristics of the laser ablation for the nano-particle deposition has been verified experimentally and numerically.     

Influence of surface laser cleaning combined with substrate preheating on the splat morphology

The morphology of sprayed splats influences the coating adhesion and properties, which are determined by the spraying parameters. Many studies in this field show that the substrate surface temperature is a very relevant factor for the splat shape: the hypotheses of substrate surface wettability and contamination or adsorption layer on the surfaces are supported by the fact that the near-disk-shaped splat can be obtained by increasing the substrate temperature. In this work, a short-duration pulsed laser was used to ablate the substrate just before powder spraying. This ablation was powerful enough to eliminate the contaminants on the substrate surface and to improve the adhesion. In this study the analyses of NiAl splat morphology on the polished TA6V (Ti-6Al-4V) substrate were carried out using laser ablation with different substrate temperatures and different heating modes: the flame and another laser. Results show that the temperature at which the disk-shaped splat can be obtained decreased dramatically by laser ablation. Moreover, laser ablation combined with another laser increased the adhesion strength of the coatings. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

On the relationship between the dynamics of the power density and workpiece surface texture in pulsed laser ablation

The use of pulsed laser ablation to generate controlled 3D micro features in various materials needs to control the effect of superposition of individual laser footprints upon surface geometry. Starting from the actual footprint of a single laser shot, the paper presents a model that enables the prediction of surface texture in pulsed laser ablation; the model is simple, yet effective by considering dynamics process parameters (pulse duration, frequency, scanning speed), allows the use of different beam energy distributions while being independent on the target material. Validation trials performed in Ti6Al4V showed a maximum error in predicting surface texture of 9%.     

A time based method for predicting the workpiece surface micro-topography under pulsed laser ablation

The generation of micro-features in a predictable and repeatable manner by use of pulsed laser ablation requires an understanding of the temporal and energetic distributions of the laser beam upon the workpiece surface. Modelling the response of the material to known energetic and kinematic parameters of the pulsed laser ablation process can be carried out in a discretised time-based approach, allowing the workpiece topography to be simulated mathematically to reflect a real-life process. Considerations of the antecedent workpiece surface texture such as increases in irradiated area due to the surface gradient, and increases in laser spot size due to beam divergence throughout the elevation of the workpiece are used to predict energy densities and hence the resultant ablated depth and texture of the targeted surfaces. A fully calibrated Yb:YAG pulsed fibre laser (SPI G3.0 RM) was used to validate the model on three materials, highlighting the models strengths for different material types. It was found that Ni based workpieces presented redeposition phenomena under these laser ablation conditions. To analyse the model without redeposition, validations trials on materials that do not present such side effects, e.g. diamond, were carried out and differences were found to be up to 9.39%.     

Optimisation of a paint coating ablation process by CO2 TEA laser: Thermal field modelling and real-time monitoring of the process

The ablation of coatings (paint, varnish, oxides, etc.) using a pulsed laser beam is a process that has a number of major benefits compared with conventional cleaning techniques (mechanical and chemical), the main advantage being the possibility of removing the surface layer without any risk of damaging the substrate. Another benefit, and certainly not the least, is the non-use of toxic solvents or chemical products that are difficult to eliminate and, in any case, will be banned from use in the near future. However, it must be stressed that, notwithstanding the above benefits, laser ablation of painted surfaces can only give good quality results if the process is properly controlled. The procedure discussed in this paper has two aims: the first involves establishing a thermal model of the interaction, applied to the ablation of paint from metal surfaces by infra-red laser. The second involves proposing an optimised paint removal process validated by experimental measurements. This second objective will be based on an analysis method of the acoustic signal transmitted by the ablation plasma beam.     

纳米材料的激光制备技术

本文综述了纳米材料的激光制备方法。其中重点介绍了激光消融法制备纳米颗粒、纳米纤维、纳米薄膜的原理和现状。     

Synthesis of single-wall carbon nanotubes by excimer laser ablation

We report the KrF excimer laser ablation of carbonaceous targets in an innovative laser ablation chamber. The targets have been prepared using a new approach, without pressing or hot pressing of the composition. The Co/Ni doped target has yielded single-wall carbon nanotubes with a narrow diameter distribution. High-resolution transmission electron microscopy has been used along with the confocal Raman microscopy to characterize the products obtained. Thermogravimetric analysis confirms the presence of multiple carbonaceous species with different oxidation temperatures.     

激光技术在材料科学中的应用

综述了激光技术在纳米村料制备，表面改性和成形技术等先进制造工艺中的应用和进展。重点介绍了激光消融法制备纳米颗粒、薄膜的原理、特点段激光淬火、激光表面熔敷和激光表面合金化等材料表面改性技术。同时，对先进的激光立体成形技术及其应用现状做了慨述。     

Potential and limitations of laser technology in restoration of metallic objects of art and cultural heritage

Potential applications of laser techniques, such as precision cutting, micro welding, and ablation of stains, are presented. Concerning laser beam ablation, we explain the mechanism, and then discuss potential applications (cleaning of gilded or solid metal surfaces, cleaning of verdigris) and limits (thinning of verdigris, removing rust from verdigris). This research has been performed within the frame of a joint project sponsored by Deutsche Bundesstiftung Umwelt (DBU, German Foundation for the Environment) as well as in immediate co‐operation with restorers. The presentation of these results is to propose laser technology as a promising option to be taken into account in planning and doing restoratory work.