Study of the fluence dependent interplay between laser induced material removal mechanisms in metals: Vaporization, melt displacement and melt ejection

Three quantitative methods, namely profilometry, high speed imaging and recoil momentum measurements using a ballistic pendulum, are used to determine the interplay of vaporization, melt displacement and melt ejection on nanosecond laser induced material removal. At low to moderate fluences (<7 J cm⁻²) material removal occurs via vaporization and melt displacement in aluminium. At high fluences (>7 J cm⁻²), material removal occurs predominantly via the explosive ejection of liquid droplets from the melt pool.     

Li2O-2B2O3熔体的物性研究

系统测量了四硼酸锂(Li₂Ｏ-２Ｂ_２Ｏ_３)熔体的密度ρ、表面张力γ随着温度的变化规律，实验结果表明在1100K到1500K范围内Li₂Ｏ-２Ｂ_２Ｏ_３高温熔体的密度和表面张力随着温度的升高均线性减小.通过实验数据拟合得出熔体密度与温度关系为ρ(T)=2.574—4.89×１０^－４T，熔点处Li₂Ｏ-２Ｂ_２Ｏ关键词： 功能晶体 四硼酸锂 密度 表面张力     

片状铌酸锂单晶的生长及其特性

采用润湿导模技术生长出大块的片状铌酸锂单晶,尺寸可达200mm长、20mm宽、3mm厚。经测试,性能与柱状单晶相同。为了做好此项工作,我们测定了铌酸锂熔体的密度ρ_m与表面张力α等物理常数,得出在1270℃时ρ_m=3.57×10³kg/m³,α=204dyne/cm;在1300℃时ρ_m=3.42×10³kg/m³,α=192dyne/cm。 关键词：     

Nanosecond laser ablation for pulsed laser deposition of yttria

A thermal model to describe high-power nanosecond pulsed laser ablation of yttria (Y₂O₃) has been developed. This model simulates ablation of material occurring primarily through vaporization and also accounts for attenuation of the incident laser beam in the evolving vapor plume. Theoretical estimates of process features such as time evolution of target temperature distribution, melt depth and ablation rate and their dependence on laser parameters particularly for laser fluences in the range of 6 to 30 J/cm² are investigated. Calculated maximum surface temperatures when compared with the estimated critical temperature for yttria indicate absence of explosive boiling at typical laser fluxes of 10 to 30 J/cm². Material ejection in large fragments associated with explosive boiling of the target needs to be avoided when depositing thin films via the pulsed laser deposition (PLD) technique as it leads to coatings with high residual porosity and poor compaction restricting the protective quality of such corrosion-resistant yttria coatings. Our model calculations facilitate proper selection of laser parameters to be employed for deposition of PLD yttria corrosion-resistive coatings. Such coatings have been found to be highly effective in handling and containment of liquid uranium.     

Mass ejection from shock-loaded free surfaces of steel

Abstract

The mass ejection from shock-loaded free surfaces of steel specimens under several condition of machining were measured using the piezoelectric quartz crystal gauge technique. At shock pressure of 45 GPa, the ejected mass quantities of steel with respect to the surface smooth finishes 0.4 μm (machinework) and 0.11 μm (vibration- roll compression) are 0.2 g/m² and 0.15 g/m², respectively. The ejected mass quantities of steel with respect to the surface smooth finishes 0.2 μm (polishing) and 0.1 μm (abrasiveness) are very small. Experiments show that the mass ejection is dependent on conditions of free surfaces. Surfaces defects, such as pits, scratches and machine marks are thought to be the primary reason for mass ejection.     

液态结构与性质关系Ⅲ——剩余键理论模型

利用剩余键的思想, 通过数学推导, 建立定量描述金属熔体结构与黏度关系的物理模型, 提出黏度的微观结构变化特性为熔体剩余键结构尺寸d的演变; 应用建立的模型理论计算液相线以上一定温区镁熔体和铝熔体的运动黏度, 得到函数关系式分别为v_Mg=3.17×10^-7+3.04×10^-7· d和v_Al=1.65×10^-7+1.05×10^-7·d, 这与采用坩埚扭摆振动法的实验测量结果相符合. 该模型从化学结合键角度揭 示了金属熔体的结构微观不均匀性及其黏度的微观物理本质, 为金属熔体黏度的理论计算提供了一种新的途径. 这对于深入认识液态金属的微观结构及其与宏观物性之间的关系具有重要意义.     

Topographical and surface chemical characterization of nanosecond pulsed-laser micromachining of titanium at 532-nm wavelength

Electropolished titanium was micromachined by single, 5-ns pulses from a frequency-doubled (532 nm) Nd:YAG laser. The focal spot size was varied from 10 to 100 m and the applied fluences varied from the melting threshold (1 J/cm²) to more than 100 J/cm². The resulting craters were imaged by optical microscopy, topographically characterized by interferometry and chemically characterized on the surface by small-spot depth-profiling Auger electron spectroscopy and small-spot X-ray photoelectron spectroscopy. The prevailing ablation regime for the studied fluences and focal spot sizes was found to be melt ejection. The surface chemical characterization showed growing oxide thickness in the heat-affected zone (around the center spot) for increasing fluence but no difference inside the crater. Titanium nitride formation was found inside the crater. PACS 81.65; 87.80Mj     

Study of the mechanism of the negative secondary ionic emission by selective superficial oxidation and adsorption

With the aid of a speed analyser mass spectrograph able to function in an ultra vacuum, it was shown in previous work by conducting a study of the speed of ejection of secondary ions O^? emitted under the impact of ions K⁺ by copper target, that under certain experimental conditions at least two different origins of this emission could be demonstrated. The hypothesis that we thus adopted was that the O^? ions having the greatest ejection speed could originate from the layer adsorbed on the surface, whereas the O^? ions having the weakest ejection speed could originate from the oxide. In order to confirm this hypothesis, we conducted the following experiments.We oxidized a copper target with a natural isotope ¹⁸₈O of oxygen. The spectrogram of the negative secondary emission involved then, in addition to the ions of the adsorbed layers (H^?, C^?, CH^?, ¹⁶₈O^?, ¹⁶₈O^?,...) the ion ¹⁶₈OH^?, which could only originate from a chemical compound formed at the time of the reaction of the oxidation.In a second series of experiments, we introduced molecular oxygen ¹⁸₈O₂ into the system, beginning with an initial pressure of 10^?8 torr up to a pressure of 10^?5 torr. The mass spectrogram again contained, over and above the aforementioned ions, the ion ¹⁸₈O^?. Here this ion could only originate from the adsorbed layers, since the experimental conditions favored the chemical and physical adsorptions.In addition, the energy analysis of the ions ¹⁸₈O^? obtained in the two instances allows us to say that the ion O^? originating from the rupture of the chemical junction Cu₂O has a weaker speed of ejection than the ion O^? originating from the adsorbed layer of gas. In conclusion, the experiment shows that the energy transfer from the incident particle to the ejected particle, by means of the atoms in the target, leads to a speed of ejection which is discrete but weak, in the case of the particle originating from the rupture of a chemical junction; whereas in the case of particles originating from the adsorbed layers of gas, the speeds of ejection are greater and have a widespread distribution.     

Classical trajectories studies of DIET from calcium fluoride

We investigate desorption of positive ions resulting from the repulsive environment created by core-hole Auger decay from relaxed CaF₂ surfaces. The molecular dynamics simulations in the lamina geometry (with two-dimensional ion-lattice summation) is used. For both (011) and (111) surfaces the simulation with changed charge without providing additional kinetic energy does not lead to the ejection of F⁺ ion due to the lattice rearrangement and trapping of the ion. We also assume that the positive ion gains a substantial amount of kinetic energy at the onset of simulations, crudely mimicking ion-stimulated desorption. For the (011) surface the results are extremely sensitive to the size of the considered system, in sharp contrast to the ejection of positive ions from alkali halides. For a 384 ion system, ejection occurs if the kinetic energy, equal to 0.25 eV or more, is delivered to the F⁺ ion at the start of the simulation. For a 768 ion system ejection occurs only for the initial kinetic energy of 4 eV. This result is probably caused by inadequate classical potential and lack of full convergence of the two-dimensional Ewald summation scheme for a highly disordered system. For the (111) surface with 1536 ions in the cell, ejection occurs for an initial kinetic energy of 0.4 eV.     

On the control of microstructure in rapidly solidified Nd–Fe–B alloys through melt treatment

Rapidly solidified (RS) Nd₂Fe₁₄B alloys were prepared by melt-spinning under different melt treatment conditions i.e., the melt temperature was varied prior to ejection onto the quenching wheel. X-ray diffraction, transmission electron microscopy, thermal analysis and magnetic measurement were conducted on the as-quenched alloys to investigate their structural and magnetic characteristics. RS Nd₂Fe₁₄B alloys may display a variety of microstructures depending upon the thermal history of the melt before ejection: it was possible to synthesize an entirely amorphous structure, a partially amorphous structure containing nuclei and/or nanophases and a nanocrystalline structure. The relationship between the formation of crystalline nuclei or nanophases and the thermal history of the melt was studied. A lower melt ejection temperature produced a nanocrystalline microstructure, while higher melt ejection temperatures (T>1723 K) largely eliminated the presence of nuclei and associated nanophases and produced an amorphous product. The experimental results indicated that optimization of the melt treatment conditions will produce rapidly solidified Nd–Fe–B alloys with a more uniform microstructure.     

Ni熔体凝固过程中临界晶核和亚临界晶核的分子动力学模拟

本文用分子动力学模拟研究了Ni熔体以不同冷速凝固后微观结构的演变规律, 并通过理论计算确定出了Ni熔体凝固后获得理想非晶的临界条件. 模拟结果发现冷速小于10¹¹ K/s时, Ni 熔体凝固后形成晶态组织; 冷速在10¹¹ K/s到10^14.5 K/s之间时, Ni熔体凝固后形成由晶态结构与非晶态结构组成的混合组织. 冷速小于10¹⁰ K/s, Ni 熔体凝固后形成的晶态组织具有fcc结构; 冷速在10¹⁰ K/s到10^14.5 K/s之间时, Ni熔体凝固后组织中的晶态由fcc和hcp结构层状镶嵌排列构成. 通过分析模拟结果和计算结果, 确定出了Ni熔体凝固后形成理想非晶的临界冷速为10^14.5 K/s. 并发现Ni熔体中临界晶核(冷速等于10^14.5 K/s)和亚临界晶核(冷速大于10^14.5 K/s) 均由fcc和hcp组成层状偏聚结构, 这表明Ni熔体中生长的晶体、临界晶核和晶胚的结构是相同的. 关键词： 分子动力学模拟 晶体团簇 临界冷速 结构     

Thermophysical properties of Ni-5%Sn alloy melt

The surface tension and specific heat of Ni-5%Sn alloy melt were measured by the oscillating drop method and the drop calorimetric method using electromagnetic levitation, respectively. The temperature coefficient of surface tension is 6.43×10⁻⁴ N·m⁻¹K⁻¹ within the temperature regime of 1464–1931 K. The enthalpy change was measured in the temperature range from 1461 to 1986 K, and the average specific heat was obtained as 43.03 J·mol⁻¹K⁻¹. Some other thermophysical properties, such as viscosity, solute diffusion coefficient, density, thermal diffusivity and thermal conductivity of this alloy melt, were derived based on the experimentally measured surface tension and specific heat. Using these thermophysical parameters, the relation between solute trapping and undercooling in rapidly solidified α-Ni was calculated, and the theoretical prediction shows a good agreement with experimental data.     

A New Model of Crater Formation by Arc Spots

The forces acting on the cathode arc spot surface and removing the molten layer from the crater bottom are composed mainly of the ion pressure, the neutral gas pressure and the evaporation recoil whilst electrostatic forces diminish the effective pressure that is in the order of some 10⁹ dyn/cm². The motion of the liquid layer caused by these forces is treated with the hydrodynamic equations. A simple solution exists in the special case of constant layer depth, that is achieved a few nanoseconds after spot formation. From this model the layer depths (some 0.1 μm) and the ejection velocities at the crater rims (few 10⁴ cm/s) are calculated. The real spot velocity agrees with the velocity of the melting front below the spot surface, but because of the stochastic character of the spot motion the apparent velocity decreases with growing observation time intervals Δt according to Δt^?1/2.     

Effect of pulses from a high-power ytterbium fiber laser on a material with a nonuniform refractive index. I. Irradiation of yttrium oxide targets

The irradiation of Nd:Y₂O₃ targets with an absorption coefficient of 13–1.7 × 10³ cm^?1 using laser pulses with a duration of 0.1–3.5 ms and peak power of 200–700 W at a power density of (0.2–1.3) × 10⁶ W/cm² is studied. A relatively large spread of the delay times of laser plume, spike emission of the laser plume, cleavage of the front surface of the target, and greater ejection of substance from the crater in comparison with the effect of the CO₂-laser radiation with almost the same power are demonstrated. A numerical model of the effect of radiation on a target with a nonuniform refractive index is proposed to interpret the destruction of dielectric material (cleavage of the front surface) and the large spread of the delay times of the plume.     

Randomisation of crystal structure within energetic collision cascades revealed by the sputtering of Au single crystals during ion bombardment

The ratio of random to preferential atomic ejection which occurs from the surface of a gold single crystal during ion bombardment has been found to be very sensitive to the ion mass. For instance, in the case of 50 keV Ne⁺ ion bombardment preferential ejection dominates, whereas in the case of 50 keV Au⁺ ion bombardment preferential ejection is only a small fraction of the total. These results are interpreted in terms of a dynamic randomisation of the crystal lattice which occurs during the creation of energetic atomic collision cascades. The disorder occurs so rapidly that a substantial fraction of atoms sputtered from the surface are ejected from an essentially random structure. It is thought that such gross disorder is only a transient phenomenon, and leaves no significant permanent effects, except perhaps for a few clustered defects.     

Shear‐Enhanced Nucleation of Isotactic Polypropylene in Limited Space

The nucleation rate was measured by directly counting the number of nuclei, which were developed while an isotactic polypropylene melt was flowing under shear in a thin film. The nucleation rate was enhanced with an increased rate of shear, e.g., by a factor of 10 larger at the rate of shear of 14 s^?1 compared with the quiescent state, at 134°C. The ratio of the shear‐enhanced nucleation rate to the nucleation rate in the quiescent state was larger at a higher temperature of crystallization, i.e., about 10 times at 134°C to 590 times at 140°C. The increase of the nucleation rate under shear flow was explained by a reduction of the lateral and end (fold) surface free energies; the product σ _s ² σ _e decreased to 3.2×10^?7 for the sheared melt, from 6.0×10^?7 (J m^?2)³ for the isotropic state. The free energy reduction was caused by transition of the nucleus formation mode from three‐dimensional folded chain nuclei to two‐dimensional bundle nuclei, in which chains lie down on the glass substrate, aligning parallel to the flow direction.     

Microstructure and phase composition of a zirconium coating-silicon substrate system treated using high-current electron beams

The results of structural-phase conversions in a zirconium coating-silicon substrate system treated with low-energy high-current electron beams (LHEBs) are presented. It is revealed that the action of LHEBs with energy densities of 8–10 J/cm² leads to the formation of a eutectic layer containing clusters with a characteristic size of 40–50 nm (the melt of ZrSi₂ and silicon) at the zirconium-silicon interface. After the energy density reaches 12 J/cm², silicon dendrites and regions of silicide crystallites are formed in the surface layer 20–30 μm thick. Data on the characteristic sizes of dendrites and eutectic clusters have made it possible to perform numerical estimation of the surface-melt overcooling (12–25 K), the temperature gradients (3.7 × 10⁷?1.6 × 10⁸ K/m), and the interphase boundary velocity (0.8?3.2 × 10^?4 m/s).     

Polytherms of the Wetting Angles of Pb-Ni (0.3 at %) Melt on Nickel Substrates

Polytherms of the wetting angle of Pb-Ni (0.3 at %) melt on pressed nickel substrates and substrates cut from a plate of nickel of grade NP-2 are studied in the temperature range from the melting point to 850°C by the sessile drop method in a vacuum chamber with a residual pressure of 10^-2 Pa. The wettability thresholds are found. The wetting of both pressed substrates and NP-2 nickel begins at a temperature of 500°C or higher. After crystallization, the morphology of the droplet surface and the zones near it is studied using scanning electron microscopy. Pb _n Ni _m intermetallides of pyramidal form are detected, and zones where the melt spreads along the grain boundaries followed by the crystallization and formation of fibrous structures are found.     

InSb quantum dots and quantum rings on InAs-rich surface

We report a study of InSb nanoobjects (quantum dots and quantum rings) grown on InAs-rich surface by liquid phase epitaxy. Characterization of the sample surface was performed using atomic force microscopy (AFM). The bimodal formation of the uncapped InSb quantum dots (QDs) was observed for the growing on a binary InAs substrate. Uniform high-density (1 × 10¹⁰ cm⁻²) quantum dots with a height of 3 nm were obtained at T = 420-430 °C, whereas low-density (5 × 10⁸ cm⁻²) big quantum dots were 9 nm in height. As a buffer layer, lattice-matched InAsSb_0.12P_0.25 solid solution was deposed on InAs substrate using metal-organic vapour phase epitaxy. Deposition from the InSb melt on the buffer layer resulted in the formation of InSb nanoobjects with density as high as 3 × 10¹⁰ cm⁻².     

Nanospallation induced by an ultrashort laser pulse

A femtosecond laser pulse with power density of 10¹³ to 10¹⁴ W/cm² incident on a metal target causes ablation and ejection of the surface layer. The ejected laser plume has a complicated structure. At the leading front of the plume, there is a spall layer where the material is in a molten state. The spall layer is a remarkable part of the plume in that the liquid-phase density does not decrease with time elapsed. This paper reports theoretical and experimental studies of the formation, structure, and ejection of the laser plume. The results of molecular dynamics simulations and a theoretical survey of plume structure based on these results are presented. It is shown that the plume has no spall layer when the pulse fluence exceeds an evaporation threshold F _ev. As the fluence increases from the ablation threshold F _a to F _ev, the spall-layer thickness for gold decreases from 100 nm to a few lattice constants. Experimental results support theoretical calculations. Microinterferometry combined with a pump-probe technique is used to obtain new quantitative data on spallation dynamics for gold. The ablation threshold is evaluated, the characteristic crater shape and depth are determined, and the evaporation threshold is estimated.