Michel L. Autric

MONTE CARLO SIMULATION OF PULSED LASER ABLATION FROM TWO-COMPONENT TARGET INTO DILUTED AMBIENT GAS

Laser ablation from a binary target into a diluted gas background is studied by means of a Monte Carlo simulation. The influence of the ambient gas on the spatial and mean energy distribution of particles deposited at the distant detector is considered. Thermalization of the particles, the random scattering effect and the backscattering of particles were observed. Considerable modification of the deposited film thickness profiles due to collisions of the ablated particles with the ambient gas is shown. The increase of the ambient gas pressure was found to affect the stoichiometry distribution of deposited and backscattered particles. The study is of a particular interest for the development of the thin film growing technique known as pulsed laser deposition.

Optical spectroscopy of emission from Si–SiOx nanoclusters formed by laser ablation

Abstract This paper reports an experimental study of optical emission from Si—SiOx nanoclusters in a laser-induced plasma plume produced by conventional laser ablation techniques. Emission spectroscopy and optical time-of-flight measurements (TOF) were carried out during reactive ablation of Si targets in He, Ar or O2 atmosphere. Several broad emission bands detected and identified as emission from Si nanoclusters formed at early stage of laser induced plasma expansion. The bands observed show a good spectral correlation with photoluminescence bands of Si—SiOx nanoclusters thin films and Si nanoclusters absorption bands. Analysis of plume dynamics showed considerably slower translational velocities and broader energy distribution of nanoparticles than those of neutrals and ions. From TOF measurements large spectral band widths can be explained by superposition of radiation from the clusters of different size. Two fluxes of clusters, forward and reversed, found within the plasma plume. Their spatial behavior and pressure dependence can be explained by the character of their motion, which is determined by complex hydrodynamics of plume expansion.

Numerical study of the role of a background gas and system geometry in pulsed laser deposition

The transport of laser ablated particles through a Maxwell-distributed ambient gas is simulated by Monte Carlo method. Three system geometry configurations frequently appearing in laser ablation experiments are considered: plume tilting, use of an interacting gas jet, and deposition on a substrate placed perpendicular to the laser-irradiated surface. The influence of the ambient gas on the formation of film thickness profiles and kinetic energy distributions of the deposited particles is studied. The thermalization of the laser plume and the backscattering of the ablated particles due to collisions with the background gas are investigated from two-dimensional film thickness distributions.

Monte Carlo simulation study of the effects of nonequilibrium chemical reactions during pulsed laser desorption

Monte Carlo simulation is used to study the role of chemical reactions in the gas flow of particles laser desorbed from the target into a vacuum. The influence of recombination and dissociation processes on the properties of the gas flow is considered. It was found that chemical reactions have a significant effect on the composition of the desorption jet and on the angular and mean energy distributions of the desorbed particles. The study of these phenomena is of a particular interest for the understanding of the process of thin film deposition by pulsed laser ablation.

Laser Welding of AZ91 and WE43 Magnesium Alloys for Automotive and Aerospace Industries

Magnesium alloys have interesting specific characteristics including the best strength/weight ratio of all commercial alloys and makes it especially interesting for the automobile and aerospace industry. The machining of magnesium alloys requires processes with high energy density and shielding gases that effectively protect the metal from the action of oxygen. Therefore, the laser beam may be a favorable machining tool for this material. In this communication, the authors present experimental results for CO 2 laser welding of AZ91 for the automotive industry and WE43 for the aerospace industry.

Laser induced surface modifications on ZrO2 ceramics

Zirconia is widely used as a solid electrolyte due to its high ionic conductivity and as a biocompatible material, and tailoring its properties is of great importance. In the present paper, experimental results are presented on modifications of surface properties of ZrO2 ceramics doped with 3% of Y2O3 exposed to radiation of either an excimer KrF (λ=248 nm) or ArF laser (λ=248 nm), pulse duration of 15–20 ns in air. The modification of porosity of irradiated areas and its diffuse reflectance spectra (DRS) are studied as a function of laser beam parameters. The morphology of laser treated zirconia ceramics along with its microhardness and roughness are shown to depend significantly on the laser fluence. The catalytic activity of laser treated zirconia ceramics towards the electroless deposition of Cu and Ni is analyzed with respect to DRS modifications.

Nonstationary effects in pulsed laser ablation

Monte Carlo simulation is used to study the influence of the time evolution of the target temperature on the characteristics of the laser-ablated flow. The time dependencies of the flow parameters at the border of the Knudsen layer are compared with these at the target. Moreover, the mean number of collisions and the backscattered flow are obtained for different desorption fluxes. The effect of the time evolution of the desorption flux on time-of-flight (TOF) distributions is considered, and a possible explanation of the inconsistencies that frequently appear when TOF is fitted by Maxwell–Boltzmann distribution is proposed.

Crossed Beam Pulsed Laser Deposition of cryolite thin films

To deposit cryolite thin films with a composition close to the correct stoichiometry on a silicon substrate, a Direst Pulsed Laser Deposition (DPLD) set-up and a Crossed Beam Pulsed Laser Deposition (CBPLD) set-up have been used. In the case of CBPLD two targets are simultaneously ablated and the two ablation plumes are crossing at 15 min from the targets. With the two different set-up thin films present a composition close to the correct stoichiometry (Na3AlF6). The surface of thin films deposited by DPLD shows a very high droplets density. At the opposite the CBPLD allows to decrease dramatically this density. Furthermore fast CCD-photographs of the plasmas have been used to study plumes expansion with the two different set-up. With the CBPLD one, the two plasmas interaction has been studied and their expansions have been compared with the one observed when only one target is ablated. The trajectories redistribution of the ablated species due to the collision of the two plasma clouds is clearly visible

Modeling and diagnostics of pulsed laser-solid interactions: applications to laser cleaning

The present study concerns the cleaning of materials using pulsed laser irradiation and it summarizes the most recent results obtained by the collaborative research of different European groups, within the framework of a European program for training and mobility of researchers. A series of pulsed lasers, which emit at various wavelengths (from UV to IR) with short duration of pulse (few nano-, pico- or femto- seconds), is used for the removal of metallic, ceramic and organic pollutants from contaminated solid surfaces of different natures. The scientific results obtained so far are focused on the laser cleaning of silicon wafers from sub-micrometer particles, the theoretical modeling of particles removal mechanism during dry laser cleaning, the removal of oxide layers from oxidized metals and alloys, as well as on the development of laser imaging as a diagnostic tool for the estimation of the efficiency of the proposed cleaning technique.

CO2 laser welding of magnesium alloys

Metallic alloys with a low mass density can be considered to be basic materials in aeronautic and automotive industry. Magnesium alloys have better properties than aluminum alloys in respect of their low density and high resistance to traction. The main problems of magnesium alloy welding are the inflammability, the crack formation and the appearance of porosity during the solidification. The laser tool is efficient to overcome the difficulties of manufacturing by conventional processing. Besides, the laser processing mainly using shielding gases allows an effective protection of the metal against the action of oxygen and a small heat affected zone. In this paper, we present experimental results about 5 kW CO2 laser welding of 4 mm-thick magnesium alloy plates provided by Eurocopter France. The focused laser beam has about 0.15 mm of diameter. We have investigated the following sample: WE43, alloy recommended in aeronautic and space applications, is constituted with Mg, Y, Zr, rare earth. More ductile, it can be used at high temperatures until 250 degrees Celsius for times longer than 5000 hours without effects on its mechanical properties. A sample of RZ5 (French Norm: GZ4TR, United States Norm ZE41) is composed of Mg, Zn, Zr, La, rare earth. This alloy has excellent properties of foundry and it allows to the realization of components with complex form. Also, it has a good resistance and important properties of tightness. The parameters of the process were optimized in the following fields: laser power: 2 to 5 kW, welding speed: 1 to 4.5 m/min, focal position: -3 mm to +3 mm below or on the top of the metal surface, shielding gas: helium with a flow of 10 to 60 l/min at 4 bars. Metallurgical analyses and mechanical control are made (macroscopic structure, microscopic structure, interpretations of the structures and localization of possible defects, analyse phases, chemical composition, hardness, tensile test etc.) to understand the parameters influence of welding on the obtained beads. For a given laser power, we considered that the welding speed as well as the focal position strongly influence the macroscopic and microscopic welding aspect, whereas the dependence with the flow of the protection gas is weak. For WE43, the bead appears correct in the macroscopic scale for a laser power of 2 kW, a speed of 2 m/min, a focal position on the metal surface or 1 mm under; and an output helium gas of 50 l/min. For RZ5, a correct weld is obtained with a 3 kW laser power, a welding speed of 2 m/min, a focal position of 1.5 mm under the surface and a 50 l/min output helium gas. The microscopic examination showed that the size of the grains has clearly reduced (reduction factor can be up to 35) without formation of porosities, neither cracks nor inclusions; indeed the measured Vickers microhardness of the weld bead is slightly higher than the basic metal. Experiments show that we obtained adequate parameters for high quality welding without using filler material. In future, we plan to weld at higher speed by optimizing the various parameters of the laser welding (power, focal position welding speed and gas flow, ...). Furthermore, we will try to weld samples with a thickness superior than 4 mm.