L. Lavisse

Laser plasma plume structure and dynamics in the ambient air: The early stage of expansion

Laser ablation plasma plume expanding into the ambient atmosphere may be an efficient way to produce nanoparticles. From that reason it would be interesting to study the properties of these laser induced plasmas formed under conditions that are known to be favorable for nanoparticles production. In general, plume behavior can be described as a two-stage process: a “violent” plume expansion due to the absorption of the laser beam energy (during the laser pulse) followed by a fast adiabatic expansion in the ambient gas (after the end of the laser pulse). Plasma plume may last a few microseconds and may have densities 10−6 times lower than the solid densities at temperatures close to the ambient temperature. Expansion of the plasma plume induced by the impact of a nanosecond laser beam (λ = 1064 nm) on the surface of metallic samples in the open air has been investigated by means of fast photography. Spatio-temporal evolution of the plume at the early stage of its expansion (first 330 ns) has been recorded. ...

Influence of laser–target interaction regime on composition and properties of surface layers grown by laser treatment of Ti plates

Surface laser treatment of commercially pure titanium plates was performed in air using two different Nd : YAG sources delivering pulses of 5 and 35 ns. The laser fluence conditions were set to obtain with each source either yellow or blue surface layers. Nuclear reaction analysis (NRA) was used to quantify the amount of light elements in the formed layers. Titanium oxinitrides, containing different amounts of oxygen and nitrogen, were mainly found, except in the case of long pulses and high laser fluence, which led to the growth of titanium dioxide. The structure of the layers was studied by x-ray diffraction and Raman spectroscopy. In addition, reflectance spectra showed the transition from a metal-like behaviour to an insulating TiO2-like behaviour as a function of the treatment conditions.Modelling of the laser–target interaction on the basis of the Semak model was performed to understand the different compositions and properties of the layers. Numerical calculations showed that vaporization dominates in the case of short pulses, whereas a liquid-ablation regime is achieved in the case of 35 ns long pulses.

Thermal Effects of Continuous Laser Treatment on Ti-6 Al(wt%)-4V(wt%) Titanium Alloy

A titanium alloy Ti-6 Al(wt%)-4V(wt%) was treated in air by Nd:YAG laser radiation (wavelength of 1.064 %m) in continuous mode. Targets were irradiated globally with different levels of energy (accumulated fluence) at constant power. Different focal lengths and beam displacement velocities were used. Cross section microstructural observations were carried out by scanning electron microscopy. Backscattered electron imaging reveals microstructural modifications in samples. A structural phase transformation of beta (bcc) to alpha prime (hcp) phase was observed. The depth of the transformed zone observed by phase transformation is dependent on the treatment parameters. Conformity between microstructural observations and the treatment parameters is discussed with reference to thermal simulations.

A Thermo-Kinetic Study of Titanium Oxidation Assisted by a Nd-YAG Pulsed Laser

The nitriding or the carburation of a titanium substrate assisted by a laser source has been extensively studied, but these have been fewer investigations of laser induced oxidation phenomena. Taking account of the problem complexity due to the large number of possible oxidation stages, we have elaborated and carefully characterized the oxidized layers. Physico-chemical analysis (Auger and XPS), phase identification (XRD in θ 2 θ and grazing mode), and morphological observations (SEM) have been performed. Elementary analyses using microprobe (EDS) and elementary nuclear analysis (NRA) have been undertaken to precisely determine the oxygen diffusion profile. Explanations of the formation of the oxidized compounds are suggested by reference to the Ti – O diagram. The thermal cycles undergone by the titanium substrate during the laser treatment have also been simulated to help clarify the role of the cooling rate as well as the repetition rate on the presence of some metastable phases. The aim of this study is the establishment of phase/structure mapping. This would allow a predictive analysis of the phases to be formed as a function of the laser treatment parameters and the reactive atmosphere conditions.