EXCIMER LASER-INDUCED HYDRODYNAMICAL EFFECTS AND SURFACE MODIFICATIONS ON SILICON-CARBIDE

The use of lasers in many applications requires an understanding and c ontrol of the fundamental processes involved during the laser radiatio n-material interaction. The importance and the duration of the phenome na involved (optical, thermal, mechanical, electromagnetic, radiative) depend on parameters such as the power density of the laser radiation , the nature and the surface morphology of the material, the surroundi ng gas and the wavelength of the radiation. A part of the incident ene rgy is reflected by the surface, while the remaining quantity of this energy is absorbed by the material. This incident energy contributes t o heating, melting, vaporization and plasma formation if the laser int ensity is sufficiently high. The present study is devoted to the ceram ic material irradiation in air by an excimer laser at a wavelength of 248 nm and a pulse duration of 20 ns. The objective is to understand t he different phenomena induced by radiation and to improve mechanical properties of the surface. Presented results concern the dynamics of p lasmas and shock waves, plus surface modifications (porosity, roughnes s, composition changes) on silicon carbide samples (SiC). A part of th e study has resulted in the characterization of the flow created in fr ont of the sample revealing different hydrodynamical regimes. Visualiz ations of the luminous plasma front and shock waves have been obtained using a fast electronic camera IMACON 790 and a ICCD camera. These op tical devices have permitted us to observe simultaneously the formatio n and expansion of different fronts (plasma and shock waves) propagati ng into the surrounding gas. The results have been compared to the the ory. Complex structures inside the plume have been observed inducing a turbulence phenomena some milliseconds after the irradiation. On the other hand, the surface morphology has been characterized by observati ons with a scanning electron microscope (SEM) showing important modifi cations of the morphology according to the energy density and number o f pulses applied. A decrease of surface porosity is revealed using ima ge processing technique. This improvement is accompanied with an incre ase of roughness which has been measured by mechanical profilometry an d might be detrimental for friction applications. In addition, an anal ysis of the material in depth by Auger electron spectroscopy (AES) has shown chemical composition changes (an increase in the percentage of silicon) in the zone irradiated.