Characterization of the behavior of ceramic materials submitted to UV laser radiation: applications to surface treatment

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Abstract
Due to their fabrication process, the ceramic materials usually exhibit a granulary surface. That leads to a non negligible roughness and porosity, in so far as the intergranular interstices could be regarded as pores. On the other hand, the hardness of such materials makes the processing (polishing for instance) somewhat difficult. The purpose of this work is the surface modification of different ceramic samples using UV laser radiations in order to improve the tribological properties of the workpieces. Experiments have been performed on alumina (Al2O3), aluminum nitride (AIN) and silicon carbide (SiC) in order to determine the influence of different parameters on the roughness and porosity of their surface. Three laser wavelengths (193 nm, 248 nm and 308 nm), a range of energy density up to 8 J/cm2, a pulse duration of 25 ns and different repetition rates (1 - 200 Hz) were the experimental parameters. Results concerning roughness were analyzed using a mechanical profilometer. The photographs of treated surfaces obtained by scanning electron microscopy have been numerized; the porosity of the material has been measured by image processing. For instance, in case of alumina, we obtain a 90 surf.% decrease of porosity after laser treatment (2 J/cm2). At such a fluence, the interstices (which typical width is about 1 micrometers ) are no more visible, resulting in a minimal porosity in spite of the appearance of numerous circular pores and microcracks. A good choice of laser parameters allows us to obtain a significant improvement of the surface: the granular structure and microcracks no longer appear. Absorptivity was determined at 248 nm using a thermocouple.

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