Study On The Interaction Mechanism Of Laser Surface Treatment Of Materials

The mechanism of laser damage is deduced from the morphological changes on surface of crystal Si, Hg_0.8Cd_0.2Te and high-molecular polymer PTFE. During this process, the SEM and Optical Microscope are used to test the surface morphology. The XPS indicates that Physical property and Chemical structure altered after KrF excimer laser irradiated the surface of PTFE.Theoretical calculations of damage thresholds of Si and Hg_0.8Cd_0.2Te are P0=4.44×10⁸ W·cm^-2 and P0=9.48×10⁶ W·cm^-2, respectively, when laser beam ablates materials with wavelength 10⁶4 nm at atmospheric pressure, while the experimental values are 1.82×10⁸ W·cm^-2 and 8.42×10⁶ W·cm^-2. The order of the theoretical value is the same as the experimental. And it can be the reference for our experiments for the future.The theoretical explanation to crystal damage is given in detail. We also compare the differences between Si and Hg_0.8Cd_0.2Te when the two materials are radiated by different lights. The common phenomena are melting and vaporizing. And the common mechanism of laser damage is the presences of three basic kinds of forces: the thermal stress which is caused by the existence of a temperature gradient, shock pressure of a laser supported detonation wave (LSDW) and the recoil pressure of the vaporizing wave. Then the material surface can be cracked by these forces. When Si and Hg_0.8Cd_0.2Te are irradiated by the same excimer laser, the differences on configuration are determined not only by the Physical characters of materials (forbidden band, absorption coefficient, thermal diffusivity, melting point, latent heat of fusion, et al.), but by the nature of laser. For the same material Hg_0.8Cd_0.2Te, the fusion effect which is produced by laser with wavelength 10⁶4 nm is clearer than that with 248 nm. The reason is that evaporation and vaporization effects are stronger when Hg_0.8Cd_0.2Te is being irradiated by laser with 248 nm. No less than Hg_0.8Cd_0.2Te, the surface of Si also alters according to the kind of laser. The results indicate that the morphological changes on surface are mostly owed to the distinct absorption coefficient for different lights. Laser-induced periodic structures have been observed on the surface under ultraviolet radiation as a result of melting subsequent solidification. But there is no crack clearly. So the mechanical damage is the major reason for 1064 nm laser ablation on Si. While for 248nm laser ablation, the thermal effect is much clearer.The differences of surface morphology between Si and PTFE are discussed when the surfaces are ablated by an excimer laser. The former is photothermal effect, the later is photochemical effect. One is semiconductor and the other is nonconductor. The different characters are keys. There are different binding energy of chemical bond, different melting point and absorption coefficient. Even there is higher latent heat of fusion for crystal Si.Finally, we analyze the element changes by means of XPS when PTFE is treated by excimer laser. The surface of PTFE irradiated is carbonized and the surface roughness increases. XPS indicates that the content of F reduces greatly after laser irradiation, and the oxygen function group is found on PTFE surface because of the participation of the background gas. These can bring the polarization of molecules. That will do good to further graft copolymer and reaction with some chemical reagent.As a fundamental research, we study the mechanism of laser ablation on materials Si, Hg_0.8Cd_0.2Te and PTFE. The theories of sputtering and laser damage threshold are reinforced in this paper. Meanwhile, the factors contributing to the crystal model change are the thermal stress, shock pressure of LSDW and the recoil pressure of the vaporizing wave. Valuable conclusion can help to study the interaction between laser and materials for future.