Laser induced deformation and structural modification of crystalline and amorphous materials.

摘要

The micro scale laser shock peening process (muLSP) in the study presented herein is limited to the surface processing of metallic components, while the structural modification is utilized for changing the properties of the interiors of amorphous transparent dielectrics. However, both processes are analyzed within the framework of laser processing with an emphasis on the laser induced changes in material properties. The work presented in this thesis seeks to investigate the change in these properties through analytical, numerical and experimental studies. The effects of anisotropy and heterogeneity on laser processing are the subject of the investigation of micro scale laser shock peening whereas the factors that influence the morphology of the laser induced features and change of structure are investigated for the laser treatment of transparent dielectrics.;For micron-sized laser beams, the size of the laser-target interaction zone is of the same order of magnitude as the grains of the target material. Thus the target material must be considered as being anisotropic and inhomogeneous. The analytic, numerical and experimental aspects of laser shock peening on two different aluminum single crystal surfaces, (110) and (114) are studied. The analytical solutions for the stress and deformation fields are derived for the (110) case utilizing anisotropic slip line theory and compared against the (114) case and numerical results. A finite element model which considers inertial and strain hardening effects is developed to investigate the dynamic response of materials during the muLSP.;In addition to the effects of anisotropy in muLSP, at the micro scale, heterogeneity is studied as well. Experimental and numerical studies have been performed in order to investigate the response of the grain boundary under muLSP. For this purpose aluminum bicrystals were studied. The orientations of the crystals in the bicrystal have been chosen such that an approximate plane strain condition is achieved. This enables a meaningful comparison between experimental findings and a developed finite element model which accounts for anisotropy, heterogeneity and inertia.;Non-linear absorption of femtosecond laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. Features were generated in the interior of glass samples through the employment of single pulses as well as pulse trains. The illumination point spread function was utilized for the first time to quantify the morphology of the produced features. It is also experimentally shown that, even with a lower numerical aperture, cavities can be formed in the interior of glass if a single laser pulse with energy in the micro joule range is deposited into the material. Cross-sections of the induced features were examined via the spatially resolved Raman spectra and a new method for the quantitative characterization of the structure of the fused silica was developed. The proposed method identifies the volume fraction distribution of ring structures within the continuous random network of the probed volume of target material.