Abstract
Since the recent decades laser pulses have been used for micro material processing. Two regions of energy density have to be distinguished: the one where ablation of the material takes place and the resulting morphology can be observed by direct imaging techniques, and the other one where the fluence domain is characterized by physicochemical modifications of the substrate surface in the condensed state. The aim of this work was the documentation and analysis of the laser-material interaction in respect to resulting physicochemical processes. Methodologically, a double comparative approach is used: between two different pulse durations and between different chemical compositions.
Nanosecond pulses in the ultraviolet wavelength range (t
= 10 ns, l
= 266 nm) and femtosecond pulses in the infrared region (t
= 130 fs, l
= 800 nm) were employed. These varying parameters lead to completely different results, common to all the treated materials. A high-tech glass (barium-alumo-borosilicate glass) and three non-oxide ceramics (aluminium nitride, silicon carbide and a composite of SiC-TiC-TiB2) were used. All of them were not yet systematically investigated in respect to laser micro material processing. Only rare literature data exist for the electrical insulator AlN and the direct semiconductor SiC. The composite compound SiC-TiC-TiB2 was developed and used for tribological applications. The laser treatments were undertaken with direct focussing at air. Surface analytical techniques - such as XPS, Nanoindenter, ESR, µ-Raman, EDX, XRD - were used to identify the chemical changes between untreated and laser-treated areas. Single-pulse irradiation led to material modifications in the condensed state in most instances. Only in a few cases, bubble formation could be detected, too. Multi-pulse results differed depending on the pulse duration. In the nanosecond case, melting of the surface and redeposited material - so called debris - were observed. With femtosecond pulses instead, only negligible melting and few debris could be detected. Additionally, periodic structures appeared - so called ripples.
Laser treatment of binary work pieces exhibited preferential ablation of the lighter component. Crystal structure changes, metallization or oxidation processes of the irradiated surface as a consequence of laser-induced melting and resolidification could be observed. It was shown that for the ablation of inorganic chemical compounds, the pulse duration of the laser radiation had an influence on the physicochemical modification of the materials. The application of pulses with a duration less than a picosecond resulted in morphological and chemical changes of the treated area in a thickness range of several hundreds of nanometres. The nanosecond laser treatment instead yielded a bigger conversion layer of about one order of magnitude compared to the fs case. Technical consequences were discussed. |
