| Abstract [eng] |
This dissertation focuses on the development of micromachining technology based on femtosecond ablation required for the construction of the wear sensor. The required laser processing operations were grooves cutting in steel and aluminium alloys, as well as microholes drilling in conducting ceramic fibres. In addition, ablation with filamented beams in water was expanded on metallic materials. Two micromachining systems in which the same high-average-power high-repetition-rate ytterbium femtosecond laser was used were constructed: one based on linear direct-drive servomotors stages employing the sample positioning approach, and the second using galvoscanner to scan the laser beam over the stationary sample. Because of the higher beam scanning velocity, the use of a galvoscanner shortens the fabrication time by an order of magnitude, and the quality of the grooves could even be improved by using advanced scanning algorithms. Great attention was paid to the sample positioning and laser beam scanning algorithms. Invented algorithms (bidirectional spiral from lines and composite spiral) enabled the achievement of symmetrical rectangular grooves of the required dimensions (50 – 300 μm) with less than ±5 % deviation and more than 80° wall inclination angle. The influence of wavelength, working gas and beam polarization on the structure morphology was studied. It was found that the proper selection of processing parameters and the use of particular scanning algorithms provide the means for micromachining micronotches and blind microholes in Ti4O7 ceramics without an electro-conductivity change in the material. Chromium thin-film ablation patterns signify that a thin (0.3 - 1 mm) water layer above the sample nonlinearly transforms a femtosecond beam - with power exceeding >50 times the critical power for self-focusing - into multiple filaments, at least in the case of single pulses. A thin water layer above the metallic materials provides higher ablation efficiency, better cutting quality and enables the cutting of material samples up to 3 mm in thickness. A laser-cutting parameter optimization for 1 mm thick bulk steel ended up with a maximum 0.2 mm/s cutting speed, with the following laser, galvoscanner and focusing system parameters: highest pulse energy (300 μJ at 10 W average laser power and 33 kHz pulse repetition rate); shortest pulse duration (280 fs); minimal (~300 μm) water buffer layer; focusing 200 μm above the sample surface (for 100 mm F-theta objective); 500 – 800 mm/s scanning speed range. Data statistical analysis revealed that pulse energy and beam scanning speed are the main factors that affect the cutting speed. The results of the dissertation will undoubtedly be beneficial not only for the wear sensors production, but for much wider application of the femtosecond ablation in micromachining of metals and nontransparent ceramics. |
