Excimer Laser Machining of Silicon Nitride
Surface cracks often formed e.g. by grinding can dramatacilly lower the strength of ceramic components. UV-excimer laser machining is proposed as a finishing process for reducing surface defects by ablation of thin surface layers. Due to the high pulse power the vapourization temperature is reached within some ns and a thin layer (depth ≦ 1 micron) is ablated. The rapid and highly localized energy input leads to a small heat affected zone and a fast decomposition of the ceramics.
The formation of surface cracks is investigated on hot-isostatically pressed silicon nitride specimen in 4-point-bending tests. Different sets of laser parameters as wavelength, fluence, beam shape and pulse duration are tested. The beam size is determined as the most important parameter providing high strength and high surface quality. Bending strengths of more than 600 MPa can be achieved for ablation depths of 30–40 microns.
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