Picosecond-laser polishing of CVD-diamond coatings without graphite formation

Abstract

Abstract Laser polishing of polycrystalline diamond coatings was up to now investigated by the use of nanosecond-lasers or femtosecond–lasers. We investigate polishing diamond coatings by the use of a picosecond-laser (ps-laser) to combine a high removal rate with an interaction time in the range of the free time between lattice collisions to prohibit heating up of the sample and thereby the formation of graphite. Polycrystalline CVD-diamond coatings are deposited by a laser based plasma CVD process. 10 µm thick diamond coatings with an initial surface roughness Sa of 1.85 µm are deposited on tungsten carbide substrates (6% Co). The ps-laser TruMicro 5050 with a wavelength of 515 nm was used under an inclination angle of 80°. The coating is analyzed by laser scanning microscopy, scanning electron microscopy and Raman spectrometry. High ablation rates are possible, but quickly lead to a formation of indentations in case of working in the focus of the laser beam. Therefore the laser was used 4.5 mm defocused which enables polishing a track width of 600 µm. The surface roughness could be reduced by 59% from 1.85 µm to 0.76 µm. By rotating the sample three times by 90° the roughness could be further reduced to 0.51 µm, which means a relative reduction of 72%. The results of Raman spectroscopy showed that diamond coatings can be polished by a ps-laser without the formation of graphite.