Femtosecond laser microstructuring of diamond-like nanocomposite films

We report on femtosecond laser surface modification and microstructuring of diamond-like nanocomposite (DLN) films (a-C:H,Si:O), and investigation of the frictional properties of laser-micropatterned DLN films on the nano, micro and macroscale. DLN films of 2–3μm thickness were irradiated using a femtosecond laser (wavelength 1030nm, pulse duration 320fs, pulse repetition rate 101kHz) to produce periodic linear micropatterns over the areas of 40mm2. Laser irradiation was performed at low fluences (below the single-pulse ablation threshold) corresponding to the conditions of surface graphitization and incipient ablation developing during the multipulse irradiation. Frictional properties of laser-micropatterned DLN films were studied using (i) lateral force microscopy (LFM) and (ii) ball-on-flat tribometer under linear reciprocating sliding against 100Cr6 steel and Si3N4 balls. The LFM measurements revealed significant changes in the friction behavior of the laser-patterned films during transition from nano to microscale, demonstrating much lower friction forces within laser-graphitized strips than on the original film. Such microfriction behavior was attributed to (i) higher hydrophobicity of laser-graphitized nanostructured surface and (ii) strong influence of capillary forces of adsorbed water layers on friction under the ‘nano’ loads. Macroscopic friction properties of the fs-laser-patterned DLN films were shown to depend on the friction pair (DLN vs steel ball, DLN vs Si3N4 ball), both at the initial stage of sliding and during prolonged sliding tests.