Tribological properties of gradient Mo-Se-Ni-C thin films obtained by pulsed laser deposition in standard and shadow mask configurations

Solid lubricant films were obtained by pulsed laser deposition (PLD) on steel substrates in such a way as to form an underlayer of diamond-like carbon film, and then the concentration of carbon was gradually reduced in Mo-Se-Ni-C films to obtain a pure MoSex(Ni) top layer. The use of a shadow mask configuration (SMPLD) avoids the deposition of micron- and nanometre-sized metallic particles (Ni, Mo), but the SMPLD films were characterised by relatively high Se content, reduced density and low hardness. The tribological properties of the films were evaluated using a ball-on-disk sliding test in humid air after long-term storage in laboratory conditions. For the PLD films, a relatively high friction coefficient was measured during running-in (~ 0.08). The friction coefficient decreased to 0.06 and did not change during the deepening of the wear track. The micro- and nano-particles were embedded into the film matrix during initial running-in and did not cause any apparent acceleration of the wear that was inhibited by the formation of a thick MoSe2 tribolayer on the surface of the wear track. However, an increased wear rate of the counterpart was detected. The smooth surface of SMPLD films provides a low friction at the beginning of the test; then the friction coefficient increased gradually from 0.05 to 0.2 during the film wear, and sufficiently thick low-friction tribolayer was not found. The improved ability of the PLD films to form tribolayers could be due to their structure peculiarity and the relatively low concentration gradient of carbon in these films.