Pulsed laser deposition of antifriction thin-film MoSe x coatings at the different vacuum conditions

Abstract

MoSe x coatings were obtained by pulsed laser deposition in vacuum at the pressure of background Ar gas up to 10 Pa. The deposition temperature was 200 °С. The films were studied by means of X-ray diffraction, scanning and transmission electron spectroscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy of helium ions. The tribological properties of thin-film coatings were investigated by pin-on-disk testing in air with 50% relative humidity. In addition, wear tracks were studied by micro-Raman spectroscopy. Chemical composition, structure, and tribological properties of the coatings were found to be sensitive to the presence of the inert gas. Thus, increasing the gas pressure from 10 − 4 to 10 Pa changes the chemical composition, so that the ratio of the atomic concentrations of Se and Mo ( x = Se/Mo) increases from 1.5 to 2.4 in the principal deposition zone. The changing of the structure concerns the accumulation of distortions in the lattice of MoSe x nano-crystals as increasing the distance between the basal planes and intensive formation of nano-sized inclusions of the amorphous phase and Mo nano-crystals in the volume of the coatings. At the optimal gas pressure (∼ 2 Pa), the composition of the coating was close to the stoichiometric one, and the layer adjacent to the substrate consisted of MoSe x nano-crystals with the basal planes parallel to the substrate surface or oriented at small angles to the surface. The thickness of the oriented layer in such coatings was greater than the thickness of the similar layer in the coatings deposited in vacuum (10 − 4 Pa). The tribological properties of MoSe x coatings deposited on substrates of stainless steel type 95 × 18 (18 at.% Cr) depend on the gas pressure. The friction coefficient in air decreases from 0.08 for deposition at the background pressure of 10 − 4 Pa to 0.04 for deposition at the optimal pressure. This change in the deposition conditions has only a marginal effect on the coating durability. Means to increase the durability are also considered.