Molybdenum disulfide (MoS₂) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD) process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS₂coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS₂ coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper.
Main focus was on the deposition of carbon nanofibers (CNFs) onto the hard nanocomposite (nc-Ti1 - xAlxN)/(a-Si3N4) (nACo®) coating surface and the investigation of the structure and tribological properties of CNFs. The alcohol chemical vapor deposition (ACCVD) method was employed to prepare CNFs and the deposition temperatures were 600 and 700 °C, respectively. Prior to the CNF deposition, Ni catalyst was deposited onto the nACo® surface using the magnetron sputtering. The influence of the deposition temperature on the carbon nanofibers structure was investigated by Raman spectroscopy and scanning electron microscopy (SEM). The higher order degree of CNF structure is observed with increasing deposition temperature. Tribological tests were carried out under fretting contact conditions against Al2O3 ball. It is shown that the coefficient of friction (COF) decreases from 1.0 to 1.2 for the clean nACo® surface to 0.2–0.4 for the CNF layers deposited on the nACo® surface. The roughness of the nACo® surface was varied and a higher durability of the CNF layers deposited on the rougher nACo® surface is found.
An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO2), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO2-x Magnéli phases and/or graphite.