Comparative Study of DC and RF Sputtered MoSe2 Coatings Containing Carbon—An Approach to Optimize Stoichiometry, Microstructure, Crystallinity and Hardness

Abstract

Low stoichiometry, low crystallinity, low hardness and incongruencies involving the reported microstructure have limited the applicability of TMD-C (Transition metal dichalcogenides with carbon) solid-lubricant coatings. In this work, optimized Mo–Se–C coatings were deposited using confocal plasma magnetron sputtering to overcome the above-mentioned issues. Two different approaches were used; MoSe2 target powered by DC (direct current) or RF (radio frequency) magnetron sputtering. Carbon was always added by DC magnetron sputtering. Wavelength dispersive spectroscopy displayed Se/Mo stoichiometry of ~2, values higher than the literature. The Se/Mo ratio for RF-deposited coatings was lower than for their DC counterparts. Scanning electron microscopy showed that irrespective of the low carbon additions, the Mo–Se–C coatings were highly compact with no vestiges of columnar growth due to optimal bombardment of sputtered species. Application of substrate bias further improved compactness at the expense of lower Se/Mo ratio. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy confirmed the presence of MoSe2 crystals, and (002) basal planes. Even very low carbon additions led to an improvement of the hardness of the coatings. The work reports a comparison between RF and DC sputtering of MoSe2 coatings with carbon and provides a guideline to optimize the composition, morphology, structure, and mechanical properties.

On the Microstructural, Mechanical and Tribological Properties of Mo-Se-C Coatings and Their Potential for Friction Reduction against Rubber

“…Additionally, the H/E* ratio, which relates to elastic strain at failure, also increased, meaning the wear resistance should improve [ 31 ]. Nanoindentation results are generally in agreement with the previous study on similar Mo-Se-C coatings [ 23 ]. It is noteworthy that although the coatings in the present study exhibited a slightly underdense columnar morphology, their hardness was quite similar to those obtained in laboratory-scale deposition conditions with similar compositions and superior morphological appearance.…”

“…The surface morphology of the MoSe coating showed features akin to sponges. Hence, the film was highly porous, as observed in previous works of Yaqub et al [14,23]. With carbon doping, the columnar cross-section gradually vanished.…”

“…In this context, the coatings with higher carbon content were likely deposited under a condition of increased adatom mobility, ultimately leading to an increase in the density of the films [24]. Compared to a previous work where Mo-Se-C films were deposited in smaller scale laboratory equipment [23], the morphological features for the MoSeC24 and MoSeC33 indicated a reduced density. For example, the coating with 27 at.…”

“…The cause of this was an increase in the interplanar distances. The former roots diminished van der Waals adhesive forces between TMD basal planes and can facilitate the easy shearing properties of MoSe 2 [ 23 ]. Another trend observed was the presence of smaller MoSe 2 crystallites, as suggested by the broader (002) peaks in the coatings containing carbon.…”

Specific Features of Reactive Pulsed Laser Deposition of Solid Lubricating Nanocomposite Mo–S–C–H Thin-Film Coatings

“…), and the interest in new processes that can bring about a radical change in the patterns of friction and wear. Modern studies show that it is necessary to regulate the architecture of the coating, i.e., morphology and phase composition, at the nanoscale to improve the tribological properties of nanocomposite TMD-based coatings [ 9 , 10 , 11 , 12 , 13 , 14 ]. In this case, triboinduced processes in the contact layer can change significantly and new nanophases can form in the tribofilm.…”