Selective-releasing-affected lubricant mechanism of a self-assembled MoS₂/Mo–S–C nanoperiod multilayer film sliding in diverse atmospheres

Abstract

A self-assembled MoS/Mo-S-C multilayer film prepared by r.f. co-sputtering of MoS and graphite targets was tribotested in diverse sliding atmospheres, and the lubricant mechanism and its correlations to the selective releasing behavior of non-lubricant component were analyzed in detail. Based on the analysis of composition and structural transformation of transferred materials by Raman and HRTEM characterizations, selective releasing behavior according to the sliding atmospheres were found to be particularly apparent in vacuum and dry inert atmospheres. Under these conditions, a-C in graphitic form was selectively released outwards from the topmost surfaces of contacts, leaving MoS layers finely reordered in (002) orientation playing a lubricant role. Composition and structural transformation were observed not only on the topmost surface of wear track but also on the underlying layer at a thickness of tens of nanometers. Moreover, it was found that the selective releasing of a-C initiated from the center of contacts with higher pressure, and then developed toward the edges, resulting in a gradual change in the composition and microstructure of the transferred materials from the well-aligned MoS layers in the center to the graphitic a-C dominant composites at the edges. The short running-in periods and low-friction performance in all the tribotests suggested that the initial preference of MoS-riched sublayers in (002) orientation may facilitate the formation of shearless tribofilms and therefore provide a feasible way of structural tailoring in the MoS-based lubricant films for an improved triboactive response.

On the S/W stoichiometry and triboperformance of WSxC(H) coatings deposited by magnetron sputtering

“…9d one can distinguish the long (> tens of nm) WS 2 platelets formed in the tribofilm from the randomized short WS 2 platelets (5 nm) in the raw coating. Most interestingly, perfectly aligned lamellae, with d = 0.63 nm characteristic of (002) WS 2 plane, extend up to > 60 nm, which is much thicker than reported in earlier work (several nm) [37,54]. Fig.…”

Instant WS2 platelets reorientation of self-adaptive WS2/a-C tribocoating

“…4a confirms thick (002)-orientated WS 2 layers are transferred onto the counterpart ball. Previous study [4,10,11] reported that amorphous TMDs crystallize initially from the bottom of the wear track and become mostly ordered at the outmost tribofilm. On the contrary, Fig.…”

Self-healing of a pre-notched WS2/a-C coating

“…It is in strong contrast to Figure 4(b) where WS 2 platelets are apparently aligned and connected over the interface, suggesting the potential of such patches to heal irregular notches/cracks. WS 2 platelets are also elongated and densified near the interface, as illustrated by Figure 4(c), due to the crystallization of WS 2 even from an amorphous state under cyclic loading [21]. HR-TEM observations in Figure S6 show that the oxidation of WS 2 into WO 3 may locally block or diverge the perfect alignment of WS 2 basal plane.…”