On the Self‐Repair of WS₂/a‐C Tribocoating

Abstract

This study investigates the self‐healing capacity of a WS2/amorphous carbon (a‐C) tribocoating. It is found that prenotches up to 45 µm wide in the WS2/a‐C coating surface can be completely healed under the stimulus of sliding operations. The in situ tribotest of 100, 500, 2000, and 6000 laps confirms a dynamic filling of tribofilms that patch the voids and prenotched damages. The stabilized coefficient of friction (CoF) remains at an ultralow value down to 0.02, independent of the prenotched damage at the top of coating. The sites of notched damage in fact act as lubricant reservoirs to accumulate the otherwise “wasted” debris, which are restored as a superlubricant by the sliding operation. High resolution transmission electron microscopy reveals that WS2 (002) nanoplatelets in the healed notch are parallel to the top coating surface but conformal to the coating/notch interface. The patchy tribofilm holds excellent promise for the self‐repair of damages in the field of tribology.

Temperature-Adaptive Ultralubricity of a WS2/a-C Nanocomposite Coating: Performance from Room Temperature up to 500 °C

“…008-0237), indicating that the WS 2 layers are slightly expanded. 33 Note that such thick, long-range, almost fully well-aligned WS 2 platelets have rarely been reported in earlier studies, 5 , 34 where most often only a few nanometer thick layers of such platelets were observed. In addition, previous studies 35 , 36 suggested that amorphous TMDC platelets gradually crystallize from the bottom of the wear track and only become fully crystalline in the outmost part of the tribofilm in direct contact with the counterpart; in other words, these works describe a gradient of crystallinity in the layer, from amorphous to crystalline.…”

“…Note that sulfur content and S/M (M = W, Mo) ratios have been identified as crucial factors in TMDC-based coatings to enable the formation of enough lubricous phases for a favorable tribological behavior. 33 , 48 Stable ultralow CoFs of 0.02 have been observed in the range S/W = 1.33–1.79. 29 According to Table 1 , the as-deposited coating and the one annealed at 200 °C have a S/W ratio of 1.30.…”

“…The Raman spectra of the as-deposited and annealed coatings are shown in Figure S6 ; weak peaks associated with the E 1g (∼306 cm –1 ) and A 1g (∼421cm –1 ) vibrational modes of WS 2 12 , 43 can be distinguished except for the coatings annealed at 500 and 600 °C. However, the as-deposited coating and those annealed at 200, 400, and 450 °C all present typical peaks associated with amorphous carbon; for instance, the spectral region from 1000 to 1700 cm –1 in the as-deposited coating can be deconvoluted into a D peak at around 1370 cm –1 and a G peak at 1560 cm –1 , 33 , 35 which are characteristics of DLC phases. Note that the D peak is assigned to the breathing modes of sp 2 C in the rings, while the G peak is ascribed to the stretching mode of all pairs of sp 2 C both in rings and chains.…”

“…Some “convective-like” or “turbulent-like” densely but randomly distributed WS 2 were also reported in the middle part of the tribofilm, suggesting the occurrence of relatively easy localized sliding displacements between WS 2 nanocrystallites in the tribofilm. 33 , 51 …”

“…Most of the carbon is released from the contact 33 , 35 during sliding and turns into debris on the sides of the wear track, while a few carbon atoms intercalate between WS 2 planes (see hollow arrows in Figure 11 d) and cause the lattice to expand.…”