Dissipation Mechanisms and Superlubricity in Solid Lubrication by Wet-Transferred Solution-Processed Graphene Flakes: Implications for Micro Electromechanical Devices

Abstract

Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS₂ and WS₂. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.

Figure 1

Method used to coat AFM probes with graphene from the EdG aqueous solution. (a) Schematics of the experimental set-up (not in scale). (b) Top-view optical micrograph of a commercial nanoprobe, with the cantilever fully dipped into the EdG drop (the dotted line is the position of the drop lateral meniscus driven by the micromanipulator). High-resolution optical micrographs of the same cantilever respectively: (c) pristine uncoated; (d) before retraction of the tipped-end from the EdG drop; (e) coated by a FLG liquid layer, immediately after retraction from the EdG drop; (f) after several ‘dip-retract’ cycles; (g) coated by a FLG dry deposit.

Figure 2

(a) Optical micrograph of a graphene patches formed on a SiO₂ substrate by drop-casting the water-based FLG flakes dispersion. (b) Topography (top, scale bar 530 nm, F_N = 12 nN), associated friction map (bottom) and (c) F_f vs F_N curves contrasting the response of FLG flakes and uncovered SiO₂ regions. (d) Representative Raman spectrum acquired on a micrometric region from the FLG patch in (a) (highlighted with the red dotted square).

Figure 3

SEM micrographs of the graphene-coated AFM probes at different magnifications. (a, b) Commercial rectangular-shaped silicon cantilever (HQ:CSC37AlBs by Mikromasch), with evidence of the graphene-wrapped nanotip (inset in (b)). (c, d) Colloidal AFM probe with a silica bead glued onto a rectangular-shaped cantilever, after a total number of 200 ‘dip-retract’ deposition cycles. At higher magnification, the silica surface appears partially covered by FLG flakes (inset in (d)). (e, f) Colloidal AFM probe as in (c), but after 600 ‘dip-retract’ deposition cycles: the silica surface is coated by a thicker, still inhomogeneous, deposit of flakes. Crumpled flakes are easily discerned at a higher magnification (e.g., see inset in (f), field of view 1.1 × 1.0 μm²).

Figure 4

(a) AFM morphology and (b) cross-section height (along the dash line in (a)) for a pristine silica bead. The white arrow and dotted circle in (a) highlight respectively the position and size of the contact spot with HOPG (see text). (c–j) Evolution of the surface morphology and of the cross-sectional height for the same bead upon four ‘deposition runs’. Only after ‘deposition run’ 4, the topographically highest contact asperity becomes off-centered and located over a graphene deposit (see (i and j)). (k) Adhesion force F_A measured on HOPG after each ‘deposition run’: adhesion breakdown occurs after the fourth run. (l) Cross-section height and friction force along the dotted line in (i): it shows the lubricious behavior of deposited FLG flakes compared to SiO₂.

Figure 5

(a) Normal force vs displacement (F_N vs z) curve on HOPG for a pristine colloidal probe. (b) Normal force vs tip–sample distance (F_N vs D) curve for a graphene-coated colloidal probe named ‘coated probe 1’. Besides adhesion reduction, a long-ranged repulsive interaction at ∼50 nm signals the response of the elastically-soft graphene coating. (c) As in (b) but for a different probe named ‘coated probe 2’. Adhesion is still reduced compared to the pristine contact in (a), but there is no evidence of the coating compliance.

Figure 6

(a–c) Atomic-scale lateral force maps acquired respectively on HOPG (F_N = 370 nN), WS₂ (F_N = 205 nN) and MoS₂ (F_N = 218 nN), using the ‘coated probe 1’ of Figure 5. In the inset of each panel is the 2D Fast Fourier Transform calculated from the corresponding force map (fast sliding directions indicated by black arrows). (d) Set of representative F_f vs F_N characteristics measured for the three-layered sliding junctions (v = 33 nm/s). The sharp friction jumps (highlighted by black arrows) reflect the specific normal-force response of the used probe. Negative (differential) friction coefficients for FLG/WS₂ and FLG/MoS₂ contacts originate from the friction force fluctuations and the weak F_N-dependence compared to the FLG/HOPG case. (e) Load-dependent friction loops for the FLG/HOPG interface in (d). (f–h) Load-dependent variation of the interfacial parameters E₀, k, and η, extracted from the friction characteristics in (d). (i) Comparison of experimental F_f^* vs F_N data with predictions from the PT model, for three different graphene-coated colloidal probes (indicated respectively by circles, squares and triangles). The PT confidence level is shown in orange.