Unraveling the Friction Evolution Mechanism of Diamond‐Like Carbon Film during Nanoscale Running‐In Process toward Superlubricity

Abstract

Diamond‐like carbon (DLC) films are capable of achieving superlubricity at sliding interfaces by a rapid running‐in process. However, fundamental mechanisms governing the friction evolution during this running‐in processes remain elusive especially at the nanoscale, which hinders strategic tailoring of tribosystems for minimizing friction and wear. Here, it is revealed that the running‐in governing superlubricity of DLC demonstrates two sub‐stages in single‐asperity nanocontacts. The first stage, mechanical removal of a thin oxide layer, is described quantitatively by a stress‐activated Arrhenius model. In the second stage, a large friction decrease occurs due to a structural ordering transformation, with the kinetics well described by the Johnson–Mehl–Avrami–Kolmogorov model with a modified load dependence of the activation energy. The direct observation of a graphitic‐layered transfer film formation together with the measured Avrami exponent reveal the primary mechanism of the ordering transformation. The findings provide fundamental insights into friction evolution mechanisms, and design criteria for superlubricity.

Reactive molecular dynamics simulations of thermal and shear-driven oligomerization

“…AFM experiments on diamond-like carbon (DLC) showed that the running-in period before achieving superlubricity consisted of removal of the native oxide layer followed by structural ordering to form a graphitic-layered transfer film [28], consistent with the findings from the ballon-flat testing [29]. The removal of the oxide layer was explained by the Bell model while the structural ordering transformation was explained by a modified Bell model.…”

A Novel Finding of Tribological and Mechanical Linking to Micro‐Convex Texture on Hydrophilic Composites Surface under Water‐Lubricating Conditions

“…Generally speaking, the adhesive wear constant k s was related to the friction type, friction temperature, lubrication, and sliding distance, while the abrasive wear constant k a was related to the distribution of abrasive particles, elastic deformation, and quantity of materials. [71][72][73] Obviously, this was a complex system, and we could not qualitatively analyze it through a single variable. However, it was undeniable that the PVA fibers addition greatly improved the sample hardness, which may be the main reason why its wear loss was significantly lower than that of the pure sample.…”

Ion energy-induced nanoclustering structure in a-C:H film for achieving robust superlubricity in vacuum

“…The bonding states of a-C:H films were acquired by XPS and Raman characterization. For XPS analysis of Friction | https://mc03.manuscriptcentral.com/friction a-C:H films, the outermost ~10 nm layers were removed by low-energy Ar + sputtering to avoid any potential interference of surface contaminations and oxidization (which was estimated to be less than 1 nm by time-offlight secondary ion mass spectrometry (TOF-SIMS) characterization [56]). Considering the relative intensities of C 1s, O 1s, and Si 2p peaks with their sensitivity factors, the impurities of oxygen and silicon in all the films were below 1%, which was considered to be negligible for C 1s peak fitting [73].…”

“…For establishing stable selflubricating state, an appropriate hardness was needed for the films to resist the asperities of the counterparts against pressing in, which could reduce the energy dissipation and material dislocation caused by the plastic flow of the material and lead to low friction and wear [103]. In addition, some researchers believe that the superlubricious property of a-C:H films is also attributed to the easy-searing graphene-like layered structure, which is generated from the rehybridization of sp 3 spatial network during friction contact [24,48,53,56,115]. Under this hypothesis, the barrier to achieve superlubricity for a-C:H films deposited under low energy may also be related to their chain-developed, passivated bonding structures, which could hamper the surface structure evolution towards the graphene-like layered structure.…”

“…In addition, the bonding state of carbon skeleton is also important for the realization of superlubricity. Both experimental and simulation results suggest that the tribo-induced rehybridization of carbon network from sp 3 to sp 2 and their reorientation towards layerlike structure may also account for the formation of easy-shearing interfaces, which contributes to the selflubricating behavior [47][48][49][50][51][52][53][54][55][56][57][58][59]. However, for traditional a-C:H synthesized with hydrogen and alkanes, the sp 2 ratio decreases with hydrogen content [60], which makes it hard to investigate the influences of bonding hybridization and surface passivation independently.…”