Superlubricity of Silicon-Based Ceramics Sliding against Hydrogenated Amorphous Carbon in Ultrahigh Vacuum: Mechanisms of Transfer Film Formation.

Tribological interfaces between silicon-based ceramics, such as Si3N4 or SiC, are characterized by high friction and wear in unlubricated conditions. A solution to this problem is to use them in combination with a hydrogenated amorphous carbon (a-C:H) countersurface from which a passivating carbon film is transferred onto the ceramic surface. However, the mechanisms underlying a stable film transfer process and the conditions that favor it remain elusive. Here, we present friction experiments in ultrahigh vacuum in which friction coefficients lower than 0.01 are achieved by sliding Si3N4 against a-C:H with 36 at. % hydrogen but not against a-C:H with 20 at. % hydrogen. Chemical surface analyses confirm that the superlubric interface forms via the transfer of a hydrocarbon nanofilm onto the Si3N4 surface. Quantum-mechanical simulations reveal that a stable passivating a-C:H film can only be transferred if, after initial cold welding of the tribological interface, the plastic shear deformation is localized within the a-C:H coating. This occurs if the yield shear stress for plastic flow of a-C:H is lower than that of the ceramic and of the shear strength of the a-C:H-ceramic interface, i.e., if the a-C:H hydrogen content ranges between ∼30 and ∼50 at. %. While the importance of a relatively high hydrogen content to achieve an efficient passivation of a-C:H surfaces in a vacuum is well-documented, this work reveals how the hydrogen content is also crucial for obtaining a stable a-C:H transfer film. These results can be extended to glass, SiC, and steel, supporting the generality of the proposed mechanism.

Interplay of mechanics and chemistry governs wear of diamond-like carbon coatings interacting with ZDDP-additivated lubricants.

Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC's tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear.

Role of OH Termination in Mitigating Friction of Diamond-like Carbon under High Load: A Joint Simulation and Experimental Study.

Diamond-like carbon (DLC) has recently attracted much attention as a promising solid-state lubricant because it exhibits low friction, low abrasion, and high wear resistance. Although we previously reported the reason why H-terminated DLC exhibits low friction based on a tight-binding quantum chemical molecular dynamics (TB-QCMD) simulation, experimentally, the low-friction state of H-terminated DLC is not stable, limiting its application. In the present work, our TB-QCMD simulations suggest that H/OH-terminated DLC could give low friction even under high loads, whereas H-terminated DLC could not. By using gas-phase friction experiments, we confirm that OH termination can indeed provide much more stable lubricity than H termination, validating the predictions from simulations. We conclude that H/OH-terminated DLC is a new low-friction material with high load capacity and high stable lubricity that may be suitable for practical use in industrial applications.

Diamond-like carbon coating under oleic acid lubrication: Evidence for graphene oxide formation in superlow friction.

The achievement of the superlubricity regime, with a friction coefficient below 0.01, is the Holy Grail of many tribological applications, with the potential to have a remarkable impact on economic and environmental issues. Based on a combined high-resolution photoemission and soft X-ray absorption study, we report that superlubricity can be realized for engineering applications in bearing steel coated with ultra-smooth tetrahedral amorphous carbon (ta-C) under oleic acid lubrication. The results show that tribochemical reactions promoted by the oil lubrication generate strong structural changes in the carbon hybridization of the ta-C hydrogen-free carbon, with initially high sp3 content. Interestingly, the macroscopic superlow friction regime of moving mechanical assemblies coated with ta-C can be attributed to a few partially oxidized graphene-like sheets, with a thickness of not more than 1 nm, formed at the surface inside the wear scar. The sp2 planar carbon and oxygen-derived species are the hallmark of these mesoscopic surface structures created on top of colliding asperities as a result of the tribochemical reactions induced by the oleic acid lubrication. Atomistic simulations elucidate the tribo-formation of such graphene-like structures, providing the link between the overall atomistic mechanism and the macroscopic experimental observations of green superlubricity in the investigated ta-C/oleic acid tribological systems.