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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.

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Overbased detergents are well known in the tribology field as anti-wear additives. In boundary lubrication, they generate a quite thick tribofilm on rubbing surfaces. They were studied by coupling XPS and AES depth profiles with XANES and ToF-SIMS analyses. Under friction, we show by ToF-SIMS analysis that detergent molecules are split into smaller structural units. Moreover, ionic bonds do not resist high pressure and shearing, and sulfur disappears from the contact zone. The overbased calcium carbonate core finally collapses and crystallizes to give a good anti-wear film between rubbing surfaces.

Transition-metal-dichalcogenide coatings provide low friction because of characteristic low shear strength along the basal plane of the lamellar structure; however, the material can easily degrade through exfoliation and poor adhesion to the metallic substrates. In this work, an innovative approach was employed to improve the coating's adhesion. A secondary plasma source was used during deposition to generate an additional charged particle flux which was directed to the growing film independently of the magnetron cathode. Therefore, Mo-S-N solid lubricant films were deposited by DCMS from a single molybdenum disulphide (MoS 2 ) target in a reactive atmosphere. Nitrogen was introduced during the deposition with increasing partial pressures, resulting in a high N 2 content in the doped films (37 at. %). The variation in incident ion energy and flux of energetic species bombarding the growing film allows for the control of the S/Mo ratio through selective re-sputtering of sulphur from the film. The S/Mo ratio was progressively increased to the range of 1.2-1.8, having gradient from metallic layer upto-the lubricious sulphide. Combining the ion bombardment with nitrogen incorporation, cohesive critical load (Lc1) reached 38N, 10 times more than MoS 2 coating. Observation using HRTEM revealed an amorphous structure and strong bonding with the substrate.

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International audienc

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