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.

Organophosphorus compounds are common additives included in liquid lubricants for many applications, in particular automotive applications. Typically, organic phosphites function as friction-modifiers whereas phosphates as anti-wear additives. While the antiwear action of phosphates is now well understood, the mechanism by which phosphites reduce friction is still not clear. Here we study the tribochemistry of both phosphites and phosphates using gas phase lubrication (GPL) and elucidate the microscopic mechanisms that lead to the better frictional properties of phosphites. In particular, by in situ spectroscopic analysis we show that the friction reduction is connected to the presence of iron phosphide, which is formed by tribochemical reactions involving phosphites. The functionality of elemental phosphorus in reducing the friction of iron-based interfaces is elucidated by first principle calculations. In particular, we show that the work of separation and shear strength of iron dramatically decrease by increasing the phosphorus concentration at the interface. These results suggest that the functionality of phosphites as friction modifiers may be related to the amount of elemental phosphorus that they can release at the tribological interface.

Sulfur and phosphorus are key elements for the functionality of lubricant additives used in extreme pressure applications, such as synchronizer systems in cars. To understand their mechanism of action we combine first principles calculations and gas phase lubrication experiments. The surface spectroscopy analysis performed in situ after the tribological test indicates that iron sulfide (phosphide) is formed by rubbing steel-on-steel in the presence of organo-sulfur (–phosphorus) molecules. We, thus, study the effects of elemental sulfur and phosphorus on the interfacial properties of iron by spin-polarized density functional theory calculations. The results show that both the elements are very effective in reducing the adhesion and shear strength of iron. Sulfur is predicted to be more effective than phosphorus, especially at high pressure. Gas phase lubrication experiments confirm these results, indicating that the friction coefficient of iron-sulphide is lower than that of iron-phosphide and both S and P dramatically reduce the friction of steel-on-steel. These results indicate that the release of elemental sulfur and phosphorus may be the key mechanism to controlling the tribological properties of the metal interface and elucidate that the underling microscopic phenomenon is metal passivation.

The impact of fatty acids on the lubricity of diesel and biodiesel fuels was investigated in steel/steel contacts at two different temperatures of 40 °C and 100 °C. The addition of 7% v/v Fatty Acid Methyl Esters (FAMEs) to fuels, to form biodiesel, lubricates the steel–steel contact and the friction coefficient is reduced by about 30%. Further addition of fatty acids leads to a significant reduction in friction by a factor of 2 at the highest temperature of 100 °C. Some unsaturated fatty acid additives boost the friction reduction by a synergistic effect with the biofuel. The analysis of worn surfaces by XPS revealed the formation of an insulating film mainly located at the center of the tracks and the disappearance of carboxylic groups. PM-IRRAS analysis demonstrated the lack of carbon double bonds in the tribofilm and the presence of some esters (iron and/or organic soaps). By combining high-resolution photoemission soft X-ray absorption and SIMS studies, we found that the extreme surface of the tribofilm is formed by planar arrangements of sp²-rich molecules such as quinones and/or graphene oxides. It is thought that under the effects of shearing, temperature and possibly oxygen from air or fuel, complex chemical reactions between FAME and unsaturated additives occur in the diesel matrix generating a new tribofilm material on the steel surface. As double bonds are involved in the process, the basic tribochemistry mechanism is thought to be related to a reticulation process catalyzed by the formation of oxide particles in the sliding interfacial contact zone accompanied by an aromatization of the fatty acids at the extreme surface.

To meet the surging needs in energy efficiency and eco-friendly lubricants, a novel superlubricious technology using a vegetable oil and ceramic materials is proposed. By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp² and sp³ hybridized carbon in presence of a commercially available silicon nitride bulk ceramic, castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most. Besides a partial liquid film possibly separating surfaces in contact, local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds. High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities, thus helping both the chemical polishing of surface and its chemical passivation by H and OH species. Particularly, the formation of the formation of −(CH₂−CH₂)_n− noligomers have been evidenced to have a major role in the friction reduction. Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp²-hybridized carbon atoms in the amorphous carbon structure. Hence, tuning sp²-carbon content in hydrogen-free amorphous carbon, in particular, on the top layers of the coating, provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.