This is a repository copy of Single-asperity study of the reaction kinetics of P-based triboreactive films.

Despite the ubiquitous use of the zinc dialkyldithiophosphate (ZDDP) as antiwear additive, no complete information is yet available on its exact decomposition reactions and kinetics to form triboreactive protective films on contacting surfaces. This hinders the replacement of ZDDP with more environmental friendly additives of similar antiwear capabilities. Using a multi-technique approach, this study shows that before the formation of a phosphate-rich protective film, the decomposition of ZDDP proceeds by forming intermediate zinc sulfide and sulfate species, which can be mechanically mixed with the iron oxides on the rubbing steel surfaces. The mixed sulfuroxide layer can play different vital roles including binding the subsequently formed phosphate layers with the metal surface. These layers consist mainly of zinc thiophosphate of initially short chains, which are formed due to the excess concentration of metal oxide on the surface. As the concentration of the oxide decreases in the subsequent layers, the short chains start to polymerize into longer ones. The polymerization process follows first-order reaction kinetics with two distinctive phases. The first one is a fast transient burst phase near the steel surface, whereas the second phase dominates the formation process of the layers away from the substrate and is characterized by slow kinetics. The findings of this study provide new insights into the decomposition mechanisms of the currently most widely used antiwear additive and open future opportunities to find green alternatives with similar superior antiwear properties.

The complex tribochemical nature of lubricated tribological contacts is inaccessible in real time without altering their initial state. To overcome this issue, a new design of a pin-on-disc tribological apparatus was developed and combined with synchrotron X-ray absorption spectroscopy (XAS). Using the designed apparatus, it is possible to study in situ the transient decomposition reactions of various oil additives on different surfaces under a wide range of realistic operating conditions of contact pressure (1.0–3.0 GPa), temperature (25–120 °C), and sliding speed (30–3000 rpm or 0.15–15 m/s). To test the apparatus, several tribological tests were performed at different shearing times ranging from 2.5 to 60 min. These tests were carried out under helium atmosphere at a temperature of 80  °C, contact pressure of 2.2 GPa, and sliding speed of 50 rpm. The XAS experiments indicate that the zinc dialkyldithiophosphate antiwear additive decomposes in the oil to form a tribofilm on the iron surface at different reaction kinetics from the ones of the thermal film. The tribofilm composition evolves much faster than the one of the thermal film, which confirms that the formation of the tribofilm is a thermally activated process similar to the one of the thermal film but accelerated by shear. Furthermore, the results indicate that the sulfur of the formed film, whether a tribofilm or a thermal film, appears initially in the form of sulfate, with some sulfide, which under heat or shear is reduced into mainly sulfide.

One of the major obstacles in replacing the widely used zinc dialkyldithiophosphate (ZDDP) antiwear additive with a more environmentally friendly one is the difficulty of time-resolving the surface species resulting from its decomposition mechanism under high contact pressure and temperature. To tackle this issue, a newly developed miniature pin-on-disc tribotester was coupled with synchrotron X-ray absorption spectroscopy (XAS) to perform in situ tribological tests while examining the composition of the formed triboreactive films. The results showed that in the case of bare steel surfaces the initial decomposition products are mainly zinc sulfate species, which with further shearing and heating are reduced to zinc sulfide mixed with metal oxides. The mixed base layer seems to enhance the tenacity of the subsequently formed zinc phosphate layers composing the main bulk of the protective triboreactive film. This base layer was not observed in the case of coated substrates with hydrogenated diamond-like carbon (a-C:H DLC) coating, which results in the formation of less durable films of small volume barely covering the contacting surfaces and readily removed by shear. Comprehensive decomposition pathways and kinetics for the ZDDP triboreactive films are proposed, which enable the control and modification of the ZDDP triboreactive films.

This is a repository copy of A simple deterministic plastoelastohydrodynamic lubrication (PEHL) model in mixed lubrication.

The ability to simulate mixed lubrication problems has greatly improved, especially in concentrated lubricated contacts. A mixed lubrication simulation method was developed by utilizing the semi-system approach which has been proven to be highly useful for improving stability and robustness of mixed lubrication simulations. Then different variants of the model were developed by varying the discretization schemes used to treat the Couette flow terms in the Reynolds equation, varying the evaluation of density derivatives and varying the contribution of terms in the coefficient matrix. The resulting pressure distribution, film thickness distribution, lambda ratio, contact ratio, and the computation time were compared and found to be strongly influenced by the choice of solution scheme. This indicates that the output from mixed lubrication solvers can be readily used for qualitative and parametric studies, but care should be taken when making quantitative predictions.

A new mechanism for the action of antiwear tribofilms is proposed. The antiwear action of ZDDP additive is believed to be mainly due to the formation of tribofilms that reduce wear by chemical action. In this study, a mixed lubrication model is developed and tribofilm growth integrated into this model to simulate the effects of tribofilms on lubrication. The dynamic evolution of the contacting surfaces due to plastic deformation, wear and tribofilm growth continuously change the lubrication characteristics inside the contact. It is observed that the growth of tribofilm roughens the contact and increase contact severity. It was found that this roughness increase also helps to entrain more lubricant, resulting in thicker lubricant films. Therefore, the plot of the evolution of film thickness ratio (h central (t)/R q (t)) shows that the lubrication regime is improved by the presence of tribofilm. Therefore, not only the chemical presence but the physical presence of the tribofilm on the surfaces also helps to improve contact performance by retaining more lubricant and improving the lubrication regime.

It has been long speculated that the good antiwear properties of the nano-thin triboreactive interfaces might have a rheological origin that can greatly influence their friction, lubrication and antiwear characteristics, which account for nearly one quarter of the worldwide total energy consumption. However, the measurement of the nanoscale viscosity of such tribological interfaces is still a challenging task. This is mainly due to their ultralow thickness, i.e. typically < 150 nm, high viscosity and reactivity, which make the currently used bulk, interfacial or micro-gap rheological techniques inadequate. Here we demonstrate two methods, i.e. creep and squeeze flow, that can be used to quantify the viscosity of triboreactive films in-situ as they form and ex-situ after formation. Films generated from the zinc and ashless dialkyldithiophosphate (ZDDP and DDP) antiwear additives were analysed as model systems because of their industrial and academic importance. The results confirm that the formed tribofilms behave as molten glass with an average viscosity ranging from 2 × 10 11 to 7 × 10 12 Pa.s. During its formation, the molten glass showed rich intrinsic rheological properties that allowed them to maintain local order on the nanoscale through the motion and reconfiguration of single and multiple patches within the formed film, which can significantly predetermine its superior antiwear properties. The findings of this study open future opportunities for optimising the nano-flowable glass to efficiently control the lubrication of tomorrow's engines without the need of any environmentally harmful oil additives.