Molybdenum Dithiocarbamate (MoDTC) is an excellent friction modifier used in oil formulations for internal combustion energies. In the search of minimising transition metals contents in lubricating oils, extensive work has been carried out in benchtop tribometers to produce optimised formulations. However, tribometers present major limitations mimicking the extreme environment present within the engine cylinders and assessing the effect of this environment on the tribochemistry of this additive. This work presents a combined analysis using single-cylinder engine tests and surface analysis techniques: Raman and XPS. Raman microscopy maps the distribution of MoS 2 and XPS identifies all the molybdenum species formed by tribochemistry. This approach enabled assessment of the effect of the combustion on MoDTC by comparing the results of experiments executed under fired and motored conditions. Areas where the MoS 2 solid lubricant and other molybdenum species formed were identified on the liner and linked to the corresponding friction force attained during the engine test cycle. As expected, the effective MoS 2 formation on returning points on the liner delivered efficient friction reduction. Noticeable differences in the tribochemistry of MoDTC and distribution of molybdenum species were found between fired and non-fired tests. In motored tests, MoS 2 was generated in both returning points, whereas in fired tests MoS 2 was generated only at the bottom, with oxidised molybdenum species present close to the top dead centre. Tribochemistry mechanisms under fired conditions are proposed. Understanding these mechanisms constitutes an invaluable tool to transfer the results obtained with bench tribometers to real engine operating conditions.

Environmental concerns have made the development of non-flammable, high-specific heat capacity, and high-performance lubricants an urgent priority, driving an increased demand for aqueous-based formulations. A key challenge for their widespread application is achieving low friction across a broad range of speeds. A low-viscosity system composed of polyalkylene glycol (PAG), water, and diethylene glycol offers superlubricity (i.e., friction coefficient <= 0.01) at rolling speeds above 150 mm/s; however, significant friction remains at lower rolling or sliding speeds. This limitation can be addressed by introducing eco-friendly and non-toxic gallate molecules. For example, adding 1 % lauryl gallate stabilizes the friction coefficient at approximately 0.04 with no measurable wear. To activate the anti-friction and anti-wear properties of gallates, the molecules must have alkyl chains with eight or more carbon atoms. Large-scale molecular dynamics simulations have been conducted to explore the key mechanisms by which gallate molecules achieve such superior lubricity. This is made possible through innovative machine learning techniques that enable simulations with Density Functional Theory (DFT) accuracy, allowing the modeling of large systems over extended timescales. Simulations reveal that the superior lubricity of gallates results from the strong anchoring of molecular patches that chemisorb onto the iron surface in specific orientations, enabling the alkyl chains to form an inert cushion at the steel/steel interface. Lubrication occurs thanks to this chemical inert buffer region, which effectively separates the metal surfaces realizing a beneficial friction and wear reducing tribofilm, with a clear dependence on the chain length. These findings by a combined experimental-computational approach provide valuable insights for the development of sustainable lubricants, advancing the field of green tribology.

As much as 23 % of the world's energy is consumed in tribological contacts and reducing friction and wear can have a substantial environmental impact. However, the large majority of lubricants and additives, still based on crude oil, are becoming less and less sustainable: environmentally friendly alternatives are in dire need. Hypericin, a natural constituent of St. John's wort commonly known as antidepressant/antiviral/antibiotic medicinal, is here revealed to have an outstanding potential as friction modifier. Indeed, sustainable superlubricity performance, with friction coefficient below 0.01 under boundary lubrication, is observed when adding hypericin to glycerol to lubricate steel/SiC tribopairs. XPS and Raman spectroscopies revealed the formation of graphitic structures on surfaces rubbed in the presence of hypericin and High-Resolution-Transmission-Electron microscopy on focused ion-beam cross-sections evidenced the first stage of tribofilm polymerization and subsequent formation of graphene. First-principles calculations elucidate the thermodynamic forces driving graphene-formation induced by mechanochemical reactions. The process, tribologically promoted, was monitored in real-time by (ab-initio) molecular dynamics. Formation of graphene patches was found to strongly correlate with friction coefficient reductions up to the superlubricity limit. This work suggests an unconventional way towards formation of graphene-like materials through mechanochemistry and reveals the great potential of pharmacopeia-derived molecules as newly-emerging environment-friendly lubricant additives for industrial applications, opening new ways to formulate water-based lubricants, nowadays considered a strategic oils-alternative to reduce carbon footprint.

Molybdenum dithiocarbamates (MoDTC) are widely used in automotive industries as lubricant additives to reduce friction and to enhance fuel economy. Sulfur-containing additives such as zinc dithiophosphates (ZnDTP) are proposed to play a key role in the improvement of friction reducing properties of MoDTC in formulated lubricants by facilitating the formation of MoS2 tribofilm at the rubbing contacts. This study focuses on the interactions between MoDTC and ZnDTP under conditions comparable with those prevailing in operating engines. The capacity of ZnDTP to sulfurize MoDTC in solution in a hydrocarbon base oil could be demonstrated. Sulfurized Mo complexes bearing one or two additional sulfur atoms (1S-MoDTC and 2S-MoDTC, respectively) which have replaced the genuine oxygen atom(s) from the MoDTC core were detected and quantified using a specifically developed HPLC-MS analytical method. A possible sulfurization mechanism relying on the higher affinity of phosphorus from ZnDTP for oxygen could be proposed. In parallel, the evolution and molecular transformation of the prepared 2S-MoDTC in hydrocarbon base oil under thermal and thermo-oxidative conditions were followed using HPLC-MS and compared with the evolution of their friction coefficients. 2S-MoDTC complexes were shown to exhibit a better retention of friction reducing capability under oxidative conditions than the "classical" MoDTC, although they did not seem to significantly reduce the friction coefficients of lubricants as compared to the "classical" MoDTC. Therefore, sulfurization of MoDTC by ZnDTP might contribute to delaying the progressive consumption of MoDTC and the loss of their friction-reducing efficiency in lubricants under thermo-oxidative conditions.

Ascorbyl palmitate (AP), known as a nutrition pill, and an antioxidant agent in food, has demonstrated excellent lubricity as an additive in PAO4. Adding one wt% AP in PAO4 drastically decreases friction Coefficient (CoF) up to 66% and protects the steel surface from wear. Meanwhile, it shows a more vital friction reduction ability than conventional Mo-based additives and fatty acids, especially palmitic acid. Ascorbic acid core on AP optimises palmitic acid lubricity by forming robust chemical C-O-Fe bond on steel, increasing surface coverage rate. Masked by AP self-assembled layers, steel surfaces can also handle extreme pressure (up to 2.34 GPa) and temperature (150 °C) with unmeasurable wear. This work broadens human-friendly AP vitamin C application for industrial use and introduces a new pathway for optimizing fatty acids lubricity.

The presence of zinc complexes or of oxidative conditions induces dithiocarbamate transfer from methylene-bis(dithiocarbamates) to molybdenum dithiocarbamates via zinc dithiocarbamate species.

The piston-liner tribological contact has a key role in controlling friction caused energy losses in a passenger car engine. This study analyses the MoDTC-containing low viscosity lubricant performance and the resulting low-friction MoS2 tribofilm distribution on the liner surface. The engine liners were tested in a floating liner measurement system, where fully-formulated lubricants with different MoDTC concentrations were utilised to assess the impact of friction modifiers on the friction performance. Tested engine liners were dismantled from the measurement system, and MoS2 tribofilm formation on the liner was quantitatively characterised by Raman Microscopy mapping. The MoS2 tribofilm chemical composition and lamellar structure were further investigated by Transmission Electron Microscopy (TEM), High-Angle Annular Dark-Field Scanning (HAADF) and Energy Dispersive X-ray (STEM-EDX) Microscopy. The findings presented in this paper confirm that MoS2 tribofilm spatial distribution plays a pivotal role in reducing friction in piston-liner contact. 60% friction torque reduction was achieved at BDC area with 0.7% MoDTC concentration in the fully formulated lubricant. Moreover, MoS2 presence in the middle of the liner indicates MoDTC effectiveness throughout the whole engine cycle. Therefore, contributing to the overall energy savings in passenger cars.

During engine tests, it has been observed that the combined use of molybdenum dithiocarbamates (MoDTC) and methylene-bis(dithiocarbamates) (MBDTC) in formulated engine oils resulted in better fuel efficiency, keeping the friction coefficient stable at low values for a longer period of time as compared to the same oil devoid of MBDTC. Therefore, the interactions between MBDTC and MoDTC have been investigated at the molecular level. The qualitative and quantitative evolution of MoDTC in two engine oils similarly formulated, but with and without MBDTC, were compared during engine tests using a specifically developed high performance liquid chromatography-mass spectrometry (HPLC-MS) analytical method. Parallel to the molecular study, the evolution of the friction coefficients of both lubricants as well as the evolution of the fuel consumption of the engine were determined. The combined use of MoDTC and MBDTC was shown to exhibit better fuel efficiency and to maintain a relatively low friction coefficient for longer periods of time as compared to the oil devoid of MBDTC. It could be determined that the enhanced performances observed were presumably related to an extension of the lifetime of MoDTC in the engine oil containing MBDTC. Since the MoDTC remaining at the end of the engine test in oil containing MBDTC exclusively bear ligands corresponding to the dithiocarbamate moieties of MBDTC, it can be concluded that the prolonged existence of MoDTC was due to the progressive replacement of the degraded dithiocarbamate ligands on MoDTC educts by those released from MBDTC during engine functioning. As a result, the concentrations of MoDTC could be maintained at a useful level for a longer period in the engine oil containing MBDTC, leading to better fuel consumption performances.

The impact of a fatty triamine (Triameen YT) additive was investigated on the friction performance and stability of molybdenum dithiocarbamate (MoDTC) in the formulations containing polyalphaolefin synthetic base oil (PAO) and zinc dialkyldithiophosphate (ZDDP). Triamine has no significant effect when mixed with MoDTC and ZDDP, but it improves the performance of MoDTC alone. However, in the MoDTC—Triamine—PAO solutions, a chemical reaction easily occurred and a reddish precipitate was formed upon storage. According to IR, XPS, TEM, and XAS characterizations, this precipitate is poorly crystalline layered alkylammonium oxothiomolybdate. Formation of the precipitate impaired the tribological performance by decreasing the number of active species delivered at the sliding contact interface. However, low friction coefficients were recovered by redispersion of the precipitate in PAO.

The effect of three different film-forming additives mixed in PAO oil on the elasto-hydrodynamic (EHD) friction was investigated and compared to cases with the base oil only. When the boundary films were formed on the surfaces, the organic friction modifier (OFM) decreased the EHL friction by up to 8.7%, the ionic liquid (IL) decreased it by up to 6.4%, while the polymeric organic friction modifier (pOFM) increased it by up to 4.2%. In contrast, if the boundary films were not formed, the EHL friction remained the same as with the base oils only. The oils' surface tension and viscosity were analysed at 25 degrees C and 100 degrees C as potential influencing effects, and the most important parameter for friction changes was found to be the surface energy. The mechanism behind this EHL friction reduction is, therefore, the effect of the boundary films on the poor oil-surface wetting and the consequent boundary slip. This study confirms the poor wetting arising from additive boundary-film formation as a relevant EHL friction-reduction mechanism. This observation is similar to that observed previously for surface coatings, thus suggesting that the boundary films of the surface-energy-reducing additives is a parameter with the potential to reduce the EHL friction.

Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient in reducing the friction of steel, and they are used in a number of industrial applications. The functionality of these additives is ruled by the chemical interactions occurring at the buried sliding interface, which are of key importance for the improvement of the lubrication performance. Yet, these tribochemical processes are very difficult to monitor in real time. Ab initio molecular dynamics simulations are the ideal tool to shed light on such a complicated reactivity. In this work, we perform ab initio simulations, both in static and tribological conditions, to understand the effect of surface oxidation on the tribochemical reactivity of MoDTC, and we find that when the surfaces are covered by oxygen, the first dissociative steps of the additives are significantly hindered. Our preliminary tribological tests on oxidized steel discs support these results. Bare metallic surfaces are necessary for a stable adsorption of the additives, their quick decomposition, and the formation of a durable MoS2 tribolayer. This work demonstrates the importance of the catalytic role of the substrate and confirms the full capability of the computational protocol in the pursuit of materials and compounds more efficient in reducing friction.

Molybdenum dithiocarbamates (MoDTCs) are a class of lubricant additives widely employed in automotives. Most of the studies concerning MoDTC take into account the dimeric structures because of their industrial relevance, with the mononuclear compounds usually neglected, because isolating and characterizing subgroups of MoDTC molecules are generally difficult. However, the byproducts of the synthesis of MoDTC can impact the friction reduction performance at metallic interfaces, and the effect of mononuclear MoDTC (mMoDTC) compounds in the lubrication has not been considered yet in the literature. In this study, we consider for the first time the impurities of MoDTC consisting of mononuclear compounds and combine experimental and computational techniques to elucidate the interaction of these impurities with binuclear MoDTC in commercial formulations. We present a preliminary strategy to separate a commercial MoDTC product in chemically different fractions. These fractions present different tribological behaviors depending on the relative amount of mononuclear and binuclear complexes. The calculations indicate that the dissociation mechanism of mMoDTC is similar to the one observed for the dimeric structures. However, the different chemical properties of mMoDTC impact the kinetics for the formation of the beneficial molybdenum disulfide (MoS2) layers, as shown by the tribological experiments. These results help to understand the functionality of MoDTC lubricant additives, providing new insights into the complex synergy between the different chemical structures.

The aim of this paper is to investigate the tribological conditions required to obtain low friction with a diblock PIB-PEG polymer friction modifier (PFM) blended in base oil (PAO4 + 1% wt PFM) under a severe lubrication regime. Two tribological conditions, rolling/sliding and reciprocating pure sliding, were investigated. A very low friction coefficient (mu similar to 0.035) was obtained at a temperature of 100 degrees C whatever the tribometer used. ECR measurements, ToF-SIMS characterizations of wear tracks and AFM analysis suggested the presence of an adsorbed polymer film on the rubbing surfaces. ToF-SIMS characterizations showed that the polymer bonds to the steel substrate through polar functions.

The current study analyses the friction performance of low viscosity fully-formulated oils containing the Molybdenum Dialkyl Dithiocarbamate (MoDTC) friction modifier at different concentrations. The MoDTC friction modifier is known to produce MoS2 sheets in the tribocontact providing a low coefficient of friction under boundary lubrication conditions. However, there is a little knowledge around the quantitative relationship between the concentration of MoDTC in the oil and MoS2 amount and distribution in the contact. The study uses Raman spectroscopy mapping capability to characterise the tribofilm formed from different chemistry lubricants and under different tribological conditions as defined by the lambda ratio. After qualitative and quantitative chemical surface characterisation a discussion is presented to highlight some important aspects to relate the formed MoS2 sheets, their spatial distribution in tribofilms and the subsequent tribological performance.

In order to tackle the new challenges towards the reduction of carbon emissions in transport industry, the present work aims to understand the effect of the friction modifier (FM) molybdenum dithiocarbamate (MoDTC) on the performance of an automobile engine. A petrol engine has undergone motored test trials, measuring the friction torque reduction when the FM additive is blended into a fully formulated SAE 5W30 oil. Moreover, the engine has been dismantled after the test, investigating the tribochemistry of MoDTC at different key engine components undergoing boundary lubrication, using Raman microscopy. This work demonstrates that materials and contact pressure play a crucial role in MoDTC tribochemistry to form a low friction tribofilm, contributing to global engine friction reduction.

The remarkable lubricant properties of molybdenum dithiocarbamates (MoDTCs) make this class of oil additives well-known in the automotive industry. However, the mechanism of function of these compounds is still not completely understood at the atomistic level. We provide new insights into the dissociation of MoDTCs in tribological conditions, which are the key to describe the debated mechanism to form MoS2. Quantum mechanics/molecular mechanics (QM/MM) dynamic simulations allowed us to monitor in real time the tribochemical reactions occurring at the iron interface and revealed that the presence of the iron substrate and the mechanical stresses alter the dissociation path with respect to what is expected for the isolated MoDTC molecules. Moreover, they uncovered the important role of molecular oxidation on the dissociation pattern: the presence of oxygen atoms in the ligand position of MoDTCs favors the release of the central units of the molecules, containing just Mo and S atoms with the correct stoichiometry to form MoS2. This work demonstrates how the predictive power of ab initio simulations can be very valuable to design new lubricant additives.

The tribological performance of lubricants containing TiO2 nanoparticles and Molybdenum DiThioCarbamate (MoDTC) was investigated using a reciprocating ball-on-flat tribometer in steel–steel contacts. Lubricants containing only MoDTC were used for comparison. The influences of the additive concentration (0.1 wt% and 0.5 wt%) and of the roughness of the counterparts (Ra from 10 to 200 nm) on the performance of the lubricant were studied. Improved friction modification and anti-wear properties were found when TiO2 nanoparticles were blended with MoDTC compared to MoDTC alone, even at low concentration and with rough surfaces. XPS characterizations and FIB-TEM analyses of tribofilms were performed and suggested that the formation of MoS2 from MoDTC is favored in the presence of TiO2 nanoparticles. The results are discussed, taking into account the tribocatalytic properties of TiO2 nanoparticles.

First direct molecular evidence of ligand exchange between Mo and Zn complexes from lubricants.

The tribological behavior of different linear amines blended to PAO4 alone or in combination with ZDDP was investigated under boundary lubrication regime via the coupling of tribometry and XPS measurements. Using a reciprocating ball-on-flat tribometer, it was evidenced that all tested amines were able to reduce both friction and wear. The combination of primary monoamine (1 wt %) and ZDDP (1 wt %) produced synergistic effect on reduction of both friction and wear in most cases. Experimental investigations suggest that this synergistic effect is due to the fast formation of a zinc-oxide-enriched tribofilm depleted in phosphates. Moreover, our results show that the lubrication mechanism and the composition of the tribofilm are strongly dependent on the amine/ZDDP molar ratio.