Organic Friction Modifiers Research Articles

Gas phase lubrication study with an organic friction modifier

AbstractFriction modifier additives play a crucial role in controlling friction and wear of lubricated tribological systems. Model experiments in a controllable atmosphere performed by integrating a tribometer into a system of in situ surface analytical methods in vacuum can give insights into the additives functionality. In this work, thin, well‐defined layers of an organic friction modifier (OFM) are adsorbed onto an iron oxide surface by means of an effusion cell immediately before measuring friction and wear. The results show that contrary to the assumption that homogeneous layers are formed, this OFM accumulates in droplets on the surface. Droplet number and radius increase with evaporation time. In friction tests, the smallest friction values are found for a low coverage of droplets. For larger droplets, friction increases due to a capillary neck of additive that forms between the sliding surfaces and is dragged along during the friction test.

Achieving Superlubricity of Ricinoleic Acid in the Steel/Si3N4 Contact Under Boundary Lubrication

To improve fuel efficiency and minimize environmental pollution, the interest in organic friction modifiers has resurged, especially fatty acids. Efforts have been made to enhance fatty acids’ performance by tuning their saturation state and adding OH lateral groups in the alkyl chain. Here, we compared lubrication properties in the boundary regime for oleic acid (OA), ricinoleic acid (RA), and linoleic acid (LA) under the same viscosity for a steel/Si3N4 tribo-pair. The presence of the OH group in the OA chain reduces not only friction coefficient (CoF) by ~ 70% but also prevents scratches from generating on both surfaces. In the meantime, it determines local friction distribution by controlling top surface terminations. Even at 100 °C, the steady-state friction coefficient of steel/Si3N4 lubricated by RA is only 0.014. A temperature decrease to 80 °C leads to a superlubricity regime (CoF < 0.01) resulting from the cooperation of nanometer-thick liquid film (possibly containing water) and low friction of contact asperities terminated by OH hydroxyl groups provided by RA. In summary, steel/Si3N4 contact lubricated by ricinoleic acid gives unsurpassed tribological performances reaching superlubricity under a wide range of operating conditions.

Combining micro-infrared reflection absorption spectroscopy with density functional theory for investigating the adsorption of organic friction modifiers on steel surfaces

Friction plays a major role in contributing to the global energy consumptions and reducing it would bring about major economic and environmental advantages. Organic friction modifiers are cost-effective and environmentally friendly additives which are included in the lubricant packages to improve performances. Their friction-reducing properties are connected to their adsorption state on steel surfaces. This study applies Infrared Reflection Absorption Spectroscopy (IRRAS) to probe the oleic acid tribofilm on microscopic areas of the sample (micro-IRRAS). The recorded spectra are interpreted by density functional theory (DFT) calculations and provide useful information about the effect of the tribological phenomena on the adsorption state and molecular orientation of the oleic acid friction modifier. The oleic acid molecules adsorb on the surface forming both monodentate and bridging configurations on the iron atoms. IRRAS data show that under the effect of friction the molecules tilt towards the steel surface.

Molecules with a TEMPO-based head group as high-performance organic friction modifiers

High-performance organic friction modifiers (OFMs) added to lubricating oils are crucial for reducing energy loss and carbon footprint. To establish a new class of OFMs, we measured the friction and wear properties of N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide referred to as C12Amide-TEMPO. The effect of its head group chemistry, which is characterized by a rigid six-membered ring sandwiched by an amide group and a terminal free oxygen radical, was also investigated with both experiments and quantum mechanical (QM) calculations. The measurement results show that C12Amide-TEMPO outperforms the conventional OFMs of glyceryl monooleate (GMO) and stearic acid, particularly for load-carrying capacity, wear reduction, and stability of friction over time. The friction and wear reduction effect of C12Amide-TEMPO is also greatly superior to those of C12Ester-TEMPO and C12Amino-TEMPO, in which ester and amino groups replace the amide group, highlighting the critical role of the amide group. The QM calculation results suggest that, in contrast to C12Ester-TEMPO, C12Amino-TEMPO, and the conventional OFMs of GMO and stearic acid, C12Amide-TEMPO can form effective boundary films on iron oxide surfaces with a unique double-layer structure: a strong surface adsorption layer owing to the chemical interactions of the amide oxygen and free radical with iron oxide surfaces, and an upper layer owing to the interlayer hydrogen-bonding between the amide hydrogen and free radical or between the amide hydrogen and oxygen. Moreover, the intralayer hydrogen-bonding in each of the two layers is also possible. We suggest that in addition to strong surface adsorption, the interlayer and intralayer hydrogen-bonding also increases the strength of the boundary films by enhancing the cohesion strength, thereby resulting in the high tribological performance of C12Amide-TEMPO. The findings in this study are expected to provide new hints for the optimal molecular design of OFMs.

Adsorption Behavior of TEMPO-Based Organic Friction Modifiers during Sliding between Iron Oxide Surfaces: A Molecular Dynamics Study.

Organic friction modifiers (OFMs) added to lubricating oils to reduce friction and wear are crucial for reducing energy loss and CO2 emissions. In our previous studies, we have developed N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide, referred to as C12TEMPO, as a new type of OFM and experimentally demonstrated its superior performance to conventional OFMs of stearic acid and glycerol monooleate. However, the behavior of C12TEMPO adsorbing onto solid surfaces from base oil during sliding, which largely dictates the lubrication performance, is yet to be elucidated. Here, we performed molecular dynamics simulations for confined shear of a C12TEMPO solution in poly-α-olefin between hematite surfaces. Unlike conventional OFMs, which typically have one functional group or multiple functional groups of the same type, C12TEMPO features two functional groups of different types: one amide and one terminal free oxygen radical. The results showed that adsorbed boundary films with a double-layer structure form stably during sliding, owing to double- or single-site surface adsorption and interlayer hydrogen bonding via the two functional groups. Additionally, some molecules in each of the first and second layers also form intralayer hydrogen bonding. Such multitype adsorption is unique and favorable for enhancing the strength of boundary films to withstand heavily loaded and prolonged sliding. The velocity distribution indicates that the first and second layers are solid- and liquid-like, respectively. The second layer could act as a buffer for the first layer, which is the last barrier to prevent solid-solid contact, against shear. We also found that the second layer can act as a reservoir to rapidly repair the once depleted region in the first layer because of the interlayer hydrogen bonding. The combination of the high strength and self-repair ability of the C12TEMPO boundary films can rationally explain the experimentally observed properties of high load-carrying capacity, excellent antiwear effect, and high stability of friction over time.

Effect of Steel Hardness and Composition on the Boundary Lubricating Behavior of Low-Viscosity PAO Formulated with Dodecanoic Acid and Ionic Liquid Additives.

Two ionic liquids, tributylmethylphosphonium dimethylphosphate (PP) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP), as lubricant additives in polyalphaolefin (PAO8) were studied under boundary lubricating conditions on two types of steel (AISI 52100 bearing steel and AISI 316L stainless steel). The tribological behavior of these ILs was compared with dodecanoic acid, a well-known organic friction modifier. This study employs a ball-on-disk tribometer with an alumina ball as a counterpart. A range of advanced analytical tools are used to analyze the tribofilms, including scanning electron microscopy equipped with a focused ion beam, scanning transmission electron microscopy equipped with X-ray energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. A quartz crystal microbalance with dissipation was used to study the surface adsorption of the additives on iron- and stainless steel-coated sensors to reveal the adsorption kinetics, adsorbed layer mass, and bonding strength of the adsorbed layer on the metallic surfaces. The most important factors controlling friction and wear are the thickness and viscoelastic properties of the adsorbed layer, the thickness and chemical composition of the tribofilm, and the hardness and chemical composition of steel. Among all additives studied, BMP on stainless steel gives a strongly adsorbed layer and a durable tribofilm, resulting in low friction and excellent antiwear properties.

Ionic Liquid Additives in Water-Based Lubricants for Bearing Steel – Effect of Electrical Conductivity and pH on Surface Chemistry, Friction and Wear

Water-based lubricants have the potential to become the largest environmentally friendly lubricants in applications such as electric vehicles and the newly emerging green technologies of the future due to their inherent low viscosity and cooling properties. In order to be environmentally acceptable (EAL), both base lubricants and additives should comply with biodegradability, non-toxicity, and non-bioaccumulation requirements. Additives for water-based lubricants should ideally be polar and soluble in water and, at the same time, should not increase the electrical conductivity to critical levels for corrosion. However, most additives used in synthetic or mineral oils are non-polar. Ionic liquids have recently gained attention as lubricant additives due to their high polarity, making them highly surface-active (i.e. high tendency to adsorb on metal surfaces). However, they are seen as highly corrosive for many metal alloys. In this work, a water-glycol lubricant containing two different ionic liquids has been investigated as a potential green lubricant for a bearing steel AISI 52100 with accurate control on electrical conductivity and pH. The selected ionic liquids were tributylmethylphosphonium dimethylphosphate (PP) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP). The tribological behaviour of the ionic liquids was compared with a well-known organic friction modifier, dodecanoic acid (C12). The ionic liquids showed lower friction and wear rate than the water-based lubricant alone. However, they showed higher friction than the lubricant formulated with C12, in which PP gave lower friction than BMP due to low pH. A detailed subsurface analysis of the wear track using scanning-transmission electron microscopy (STEM) showed that a thick oxide tribofilm was built on the wear track for both lubricants formulated with ionic liquids due to high electrical conductivity. This tribofilm gave beneficial effect on wear. Although PP and BMP gave thicker tribofilms than C12, it was not durable, resulting in cracking and detachment.

Investigation of the Lubrication Mechanism by In-Situ AFM Observation for the Formation Process of the Tribo-Films under Lubrication with Combined Solutions of ZDDP and Organic Friction Modifiers

Investigation of the Lubrication Mechanism by <i>In-Situ </i>AFM Observation for the Formation Process of the Tribo-Films under Lubrication with Combined Solutions of ZDDP and Organic Friction Modifiers

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents.

The performance of organic friction modifiers (OFMs) depends on their ability to adsorb onto surfaces and form protective monolayers. Understanding the relationship between OFM concentration in the base oil and the resulting surface coverage is important for improving lubricant formulations. Here, we use molecular dynamics (MD) simulations to study the adsorption of three OFMs─stearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO)─onto a hematite surface from two hydrocarbon solvents─n-hexadecane and poly(α-olefin) (PAO). We calculate the potential of mean force of the adsorption process using the adaptive biasing force algorithm, and the adsorption strength increases in the order SA < GMS < GMO. We estimate the minimum area occupied by OFM molecules on the surface using annealing MD simulations and obtained a similar hard-disk area for GMS and GMO but a lower value for SA. Using the MD results, we determine the adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one previous experimental data set for SA on hematite. For two other experimental data sets for SA, lateral interactions between surfactant molecules need to be accounted for within the MTT framework. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations but also has the highest plateau coverage. We validate the adsorption energies from the MD simulations using high-frequency reciprocating rig friction experiments with different concentrations of the OFMs in PAO. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the simulations and the experiments. The results deviate for OFMs containing Z-unsaturated tailgroups (GMO) due to the additional steric hindrance, which is not accounted for in the current simulation framework. This study demonstrates that MD simulations, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid-liquid interfaces.

Relationship between interfacial adsorption of additive molecules and reduction of friction coefficient in the organic friction modifiers-ZDDP combinations

Organic friction modifiers (OFMs) are often used with zinc-dialkyl-dithio-phosphate (ZDDP) in engine oil formulations to reduce boundary friction while maintaining the anti-wear property of ZDDP. This study investigated the interaction between fatty acids and ZDDP. The results showed that the adsorption of fatty acids determines the friction coefficient when fatty acids are used alone. The friction coefficient was smaller when a denser fatty acid adsorbed layer was formed. Neutron reflectivity (NR) measurement revealed that, for a fatty acid-ZDDP combination, the fatty acid also contributes to friction behavior; a synergistic effect occurred only with a reduction in the interface metal thickness. These results suggest that the fatty acid-ZDDP combination promotes metallic soap formation, which reduces friction.

Ionic liquids as boundary additives in water-based and PAO lubricants

Ionic liquids have been widely discussed as potential lubricants, however, their properties make them also very good potential candidates as lubricant additives (e.g., friction modifiers and anti-wear). In this work, the tribological study of two ionic liquids (tributylmethylphosphonium dimethylphosphate (PP), and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP)) as lubricant additives has been performed on stainless steel (AISI 316L) exposed to polar (water-glycol) and non-polar (polyalphaolefin) based lubricants under boundary lubricating conditions. The performance of these ionic liquids as lubricant additives has been compared to a classical organic friction modifier (dodecanoic acid (C12)). The water-glycol lubricant formulated with the two ionic liquids showed friction values higher than the same base lubricant formulated with dodecanoic acid, however, opposite results were observed for polyalphaolefin (PAO). A detailed surface chemical analysis using X-ray photoelectron spectroscopy (XPS) revealed differences in the passive/tribofilm thickness and chemical composition of the stainless steel surface tested in all lubricants. In the case of the polar lubricant additivated with ionic liquids, the tribochemical reaction accompanied by a tribocorrosion process led to the formation of an unstable passive/tribofilm resulting in high friction and wear. However, in the absence of tribocorrosion process (polyalphaolefin base lubricant), the tribochemical reaction led to the formation of a stable passive/tribofilm resulting in low friction and wear. A detailed surface and subsurface investigation of the microstructure using scanning electron microscopy equipped with a focused ion beam (SEM-FIB) showed that high wear rates resulted in thicker recrystallization region under the wear track surface. Among all lubricant additives tested in this work, BMP in non-polar lubricant media showed the best tribological performance.

Boundary lubrication performance of polymeric and organic friction modifiers in the presence of an anti-wear additive

The effect of oleic acid, oleyl alcohol and oleyl amine based organic friction modifiers (OFM), ester and ethoxylated fatty ester based di-block polymeric friction modifiers (PFM) and Zinc dialkyldithiophosphate (ZDDP) on the boundary lubrication of steel surfaces was studied using a ball-on-disc tribometer equipped with optical interferometry. Frictional performance of oil blends containing the OFM/PFM, with and without ZDDP, was investigated at a slide roll ratio (SRR) of 100% and 130 °C for a rubbing duration of 24 h. Surface topography measurements using 3D surface profilometer and scanning electron microscopy (SEM) were used to quantify the anti-wear performance. Comparison of optical interferograms and friction data showed that formation of a thicker tribofilm does not always translate to effective friction reduction. Our experiments showed that PFMs generally provide better friction reduction than OFMs with or without ZDDP by effectively eliminating the boundary regime. This was attributed to the ability of PFMs to form a tenacious low shear strength film on the metal surface due to its polymeric nature. In contrast, both the friction modifiers had a negative effect on the anti-wear performance of ZDDP. The observed tribological response is attributed to either preferential adsorption of the friction modifier over ZDDP or ZDDP decomposition products. This notion was further corroborated by results obtained from energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) with depth profile analysis, which showed lower concentration of ZDDP derived products for the formulation containing both the friction modifier and ZDDP.

Mechanistic insights into the effect of structural factors on film formation and tribological performance of organic friction modifiers

The influence of molecular structure of organic friction modifiers (OFMs) on their adsorption behaviour on iron oxide surface immersed in PAO oil was investigated using neutron reflectometry (NR). The effects of carbon chain length and levels of unsaturation on film formation was studied using a series of fatty acids with 18–20 carbon atoms in the temperature range between 25 and 100 °C. The results revealed that saturated OFM molecules generate thicker films compared to its unsaturated counterparts. Further, a decrease in film thickness was observed with the degree of unsaturation with the exception of linolenic acid. The film thickness and grafting density of the OFM increased with temperature. A decreased film thickness for the binary additive system comprising GMO and saturated molecules suggested that the adsorption of OFMs on the surface is hindered in the presence of glycerol mono oleate (GMO). A lower boundary friction coefficient and wear exhibited by OFMs with longer chain and linear structure suggest that their tribofilms are more effective in withstanding shear. The tribofilm thickness measured using optical interferometry showed no correlation with the adsorbed layer thickness determined from NR data; this lack of correspondence is attributed to the load and shearing forces experienced by the OFM molecules at the contact surfaces during friction measurements.

Elasto-hydrodynamic friction changes on steel surfaces arising from the modified surface energy of the steel due to additive boundary films

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 °C and 100 °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.

Tribological performance of organic molybdenum in the presence of organic friction modifier.

The tribological performance of organic molybdenum in the present of organic friction modifier was investigated in this study. Three types of organic friction modifiers were selected, which are Glycerol monooleate, Pentaerythritol and N,N-Dimethylhexadecylamine. The organic molybdenum are MoDTC, MoDDP and molybdenum amide. Friction coefficient and wear were studied in block-on-ring test rig with steel test specimens. Experimental results indicate the Pentaerythritol shows synergistic effect with MoDTC in wide range temperature, while increased the friction coefficient of molybdenum amide in high temperature. N,N-Dimethylhexadecylamine shows synergistic effect with molybdenum amide, while hindered the friction reduction performance of MoDTC in low temperature. The presence of Glycerol monooleate reduced friction coefficient of MoDTC in low temperature, while increased the friction coefficient of molybdenum amide in most situations. All the tested organic friction modifiers improved the friction reduction performance of MoDDP. Most of the tested organic friction modifiers reduced the wear of organic molybdenum. The PT shows the best anti-wear performance with MoDTC. The tribo-chemical products in test specimens lubricated with different lubricant formulas indicate that the presences of Pentaerythritol promotes the production of MoS2 in MoDTC. N,N-Dimethylhexadecylamine promotes the production of MoS2 in molybdenum amide. The side products of MoO1.6S1.6 and Cr/MoS2 of MoDDP in high temperature lead to high friction coefficient.

Influence of Base oil Polarity on the Tribological Performance of Surface-Active Engine Oil Additives

Friction, wear and tribofilm growth of organic friction modifiers (glycerol monooleate and oleamide), anti-wear additive (ZDDP) and binary additive system comprising the organic friction modifiers and ZDDP were studied in polyalphaolefin (PAO) and ester oil. The mechanisms underlying base oil polarity-dependent frictional performance of the OFM and AW additives at high temperature (140 ℃), either singly or in combination, were investigated in the light of chemical composition analysis of the tribofilms post friction measurements using energy dispersive X-ray spectroscopy (EDX), static and dynamic time-of-flight secondary ion mass spectrometry (ToF–SIMS). Depending on the rubbing conditions, the boundary friction coefficient of the binary additive systems was found to be either lower than that of individual additives or to lay between the values for the individual additives. Chemical composition analysis of the tribofilms indicated that the nature of base oil controlled interactions between ZDDP and OFM and consequently adsorption and reactive tribofilm formation in the boundary lubrication layer. Surface roughness and wear scar width measured post tribological tests using 3D surface profiler showed improved wear performance in both PAO and ester-based additive formulations.

Towards a neutron and X-ray reflectometry environment for the study of solid\u2013liquid interfaces under shear

A novel neutron and X-ray reflectometry sample environment is presented for the study of surface-active molecules at solid–liquid interfaces under shear. Neutron reflectometry was successfully used to characterise the iron oxide–dodecane interface at a shear rate of 7.0times {}10^{2}hbox {s}^{-1} using a combination of conventional reflectometry theory coupled with the summation of reflected intensities to describe reflectivity from thicker films. Additionally, the structure adopted by glycerol monooleate (GMO), an Organic Friction Modifier, when adsorbed at the iron oxide–dodecane interface at a shear rate of 7.0times {}10^{2}hbox {s}^{-1} was studied. It was found that GMO forms a surface layer that appears unaltered by the effect of shear, where the thickness of the GMO layer was found to be 24.3^{+9.9}_{-10.2} Å under direct shear at 7.0times {}10^{2}hbox {s}^{-1} and 25.8^{+4.4}_{-5.2} Å when not directly under shear. Finally, a model to analyse X-ray reflectometry data collected with the sample environment is also described and applied to data collected at 3.0times {}10^{3}hbox {s}^{-1}.

Failure of Solid Lubricant W-S-C Coatings under Boundary Lubrication Conditions

A boundary lubrication regime imposes a significant challenge on mechanical systems in terms of both friction and wear. A standard approach to the boundary lubrication regime problems involves coating the contacting parts and lubrication with additivated lubricant. The presented paper explores the application of TMD-based coatings combined with a base oil and an organic-based friction modifier to provide a more environmentally friendly and cheaper solution. An investigation has been performed using a reciprocating tribometer across a range of applied loads, with post-test characterisation performed by a 3D optical profilometer, Raman microscopy, SEM, and TEM. Results show that the coating with higher hardness performs better and that the organic friction modifier increases coefficient of friction but decrease wear in the boundary lubrication regime. Moreover, it was established that without an external source of sulphur TMD-based coatings do not have a satisfactory performance in the boundary lubrication regime.

Scale-Dependent Friction-Coverage Relations and Nonlocal Dissipation in Surfactant Monolayers.

Surfactant molecules, known as organic friction modifiers (OFMs), are routinely added to lubricants to reduce friction and wear between sliding surfaces. In macroscale experiments, friction generally decreases as the coverage of OFM molecules on the sliding surfaces increases; however, recent nanoscale experiments with sharp atomic force microscopy (AFM) tips have shown increasing friction. To elucidate the origin of these opposite trends, we use nonequilibrium molecular dynamics (NEMD) simulations and study kinetic friction between OFM monolayers and an indenting nanoscale asperity. For this purpose, we investigate various coverages of stearamide OFMs on iron oxide surfaces and silica AFM tips with different radii of curvature. We show that the differences between the friction-coverage relations from macroscale and nanoscale experiments are due to molecular plowing in the latter. For our small tip radii, the friction coefficient and indentation depth both have a nonmonotonic dependence on OFM surface coverage, with maxima occurring at intermediate coverage. We rationalize the nonmonotonic relations through a competition of two effects (confinement and packing density) that varying the surface coverage has on the effective stiffness of the OFM monolayers. We also show that kinetic friction is not very sensitive to the sliding velocity in the range studied, indicating that it originates from instabilities. Indeed, we find that friction predominately originates from plowing of the monolayers by the leading edge of the tip, where gauche defects are created, while thermal dissipation is mostly localized in molecules toward the trailing edge of the tip, where the chains return to a more extended conformation.

Effect of temperature on tribological performance of organic friction modifier and anti-wear additive: Insights from friction, surface (ToF-SIMS and EDX) and wear analysis

Tribological performance of an organic friction modifier (glycerol monooleate – GMO), an anti-wear additive (zinc dialkyldithiophosphate – ZDDP) and its combination was studied using a ball-on-disc tribometer equipped with optical interferometry at 90 °C and 140 °C. Higher temperatures were needed for the formation of additive layers and chemically reactive anti-wear protective layers. Comparison of Stribeck curves obtained after different rubbing durations showed that GMO reduced friction at low speed sliding-rolling contact. Despite notable decrease in the base oil viscosity with increase in temperature — in turn, increasing the severity of asperity contact — GMO exhibited enhanced frictional performance. The presence of ZDDP alone in the formulation led to significant increase in friction at both low and high temperatures, with the thickness of tribofilm unaffected after prolonged rubbing. The addition of friction modifier to ZDDP-based formulation reduced the friction in the boundary lubrication regime where longer rubbing aided in effective friction reduction at higher temperature. For the binary additive system, the friction coefficient was found to lie between the values corresponding to the single additives. The observed tribological response is attributed to preferential adsorption of the friction modifier over ZDDP and/or ZDDP decomposition products. This notion was corroborated by the results obtained from static and dynamic time-of-flight secondary ion mass spectrometry (ToF-SIMS) and energy-dispersive X-ray spectroscopy (EDX) analysis of the disc surface post tribological measurement. The concentration gradient of different chemical species detected in the tribofilm correlated with the frictional performance of the additives. Analysis of surface roughness and wear scar width showed an improvement in wear performance at higher temperature suggesting the friction modifier and anti-wear additive adsorbed on the surface providing a mechanical barrier. A synergistic effect on the wear performance was observed for GMO + ZDDP-based formulations, which resulted in a smaller wear scar compared to the individual additives.