MoS2 Tribofilm Research Articles

Effect of Molybdenum Bisulfide (MoS2) Particle Size on Tribological and Vibration Reduction Properties of Thermoplastic Polyurethane

The incorporation of lubricating filler can improve the self-lubricating properties of polymers and mitigate friction-induced vibration under harsh conditions. However, filler performances significantly vary depending on its size. In this study, molybdenum bisulfide (MoS2) particles of various sizes were utilized as lubricating additives to reinforce thermoplastic polyurethane (TPU). The results showed that an appropriate decrease in the particle size of MoS2 facilitated more efficient stress transfer from the polymer to the particle due to strengthened interfacial interaction, resulting in improved mechanical properties of the reinforced TPU composites. Additionally, the adsorption property of the MoS2 particle improved, extending the duration of the MoS2 tribofilm. These beneficial phenomena endow TPU composites with excellent tribological and frictional vibration reduction properties, with the 500 nm MoS2 presenting the best reinforcing effect. However, further reduction in particle size caused particle agglomeration, which adversely affected the mechanical and self-lubricating properties of the composite and eventually reduced the frictional vibration reduction properties. The findings obtained herein provide a valuable reference for developing a novel polymer with excellent friction and vibration reduction properties.

Lubrication antagonism mechanism of nano-MoS2 and soot particles in ester base oil

Spherical nano-MoS2 (S-MoS2) has excellent lubricating properties and potential application value in engine oil additives. Engine soot can enter the engine oil, so the tribological interaction between S-MoS2 and diesel combustion soot (DCS) should be investigated. In this study, DCS was used to simulate engine soot. The interaction was investigated in dioctyl sebacate (DOS), and the interaction mechanism was full characterized. Results showed that S-MoS2 and DCS had obvious antagonism effects on lubrication. The 0.5% S-MoS2 exhibited good lubricating properties in DOS, which could reduce friction by ∼22% and wear by ∼54%. However, after 0.5% S-MoS2 was added to the 0.5% DCS contaminated DOS, the lubrication performance was not improved and was even worse than that without S-MoS2. When S-MoS2 was added for DOS lubrication, a tribofilm containing MoS2 formed on the friction surface, but simultaneously adding 0.5% DCS resulted in the disappearance of the MoS2 tribofilm. Moreover, under the action of friction heat, DCS and S-MoS2 could form hard MoxCy, thereby increasing abrasive wear. Finally, a preliminary deantagonism method was provided. After 2.0% zinc isooctyl dithiophosphate was added to the above antagonistic system, the friction coefficient did not show visible changes, but the wear recovered to a level close to that when only S-MoS2 was added. The antiantagonism method is not very satisfactory and some more efficient methods need to be further explored.

Current Knowledge on Friction, Lubrication, and Wear of Ethanol-Fuelled Engines—A Review

The urgent need for drastic reduction in emissions due to global warming demands a radical energy transition in transportation. The role of biofuels is fundamental to bridging the current situation towards a clean and sustainable future. In passenger cars, the use of ethanol fuel reduces gas emissions (CO2 and other harmful gases), but can bring tribological challenges to the engine. This review addresses the current state-of-the-art on the effects of ethanol fuel on friction, lubrication, and wear in car engines, and identifies knowledge gaps and trends in lubricants for ethanol-fuelled engines. This review shows that ethanol affects friction and wear in many ways, for example, by reducing lubricant viscosity, which on the one hand can reduce shear losses under full film lubrication, but on the other can increase asperity contact under mixed lubrication. Therefore, ethanol can either reduce or increase engine friction depending on the driving conditions, engine temperature, amount of diluted ethanol in the lubricant, lubricant type, etc. Ethanol increases corrosion and affects tribocorrosion, with significant effects on engine wear. Moreover, ethanol strongly interacts with the lubricant’s additives, affecting friction and wear under boundary lubrication conditions. Regarding the anti-wear additive ZDDP, ethanol leads to thinner tribofilms with modified chemical structure, in particular shorter phosphates and increased amount of iron sulphides and oxides, thereby reducing their anti-wear protection. Tribofilms formed from Mo-DTC friction modifier are affected as well, compromising the formation of low-friction MoS2 tribofilms; however, ethanol is beneficial for the tribological behaviour of organic friction modifiers. Although the oil industry has implemented small changes in oil formulation to ensure the proper operation of ethanol-fuelled engines, there is a lack of research aiming to optimize lubricant formulation to maximize ethanol-fuelled engine performance. The findings of this review should shed light towards improved oil formulation as well as on the selection of materials and surface engineering techniques to mitigate the most pressing problems.

Tribological investigation of graphene or/and MoDTC as additives in PAO base oil

In this work, the anti-friction and anti-wear capacity of 0.05 wt% graphene or/and 0.5 wt% MoDTC were investigated in PAO base oil at different test temperatures. Sliding surfaces were analyzed by scanning electron microscope, X-ray energy dispersive spectrometer, and Raman spectroscopy. Results indicated that graphene alone as an additive mildly reduced the friction coefficient at low test temperatures due to its easy shear and ultrathin lamellar structure. MoDTC alone effectively reduced the friction coefficient at different test temperatures because of the formation of MoS2 tribo-film. The excellent mechanical and thermal properties of graphene tribo-film and the good lubricity of MoS2 tribo-film contributed to the enhanced anti-wear capacity of graphene and MoDTC additives, respectively. Graphene-MoDTC dual additive further improved the lubricating performance of PAO base oil. It was shown that MoS2 tribo-film derived from MoDTC dominated the friction behavior of graphene-MoDTC dual additive and graphene nanosheets further improved the anti-wear characteristics of the dual additive. It suggests that the combination of graphene and MoDTC supplies a feasible additive formulation design for PAO base oil at steel/steel interface.

Environmentally adaptive lubrication enabled by gradient structure design of TiN-MoS2/Ti

Molybdenum disulfide (MoS2) films have been widely employed as lubricants for mechanical components but continue to face significant engineering challenges including their low hardness, high humidity sensitivity and weak adhesion to substrates. Although the co-deposition of MoS2 with additives provides enhanced functionality in some cases, the weak adhesion of MoS2 to the substrate is the key to the localized delamination or even early failure of the film. Herein, a novel MoS2-based film is designed and fabricated by magnetron sputtering deposition technique, which consists of Ti bonding layer, TiN load-carrying layer, TiN/Ti/MoS2 gradient layer, and top layer of Ti-doped MoS2 lubricating film. Results demonstrate that the rationally gradient design not only improves the adhesion strength between MoS2 and substrate, but also promotes the preferential growth of MoS2 along [001] zone axis perpendicular to the (002) crystal plane. The 3.2 at.% Ti-doped MoS2 film exhibits a low friction coefficient and wear rate before and after salt spray test, which is ascribed to the formation of a continuous and dense MoS2 tribofilm. Additionally, Ti additive can preferentially absorb oxygen in air, thereby preserving the self-lubricating ability of MoS2. This novel structure design provides an example for obtaining functional lubricants with good mechanical properties and environmental adaptability.

Augmenting the lubrication performance of nano-oils through synergistic co-functioning of nanoparticles

The lubricant industries are desperate for novel lubricants that can withstand extreme pressure (EP) conditions apart from having excellent antiwear (AW) and anti-friction (AF) properties. This work aims to augment the overall lubrication potential of Polytetrafluoroethylene (PTFE) based nano-oils (oval-shaped, 200 nm) by incorporating nanoparticles (NPs) of various types and shapes (graphite 55 nm spherical, lamellar multilayer graphene, lamellar talc (50–100 nm), Titanium 70 nm spherical, and MoDTC 20 nm spherical. Hybrid nano-oils comprising PTFE NPs in combination with selected NPs in a 3:1 ratio were formulated and explored for possible tribo-synergism for AW, AF, and EP capabilities. All hybrid oils proved superior to the solo oils, indicating their synergistic capability. Overall, formulations with MoDTC (commercial friction modifier) yielded the highest tribo-synergism for all parameters, mainly owing to the formation of a mechanically mixed tribo-layer of PTFE/MoS2 at interfacial contacts at high loads. At lower loads, MoDTC quickly formed a low-shear, thin MoS2 tribo-film, which boosted the AW performance of PTFE NPs. The inclusion of just 1 % of MoDTC showed the highest synergistic benefits with PTFE NPs, enhancing weld loads from 560 to 700 kg with a nearly 25 % increment in performance. The tribo-properties of oils can be tuned by incorporating a unique combination of NPs as additives due to cooperativity in lubrication mechanisms.

Low friction tribofilm formation and distribution on an engine cylinder tested with MoDTC-containing low viscosity engine lubricants

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.

In situ observation of the formation of MoDTC-derived tribofilm on a ta-C coating using reflectance spectroscopy and its effects on friction

We clarify the relationships between tribofilm thickness, chemical composition, and friction coefficient by combining in situ and ex situ analyses. An in situ reflectance observation is conducted for MoDTC-derived tribofilm on a ta-C coating during friction tests. The results show that the thicker the tribofilm is, the lower the friction coefficient. In addition, the refractive index increases as friction decreases. XPS analysis shows that the topmost surface of the thick tribofilm contains a higher MoS2 concentration than the deep position. Therefore, the generated MoS2-rich tribofilm is considered to reduce friction. In addition, as the MoS2 tribofilm wears out, friction increases. In conclusion, we have developed a MoDTC-derived tribofilm formation model using a new method of in situ optical measurement.

A new polyimide matrix composite to improve friction-induced chatter performance through reducing fluctuation in friction force

Stabilizing friction force of polymer composites’ wear interfaces during frictional processes is useful to suppress or eliminate the friction-induced chatter and noise behaviors in low velocity. A new Polyimide (PI) matrix composite material composed of Polyimide and MoS2 nano particles was fabricated and tested. The experimental wear tests were conducted to investigate relationship between friction forces, friction-induced chatters and noises under dry and water conditions in low friction velocity. The typical tribological properties of modified composite and some typical mechanical properties were analyzed to reveal the mechanism of MoS2 nano particles on frictional behaviors reduction. The results showed that the transformations and formations of MoS2 tribo-film presented excellent COF reduction on modified PI composite by more than 50%, and also could effectively reduce PI deformations on wear surface which obviously decreased friction forces and their fluctuating amplitudes. These cases endowed the modified PI composite excellent performances on friction-induced chatter and noise reductions compared to the pure PI even though some mechanical properties were weakened. The understanding gained herein is helpful for comprehending the friction-induced chatter reduction of the MoS2 filler on polymer matrix composites and future designing of polymer matrix composites with low friction and good chatter and noise reductions in low friction velocity.

Novel Uses of Al2O3/Mos2 Hybrid Nanofluid in MQCL Hard Milling of Hardox 500 Steel

In recent years, the application of environmentally friendly cutting fluids in the metal cutting industry has been a growing concern in all over the world. In this study, the minimum quantity cooling lubrication (MQCL) technique, which uses very small amount of cutting oil, is motivated to apply to the hard milling process of Hardox 500 steel. Further, rice bran oil, a natural biodegradable oil, is used as the base fluid of Al2O3/MoS2 hybrid nanofluid. ANOVA analysis is used to study the influences of nanoparticle concentration, cutting speed, and feed rate on surface roughness. The obtained results indicate that good surface quality is achieved and the cutting speed is significantly increased to 140 m/min (about 2.55–2.80 times higher than the recommended values) due to the better cooling and lubricating effects from MQCL system and Al2O3/MoS2 hybrid nanofluid. Moreover, the microstructure of the machined surface proves the formation of MoS2 tribo film by using Al2O3/MoS2 hybrid nanofluid, indicating that the effectiveness of each type of nanoparticle in hybrid nanofluid has been promoted. Furthermore, the important technical guides for machining Hardox 500 steel are provided.

Temperature dependence of molybdenum dialkyl dithiocarbamate (MoDTC) tribofilms via time-resolved Raman spectroscopy

Molybdenum dialkyl dithiocarbamate (MoDTC) is a friction reducing additive commonly used in lubricants. MoDTC works by forming a low-friction molybdenum disulphide (MoS2) film (tribofilm) on rubbed surfaces. MoDTC-induced MoS2 tribofilms have been studied extensively ex-situ; however, there is no consensus on the chemical mechanism of its formation process. By combining Raman spectroscopy with a tribometer, effects of temperature and shear stress on MoS2 tribofilm formation in steel-steel contacts were examined. Time-resolved Raman spectra of the tribofilm were acquired, together with the instantaneous friction coefficient. The tribofilm is constantly being formed and removed mechanically during rubbing. Increasing shear stress promotes MoS2 formation. The nature of the tribofilm is temperature-dependent, with high-temperature tribofilms giving a higher friction than lower temperature films. Below a critical temperature Tc, a small amount of MoS2 gives significant friction reduction. Above Tc, a patchy film with more MoS2, together with a substantial amount of amorphous carbon attributed to base oil degradation, forms. The composition of this tribofilm evolves during rubbing and a temporal correlation is found between carbon signal intensity and friction. Our results highlight the mechanochemical nature of tribofilm formation process and the role of oil degradation in the effectiveness of friction modifier MoDTC.

An insight on the MoS2 tribo-film formation to determine the friction performance of Mo-S-N sputtered coatings

Amorphous Mo-S-N coatings are known to provide excellent tribological properties in diverse environments due to easy sliding under the influence of MoS 2 tribo-films. However, the role of nitrogen incorporation, the formation mechanism of MoS 2 tribo-film at the sliding interface and the changes in the friction behaviour under different environments are not fully understood. In this study, an amorphous coating with 30 at. % N was deposited in a semi-industrial reactive direct current magnetron sputtering (DCMS) system, using a single MoS 2 target in combination with a secondary plasma source. The coating was predicted to have either a Mo-S-N phase with N filling some of the S sites or a MoS 2 (N 2 ) structure where the gas molecules prevent the formation of a crystalline lamellar structure. Tribological studies performed in vacuum and ambient air resulted in steady-state COF values of 0.03 and 0.15, respectively. High-resolution transmission electron microscopy (HRTEM) analysis performed on the wear-tracks revealed that the low coefficient of friction (COF) in vacuum was attributed to the formation of a thick and continuous lamellar tribo-film with a low amount of nitrogen. Contrarily, in ambient air, the surface oxidation disturbed the formation of a continuous MoS 2 tribo-film from the amorphous coatings, leading to an increase in the COF and wear rate. This study shows through indirect measurements of the chemical composition of the as-deposited coating and wear debris that nitrogen is stored in gaseous form (N 2 ) within the amorphous matrix and is released from the contact during sliding. • Amorphous Mo-S-N coatings with 30 at. % N deposited with high adhesion to steel. • Tribo-film formation and role of N on the lubricating behaviour of Mo-S-N coatings. • COF in vacuum (10 −2 Pa) of 0.03 and in ambient air (24 °C, 20–30% RH) of 0.15. • Low friction in vacuum due to formation of a MoS 2 tribo-layer at the sliding interface. • Higher friction in ambient air due to a surface oxide formed at the interface.

The effect of S/Mo ratio on structure and properties of MoSx-Ti composite coatings deposited by magnetron sputtering

MoSx-Ti composite coatings with different S/Mo ratios were prepared by varying the substrate bias voltage of magnetron sputtering. It was found that the S/Mo ratio of the as-deposited MoSx-Ti coating decreased gradually from 1.7 to 1.3 as substrate bias voltage increased from −30 V to −110 V. The MoS1.7-Ti coating exhibits a nanocrystalline and amorphous phase mixed structure while the other MoSx-Ti coatings were all amorphous structured. The hardness of the MoSx-Ti coatings increased from 8.8 GPa to 9.8 GPa as the S/Mo ratio decreased from 1.7 to 1.3. The MoSx-Ti coatings with the S/Mo above 1.4 exhibit COF with values smaller than 0.07 at RH of 50% and increased wear rate in the magnitude of ∼10–7 mm3 Nm−1. The tribological behavior of the MoSx-Ti coatings were determined by the formation of the imperfect crystalline MoS2 tribofilm. With the decrease of the S/Mo ratio, the increased defects in imperfect crystalline MoS2 tribofilm lead to quicker consumption of the tribofilm during the trio-test, which result in a higher wear rate.

MoS2 tribofilm distribution from low viscosity lubricants and its effect on friction

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.

Effect of boron content on the structure, mechanical and tribological properties of sputtered Mo–S–B films

Molybdenum disulfide (MoS2) is a widely applied solid lubricant due to its unique layered structure. However, low hardness and environmental sensitivity severely limit its practical application in industry. Here, Mo–S–B films were deposited on Si (100) substrates by magnetron sputtering. The structure, mechanical and tribological properties showed a significant dependency on composition and chemical bonding. Increasing the B content from 0 to 23.4 at.%, the surface became smooth, corresponding to a transition from columnar structure to dense feature, along with a phase transition from hexagonal-MoS2 to amorphous phase. The hardness lineally increased from 0.75 GPa for MoS2 to 8.38 GPa for the Mo–S–B film with 23.4 at.% B. The tribological properties of films were evaluated against steel ball using ball-on-disc tribometer. The tribological test results indicated that the increased B content led to the enhancement of wear resistance for Mo–S–B films, the wear rate of Mo–S–B films with 23.4 at.% B showed the lowest value of 5.5 × 10−7 mm3/Nm, which was two orders of magnitude lower than pure MoS2 (5.66 × 10−5 mm3/Nm). The friction coefficient just rose slightly, keeping it at the same level as pure MoS2. Raman analysis for wear debris after rubbing revealed that MoS2 tribofilm improved lubrication behavior.

In-Situ Formation of MoS2 and WS2 Tribofilms by the Synergy Between Transition Metal Oxide Nanoparticles and Sulphur-Containing Oil Additives

This works investigates the in-situ formation of MoS2 and WS2 tribofilms by the synergy between transition metal oxide nanoparticles and conventional sulphur-containing anti-wear and extreme pressure additives. The formation of these low friction tribofilms can be obtained under reciprocating sliding contact and under extreme pressure conditions, as evidenced using X-ray photoelectron spectroscopy. Under reciprocating sliding conditions, the synergy between transition metal oxide nanoparticles and the ZDDP leads to coefficients of friction around 0.06 before they rise as consequence of oxidation. The synergy is more outstanding in extreme pressure conditions, particularly for MoO3 nanotubes combined with extreme pressure additive. This combination outperforms base oil mixtures containing EP additive or MoS2 nanotubes. While MoS2 nanotubes build superb extreme pressure tribofilms containing iron and molybdenum oxides and sulphides, MoO3 nanotubes are able to build similar tribofilms that can continuously re-sulphurize in the presence of the extreme pressure additive. Despite having a similar chemistry, MoO3 nanotubes are observed to sulphurize more easily when compared to WO3 nanoparticles. The work highlights the tribological potential of these nanoparticles otherwise typically used as precursors for the synthesis of transition metal dichalcogenide nanoparticles.

Comparative studies of the tribological behaviors and tribo-chemical mechanisms for AlMgB14-TiB2 coatings and B4C coatings lubricated with molybdenum dialkyl-dithiocarbamate

Abstract We compared the tribological behaviors and the tribo-chemical mechanisms of AlMgB14-TiB2 coatings and B4C coatings under lubrication with molybdenum dialkyl-dithiocarbamate (MoDTC). In unidirectional sliding contact, the boride coatings exhibited an unstable tribological performance, while the friction coefficients of these coatings maintained a very steady low level in reciprocating sliding contact. No severe wear occurred to the boride coatings. However, significant abrasive wear was observed on the steel balls rubbing against the boride coatings under unidirectional sliding contact. Analysis indicated incomplete dissociation of MoDTC at coating wear tracks and no formation of MoS2 tribofilms on steel counterfaces in the case of unidirectional sliding contact, whereas well-developed MoS2 tribofilms were formed on AlMgB14-TiB2 coatings under reciprocating sliding condition.

In situ tribochemical sulfurization of polyisobutylene-based molybdenum species for enhanced tribo-performance

Abstract Herein, a zero-phosphorus and sulfur polymer-derived molybdenum-based compound (PIB Mo) was synthesized and corresponding researches on synergistic or antagonistic effects between the PIB-Mo and sulfur-containing species as well as arylamine derivatives are reported. The in situ sulfurization of PIB-Mo leads to the tribochemical formation of MoS2 tribo-film as verified using EDS, Raman spectra and XPS, which is continuously formed and oxidized acting as a sacrificial layer, thus contributing an excellent friction and wear performance. Other antagonistic and synergistic effects are observed. The anti-oxidation property of ZDDP is weaken in the presence of PIB-Mo with the increase contents, however, a significantly synergistic effect of PIB-Mo and arylamine derivatives obtained, which dominate the anti-oxidation property and boost the anti-oxidation property in formation application.

Structure, Mechanical and Tribological Properties of MoSN/MoS2 Multilayer Films

MoSN/MoS2 multilayer films were deposited by a sputtering MoS2 target in alternate Ar and Ar/N2 mixed atmospheres with different nitrogen flow rates. The influence of nitrogen flow rates on the microstructure, mechanical and tribological properties of the prepared films were investigated. The multilayer film exhibited the preferred orientation of (002) plane for MoS2 sublayers and amorphous structure for MoSN sublayers. Introducing N2 into the source gas resulted in a much more compact structure for multilayer films due to the suppression of columnar growth of MoS2 film. With the increase of the nitrogen flow rate, the hardness of the multilayer film firstly increased from 2.3 to 10.5 GPa as the nitrogen flow rate increased from 4 to 10 sccm and then turned downwards to 6.5 GPa at 20 sccm. MoSN/MoS2 film deposited with an optimized microstructure exhibited low friction coefficients below 0.03 and a wear life higher than 1.8×105 revolutions in vacuum. Meanwhile, the optimized film showed an ultralow friction coefficient of 0.004~0.01 and wear rate of 4.7 × 10−7 mm3/N·m in an ultrahigh vacuum. Both the enhanced hardness by N-doping and sustainable formed MoS2 tribofilm contributed to the improved tribological property of MoSN/MoS2 multilayer film.

Understanding the Friction Reduction Mechanism Based on Molybdenum Disulfide Tribofilm Formation and Removal

Among friction modifier lubricant additives, molybdenum dialkyldithiocarbamate (MoDTC) provides excellent friction behavior in boundary lubricated tribocontacts. It is well established that the low friction obtained with MoDTC is as a result of the formation of lattice structure MoS2 nanosheets. However, the relationship between the molybdenum species quantity, its distribution on the contact surface, and the friction behavior is not yet fully understood. In this work, Raman microscopy and atomic force microscopy (AFM) have been used with the aim of understanding the link between the friction behavior and the MoDTC/ZDDP tribofilm formation and removal. Tribotests were coupled with a collection of ex-situ Raman intensity maps to analyze the MoS2 tribofilm buildup. Post-test AFM analyses were implemented on the ball wear scar to acquire the average MoDTC/ZDDP tribofilm thickness. In-situ Raman spectra analyses were carried out to detect the MoS2 tribofilm removal. A good correlation was achieved between the friction coefficient measurements and Raman maps when using a linear relationship between the microscopic friction and the local amount of MoS2 tribofilm. After a rapid increase, the average MoDTC/ZDDP tribofilm thickness levels out to a steady state as the friction drop ceases. The removal rate of MoS2 from tribofilms, obtained at different temperatures, suggests that the MoS2 tribofilms are much easier to remove from tribocontacts compared to antiwear ZDDP tribofilms. This is the first study that sets out a framework to link MoS2 amount and coverage to the friction behavior, providing the basis for developing numerical models capable of predicting friction by taking into account tribochemistry processes.