Molybdenum Dithiocarbamate Research Articles

Synergistic lubrication effect of OLC and MoDTC for reducing friction and wear of MAO ceramic coating on TC4 alloy

AbstractTC4 titanium alloy has been widely used in the automotive field due to its exceptional properties. However, inherent defects such as low hardness and poor wear resistance for TC4 alloy limited its wider application. The microarc oxidation (MAO) technique was employed in this paper to prepare MAO coatings on TC4 titanium alloy. The microstructure, phase structure, mechanical properties, and tribological performance were systematically evaluated. The results show that the coating contains a large amount of rutile TiO2 hard phase after MAO treatment, which significantly improves the mechanical properties of the substrate. The hardness of the MAO coating can reach 581 HV.05. Furthermore, the synergistic lubrication effect of onion‐like carbon (OLC) nanoparticles and organic molybdenum dithiocarbamate (MoDTC) in PAO oil was observed for MAO‐treated TC4. Particularly, when .01 wt.% OLC is used with 1 wt.% MoDTC oil, the coefficient of friction (COF) decreases to .062, and the wear rate decreases to 4.3 × 10−7 mm3/Nm. Combined Raman and X‐ray photoelectron spectroscopy (XPS) analysis indicate that OLC is deposited on coating area to form a lubricating carbon film. Additionally, OLC can promote the decomposition of MoDTC during sliding to generate a tribofilm containing MoS2.

The effect of ethanol fuel dilution on oil performance and MoDTC tribofilms formation and composition

Ethanol has emerged as a promising alternative to fossil fuels, but its use can lead to significant dilution in lubricants, particularly during cold start or heavy traffic. This dilution can affect the performance of additives, including friction modifiers like molybdenum dithiocarbamate (MoDTC), which are designed to reduce friction under extreme contact conditions. Prior research suggests that ethanol may impact the performance of MoDTC, prompting this study’s goal to investigate the effects of ethanol on MoDTC tribofilms and their friction response under boundary lubrication conditions. Therefore, reciprocating tribological tests were performed with fully formulated lubricants containing MoDTC with varying ethanol concentrations. The results indicate that a critical ethanol dilution level inhibits friction reduction by MoDTC activation, resulting in friction coefficients (COFs) similar to the base oil. Surfaces tested with simple mixtures of polyalphaolefin (PAO) + MoDTC showed increased COFs with added ethanol. Analysis of tested surfaces using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy near the edge structure (XANES) revealed the presence of sulfates, MoO3, MoS2, and MoSxOy compounds in the tribofilms formed on the surfaces, with and without ethanol diluted in the lubricant. However, the addition of ethanol increased the sulfates and MoO3 content of the tribofilms at the expense of friction-reducing compounds such as MoS2 and MoSxOy. These findings suggest that ethanol dilution in lubricants containing MoDTC creates an oxygen-rich interfacial medium that favors the formation of compounds with insufficient friction-reducing capabilities.

The Effect of Lubricity of Calcium Sulfonate on ZnDTP and MoDTC

Reduction of friction and wear by lubrication is necessary to improve the efficiency and prolong the lifespan of industrial machinery. However, several aspects regarding the lubrication effects in additive-combined oil are still unresolved. This study focuses on the effects of combining zinc dialkyldithiophosphate (ZnDTP), molybdenum dithiocarbamate (MoDTC), and Ca sulfonate on the frictional behavior of poly-α-olefin 4 (PAO4) lubricant oils. Ball-on-disk friction tests demonstrated that the exclusive addition of high-base Ca sulfonate solution reduced friction and wear of the lubricating oil. When combined with ZnDTP and MoDTC, the friction coefficients of PAO with high-base and low-base Ca sulfonate are affected by competitive reactions on the sliding surface. Quartz crystal microbalance with dissipation monitoring measurements suggests that high-base Ca sulfonate forms a stronger adsorption film than low-base Ca sulfonate. Atomic force microscope observations confirmed that the addition of low-base Ca sulfonate reduced friction and wear, whereas high-base Ca sulfonate induced competitive reactions with ZnDTP and MoDTC. This study provides valuable insights into the effects of the combination of additives on the lubricity of PAO and their impact on friction and wear characteristics, which are valuable for the design of suitable lubricants for industrial machinery.

On the Role of Friction Modifier Additives in the Oil Control Ring and Piston Liner Contact

Abstract In-cylinder internal combustion engine parasitic frictional losses continue to be an area of interest to improve efficiency and reduce emissions. This study investigates the frictional behavior at the oil control ring–cylinder liner conjunction of lubricants with anti-wear additives, varying dispersant concentration, and a range of friction modifiers. Experiments are conducted at a range of temperatures on a cylinder liner with a nickel silicon carbide coating. A novel motored reciprocating tribometer, with a complete three-piece oil control ring and cylinder liner, was used to isolate the friction at the segment–liner interfaces. Four lubricants were tested, three with the same 3% dispersant concentration and 1% zinc dialkyl dithiophosphate (ZDDP) anti-wear additive: the first with no friction modifier, the second with inorganic friction modifier (molybdenum dithiocarbamates), and the third with organic friction modifier (amide). A fourth lubricant with an organic friction modifier with a 9% dispersant concentration was tested to compare the effect of the level of dispersant with the friction modifier. Results indicate that the inorganic friction modifier reduces friction comparatively to the other lubricants, showing the importance of friction modifier selection with anti-wear additives.

Enhancement of Diarylamine Antioxidant Activity by Molybdenum Dithiocarbamates.

Molybdenum dithiocarbamates (MDTCs) are indispensable lubricant additives. Although their role as antiwear agents is well established, they have also been attributed antioxidant properties that are not understood. MDTCs do not inhibit autoxidation, but they markedly enhance the capacity of diphenylamines (DPAs)─ubiquitous radical-trapping antioxidants (RTAs)─to do so. We find this synergy to be evident not only at elevated temperatures (160 °C in n-hexadecane) but also at moderate temperatures, where autoxidations can be continuously monitored and kinetics more easily interpreted (100 °C in squalane). Interestingly, the synergy disappeared in an unsaturated hydrocarbon (n-hexadec-1-ene), where the RTA activity of the DPA is known to result from the diarylnitroxide derived therefrom. Autoxidations of squalane carried out in the presence of the diarylnitroxide─wherein it is a poor inhibitor─were much better inhibited in the presence of MDTC, suggesting that it converts the nitroxide to (a) more competent RTA(s). Indeed, preparative experiments revealed two species: DPA and a DPA dimer into which a single oxygen atom had been incorporated. This conversion is accelerated by the oxidation of MDTC to a dioxo molybdenum species. A mechanism is proposed to account for these observations, and the implications of our findings and their interpretation are discussed.

Effect of Friction Reducers with Unreinforced PEEK and Steel Counterparts in Oil Lubrication

The increasing adoption of PEEK (polyetheretherketone) in many industrial applications has promoted intense research to optimize its lubrication along with the development of friction reducers (FRs), additives that help in reducing fuel consumption and, consequently, CO2 emissions. In this study, the effect of FRs in improving the lubrication of PEEK–steel couplings was evaluated and their mechanism studied using the Mini Traction Machine (MTM) tribometer. Different types of FRs (such as Molybdenum dithiocarbamate, glycerol monooleate, amine and polymeric derivatives) and coupling combinations (steel/steel, steel/PEEK and PEEK/steel) were considered. The oil samples were evaluated as fresh and after a rubbing time considering different operative conditions (from high to low T, fixed load and type of contact motion), and a measurement of the tribofilm was acquired. The experimental campaign showed a ranking among FRs friction-reducing behavior and, in some cases, a synergistic effect was noted between the tribofilm containing the friction modifier and the PEEK surface. Comparing the top performing FRs with reference oil showed a reduction in friction of 22%, 21% and 37%, respectively, in steel–steel, PEEK–steel and steel–PEEK couplings, while in the standard steel–steel coupling, two out of four FRs did not reduce the friction. After conditioning in the presence of PEEK, all friction-modifier additives reduced the friction effectively. This demonstrates the promising performance of PEEK, its compatibility with friction-reducing additives, and its applicability to sliding machine parts in order to improve efficiency and thus reduce CO2 emissions.

Characterization of Base Oil and Additive Oxidation Products from Formulated Lubricant by Ultra-High Resolution Mass Spectrometry

Automotive formulated lubricants are high value products composed of 80% base oil and 20% various additives. During their life service, lubricants are exposed to several factors that will cause degradation over time, such as high temperature, shear, and oxidation. Base oil is a complex combination of hydrocarbons that are relatively sensitive to oxidation. During the initiation phase of oxidation, free radicals are formed, leading to the production of hydroperoxide ROOH and an alkyl radical R•. These compounds will react with the base oil molecules to form aldehydes, ketones, and carboxylic acids in the termination phase. Owing to the molecular complexity of these mixtures, Fourier transform mass spectrometry seems to be the most appropriate tool to cover their wide range of compounds due to its ultra-high resolving power and mass accuracy. In this study, a native formulated lubricant and its different oxidized states at 140 °C under air flow (3, 5, 7, 8, and 9 days of oxidation) were analyzed by FTICR MS. The combination of atmospheric pressure chemical ionization (APCI) was used to achieve a non-selective ionization of molecules, including base oils, while Electrospray ionization (ESI) was used to selectively ionize acidic molecules. Apparent Kendrick mass defect (aKMD) plots were used to separate homologous series of molecules on different horizontal lines on the basis of the CH2 repetition unit. Aging of lubricants was mainly characterized by a rapid consumption of certain additive families, such as molybdenum dithiocarbamates (MoDTCs) and zinc dithiophosphate (ZnDTPs), but also by the emergence of many oxidation products. Thus, the presence of aldehydes, ketones, and acids was characterized in the early stage of aging while larger products from polymerization were observed in a more advanced stage of aging. Interaction products between peroxy radicals and hindered phenols/alkyl diphenylamines (ADPAs) antioxidations were elucidated toward the high m/z. The formation of such products can be explained by trapping mechanisms of these additives at high temperature (>120 °C). Other types of interaction products were observed with the formation of antioxidant complexes. Additive degradation products were also characterized. For instance, polyisobutenyl succinimide dispersant oxidation products were clearly evidenced on the aKMD plots due to the gaps of 56 Da between each point. Overall, this study demonstrated the efficiency of the aKMD approach, and the use of ESI/APCI to characterize base oil and additive oxidation products.

Tribological properties and synergetic self-lubricating mechanism of PS-PAO based nanocapsules with MoDTC addtives

With development of encapsulation technology, analysis of dependent nano-scaled microcapsule is significant to intensively understand self-lubrication behavior. In this paper, polystyrene (PS)-poly alpha olefin (PAO) capsules were synthesized with PAO as core and PS as shell. The diameter, shell thickness and encapsulated rate of PS-PAO4 were 381.51 ± 25.44 nm, 40 nm and 43.29 wt.%. The friction coefficient (COF) of PS-PAO4 by SRV friction test was 0.089, which played intermediate between PS nanosphere and PAO oil. Comprehensive analysis with Hamrock-Dowson equation and classical Stribeck curve manifested that the PS-PAO nanocapsules and PS nanospheres stayed in mixed lubrication regime, and PAO oil stayed in the boundary lubrication regime. Due to the absence of lubrication, PS dry friction and asperities contact status were more significant to determine the lubrication state of PS-PAO4 sample. The influence factors including load, operating temperature, PAO types and molybdenum dithiocarbamate (MoDTC) addtives were respectively investigated. A three-stage decline curve of PS-PAO4/MoDTC was found with final COF of 0.015, which the COF value decreased by 83.15% compared by PS-PAO4. The synergetic self-lubricating mechanism was mainly determined by asperity contacts, PS debris grinding, FeS tribofilm formation of MoDTC additives and counterparts, transfer film and third body formation by PS, MoDTC, PAO oil and counterparts. This work provides intensive understanding of nanocapsules for mixed lubrication regime and is expected to apply in ultrathin coating, liquid lubricant additives and functional polishing abrasives under heavy-duty and low speed conditions.

Friction Reduction by Laser Irradiation for a Friction System Using Bearing Steel and Aluminum Alloy in Engine Oil

Aluminum alloy sliding components are widely used in internal combustion engines. However, aluminum easily adheres to the countersurface, resulting in high friction. Hence, laser irradiation was used to oxidize the aluminum surface and promote tribofilm formation using molybdenum dithiocarbamate and zinc dialkyldithiophosphate (ZnDTP). This study investigated laser irradiation to reduce friction between steel balls and aluminum disks in fully formulated engine oil. Laser scanning was used to create 100-μm-pitch concentric circles on the disks. The friction behavior was classified into two modes: no friction reduction (mode-I, μ: 0.09–0.12) and friction reduction after the high-friction period (mode-II, μ: 0.06–0.09). Friction mode transition occurred when the laser energy density exceeded the critical value (Ec). Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy showed that, for mode-I, aluminum adhered to the ball, and no sulfur-containing tribofilm formed. For mode-II, the ZnDTP-derived phosphate film formed on the disk suppressed aluminum adhesion, and a sulfur-containing tribofilm formed on the ball. Micro-Vickers tests and X-ray diffraction showed that an amorphous/nanocrystalline structure formed in the unirradiated area owing to heat diffusion under high-energy-density laser irradiation (>Ec). Results suggest that metallographic structural change in the unirradiated area promotes ZnDTP reactions, causing the mode transition.

Impact of Fatty Triamine on Friction Reduction Performance of MoDTC Lubrication Additive

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.

Unraveling the mechanism to form MoS[formula omitted] lubricant layers from MoDTC by ab initio simulations

The morphology of molybdenum disulfide (MoS2) is a crucial aspect to ensure the functionality of this remarkable 2D-material both in electronic and tribological applications. Indeed, molybdenum dithiocarbamates (MoDTCs) can be tribochemically transformed into MoS2, which is able to reduce the friction coefficient of metallic moving parts. However, this transformation is influenced by temperature, sulfur/oxygen ratio, normal and shear stresses, making the mechanism of this process particularly challenging to explain. Ab initio simulations based on density functional theory (DFT), including a quantum mechanics/molecular mechanics (QM/MM) approach, are used here to shed light on the crystallization of MoS2 promoted by mechanical stresses. Chemistry plays an important role during the reorganization of the units of MoSx obtained from MoDTC, because sulfur and oxygen atoms tend to move outside of the amorphous layer, surrounding the molybdenum atoms and creating a structure that can crystallize into MoS2. Normal load and sliding have a synergistic effect in rearranging the amorphous units into a crystalline structure, as the former helps overcoming the energy barriers associated to bonds breaking and forming, while the latter allows misplaced atoms to be pulled towards the crystalline sites. A crystalline MoS2 was obtained by ab initio calculations below 1000 K.

Frictional Power Evaluation of Additive-Mixture in Trimethylolpropane Trioleate Oil using Single Cylinder Diesel Engine

Rapid depletion of petroleum sources and increase of demand for environmentally friendly products has urged the need for biodegradable lubricants to be utilized in various applications, especially for internal combustion (IC) engine lubrication. To perform on par with the commercial engine oils, bio-based lubricants need to be further improved through the addition of appropriate additives. This study investigates the synergistic performance of additive-mixture comprising of ionic liquids (IL), glycerol monooleate (GMO), molybdenum dithiocarbamate (MoDTC) and titanium dioxide (TiO2) nanoparticle in trimethylolpropane trioleate (TMPTO) ester base oil. Lubricity performances of the prepared sample were evaluated in the context of tribological improvement using laboratory tribotester as well as friction power reduction in actual single-cylinder diesel engine. From a prior study, it was found that additive mixture containing 0.93 wt.% IL, 1.49 wt.% GMO, 0.52 wt.% MoDTC, and 1.25wt.% TiO2 was able to provide substantial friction and wear as well as extreme pressure improvements. The present study was conducted to evaluate whether the formulation can offer the same performance on the actual engine in conjunction to the laboratory bench test. To do so, two different types of engine tests were conducted which are that standard engine performance test and also the Willans line test to evaluate the engine frictional power. The performance of the additives-TMPTO mixture was compared directly to those of commercial SAE 0W-30 engine lubricant. In the standard engine performance test, it was found that the additives-TMPTO mixture performed better than commercial SAE 0W-30 engine lubricant in terms of 2.5% higher maximum torque value, 3% higher overall brake power output, 10% lower brake specific fuel consumption and 12% higher brake thermal efficiency throughout the conducted test speed of 800 to 2300 rpm. Through the Willans line test, it was found that the additives-TMPTO mixture showed better results than the commercial SAE 0W-30 engine lubricant in terms of 20-25% lower frictional power at 900, 1200 and 1500 rpm. Overall, these results further justify the enhanced lubrication properties of the formulated bio-based oil in addition to the prior conducted tribological analyses.

Efficient friction and wear reduction of Al-Si alloy via tribofilms generated from synergistic interaction of ZDDP and chemically functionalized h-BN additives

Zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) are conventional lubricant additives for reducing friction and wear of metallic components. However, their efficacy for lightweight aluminum (Al)-based alloys is limited due to the wear of soft Al matrix regions. The present work reports significant improvement in the tribological properties of Al-Si alloys (ADC12) under liquid lubricants containing a combination of ZDDP and hexagonal boron nitride (h-BN) nanosheets. In situ monitoring of tribofilm growth, friction, and wear was carried out using atomic force microscopy (AFM)-based measurements. At high temperature (110 °C), experiments were carried out at the Al matrix and Si phase of ADC12 alloy under the lubricant containing ZDDP (1 wt%) and tert-butoxy (TBO) functionalized h-BN nanosheets [(h-BN-TBO), (0.02 mg/mL and 0.05 mg/mL)]. The addition of h-BN-TBO in ZDDP containing lubricant significantly improved the tribological properties due to the synergistic interaction between h-BN-TBO and ZDDP; leading to the formation of lubricious and robust antiwear tribofilms on Si and Al phase regions. A synergistic combination of h-BN-TBO and ZDDP as lubricant additive showed superior performance compared to ZDDP or a combination of ZDDP and MoDTC-based lubricant systems.

Optimizing the Mo concentration in low viscosity fully formulated oils

Molybdenum dithiocarbamate (MoDTC) is usually added in low viscosity engine oils to improve its ability to reduce friction. However, increasing the amount of MoDTC added results in unwanted sulfated ash increases and other adverse effects. Thus, it is necessary to investigate the optimization of the amount of MoDTC added. The current work investigates this topic in terms of the Mo concentration in the lubricant. Seven different Mo concentrations with the same additive package are tested, whose tribological properties and chemistry of the tribofilm are investigated. For the selected operating conditions in this study, the critical concentration of Mo is determined to be ≈ 350 ppm due to its ability to reduce the friction to ≈ 0.04 under a constant and varying lambda ratio by forming the required threshold thickness of MoS2 within the tribofilm matrix, dominated by zinc dialkyl dithiophosphate (ZDDP) species. In general, increasing the Mo concentration increases the formation rate of MoS2 in the initial stages of the traction, directly forming a thicker MoS2 layer within the tribofilm up to a specific Mo concentration. Induction time also decreases with increasing Mo concentration, and increasing the Mo concentration from 350 to 1000 ppm does not negatively impact the wear generated.

Molecular evidence for improved tribological performances of MoDTC induced by methylene-bis(dithiocarbamates) in engine lubricants.

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.

Molecular evidence for sulfurization of molybdenum dithiocarbamates (MoDTC) by zinc dithiophosphates: a key process in their synergetic interactions and the enhanced preservation of MoDTC in formulated lubricants?

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.

Dithiocarbamate transfer reaction from methylene-bis(dithiocarbamates) to molybdenum dithiocarbamates in engine lubricants investigated using laboratory experiments

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

Tribochemical reaction and wear mechanism of MoDTC based friction modifier

In this study, molybdenum dithiocarbamate (MoDTC) coatings were fabricated on 304 stainless steel substrates via an electrodynamic spraying process with the aim to clearly understand the evolutionary process from MoDTC to MoS2 during contact sliding. Experiments were performed using an in-situ tribo-tester equipped with Raman spectroscopy to continuously monitor the chemical state of the wear track. The tribochemical reaction and removal process of MoDTC were assessed, and the evolution process of MoDTC was determined systematically. Additionally, it was determined that the tribological performance of the MoDTC coating was dominated by a tribochemical reaction at the microscale and by surface topography at the macroscale.

Importance of the catalytic effect of the substrate in the functionality of lubricant additives: the case of molybdenum dithiocarbamates

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.

Raman and TEM characterization of 2D layered MoS2 crystals grown on non-metal surfaces by friction-induced synthesis

Thin films of crystalline molybdenum disulfide (MoS2) are grown by friction-induced synthesis at the interface between a n+-Si(100) substrate and a synthetic quartz ball. The thin films are formed as a result of the tribochemical reaction of molybdenum dithiocarbamates (MoDTC) in synthetic oil under sliding friction conditions. Raman mapping shows the width of the grown MoS2 film to be about 7 µm, and the difference between the Raman frequencies of the E12g and A1g modes indicate that there are more than 6 monolayers in the MoS2 film. A TEM image indicates that a layer-by-layer structure is formed at the top surface of the stacked films. These results show that it is possible to fabricate a 2D layered MoS2 film on a non-metal surface using this friction induced crystal growth method.