Experimental simulation of phosphites additives tribochemical reactions by gas phase lubrication

Abstract

Tribochemical reactions of phosphites additives on steel surface have been simulated by gas phase lubrication. Trimethylphosphite (TMPi), P(OCH₃)₃, has been used as model molecule for phosphites additives. It has been introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests have been carried out at ambient temperature and 100°C. Chemical analyses by X-ray photoelectron spectroscopy and by Auger electron spectroscopy have been conducted inside and outside of the track. Two kinds of analysis have been carried out: ex situ and in situ surface analyses after tribological test. Indeed, a new environmentally controlled tribometer allows friction test then accurate analyses without air exposure of the formed tribofilm. Tribotests conducted under TMPi gas phase show a reduction of friction coefficient until 0˙2 instead of 1˙4 under high vacuum. Jointly, formation of tribofilm has been confirmed by optical microscopy and ex situ chemical analysis. Comparison between analyses performed inside and outside of the wear scar indicates that the friction induces the formation of phosphide compound that could reduce friction. Moreover analyses show the formation of methoxy group (CH₃O) and carbonate originally from the decomposition of TMPi under friction into H₂ and CO. In situ analyses clearly show the importance to investigate an uncontaminated tribofilm in order to obtain a better characterisation of it and then a better comprehension of the tribochemical mechanisms.     

Understanding the Tribochemical Mechanisms of IF-MoS<Subscript>2</Subscript> Nanoparticles Under Boundary Lubrication

Inorganic fullerene-(IF)-like nanoparticles made of metal dichalcogenides (IF-MoS₂, IF-WS₂) have been known to be effective as anti-wear and friction modifier additives under boundary lubrication. The lubrication mechanism of these nanoparticles has been widely investigated in the past and it is now admitted that their lubrication properties are attributed to a gradual exfoliation of the external sheets of the particles during the friction process leading to their transfer onto the asperities of the reciprocating surfaces. However, the chemical interaction between these molecular sheets and the rubbing surfaces has so far never been investigated in detail. In this study, the tribochemistry of the IF nanoparticles was carefully investigated. A series of friction test experiments on different rubbing surfaces (Steel, Alumina, Diamond-Like Carbon) were performed with IF-MoS₂ nanoparticles. High-resolution transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the tribostressed areas on rubbing surfaces. A tribofilm composed of hexagonal 2H-MoS₂ nanosheets was only observed on the steel surface. This transfer film was found to be incorporated into an iron oxide layer. A tribochemical reaction between the 2H-MoS₂ nanolayers and the iron/iron oxide has been proposed as an explanation for the adhesion of this tribofilm. The tribochemical mechanism of the IF-MoS₂ nanoparticles is discussed in this article.     

Tribochemical processes in engine oil with copper nanoparticles and azomethine ligand

The tribochemical processes in SAE 10W40 engine mineral oil with complexing compound and dioctylphtalate have been studied. A tribofilm that reduces the coefficient of friction is found to be formed on the friction surfaces; it is probed via optical and scanning electron microscopy, FTIR infrared spectroscopy, and X-Ray fluorescence and is established to be composed of copper covering the steel surface and a tribopolymer film. The effect of tribochemical processes with involving the steel surface, copper nanopowder, ligands, coordination compounds, and complex ether in the formation of a tribofilm has also been elucidated.     

Micellar calcium borate as an antiwear additive

The mechanisms of action of a new generation of antiwear additives is studied here by means of energy‐filtering transmission electron microscopy (EFTEM) carried out on the wear particles generated during friction tests between two ferrous surfaces (under boundary lubrication conditions). This paper deals with the structural and physico‐chemical changes that colloidal particles, calcium carbonate (CC) and calcium borate (CB) overbased salicylates detergents, have undergone during the build‐up of the interfacial antiwear tribofilm. EFTEM allowed us to investigate the nature of wear fragments originating from the film, stemming from CC and CB micelles, and to make a comparison regarding the tribofilm formation mechanisms. It appears that the CC wear debris are mainly crystalline and contain a high concentration of iron (as abrasive iron oxide Fe₂O₃), limiting their antiwear action. Consequently, CC micelles do not lead to an effective protective tribofilm. On the other hand, CB micelles do have an antiwear action, which we explained by the formation of a glassy iron borate tribofilm during the friction tests. Many of the CB wear fragments are composed of this amorphous material containing very small crystallites of residual calcite. Boron (contained in the CB micelles) is responsible for the presence of amorphous zones of the film and acts as a glass former, in a comparable way to phosphorus in zinc dithiophosphate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lubrication of carbon coatings with MoS2 single sheet formed by MoDTC and ZDDP lubricants

The fuel economy and reduction of harmful elements of lubricants are becoming important issues in the automotive industry. One approach to these requirements is the potential use of low‐friction coatings in engine components exposed to boundary lubrication conditions. Diamond‐like carbon (DLC) coatings, extensively studied as ultra‐low friction films to protect ductile metals surfaces for space applications, are expected to fit the bill. The main purpose of this work is to investigate the friction and wear properties of DLC coatings lubricated with molybdenum dithiocarbamate (MoDTC) and zinc dithiophosphate (ZDDP) under boundary lubrication conditions. The mechanisms by which MoDTC reduces the friction in the centirange were studied using ultra‐high vacuum (UHV) analytical tribometer. The UHV friction tests were performed on a tribofilm previously formed on selected DLC material with MoDTC and ZDDP containing oil. Ex‐situ characterizations show that the composition of this tribofilm is similar to that of a tribofilm obtained on steel surfaces in the same lubrication conditions with MoS₂ single sheets dispersed inside zinc phosphate zones. However, analyses by X‐ray photoelectron spectroscopy (XPS) indicate that MoDTC and ZDDP additives seem to be more active on steel surfaces than carbonaceous ones. After UHV friction with the tribofilm formed on selected DLC and steel pin counterpart, the wear scars of both sliding surfaces were characterized by in‐situ analytical tools such as Auger electron spectroscopy, scanning Auger microscopy and micro‐spot XPS. Low friction is associated with the transfer of a thin MoS₂ film to the steel pin counterpart. Copyright © 2006 John Wiley & Sons, Ltd.     

Antiwear Tribofilm Formation on Steel Surfaces Lubricated With Gear Oil Containing Borate,Phosphorus, and Sulfur Additives

The formation of antiwear tribofilms plays a critical role in the longevity of automotive gears. The focus of this experimental study was on the lubrication efficacy of gear oils with different contents of borate-, phosphorus-, and sulfur-containing additives leading to the formation of protective tribofilms. Experiments were performed with AISI 52100 steel balls sliding against AISI 52100 steel disks in baths of different oils at ambient (～32 °C) and elevated (～100 °C) temperatures under load and speed conditions favoring sliding in the boundary lubrication regime. Friction coefficient responses accompanied by electrical contact voltage measurements provided real-time information about the formation and durability of the antiwear tribofilms. The wear resistance of the tribochemical films was quantified by wear rate data obtained from surface profilometry measurements of wear tracks on the disk specimens and sliding tests performed at ambient temperatures after the formation of the tribofilms during elevated-temperature sliding. Results indicate a strong dependence of tribofilm formation on temperature and type of additives. The slightly lower friction and higher wear resistance obtained at elevated temperatures with blended oils is attributed to the increased chemical reactivity of additives containing borate, phosphorus, and sulfur, leading to the formation of durable tribofilms. Relatively higher wear resistance and faster tribofilm formation were obtained with the borate-enriched gear oil formulations.     

A dual‐analysis approach in tribochemistry: application to ZDDP/calcium borate additive interactions

Tribochemical interactions between zinc dithiophosphate (ZDDP) and micellar calcium borate (CB) under boundary lubrication were investigated by coupling, in the same location of the wear track, both analytical TEM analysis of collected wear fragments from the tribofilm and XPS surface analysis of the tribofilm directly underneath. This is the so‐called dual‐analysis approach, which improves the interpretation of tribochemical reactions. The elemental composition inside the wear scars was analysed by micro‐spot XPS. By depth profiling, the film thickness could also be determined. In particular, the efficiency of the additive combination could be proven by quantification of iron oxide. The nature of wear particles was investigated in the TEM by using EELS and EDX simultaneously, with the result that phosphorus, boron and sulphur contributions have been carefully distinguished. The technique is very powerful for determining the composition of the material through quantification of both EELS and EDX spectra on the same specimen. The main result, when ZDDP and CB additives are used together, is the formation of a calcium and zinc borophosphate glass tribofilm. The overall data confirm the general friction‐induced glass model as being a unifying concept that explains the mechanisms of antiwear additives under boundary lubrication. Moreover, the analytical results strongly suggest the role of viscous flow of the magma state glass tribofilm above its glass transition temperature to be a main contribution to the antiwear mechanism under mild wear conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tribochemical interactions between Zndtp,Modtc and calcium borate

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and overbased detergent calcium borate (OCB) lubricant additives have been investigated. Friction tests were performed in mild wear conditions under boundary lubrication, in order to enhance tribochemical surface effects. The nature of tribofilms formed was studied by coupling high‐resolution TEM on wear fragments and inside‐wear‐scar, micro‐spot XPS in the same location of the wear track (so‐called dual analysis). The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS₂ single sheets and the digestion of MoO₃ in the zinc polyphosphate glass formed. The final result of the tribochemical reaction is a two‐phase tribofilm composed of (i) non‐oriented MoS₂ sheets (friction modifier) embedded in a carbon‐rich phase and (ii) a mixed Zn/Mo polyphosphate glass (antiwear). The Modtc/OCB mixture has a similar antiwear mechanism except that the oxide is not completely eliminated, due to the softer action of borate anion compared with phosphate one. Compared to the data obtained with binary combinations (Modtc/Zndtp, Modtc/OCB and Zndtp/OCB), we show here that the ternary system Modtc/Zndtp/OCB provides both a low wear rate and an ultralow friction value, while adding detergent and anti‐corrosive properties to the formulation. Our analytical data indicate that the synergistic effect can be attributed to an outstanding nanostructure of the tribofilm formed. It is composed of a single‐phase material containing perfectly oriented MoS₂ single sheets embedded in a calcium and zinc borophosphate glass. The ternary system produces a smart material in the interface, because both functions (antiwear and friction reduction) are correlated. Compared to phosphate alone, the mechanism by which MoS₂ sheets have been oriented in the borophosphate could be related to aligned molecules of the glassy polymer in the direction of sliding. This revised version was published online in July 2006 with corrections to the Cover Date.     

渗氮GCr15钢在含氮硼酸酯添加剂润滑油作用下的摩擦学性能

针对提高滚动轴承使用寿命的问题,提出离子渗氮处理与添加剂结合的解决办法。用等离子渗氮炉在GCr15轴承钢表面制备了渗氮层,并用扫描电子显微镜(Scanning electron microscope, SEM )、X射线衍射仪(X-ray diffractometer, XRD)和显微硬度计观测了渗层的形貌、相结构和硬度变化。利用四球摩擦磨损试验机对比考察渗氮钢和未渗氮钢在含氮硼酸酯润滑条件下的摩擦学性能,并用X射线光电子能谱仪(X-ray photoelectron spectroscope, XPS)分析摩擦反应膜的成分与化学结合态。结果表明,渗氮层与润滑油添加剂之间发生了意想不到的良好交互作用,渗氮钢在质量分数为1.25%氮硼酸酯的润滑条件下表现出最低的摩擦因数和磨斑直径,比未渗氮钢分别降低了34%和45%;分析证明,渗氮层的摩擦表面生成了高BN质量分数的摩擦反应膜是获得优异摩擦学性能的主要原因,而在未渗氮钢摩擦表面未检测到BN。     

Deactivation Effect of Tricresyl Phosphate (TCP) on Tribochemical Decomposition of Hydrocarbon Oil on a Nascent Steel Surface

A nascent surface has high activity to catalyze the decomposition of a lubricant under boundary lubrication conditions. To reduce the decomposition of a lubricant (multialkylated cyclopentane, MAC), tricresyl phosphate (TCP) was introduced as an additive. The tribological properties and decomposition process of lubricants on the nascent surface of bearing steel 52100 were investigated by a ball-on-disk friction tester in a vacuum chamber with a quadrupole mass spectrometer (Q-MS). The addition of TCP prolonged the induction period for decomposition of the lubricant. During the friction processes, hydrogen and gaseous hydrocarbons desorbed as tribochemical reaction products. XPS analysis revealed that the tribofilm from the additive was mainly composed of iron phosphate, which decreased the probability of generating a nascent surface, resulting in the reduction of desorption rate of gaseous products. The critical load for the mechanical activation of the decomposition correspondingly doubled.     

Tribological properties of plasma nitrided stainless steel against SAE52100 steel under ionic liquid lubrication condition

1Cr18Ni9Ti stainless steel was modified by plasma nitriding. The phase composition of the plasma nitrided layer was examined by means of X-ray diffraction. The friction and wear properties of the modified and unmodified 1Cr18Ni9Ti stainless steel specimens sliding against SAE52100 steel under the lubrication of ionic liquid of 1-ethyl-3-hexylimidazolium hexafluorophosphate (L-P308) and poly α-olefin (PAO) were investigated on an Optimol SRV oscillating friction and wear tester, with the interactions among the modified surface layer and the ionic liquids and PAO to be focused on. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces of the modified steel surfaces were examined by means of X-ray photoelectron spectroscopy. Results showed that the modified sample had better anti-wear abilities than the unmodified one, but the modified sample had a slightly higher friction coefficient than the untreated one. This was partly attributed to the change in the hardness and phase composition of the stainless steel surfaces after plasma nitriding and tribochemical reactions between the steel and the lubricant. The resultant surface protective films composed of various tribochemical products together with the adsorbed boundary lubricating film contributed to reduce the friction and wear.     

Extreme Pressure Lubricant Additives Interacting on the Surface of Steel- and Tungsten Carbide–Doped Diamond-Like Carbon

Certain diamond-like carbon (DLC) coatings offer excellent tribological properties under both dry and oil-lubricated sliding conditions. However, the underlying mechanisms under lubricated conditions are generally not fully understood, especially when performance depends on strong tribochemical interactions with lubricant additives. The aim of the present work is to explore the friction and wear performance of steel and tungsten carbide (WC)-doped DLC (WC-DLC) surfaces in the presence of different types of extreme pressure (EP) and nitrogen–sulfur-based (NS) additives. Tribological tests were performed on a ball-on-disc test rig, and X-ray photoelectron spectroscopy (XPS) was used for physical and chemical characterization of the tribofilms. It was observed that EP and NS additives significantly reduced the wear of WC-DLC surfaces in comparison with tests conducted on steel surfaces. XPS indicated that the additive interactions on the WC-DLC surface formed a distinctive tribofilm that promoted better friction and wear performance. The higher concentration of carbon compounds and lower concentration of oxygen compounds in the tribofilm significantly improved friction and wear characteristics.     

Frictional Characteristics of IF-WS2 Nanoparticles in Simulated Engine Conditions

The efficacy of inorganic fullerene-like (IF) tungsten disulfide (WS₂) nanoparticles as engine oil friction modifier additive was investigated using a high-stroke reciprocating piston ring–cylinder bore bench test system. Frictional characteristics of various IF-WS₂ nanoparticle concentrations in mineral oil were experimentally analyzed in simulated engine conditions controlling oil temperature, speed, and normal load. Though the effect of IF-WS₂ nanoparticles on piston ring and cylinder bore friction was minor in low concentrations, after a running period in a mixed lubrication regime with 10% additive, a considerable reduction in friction coefficient was observed when the IF-WS₂ formulated oil was used. This reduction remains to some extent with reference mineral oil after solvent cleaning of the mating components. The results show that a thin tribofilm gradually forms on the piston ring and the cylinder bore surfaces, reducing the friction coefficient in a mixed lubrication regime. However, in order to obtain the friction reduction there seems to be a threshold in the concentration of IF-WS₂ in mineral oil and film development period. The effects of lubrication regime, contact load, speed, and surface roughness on tribofilm formation are also discussed. The presence of WS₂ tribofilm formed on the piston ring and the cylinder bore surfaces was further verified using Raman spectroscopy.     

Influence of Steel Type on the Propensity for Tribochemical Wear in Boundary Lubrication with a Wind Turbine Gear Oil

Tribochemical wear may occur at the interface between a surface and a lubricant as a result of chemical and mechanical interactions in a tribological contact. Understanding the onset of tribochemical wear damage on component surfaces requires the use of high resolution techniques such as transmission electron microscopy (TEM). In this study, two steel types, case carburized AISI 3310 and through-hardened AISI 52100, were wear tested using a ball-on-disk rolling/sliding contact tribometer in fully formulated commercial wind turbine gearbox oil under boundary lubrication conditions with 10% slip. With the exception of steel type, all other test conditions were held constant. Conventional tribofilm analysis in the wear tracks was performed using X-ray photoelectron spectroscopy, and no significant composition differences were detected in the tribofilms for the different steel disk types. However, TEM analysis revealed significant tribochemical wear differences between the two steel types at multiple length scales, from the near-surface material microstructure (depth < 500 nm) to the tribofilm nanostructure. Nanometer-scale interfacial cracking and surface particle detachment was observed for the AISI 52100 case, whereas the tribofilm/substrate interface was abrupt and undamaged for the AISI 3310 case. Differences in tribofilm structure, including the location and orientation of MoS₂ single sheet inclusions, were observed as a function of steel type as well. It is suggested that the tribochemical wear modes observed in these experiments may be origins of macroscopic surface-initiated damage such as micropitting in bearings and gears.     

Tribochemistry of Bulk Zinc Metaphosphate Glasses

Zinc polyphosphate glasses are the principal component of the antiwear tribofilms formed on steel surfaces in the presence of additives, such as zinc dialkyldithiophosphates. In this work, amorphous, zinc metaphosphate glasses have been synthesized and characterized by means of X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), elemental analysis, and X-ray photoelectron spectroscopy (XPS). Tribological tests were performed by rubbing steel balls against the zinc metaphosphate discs in a poly-α-olefin (PAO) bath at room temperature. XPS was used in order to characterize the tribostressed areas on both metaphosphate discs and steel balls. A transfer film, constituted of iron and zinc polyphosphates, was formed on the contact area of the balls. This transfer film was found to reduce friction and prevent ball wear. A reduction in the relative intensities of XPS signals related to bridging-oxygen species and a binding energy shift of 0.4 eV of the P 2p toward lower values demonstrated the presence of shorter-chain-length phosphates inside the tribo-tracks on the discs. Furthermore, iron was transferred to the glass during the tribological tests. A tribochemical reaction between zinc metaphosphate and iron oxide has been proposed as an explanation for the depolymerization of the glass and the formation of iron phosphate.     

The Formation of Tribofilms of MoS2 Nanotubes on Steel and DLC-Coated Surfaces

Solid-lubricant nanoparticles as additives in oil provide good tribological properties based on the physical lubrication mechanisms in the contact. For this reason, they are strong candidates for use in the lubrication of diamond-like carbon (DLC) coatings, which only poorly interact with the traditional, chemically based additives. In this study, we focused on how a tribofilm formed from MoS₂ nanotubes is related to the tribological properties of these nanotubes, and then, we analysed such a tribofilm on steel and DLC-coated surfaces using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. We demonstrated that when using oil containing MoS₂ nanoparticles, the formation of a tribofilm is a key factor in decreasing the friction for the steel and DLC-coated contacts. The major difference between the steel and the DLC contacts is the extent to which the MoS₂-based tribofilm covers the surface, which is 20 % in the case of the DLC/DLC contacts, but almost 40 % in the case of the steel/steel contacts. Moreover, the MoS₂-based tribofilm was found to be more oxidized on the DLC surface than on the steel surface. Nevertheless, we found that the chemical and functional properties of the MoS₂-based tribofilm are very similar, or even the same, for both the steel and DLC-coated surfaces. No direct evidence of any chemical reactions between the MoS₂ and the steel or DLC coating was observed.     

X-Ray Photoelectron Spectroscopy Analysis of Antiwear Tribofilms Produced on Boundary-Lubricated Steel Surfaces from Sulfur- and Phosphorus-Containing Additives and Metal Deactivator Additive

X-ray photoelectron spectroscopy (XPS) was performed on AISI 52100 steel surfaces subjected to sliding in the boundary lubrication regime at 32 and 100°C. The specimens were lubricated with base oil blended with individual additives containing sulfur (S), phosphorus (P), or metal deactivator, as well as base oil with all the previous additives in the same amounts as in the single blends. XPS spectra were analyzed to confirm the formation and determine the chemical composition of the antiwear tribofilms produced on the steel surfaces during sliding. The use of S- and P-containing additives on the tested disk surfaces revealed that tribochemical reactions resulted in the formation of antiwear tribofilms containing S- and P-rich components. Results for the multi-additive blend provided evidence for two components in the produced tribofilm, appearing to consist primarily of sulfide and phosphate. This investigation provides new insight into the competing roles of these compounds on the tribological properties of the antiwear tribofilms. The significance of the sulfide components is demonstrated by the more pronounced antiwear effect of the S-containing additive in the multi-additive formulation.     

Non-ferrous coating/lubricant interactions in tribological contacts: Assessment of tribofilms

Use of low friction non-ferrous coatings for engine tribo-components exposed to boundary lubrication is becoming popular in automotive industries. The excellent tribological behaviour of some non-ferrous coatings also reduces dependence on some harmful components of lubricants. In this work, hydrogenated diamond like carbon (HDLC) and chromium nitride (CrN) coatings sliding against cast iron counterbody have been used to study the interaction with friction modifiers (Moly dimer and Moly trimer) and antiwear additive zinc dialkyldithiophosphate (ZDDP) under boundary lubrication condition. The tribological results of the non-ferrous coatings are compared with those of uncoated steel. Tribofilms are formed using a reciprocating pin-on-plate tribometer. The chemical analysis of the tribofilms has been accomplished using X-ray photoelectron spectroscopy (XPS). The XPS analysis shows that the friction modifiers form a low friction tribofilm on the non-ferrous coatings. No antiwear tribofilm derived from ZDDP was observed on the HDLC coating but a stable antiwear tribofilm was found on the CrN coating. Moly dimer together with ZDDP+Base Oil showed the lowest friction coefficient for the CrN coating while Moly trimer along with ZDDP+Base Oil gave the lowest friction for the HDLC coating. This study will investigate the generic differences between the tribofilms formed on the DLC and CrN coatings by two additive-containing oils.     

Synergistic effects in binary systems of lubricant additives: a chemical hardness approach

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and detergent overbased calcium borate (CB) lubricant additives have been investigated by coupling analytical TEM and micro‐spot XPS in the tribotester Optimol of SRV GmbH (mild wear conditions in boundary lubrication). Synergistic effects have been observed on both friction and wear data, especially in the Modtc/Zndtp combination. Results have been interpreted on the basis of a chemical hardness concept: the hard and soft acids and bases (HSAB) principle, stabilisation of hard–hard pairs and the maximum hardness principle. The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS₂ single sheets and the digestion of MoO₃, which is also formed, by the zinc polyphosphate glass. The final result of the tribochemical reaction is a tribofilm composed of MoS₂ sheets embedded in a mixed Mo/Zn polyphosphate glass. The CB/Modtc mixture has a similar mechanism except that the oxide is not completely eliminated, due to the softer borate anion compared with the phosphate one. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transient processes of MoS2 tribofilm formation under boundary lubrication

A tribochemistry study that involves the application of Raman spectroscopy surface analysis has been undertaken to understand the time‐dependent tribochemical reactions, for lubrication by Molybdenum dialkyl‐dithiocarbamate (MoDTC) occurring in boundary lubricated conditions. Under the conditions of rubbing and high temperature, time‐resolved Raman spectroscopy results show the intermediate steps that lead to the MoDTC additive to be tribochemically structured on the wear scar of the contacting surface. A MoS₂ tribofilm with a lattice layer structure is observed on the wear scar whenever the lowest friction was achieved. An apparent shift of the A_1g and E_2g Raman modes, indicating qualitative and quantitative information on the MoS₂ tribofilm formed, is observed to be related to low friction. Detailed analyses of Raman spectra obtained on wear scars at different test durations and temperatures indicate that both temperature and rubbing are needed for the formation of low friction MoS₂ tribofilm. Copyright © 2016 John Wiley & Sons, Ltd.