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 A1g and E2g 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

Investigation of pure sliding and sliding/rolling contacts in a DLC/Cast iron system when lubricated in oils containing MoDTC-Type friction modifier

Diamond-like carbon (DLC)/cast iron (CI) systems have been widely investigated due to their important application in engine components such as cylinders, pistons and more specifically for the cam/follower interface. The pure sliding contact of the DLC/CI system has traditionally been the focus of research; consequently less is understood about sliding/rolling contact systems. In addition, the tribological and tribochemical characteristics of the Molybdenum Dialkyl Dithiocarbamate (MoDTC) as a lubricant additive in such sliding/rolling contacts are not fully understood. In this study, a Mini Traction Machine (MTM) was used to run the experiments using alloy steel balls coated with 15 atomic percent (at. %) hydrogenated DLC (a-C: 15H) rubbing against uncoated cast iron discs. Results showed that the sliding/rolling ratio affects friction, wear and tribochemistry in CI/DLC systems; pure sliding enhances MoDTC activation. MoDTC decomposes to form MoS₂, FeMoO₄ and not MoO₃. In addition, it was observed that MoS2/FeMoO₄ ratio depends on test conditions and affects to the friction performance

New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study

Molybdenum DialkyldiThioCarbamate (MoDTC) is a friction modifier that has been used in automotive engines for many years. However, its exact decomposition mechanism within tribocontacts is not fully understood. In this study, an attempt has been made towards understanding the mechanism of MoDTC decomposition in steel/steel contacts by employing Raman spectroscopy. Results show that the main MoDTC decomposition products are MoS2, FeMoO4 and sulphur-rich molybdenum compounds, MoSx (x>2), in contrast to the previously reported MoS2 and MoO3. Formation of these products is dependent on tribological parameters. Raman results from this study indicate that the Mo6+ species previously observed in X-ray Photoelectron Spectroscopy (XPS) analysis are probably from FeMoO4 and not MoO3. This paper presents an alternative reaction pathway for MoDTC decomposition in steel/steel contacts with MoSx as an intermediate product and MoS2 as the final product. FeMoO4 is formed from a side reaction of iron oxides with molybdenum compounds at low temperatures and low MoDTC concentrations. Results also show that friction is dependent on the nature of decomposition products at the tribocontact. Knowledge of MoDTC decomposition reaction pathway will enable the friction performance of MoDTC lubricants to be optimized

Raman spectroscopic studies of friction modifier Molybdenum DialkyldiThioCarbamate (MoDTC)

Presently, there is a strong push towards improving fuel economy in passenger cars. Poor fuel economy is attributed to high friction in various components within car engines. About 5% of the friction losses in internal combustion engines occur in the boundary lubrication regime where metal metal contact is present. Lubrication in boundary lubrication regime is achieved by using lubricants containing chemically active additives which react with the surfaces to form thin films known as tribofilms. The formed tribofilms provide friction reduction and wear protection due to their physicochemical properties. Molybdenum DialkyldiThioCarbamate (MoDTC) is an additive added in engine oil mainly as a friction modifier. MoDTC reduces friction by degradation of the additive to form discrete MoS2 at the tribocontact. There is however little knowledge on the degradation process of MoDTC at the tribocontact. In this thesis, tribochemical reactions that occur in steel/steel tribocontacts in the presence of MoDTC additive have been investigated. Tribological tests were conducted using model oils comprising of MoDTC additive in mineral base oil. Tests were conducted under unidirectional linear sliding and sliding/rolling conditions. Raman spectroscopy was used to conduct chemical characterisation of the rubbed surfaces. Results show that mechanical activation accelerates the rate of MoDTC degradation. Under tribological conditions, MoDTC decomposes to form three main compounds; MoS2, MoSx and FeMoO4. MoDTC decomposition products formed at the tribocontact are dependent on test conditions. The mechanism for the degradation process has been proposed. MoDTC tribofilms were observed to grow rapidly within generated wear scars until a limiting thickness was achieved. The limiting thickness was dependent on contact parameters. The chemical composition of MoDTC tribofilms determined the friction observed in tribotests. In sliding/rolling conditions, low friction values (µ=0.04-0.05) were obtained when the tribofilms are composed of MoS2 while higher friction (µ=0.06-0.08) was observed when the tribofilms were composed of Fe2O3, Fe3O4, MoSx and FeMoO4. MoDTC provided wear protection to the steel substrates only at test conditions which allowed MoDTC tribofilms to be present at the contact. In conditions where MoDTC tribofilms were missing from the contact, severe wear of the substrate was observed. The durability of MoDTC tribofilms when rubbed in MoDTC free lubricant was observed to be dependent on the sliding configuration. MoDTC tribofilms were less durable in sliding/rolling contacts than in unidirectional sliding contacts. In unidirectional sliding conditions, it was also observed that MoDTC tribofilms formed on fresh steel samples were more durable than those formed on oxidised steel samples. This is because the adhesion of MoS2 on iron oxide is less than on steel

In-situ Interface Chemical Characterisation of a Boundary Lubricated Contact

An increasing demand for improved fuel efficiency and more reliable automotive engines has seen a number of approaches made to further improve the tribological performance in automotive engine parts. Engine oil lubricants extend the life of the moving parts operating under different conditions and also preventing any damages to these parts. However, although its applications are beneficial towards the moving parts, the environmental implications of these lubricants are somewhat harmful, leading to stricter regulations against its emissions. Strict emission requirements have led to a greater interest in understanding the tribological performance of these lubricant additives. Hence, in order to develop more environmentally friendly additives, it is necessary to understand the tribochemical mechanism that occur at the lubricated systems. However, to date despite considerable efforts, a model to predict friction coefficient is only limited to elastohydrodynamic and hydrodynamic lubrication systems. Under boundary and mixed lubrication conditions, the friction and wear behaviour of the tribological system are characterised by the surface asperities of real contact and with the formation of thin surface films. Thus, sophisticated and reliable experimental techniques are required to investigate and assess the tribological systems under this conditions. In-situ approaches can greatly enhance our understanding on the progressive developments between the contacting interfaces, including the detailed chemical, structural and physical interactions governing friction and wear. The research focuses on developing a methodology for in situ and real time boundary lubricated surface optical and chemical characterisation with the aid of Raman Spectroscopy. The techniques are developed with the lubricant additive of Molybdenum Dialkyldithiocarbamate (MoDTC) and used to experimentally evaluate the interface phenomena occurring in a bench tribometer. MoDTC under defined tribological conditions forms MoS2 tribofilms which reduces friction. Surface analytical methodology of ex-situ and in-situ analysis is applied for the lubricant additive to understand the tribochemical process occurring at the tribological contacts

Tribological and Tribochemical Behaviour of Machine Elements Having DLC/Cast Iron Interfaces and Lubricated with MoDTC-Type Friction Modifier

Due to the importance of reducing CO2 emissions and improving fuel economy, many researchers have focused on studying the cam/follower contact and the ways to reduce friction (improve engine efficiency) and avoid wear (enhance engine durability). This thesis addresses aspects of tribology and tribochemistry of a cam/follower interface with consideration of how the surface/lubricant interaction affect the system performance. A Mini Traction Machine (MTM) was used to evaluate different lubricant formulations as well as to examine the tribofilm formation under pure sliding and sliding/rolling contacts. A Single Cam Rig (SCR), taken from 1.25L FORD Zetec (SE) engine, was also used in this work in order to investigate the friction, wear and Tribochemical performance of the camlobes/tappets as a function of tappet clearance and type of coating. In addition, a new technique of measuring tappet rotation using a giant magnetoresistance (GMR) sensor coupled with a split pole ferrite disk magnet has also been developed in this study. Raman Spectroscopy, Scanning Electron Microscope (SEM), Energy-Dispersive X-Ray (EDX), X-ray Photoelectron Spectroscopy (XPS), Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) were performed on the surfaces to understand the tribochemical interactions between oil additives and the cam/follower interface. Results obtained from MTM tribometer showed that the sliding/rolling ratio affects friction, wear and tribochemistry in CI/DLC systems; pure sliding enhances MoDTC activation. MoDTC decomposes to form MoS2, FeMoO4 and not MoO3. In addition, it was observed that the MoS2/FeMoO4 ratio depends on test conditions and affects the friction performance. The results obtained from SCR tribometer closely support those in the MTM tribometer and that link between tribometer and component testing is discussed. In addition, the chemistry of the tribofilm derived on camlobes and tappets varies as a function of tappet clearance and cam profile. In terms of tappet rotation, results showed that Molybdenum Dialkyl Dithiocarbamate (MoDTC) and Diamond-Like Carbon (DLC) coating promoted the rotation of the tappet. Furthermore, the tappet rotation is strongly dependent on oil formulation, clearance, speed/temperature, and surface roughness of the coating

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

The effect of clearance between tappet insert and camlobe on the tribological and tribochemical performance of cam/follower surfaces

This paper examines the effect of tappet insert clearance on the tribological and tribochemical performance of the camlobe/follower tribopair when lubricated in a fully-formulated oil containing 1 wt% of Molybdenum Dialkyl Dithiocarbamate (MoDTC). Tests were performed on a Single Cam Rig (SCR), taken from 1.25 l FORD Zetec (SE) engine. White Light Interferometry and Talysurf contact profilometry were used to characterise the wear scar on the tappet inserts and camlobes respectively. In addition, Scanning Electron Microscopy (SEM) was used on both (i.e. camlobes and tappet inserts) for wear mechanisms assessment as well as to access the durability of coatings used on tappet inserts. Energy-Dispersive X-ray (EDX) and Raman spectroscopy analyses were also used to understand the tribochemical interactions between oil additives and the cam/follower interface. Results show that the chemistry of the tribofilm derived on camlobes and tappet inserts vary as a function of tappet insert clearance and cam profile. Also, regardless of the type of coating, the smaller clearance of tappet inserts exhibited higher friction and wear. Therefore, based on this work, the use of the thicker tappet insert would be inadvisable as this possibly can cause higher fuel consumption and inefficient performance of the intake/exhaust valves of the engine

Friction and wear phenomena of vegetable oil based lubricants with additives at severe sliding wear conditions

The tribological responses of palm oil and soybean oil, combined with two commercial antiwear additives (zinc dialkyl dithiophosphate and boron compound), were investigated at a lubricant temperature of 100 °C and under severe contact conditions in a reciprocating sliding contact. The friction coefficient of palm oil with zinc dialkyl dithiophosphate was closest to the commercial mineral engine oil, with a 2% difference. The soybean oil with zinc dialkyl dithiophosphate produced a 57% improvement in wear resistance compared to its pure oil state. The existence of boron nitride in vegetable oils was only responsive in reduction of wear rather than friction. The response of commercial antiwear additives with vegetable oils showed a potential for the future improvement in the performance of vegetable oils

Formation of interfacial molybdenum carbide for DLC lubricated by MoDTC: Origin of wear mechanism

A large amount of research has been devoted to the effect of molybdenum dithiocarbamate (MoDTC) additives on the lubricating performances of carbon-based coatings, showing that a high wear rate is produced when the MoDTC is blended with the base oil. However, the mechanisms leading to the coating removal are not fully understood yet. In this work, the friction and wear performances of an amorphous hydrogenated DLC coating doped with silicon and oxygen have been analysed when lubricated by MoDTC-containing oils. Tribological experiments have been conducted with DLC/steel and DLC/DLC contacts under boundary lubrication conditions using a ball-on-flat tribometer. To understand the wear mechanism, the chemical composition of the tribofilm formed on the steel ball counterpart was investigated by X-ray Photoelectron Spectroscopy (XPS). Transmission Electron Microscopy (TEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDX). A new DLC wear model has been proposed and validated