Raman spectroscopic studies of friction modifier Molybdenum DialkyldiThioCarbamate (MoDTC) (P11).
This project aims to provide a clear understanding of the chemical processes that take place in the interfacial region of two rubbing surfaces that are in the boundary lubricated regime. The aim of this study is to develop a better understanding of the interfacial processes (additive decomposition, tribofilm formation and removal) that occur within steel/steel contacts during tests with MoDTC-containing lubricant.
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 study, 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.
Doris Nekesa Khaemba
IInstitute of Functional Surfaces (IFS)
Mechanical Engineering Department
University of Leeds, LS2 9JT
E-mail:d.n.khaemba@leeds.ac.uk
Supervisors: Prof. Ardian Morina and Prof. Anne Neville.
Monthly reports are available here
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 in tribocontacts.
In this project tribochemical reactions that occur in steel/steel tribocontacts in the presence of MoDTC additive have been investigated. The main objectives of this project were:
Tribological tests were conducted using model oils comprising of MoDTC additive in Group III mineral base oil. Tests were conducted under unidirectional linear sliding and sliding/rolling conditions using high speed pin-on-disc (HSPOD) and MiniTraction Machine (MTM), respectively. Friction results were obtained during tests. Tribological tests were conducted at varying temperature, MoDTC concentration, contact pressure, speed, surface roughness and slide-roll ratio. Space Layer Image Mapping (SLIM) was used to monitor the changes that occurred at the tribocontact with rubbing time. Optical microscopy and white light interferometry were used to study the wear of the tribopair after tribotests. Raman spectroscopy was used to conduct chemical characterisation of the rubbed surfaces. The potential for employing Raman spectroscopy in in-situ analysis of lubricated surfaces was investigated in detail.
This project addressed tribochemical processes in boundary lubricated systems which was one of four key areas highlighted in the ENTICE project proposal
Mechanical activation accelerates the rate of MoDTC degradation. Under tribological conditions, MoDTC decomposes to form three main compounds; MoS2, MoSx (x>2) and FeMoO4. The formation of MoS2 in tribological contacts has been reported in literature before however presence of amorphous MoSx and FeMoO4 has not been reported. These findings are thus novel. MoDTC decomposition products formed at the tribocontact are dependent on the test conditions. A new mechanism for the degradation process has been proposed. This mechanism is shown in Figure 1. According to this mechanism, MoDTC decomposition process begins by rupturing of C‑S bonds in stage 1 forming molybdenum intermediate compound which undergoes intramolecular sulphonation forming amorphous MoSx. In stage 2, MoSx is converted to MoS2. The conversion of MoS2 to MoSx is dependent on temperature, shear stress and MoDTC concentration. FeMoO4 is formed from a side reaction of iron oxides on the steel surfaces with MoSx.
The chemical composition of MoDTC tribofilms formed at the rubbing surfaces determine 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 values (µ=0.06‑0.07) was observed when the tribofilms were composed of a mixture of Fe2O3, Fe3O4, MoSx and FeMoO4 in varying proportions. Therefore, in order to achieve low friction ideal test conditions that ensure complete MoDTC decomposition to MoS2 are necessary. Results from this study show that the ideal test conditions are; high temperatures (>80ºC), high MoDTC concentration (>0.3 wt%) and high contact pressure. Local contact pressures can be increased by having at least one of the tribopair having a rough finish (Ra >100 nm). Previous studies had reported that friction performance of MoDTC was dependent on contact parameters. It was however unclear how the contact parameters affected friction. In this study we have found that the link between contact parameters and friction lies on the chemical composition of MoDTC tribofilms.
MoDTC decomposition products formed at the rubbing surfaces results in formation of MoDTC tribofilms. MoDTC tribofilms grow rapidly within the generated wear scars until a limiting thickness is achieved. This limiting thickness is dependent on contact parameters. The presence of the tribofilms at the tribocontact provides wear protection to the steel substrate. There are however certain test conditions (low MoDTC concentrations, high temperatures) where MoDTC is incapable of providing wear protection. The reason for this is that MoDTC tribofilms formed at the tribocontact are removed as soon as they formed. This happens when the rate of tribofilm formation is lower than the rate of tribofilm removal. To ensure that tribofilm remains within the contact, the rate of tribofilm formation should be increased by using higher MoDTC concentrations.
The durability of MoDTC tribofilms when MoDTC additive is depleted within a tribocontact is dependent on the sliding configuration. MoDTC tribofilms were less durable in sliding/rolling contacts than in unidirectional linear sliding contacts. In unidirectional linear sliding conditions, 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.