| Summary: | 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.
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