LAUSR

Abstract Molybdenum dialkyldithiocarbamates (MoDTCs) are very effective friction modifier additives for use in engine oils and other lubricants. However, as engine oils age during extended drain intervals, MoDTCs can lose some or all of their ability to reduce friction and this is generally believed to result from their oxidative degradation. In this study, MoDTC solutions in base oil have been subjected to oil ageing in a controlled NOx/air flow rate, controlled temperature test apparatus and the effect of ageing on the ability of the MoDTC to reduce friction has been explored. As shown in previous studies, the additive’s friction-reducing properties are completely lost after a quite short period of ageing at 160 °C. However, it was found that at this temperature the presence of NOx has little if any influence on the rate of friction loss, indicating that the latter is controlled primarily by the rate of reaction of oxygen with the base oil and thus the rate of consumption of MoDTC as a peroxide decomposer. By contrast, when ageing tests are carried out at lower temperatures it is found that NOx has a very strong effect on the rate at which MoDTC loses its ability to reduce friction, so that at 100 °C NOx accelerates this rate by two orders of magnitude compared to air alone. This suggests that at low temperatures the rate at which MoDTC is consumed is controlled by its reaction as a radical inhibitor with NOx species.