Kinetic study on lubricity of MoDTC as a friction modifier

The temperature dependence of friction coefficient of lubricant oils containing MoDTC was evaluated using a ball on disk tribometer at constant temperatures of 25, 40, 60 and 80°C. In the experiments, the friction coefficient gradually decreased and tended to become lower at steady state. The decreasing slope of the friction coefficient was steeper and the lower friction coefficient at the steady state were obtained at a higher oil temperature. The friction coefficient was also controlled by the concentration of MoDTC in oil. Considering that die results of lubricating tests supported the successive processes involving production and wear of MoS₂, the friction coefficient can be controlled by the coverage of M0S2 which is determined by a balance of formation and wear of MoS₂. An analysis formula was established based on kinetic assessment of the process in order to obtain the Arrhenius plots of the test results. There were two regions which had different slopes in the plot. The activation energies were 70 and 4 kJ/mol for the low and the high temperature regions, respectively. This result suggested that MoS₂ production from MoDTC was reaction rate-limiting at low temperature and transport rate-limiting at high temperature.