Abstract
The effectiveness of the lubrication of hard smooth steel surfaces by molybdenum disulphide (MoS 2) dispersed in a mineral oil, both with and without a zinc dialkyldithiophosphate (ZDDP) additive, has been investigated using a four-ball extreme pressure lubricant test machine. Each oil, additive-oil or MoS 2 dispersion, exhibited a wear scar diameter-load curve with an abrupt transition from mild to severe wear at a characteristic load. When ZDDP was present in the oil, MoS 2 was effective at pre-transition but not post-transition loads, whereas in the absence of ZDDP the converse applied, with MoS 2 only beneficial above the transition load. These results are explained in terms of the adherence (or not) of MoS 2 particles to the films physically adsorbed on the steel surfaces at pre-transition loads. At the transition load the surface temperature reaches the desorption (or decomposition) temperature of the particular oil or additive film and MoS 2 then adheres to exposed metal, unless the latter chemically reacts with oil constituents to form a chemisorbed film.
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