Strain rate and temperature effects on mechanical properties of zinc dithiophosphate tribofilms measured by nanoindentation

Zinc dialkyldithiophosphate (ZDTP) is an additive extensively used in motor oil because of its good anti-oxidant and anti-wear properties under boundary lubrication conditions. Its anti-wear action is associated to the formation of a thin protective solid film onto the rubbing surfaces, due to a tribochemical reaction. Because ZDTP is pollutant, the knowledge of its anti-wear mechanisms is a crucial point in order to help to reduce its use or to design substitution additives. On a mechanical point of view, the efficiency of ZDTP tribofilms results from the equilibrium between film formation and wear rates, associated with appropriate mechanical properties. Their measurement contributes to better understand how they work, but the patchy nature of the tribofilms and their low thickness, hundreds of nanometers, make them difficult to measure. Nanoindentation tests have been performed at several temperatures (24 to 80°C) and several strain rates (0.003 to 0.1 s-1) on a ZDTP tribofilm using a Nanoindenter XP® entirely set into a climatic chamber. For all tests, an increase of the elastic modulus was observed from a threshold contact pressure value. This effect is similar to the Anvil effect observed on polymers: in confined geometry, the elastic modulus increases versus hydrostatic pressure. For the tribofilm, this effect is enhanced at high temperature and low strain rate. Furthermore, when the temperature increases, a change in the rheological behaviour of the tribofilm is observed. Up to about 50°C, the tribofilm exhibits a viscoplastic behaviour - the hardness increases versus strain rate - and above 50°C, the hardness decreases versus strain rate (“shear thinning-like” behaviour)