This study aims to contribute to better understand the antiwear action of zinc dialkyldithiophosphate (ZDTP) additives used in car engine lubrication. The antiwear action of ZDTP is associated to the formation of a protective tribofilm onto the rubbing surface. On a mechanical point of view, the efficiency of ZDTP tribofilms results from equilibrium between film formation and wear rates, associated with appropriate rheological properties. In this work, the mechanical properties of a ZDTP tribofilm have been measured by nanoindentation in different test conditions in order to investigate the effect of temperature and strain rate. A Nanoindenter XP® entirely set into a climatic chamber was used to perform the nanoindentation tests. 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, in the studied range, this effect is enhanced at high temperature and low strain rate. Furthermore, when the temperature increases, a change in the rheological behavior of the tribofilm is observed. Up to about 50°C, the tribofilm exhibits viscoplastic behavior — the hardness increases versus strain rate — and above 50°C, the hardness decreases versus strain rate (“shear thinning-like” behavior).

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