Mechanical behaviour and constitutive models of ZDDP tribofilms on DLC coatings using nano-indentation data and finite element modelling

The mechanical behaviour of zinc dialkyldithiophosphate (ZDDP) tribofilms on diamond-like carbon (DLC) coatings has been studied by combining the nano-indentation experimental data and finite-element modelling. Different constitutive models, whose analysis was performed using a global algorithm that consists of forward and inverse algorithms, were chosen to establish which of them more accurately predicts the actual mechanical behaviour of the ZDDP tribofilms. A non-homogeneous, dual structure, which was also verified by the local stiffness using the AFM force-modulation mode, was found. Based on the proposed algorithm, the mechanical properties of the ZDDP tribofilms were found, e.g. the elastic modulus E=26 GPa and the yield stress Y=1.55 GPa for the harder component and approximately E=1.65 GPa and Y=0.144 GPa for the softer component.