Sliding Evolution of the Mechanical Behaviour of Zinc Dialkyldithiophosphate Tribofilms on Diamond-Like Carbon Coatings

The evolution of the mechanical behaviour of zinc dialkyldithiophosphate (ZDDP) tribofilms on diamond-like carbon coatings during sliding contact has been studied by combining nano-indentation experimental data and finite element modelling. The nano-indentation data from the ZDDP tribofilms were obtained for two different sliding distances of the tribological tests after 15,000 and 216,000 cycles, which correspond to 25 min and 6 h, respectively. A non-homogeneous structure was found for the ZDDP tribofilms after both sliding distances, which were also verified by the local stiffness using the AFM force modulation mode. A dual structure is observed for the ZDDP tribofilms after 15,000 cycles, while three distinct components with different mechanical behaviours were found after 216,000 cycles. It was also found that the mechanical behaviour of the harder component of the ZDDP tribofilms remains almost unchanged for both sliding times, while the softer component tends to stiffen slightly over the sliding time. In addition, using the proposed algorithm with finite element simulations, the mechanical properties of the different components have been obtained, e.g. the elastic modulus E = 25.5 GPa and the yield stress Y = 1.585 GPa of the harder component for both sliding times. The softer component exhibits E = 1.65 GPa and Y = 0.144 GPa after 15,000 cycles and E = 3.5 GPa and Y = 0.17 GPa after 216,000 cycles. Finally, an intermediate component with E = 11.7 GPa and Y = 0.94 GPa has been only found at 216,000 cycles.