Atomic Force Microscopy to evaluate the effect of concentration and reciprocal position on ZDDP tribofilm topography and an assessment of its ability to determine tribofilm durability

ZDDP has been used in most vehicles since the early 1950s; however, it has been proven to reduce the efficiency of catalytic converters. The incoming Euro 6 legislation introduces a stricter restriction on harmful emissions, increasing the importance of achieving maximum efficiency from catalytic converters. Therefore, it is necessary to remove ZDDP from cars. However, ZDDP is an effective anti-wear additive, without which modern cars, with tight design tolerances, are unable to function. Previous work has shown that the morphology of ZDDP tribofilms can relate to their anti-wear efficacy; however, no previous work has undertaken a robust characterisation, considering both the topography and durability, of ZDDP tribofilms. This research activity has shown that the concentration of ZDDP within a base oil affects the resultant pad topography in the tribofilm, with higher concentrations resulting in smaller but more evenly distributed pads. It has also shown that for lower concentrations the position along a reciprocating stroke influences the topography, with regions that experience lower lambda ratios displaying topography similar to that seen for higher concentrations. This effect is understood to occur as a result of increased shearing of the ZDDP resulting in the adherence of more, but shorter, polyphosphate chains to the surface and a hypothesis for how this occurs is presented. This thesis also demonstrates the potential of AFM for use in assessing the durability of ZDDP tribofilms. It has been shown that the AFM can be used to conduct a semi-quantitative assessment, with the potential for further improvements to be developed. It has also been shown that there can be an effect of the ZDDP concentration used to form a ZDDP tribofilm on its durability. The upper layer of the ZDDP tribofilm can be easily manipulated by the AFM tip, and two hypotheses are presented to explain this phenomenon.