New insights into lubricated tribological contacts

The emissions of carbon dioxide, CO2, must rapidly be reduced to avoid severe climate change. Tribology-related solutions can be an important part in reducing these emissions. The aim of this thesis is to improve the fundamental knowledge in two tribology-related fields, both concerning lubrication, to enable such solutions in the future. The first research topic is focused on a fuel additive based on boric acid that has shown potential to reduce the fuel consumption when used in combustion engines. The second topic focuses on grease-lubricated tribological contacts, particularly fretting contacts. The friction-reducing potential of the boric acid fuel additive was evaluated and confirmed in simplified lab experiments. The friction reductions are attributed to the formation of an easily sheared boric acid tribofilm on the sliding surfaces. Estimates of how these friction reductions would affect the fuel consumption in a passenger car were undertaken. Under a few assumptions, it was shown that the considerable friction reductions measured in the lab experiments, transferred to the piston/cylinder contact, could lead to the fuel savings around the 6% previously observed in field-tested combustion engines. The boric acid tribofilms that form on the sliding surfaces differ in appearance depending on the tribotest temperature. Further, they have poor stability over time as well as at high temperatures, and may easily become cleaned off by the sample preparation prior to analysis. Further, they are sensitive to the electron irradiation used in various surface analysis techniques. Possibly also experiencing vacuum alter their properties. Based on the demonstrated instabilities, it is recommended to perform analysis of lab-tested and field-tested surfaces as soon as possible after testing to limit the ageing of the boric acid tribofilms. From grease-lubricated fretting tests it was found that the grease properties (thickener type and base oil viscosity), together with the applied load, displacement amplitude and material combination, all influence the prevailing fretting regime. The thickener type, here lithium complex (LiX) and polypropylene (PP), can have a stronger influence on the grease performance than the addition of graphite, GO and rGO additives, to an already well-working grease. Cross sections of selected wear marks from the fretting tests are presented and they are, together with fretting maps, used as tools to better understand the lubrication mechanisms of LiX and PP greases. The exact lubrication mechanisms are not yet fully understood but a lubrication model is suggested, based on the present knowledge and hypotheses