| Summary: | Friction has enormous effect on energy consumption and wear limits the life time of many mechanical devices with relative motion components. The challenges in the automotive industry have been driven by fuel economy as well as stringent emission legislation for environmental sustainability. Improvements of coating deposition technology enable the mass production of high quality Diamond-Like Carbon (DLC) coatings at an industrial scale and also increase their use in lubricated contacts. However, the understanding of the interactions of different lubricant additives with this material is not yet fully developed. This study focused on several fundamental aspects of the tungsten-doped DLC coating (denoted as WDLC) behaviour under boundary lubrication conditions with model lubricants. Zinc dialkyldithiophosphate (ZDDP), glycerol monooleate (GMO) and molybdenum diakyldithiocarbamate (MoDTC) were used in these model lubricants. The primary test contact was semi-coated, i.e., WDLC vs. cast iron (CI). As references, steel/CI, non-doped DLC coating (NDLC)/CI and fully coated WDLC/WDLC interfaces were also studied. As the primary focus, WDLC/CI interface was tested not only at 100 °C, but also a wider service temperature from 30 °C to 150 °C. For the other references, tests were only carried out at 100 °C. The tribological tests were carried out by Cameron Plint TE77 reciprocating tribometer. Test simulation was based on the liner motion part of the camshaft/follower system in an internal combustion engine (ICE). This study demonstrated the characterization of WDLC coating lubrication by Raman spectroscopy. The effect of lubricant additives on the coating structure change was discussed in terms of carbon structure and the tungsten dopant. Electron Energy Loss Spectroscopy (EELS) and Raman spectroscopy characterization for the upper carbon layers indicated that the WDLC coating interacted chemically with selected lubricant additives. The testing temperature was proved to be critical in WDLC lubrication. The study also clarified the role of doping tungsten and its unique role in changing the tribological behaviours of DLC lubrication with references to NDLC coating tests. The fully coated WDLC/WDLC tests clarified further the tribofilm formation dependence of ferrous surface for ZDDP and MoDTC additives. The formation of tribofilm of GMO on WDLC coated surface had less dependence than on ferrous materials. Different models of lubrication in semi-coated WDLC/CI interface were proposed individually in the concluding chapters and the role of tungsten carbide in changing the tribochemical interaction of GMO+ZDDP lubricant compared with NDLC/CI interface was further discussed. Overall, this study provided fundamental aspects for better understanding of interactions between current ferrous based additives (with a further reduced treat rate) and WDLC coating in a semi-coated tribological system. The methodology and results should contribute both the fundamental science and engineering practices which the engineering science community should be benefited from.
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