The lubrication of aluminium-silicon surfaces with a novel antiwear additive

Even though research into aluminium-silicon alloys is becoming increasingly fashionable, very little is known regarding the tribochemistry of these substrates when lubricated with conventional engine oil additives, especially when compared against ferrous surfaces. In this thesis, many advancements and thus contributions have been made in the field of aluminium-silicon lubrication. Firstly, using a Cameron Plint TE77 tribometer, the tribological performance of overbased calcium sulfonate, zinc dialkyldithiophosphate and an organic antiwear additive on aluminium-silicon surfaces were evaluated. Secondly, contact conditions on silicon grains within the AI-Si alloy were replicated using a silicon crystal substrate, as the silicon regions were vital in the effective lubrication of the aluminium-silicon alloy. Simulation was validated through tribological testing. Thirdly, the role of the aluminium matrix under boundary lubricated wear was evaluated using a replicate aluminium alloy. Tribofilm formation and morphology on the three substrates was analysed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Tribofilms were generated on silicon surfaces more effectively than on the aluminium alloy. Wear levels were lower on the non-conductive substrate compared to the aluminium substrate. The mechanical properties of tribofilms on silicon crystal surfaces were analysed using AFM and nanoindentation, and related to observed wear levels. Fourier transform infrared (FTIR) and secondary ion mass spectrometry (SIMS) determined the tribochemistry of generated tribofilms on all three tribosystems to be very similar to those on ferrous substrates. The addition of the organic antiwear additive to lubricants containing detergent, ZOOP or a combination of the two, consistently improved film formation and wear levels in the three tribosystems. The mechanisms by which this occurred were identified using labelling techniques. This research has provided great insight into the effective lubrication of aluminium-silicon substrates, with the results compiled to provide a definitive mechanism by which the boundary lubrication of aluminium-silicon alloys occurs.