Surface interactions and tribochemistry in boundary lubrication of hypereutectic aluminium-silicon alloys

The tribofilms formed on a hypereutectic Al-Si alloy after sliding against AISI 52100 steel pins in a reciprocating pin-on-plate tribometer with polyalpha olefin (PAO) base oil containing zinc diakldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives have been studied. Surface-sensitive analytical techniques (X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), Raman spectroscopy, and nanoindentation) have been used for tribofilm chemical and mechanical characterization. The thickness of the tribofilms has been evaluated using depth profiling and focus ion beam (FIB) milling. This is the only reported work to analyse this kind of tribofilm on AI-Si alloys in this detail. From the analysis obtained from miniSIMS, differences in the composition of the top layer of the tribofilms formed from ZDDP and ZDDP+MoDTC lubricants have been observed. When using only ZDDP, this top layer is rich in Zn and Fe; with the addition of MoDTC the top layer contains phosphates, probably from the decomposition of the ZDDP, and MoS(2). The effect of adding MoDTC can also be observed in the overall tribofilm thickness and coverage of the samples.