Additive–additive interaction: an XPS study of the effect of ZDDP on the AW/EP characteristics of molybdenum based additives

The effect of zinc dialkyldithiophosphate (ZDDP) addition on the antiwear (AW) and extreme pressure (EP) properties of molybdenum dialkyldithiocarbamate (MoDTC) and molybdenum dialkyl dithiophosphate (MoDTP) are evaluated by standard Four-Ball friction test and also by the determination of coefficient of friction using an oscillating SRV apparatus. The boundary lubrication film formed on the worn surface using the two molybdenum additives and their combination with ZDDP is investigated by depth profile X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to find out the chemistry of tribochemical reaction occurring at the surface during friction. It is seen that MoDTP possesses better AW properties than MoDTC due to its high reactivity with the metal surface. Nevertheless, the AW characteristics of MoDTC could be improved by the addition of ZDDP. The frictional properties of MoDTP, however, do not change by the addition of ZDDP. The synergistic action of ZDDP on MoDTC is attributed to the enhanced decomposition of MoDTC in presence of ZDDP. This is tentatively explained in terms of some interaction of zinc with the electron donating nitrogen present in MoDTC, which would have helped to increase its tribo-reactivity. XPS studies revealed that in the presence of ZDDP, MoDTC form mainly metal sulphides like MoS2 and FeS under friction. The MoDTP+ZDDP derived surface, on the other hand, produced mainly metal phosphate along with molybdenum oxysulphides and small amount of MoS2 and FeS. The mechanism of action of additives is explained.