Tribological performance of an Al-Si alloy lubricated in the boundary regime with zinc dialkyldithiophosphate and molybdenum dithiocarbamate additives

The tribological performance of the cast iron/Al-Si alloy system lubricated in the boundary regime, using lubricants containing zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives have been investigated. The results obtained are discussed in relation to the performance of these additives in cast iron/steel (ferrous)-based material systems. Tribological tests were conducted on a Plint reciprocating test machine (TE 77) in the contact conditions comparable to the conditions in the piston ring/cylinder liner system of the internal combustion engine. Surface sensitive analytical techniques such as environmental scanning electron microscopy and energy dispersive X-ray analysis have been used to determine the nature of the wear occurring as well as the chemical nature of the tribofilms formed on the AI-Si material. It has been shown that the MoDTC additive, added into a ZDDP-containing lubricant, improves the effectiveness of friction reduction in a completely ferrous system and a cast iron/Al-Si system. In terms of wear, an increase in MoDTC concentration (from 0.22 to 1.2 wt%) in the ZDDP-containing lubricant resulted in an antagonistic effect on the anti-wear performance of ferrous materials but improved the effect on Al-Si alloys. The tribofilm formation and wear mechanisms are discussed in relation to recent literature and it is shown that additives strongly affect both the processes.