Antagonistic Interaction of Antiwear Additives and Carbon Black
It is well known that the presence of soot in engine oils can lead to an increase in wear of engine parts. This is a growing problem as soot levels in diesel engine oils are rising due to a combination of extended drain intervals and the various methods employed to reduce NOx formation such as retarded ignition and exhaust gas recirculation. Several different mechanisms have been proposed by which soot might lead to an increase in wear in mixed lubrication conditions, of which the most widely favoured is abrasion by soot, either of the rubbing metallic parts in engines or of the antiwear additive films formed on rubbing metal surfaces. In this study it is shown that the combination of mixed alkyl ZDDP and carbon black (used as soot surrogate) is strongly antagonistic in terms of wear. In a lubricant containing carbon black, the presence of ZDDP leads to considerably more wear than if ZDDP is left out. A similar, though less severe antagonism is also seen with primary ZDDP and other antiwear and EP additives. By varying the lubricant film thickness it is shown that the effect of carbon black in ZDDP-containing oils is to promote wear up to quite thick hydrodynamic film conditions, approaching the secondary carbon black particle size. It is proposed that the antagonistic wear effect results from a corrosion-abrasive mechanism in which the reaction film formed by antiwear additive and rubbing metal surface is very rapidly and continually abraded by carbon black. At most carbon black concentrations, wear rate then becomes controlled by the rate of initial antiwear additive film formation, which for secondary ZDDP is very rapid, rather than by the kinetics of the abrasive process. From this understanding, strategies for reducing the impact of engine soot on wear can be deduced.