The influence of zinc dialkyldithiophosphate and other lubricant additives on soot-induced wear

Abstract

Diesel engines are becoming increasingly popular in both passenger and commercial vehicles because they offer better fuel efficiency than their gasoline counterparts. However, a disadvantage of this type of engine is the high volumes of soot it produces, which can contribute to an increase in wear of the engine components. A number of wear mechanisms have been proposed to explain wear by soot, of which the abrasive mechanism is still the most widely accepted. Lubricant additives such as zinc dialkyldithiophosphate (ZDDP) are frequently used as antiwear (AW) agents but the combined effect of soot and additives such as ZDDP on wear is not well understood. The aim of the work described in this thesis is to explore the impact of soot on wear both in the absence and presence of lubricant additives, of particular interest is the additive ZDDP. A unidirectional sliding/rolling test is used to explore the impact of film-thickness on wear while a reciprocating wear tester enables accurate wear measurements for individual test lubricants. The AW additives studied, most especially ZDDP, show excellent antiwear behaviour in the absence of carbon black (CB) – used as a substitute for soot. However, once CB is combined with AW additives, an unusual wear pattern emerges and an increase in wear is observed. A new mechanism of wear by soot is therefore suggested and discussed to explain this effect.