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
