The anti-wear performance and action mecha-
nisms of zinc dithiophosphate (ZDDP) have been investi-
gated under various test conditions. The Mini Traction
Machine–Space Layer Imaging (MTM–SLIM) is a widely
used and useful method for monitoring tribofilm formation
by ZDDPs. However, tests are generally carried out in
mixed sliding–rolling conditions, typically between 50 %
SRR (slide–roll ratio) and 100 % SRR. In this paper, the
authors describe an investigation of ZDDP film formation
at SRRs much higher than 100 % SRR, including pure
sliding conditions using a novel MTM–SLIM technique. At
high SRRs, ZDDP tribofilms form without damaging the
ball surfaces so long as both surfaces move above a
threshold speed with respect to the contact, regardless of
whether the two surfaces move in the same or opposing
directions. In pure sliding conditions, although the worn
area expands with time under pure sliding conditions
showing that wear takes place, tribofilms are still built up
throughout a test and the ZDDP has a beneficial effect on
wear rate. The very early stages of film formation are
studied to show that a tribofilm with a high concentration
of S is formed initially and then replaced with a film having
a high concentration of Zn and P.