Abstract
Recent years have seen an increase in the concentration of dispersant present in formulated engine oils, while the concentration of antiwear additives has been progressively reduced. However, it is known that the presence of dispersant can, in some cases, detract from the performance of antiwear additives in lubricant blends. In this article, the influence of three succinimide dispersants on the film formation and wear-reducing properties of a secondary zinc dialkyldithiophosphate (ZDDP) has been studied. Both posttreated and non-post-treated dispersants reduce steady-state ZDDP tribofilm formation to a certain extent depending on the dispersant concentration. At very high dispersant concentrations, ZDDP film formation is suppressed almost entirely. This can be restored only marginally by increasing ZDDP concentration, which implies that the absolute dispersant concentration rather than the dispersant : ZDDP ratio controls the impact of the dispersant on ZDDP film formation. Addition of dispersant to ZDDP also caused an increase in wear rate for all three dispersants tested. For one succinimide reported in detail in this article, it is has been shown that the wear rate increases approximately linearly with dispersant concentration and is largely independent of ZDDP concentration over the P weight percentage range studied.
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