Zinc dialkyldithiophosphate (ZnDTP) is a well-known multi-functional additive for engine oils that has superior oxidation inhibition and antiwear performance. However, since it was pointed out that ZnDTP poisons the exhaust catalyst, it has become important to reduce ZnDTP concentrations in engine oil without losing the superior antiwear performance of ZnDTP. A previous study presented at the 30th Leeds-Lyon Symposium on tribology at Lyon examined the friction and wear characteristics of intentionally deteriorated ZnDTP, which can simulate used engine-oil, and this paper is the subsequent written report of that study.
Sec-C6 ZnDTP was degraded by cumenehydroperoxide (CHP), which modeled the hydroperoxide present in combustion gas. Changing the degree of degradation simulated the gradual changes in deterioration observed during actual use. ZnDTP produced various types of phosphorous-containing oil-soluble compounds after reacting with CHP. At the same time, Zinc-containing solids were precipitated from the oil during the process of deterioration. In our previous study, the precipitates were removed from the oil, and only the liquid layers were used to study the influence of oil-soluble compounds on performance in terms of friction and wear. It was found that the simulated used-oils accelerated wear, and some of the degradation products from ZnDTP led to the acceleration of wear. In this paper, polyisobutenyl succinimide (PIBSIM) was used as a dispersant in order to study the influence of solid precipitates on wear when they are dispersed in degraded oils as invisible fine particles. It was found that PIBSIM could adsorb on the friction surface, and thereby decreased the wear rate. Zn-containing solids played no role in the control of wear if they were dispersed as invisible fine particles in the oil. PIBSIM was shown to exert no effect on the reduction of the wear rate of the degraded oils when it was used for dispersing the invisible fine particles in the oil. The effect of the trace amounts of CHP, which could be remained in the samples, on antiwear performance of degraded samples was also investigated, and the direct acceleration of wear due to CHP was found not to be the major path showing an acceleration of wear by highly degraded ZnDTP. Disulfide (DS) and some degraded samples showed an initial high-wear region followed by a low-wear region. The initial high level of wear was considered to be corrosive wear due to the excessive reaction with sulfur. ZnDTP produced a surface protective film as soon as the experiment began, whereas in the case of slightly degraded samples and the DS sample, formation of the P-containing films was delayed.