Mechanochemistry of Zinc Dialkyldithiophosphate on Steel Surfaces under Elastohydrodynamic Lubrication Conditions

Abstract

Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerisation.

Stress Anisotropy Severely Affects Zinc-phosphate Network Formation

“…Films generated predominantly by shear stress turn out comparatively soft, potentially too soft, like thermal ZnPs films, to protect surfaces from wearing. This speculation is certainly consistent with the lower durability of ZDDP tribofilms formed in the absence of asperity contact [27].…”

“…In yet another step, initially disconnected ZnPs form networks, which are sufficiently stiff to protect the surfaces from counter faces but also compliant enough to be sacrificial under extreme rubbing conditions [24,25,7]. This step has traditionally been argued to occur through nucleophilic substitution [26,27], which, however, does not explain why mechanical properties of zinc-phosphates films differ substantially between valleys and peaks [12][13][14]. Thus, the final aspect of film formation should be film strengthening, which is the topic of this work.…”

Boundary Friction of ZDDP Tribofilms

“…The detailed molecular mechanism by which ZDDP polymerises to form polyphosphate is not known but must involve loss of alkyl-containing species. Two suggested products are alkenes and dialkyl sulphides [29]. However these are generally quite volatile and would not be expected to adsorb on solid surfaces at the temperature employed.…”

Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces

“…It has recently been shown that the initial removal of alkyl groups is the rate determining step for ZDDP tribofilm formation. 90 Several researchers have attempted to link the thermal stability of antiwear additives with their tribological performance. For example, it has been suggested that pure aryl phosphates (with higher thermal stability 20 ) yield improved antiwear performance compared to alkyl phosphates; 22 however, the opposite trend has been reported when they were dissolved in a base oil.…”