The influence of base oil polarity and slide–roll ratio on additive-derived reaction layer formation

Abstract

Functional additives, particularly extreme-pressure and antiwear additives, in formulated oil will compete to adsorb and form a protective layer in tribological contacts. The thickness of the layer is determined by the equilibrium between the formation and removal processes. In this article, the interactions between additives and base oil molecules and operating conditions influence friction and wear are studied. One polar (ester oil) and one non-polar (poly-a-olefin) commercial base oil blended with zinc dialkyl dithiophosphates were studied. The tribological performance was evaluated using a ball-on-disc test rig under mixed rolling-sliding conditions in the boundary lubrication regime. An adapted in situ interferometry technique was used to monitor the additive-derived reaction layer formation. The properties of the additive-derived reaction layers were studied using surface analysis techniques, X-ray photoelectron spectroscopy and atomic force microscopy. A thicker layer was formed when the additive is blended in the nonpolar oil. This observation suggests that base oil polarity determines the transport of additives to the surface, thereby controlling the maximum reaction layer thickness, friction and wear, as well as the morphology of the additive-derived reaction layer. However, the reaction layer chemical composition is not strongly influenced by the base oil polarity. Among the operating conditions, shear was identified as a fundamental parameter for the activation of additives on rubbing steel surfaces and the properties of the derived reaction layer.

A Deterministic Stress-Activated Model for Tribo-Film Growth and Wear Simulation

“…They also showed that the growth rate is governed by the shear stress, rather than the normal stress. This fact is confirmed by experiments showing that tribo-films do not grow in pure rolling contacts, not even at high pressures [21].…”

“…Inputs for the model are, in addition to the traditional mechanical properties of the substrate materials, the constants in the stress-activated Arrhenius equations and the constants a and C 1 in the wear equations. These constants depend on the type of additive, base oil, and concentration of ZDDP molecules [21,43]. It is important to notice that these constants may be independent of load or temperature.…”

“…However, an important feature, i.e., an overshoot of the tribo-film thickness in the experimental data, cannot be reproduced. This overshoot is frequently observed in macroscale contacts as well [1,2,21]. The fact that it occurs on both micro-and macroscale makes is an intriguing feature of the tribo-film growth, which needs to be studied further.…”

Evolution of ZDDP‐derived reaction layer morphology with rubbing time

“…At this stage, primarily the zinc rich top layer will be worn during rubbing and as there are few (or none) iron ions available the new reaction layer will be formed with zinc as counter ion, leading to an equilibrium in the rates of formation and removal (Naveira‐Suarez et al . 2010). The initial WTW measured in sample ZDDP1, after 5 min rubbing, corresponds with the Hertzian contact diameter.…”

“…The same behavior was observed when using ZDDP in solution with commercial base oils of different polarities and among the operating conditions, shear was identified as a fundamental parameter on the activation of additives on rubbing steel surfaces and the properties of the derived reaction layer (Naveira‐Suarez et al . 2010). In this article, we study the evolution of the reaction layer with rubbing time and its nanofriction and nanowear behavior, to further understand the differences observed in the friction and wear performance.…”