Parameters affecting the functionality of additives in lubricated contacts
Traditionally rolling contact fatigue observed in bearing field applications was subsurface initiated. However, despite the improvement of steel properties, some factors such as downsizing in bearing design, extreme loading of the bearings as well as demanding application conditions (start up-stop cycles) have led to an increase on the cases of surface damage related to surface initiated fatigue, that comes basically from surface distress. Possible causes leading to surface initiated fatigue are: material and surface properties, marginal lubrication and lubricant chemical composition. Lubricants are formulated products composed of base oil, and an additive package designed for a specific application. Extreme-pressure (EP) and antiwear (AW) additives are chemically active additives, they react with the steel surfaces in contact to form a protective additivederived layer, thus reducing friction and controlling wear. However, certain EP/AW additives that increase the performance of other machine elements, such as gears, can be detrimental for the bearings running in the same lubrication environment. In order to identify the plausible mechanisms that govern the detrimental effect of EP/AW additives on bearing performance, it is necessary to study closely the interactions occurring in the system form by the base oil, the additives present and the steel surface, as well as the influence of operating conditions.
The focus of the present work is to identify the parameters affecting the additive-derived layer formation, as it is directly related to the additive reactivity towards the surface, and the tribological properties of the layer, that will determine the tribological performance. Zinc dialkyldithiophosphate (ZDDP), and two low viscosity model oils with different polarity were selected. The influence of base oil polarity on the additive performance was studied in the nanoscale using Atomic Force Microscopy and the tribological performance was evaluated using a ball-on-disc test rig under mixed rolling-sliding conditions in the boundary lubrication regime. An in-situ interferometry technique was used to monitor the additive derived reaction layer formation, and the chemical composition, morphology and nanomechanical properties were studies using X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Nanoindentation respectively.
It was found 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 and nanomechanical properties of the additive-derived reaction layer. However the reaction layer chemical composition is not determined by the base oil polarity. 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.