The behaviour of antiwear additives in lubricated rolling-sliding contacts
Traditionally rolling contact fatigue observed in bearing field applications was subsurface initiated. However, despite an improvement in the properties of steel, some factors such as downsizing in bearing design, extreme loading of bearings as well as demanding application conditions (start-stop cycles) have led to an increase in cases of surface damage related to surface initiated fatigue, which essentially comes 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 a base oil and an additive package designed for a specific application. Extreme-pressure (EP) and antiwear (AW) additives are chemically active, they react with the steel surfaces in contact to form a protective additive-derived 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 bearings running in the same lubrication environment.
Therefore, there is a need to gain a fundamental understanding of the mechanisms of interaction of the lubricant additives affecting bearing performance and to predict the bearing performance in terms of lubrication environment and application conditions. Antiwear and extreme pressure additives form a protective layer on the surface of the contacting steel surfaces. Therefore, in order to identify the plausible mechanisms that govern the detrimental effect of additives on bearing performance, it is necessary to identify the parameters affecting the additive-derived layer formation and the tribological properties of this layer, as they are directly related to the additive reactivity towards the surface.
To identify these parameters this thesis has been divided into three areas of study; the lubricant composition, the operation conditions relevant for bearing applications, and the properties of the contacting surfaces. The work on the effect of lubricant composition on additive-derived reaction layer formation, especially when related to ZDDP chemistry, has mainly been focused on the different formulations of the additive and the inherent properties due to the different chemical structures, concentrations and interaction with other additives present in the lubricant. The role of the base oil has not been sufficiently addressed, however the relevance of synthetic base oils has brought a new focus on the effect of the interactions between the base oil and additive molecules on tribological performance. The polarity of the base oil was selected as the key parameter to study.
Several operating conditions (in terms of lambda ratio, temperature and additive concentration) have been previously studied. It has been shown that low lambda ratio, meaning high metal to metal contact, high temperature and high additive concentration lead to a high reactivity of the additives and therefore to thicker reaction layers. However, the majority of these studies have been performed under pure sliding conditions or at high slide-roll ratios (SRR > 50%), corresponding to typical operating conditions for gears. The behaviour of the additives when the slide-roll ratio ranges from between 0 to 10%, conditions that can be found in bearing applications, has hardly been addressed. Finally, the influence of the surface on the activation and reactivity of the additive was studied using different counterparts (steel and stainless steel), all present in bearing applications.
The nature and properties of the derived reaction layers, as a function of base oil-additive interaction, operating conditions and contacting materials, in terms of thickness, morphology, nanomechanical properties and chemical composition, were studied using a series of surface analysis techniques.
Zinc dialkyldithiophosphate (ZDDP) and low viscosity model oils as well as commercial basestocks, with similar physical properties but different polarities were selected for this study. The influence of base oil polarity, operating conditions and contacting surfaces on the additive performance was studied at the nanoscale level 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 studied using X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Nanoindentation respectively.
The polarity of the base oil influences the tribological performance of ZDDP additives in rolling-sliding lubricated contacts. The same additive presents differences in friction and wear performance as a function of the type of oil it is blended in.
The polarity of the oil influences the growth rate and reaction layer thickness of ZDDP antiwear layers. The polarity of the molecules determines the way they approach and attach to the surface, influencing the final reaction layer thickness, as well as the structure and characteristics of the reaction layer. A thinner layer is formed when the additive is blended in the polar oil, due to the higher affinity of the polar base oil molecules for the steel surface, that limit the access of the additive molecules to the surface and therefore their ability to attach and react with it to form a protective reaction layer.
The morphology of the reaction layers derived from polar base oils solutions consist of large, smooth pads, identified as features with load carrying capacity. The formation of this type of structure, and the nanomechanical properties of the layer, explain the better wear performance exhibited by those layers.
A model for the formation of the reaction layer, consisting of three stages: activation, wearing-out and equilibrium, is proposed.
Regarding 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.
The influence of different metallic materials was studied using different steel/stainless steel pairs. The results show how different metallic materials lead to the formation of layer, similar in thickness but with very different topographies. The presence of extensive cracks on the layers formed on the stainless steel surface indicates that the nature of the oxides present on the surface influence the adhesion properties of the reaction layer.