Friction in highly-loaded lubricated contacts: a discussion on the mechanisms behind the limiting shear stress concept
Résumé
INTRODUCTION
An efficient lubrication of machine elements is essential everywhere, in our everyday life activity as well as in manufactures, energy plants … New demands related to environmental considerations and economic competition have emerged. Combined with the continuous increase in the severity of the operating conditions, friction reduction in lubricated systems is more than ever a critical issue. As the contacts are highly loaded, the limiting shear stress (LSS) becomes one if not the most important factor to influence friction.
Even if the expression "limiting shear stress" was not employed at this time, it was already underpinned in the earliest papers dealing with the link between traction and shear behavior of the lubricants. A strong non-linear response was deduced from friction measurements as the friction coefficient displays a plateau beyond a critical state in the lubricant. [1]. The first evidence of LSS determined from shear strain vs shear stress experiments under high pressure was published a couple of years later [2]. This yield stress was believed to be the property which determines the maximum friction in highly loaded EHD contacts.
Since then, it was not observed more remarkable advancements, probably due to the difficulty in repeating those experiences to other conditions. Most of the LSS values were again deduced from friction measurements, i.e. not independently from the contact. This is an ongoing weakness in the understanding of the response of highly loaded contacts. Thus, there is still an important challenge for improving our knowledge in that matter, and to propose a quantitative, physically-based approach.
MECHANISMS BEHIND LSS
In the current literature, the LSS is mainly assigned to either film slip at the wall or a transition in the lubricant behavior.
In the former case, some researchers measured the lubricant slip at the interface and found a linear dependence with pressure and an exponential dependence with shear stress [3]. However, the LSS has been found to be only weakly pressure dependent and almost SRR [4, 5] independent.
In the latter case, many authors invoke the behavior of the lubricant under very high pressures. Indeed, at pressures higher than 1GPa, most lubricants experience glass transition and display a solid-like behavior. This transition was widely explored in the case of lubricant submitted either to low temperature or to high frequencies or high pressures [6]. This resulted in a heterogeneous pressure inside the fluid which makes difficult stress measurements. It is however not clear whether the lubricant solidifies in the whole contact area or only in some nucleation zones. Indeed, some authors mentioned solidification zones due to cavitation bubbles initiated in the lubricant by the very high shear stress. For others, the lubricant experiences a metastable fluid state from which solidification triggers beyond a critical deformation. It then propagates in the contact, causing here again lubricant slip. Finally, some authors observed shear bands forming inside the lubricant and sliding on each other [7], which have never been directly related to the lubricant solidification.
However, these scenarios do not provide a full picture of the LSS and many questions are still pending. Indeed, the role of some parameters remains unclear, as the influence of the flow time scale on the lubricant behavior. As far as the authors are aware, today no single theory provides a full understanding of the whole process.
We propose a review of the main mechanisms found in the literature which have been assumed to initiate friction plateau in a highly-loaded contact. The main physical processes involved in a very high shear stress contact will be detailed. Some dimensionless numbers describing the flow will be derived from the orders of magnitude involved. This paper should finally lead to a better understanding of the limiting shear stress concept, which is a first step for a physical-based friction prediction.