Lubrication at Extreme Conditions: A Discussion About the Limiting Shear Stress Concept

Although limiting shear stress is a common behavior that is observed in many highly loaded lubricated contacts, its physical origins are still a matter of debate. Several scenarios are proposed in the literature to explain the macroscopic manifestations of the shearing response of a thin film of highly pressurized lubricant. Experimental measurements have been taken to capture these scenarios. However, limiting shear stress has never been simultaneously measured to validate any of them. Over the last decade, more and more molecular dynamics simulations have been performed to go further into details that are experimentally unattainable. This paper aims at comparing both approaches to studying the limiting shear stress, and more specifically at experimentally validating a lubricant phase diagram derived from molecular dynamics simulations. It suggests that the physical mechanism that initiates limiting shear stress is shear localization.