On the Influence of Phase Change in Highly Loaded Frictional Contacts

Abstract

Highly loaded lubricated contacts may present a friction plateau on which the friction force becomes independent from the sliding velocity. Although this phenomenon has been known for a long-time, its physical origin remains poorly known. The present paper aims at giving further insight on the physical mechanisms triggering the friction plateau. The study specifically focuses on the influence of pressure on the physical state of the lubricant. An uncoupled experimental approach (rheology, tribology, spectrometry) has been conducted on a model lubricant. The lubricant behavior has been probed both at rest by Brillouin light scattering spectroscopy and dynamically under stationary conditions in a lubricated contact. The first experimental approach led to evidence a transition pressure associated with the lubricant glass transition, which has then been correlated to the lubricant macroscopic response in friction. We show that the onset of the plateau behavior occurs for Hertzian pressures close to the glass transition pressure, corresponding to a very small fraction of lubricant in a "nominal glassy state" within the contact.

Traction behaviour of elastohydrodynamic lubrication films with anomalous shapes

“…14,[24][25][26][27] Measurements of viscosity and limiting shear stress have been conducted using high-pressure rheometers. [28][29][30][31] Shear localisation in accordance with the appearance of the limiting shear stress has also been investigated. [32][33][34][35][36][37] It has been recognised that traction is generated within the lubricated area, while a film is formed by the lubricant flow based on a Newtonian fluid at the entrance.…”

Approach to Determine the Limiting Shear Stress of Lubricants at High Pressures Based on Traction Mapping

“…However, many studies have shown that when the lambda ratio is less than three, e.g., about one, the lubrication regime has already been in the full film EHL for the studied The LSS is usually believed to be a kind of shear localization such as a wall slip [7,8] or a plug flow [9,10]. Some other work suggested that LSS may be related to the thermal effects and glass transition of lubricants [11][12][13][14]. Over the past decades, molecular dynamics (MD) simulations have been used to simulate the rheological properties of fluids under conditions of high pressure and high shear [15][16][17].…”