The viscosity at the glass transition of a liquid lubricant

Scott Bair

doi:10.1007/s40544-018-0210-1

Abstract

In the classical study of elastohydrodynamic lubrication (EHL) which does not employ real, measurable viscosity in analysis, the possibility of a glass transition has not been considered in many years. Indeed, the two rheological assumptions of classical EHL, the Newtonian inlet and the equivalence of a traction curve to a flow curve, would not have persisted so long had the pressure dependence of the viscosity been accurately stated. With the recent appearance of viscosity obtained from viscometers in EHL analysis, the possibility of a glass transition in the contact should be reexamined, especially for the fragile traction fluids. This article employs published data for a synthetic cycloaliphatic hydrocarbon to estimate the glass transition viscosity so that, when using real viscosities in EHL simulations, the state of the liquid may be assessed. Far into the glassy state the liquid should be treated as an elastic solid with a yield stress.

Highlights

The glass transition in elastohydrodynamic lubrication (EHL) liquids under pressure has been the subject of discussion and speculation for more than forty years [1, 2]
The glass transition has not been a recent topic of classical EHL as the viscosity there is described by simple pressure relations based on fictional accounts [3] of viscometer measurements in which the viscosity does not usually reach to the large values for which the transition occurs
For much of the last forty years, the temperature and pressure dependence of viscosity in EHL has been specified by simple models which prevent the viscosity from reaching realistic values

Summary

Introduction

The glass transition in elastohydrodynamic lubrication (EHL) liquids under pressure has been the subject of discussion and speculation for more than forty years [1, 2]. The glass transition has not been a recent topic of classical EHL as the viscosity there is described by simple pressure relations based on fictional accounts [3] of viscometer measurements in which the viscosity does not usually reach to the large values for which the transition occurs. Experimental measurements of the glass transition and the viscosity of liquid lubricants over a range of temperature and pressure more than thirty years ago [7] indicated that the glass viscosity must be much smaller, 107 Pa·s to 109 Pa·s.

Glass transition temperature and pressure

Temperature and pressure dependence of viscosity

Viscosity at the glass transition

Conclusion