The Role of Fragility in Thermal Elastohydrodynamics

Abstract

Temperature primarily influences thermal elastohydrodynamic lubrication (TEHL) through the temperature dependence of the viscosity of the liquid. The pressure and temperature dependences of viscosity increase rapidly as the glassy state is approached from the liquid state, a property known as fragility. The glass temperature increases with pressure and reaches to ordinary temperatures at TEHL pressures. It is astounding, therefore, that most TEHL analyses have ignored fragility by utilizing a viscosity correlation incapable of describing this behavior. Here, a low viscosity fragile oil is characterized for low-shear viscosity to 1.6 GPa and TEHL line contact simulations show, not only a substantial effect on friction, but significant differences in minimum film thickness when fragility is not ignored, as is customary in classical TEHL. The influence on friction manifests even under moderate load and speed conditions, while that on film thickness seems to be restricted to high loads.

Evaluation of Various Shear-Thinning Models for Squalane Using Traction Measurements, TEHD and NEMD Simulations

“…Starting from this value, the low-shear viscosity η 0 increases approximately exponentially with pressure (Figure 1, centre), which is shown in a single logarithmic plot as a straight line with the slope α p . Actually, the isothermal curves in this representation often have a slightly degressive or degressive-progressive progression, which indicates a transition to the glassy state [9]. In addition, the curves usually spread, i.e., α p = f (ϑ, p).…”