Lubricants With Non-Newtonian Rheology and Their Degradation in Line Contacts

Abstract

A plane isothermal elastohydrodynamic problem for a line contact lubricated by a degrading fluid with non-Newtonian rheology is studied. The lubricant is represented by a base stock with a polymer additive which undergoes stress-induced degradation caused by scission of polymer molecules. The polymer molecules are considered to be of linear structure. The effective lubricant viscosity experiences reversible and irreversible losses. The reversible loss of the effective lubricant viscosity (shear thinning) is due to the non-Newtonian rheology of the fluid and variations in the fluid shear rate. The irreversible loss of the effective lubricant viscosity is caused by the degradation process of the polymer additive dissolved in the lubricant. The degradation process of the polymer additive while it passes through the contact is described by a kinetic equation. The kinetic equation is solved along the lubricant flow streamlines. The solution of the kinetic equation predicts the density of the probabilistic distribution of the polymer molecular weight versus polymer molecule chain length. The changes in the distribution of polymer molecular weight affect local lubricant properties. In particular, the lubricant viscosity experiences reversible and irreversible losses and, in general, is a discontinuous function. The changes in the lubricant viscosity alter virtually all parameters of the lubricated contact such as film thickness, friction stresses, pressure, and gap. The considered non-Newtonian rheology of the lubricant causes a small reversible loss of its viscosity. As a result of the polymer additive degradation the lubricant may experience a significant irreversible loss of its viscosity which, in turn, leads to a noticeable reduction in the lubrication film thickness in comparison with the case of a non-degrading lubricant with similar rheology. Some comparisons between the cases of lubricants with Newtonian and non-Newtonian rheologies with and without lubricant degradation are considered.