ABSTRACT
Modern lubricating oils are formulated with a variety of additives designed to (1) provide beneficial rheological characteristics to lubricants, (2) to stabilize their physical and chemical properties, and (3) to protect lubricated equipment against wear, fatigue, and corrosion. Under the influence of chemical and mechanical stresses and elevated temperatures lubricants tend to undergo certain reversible and irreversible changes. The reversible changes are caused by temporary alignment of polymeric additives in the direction of flow, resulting in an apparent drop in viscosity. When the liquid returns to a state of rest, the viscosity returns to its initial value. This is known as non-Newtonian rheology. The irreversible changes are due to a number of ongoing processes such as stress-induced scission of polymeric additives, oxidation, contamination, etc. The latter detrimental processes limit the useful life of lubricants and can lead to costly repairs and down time if a lubricated system is not properly maintained. In this chapter we focus on the combined effects of the lubricants’ non-Newtonian rheology and stress-induced polymer molecule scission and on changes in lubricant contact parameters.
