Lubricants and Lubricant Additives Under Shear Studied Under Operating Conditions by Optical and Infrared Spectroscopic Methods

Abstract

A physical basis was sought for differences in fuel economy with a typical petroleum lube base stock when various additives were present. The kinematic viscosity was maintained at 7.1 cS at 100°C by blending. Film thickness, temperature, and traction were determined for a ball/plate contact under elastohydrodynamic conditions. Different additives produced considerably different results. Invariably, small film thicknesses paralleled low temperatures in the conjunction region, low traction and high fuel economy. Zinc dithiophosphate (ZDDP) was unique in that it always increased film thickness, but increased traction only at high loads and low speeds while decreasing it otherwise. Other additives, e.g. friction modifier (FM) and viscosity index (VI) improver and combinations decreased film thickness and traction. To help determine the reasons for these behaviors—whether surface or bulk effects—polarized infrared emission spectra were collected from an operating mock journal bearing. Adherent layers do not change polarization with increase of shear rate and could therefore be distinguished from the bulk material. ZDDP did change polarization with shear rate us did VI improver, but FM did not. The formation and breakup in the bulk phase of insoluble particles or globules—similar to grease thickeners held together by ZDDP is proposed as a novel possible explanation for its behavior. The established theories about the action of the other additives were confirmed.

Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987

Notes

Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987