Film forming and friction properties of single phase and two phase lubricants in high-speed rolling/sliding contact

Abstract

Single-phase (neat oil) and two-phase (oil-in-water emulsions) lubricants are widely used in metal forming processes, where speeds as high as 20 m s-1 are reached. Most of the previous work done on both neat oil and on oil-in-water emulsions has focused on low speed behaviour (below 5 m s-1) and, as a result, the low speed behaviour of oil-in-water emulsions is well understood. Under these conditions, the lubricating oil film is composed predominantly of oil and the thickness of the film is similar to that for neat oil. However, the behaviour at high speed is entirely different. No experimental film thickness and friction results at speeds above 5 m s-1 are available for neat oil and only one study (Zhu et al., 1994) has reported the film thickness behaviour of oil-in- water emulsions above this speed whereas no friction measurements at speeds above 3.5 m s-1 have been carried out using oil-in-water emulsions. Consequently, to date, the behaviour of neat oil and the relation of emulsion composition to film forming ability at high rolling speeds could not be described. This project is aimed at investigating the mechanism of film formation and the film forming and friction properties of single-phase and two-phase lubricants in high speed rolling/sliding contacts. An EHD test rig was modified to measure film thickness and friction of oil-inwater emulsions in very high speed, rolling/sliding conditions (up to a mean rolling speed of 20 m s-1). Ultrathin film interferometry was used to investigate film thickness while infrared temperature mapping of the contact was used to obtain maps showing the rate of heat input into the surface, from which shear stresses and friction could be calculated. Light induced fluorescence was also employed using a water-soluble and an oil-soluble dye to allow visualization of the contact (at low speeds) and help in investigating the composition of the entrained lubricant at these high speeds. Results showed that, for neat oils, the major factor affecting the film formed at high speed is shear heating. For dilute emulsions at the highest speeds, the film thickness and friction are close to those obtained with pure water. More concentrated emulsions give slightly higher film thicknesses. A comparison of the results with earlier theoretical predictions was carried out. Together with the fluorescence results, this suggested that high speed leads to the entrainment of a micro-emulsion.