Thick‐boundary‐film formation by friction modifier additives

Abstract

Abstract The addition of friction modifiers to lubricants has been the subject of study for a number of years. Two main models have emerged, a thick‐film and a thin‐film or monolayer model, but it is uncertain which materials and conditions produce which results. The present work, therefore, looked at whether certain additives form thin or thick films in rolling and/or rolling‐sliding contacts; the practical effect of any thick‐film formation behaviour; and the nature and structure of boundary films formed. Using ultrathin‐film interferometry, tests were performed on a rig, with two commercial friction modifiers, a copper carboxylate soap, and a dimerised unsaturated carboxylic acid, in hexadecane and in a solvent‐refined mineral oil. The results showed that both additives formed thick films in high‐pressure rolling and sliding contacts at slow speeds. However, the nature of the films is determined by temperature and speed, and under certain conditions they can be destroyed.