Effect of temperature, substrate type, additive and humidity on the boundary lubrication in a linear perfluoropolyalkylether fluid

Abstract

There is a need to better understand the behavior of perfluoropolyalkylether (PFPAE) fluids under boundary lubrication so that their interaction with metal surfaces may be modified by additives or by other means. In this study, the effects of temperature, substrate steel type, an antiwear additive, and atmospheric humidity on the protective surface film formation in a linear PFPAE fluid were studied. The measured performance characteristics were coefficient of friction, area of wear scar, final surface roughness and rate of surface film formation. The high temperature (150 °C) caused higher friction and wear as compared to lower temperatures (50 and 100 °C). The high friction and wear at 150 °C was explained by the lack of organic adsorbate film formation over the sliding surfaces at 150 °C. It was proposed that humidity reduces friction and wear by increasing the rate of formation of perfluorocarboxylate species over inorganic surface layers. A fluorinated tertiary alcohol additive reduced the friction at the first hour of sliding at 150 °C. However, the friction was increased towards the end of the five-hour sliding tests due to the loss of the entire additive by evaporation. The rate of surface film formation and wear protection was higher on M50 steel than on M50–NIL steel because the M50 steel surface reacts more with the PFPAE degradation products.