Interfacial interaction of PTFE sub-micron particles in oil with steel surfaces as excellent extreme-pressure additive

Abstract

This work investigates on the plausible interfacial mechanisms of PTFE (submicron particle) SMPs with the metal counterface during the growth of PTFE tribo-film. A series of submicron-oils was prepared to examine the effect on oil performance with variation in concentration of SMPs (0–9 wt%) in a Group III base-oil. Commercially available submicron suspension containing an adequate amount of dispersant for suspending the PTFE particles (avg size 230 nm) was used in various doses. In order to roughly estimate the shelf life of the formulated oils; UV–Vis spectroscopy technique was employed on the diluted oil samples since original samples were too thick. Dynamic light scattering (DLS) studies were carried in order to estimate the changes in the particle size with storage time. Nano-oils were characterized for their physical properties viz. (density, viscosity and viscosity index) and tribo- performance. Results revealed that 6 wt% of PTFE SMPs proved to be an optimum concentration for the highest EP performance with 392% enhancement compared to the base oil, which is not reported in the literature. To understand the mechanisms behind this performance, worn surfaces of pre-weld balls were inspected by FESEM (Field emission scanning electron microscope), 3D profilometry, goniometry and XPS (X-ray photo-electron spectroscopy), which revealed exact reasons for the varied performance of nano-oils as a function of increasing concentration of SMPs. XPS spectroscopy on worn surfaces revealed the formation of chemically modified tribo-film over the asperities. Furthermore, the thickness of tribo-film formed was also evaluated. Since EP trends were so promising, preliminary oils were characterized for AW and AF performance also. AW and AF performance got enhanced by 30% and 40% respectively.