Diprotic ammonium palmitate ionic liquid crystal and nanodiamonds in aqueous lubrication. Film thickness and influence of sliding speed

Ionic liquid based lubricants offer potential to reduce the sliding friction and wear of ferrous bearing surfaces such as those used in cutting and machining processes. In this study, two types of promising additives were investigated: (1) diprotic bis(2-hydroxyethyl)ammonium palmitate (DPA) ionic liquid crystal and (2) a 0.1 wt% dispersion of nanodiamonds (ND) in DPA. Both have been used at 1 wt% concentration in aqueous lubricants. Those were Water+DPA, and Water+DPA+ND. The stability of the dispersions has been studied by dynamic light scattering, finding that the interaction between the additives and water changes particle size distribution. Glass-against-steel optical interferometry using water film alone indicated that the film thickness decreased with number of sliding strokes and that in turn increased the friction coefficient. By comparison, Water+DPA and Water+DPA+ND showed lower friction coefficients and increased film thickness. In steel-steel pin-on-disk contacts lubricated only by water, electrical resistance increases at speeds higher than 100 mm/s. DPA and DPA+ND additives displace the electrical resistance increase to lower sliding speed, around 10 mm/s. The additives reduced the coefficients of sliding friction up to 82%, and wear rates of AISI 316L disks by more than two orders of magnitude. The combination of the palmitate ionic liquid crystal and nanodiamonds in Water+DPA+ND gave the lowest wear rates. The wear mechanism and surface analysis associated with these results are discussed.