Ionic liquids as lubricants of polystyrene and polyamide 6-steel contacts.: Preparation and properties of new polymer-ionic liquid dispersions

Room-temperature ionic liquids (ILs) have been used as external lubricants in polystyrene (PS) and polyamide 6 (PA6)-steel contacts and as internal lubricants in new polymer-IL dispersions. 1-C (n) H2n+1-3-CH3-imidazolium X- [X=BF4; n=2 (IL1), 6 (IL2), 8 (IL3). X=PF6; n=6 (IL4). X=CF3SO3; n=2 (IL5). X=(4-CH3C6H4SO3); n=2 (IL6)] ionic liquids give low friction and extremely mild wear in PS/AISI 316L stainless steel contacts, independently of IL composition. For AISI 52100 steel pins a tribocorrosion reaction produces FeF2 and increases friction. PS+IL1 (1; 1.35; 3 wt.% IL1) dispersions show lower dry friction and wear against AISI 52100 as IL1 proportion increases, but the lowest friction, with a one order of magnitude reduction with respect to PS, is reached for PS+1%IL1 once the skin layer has been worn out. Increasing IL1 content to 10 wt.% produces an heterogeneous material with non-uniform IL distribution. IL4 reduces friction and wear in PA6+3%IL4 dispersions against AISI 316L, although the lowest values are obtained with IL4 as external lubricant. The cryofracture surfaces of the polymers have been examined and the thermal stability of the polymers in the presence of ILs has been determined.