Bio-based ionic liquid crystal for stainless steel-sapphire high temperature ultralow friction

In the present work, the biobased protic ionic liquid crystal bis(2-hydroxyethyl) ammonium palmitate (DPA) has been studied as neat lubricant under linear reciprocating sliding at 75 °C, in the liquid crystalline region, and at 110 °C, above its melting point. Three different tribopairs have been studied using AISI 52100 steel, AISI 316L stainless steel and sapphire balls against AISI 316L disks. Tribological results at 75 °C are in agreement with the different sliding pairs and contact conditions. At 110 °C, a sharp friction coefficient reduction to reach an ultralow steady state value of 0.007–0.009, is observed for the sapphire-AISI 316L contact. Wear rate is also reduced in one order of magnitude. Results are related to water content in DPA, as determined by TG-MS. Wear mechanism are discussed upon the basis of optical and scanning electron microscopy (SEM/EDX) and of surface analysis by XPS.