Nanotribology of Ionic Liquids as Lubricant Additives for Alumina Surfaces

Common lubricants, optimized for steel, perform poorly for alumina surfaces, making ionic liquids (ILs) attractive potential alternatives, either in neat form or as oil additives. Here, atomic force microscopy (AFM) has been used to study the lubricity of two oil-miscible ILs between a silicon AFM tip and an alumina surface. Trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (P6,6,6,14(iC8)2PO2) and trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate (P6,6,6,14DEHP) were mixed with hexadecane at concentrations from 0 to 100 mol % IL. Both ILs are effective lubricants for alumina, reducing lateral forces to approximately one third of the forces measured in hexadecane. When used as an additive in hexadecane, increasing the IL concentration generally decreased the friction, as the IL adsorbs on alumina and forms a strong boundary layer. P6,6,6,14DEHP mixtures reduce friction more effectively than P6,6,6,14(iC8)2PO2. For both ILs, a 2 mol % mixture of IL and hexadecane reduced friction most effectively, even more than the neat IL.