Ultralow friction of 5CB liquid crystal on steel surfaces using a 1,3-diketone additive

Liquid crystals (LCs) are considered attractive lubricating materials due to their controllable molecular alignment and rheological behavior using an external electric field or a surface-alignment layer. In this study, a 1,3-diketone EPND (1-(4-ethyl phenyl) nonane-1,3-dione) fluid was tribologically investigated as an additive in a nematic liquid crystal 5CB (4-pentyl-4'-cyanobiphenyl) on steel surfaces. It shows that the chemically bonded EPND monolayer can reduce the surface energy of steel to a value lower than the surface tension of 5CB. According to the FCK rule, the EPND monolayer promotes a homeotropic alignment of 5CB, which facilitates its boundary lubrication witnessed by a Stribeck behavior test. Concerning on the influence of surface roughness change during long-term friction, it suggests that severe wear and deep grooves should be avoided by applying 5CB/EPND hybrid fluid at a low initial contact pressure (~137 MPa). In this situation the surface energy effect of EPND monolayer can dominate the molecular alignment of 5CB in the presence of shear, and lead to an ultralow coefficient of friction (= 0.008). This study shows that EPND is a promising additive for nematic liquid crystal lubricants, especially in applications with low load and high requirement for low friction.