Effects of nanoscale surface texture and lubricant molecular structure on boundary lubrication in liquid.

Nanoconfinement effects of boundary lubricants can significantly affect the friction behavior of textured solid interfaces. These effects were studied with nanotextured diamond-like carbon (DLC) surfaces using a reciprocating ball-on-flat tribometer in liquid lubricants with different molecular structures: n-hexadecane and n-pentanol for linear molecular structure and poly(α-olefin) and heptamethylnonane for branched molecular structure. It is well-known that liquid lubricants with linear molecular structures can readily form a long-range ordered structure upon nanoconfinement between flat solid surfaces. This long-range ordering, often called solidification, causes high friction in the boundary lubrication regime. When the solid surface deforms elastically due to the contact pressure and this deformation depth is larger than the surface roughness, even rough surfaces can exhibit the nanoconfinement effects. However, the liquid entrapped in the depressed region of the nanotextured surface would not solidify, which effectively reduces the solidified lubricant area in the contact region and decreases friction. When liquid lubricants are branched, the nanoconfinement-induced solidification does not occur because the molecular structure is not suitable for the long-range ordering. Surface texture, therefore, has an insignificant effect on the boundary lubrication of branched molecules.