Effects of Nanoscale Surface Texture and Lubricant Molecular Structure on Boundary Lubrication in Liquid Ala’ A. Al-Azizi, † Osman Eryilmaz, ‡ Ali Erdemir, ‡ and Seong H. Kim* ,† † Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ABSTRACT: 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. ■ INTRODUCTION The roughness of sliding surfaces can have significant impacts on not only the asperity contacts and effective contact areas but also the fluidic properties of the molecules sandwiched by the solid surfaces. When the topographic features of the interface are much larger than the deformation depth of the surface under compression, then their effects are relatively simple and predictable; the surface topographic features can reduce the effective contact area or act as a reservoir for lubricant supply or flow. In the hydrodynamic lubrication regime, the textured surfaces give a larger effective clearance between the sliding surfaces compared to nontextured surfaces. 1,2 The larger clearance increases the load carrying capacity and reduces friction and wear. In the boundary lubrication regime, on the other hand, surface texture reduces the friction coefficient mainly by acting as reservoirs for constant liquid replenish- ment. 3,4 However, if the size and orientation of large grooves or dimples were not chosen properly, they can act as drain channels for lubricants and result in a severe wear and increase in friction compared to flat surfaces. 5-7 If the asperity contacts between protruded regions can support the contact pressure, then the net contact area is reduced, which may result in a lower friction. This effect could be observed when the contact pressure is sufficiently low that the topographic features are not damaged during the sliding action. 8-10 The completely opposite situation would occur when the sliding solid surfaces are atomically flat. In this case, liquid lubricants can form a long-range ordered structure upon nanoconfinements between flat surfaces. 11-15 As the separation between two surfaces decreases, the confined liquid starts to arrange itself in a layered structure. The long-range structuring of confined liquids can start when the distance between confining solids is smaller than ∼10 molecular layers of the liquid lubricants. 13 Studies using a surface force apparatus (SFA) showed that properties of confined liquids deviate from the properties of the bulk liquids and exhibit a “solid-like” behavior. 16-20 Shear viscosity and relaxation times of nano- confined liquids are higher than those of the bulk liquid due to the long-range ordering into layered structure. Solidified liquid layers offer high load bearing capacity and are not easily expelled from the contact region. 21 The solidification behavior is strongly dependent on the molecular structure of the sandwiched liquid. Simple linear and cyclic molecules solidify readily upon nanoconfinement, but branched molecules have a lower tendency to solidify. 14-16,18,22-27 The heavily branched molecular structure disrupts the extent of layering of confined liquid and prevents solidification. 22-24,27 Because of the polycrystalline nature of many engineering materials, most tribological interfaces are not atomically flat. Solid surfaces subject to sliding contacts are often polished so that detrimental asperity contacts are minimized. These surfaces undergo elastic or plastic deformations during sliding Received: July 6, 2013 Revised: September 6, 2013 Published: October 24, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 13419 dx.doi.org/10.1021/la402574d | Langmuir 2013, 29, 13419-13426