The influence of roughness orientation on the friction-speed characteristics of adsorbed films was investigated by changing the angle of the anisotropically striated roughness to the sliding direction of the counter surface. As the angle of the striated roughness becomes perpendicular to the sliding direction, the friction coefficient is decreased. The friction-speed characteristics of the adsorbed film in the transverse direction of the striated roughness having different height or shape of asperities were also investigated. The friction coefficient obtained in the low speed region decreased with increasing arithmetic mean peak curvature (S pc ) when stearic acid-formulated oil was used as a lubricant oil. The results demonstrated that the roughness orientation and the asperity shape of the transverse roughness were key factors for generating additional load-bearing pressure so that the adsorbed films could exert their intrinsic friction-reducing effects.

This study aims to investigate the influence of surface morphology on boundary-lubricated friction in a stearic acid solution. The surface morphology was controlled by fabricating submicrometer line-and-space patterns on Si(100) surface via photolithography. The boundary-lubricated friction on the patterns was measured by in-liquid lateral force microscopy for both transverse and longitudinal ridges, with respect to the sliding direction; the highest friction was observed on longitudinal ridges and grooves, which is in agreement with the tendency observed in our previous friction studies on steel surfaces. To further investigate this phenomenon, some additional patterns having different submicrometer morphologies were prepared and their friction characteristics were investigated. On the patterns not allowing the fluid to flow along the grooves, the frictional forces were equivalent for transverse and longitudinal grooves and ridges. Therefore, the high friction observed on the longitudinal ridges was caused by flowing out of fluid along the grooves, and it was possible to conclude that the fluidity around the submicrometer ridges and grooves influences the friction-reducing effect of stearic acid in boundary lubrication regime.