There is strong anticipation for contributions from tribology in reducing greenhouse gas emissions from mechanical systems and achieving a carbon-neutral society. Surface texturing has long been implemented as one method to improve tribological properties, focusing on reducing friction losses and wear damage. However, recent advancements in surface processing techniques and lubrication analysis have significantly progressed in this area. This special issue aims to cover various aspects of surface texturing technologies for enhancing tribological properties, ranging from fundamental lubrication mechanisms to practical applications, including theoretical analysis, surface design, surface processing, surface modification, and evaluation methods. Through this special issue, the journal aims to showcase current trends and future prospects in this field.

 On behalf of the editorial board of Tribology Online, the Guest Editors sincerely thank all the contributors to this Special Issue and hope the readers find these papers interesting and useful.

Friction control is the most fundamental and important issue in the field of tribology. In recent years, friction control technology, which aims to reduce the energy loss in mechanical systems to address global environmental issues, has received significant attention. Friction control involves minimizing the friction coefficients and maintaining high frictional forces, such as in power transmissions and brakes. Surface texturing plays a crucial role in controlling the friction of tribological elements. In the future, technologies for active friction control at the interface between living organisms and artifacts will be indispensable for realizing a collaborative society with the coexistence of humans and robots. This article reviews the role and key findings of surface texturing and outlines its potential and future trends.

The low-friction mechanism of graphene oxide-dispersed lubricants remains to be elucidated. In this study, pin-on-disk friction tests were conducted on specimens with various surface properties to clarify the effect of the lubrication condition of the contact point on the low-friction by dispersing graphene oxide (GO) particles into lubricating oil. The relationship between the lubrication state and the friction coefficient was investigated using Λ ratio calculated from the surface roughness of the sliding area after the test and the calculated minimum oil film thickness. The friction coefficient decreased as Λ ratio increased, that is, as the lubrication state approached fluid lubrication. For the disk specimen containing pit shapes on the surface, the friction coefficient was significantly reduced by up to 70% for the graphene oxide-dispersed lubricant compared to the base oil alone. On the other hand, the friction reduction was only 20% for the specimen without pits. The composite surface roughness of the sliding area after the test was significantly improved for the disk with pits, which indicates that the progress of running-in process during the test affects the friction properties of the GO containing lubricant.

This paper proposes a method of topographical characterization for tribo-surface textures based on contact asperities. BEM (boundary element method) simulation using the Boussinesq-Cerruti theory was employed to estimate the contact condition. Parameters such as contact area ratio, contact pressure, radius of curvature, etc., were evaluated from the contact condition. The proposed method was applied for understanding the mechanism of higher friction for a belt-CVT (continuously variable transmission). Results showed that the contact summit density and mean contact summit area have clear correlations with the coefficient of friction obtained by experiments.

The present study describes an improvement of damping performance of a mechanical tensioner driven by friction force at the power spring interface, used for the timing chain system for an internal combustion engine. The mechanical tensioner consists of a power spring subjected to face-to-face contact with a reciprocal motion and a leaf spring to apply the reaction load. The concave and convex surface texture was fabricated with an electrochemical etching on the power spring surfaces. Since the reaction load is affected by the friction force at the power spring interface, the damping performance is evaluated from a hysteresis loop resulting in the reaction load and the plunger displacement trajectory. Superior stabilization of the reaction load and a remarkable decrease of the wear loss including the restriction of the oxidation was found on the convex textured tensioner in lubricated condition. The tensioner performance was also evaluated in staved lubricating conditions. Results showed that the change of the reaction load of the convex textured tensioner was small just before the stiction occurrence. In contrast, the reaction load of the concave textured tensioner increased with the increase in the number of cycles. Therefore, the applicability of the convex surface texture to the power spring surface of the tensioner was confirmed. It was also estimated that the discharge of the wear debris is an important factor in maintaining the tensioner performance.