The effects of nanotexturing on oil film thickness and shape under pointcontact elasto-hydrodynamic lubrication (EHL) conditions were experimentally investigated. A disk-onball friction tester with an optical interferometer was used to measure oil film thickness and to observe the oil film shape. Periodic groove structures with a spiral, perpendicular, or parallel shape and with various groove depths and distances were formed by irradiation of a femtosecond laser onto the surface of steel balls. These nanotextured balls were tested under a load of 20 N and at rotational speeds from 1.0 to 3.0 m/s. Most of the balls with nanotexturing had a thicker oil film than those without texturing. The groove depth and angle were the key parameters determining the thickness of the oil film as they controlled the amount of side leakage of oil from the contact point. Optimization of these parameters resulted in an oil film that was almost twice as thick as that on the ball without texturing.

Boundary lubrication is one of the most interesting topics in the field of tribology, and many studies have been conducted in the past to gain deeper understanding of the behaviour of boundary lubrication layers formed by additives mixed in lubricants because the layer largely affects the tribological property of the target surface. With a strong demand to clarify the behaviour of boundary lubrication layers, several new approaches to in situ (=in lubricant) analysis have been proposed and applied. This paper focuses on the effectiveness of combined use of neutron reflectometry and frequency-modulation atomic force microscopy for the quantitative analysis of boundary lubrication layers formed at the interface between the metal substrate and lubricant and introduced their application with concrete case examples.

Lubrication is critical to achieve stable rolling during the cold rolling of flat steel products. However, the oil film thickness distribution in the roll bite and its effect on the friction between the work roll and strip has not yet been clarified. This study aims to elucidate the relationship between the oil film thickness distribution and friction by focusing on the rolling oil viscosity and steel grades because they significantly affect the friction between the work roll and the strip. Rolling oil was prepared with quantum dots (QDs) as the fluorescent additive and used in rolling experiments to determine its distribution. Furthermore, cold rolling experiments were conducted using two types of oils with different viscosities and three steel grades: low-carbon steel (LCS), high-strength steel (HSS), and advanced high-strength steel (AHSS) with tensile strengths of 270, 590, and 1180 MPa, respectively. Subsequently, the oil film thickness distribution on the steel strip surface was visualized by fluorescence microscopy using QDs. The idea that the higher the tensile strength of the steel or the higher the oil viscosity, the wider the rolling oil distribution on the strip surface was demonstrated. The numerical analyses revealed that the rolling oil distribution on the steel sheet surface was wider for AHSS and HSS than that for LCS. The high surface pressure between the roll and the steel plate may have increased the oil leaching area by increasing the oil viscosity. These findings demonstrate that rolling oil permeation from oil pits reduces the friction between the work rolls and the strip.