In this paper, we discuss direct connections between interface science and tribology. After giving explanations on fundamentals of thermodynamics and dynamics of interface, we will introduce experimental or theoretical studies on Schallamach waves in sliding friction of polymer gels, as an example of wetting and de-wetting in tribology.
Soft matter such as polymers, colloids, and liquid crystals has been applied in various industrial fields where slip resistance,sealing performance or high slidability is needed. To improve slip resistance or sealing performance, rubber is applied to shoe soles, vehicle tires, and packing because the low elastic modulus of rubber induces a high real contact area. Furthermore, it is expected that polyvinyl alcohol hydrogels will be applied to artificial joints because of their low friction in water-lubricated conditions. The reason for soft mater having such low or high friction properties in wet conditions is based on wettability, which is quantified by spreading coefficient S. For the interface of rubber and a floor at S > 0, for example, the lubricant enters the interface thermodynamically, which leads to a low real contact area and low friction force. Conversely, at S < 0, the lubricant is dewetted from the interface, which causes increases in the real contact area and friction force. In addition to viscosity of lubricant,this unforced dewetting behavior can be changed by enforced wetting, e.g. contacting and sliding.
Inspired by the water-repellent and the self-cleaning properties of the lotus leaf and the nepenthes pitcher plant in the natural world, artificial super hydrophobic surfaces have generated extensive attention in academia and industry. Hydorophobicity and sliding behavior of water droplet are a very important phenomenon in our daily life as well as in many industrial processes. However, the difference between these two phenomena is not well understood. In this article, first, Hydrophobicity and sliding behavior of water droplet are explained, and the research trend of super hydrophobic surface and water slip surface in recent years are introduced. Then, it is explained that hydrophobicity and sliding behavior of water droplet can be controlled separately.
Even with a solid-solid contact with dry surfaces, it is more reasonable to assume that some oil or moisture is present at the interface, except with special environments such as in high vacuum condition. With the cases, liquid bridges can be formed at the interface. Effect of the liquid bridge on adhesion force and friction force are described in this report. Adhesion force arises as a consequence of meniscus shape of liquid-air interface and of a normal component of surface tension at contact line. The adhesion force enhances contact pressure at the frictional interface, and results in an increase of friction force. Asymmetrical shape of the liquid bridge also has a frictional action even without the solid-solid contact. In addition, examples of particle based numerical method are introduced. The method has advantages on simulating capillary phenomena with large deformation of liquid-air interface.
On the surface of resin, additives such as oil agent for metals cannot be used, and the affinity with oil is poor. It is considered to improve the wettability of oil and avoid the direct contact with solids is effective to reduce the friction of resin under oil lubrication. This article introduces the plasma surface modification as a method to improve the oil wettability of resin surface. Then, the practical example that improved tribological characteristics by the plasma modification is introduced with explanation of mechanisms.
Ski wax plays an important role to improve the skiing performance. The following three functions are required for the high-performance wax: high water-repellency, low friction, and high durability. Fluorinated waxes have high water-repellent and low frictional properties; however, they are less durable. Therefore, first the high affinity paraffin is waxed to a ski running surface (ski sole) made of ultra-high molecular weight polyethylene, then the fluorinated paraffin is over-waxed, by using a hot iron. Each wax performance especially depends on the temperature, thus the accurate measurements have been made to evaluate the absolute temperature at the interface between sole and snow during skiing. As a result, the temperature increase due to frictional heating actually depended on the type of wax.
Wiper blade rubber is a very important part in the automotive wiper system. And, Wiping ability, quietness, durability, and stain resistance are important for blade rubber. In recent years, quietness in a car room has been increased due to idling stop, EV conversion, etc., so demands for wiper operation sound are getting stricter. To satisfy the demands, the rubber material, surface treatment and coating are important technologies. In this paper, we introduce the basic performance and tribology of the wiper blade rubber.
Bump foil journal bearings are prospective applicants that can support a small-sized rotor of high-speed rotary machinery. Some model bearings are used in predicting the characteristic performances of the bearings. In the previous study, the finite element (FE) method that adopted a curved-beam element was applied in analyzing the displacement of the bearing surface that is called as top foil. However, the model assumes that the air film thickness in the axial direction is constant, so the performance of foil journal bearings that has uneven air film thickness in the direction cannot be obtained. In this study, the top foil is assumed to be a thin shell. The displacement is analyzed by replacing the curved-beam element to a plate shell element. Although the replacement of the FE model is found to have a large influence on the performances, this is because the shell model cannot couple the membrane and the bending effects. It is concluded that the shell model is inappropriate to the prediction of the bearing performance and should be replaced to other type of shell model that can couple the two effects.