In recent years, global warming caused by greenhouse gases has become an issue worldwide, and various efforts have been made to achieve carbon neutrality. In the automotive industry, polymer additives such as viscosity index improvers have contributed to higher performance of lubricants. In addition, polymer additives can provide various functions such as improvement of friction, wear, and seizure depending on the molecular design, and it is important to understand the parameters necessary to provide such functions. This paper describes the fundamentals to be considered in the design of polymer additives and introduces the latest developments of polymer additives with friction and wear reducing effects.

The ongoing trend for using low viscosity lubricants, with the aim of improving the efficiency of mechanical systems, means that machine components are operated for long periods under thin film, boundary and mixed lubrication conditions. This increases the risk of surface damage such as wear, scuffing and pitting. In order to provide adequate surface protection, the role of lubricant additive has become increasingly important. Among such additive, polymeric additives have been widely used, especially to control lubricant viscosity. Recently, these are used to form adsorption film to reduce surface damage under boundary and mixed lubrication and to reduce friction. This paper reviews the effect of polymeric additives on lubricant performance and outlines the research and the mechanisms by which they are currently believed to function.

We developed a cell capable of small-angle X-ray scattering (SAXS) measurements under high shear (~10⁵s^-¹) and attempted to observe the morphological changes in polymers added as additives in lubricants under high shear by using a powerful X-ray source of synchrotron radiation. In this paper, we first outlined the physics of the change in viscosity of the lubricant due to the addition of polymers and the principles of small-angle X-ray scattering. Then, experimental results under high shear are discussed. It was found that in PMA systems, polymer chains are stretched to parallel to the shear direction by shear, and that the higher the molecular weight, the more the polymer is stretched. In comb-shaped PMA, it was found that monomolecular micelles were formed and that under high shear, only the corona portion was deformed.

In the recent trend towards lower viscosity lubricants, the demand for polymer additives is further increasing. A mixture of polymer additives in lubricants can be expected to provide low friction in boundary lubricated condition, high viscous behaviour at low speeds in fluid lubricated condition and thin oil film retention in elastohydrodynamic lubricated condition. This paper outlines recent researches on polymer additives.

The “Concentrated Polymer Brush”(CPB) system is one of the advanced soft materials that can achieve high lubrication and low friction. The density of graft chains growing from the surface increased by one order of magnitude by applying the living radical polymerization method to graft polymerization, and the graft chain length, or graft film thickness, increased by more than one order of magnitude by optimizing the polymerization conditions in terms of reaction kinetics, such as high-pressure conditions. The former (high-density grafting) has greatly advanced our systematic understanding of the fundamental properties of CPB systems especially through microscopic measurements, while the latter (film thickening) has led to the development of excellent tribological properties in macro (practical) systems, paving the way for application to practical mechanical sliding parts. This article outlines the fundamental and tribological properties of the swollen CPB system and discusses strategies and possibilities for application in mechanical sliding systems. In particular, we focus on a unified understanding of the concept of soft and resilient tribo-materials (SRT materials) based on the Stribeck curve.

With the development of high-performance computers, the importance of computer simulation is growing in tribology. In this article, we introduce the molecular dynamics-based tribology simulation method for polymer materials with our tribology simulation results for polymer brush.

Alkyl diphenyl ether, ADE, synthetic lubricant oil is often used in severe lubrication conditions by its high oxidation stability and lubrication properties. In this study, pure rolling and rolling-sliding contact fatigue test was conducted lubricated with ADE’s. Poly alpha olefin, PAO was also used as reference lubricant. Some polyphenyl ethers were added to ADE to know the effects of polarity of ether group under severe rolling contact condition. In pure rolling condition at 150°C in nitrogen, ADE showed higher fatigue life than PAO. The bearing life was extended further by polyphenyl ether additives. ADE and ADE with polyphenyl ether oils formed carbon rich surface film on the rolling track during the rolling contact fatigue test. ADE also showed a higher bearing life than PAO under rolling-sliding contact. Traction force and wear was slightly higher with ADE than PAO in a higher slip range.

Prediction technology of rubber block friction properties on ice has been studied for the fundamental understandings of tribological mechanisms and the improvement of winter tire grip performance on icy road. The 2-D coupled rubber block model with sipes (narrow grooves) and the mechanical analysis methodology have been applied in order to figure out the complex tribological phenomena on ice. The ploughing effects of sipe edges on ice surface and the decrease of actual contact area caused by the block deflection are considered to the block model in order to evaluate effects of the sipe density and the block stiffness of tire tread pattern. Predicted friction coefficients were shown as the diagram of the sipe density and the actual contact area. The diagram expressed that the friction coefficient on ice could be improved effectively by increase of these two prime factors at the same time. Improvement study of the sipe structure has been conducted applying this prediction results. The new conceptual structures with high density short sipes have been proposed. Tentative test results with block specimens indicated the effectiveness of the new sipe structures.