Environmental problems have had a great impact on the development of friction materials for disc brake. Therefore, it becomes important for the brake industry to understand the regulatory trends as to friction materials. Focusing on the environmental regulation trends about friction materials in the last decade, the following two topics are reviewed. One is copper free initiative which regulates the copper content of friction materials in the United States. The other is the emission of brake wear particles which is being studied mainly in Europe.

A disc brake system is a typical friction brake and has been widely used as a service brake for automobiles. It converts kinetic energy of a running vehicle into heat through friction between a brake disc and pads. Surface finish and corresponding surface texture of gray cast iron brake discs affect friction and wear particularly during running-in. Brake judder is a significant issue concerning automobile noise and vibration claims. It is a vibration forced by disc thickness variation (DTV), which are caused by several mechano-thermo-chemical mechanisms. Vehicle electrification causes significant effects on friction, wear, and judder of brake systems since regenerative braking of xEVs reduces operating frequencies and loads of friction brakes. Continuous improvement of gray cast iron brake discs will be expected to maintain safe and secure braking with comfortableness.

In recent years, due to heightened awareness of health promotion and energy conservation, demand for bicycle use for commuting, school, leisure, etc. have been increased. In sport-type bicycle, caliper brake and disk brake are common. Compared to a disk brake, braking performance of a caliper brake under wet condition reduces when driving at a high running speed. To increase the braking performance of a caliper brake of bicycle under wet condition, a brake shoe made of a fluorine-contained rubber (FKM) compound filled with rice bran ceramics (RBC) particles was developed. This article introduces the braking performance and tribological properties of the FKM/RBC shoe under wet condition from high running speed to stopping. The RBC filler possessed the characteristics to maintain the braking performance irrespective of duration of use and to reduce the wear of the shoe and rim under rainy conditions.

Paper-Based Wet Type Friction Material (friction material) is used for Wet Clutch in Automatic Transmission (AT) of Vehicles. Friction material technologies have been developed for various requirements that depend on AT Types. Recently, the requirements for friction material are changing and becoming more complicated because of HEV and BEV applications. Basic study is necessary to understand the mechanism of friction behavior and to look for further possibilities of new applications.

Paper-based friction materials which are used for clutch discs in automobiles play a role in power transmission. The friction materials are subjected to cyclic compressive-shear loading in service, and it is important to evaluate their fatigue properties taking into account such a load condition. The friction materials are porous materials consisting of various fibers and resins, which have complicated microstructures, and the techniques to evaluate their mechanical properties and lifetime have not been established. This review first describes the technique of tensile testing to evaluate basic mechanical properties such as tensile strength of the materials, and the testing results are presented with special attention paid to the difference in the properties between in-plane and out-of-plane directions of the materials. Then, the techniques and experimental results of fatigue testing are demonstrated, and the fatigue failure mechanism under cyclic compressive loading is discussed. Summarizing the review, future predictions have been presented.

The wet clutch is a system in which friction is generated by pressing a friction material composed of aramid fiber, phenol resin, diatomaceous earth, carbon fiber, etc. against the friction plate and steel plate, and the power is controlled using that friction. It is used in automobiles, construction machines, and agricultural machines. In particular, it is often used in transmission mechanisms such as automatic transmissions for automobiles, and we will continue to apply clutch packs equipped with wet clutches to transaxles (e-Axles) for electric vehicles. In this report, we will explain the history of ATF specifications and the required performance, especially clutch characteristics.

Ionic liquids containing halogens have good lubricity, however, they are reported to cause corrosive wear. In contrast, halogen-free ionic liquids do not cause such corrosive wear, but they do not have good lubricity comparable to the halogen-containing counterparts. In this study, we focused on specific halogen-free boron-containing ionic liquids that exhibited low friction performance. The performance was mainly due to its boron-containing anions, because it was realized with both phosphonium-based and ammonium-based cations. Surface analysis and scratch test with atomic force microscope (AFM) were conducted with phosphonium-based cations to clarify the low friction mechanism. The boron-containing ionic liquid, [BOB][P₆₆₆₁₄], showed lower friction of around 0.01, which was much lower than MoDTC-containing engine oils did. The cause of the ultra-low friction with [BOB] [P₆₆₆₁₄] is thought to be due to the fact that the reaction film formed by friction has a very smooth surface and suppresses contact between metals. On the other hand, another boron-containing ionic liquid [BMB] [P₆₆₆₁₄], which did not form a smooth surface, showed relatively high friction compared to [BOB] [P₆₆₆₁₄]. Scratch tests with AFM indicated that the softer reaction film derived from [BOB] [P₆₆₆₁₄] led to the formation of a smooth sliding surface.

The vapor phase carbon coating such as diamond-like carbon (DLC) coating can form a hard carbon layer with a uniform and large area on the base materials, and is widely used in industry. However, it is impossible to perform carbon coating on a powder or a structure having a complicated shape by the vapor phase carbon coating method. Further, since the vapor phase carbon coating method is expensive and not suitable for mass production, therefore it is required the development of an inexpensive liquid phase carbon coating method. In this study, we developed a liquid-phase carbon coating method that enables uniform and ultrathin carbon coating on silica spherical particles by using a soluble carbon material derived from phloroglucinol, and developed carbon-coated silica spherical particles that have both high lubricity and wear suppression ability. The carbon coated silica spherical particles exhibited high lubrication characteristics as a dispersion of low and high viscosity or the form of a spray.