A friction material to be used for brakes such as disc brakes and drum brakes, clutches or the like is composed of raw materials such as a fiber base material for exerting a reinforcing function, a friction adjusting material for imparting a friction function and adjusting its friction performance and a binder for integrating these components.
With recent high-performance and speeding-up of vehicles, the role of brakes has become increasingly severe, and it is necessary to have a sufficiently high friction coefficient (effectiveness). Further, the temperature increases during braking at high speed, different from a frictional state during braking at low temperature and low speed, so that stable frictional characteristics having small changes in friction coefficient due to changes in temperature have been desired.
At present, it has been conventional that incorporation of an appropriate amount of metal fiber into a general friction material is effective for reinforcing the strength of the friction material, stabilizing the friction coefficient thereof, further maintaining the friction coefficient and improving a heat radiation effect at high temperature, improving wear resistance, and the like. Focusing attention on these characteristics of the metal fiber, Patent Document 1 discloses a friction material containing 5 to 10% by mass of steel fiber, 5 to 10% by mass of copper fiber having an average fiber length of 2 to 3 mm, and 2 to 5% by mass of zinc powder having a particle diameter of 5 to 75 μm.
According to Patent Document 1, when the friction material contains the copper fibers in an amount within a given range, the friction coefficient at low temperature can be improved, and a decrease in friction coefficient at high temperature and high speed can be suppressed. The reason for this is considered to be that during friction between the friction material and a counterpart material (disc rotor), an adhesion coating film is formed on a surface of the counterpart material due to the extensibility of copper contained in the friction material, and acts as a protective film, thereby being able to maintain the high friction coefficient at high temperature and to suppress wear of the counterpart material.
However, a wear powder of the disc rotor or a metal component contained in the friction material of the brake pad bites into the friction material and aggregates therein to form large metal masses, which remain between the brake pad and the disc rotor, in some cases. The metal masses thus formed by aggregation sometimes abnormally wear the brake pad or the disc rotor (Patent Document 2).
At present, the metal component contained in the friction material is mainly metal fiber such as steel fiber or copper fiber, in many cases. When these fibers are contained in large amounts, there is a possibility to cause the above-mentioned abnormal wear of the disc rotor.
Further, the copper component contained in the friction material is discharged as a wear powder by braking, so that the influence thereof on the natural environment has been pointed out.
Then, Patent Document 3 discloses a method for suppressing elution of the copper component in the friction material.