Generally, as the brakes mounted on automobiles and the like, disc brakes and drum brakes are mainly employed. In the disc brakes or drum brakes, for example, disc brakes pads and brake linings are used as friction materials. Examples of the friction materials used in the disc brake pads, brake linings and the like include friction materials that are formed using a friction material composition containing a binder, a fiber substrate, a metal powder, an inorganic filler, an organic filler and the like. Such friction materials are known to not only exhibit excellent friction coefficient stability, but also have excellent low-noise properties causing limited unpleasant noise, vibrations and the like. Therefore, friction materials formed using such a friction material composition have been widely adopted in major regions excluding Europe, such as Japan, North America, South America and Asia. The friction materials, such as disc brake pads and brake linings, play a part in braking by generating friction with a facing material (e.g., a disc rotor or a brake drum) and thereby converting the kinetic energy thereof into heat energy.
Incidentally, there may be a case where the conversion of the kinetic energy into heat energy involves partial conversion of the kinetic energy into vibration energy. In this case, the vibration energy generates vibrations, squeals and the like due to braking, thereby causing the user to feel discomfort and impairing the merchantability of the friction material. Therefore, friction materials are demanded not only to have high friction coefficient, friction coefficient stability and abrasion resistance, but also to be unlikely to generate squeals, vibrations and the like. Further, friction materials are also demanded to have sufficient strength such that they are not cracked during braking and to provide a long pad life, as well as to be unlikely to cause abrasion of a facing material and generation of wheel dust.
In recent years, as the needs of the automobile users, there is an increasing demand for further improvement in the comfortability (e.g., riding comfort and damping properties) and, in this respect, reduction in squeals, vibrations and the like is a very important requirement. Particularly, squeals are generally classified into: high-frequency squealing sound (1,000 Hz or higher) generated during braking; and low-frequency noise (i.e., low-frequency-band squeals of several tens to several hundreds in Hz) generated when, for example, an automobile comes to a stop late in the braking period, immediately after the stop, or when the brake pedal is released. These squeals are offensive to the ears of the users and cause discomfort to the users. Moreover, a low-frequency noise may be taken as such an automobile defect that is associated with malfunction of the automobile. Therefore, suppression of low-frequency noise is included in the important properties required for brakes and, in order to satisfy this property, there is a demand for further improvement in friction materials.
Friction materials generally contain a binder, a fiber substrate, a metal powder, an inorganic filler, an organic filler and the like and, in order to allow each friction material to express the above-described properties, each component is used singly, or in combination of two or more kinds thereof. As the fiber substrate, for example, organic fibers, metal fibers or inorganic fibers are used and, particularly, in order to improve the abrasion resistance, the friction coefficient stability after heat history, the crack resistance and the like, fibers of copper, copper alloy or the like are used in a large amount as the metal fibers.
Recently, however, it has been suggested that the use of such a copper or copper alloy-containing friction material causes, for example, pollution of rivers, lakes, oceans and the like since abrasion powder thereof generated by braking contain a large amount of copper. Thus, laws restricting the amount of a copper component used in a friction material have already been enforced mainly in the states of California and Washington. Accordingly, in order to provide a friction material that has favorable friction coefficient, abrasion resistance and rotor abrasion resistance without containing a metal such as copper or copper alloy, Japanese Patent Application Laid-Open (JP-A) No. 2002-138273 proposes a brake friction material containing magnesium oxide and graphite in an amount of from 45% by volume to 80% by volume, wherein the volume ratio of magnesium oxide and graphite (magnesium oxide/graphite) is from 1/1 to 4/1.