A sliding friction material is a material which utilizes frictional force to absorb the relative velocity of a sliding partner material in sliding contact therewith, and is used for vehicular brake pads, clutch discs, synchronizer rings, or the like.
In recent years, while performances of engines and transmissions have been increased, reduction in weight has also been required for fuel efficiency, and the size of each constituent part has been minimized. In the circumstances, in addition to reliable operation, operational characteristics sufficient even in a smaller size are required. As sliding friction materials, one with a high coefficient of kinetic friction is desired.
In order to improve the friction characteristics of sliding friction materials, various combinations of friction material components have been suggested, and sliding friction materials with a high coefficient of kinetic friction have been developed. For example, already known is a friction material layer which comprises a thermosetting resin as a binder, and a carbon material, a metallic material, and an inorganic material blended therein (see Patent Literature 1 and 2). The patent literature describes, as means for obtaining high friction performance and high durability of a friction material for synchronizer rings, blending as a main component an appropriate amount of a highly pure carbon material having a low ash content (a petroleum coke or a pitch coke) in the friction material layer.