1. Field of the Invention
This invention relates to a wet friction material used for an automatic transmission and the like and its manufacturing method.
2. Description of the Related Art
A wet friction material is used for an automatic transmission of an automobile and the like. The wet friction material has a basic structure in which a lining of high coefficient of friction is stuck on a surface of an annular core metal. A positive xcexc-V slope is required for the wet friction material so that a slippage (xcexc) increases as a relative revolution (V) to a counterpart material increases. In order to improve the positive xcexc-V slope, it is effective to contact a carbonized and inorganic surface layer with the counterpart material. Conventionally, several methods have been used such as a resin component at a very superficial layer of the wet friction material is mechanically abraded by use of a special jig so as to facilitate mixed inorganic component to be contacted with the counterpart material. A technique shown in Japanese Laid Open Patent Application No. 4-78331 relates to such method.
However, in case of the mechanical abrasion method using the special jig, special jigs are necessary for every size of products, respectively, thereby increasing production costs. It is possible to burn the very superficial layer. However, it affects irregularity of the friction surface, so that it takes much time to carbonize the surface layer and that there are generated irregularity or unevenness.
An object of the present invention is to provide an efficient and economic wet friction material and its manufacturing method that is capable of evenly carbonizing resin and fibers of a very superficial layer of a friction surface so as to flexibly deal with a variety of size of products.
According to a first aspect of the invention, there is provided a wet friction material comprising a substrate having a surface. A very superficial layer is provided on the surface of the substrate and contains resin and fibers. The resin and the fibers are heated by one of a laser beam, a heating plate and a heating roller so as to thermally change the very superficial layer in a short period of time.
According to a second aspect of the invention, there is provided a manufacturing method of a wet friction material comprising the step of heating resin and fibers contained in a very superficial layer provided on a surface of a substrate by one of a laser beam, a heating plate and a heating roller so as to thermally change the very superficial layer in a short period of time.
In each of the above aspects of the invention, the thermal change of the very superficial layer may comprise carbonizing the very superficial layer within a range of about 15 to 80 xcexcm in thickness from a surface of the very superficial layer. Alternatively, the thermal change of the very superficial layer may comprise quenching and hardening the very superficial layer within a range of about 15 to 80 xcexcm in thickness from a surface of the very superficial layer.
Moreover, the wet friction material may have a disk shape. The resin and the fibers may be heated by the laser beam so as to carbonize the very superficial layer, and the laser beam may be radiated on the surface of the very superficial layer from one of one circumference to another circumference while rotating the wet friction material on a rotating table under a laser radiation power of 30-90% and a table rotating speed of 20 to 40 rpm.