The invention relates to a friction lining for wet running, consisting of a fabric which can be applied to a carrier and is partly impregnated with resin, as well as to a method for producing such a friction lining. 
In order to achieve an advantageous porosity for receiving and conducting the respectively used lubricating liquid in the case of friction linings made of a fabric consisting of carbon fibers and partly impregnated with resin, it is known (U.S. Pat.  No.  5,662,993 A) to limit the resin share to 18 to 40%.  Despite this limitation of the resin share, comparably high hydrodynamic friction shares are obtained which reduce the coefficient of friction because the hardening of such friction linings occurs in heating presses which produce a resin accumulation in the surface region in order to reduce their operationally induced settling tendency, which can lead to surface regions with a substantially coherent resin film which allows the local build-up of a hydrodynamic lubricant film in the zone of the later friction face.  Despite the pressing of the friction linings during the hardening of the resin, the porosity of the friction linings required for taking up the lubricating liquid produces a setting of the friction linings which progresses over the service life due to the operationally induced pressure strains, which entails a reduction of the pore volume.  This setting inclination increases with decreasing resin content, so that the resin content should be increased for reducing the setting behavior of such a friction lining.  This is contrary to the demand for a sufficient porosity of the friction lining for receiving lubricating liquid. 
The invention is thus based on the object of providing a friction lining of the kind mentioned above in such a way that on the one hand the demand for the lowest possible hydrodynamic friction share can be fulfilled, and on the other hand the setting tendency can be reduced as far as possible. 
The invention achieves the object in such a way that the porosity of the fabric impregnated with resin is higher in the zone of the friction face by a lower resin content in a surface layer than in the remaining layer. 
The invention is based on the finding that the relatively high porosity which is relevant for the frictional behavior merely needs to be ensured in a surface layer of the friction face.  The porosity of the remaining layer of the friction lining, however, hardly plays any role for the frictional behavior at all.  This means that at a comparably high share of resin and the thus resulting lower porosity, a favorable frictional behavior can be achieved when a sufficiently high porosity in a surface layer is striven for by a respectively lower resin content.  As a result of this measure, the otherwise mutually contradicting requirements of a favorable discharge of the lubricating liquid by the friction face and of the lowest possible setting behavior can be fulfilled in an advantageous manner.  It was noticed surprisingly that neither the higher resin content with the thus resulting lower porosity of the friction lining outside of the surface layer of the friction lining can have a disadvantageous effect on the frictional behavior, nor can the porous layer of the friction face close to the surface with the lower resin content have a disadvantageous effect on the setting behavior.  This leads to the consequence that in comparison with the state of the art overall higher resin contents can be employed without having to fear any reduction of the coefficients of friction. 
Different methods can be employed to reduce the resin content for achieving a higher porosity in a surface layer of a friction lining whose fabric has been saturated partly with liquid resin and was subjected to a pressing pressure during the hardening of the resin.  One possibility is that the resin is partly mechanically removed again in the zone of the friction face after the hardening.  This mechanical resin removal at least partly uncovers the surface fibers of the fabric and removes locally bound resin layers, which leads to an advantageous structured friction face which as a result of its structuring is provided with the porosity needed for draining off the lubricating film.  The mechanical removal of the resin can be achieved by a brushing of the friction face.  It is also possible grind out the resin from the zone of the friction face or to use sandblasting for this purpose. 
In order to reduce the resin content in a surface layer of the friction face it is not necessary to mechanically remove the resin retroactively.  It is also possible to suck off the resin during the pressing of the impregnated fabric from the surface region of the friction face, e.g. with the help of a suction film which is placed on the fiber layer saturated with resin prior to the hardening of the resin and is subjected to a pressing pressure jointly with the fabric, so that the resin which is still sufficiently capable of flowing is pushed from the layer adjacent to the suction film into the pores of the suction film.  After the pressing process and the hardening of the resin the resin saturated suction film can be drawn off from the friction face, whereby the released surface layer of the friction lining shows a considerably lower resin content than the remaining friction lining layer.