Abstract:
Clutch linings comprising fiber-reinforced ceramic materials which contain short carbon fibers and whose matrix has a mass fraction of at least 40% of silicon carbide, process for producing them and their use in clutch systems, in particular for motor vehicles.

Description:
FIELD OF THE INVENTION 
   The invention relates to clutch linings comprising fiber-reinforced ceramic materials. 
   BACKGROUND OF THE INVENTION 
   Clutch systems are used in motor vehicles for separating the engine and gearbox. In modern vehicles, virtually exclusive use is made of friction clutches or hydrodynamic transmissions. In the customary friction clutch, force-transmitting connection between the shaft from the engine and the shaft entering the gearbox is established by means of one or more disks pressed against one another, with at least one clutch disk connected to the shaft entering the gearbox, which has linings on both sides, being located between the flywheel of the engine and a clutch pressure plate. When the clutch pedal is depressed, the pressure plate is pushed away from the clutch disk against the spring force of the disk spring, as a result of which the clutch disk is released. The connection between the engine and the gearbox is thereby interrupted. 
   The magnitude of the torque which can be transmitted depends on the area and the diameter of the clutch disk and on the spring force of the disk spring. For large torques, clutch systems having a relatively large friction area or multidisk clutches are therefore used. The clutch disk is, especially on starting, subject to wear by rubbing and is heated by the friction. 
   In view of this prior art, it is an object of the invention to provide clutch systems which can transmit large torques at small construction sizes. Preference is given to single-disk systems which have relatively small friction areas. 
   Friction pairings with ceramic materials, in particular fiber-reinforced ceramic materials, are known. When used as brake disks, high heat resistance and a coefficient of friction which has a low dependence on the load are required. In the case of clutch systems, high static friction and high torsional stiffness are desired. 
   In WO 93/11185, clutch disks are disclosed which have friction surfaces composed of a plurality of layers which alternately contain continuous carbon fibers and short carbon fibers and in each case pitch as impregnant. 
   In WO-A 01/55611, multidisk clutches immersed in an oil bath are disclosed, in which steel disks lined with braided continuous carbon fibers act against steel disks whose friction surface is coated with a thin layer of nickel powder. 
   The application of continuous carbon fibers or textiles made of these is complicated. Furthermore, the experiments on which the invention is based have shown that the preferential orientation of the fibers in the plane of the linings produced from continuous carbon fibers or textiles in which these are present is relatively unfavorable for achieving a high static friction. For this reason, linings for clutch disks which are simple to manufacture and display high static friction values are desirable. 
   In DE-A 199 39 545 clutch disks are disclosed which are provided with friction linings comprising silicon carbide reinforced with carbon fibers. It can also be provided for the flywheel and pressure plate forming a friction pairing with the linings to be made of silicon carbide reinforced with carbon fibers. Otherwise, nothing is said about the type of material used. 
   SUMMARY OF THE INVENTION 
   In view of this prior art, it has to be ensured that a sufficient static friction is realised. 
   The object of the invention is achieved by clutch linings comprising fiber-reinforced ceramic materials which contain short carbon fibers and whose matrix has a mass fraction of at least 40% of silicon carbide. 
   The linings comprising fiber-reinforced ceramic materials are preferably affixed to a carrier made of metal or fiber-reinforced ceramic. Fastening can be effected by adhesive bonding, screwing, riveting or by means of other suitable joining techniques. 
   The invention therefore also provides clutch disks which comprise a carrier made of metal or a fiber-reinforced ceramic material and are provided with linings comprising fiber-reinforced ceramic materials which comprise short carbon fibers and whose matrix has a mass fraction of at least 40% of silicon carbide. 

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The matrix of the fiber-reinforced ceramic materials preferably comprises from 60% to 90% of silicon carbide, from 0% to 30% of elemental silicon and from 0% to 10% of elemental carbon. The mass fraction of the short carbon fibers in the ceramic material is preferably from 20% to 60%, particularly preferably from 25% to 50% and in particular from 30% to 35%. 
   The weight average length of the short carbon fibers is preferably from 0.001 mm to 30 mm, particularly preferably from 0.1 mm to 15 mm and in particular from 0.2 mm to 5 mm. Their diameter is from 4 μm to 12 μm, preferably from 5 μm to 10 μm and in particular from 6 μm to 8 μm. 
   The linings comprising the fiber-reinforced ceramic material preferably have a thickness of from 2 mm to 6 mm and have the shape of rounded rectangles, rounded trapezoids or ring segments, with the dimension in the direction of the clutch disk radius preferably being from 10 mm to 30 mm and that in the circumferential direction preferably being from 10 mm to 90 mm. Preference is given to at least two, more preferably at least three and particularly preferably at least four, of these linings being arranged symmetrically around the circumference of the clutch disk. 
   Particular preference is given to choosing the length of the fibers so that the average of the projection of their length onto the normal to the largest area of the clutch linings is from 10% to 95%, preferably from 20% to 90% and particularly preferably from 30% to 85%, of the thickness of the linings. 
   The materials for the linings can be produced by a multistage process in which
         in the first step, short carbon fibers are mixed with carbonizable materials which soften upon heating (hereinafter referred to as “binders”), these mixtures are   in the second step, pressed with application of pressure and heat to form bodies whose dimensions correspond to the desired linings,   the pressed bodies are, in the third step, converted into porous carbon bodies interspersed by short carbon fibers with retention of their shape by heating in the absence of oxidizing agents, and   in the fourth step, bringing these carbon bodies into contact with a silicon-containing melt under reduced pressure, so that the melt penetrates into the porous body and fills its pores, with at least part of the carbon reacting with the silicon to form silicon carbide.       

   According to the invention, it is also possible to choose the amount of melt in the fourth step so that the pores are not completely filled with the silicon-containing melt. The remaining accessible pores can then be filled, in a preferred embodiment, with a further melt which has a melting point of at least 550° C., preferably at least 600° C. Apart from metals which meet this condition, e.g. aluminum, iron, chromium, nickel, copper, molybdenum, tungsten, vanadium, niobium, tantalum, titanium and zirconium, preference is also given here to, in particular, alloys such as brass and bronze. The mass fraction of these metals in the matrix is preferably from 2% to 20%. 
   If mounting holes are necessary for attaching the linings to the clutch disk, they can advantageously be introduced into the porous carbon bodies obtained in the third step. 
   A preferred production variant for the materials according to the invention utilizes a continuous process in which the mixture of the short fibers and the binder is produced in a mixer and is introduced into a continuous screw extruder and extruded through a die which corresponds to the shape of the desired linings. The extrudate is then cut up, and the blanks obtained are carbonized and treated with the silicon-containing melt as described above. As a result of this method of manufacture, the preferential orientation of the short carbon fibers is perpendicular to the friction surface of the linings, and this has been found to have a particularly favorable effect on the level of the static friction.