DE9422477U1 - Friction unit - Google Patents

Abstract

Friction unit is in the form of a porous ceramicized carbon body infiltrated with silicon by initiating a chemical reaction forming silicon carbide. Before being infiltrated with the liquid silicon, the carbon body is structured so that hollow chambers and/or recesses for cooling and/or reinforcing are formed in defined inner regions which maintain their shape and size after ceramicizing and which form hollow chambers on the finished friction unit. A core is inserted into the carbon body and/or green body for structuring, the core corresponding to the recesses and/or hollow chambers.

Description

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German Center for
Aerospace eV
Linder Height
51147 Cologne

30.05.2003

Utility model application

"Friction unit"

The present invention relates to a friction unit, in particular a brake or clutch body, for frictional engagement with a counter body, in the form of a porous carbon body infiltrated with liquid silicon and ceramized by initiating a chemical reaction to form silicon carbide.

A method for producing a friction unit was presented by a working group of the DLR (German Aerospace Research Institute), Stuttgart, Institute for Design and Construction Research, at the VDI Materials Day 1994 in Duisburg on March 9th/10th, 1994, which was held under the theme of "Lightweight Structures and Lightweight Components" as part of the lecture "Development of integral lightweight structures made of fiber ceramics". As part of this lecture, a technology for producing carbon fiber-reinforced carbons was presented. The carbon fiber-reinforced carbons are infiltrated with liquid silicon using a so-called "liquid siliconization process" and subjected to heat treatment, whereby the silicon is converted with carbon to form SiC. One possible area of application for these C/C-SiC materials is brake disks.

Brakes, especially in motor vehicles and aircraft, are subject to increasingly higher demands. The speeds achieved by such vehicles today are constantly increasing. When braking, this kinetic energy is converted into heat by friction, which is dissipated by the brake disc and the brake pads.

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is absorbed. Such a brake arrangement is therefore limited by the friction characteristics of the brake material and its ability to store and dissipate heat. In general, brake materials must have very good thermomechanical properties, high and constant friction characteristics and good abrasion resistance. Brakes made of carbon-reinforced carbon materials (C/C) developed in recent years, as described for example in DE-A1 30 24 200, allow temperatures of up to 1000 ⁰ C.

As mentioned above, a C/C-SiC material was presented at the VDI conference mentioned above, which shows clear advantages over a C/C material, especially with regard to thermal shock resistance, oxidation resistance, moisture absorption and friction behavior.

Based on the prior art described above, the present invention is based on the object of developing a friction unit of the type described at the outset in such a way that it can withstand increased thermal stress and which can also be produced in a simple manner.

The above object is achieved by a friction unit, in particular a brake or clutch body, for frictional engagement with a counter body, in the form of a porous carbon body infiltrated with liquid silicon and ceramized by initiating a chemical reaction to form silicon carbide, which is characterized in that before the silicon infiltration the carbon body is structured in such a way that cavities and/or recesses for cooling and/or stiffening are formed in defined inner regions, which retain their shape and size after ceramization and form cavities in relation to the finished friction unit, wherein for structuring purposes at least one core is introduced into the carbon body and/or the green body during construction, the dimensions of which correspond to the recesses and/or cavities to be formed.

As is generally known, ceramic material is extremely difficult to machine, so that structures in such previously known friction units could only be created with increased effort. In this respect, such structures were only provided on the outer circumference of such components, for example in the form of radial bores.

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taken, for example, to give the friction units the required outer contour. With the friction unit according to the invention, complicated structures are also possible, since recesses are formed in a state of the carbon body in which there is still no ceramization, which takes place by infiltration of silicon with subsequent heat treatment. In addition, it is also possible to create closed or only partially enclosed cavities within the friction unit at precisely defined locations. Such introduction of cavities and/or recesses in the body takes place practically in a green state of the carbon body in which it is still easy to machine mechanically or in a state in which the carbon fiber body itself is built up and structured. With this procedure, inexpensive production of friction units in series production is possible from a cost perspective. Such structures using recesses and/or cavities, which can be used to stiffen a friction unit, such as a brake disk, but are particularly used to create cavities for cooling, are particularly important for such ceramicized friction units, since such ceramicized friction units are used under very high loads and thus high temperatures. Areas of application include, for example, brake units in aircraft, rail vehicles and high-performance motor vehicles.

A particularly advantageous procedure is when a green body is first formed as a preliminary product of the porous carbon body, which is a carbon fiber body permeated with hardened polymers. Such a procedure, with the formation of a preliminary body made of carbon fibers coated with polymers, brings particular advantages with regard to the subsequent silicon infiltration technology. A body in the form of a carbon fiber matrix with hardened polymers represents a preliminary product that is already sufficiently hardened and solidified to be able to be mechanically processed in a defined manner. The recesses and cavities can also be introduced at a stage in the production of such friction units in which the preliminary product is already available, which can be easily designed both mechanically and in terms of shaping.

In order to easily form closed cavities or partially enclosed cavities within the friction unit, it is possible to insert a

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a ceramization, preferably in a state before its infiltration with silicon, and furthermore preferably within the framework of the green body, which is provided as a carbon fiber matrix permeated with polymers, to introduce one or more cores whose dimensions correspond to the recesses and/or cavities to be formed. Rubber, plastic, metal, ceramic or carbon are suitable materials for the cores, the materials being selected according to whether such a core is a lost core or a core that is removed at a certain construction stage of the carbon body. For example, a core made of styrofoam or rigid foam plastic has the advantage that it can be removed from the friction unit by means of a solvent, for example via a ventilation opening, before the silicon infiltration. A core made of metal, ceramic or carbon is preferably used to form recesses that are formed on an outer surface of the friction unit. In the area of this recess, the cores are inserted to shape the recess and are pulled out in a later process step when the body is no longer subject to significant deformation. Such a core made of metal, carbon or ceramic can also be used to be embedded in the fiber matrix when building the carbon fiber matrix, from which it is then removed before the recess is completely closed off, for example by a cover layer, in the construction of the carbon fiber body. After silicon infiltration, such cores that are accessible from the outside can be removed, for example from radially outward-running channels that have an approximately uniform cross-section from the inside to the outside or a slight cross-sectional expansion from a radially inner side to a radially outer side of the friction unit. Such cores are preferably made of carbon or ceramic, as they can withstand the temperatures in the individual process steps for producing the friction unit. A core made of rubber is also suitable for being pulled out of a pre-body after it has been built up; Such removal of rubber cores must take place in a state in which the rubber material of the core is not yet subjected to high temperature stress.

Furthermore, it is preferred to use cores made of a pyrolyzable material, preferably cores made of polyvinyl alcohol, in order to create cavities in the interior of the friction elements.

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unit which are not accessible later and may not have any ventilation or vent openings to the outside of the friction unit. Pyrolizable material is pyrolyzed during the pyrolysis of the carbon body, in particular a carbon body in the form of a carbon matrix with a polymeric interpenetration, and is essentially removed without leaving any residue.

In order to provide a friction unit with increased mechanical strength values, particularly in the area of the cavities or recesses, the recesses and cavities are surrounded with layers or layers of carbon fibers with a predetermined orientation when the carbon fiber-reinforced carbon body is constructed. The orientation of the fiber can be adapted to the forces and stresses occurring in the friction unit. These fiber layers therefore delimit such recesses or cavities in the friction unit. For cavities that are to be formed inside the friction unit and in which the friction unit is to have increased strength, cores are first surrounded with such fiber layers and these cores are then inserted into the carbon fiber matrix. Such cores can in turn be lost cores or cores that are removed in a predetermined process step in the construction of the friction unit.

In order to easily form cavities inside a friction unit, particularly cavities that have no connection to the outside of the friction unit, the friction unit is constructed from two individual bodies that have recesses on one outside. The two bodies are then connected to one another so that the recesses in one body are covered and closed by the other body. It is clear that such recesses can be formed in both bodies, which then complement each other in the connected state to form a common cavity; it is also possible to form the recesses only in one body and to design the other body as a cover plate. Depending on the structure of the friction unit, both bodies can be identical parts that are then connected to one another. In addition, a multi-part structure of the friction unit not only makes it possible to easily form cavities inside the friction unit, but also makes it possible to adapt the friction unit, for example in the form of a brake or clutch disc, to the requirements required for use.

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Properties, i.e. one body is designed as a friction body, while the other body can serve as a core body. The friction body carries the friction surface for frictional engagement with a corresponding counter body and is materially adapted in terms of its friction properties. The core body serves as a carrier body for the friction body and is attached to a hub of a vehicle, for example. Such a core body should then advantageously have the cooling channels and cavities and be adapted in terms of its material properties so that it has good thermal conductivity and heat storage capacity.

Further details and features of the invention emerge from the following description of embodiments with reference to the drawings. In the drawing:

Figure 1 shows a cross section through a preform of a carbon body from a

Carbon fibre matrix for a friction unit, for example a brake disc, with two recesses formed in an upper side,

Figure 2 shows a preform, similar to Figure 1, which is assembled in a forming unit

is,

Figure 3 shows a section through a friction unit with two inner, closed hollow

clear,

Figure 4 shows a pre-product in the form of a green body with two core-filled

Recesses and schematically indicated structure of the fiber direction of the carbon fiber matrix,

Figure 5 is a sectional view corresponding to Figure 4 with two enclosed,

cavities filled with a core,

Figures 6 and 7 show a sectional view of respective friction units which are made from two carbon bodies according to the preform of Figure 1 by means of a joining technique,

Figure 8 is a sectional view of a friction unit made of two bodies,

which are constructed according to the precursors of Figures 1 and 2, and

Figure 9 is a section through a friction unit made with a body corresponding to the preform of Figure 1 with an additional cover disk.

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Figures 1 to 3 show various precursors 1, 2 and 3 made from a carbon fiber matrix, which have an external shape that corresponds approximately to the finished friction unit to be produced. To produce such precursors 1, 2 and 3, carbon fibers of different lengths are preferably layered on a base (not shown) or in a mold 4 as shown in Figure 2, optionally with woven carbon fiber inserts, so that the respective precursors 1, 2 and 3 are constructed. The use of a mold 4 according to Figure 2 has the advantage that the precursor 2 can be given a reproducible external contour in a simple manner, which corresponds approximately to the external contour of the ceramized friction unit, apart from dimensional changes due to slight shrinkage during ceramization. The carbon fibers used are preferably those that are coated with a polymer layer. Alternatively, a body constructed from carbon fibers can be subsequently infiltrated with polymers and then such a preform can be cured so that a polymer matrix is achieved in the carbon matrix and at the same time the entire preform 1, 2 or 3 is given dimensional stability.

As part of the construction of the preliminary product 1, 2 or 3, cores 7 are inserted in order to form recesses 5 or enclosed cavities 6, the shape and size of which correspond to the recess 5 to be formed or the cavity 6. In Figures 1 and 2, for example, a core 7 is shown which is inserted into the carbon fiber body and which is removed from the preliminary product after solidification, so that this recess 5 remains, as shown in the right half of the sectional view in Figures 1 and 2. Depending on the shape of the core, such a recess can have a round, oval or square cross-section or can be rotationally symmetrical to the axis 8 of the respective pre-product 1, 2, so that the two recesses of the pre-products of Figures 1 and 2 each form a space which can, for example, be formed in the form of an evolute in accordance with the most favorable aerodynamic shape of the cooling channel or an evolute-shaped core is used as the core. The same applies to the cavities 6 of the pre-product of Figure 3.

Preferably, cores 7 for forming the recesses 5 are made of a material which can withstand the individual heat treatment stages for the production

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of the precursor, i.e. preferably cores made of metal, ceramic or carbon. Such cores can then be used as often as desired after they have been made from the precursor 1,

2 or 3 are removed, can be used to create additional friction units.

In order to form cavities 6, as shown in the preliminary product 3 in Figure 3, cores are also used when building the carbon fiber matrix, but these have to be removed from the preliminary product in some way. For this reason, cores made of polystyrene or rigid foam plastic are preferably inserted into these cavities, which are then dissolved out in a solidified stage of the preliminary product 3 using a solvent that is filled in, for example, via ventilation channels 9 that have a connection from the outside to the cavities 6. These ventilation channels can then be filled with pyrolyzable material that has a high carbon residue after pyrolysis and closed by subsequent ceramization. Another possibility of creating the cavities 6 by means of lost cores is to introduce a core made of a pyrolyzable material, which is essentially removed without residue in a subsequent process step in which the carbon fiber body is subjected to pyrolysis, for example in order to pyrolyze a corresponding polymer matrix. In this case, it is not necessary to provide special ventilation channels 9. A precursor, as shown in Figures 1 to

3, is then infiltrated with liquid silicon in a further process step, which is converted into silicon carbide under heat treatment, so that the finished, ceramized friction unit is obtained.

Another possibility for forming cavities 6 in a pre-product, as shown in Figure 3, is to insert rubber cores 7 in the area of the cavities. Such rubber cores are to be preferred if the cavities 6 are to have channels leading to the outside of the friction unit, corresponding to the ventilation channels 9, but with a larger cross-section, so that after completion of the pre-product 3, the elastomer core parts can be pulled out via these channels 9. In this way, internal cavities 6 can be formed which have undercuts opposite the channels 9, so that these elastomer cores can be pulled out via the channels 9 which have a smaller cross-section than the cavities.

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Figures 4 and 5 show sectional views of pre-products 10 and 11, which essentially correspond in their structure to pre-products 1 and 3 in Figures 1 and 3. In Figures 4 and 5, however, carbon fiber layers 12 are shown around the recess 5 or a core 7 inserted therein (Figure 4) or the cavity 6 and a core 7 inserted therein (Figure 5), following the contour of the recess or the cavity, which are oriented in such a way that they are adapted to the forces occurring in the subsequent friction unit and result in a particularly stable and solid structure of the carbon matrix. Such carbon fiber layers 12 can be placed around prefabricated cores 7, for example made of Styrofoam or rigid foam, as part of the construction of the carbon fiber matrix, and such prefabricated cores can then be inserted into the carbon fiber matrix. According to the procedure described above in the context of the embodiment of Figure 3, such cores can later be removed via corresponding ventilation channels not shown in Figure 5. The individual core bodies or the carbon fiber layers 12 wound around the core body can then be surrounded by carbon fiber cover layers 13. As the embodiments of Figures 4 and 5 make clear, the structures of the friction units that are to be created can be shaped and constructed as desired in a green state, and if necessary the preliminary product can be mechanically processed after hardening, but before silicon infiltration and ceramization; it is therefore not necessary to process a ceramic material.

Figures 6 and 9 show various embodiments of modular, integral friction units, which are constructed using pre-products as shown in Figures 1 and 2. Accordingly, the same reference numbers are used for the bodies 1 and 2 in Figures 6 and 9. The respective pre-products can be placed on top of one another along a connection point or connection plane, which are designated with the reference numbers in the individual Figures 6 and 9 with the reference number 14, and then connected to one another. Pre-products 1, 2 can be used in their green, i.e. not yet ceramicized, form as pre-products or as individual module parts for the friction units according to Figures 1 to 9; the connection point 14 is then filled as part of the silicon infiltration of the two pre-products 1, 2, thus creating a connection which is essentially carbide.

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contains, of the two bodies. Individual bodies 1, 2, which are each already infiltrated with silicon and ceramized, can, however, be joined together in this ceramized state, for example by means of a high-temperature-resistant brazing solder, preferably using silicon.

As Figure 6 shows, cavities 6 can be formed from two bodies 1 as shown in Figure 1, in that the individual recesses 5 of the pre-product 1 complement each other to form this cavity 6 through their identical alignment with the axis 8 or the axis of rotation of the friction unit (Figures 6 and 9). In such a structure, it is not necessary to remove any cores remaining in the structure of the carbon matrix from the cavities 6.

Figure 7 shows an embodiment in which the friction unit is made up of two pre-products, joined together along the connecting plane 14, in such a way that they are each aligned in the same direction, so that the original recesses in the pre-product 1 complement the outer surface of the other pre-product 1 to form the cavities 6, while the recesses 5 of the other pre-product 1 remain unsealed on the outside. The first pre-product 1 can, for example, serve as a friction body in this arrangement, with its smooth outer surface forming a friction surface 15, while the other pre-product 1 represents a core body. This division into a friction body and a core body aims to adapt these two bodies in terms of their material properties to the respective requirements, i.e. the friction body is equipped with good friction properties on the friction surface 15, while the core body should, on the one hand, have a high mechanical strength in order to form a supporting body of the friction unit, and, on the other hand, it should have good thermal conductivity and heat storage capacity in order to dissipate the heat generated on the friction surface 15.

The adaptation of the respective bodies which are connected to form a friction unit, as explained above with reference to Figure 7, also applies analogously to the arrangements of Figures 6, 8 and 9.

Figure 8 shows a friction unit which is composed of a pre-product 1 and a pre-product 2 according to Figures 1 and 2. In this arrangement

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recesses are formed on both outer sides, and an annular space 16 extending radially to the axis of rotation 8 is formed by the extension 17 of the body 2; with appropriate structuring, such an annular space 16 could easily have a smaller cross-section towards the outside, as shown in the upper half of Figure 8 with a broken line, for example by prefabricating a correspondingly profiled body.

Figure 9 shows a variant in which the body 1 of Figure 1 is covered on the side of the recesses 5 with a cover plate 18 so that corresponding cavities are created.

As the various embodiments show, the friction unit according to the invention enables the formation of recesses and cavities in a process stage in which the respective starting bodies are still easy to shape and work. In addition, with such a technology, particularly when a friction unit is composed of several individual bodies, especially in a not yet ceramicized state, very complex structures can also be built up by joining these individual bodies.

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Claims (15)

1. Friction unit, in particular a brake or clutch body, for frictional engagement with a counter body, in the form of a porous carbon body infiltrated with liquid silicon and ceramized by initiating a chemical reaction to form silicon carbide, characterized in that before the silicon infiltration the carbon body is structured in such a way that cavities and/or recesses for cooling and/or stiffening are formed in defined inner regions, which retain their shape and size after ceramization and form cavities in relation to the finished friction unit, wherein for structuring purposes at least one core is introduced into the carbon body and/or the green body during construction, the dimensions of which correspond to the recesses and/or cavities to be formed. 2. Friction unit according to claim 1, characterized in that the at least one core is removed before the silicon infiltration. 3. Friction unit according to claim 1 or 2, characterized in that the core is made of rubber, plastic, metal, ceramic or carbon. 4. Friction unit according to claim 3, characterized in that the core is made of polystyrene or rigid foam plastic. 5. Friction unit according to claim 4, characterized in that the core can be removed by means of a solvent. 6. Friction unit according to claim 1, characterized in that the core consists of pyrolyzable material. 7. Friction unit according to claim 6, characterized in that the core is formed from a material that can be removed without residue during pyrolysis. 8. Friction unit according to claim 6 or 7, characterized in that the core consists of polyvinyl alcohol. 9. Friction unit according to one of claims 1 to 8, characterized in that in the region of the recesses and/or cavities in the carbon body, carbon fiber layers with a predetermined orientation are introduced which delimit the recesses and/or cavities. 10. Friction unit according to claim 9, characterized in that the carbon fiber layers are built up on the core. 11. Friction unit according to one of claims 1 to 10, characterized in that the carbon body forms a first body and is connected to at least one second body, the second body forming part of the friction unit. 12. Friction unit according to claim 11, characterized in that the second body is a second carbon body produced according to one of claims 1 to 10. 13. Friction unit according to claim 11 or 12, characterized in that the first body is constructed as a friction body with a friction surface. 14. Friction unit according to claim 13, characterized in that the second body is designed as a core body with good thermal conductivity and/or heat storage capacity. 15. Friction unit according to one of claims 11 to 14, characterized in that recesses are made in the first and second bodies in the region of their connecting surfaces in such a way that they complement each other to form a common cavity.