Patent Application
20130284548
Field of the Invention
The invention relates to carbon ceramic friction disks, and a process for their preparation.
A friction disk such as a brake disk has several main tasks: it must provide sufficient torsional strength, stiffness and stability to be able to withstand, for example, the torque generated by decelerating a moving vehicle, it must provide an adequate friction coupling with a brake pad by adequate choice of materials which lead to a coefficient of friction of preferably between 0.3 and 0.7, and it must be able to limit the increase in temperature generated by the dissipation of rotational energy as heat.
Brake disks have been described in the literature that solve these tasks by adapting the geometry, for instance by introducing ventilation ducts into grey cast iron brake disks to provide air cooling and thus limiting the operating temperature. The remaining solid top and bottom layers provide the torsional stability and the friction surfaces. Carbon-fiber reinforced carbon disks have been in use in civil and military aircraft, as well as in Formula I racing cars. Also in this type of brake disk, one single material had to be adapted by different geometries to fulfill all tasks. This material had the advantage that the unsprung masses in the racing cars were kept low, due to the low density of the carbon material. Carbon fiber reinforcement accounted for the needed strength and stiffness. But carbon suffers from oxydative degradation at temperatures in excess of 400° C. Brake disks made of carbon-fiber reinforced silicon carbide are stable up to much higher temperatures. Designs including separate friction layers and carrier bodies having high mechanical strength have been described, i.e. in published, non-prosecuted German patent application DE 44 38 456 A1, corresponding to U.S. Pat. No. 6,042,935. Such carrier bodies can also be equipped with hollow spaces which allow to dissipate heat, see German patent DE 44 38 455 C1, corresponding to U.S. Pat. No. 6,086,814.
It was an object of the invention to provide a carbon ceramic brake disk that is optimized to meet all requirements by appropriate selection of the optimum material for each of these tasks, that can be built from standardized components which can be manufactured by processes that can easily be upscaled, and that has also an optimum of force transmission and traction as well as heat transfer between the regions of different materials which make up the brake disk. It has been found that a compound body is able to meet all these requirements, which compound body contains at least one part which provides the needed friction properties, at least one other part which provides the needed torsional strength and stiffness, and at least one further part which provides the needed cooling behavior. It has further been found that a technically reasonable solution is to build the brake disk in separate layers which account for the needed properties.
An object of the invention is therefore to provide a multi-layered carbon ceramic brake disk having at least one carrier body, and at least one ventilation layer that contains ventilation ducts, and preferably also at least one dedicated friction layer. The brake disk is made by joining green bodies of at least one individual carrier body, and of at least one individual ventilation layer, and preferably, of at least one individual friction layer. The green bodies contain thermoplastic or thermoset polymeric materials, in their solid or cured states, and by subsequent carbonization and ceramicization by infiltration with carbide-forming elements. For use as a brake disk in vehicle applications such as for motor-cars, trucks, and trains, the multi-layered carbon ceramic brake disk of this invention has a symmetrical structure containing a friction layer, a carrier body, a layer containing the ventilation ducts, a second carrier body, and a second friction layer. In a further configuration the brake disk has a symmetrical structure containing a friction layer, a layer containing ventilation ducts, a carrier body, a further layer containing ventilation ducts, and a second friction layer.
It is a further object of the invention to provide a process for the preparation of a multi-layered carbon ceramic friction disk which process includes preparation of a green body for a carrier body by stacking at least two layers of a mixture having a matrix polymer material and reinforcing fibers. Preparation of a green body for a ventilation layer by press molding a mixture having a thermoplastic or thermoset polymeric material, together with cores having substantially the form of the ventilation ducts to be formed, or by injection molding a thermoplastic or thermoset polymeric material into a mold having substantially the form of the ribs, fins or stubs enclosing the ventilation ducts. Optionally, there is a preparation of a green body for a friction layer by injection molding or press molding a mixture containing a thermoplastic or thermoset polymeric material and at least one of fillers and additives which influence the tribological behavior, or by tape or slip casting where a suspension of ceramic particles, preferably silicon carbide, and optionally, particulate carbon, and further optionally, at least one of fillers and additives which influence the tribological behavior, further optionally in the presence of a binder such as a phenolic resin is cast onto a metal belt, spread with a doctor blade, and solidified by drying to form a green tape which is punched according to the needed size of the friction layer. There is further optionally, an application to the interfaces formed in the stack between the green bodies, an adhesive containing at least one of a solution, a paste, and a particulate or powdery solid matter, to improve the bonding between these layers. The green bodies of the at least one carrier body, of the at least one ventilation layer containing ventilation ducts, and optionally, of the at least one friction layer, are stacked to form a stack. Optionally, the stack is subjected to a pressure and thermal treatment, to improve the bonding between these layers. The stack is subjected to pyrolysis in a non-oxidizing atmosphere under heat, to form a carbonized body, and infiltrated with a liquid carbide-forming material, which material preferably contains silicon, to form a ceramic body having a matrix containing a carbide, preferably silicon carbide.
It is a still further object of the invention to provide a process for the preparation of a green body for a friction layer by preparing and injection molding or press molding a mixture containing a thermoplastic or thermoset polymer material, and optionally, fillers and additives which modify the tribological behavior.
A still further object of the invention is to provide a process for the preparation of a green body for a friction layer by slip or tape casting where a suspension of ceramic particles, preferably silicon carbide, and optionally, particulate carbon, and further optionally, at least one of fillers and additives which influence the tribological behavior, further optionally in the presence of a binder such as a phenolic resin is cast onto a metal belt, spread to a predetermined thickness which is even over the usable width, with a doctor blade or a similar measures, and solidified by drying to form a green tape which is punched according to the needed size of the friction layer.
It is a still further object of the invention to provide a process for the preparation of a green body for a carrier body by preparing and pressing a mixture containing a resinous binder, and reinforcing fibers. This green body, and consequently also the carrier body made therefrom, has the shape of a cylinder ring disk having flat and level top and bottom surfaces.
It is a still further object of the invention to provide a process for the preparation of a green body for a ventilation layer by preparing and injection molding or press molding a mixture containing a thermoplastic or thermoset polymeric material and optionally, fillers and additives which modify the strength and stiffness and/or the thermal transfer properties of the resulting body, in a structured mold, or in a cylindrical mold together with cores, the mold structure or the cores having substantially the form of the ventilation ducts to be formed.
The green body for the carrier body is preferably a fiber-reinforced polymer composite material, wherein the fibers must provide adequate stiffness, particularly torsional stiffness which is measured by the torsional modulus, adequate strength and stiffness, particularly torsional strength, and the needed thermal stability. This means that the fibers must be able to withstand the operating temperatures of the brake disks without significant loss in the aforementioned stiffness and strength. The matrix polymer material serves to bind the fibers during the assembling steps, and is then transformed to the final ceramic matrix material by carbonization, and finally, formation of a carbide ceramic material by infiltration with at least one carbide forming element, and subsequent reaction to form the carbide. During carbonization which is a pyrolysis in the absence of air or other oxydizing agents, a porous carbon material is formed from the matrix polymer material which may be either a thermoplastic or a thermoset material, optionally in mixture with fillers and/or additives.
Preferred thermoplastic materials are predominantly aromatic polymers, i.e. polymers that have a mass fraction of aromatic moieties of at least 50%, preferably at least 60%, and particularly preferred, at least 70%. This mass fraction is calculated from the mass of aromatic residues, e. g., phenyl C6H5—, phenylene, —C6H4—, diphenylene —C6H4—C6H4—, naphthylene —C10DH6—, in a polymer such as polyethersulphone —C6H4—SO2—C6H4—O— or aromatic polyester —OOC—C6H4—COO—C6H4—C(CH3)2—C6H4-, or polyphenylene sulphide —C6H4—S—. Other useful materials are polyetherketones, polysulphone, polyphenylene sulphone, and polyetherimide.
Preferred thermoset materials are phenolic resins obtained by addition of formaldehyde to phenol or substituted phenols, and condensation of these addition products, epoxy resins derived from bisphenol A and/or bisphenol F, and furane resins.
Among the additives used, most preferred is pitch, made from distillation residues of crude oil or coal, preferably having a softening temperature of at least 100° C. (DIN 51 920), and a coke yield, measured in accordance with DIN 51905, of at least 80%. Useful fillers are preferably selected from the group consisting of particulate carbon preferably in the form of ground coke, graphite powder, carbon short fibers having an average length of not more than 5 mm, carbon microspheres, powders of carbide forming metals such as silicon, titanium, vanadium, or chromium, and other metals of the groups of the latter three, and powdery non-oxide ceramics such as silicon carbide, silicon nitride, or boron carbide.
The reinforcing fibers are preferably fibers able to withstand high temperatures of more than 500° C., more preferably of at least 800° C., which are preferably selected from the group consisting of carbon fibers, silicon carbide fibers, silicon nitride fibers, boron fibers, boron nitride fibers, boron carbide fibers, aluminum oxide fibers, and zirconium oxide fibers which are stabilized by addition of yttrium oxide to avoid conversion to the monoclinic phase upon cooling.
The reinforcing fibers for the carrier body are preferably used in the form of prepregs, viz., the so-called UD-tapes, which contain filaments in parallel alignment bound by impregnation with the thermoplastic or thermoset material as detailed supra, or in the form of non-woven or woven fiber mats which are also impregnated with the thermoplastic or thermoset material as detailed supra. It is also possible to use filament bundles that are laid in rotationally symmetric forms, such as a series of concentric circles fixed by filament bundles in radial orientation. Such reinforcing elements are commonly referred to as “tailored fiber placement”, and described in European patent EP 1 339 534 B1, corresponding to U.S. Pat. No. 7,942,993.
A preferred method to form the carrier body is to place at least two layers of impregnated UD tapes or fiber mats, woven or non-woven, on top of each other, and choosing the orientation angle so that a symmetrical and homogeneous orientation is achieved. In the case of UD tapes, two such layers are oriented in 0° and 90° with respect to each other, in the case of three UD tape layers, the orientation angles are 0°, 120°, and 240°, and for four layers, 0°, 45°, and 90°, and so forth. In the case of a woven cloth in linen weave, two layers are oriented at 0° and 45°, three layers are oriented at 0°, 30°, and 60°, and so forth. Using impregnated UD tapes or impregnated fiber cloth is preferred because the needed ring-shaped parts may simply be punched out of an impregnated tape or cloth, and then stacked to the needed height. Such process is easily automated.
The green body for the friction layer is preferably made by mixing a thermoset resin, particularly preferred, a phenolic resin or a mixture of a phenolic resin and a pitch, with additives preferably selected from the group consisting of particulate carbon preferably in the form of ground coke, graphite powder, carbon short fibers having an average length of not more than 5 mm, milled carbon fibers having lengths preferably in the range of from 0.1 mm to 2 mm, carbon microspheres, powders of carbide forming metals such as silicon, titanium, vanadium, or chromium, and other metals of the groups of the latter three, and powdery non-oxide ceramics such as silicon carbide, silicon nitride, or boron carbide. It is also possible to use a thermo-plastic resin together with the additives mentioned supra. Homogenizing is in this case preferably made with a mixing extruder, such as a twin screw extruder, which allows the fastest and most homogeneous mixing combined with a minimum of entrapped air. Pelletizing the solidified mixture allows simple and reproducible metering. A Z-arm kneader may be used for mixing both thermoplastic and thermo-set materials. The homogenized mixture is then pressed to the form of a cylinder ring and cured by heating if a thermoset, or pressed at elevated temperature and cooled in the mold if a thermoplastic material is used as matrix. An elegant method to form the green bodies for the friction layer is injection molding. In this case, the mold has to be configured in a way that joint lines are avoided as far as possible, e.g. by a circular gate at the inner circumference of the cylinder ring. Another preferred method to form the green bodies for the friction layer is slip casting or tape casting where a suspension of ceramic particles, preferably silicon carbide, and optionally, particulate carbon, and further optionally, at least one of fillers and additives which influence the tribological behavior, further optionally in the presence of a binder such as a phenolic resin is cast onto a metal belt, spread with a doctor blade, and solidified by drying to form a green tape which is punched according to the needed size of the friction layer. It is preferred in this context to use either particulate carbon, preferably ground coke or graphite flakes, in a mass fraction of at least 20%, based on the sum of masses of the solid constituents in the slip, or a resinous binder such as phenolic resins, epoxy resins or furane resins having a high yield of carbon upon carbonization is present in the slip. Other fillers and additives such as those mentioned supra may, of course, also be present. The liquid used for suspending the particles may be water, or an alcohol such as ethyl alcohol.
The green body for the ventilation layer contains a layer that has cavities and/or indentations that form the cooling channels in the friction disk. The ventilation layer is usually the only layer of those used in the present invention having hollow spaces or indentations, all other layers being massive and without hollow spaces, not counting, of course, the central hole of the ring-disk shape, and the small pores which are created mainly during the carbonization step, and remain unfilled during the infiltration with carbide-forming elements. Preferably, the green body for the ventilation layer contains a base plate which has ribs, fins or stubs on one side, or on both sides of the base plate. The space enclosed between the ribs or fins or stubs forms the cooling channel or cooling duct or ventilation duct in the multi-layered brake disk. The base material used to manufacture the green body for the ventilation layer is preferably also a thermoplastic or thermoset material, preferably also a predominantly aromatic polymer as defined supra. Among the thermosets, also phenolic resins, furane resins, and epoxy resins are preferred. The polymers may contain additives and fillers as described supra. It is also possible to use short carbon fibers up to an average length of 5 mm for reinforcement. Longer fibers are typically not used for the green bodies for the ventilation layers because of the preferred mode of injection molding to form these green bodies. The green body for the ventilation layer is preferably made by injection molding, or by press molding, both processes allowing too realize a wide range of geometries for the cooling ducts. Most preferred is injection molding. A circular gate in the mold is preferred, as in the case of the green body for the friction layer, to avoid the formation of joint lines. In a preferred embodiment, the molded green body for the ventilation layer has a circular rim on the outer and inner circumferences, preferably to both sides in the direction of the axis of rotational symmetry, which allow to geometrically fix the further layers, the green body for the friction layer, and the green body for the carrier body so that symmetrical adjustment is facilitated. These rims form a part of a cylinder jacket at the inner and outer circumferences. It is also possible, in a further embodiment, to build a green body for the ventilation layer from two parts having open recesses and channels on one side, and a base plate on the other side. These two parts have mirror symmetry, and preferably have protrusions and indentations in corresponding locations so that they can be easily fixed upon each other, with the base plates turned outwards in both parts. This is explained in more detail in
The multi-layered green body for the brake disk is then assembled, in a first embodiment, by stacking a green body for the friction layer, a green body for the carrier body which contains at least two layers of impregnated UD tapes or fiber mats, woven or non-woven, on top of each other, and choosing the orientation angle so that a symmetrical and homogeneous orientation is achieved, a green body for the ventilation layer, a further green body for the carrier body, and a further green body for the friction layer, where, in a preferred embodiment, an adhesive which is preferably a phenolic resins which may also contain powdery silicon carbide or other powdery ceramic fillers or powdery carbon or graphite, is applied between the individual layers. The stack is then pressed and heated to crosslink the adhesive, the subjected to carbonization under exclusion of oxydants at a temperature of preferably from 750° C. to 1300° C. to form a composite body of porous carbon also comprising reinforcing fibers and fillers. The composite body may be machined to remove at least those parts of the circular rim of the ventilation layer which close the cooling ducts, and is then finally subjected to infiltration with silicon or a mixture containing a mass fraction of at least 50% of silicon, and to formation of silicon carbide, and optionally, carbides of other carbide-forming elements present in the mixture with silicon, at a temperature of at least 1420° C., and preferably, under a reduced pressure of between 0.5 hPa and 10 hPa.
Depending on the brake load, other sequences in the stack to form the composite body of porous carbon are preferred, such as for extreme high duty brakes, a sequence of a friction layer, a first carrier body, a first ventilation layer, a second carrier body, a second ventilation layer, a third carrier body, and a final friction layer. The strength of the carrier body can be easily adapted to the load by choosing the number of fibrous reinforcement layers, which is preferably from two to ten. As discussed supra, the individual layers are oriented at different angles to achieve a homogeneous load distribution.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in carbon ceramic friction disks and process for their preparation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawings in detail and first, particularly to
By appropriate choice of the sequence and thickness of the individual layers, it is easily possible to adapt the brake disk to the intended purpose. Usually, if present, the friction layers have a thickness of from 1 mm to 5 mm, the ventilation layers have a thickness (which is approximately equivalent to the height of the cooling channels or cooling ducts) of from 5 mm to 20 mm, and the carrier bodies have a thickness of from 3 mm to 20 mm. Particularly in the case where carbon fiber cloth is used as reinforcing element in the carrier body, a construction as explained in
A green body for a friction layer was prepared as now described.
7.5 kg of silicon carbide powder having an average particle diameter of 40 μm were mixed with 2.5 kg of a phenol resol resin (CELLOBOND® 1203, Momentive Specialty Chemicals Inc.). This mixture was press molded to form flat cylinder ring disks having a thickness of 3 mm, an outer circumference of 400 mm and an inner circumference of 200 mm and cured at 180° C.
A green body for a ventilation layer was injection molded from a mixture of an aromatic polyester resin (bisphenol A-isophthalate-terephthalate copolymer) and a mass fraction of 25%, based on the mass of the mixture, of a powdery pitch of reduced smoking propensity having a softening temperature of 235° C., using a mold according to
For the carrier body, a woven carbon fiber tape made of 3 k filament bundles impregnated with phenolic resin (CELLOBOND® 1203) was punched to circular rings having an inner diameter of 200 mm and an outer diameter of 400 mm.
A stack of ten of these rings with a circular displacement of 36° each with regard to the predecessor ring were fixed to each side of the green body for the ventilation layer inside the rims thereof, both sides of the resulting stacks were covered with a green body for the friction layer, the assembly was put into a press mold and pressed at 180° C. with 0.5 MPa for one hour. After cooling to room temperature, the multi-layer green body was subjected to carbonization at 900° C., and was the subjected to infiltration with liquid silicon at 1680° C. After cooling, the inner and outer rims were removed by grinding, the friction layers were drilled to form perforation holes, and the surface of the friction layer was then polished.
The brake disk thus produced showed very good rotational stability (in excess of 5000 min−1).
| Number | Date | Country | Kind |
|---|---|---|---|
| 1019447.5 | Dec 2010 | EP | regional |
This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2011/074332, filed Dec. 30, 2011, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of European patent application No. 10197447.5, filed Dec. 30, 2010; the prior applications are herewith incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2011/074332 | Dec 2011 | US |
| Child | 13932454 | US |
