Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
2002-07-08
2004-07-06
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S699000, C428S110000, C428S698000, C428S697000, C428S368000, C428S642000, C428S625000, C264S241000, C264S044000, C264S645000, C264S625000, C264S640000, C264S642000, C264S046400, C156S060000, C156S182000, C156S285000
Reexamination Certificate
active
06759117
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a multilayer ceramic composite containing at least one composite material forming a supporting zone that has oxidation-sensitive reinforcing fibers, and at least one ceramic surface layer.
Carbon-containing composite materials, are used for example for brake applications, and are known in particular from the aviation industry and motor racing sector. They offer the advantage of good tribological properties right up to very high loads and temperatures.
Materials formed of carbon fiber-reinforced carbon materials (CFC or C/C) are used widely in this connection. Such materials contain carbon fibers in the form of mats, woven fabrics or other types of two-dimensional fiber structures that are stacked on top of one another and to form three-dimensional bodies and are then post-compacted with carbon. The post-compaction may be carried out by repeated impregnation with so-called carbon precursor materials (substances that decompose under pyrolysis conditions to form carbon) such as pitches or resins, and their subsequent pyrolysis to form carbon, or by deposition of so-called pyrocarbon from the gaseous phase (CVI, chemical vapor infiltration).
Composite materials for tribological applications and in particular brake discs may be subdivided according to their structure into two zones having different requirement profiles and in most cases also different material properties and composition.
The outerlying zone exposed to wear and co-operating tribologically with a second body (e.g. the brake lining) is the friction surface, which is characterized by special friction and wear properties. The underlying material is the supporting zone (core body), whose task is essentially only to absorb the frictional forces and transmit them to the securement devices, as well as absorb and dissipate the frictional or braking energy.
Composite materials with a ceramics matrix have recently been developed on an increasing scale. In this connection, materials of particular interest are those that are built up from carbon fibers and a matrix of silicon carbide-containing (SiC) or Si/SiC-containing (additional silicon phases) matrices. These materials include in particular the so-called C/SiC materials, which are composed of carbon fibers or carbon-containing fibers and a matrix of carbon (C), Si and SiC. Such composite materials are known, inter alia, from Published, Non-Prosecuted German Patent Application DE 198 56 721, German Patent DE 197 11 829 C1 and Published, Non-Prosecuted German Patent Application DE 197 10 105 A1.
For the material properties—in particular the strength and rigidity—it is very important and normal practice to protect the reinforcing fibers by coating them with carbon or carbon-containing compounds.
A common feature of all the specified materials is that they contain carbon in fiber form or in the matrix, and that the materials are heated under use conditions to temperatures at which a noticeable oxidation of the carbon takes place during prolonged use. The oxidation is assisted by the fact that the listed materials, in general in the original state—i.e. without post-treatment and/or additional protective measures for fibers or matrix phases containing elementary carbon have a not insignificant open porosity. On account of oxidation and the loss of carbon caused thereby the structure is weakened and the strength is reduced. In the case of brake discs, the weakening may be very deleterious especially as regards the functioning of the supporting zone since a material failure in the region of the device for securing the brakes or discs can lead to a total failure of the assembly.
The effect of this oxidative damage can be monitored for example by the weight loss of the composite materials during use.
For this reason composite materials for tribological applications and in particular high-performance brake discs with core bodies or supporting zones of carbon-containing material must include effective anti-oxidation mechanisms for the supporting zone. Since the physical properties, in particular the coefficient of thermal expansion, and the chemical properties of different CFC or different C/SiC or Si/SiC materials (silicon carbide-containing materials infiltrated with silicon) may differ greatly, the effectiveness of every such anti-oxidation system also varies greatly.
Various solutions have been proposed to provide antioxidation protection.
The application of anti-oxidation surface layers is one of the most commonly employed methods. Specifically in the case of CFC and C/SiC composites it is of particular importance for the anti-oxidation protection that these materials are permeated by a fine crack structure that can expand and close under alternating thermal loads. In addition, new cracks are also generated under mechanical stress.
Accordingly it is particularly those anti-oxidation protective layers that are self-healing that are of great interest. The mechanism of self-healing is based on the property that the protective layers melt at the application temperature and newly-formed cracks can reseal.
Such systems are described for example in European Patent EP 0 619 801 B1 and Published, European Patent Application EP 0 375 537 A1. The disadvantage with these systems is the fact that the protective layer formed as a glass layer has to act as a carrier of the self-healing properties in order to protect the supporting zone specifically on the surface layer, i.e. the outermost surface. On account of the softening and/or melting of this layer at the application temperature, the frictional behavior is seriously impaired.
Another possibility for crack sealing under the application conditions is to add high melting point elements, binary or multinary compounds, for example boron compounds, that oxidize at elevated temperatures under the admission of air at least partially to form oxides such as B
2
O
3
and/or low melting point glasses such as borate glasses. A double-ply cover layer system on a CFC body is described in U.S. Pat. No. 5,536,574. The CFC body is coated with a mixture of Si, SiC and Al
2
O
3
and then borated with a boron-containing mixture. A second boron-containing layer is then applied. In this case too the outermost layer softens at the application temperature and accordingly the functioning of the tribologically active surface cannot be guaranteed.
Furthermore, the B
2
O
3
glasses formed from the boron-containing compounds have turned out to be extremely harmful for the frictional properties of the cover layer.
As a further variant, the use of a boron-containing Si/SiC matrix has been proposed in U.S. Pat. No. 5,962,103. In this way cracks lying deep in the composite material can also be healed by the melting of the boron compounds formed during oxidation.
A disadvantage in use however is that the matrix properties, in particular the stiffness and strength, deteriorate markedly due to the formation of low melting point B
2
O
3
glasses in the event of oxidation and under high temperature conditions.
Those reinforcing fibers and/or matrix materials that are oxidatively degraded at elevated temperature (i.e. at temperatures that may arise during use and that are above approximately 400° C.) are termed oxidation sensitive and may therefore lead to a weakening of the composite material. In particular carbon as a constituent of the matrix or in fiber form is oxidation sensitive, in which connection the oxidation sensitivity of the latter can be reduced in a known manner (in particular according to the teaching of Published, Non-Prosecuted German Patent Application DE 197 10 105, the relevant contents of which are incorporated by reference in the disclosure) by suitably coating the fibers.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a multilayer ceramic composite and a process for forming the composite that overcome the above-mentioned disadvantages of the prior art compositions and methods of this general type.
With the foregoing and other objects in vi
Bauer Moritz
Christ Martin
Gruber Udo
Heine Michael
Kienzle Andreas
Dixon Merrick
Greenberg Laurence A.
Mayback Gregory L.
SGL Carbon AG
Stemer Werner H.
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