Brakes – Elements – Shoes
Reexamination Certificate
2001-11-28
2003-12-23
Butler, Douglas C. (Department: 3683)
Brakes
Elements
Shoes
C188S2180XL, C188S073200, C192S10700R, C428S293400
Reexamination Certificate
active
06666310
ABSTRACT:
The present invention relates to a fibre-reinforced ceramic body and a method for producing the same, in particular a brake disc of fibre-reinforced ceramic material and a method for producing the same.
Fibre-reinforced ceramic bodies and methods for producing them are known. For example a brake disc is known from DE-A-44 45 226 that is produced from a carbon fibre-reinforced material in two halves as an internally ventilated brake disc with radial ventilation channels.
The two surfaces of the brake disc are formed as friction surfaces. Ribs are formed on the inside of at least one half that rest on the other half with the formation of free ventilation channels arranged therebetween. Both halves are rigidly joined to one another by a suitable joining method, such as for example high-temperature brazing or an adhesion method process. A pot-shaped flange that is either formed integrally with one half or is connected via bolts or other joining elements to the brake disc serves for the purposes of securing to the wheel.
Such brake discs may consist for example of a carbon fibre-reinforced carbon (CFC) and have been used for some years with specially developed friction linings in motor racing. The area of use of CFC materials is however limited on account of the susceptibility of the carbon fibres to oxidation starting at about 500° C. Moreover, further composite ceramic materials have become known, from which such brake discs or other ceramic bodies can be produced (see for example DE-A-197 11 831). In this connection long fibre-reinforced or short fibre-reinforced CFC materials are used as preliminary bodies, which are then melt infiltrated with liquid silicon. Reaction-bonded carbon fibre-reinforced SiC ceramics are thereby formed.
In general fibre-reinforced ceramic materials are used as high performance materials in machine construction and plant construction and in aeronautics and space technology. A good tolerance to damage and resistance to thermal shock and oxidation, in addition to a very high specific strength and rigidity, are desired in such applications. The aforementioned properties are restricted when using long fibres since an oxidative attack can penetrate into the interior of the structural part and thereby lead to destruction if the critical crack length is exceeded.
In the case of short fibre-reinforced ceramic bodies with reaction-bound fibres based on Si/C/B/N according to DE-A-197 11 831, a better oxidation stability can, in fact, be achieved even at high temperatures since oxidation of the short fibres that are present in isolated form due to the ceramic matrix damage the structural part only on the surface. However, such ceramic bodies in many cases do not exhibit a sufficient tolerance of damage such as is required for certain high performance applications, for example for brake discs.
The object of the present invention is accordingly to provide an improved fibre-reinforced ceramic body and a method for producing such a body that is characterised by a high tolerance of damage, high strength, and high temperature resistance. In this connection, it should be possible to produce a surface that is as gas-tight and/or liquid-tight as possible to ensure a good corrosion resistance.
This object is met by a fibre-reinforced ceramic body with a novel structure that consists of a core and a boundary layer that is joined to the core and has at least one outer surface that can preferably be subjected to tribological stress, with the core being composed of one or more layers, of which at least one is reinforced with long fibres, and wherein the boundary layer is reinforced with short fibres.
With regard to the above method, this object is achieved by a method in which a core is produced from at least one layer that is reinforced with long fibres and the core is joined to a boundary layer that is reinforced with short fibres and has at least one outer surface that can preferably be subjected to tribological stress.
The object of the invention is fully achieved in this way. According to the invention a high tolerance damage is in fact achieved by means of the core that is reinforced with long fibres, while on the other hand a high temperature resistance can be achieved by means of the boundary layer reinforced with short fibres since the said boundary layer can be made gas-tight and liquid-tight by a large proportion of ceramic matrix and by using short fibres, with the result that a good corrosion resistance can be achieved also at temperatures above 500° C.
In a further modification of the invention the core has a plurality of layers reinforced with long fibres (UD layers), whose long fibres are in each case arranged in a preferred direction, with the preferred directions of at least two UD layers being arranged angularly displaced relative to one another.
In this connection, layers with woven long fibres may additionally be provided, which are preferably arranged between adjacent UD layers in order to counteract delamination.
The strength properties of the ceramic structural part can be purposely influenced by means of the combination of a plurality of UD layers arranged at an angle relative to one another and optionally layers of woven long fibres (cross-ply layers), in order thereby to achieve a particularly high strength in the specific stress directions acting on the ceramic structural part and at the same time ensure a good tolerance of damage.
In this connection the short fibres are preferably arranged statistically distributed in the boundary layer.
In order to achieve such a homogeneous distribution of the short fibres, a granulation method may for example be employed.
The term long fibres within the scope of the present application is understood to mean fibres having a length of at least 50 mm, while the term short fibres is understood to mean fibres having a length of less than 50 mm.
Such fibres are used, as a rule, in the form of fibre bundles (so-called rovings), which generally consist of ca. 3,000 to 25,000 individual filaments. 12K rovings, which consist of about 12,000 individual filaments, are widely available commercially.
All fibres that have a sufficiently high strength even at high temperatures, as long as exclusion of air is ensured, are suitable as fibres for reinforcing the ceramic structural part. Such fibres are as a rule those with a covalent bond based on silicon, carbon, boron and/or nitrogen. SiC fibres, C fibres or SiBCN fibres may preferably be used as fibres.
In an advantageous development of the invention the fibres are reaction-bonded with a matrix that may, in particular, consist of silicon carbide. The use of such a matrix material permits an advantageous production and at the same time provides a good gas-tightness in the boundary layer, and, furthermore, problems that may arise due to shrinkage during production can be largely avoided.
Such a ceramic body may be designed and fabricated as a brake disc for an internally ventilated disc brake, and may comprise two halves joined to one another, each half having an outer surface formed as a friction surface and an inner surface, wherein at least one half has on the inner surface ribs that rest on the other half, and wherein on the inner surfaces of both halves ribs are provided that run substantially radially and that engage positively between corresponding ribs of the respective other half.
With such a design of a brake disc consisting of two halves, the securing to the wheel can be considerably simplified due to the positive and material-locking connection of the interengaging ribs, since a relative rotation of the two halves can be effectively prevented by the said positive and material-locking connection. Accordingly, the securing of the brake disc to the wheel can be effected in a particularly simple manner, in which connection if necessary just a simple friction-locking connection may also be used in order to completely prevent the propagation of notch stresses into the brake disc. Overall, the stability of the brake disc consisting in such a way of two halves is furthermor
Berreth Karl
Gadow Rainer
Speicher Marcus
Antonelli Terry Stout & Kraus LLP
Butler Douglas C.
Universitat Stuttgart, Institut fur Fertigungstechnologie Kermai
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