Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-03-28
2001-07-17
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S096200, C501S088000, C501S090000, C264S658000, C264S659000
Reexamination Certificate
active
06261981
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a fibre-reinforced composite ceramic containing high-temperature-resistant fibres based on Si/C/B/N which are reaction-bonded to a matrix based on Si.
Such a process and such a composite ceramic are known from DE-A-4 438 455.
Carbon fibre-reinforced carbon (C—C, also known as CFRC or in German language usage as CFC) is among the composite ceramic materials which have been introduced successfully some time ago.
Recently developed high-performance brake systems based on CFRC brake discs with specially developed friction linings, as are used, for instance, in car racing, can, however, only be produced using numerous impregnation or carbonization and graphitization cycles, so that the production process is an extremely time-consuming, energy-intensive and costly process which may take a number of weeks or months. Furthermore, CFRC brake discs for use in production vehicles operated under normal conditions have totally unsatisfactory braking properties in the presence of moisture and when operated at low temperatures. This shows up, inter alia, in decidedly non-constant coefficients of friction as a function of the operating temperature and the surface lining which makes regulation, as hitherto customary in 4-channel ABS systems, extraordinarily difficult or even impossible. In view of this background, attempts are being made to develop improved fibre-reinforced composite ceramic materials which, for example, can be used as brake discs for high-performance brake systems in motor vehicles or railway vehicles. In addition, such fibre-reinforced composite ceramic materials are also of interest for numerous other applications, for instance as turbine materials or as materials for sliding bearings.
Although silicon-infiltrated reaction-bonded silicon carbide (SiSiC) containing from 2 to 15% by mass of free silicon has been known since the 1960s and has also been introduced commercially for some applications in the field of heat engineering, the production of SiSiC materials is also very complicated and expensive.
The abovementioned DE-A-4 438 455 discloses a process for producing a fibre-reinforced C-SiC composite ceramic in which the green body is made up of resin-impregnated fabrics. A disadvantage of this process is that the building-up or production of complicated structures from such precursors, which themselves are not exactly cheap, is very cumbersome and results in a great deal of scrap. The known process is therefore not suitable for use in mass production of components such as brake discs. The laminated structure additionally leads to strong anisotropy of various properties of the body produced in this way, which has an adverse effect on, in particular, heat removal from the brake disc. Furthermore, such brake discs have a tendency to delaminate under some conditions as they wear, which is particularly dangerous.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved fibre-reinforced composite ceramic containing high-temperature-resistant fibres and a process for producing such a composite, which makes possible simple and inexpensive production of mass-produced components such as brake discs and enables improved properties to be achieved.
This object is achieved according to the invention by a process for producing a fibre-reinforced composite ceramic containing high-temperature-resistant fibres, in particular fibres based on Si/C/B/N, which are reaction-bonded to a matrix based on Si, comprising the steps:
impregnation of fibre bundles, in particular of Si/C/B/N fibres, with a binder suitable for pyrolysis and solidification of the binder;
preparation of a mixture of fibre bundles, fillers and binders;
pressing of the mixture to produce a green body;
pyrolysis of the green body under reduced pressure or protective gas to produce a porous shaped body;
infiltration of the shaped body with a silicon melt.
The object is also achieved by a fibre-reinforced composite ceramic containing high-temperature-resistant fibres, in particular fibres based on Si/C/B/N, which are reaction-bonded to a matrix based on Si, wherein randomly distributed short fibre bundles comprising bundled individual fibre filaments with substantial retention of the individual filaments are embedded in the matrix and the short fibre bundles are surrounded, at least in the region of their surface, by a coating of carbon which has reacted completely or partially with the metallic or semimetallic matrix material.
According to the invention, it was recognized that the use of short fibre bundles for reinforcing the composite material results in considerably simplified fibre production, since it is possible to premix the various individual components and press them to form green bodies which subsequently only have to be pyrolysed and then melt-infiltrated. This makes possible a considerably simplified production process which is suitable for mass production.
It was also recognized that the favourable properties such as pseudoductile behaviour which can in principle be obtained by the fibre reinforcement can actually be achieved in production by means of silicon melt infiltration only if fibre bundles which are held together by suitable binders and are protected against attack by the silicon melt are used for fibre reinforcement. According to the invention, this is achieved by the impregnation of fibre bundles with a binder suitable for pyrolysis and subsequent solidification of the binder. This impregnation step ensures that the individual fibres in the fibre bundle hold together reliably and ensures that the fibre bundles have sufficient mechanical stability to largely prevent mechanical damage to the sensitive individual filaments during later mixing with the other constituents for producing the green body.
In the finished composite ceramic, the protection of the fibre bundles against attack by the silicon melt is shown by the fibre bundles being surrounded, at least in the region of their surface, by a coating of carbon which has reacted completely or partially with the matrix material (i.e. with silicon or with silicon compounds).
Overall, it is therefore possible to produce, in a relatively simple and inexpensive manner, a fibre-reinforced composite ceramic which has significantly improved properties compared with conventional composite ceramics and, in particular, is also suitable for use as brake body or brake disc in high-performance brake systems for production motor vehicles or railway vehicles.
For the purposes of the present invention, the term “silicon melt” encompasses not only pure silicon melts but also industrial silicon melts which contain the usual impurities or possibly additions of alloying elements.
For the fibre reinforcement, preference is given to using C fibres or, if desired, SiC fibres, although the use of other high-temperature-resistant fibres based on Si/C/B/N, some of which are just being developed, is also possible in principle and, depending on the properties of the fibres used, may even lead to further advantages. For particularly inexpensive products, aluminium oxide fibres are also conceivable.
In a preferred embodiment of the invention, the fibre bundles are produced by bundling together individual filaments with addition of a size.
The bundling together by means of a size is generally carried out by the manufacturer of the fibre bundles immediately after the production of the individual filaments by means of spinnerets. Subsequently, the fibre bundles are usually chopped to the desired length straight away.
In a further preferred embodiment of the invention, the fibre bundles are, after impregnation and solidification of the binder, additionally conditioned with an antisilicization layer suitable for pyrolysis.
This additional coating of the fibre bundles with a protective layer suitable for pyrolysis achieves an advantageous additional sheathing of the fibre bundles on their outer boundary, which firstly reduces the danger of mechanical damage to the fibre bundles during the mixing and pres
Dietrich Gerd
Gadow Rainer
Haug Tilmann
Kienzle Andreas
Schwarz Christian
Crowell & Moring , L.L.P.
Daimler-Chrysler AG
Group Karl
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