Silicon carbide articles reinforced with short graphite fibers

Details Silicon carbide articles reinforced with short graphite fibers Silicon carbide articles reinforced with short graphite fibers

1999-04-15

2001-05-15

Vargot, Mathieu D. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Carbonizing to form article

With carbonizing, then adding carbonizable material and...

C264S029700, C264S682000

Reexamination Certificate

active

06231791

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a composite material containing carbon and a matrix of silicon carbide and silicon and reinforced with carbon fibers, which material has relatively high elongation at break, and to a process for the production of such a composite material.
Composite materials having a high temperature resistant ceramic matrix reinforced with high temperature resistant fibers have met with remarkable success in many areas of advanced technology. They are used, for example, as a facing material for critical points on the outer skin of reusable space vehicles, for jet engine nozzle liners, for turbine blades, for highly stressed structural components in mechanical engineering and for friction linings. However, other than specific applications and use for development and test purposes, these materials have not hitherto been used more widely. The reason for this resides in their failure behavior. If defects, for example notches, are present in the surface or if defects occur or are present in the structure of articles consisting of these composite materials, they fail catastrophically with unrestrained crack propagation when loaded, as, unlike in metals, stress peaks cannot be dissipated by any sliding action in the crystal lattice. Such failure occurs, for example, in high strength silicon carbide reinforced with silicon carbide fibers which are perfectly bonded to the matrix. Since the occurrence of such failure is statistically highly variable, components made from these brittle composite materials frequently cannot fulfill the requirements placed upon them, especially if economic criteria must also be taken into account.
One objective of material development has thus been to reduce the brittleness of composite materials, i.e. to reduce their modulus of elasticity and to raise their elongation at break. One material which adequately exhibits this combination of properties is carbon reinforced with fibers of carbon or graphite (CFC). This material is thus used, for example, as a friction lining in high performance aircraft brakes. One disadvantage of this material is, however, the low oxidation resistance of the carbon, which results in high wear in CFC components if they cannot be kept under a protective gas. There are applications without inert gas protection such as brake material or as protective heat shields on high performance aerospace vehicles. While applying oxidation-inhibiting protective coatings has brought about some improvements, it cannot solve the problem completely.
One branch of material development then pursued the production of silicon carbide articles reinforced with fibers of carbon or graphite in which the C fibers are, on the one hand, protected from oxidation by the surrounding SiC matrix and, on the other, bonding of the C fibers is imperfect such that, while the fibers still provide a good reinforcing action, crack propagation is inhibited at the fiber interfaces by energy consumption and relatively elastic failure behavior is achieved. The production of such a material has hitherto been considered problematic because, at elevated temperatures, silicon and carbon very readily react to yield silicon carbide, i.e. the C fibers are at least partially converted into SiC, so losing their reinforcing action, and because production from C fibers and silicon carbide powder by, for example, hot pressing proved insufficiently successful.
One improvement to the situation was achieved by coating the carbon fibers using the CVD process (CVD=chemical vapor deposition) with protective layers of high-melting substances such as pyrocarbon, TiC, TIN or SiC before impregnation with liquid silicon (E. Fitzer et al. Chemielngenieur-Technik 57, no. 9, pp. 737-746 (1985), in this case p. 738, right hand column). This protective action of pyrolytically deposited carbon is also exploited in DE-PS 39 33 039 C2, in accordance with which moldings made from short carbon fibers or carbon felts are initially coated with a first layer of pyrolytic carbon, then graphitized and subsequently provided with a second layer of pyrocarbon, before they are subjected to siliconization with liquid silicon. One disadvantage of this process is the use of the comparatively costly CVD or CVI process (CVI=Chemical Vapor Infiltration), in accordance with which, if it is used only once per process stage, microcracks remain in the pyrocarbon layers, into which silicon may subsequently penetrate and at least a proportion of the C fibers may be converted into SiC (c.f. loc. cit. Fitzer et al., p. 740, column 2) or, if multi-layer coatings are provided, production of the components is very costly.
According to another process (DE 44 38 456 Al), the process starts from specially arranged layers of bundles of continuous carbon fibers which are enclosed in a synthetic resin matrix. Once this synthetic resin article reinforced with carbon fiber bundles has been carbonized, the article exhibits, thanks to the particular production method used, substantially translaminar channels which are filled with liquid silicon during siliconization. The introduced silicon is then substantially reacted with the carbon matrix to yield silicon carbide (c.f. also DLR notice PR 10/89 A: WB-BK 4./1). In this case too, the production of the basic structure from long fiber non-woven fabrics is relatively elaborate, depending upon the arrangement of the C fiber bundles, the article exhibits properties which are anisotropic overall or for each ply and if one of the layers providing protection against oxidation degrades, the underlying layer of carbon fibers may always be exposed without protection.
SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION
The object underlying the invention has been to provide a high temperature resistant, largely impervious composite material resistant to oxidation at continuous operating temperatures of up to 1700° C. which is produced from reinforcing carbon fibers and a matrix substantially consisting of silicon carbide and which exhibits high elongation at break as well as to provide a process for the production of such a composite material.
The fibers reinforcing the composite material are graphitized, high strength short carbon fibers, hereinafter also known as graphite fibers, which are enclosed in a shell of graphitized carbon. This shell was obtained by coking and subsequently graphitizing a shell of a synthetic resin enclosing the graphite fibers. This shell of graphitized carbon advantageously has at least two layers. This multi-layer structure is obtained by impregnating a carbonized article, in which the C fibers are enclosed in only one carbon shell, with a synthetic resin at least once more during the production process and then refiring, i.e. carbonizing, it before finally performing graphitization. This post-treatment also seals the cracks and pores which arose during carbonization in the first, innermost shell, and through which silicon could subsequently penetrate to the graphite fibers during subsequent siliconization.
The graphite fibers enclosed in a shell of graphitized carbon are embedded in the composite material in a matrix which predominantly consists of silicon carbide and furthermore additionally contains up to 20 wt. % of free silicon and minute quantities of unreacted carbon, and the fibers are firmly bonded to this matrix. The silicon content of the matrix is advantageously below 15 wt. % and particularly preferably below 5 wt. %. This structure of reinforcing short graphite fibers provided with a graphitized shell in a matrix substantially consisting of SiC is obtained by siliconizing a carbon article reinforced with graphite fibers. The reinforcing component of this precursor article comprises the above-described, high strength short graphite fibers provided with the shells. They are bound in the precursor article in a matrix of carbon produced from a synthetic resin/solid pitch mixture, which may be either ungraphitized or graphitized. During siliconization, substantially only the carbon or graphite matrix of the precurs

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