Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
1999-04-13
2001-04-24
Cain, Edward J. (Department: 1714)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C523S149000, C523S150000, C523S152000, C523S153000, C523S155000, C523S156000, C523S157000, C523S158000
Reexamination Certificate
active
06221475
ABSTRACT:
The present invention relates to C/C—SiC composite materials, i.e. materials having carbon fiber reinforcement densified by a combined carbon and silicon carbide matrix, for use in friction elements such as brake disks and/or brake pads.
It is well known to use friction elements made of C/C composite material manufactured by preparing a fiber preform made of carbon fibers and densifying the preform with a carbon matrix.
Preforms are prepared from a base of felts or fiber fabrics such as woven cloth, braids, knits, unidirectional sheets of threads, strands, or cables, or laminates made up of a plurality of unidirectional sheets superposed in different directions and bonded together by light needling. To prepare a preform, a plurality of layers made of base fabric plies and/or felt are superposed and bonded together until the desired thickness has been reached. Bonding can be made by needling performed individually on each layer, for example as described in document U.S. Pat. No. 4,790,052. The base fabrics or felts used are made of carbon fibers or of carbon precursor fibers with the precursor, when present, being transformed by heat treatment after the preform has been prepared.
Densification with the carbon matrix is performed by chemical vapor infiltration or by using a liquid process.
Chemical vapor infiltration consists in placing the preform in an enclosure and in admitting a gas into the enclosure which gas diffuses throughout the preform under predetermined conditions of temperature and pressure and forms a deposit of pyrolytic carbon on the fibers. As a general rule the gas comprises one or more hydrocarbons, e.g. methane, giving pyrolytic carbon by decomposition.
The liquid process of carbon densification consists in impregnating the preform with a carbon precursor in the liquid state, e.g. a resin having a non-zero coke content, and in transforming the precursor into carbon by heat treatment.
In the field of braking, C/C composites are in use at present for aircraft brake disks, but their use for land vehicles is presently restricted to F1 racing cars.
For those uses, C/C composites are generally obtained by densifying a preform with a pyrolytic carbon matrix made by chemical vapor infiltration. Unfortunately, that method is lengthy and expensive, and leads to cost prices that are generally incompatible with requirements for uses in other fields, such as rail vehicles or mass-produced private vehicles. In addition, in such other uses, the demands made of the friction elements are very different from those encountered on aircraft or F1 racing cars. Although these demands are generally less severe, tests performed by the Applicant have shown several problems. Thus, it appears that braking effectiveness varies, in particular as a function of braking intensity, and is relatively low under conditions of wet braking. In addition, wear is significant and leads to insufficient lifetime.
In order to solve those problems, at least in part, and in particular to increase resistance to wear, document EP-A-0 300 756 proposes making C/C composite friction elements that are obtained by densifying a preform by chemical vapor infiltration and by performing a final siliciding operation by impregnating the preform with molten silicon which reacts with the carbon of the matrix to form silicon carbide (SiC).
Nevertheless, chemical vapor infiltration processes as generally performed today remain relatively lengthy and expensive.
An object of the present invention is to provide C/C—SiC composite friction elements of cost and performance that makes them suitable for use in brakes for rail vehicles or for mass-produced private vehicles or for racing vehicles, or indeed for utility or industrial vehicles such as heavy trucks.
In particular, an object of the invention is to provide friction elements which provide braking effectiveness that is regular and reproducible regardless of whether braking conditions are intense or otherwise, and regardless of whether the environment is dry or wet.
Another object of the present invention is to provide friction elements which wear little and which are suitable for being used for rubbing against materials of various kinds.
These objects are achieved by a friction element having at least one friction face and made of a composite material comprising carbon fiber reinforcement and a matrix having at least a carbon phase and a silicon carbide phase, in which friction element, at least in the vicinity of the or each friction face, the matrix comprises: a first phase containing pyrolytic carbon obtained by chemical vapor infiltration in the vicinity of the reinforcing fibers; a second phase that is refractory and obtained at least in part from a liquid precursor by pyrolysis; and a phase of silicon carbide.
Such a friction element can constitute a brake disk or at least a brake disk friction lining, in a disk brake for a rail vehicle or for a mass-produced private motor vehicle, or for a racing vehicle, or for a utility or industrial vehicle.
The term “pyrolytic carbon phase” is used herein to mean a phase of pyrolytic carbon obtained by chemical vapor infiltration using one or more gaseous precursors of carbon.
The term “refractory phase” is used herein to mean a phase of carbon or of ceramic.
Advantageously, at least in the vicinity of the or each friction face, the composite material is constituted, by volume, by:
15% to 35% carbon fibers;
10% to 55% of first matrix phase containing pyrolytic carbon obtained by chemical vapor infiltration;
2% to 30% of second matrix phase of refractory material coming at least in part from a liquid precursor; and
10% to 30% silicon carbide.
The matrix phase obtained by chemical vapor infiltration forms a continuous coating of pyrolytic carbon of constant thickness on the fibers which coating is, at least initially, not cracked. By covering the fibers completely, the pyrolytic carbon can protect them during the formation of the silicon carbide phase of the matrix. In addition, pyrolytic carbon, when obtained by chemical vapor infiltration, has rather high thermal conductivity so it provides the composite material with thermomechanical properties that are sufficient at least to perform the heat sink function for evacuating the heat generated by friction. In addition to pyrolytic carbon, the first matrix phase may include one or more layers of a material capable of protecting the pyrolytic carbon, and the underlying carbon fibers, from oxidizing. A material for providing protection against oxidizing and suitable for deposition by chemical vapor infiltration is silicon carbide, a ternary Si—B—C system, or boron carbide. The material can be selected from precursors of a self-healing glass, i.e. precursors that are suitable on oxidizing for forming a glass which, on passing to a semisolid state at the temperature at which the friction element is used, plugs any cracks that appear in the first phase of the matrix.
Various types of method can be used for performing the chemical vapor infiltration, in particular constant temperature and pressure methods, temperature gradient methods, pressure gradient methods, or vaporized film methods. Temperature gradient methods can be implemented by inductive coupling between an induction coil and a core situated beside the preform that is to be densified, or by direct coupling between an induction coil and the preform. Constant temperature methods with a pressure gradient can be performed by constraining the gas constituting a precursor for the matrix to follow a path either by means of a flow that is directed under constant pressure conditions, as described in co-pending U.S. patent application Ser. No. 08/945,325 corresponding to PCT/FR96/00582, or else with a forced flow as described in international patent application WO 96/15288. Vaporized film methods consist in immersing the preform in a bath and in heating the preform to a temperature so that a film of precursor vapor forms on contact with the preform, with infiltration then taking place in the vapor phase, e.g. as described in U.S. Pat.
Domergue Jean-Marc
Georges Jean-Michel
Laxague Michel
Cain Edward J.
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten, Schurgin Gagnebin & Hayes LLP
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