Fabric (woven – knitted – or nonwoven textile or cloth – etc.) – Woven fabric – Woven fabric is characterized by a particular or...
Reexamination Certificate
2000-01-21
2001-05-08
Weisberger, Richard (Department: 1774)
Fabric (woven, knitted, or nonwoven textile or cloth, etc.)
Woven fabric
Woven fabric is characterized by a particular or...
C442S189000, C442S198000, C442S203000, C442S205000, C442S206000, C442S207000
Reexamination Certificate
active
06228786
ABSTRACT:
The present invention relates to fabricating fiber preforms for composite material parts, and in particular it relates to a yarn for such fabrication.
One field of application of the invention is that of fiber preforms for composite material friction disks such as clutch disks, and principally brake disks.
Carbon—carbon (C—C) composite material brake disks are currently used in racing cars and, on a much larger scale, in rotor and stator multiple-disk brake systems for aircraft.
The manufacture of composite C—C brake disks comprises fabricating fiber preforms of carbon fibers and densifying the preforms with a carbon matrix which fills the majority of the initially accessible internal pores in the preforms.
Densification is conventionally carried out by chemical vapor infiltration or by chemical liquid infiltration, i.e., impregnation with a liquid carbon precursor and transformation of the carbon precursor by heat treatment.
Fiber preforms can currently be manufactured by superposing plies formed from a two-dimensional fiber fabric and interconnecting the plies by needling. The two-dimensional fiber fabrics can be in the form of woven fabrics or pre-needled unidirectional sheets. The plies are needled as they are stacked, preferably keeping a constant needling depth as described in French patent FR-A-2 584 106. The plies can be stacked flat and needled to obtain flat slabs from which annular brake disk preforms are cut. In order to avoid wasting a large amount of material, the plies can be formed from juxtaposed annular sectors cut from the two-dimensional fiber fabric, the lines separating the sectors being offset from one ply to the next.
Two-dimensional fiber fabrics in the form of woven fabrics or unidirectional sheets made of carbon yarns are not suitable for continuous or twisted carbon filaments which constitute the yarns, rather than taking the fibers to place them transversely to the superposed plies. One way of solving this problem is to associate a web of carbon fibers with the two-dimensional fabric, which web provides fibers which can be entrained by the needles. Another solution consists of needling two-dimensional fabrics formed by fibers which are not of carbon but of a carbon precursor which is much more suitable for needling. The carbon precursor is transformed by applying heat treatment to the needled preform.
European patent EP-A-O 489 637 describes making two-dimensional fiber fabrics for fabricating preforms from a yarn composed essentially of discontinuous fibers (staple) which are parallel to one another and not twisted, the integrity of the yarn being ensured by a covering yarn of sacrificial material. Eliminating the covering yarn by dissolving or by heating frees the discontinuous fibers and allows needling even when the fibers are in the carbon state. Further, freeing the fibers allows them to expand into the entire volume of the preform, resulting in pores which are more easily and more uniformly accessible to the matrix material during the densification stage. Densification is thus more complete and more uniform.
For brake disks, the nature and the origin of the fibers constituting the preforms, the structure of the two-dimensional fabrics used to fabricate the preforms, the way in which the plies formed by these fabrics are connected together, in particular the needling parameters, the heat treatments to which the preforms can be subjected before densification, the nature of the matrix, and the mode of densification, are all factors which significantly influence the mechanical and tribological properties of the disks.
Particularly in the case of aircraft brakes, the disks used must not only have mechanical properties which enable them to withstand stresses both when hot and when cold, but they must also have tribological properties which enable them to behave satisfactorily in different situations: braking while taxiing cold (taxiing on runways before take-off), braking while taxiing hot (taxiing on runways after landing), braking during a normal landing, and emergency braking (interrupting take-off at the end of the take-off runway). However, in applications where the energy to be absorbed is more modest, it is desirable for the friction surfaces of the disks to reach a sufficient temperature very rapidly, and thus for the thermal conductivity of the composite material to be more limited than that which is required for emergency braking in aircraft, for example.
Bench tests carried out by the applicant have ascertained that the nature of the fibers in the preform has a great influence on the performance of brake disks and the aim of the present invention is to provide a yarn which is particularly suitable for fabricating fiber preforms for composite material parts, particularly but not exclusively brake disks. A further aim of the invention is to provide a method of fabricating such a yarn.
In one aspect of the invention, a yarn for fabricating fiber preforms for composite material parts comprises discontinuous parallel fibers which are not twisted and which are held together by a covering yarn of sacrificial material wound around the fibers, is characterized in that the yarn is a hybrid yarn in which the fibers comprise an intimate mixture of fibers of at least two different natures selected from polyacrylonitrile based carbon fibers or carbon precursor fibers, anisotropic pitch based carbon fibers or carbon precursor fibers, isotropic pitch based carbon fibers or carbon precursor fibers, phenolic based carbon fibers or carbon precursor fibers, cellulosic based carbon fibers or carbon precursor fibers and ceramic fibers or ceramic precursor fibers, and in that the mixture of fibers comprises carbon fibers or carbon precursor fibers which, in the carbon state, comprise at least 15% by weight and preferably at least 30% by weight of the mixture of fibers and constitute high strength carbon fibers, having a tensile strength of at least 1500 MPa, preferably at least 2000 MPa, and a modulus of at least 150 GPa, preferably at least 200 GPa.
The term “carbon fibers or carbon precursor fibers” here means fibers which are in the initial precursor state, for example in the polyacrylonitrile state, or in the carbon state after complete transformation of the precursor by carbonization, or in a state intermediate between the initial precursor state and the carbon state, for example pre-oxidized or semi-carbonized. Similarly, the term “ceramic fibers or ceramic precursor fibers” means fibers in the initial precursor state or in the ceramic state after complete transformation of the precursor, or in a state intermediate between the initial precursor state and the ceramic state, for example in a semi-ceramized state.
The mixture of fibers of different natures allows the fiber preforms to adapt better to the service conditions of the composite material parts. An essential characteristic of the invention resides in the fact that the hybridization is not performed by using yarns of different natures to produce the fiber preforms, but is performed at the level of the fibers making up the yarns, by using an intimate mixture thereof.
Thus in the case of brake disks, for example, the intimate mixture of fibers means that the “third body” which is created at the interface of the friction surfaces during friction is uniform over that entire surface and combines most effectively the properties contributed by the fibers of different natures.
A fiber preform for a composite material part is produced by forming a two-dimensional fiber fabric at least partially from a hybrid yarn, by superposing plies formed by the fiber fabric and, after eliminating the covering yarn, by needling the superposed plies.
The covering yarn is eliminated before or during superposition of the plies so that needling can be carried out progressively after each ply has been positioned, as described in FR-A-2 584 106 cited above.
When the hybrid yarn used to produce the fiber preform comprises fibers in the initial state or in a state intermediate between that state and the final carbon or ceramic
Coupe Dominique
Duval Renaud
Olry Pierre
Zerdouk Amina
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten, Schurgin Gagnebin & Hayes LLP
Weisberger Richard
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