Coating processes – Direct application of electrical – magnetic – wave – or... – Electromagnetic or particulate radiation utilized
Reexamination Certificate
1998-12-21
2002-02-12
Barr, Michael (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electromagnetic or particulate radiation utilized
C427S249200, C427S249400, C427S255120
Reexamination Certificate
active
06346304
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for the densification of a porous structure, a porous structure for densification by the method of the invention and a densified structure, such as a friction element for an aircraft brake, formed by the method of the invention.
The invention relates generally but not exclusively to the infiltration and densification of a porous structure, such as a carbon fiber or ceramic structure, which may be shaped as a preform for a finished product or for use in providing a finished product.
BACKGROUND OF THE INVENTION
It is known that in the manufacture of a carbon-carbon composite product, such as a brake friction element, a porous preform body, which may have approximately the desired shape and dimensions of the finished product, may be densified by a method which involves chemical vapor infiltration and deposition. The carbon-carbon composite product so formed has many useful attributes, including high strength and frictional wear resistance, but the use of such structures is limited by high costs which arise because of the slowness of the manufacturing method. Similar considerations arise in relation to the manufacture and use of other, ceramic matrix composites.
Carbon-carbon composites often are manufactured by the isothermal, isobaric chemical vapor infiltration (CVI) procedure whereby a hydrocarbon gas is caused to diffuse into a porous carbon fiber preform body and deposit carbon. To obtain a high final density and a desired microstructure the diffusion and deposition process is performed in a high temperature environment at a low pressure and takes a considerable period of time, for example typically between 500 and 2000 hours.
It is known that the rate of infiltration and deposition may be accelerated by a so-called thermal gradient technique. A temperature gradient is established within a preform and a front of deposition moves through the preform, starting at the hottest region and moving away progressively with increasing densification of the hottest region. The thermal gradient technique is discussed in U.S. Pat. No. 5,348,774 (Golecki et. al.) which describes a method of achieving a thermal gradient by the electromagnetic heating of a graphite core provided as a close fit in the bore of a porous preform body of annular shape.
Although the thermal gradient technique can accelerate the rate of infiltration and deposition, it requires the use of special equipment and process control procedures the cost of which tends to offset savings from the reduction of processing time.
Another prior art method and apparatus for densification of a porous body is described in EP O 592 239 A2. In contrast to the use of a vapor for densification, as described in U.S. Pat. No. 5,348,774, the teaching of this publication relates to the use of a liquid for densification.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved method for the densification of a porous structure, a porous structure for densification by the method of the invention and a densified structure formed by the method of the invention.
In accordances with one of Its aspects the present invention provides a method for the densification of a porous structure comprising providing the structure with a body of a material which includes a susceptor element comprising fibers of a material which is more susceptible to heating by electromagnetic radiation than the material of the body, exposing said porous structure to hydrocarbon gas and simultaneously applying an electromagnetic field to said porous structure whereby said susceptor element at least in part causes heating of the porous structure to a temperature at which the gas infiltrating the porous structure deposits carbon within the porous structure.
The present invention provides also a porous structure for densification by chemical vapor infiltration, said porous structure comprising a body which includes a susceptor element comprising fibers of a material which is more susceptible to heating by electromagnetic radiation than the material of the body, said susceptor element being positioned and arranged whereby when exposed to an electromagnetic field at least in part it causes heating of the porous structure to a temperature at which the gas infiltrating the porous structure deposits carbon within the porous structure.
The term fibers as used herein includes so-called staple fibers having a length to diameter (or width) ratio of less than 10:1, and also long length fibers, known as filaments. The fibers may be in the form of individual filaments or groups of filaments which may be twisted together, i e as yarns, tows or cords.
The susceptor element may comprise fibers of good electrical conductivity as hereinafter defined. It may be of a material which remains in the composite porous structure following densification, or it may be a material which is removed, for example by heating and melting or evaporating, or by cutting and/or machining of the composite structure.
The fibers of the susceptor element may be individually dispersed within the porous body or a plurality of fibers may be integrated, for example as a woven or non-woven fabric layer, which is then incorporated within the porous body. Whether individually dispersed or incorporated in at least one fabric layer, the susceptor element material may be selectively positioned so that susceptor element material is present to a greater extent in one part of the porous body than in the remainder or another part of the porous body.
In the case of a porous structure of annular form it is taught by the present invention that a susceptor element of fibers integrated as a layer may also be of annular shape and incorporated in the porous structure such that the susceptor element and porous structure are substantially concentric. The or at least one susceptor element layer may be positioned to lie substantially centrally between radially inner and radially outer extremities of the porous structure and/or substantially centrally between annular end faces of the porous structure.
The or each layer of susceptor element fibers preferably has a thickness less than 2.0 mm, preferably 1.0 mm or less. It is further preferred that in the case of a porous structure formed from cloth layers, the ratio of the thickness of a layer of susceptor element material to the thickness of each cloth layer is not greater than 3:1, preferably less than 1.5:1, and more preferably less than or equal to 1:1.
If the susceptor element is to remain in the composite structure following densification, preferably it and other materials of the structure are selected to be materials which do not degrade or react with one another.
Although the invention teaches that, for fibers integrated in a fabric layer, only a single susceptor element layer need be incorporated in the porous structure, it is envisaged that a plurality of said layers may be provided. The susceptor element layers may be arranged to lie co-planar and/or to lie in superimposed layers. Elements in superimposed layers may be directly superimposed, optionally spaced by porous structure material, and/or offset relative to one another.
The susceptor element(s) may be arranged within the porous structure to provide a substantially uniform heating effect or the element(s) may be arranged in a non-uniform manner which results in a thermal gradient. By selecting the uniformity or otherwise of the heating effect there may be achieved a pre-selected uniformity or variation of rate of carbon deposition within the porous body.
In this specification the reference to a susceptor element of good electrical conductivity means an element of a material having a resistivity, expressed in units of micro ohm m, of less than 20, preferably less than 10, and more preferably less than 5. It is preferred also that a susceptor element of good conductivity shall have a resistivity greater than 0.02, preferably greater than 0.05 and more preferably more than 0.1 micro ohm m.
The resistivity of the susceptor element material preferabl
Fisher Ronald
Williams Keith
Barr Michael
Dunlop Limited
Young & Thompson
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