Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1998-12-21
2001-01-30
Talbot, Brian K. (Department: 1762)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C427S249200, C427S249300, C427S255120, C428S319100
Reexamination Certificate
active
06180223
ABSTRACT:
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 fibre 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 vapour 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 vapour infiltration (CVI) procedure whereby a hydrocarbon gas is caused to diffuse into a porous carbon fibre 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 (Golicki) 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.
SUMMARY OF THE INVENTION
Another prior art method and apparatus for densification of a porous body is described in EP 0 592 239 A2. In contrast to the use of a vapour 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.
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 accordance 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 of an electrically conductive foil which is more susceptible to heating by electromagnetic radiation than the material of the body and which occupies less than 5% of the volume of the porous structure, 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 vapour infiltration, said porous structure comprising a body which includes a susceptor element of an electrically conductive foil 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 susceptor element may be an element of good electrical conductivity, by which is meant a resistivity of less than 20 micro ohm m and greater than 0.02 micro ohm m. 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.
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 only a single susceptor element foil need be incorporated in the porous structure, it is envisaged that a plurality of said foil elements may be provided. The plurality may be arranged to lie co-planar and/or to lie in superimposed layers. Elements in successive layers may be directly superimposed, optionally with porous structure material therebetween, or lie offset relative to one another.
In the case of a porous structure of annular form it is taught by the present invention that the susceptor element foil may be annular and be incorporated in the porous structure such that the foil and porous structure are substantially concentric. The or at least one annular susceptor element foil may be positioned to lie substantially centrally between radially inner and outer extremities of the porous structure and/or substantially centrally between annular end faces of the porous structure.
The foil preferably has a thickness less than 1.0 mm, more preferably 0.5 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 the foil to the thickness of each cloth layer is not greater than 2:1, preferably less than 1:1 and more preferably less than 0.5:1.
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 preferably is less than, more preferably less than one half, that of the porous body material.
A susceptor element of good electrical conductivity may be of a kind which is uniformly heated when exposed to an electromagnetic field. Alternatively the element may be of a kind which attains a temperature gradient when exposed to an electromagnetic field, for example as a result of being of a non-uniform resistance.
The frequency of the electromagnetic field is chosen in known manner to result in preferential heating of the susceptor element(s).
The shape (and/or orientation) of a susceptor element preferably also is selected to result in a preferential/efficient heating effect. To achieve a good heating effect when using a susceptor element of good electrical conductivity it is preferred typically that the element is in the form of an electrically conductive closed loop, e.g. of an annular fo
Fisher Ronald
Williams Keith
Dunlop Limited
Talbot Brian K.
Young & Thompson
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