U.S. patent application number 09/737730 was filed with the patent office on 2001-07-05 for densification.
This patent application is currently assigned to DUNLOP AEROSPACE LIMITED. Invention is credited to Fisher, Ronald, Tibbatts, Alan Frank.
Application Number | 20010006709 09/737730 |
Document ID | / |
Family ID | 10867159 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006709 |
Kind Code |
A1 |
Fisher, Ronald ; et
al. |
July 5, 2001 |
Densification
Abstract
A method of depositing a carbon-containing substance in the
pores of a porous body comprises establishing an open varying
magnetic field flux loop and placing the body such that a magnetic
field flux generated by the open magnetic field flux loop passes
through a region of said body and heats that region, creating a
thermal gradient across the body and bringing a thermally
decomposable carbon-containing gas into contact with the heated
region, thereby depositing a carbon-containing substance in the
pores of said heated region.
Inventors: |
Fisher, Ronald; (Rugby,
GB) ; Tibbatts, Alan Frank; (Coventry, GB) |
Correspondence
Address: |
Caesar, Rivise, Bernstein,
Cohen & Pokotilow, Ltd.
12th Floor, Seven Penn Center
1635 Market Street
Philadelphia
PA
19103-2212
US
|
Assignee: |
DUNLOP AEROSPACE LIMITED
|
Family ID: |
10867159 |
Appl. No.: |
09/737730 |
Filed: |
December 15, 2000 |
Current U.S.
Class: |
427/598 ;
427/249.2 |
Current CPC
Class: |
H05B 6/105 20130101;
C04B 35/571 20130101; C23C 16/045 20130101; C04B 35/80 20130101;
C04B 35/83 20130101; F16D 69/023 20130101 |
Class at
Publication: |
427/598 ;
427/249.2 |
International
Class: |
C23C 016/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 30, 1999 |
GB |
9930810.8 |
Claims
1. A method of depositing a carbon-containing substance in the
pores of a porous body, the method comprising the steps of:
establishing a varying magnetic field flux loop; placing the porous
body within the loop such that only a localised portion thereof is
heated by direct exposure to the varying magnetic field flux
generated by said varying magnetic field flux loop; creating a
thermal gradient across the porous body; exposing the heated
localised portion of the porous body to a thermally decomposable
carbon-containing gas to deposit the carbon-containing substance in
the pores of the localised portion.
2. A method according to claim 1, including passing gas about the
porous body to cool selected surface portions thereof and thereby
create the thermal gradient across the porous body.
3. A method according to claim 1, comprising establishing the
varying magnetic field flux loop at least partially within a frame
provided with a pair of jaws.
4. A method according to claim 3, including the step of locating a
stack of porous bodies in a gap defined by the jaws of the
frame.
5. A method according to claim 1, including incorporating thermally
conductive fibres in a selected zone or zones of the body to create
the thermal gradient across the porous body.
6. A method according to claim 1, wherein the thermally
decomposable carbon-containing gas is a hydrocarbon.
7. A method according to claim 1, wherein the thermally
decomposable carbon-containing gas is methyltrichlorosilane to form
a ceramic matrix comprising silicon carbide.
8. A method of depositing carbon in the pores of a porous body, the
method comprising: mounting the body in a gap defined by jaws of a
frame containing a coil and defining an open magnetic field flux
loop, such that a varying magnetic flux passes in a centre region
of the body and induces heat therein; locating the assembly so
formed in a deposition chamber; applying an alternating electric
current to the coil to generate the magnetic field flux from said
open magnetic field flux loop whereby to heat the body; causing or
allowing the passage of heat from the periphery of the body whereby
to create a thermal gradient across the body; bringing a
carbon-containing gas into contact with the heated part(s) of the
porous body; and continuing the supply of gas for a time period
until sufficient elemental carbon has deposited in the pores of the
porous body.
9. A method according to claim 8, including the step of locating a
stack of porous bodies in the gap defined by the jaws of the
frame.
10. A method according to claim 8, including passing gas about the
body so as to cool selected portions to create a thermal gradient
across the porous body.
11. A method according to claim 8, including incorporating fibres
of selected thermal conductivity in selected zones of the body to
create a thermal gradient across the porous body.
12. A method according to claim 8, including externally cooling the
deposition chamber to create a thermal gradient across the porous
body.
13. A method according to claim 8, including mounting a plurality
of frames about the body to concentrate the heat generation in
selected areas of the body.
14. A method according to claim 8, including mounting a plurality
of frames about a stack of porous bodies to concentrate the heat
generation in selected areas of the stack of porous bodies.
15. A method according to claim 8, wherein the thermally
decomposable carbon-containing gas is a hydrocarbon.
16. A method according to claim 8, wherein the thermally
decomposable carbon-containing gas is methyltrichlorosilane to form
a ceramic matrix comprising silicon carbide.
17. A method of forming a densified structure comprising the steps
of; establishing a varying magnetic field flux loop; placing a
porous body within the loop such that only a localised portion
thereof is heated by direct exposure to a varying magnetic field
flux generated by the magnetic field flux loop; exposing the body
to a decomposable carbon-containing gas; and propagating a
deposition front through the body to deposit a carbon-containing
substance in the pores of the porous body.
18. An aircraft friction brake comprising a densified structure
made according to the method of claim 17.
Description
[0001] The invention relates to densification of porous bodies. One
particular instance is in the densification of porous structural
components such as brake discs or pads for aircraft, which
typically are made of a porous carbon body infiltrated by
carbon.
[0002] Many different methods are known for the deposition of a
carbon or ceramic matrix on a fibrous substrate of carbon or
ceramic fibres. In a thermal gradient process the densification is
started at the inside of the substrate or preform and progresses
outwards until the whole article is densified. Because the exterior
pores of the preform do not become blocked until the centre of the
preform is densified, it is found that densification can proceed at
a higher rate, reducing processing times and cost compared to the
isothermal method of densification.
[0003] In all processes where a fibrous substrate is being
densified the control of deposition conditions, such as temperature
and pressure, is important to ensure the deposition reaction
continues at the required rate in order to produce a deposit of the
required structure. Where a matrix of carbon is being deposited to
manufacture friction discs for aircraft brakes, it can be desirable
that the deposit is a graphitisable carbon in order to heat treat
the composite to maximise the thermal properties. The graphitisable
form of carbon is often referred to as rough laminar and can be
distinguished from the non-graphitisable smooth laminar form by
microscopic examination using polarised light.
[0004] U.S. Pat. No. 5,846,611 to Christin discloses a method of
Chemical Vapour Infiltration (CVI) in which a substrate, such as a
needled fibrous structure, is located within a winding, the winding
extending axially over a length which is equal or greater than the
axial length of the substrate, such that the substrate lies
entirely within the electromagnetic field generated by the winding.
The frequency of the current powering the winding is selected such
that surface heating of the substrate is limited due to radiation
and convection of heat from the surface and a temperature gradient
is established between the centre region of the substrate and the
surface thereof. In order to accomplish such a temperature gradient
it is taught that the ratio of the substrates radial and
circumferential electrical resistivities must be above a known
value whilst the ratio of radial and circumferential thermal
conductivities must not be less than a known value.
[0005] Similarly, U.S. Pat. No. 5,789,026 and U.S. Pat. No.
5,652,030 both to Delperier, disclose methods of CVI in which a
substrate is located within a volume defined by the winding. In
U.S. Pat. No. 5,789,026 a temperature gradient is maintained
throughout the substrate and carbon deposition is activated, in the
substrate region at the highest temperature, by the presence of
hydrogen. U.S. Pat. No. 5,652,030 teaches that the temperature at
the surface of the substrate is maintained below a deposition
temperature of the material to be deposited, whilst the presence of
a temperature gradient ensures that a region of the substrate
toward the centre thereof is above said deposition temperature.
[0006] In thermal gradient deposition, heating may also be effected
by induction heating of a graphite susceptor see, e.g. Golecki U.S.
Pat. No. 5,348,774 which discloses a process in which the fibrous
substrate is in contact with a graphite mandrel which is heated by
the electromagnetic field from an induction coil. The fibrous
substrate is then heated by conduction from the graphite mandrel.
The main problem with this method of establishing the thermal
gradient in the preform is the requirement for a susceptor core
which might only be usable once if damage is caused during removal
from the densified composite.
[0007] Another method of producing a thermal gradient in a fibrous
substrate for densification by CVI is described by J. J. Gebhardt
et al in a paper entitled "Formation of carbon-carbon composite
materials by pyrolitic infiltration", Petroleum Derived Carbons ACS
Series No. 21 6/73, pages 212 to 227. In the method of Gebhardt the
thermal gradient is maintained by a high rate of gas flow cooling
the outer surface of the substrate and by heating only a small
volume of the cylindrical substrate in the induction coil,
densification of the whole being achieved by moving the substrate
inside the induction coil. The substrate is manufactured from
graphite fibres which have sufficient electrical conductivity to
enable heating of the substrate by direct coupling with the
induction coil. Practical constraints on the process make it
difficult to apply on an industrial scale and it is unsuitable for
other substrate geometry, such as a disc.
[0008] U.S. Pat. No. 5,025,124 (see also Materials Worlds, Journal
of the Institute of Materials, August 1998, 470-471) proposes a
method of heating called Uniform Magnetic Heating (UMH) in which an
article to be heated is placed between the jaws of a C-core
constructed of laminated material with a coil wound around each
end, through which an alternating current is passed. This
alternating current sets up a `fluctuating` magnetic field in the
laminate. A magnetic field flux loop flows through the laminate and
passes across the gap between the jaws, i.e. open ends of the
C-shaped laminate. When an article, typically a component of a
material with a higher reluctance than the material of the
laminated core is placed between the jaws the article becomes part
of the loop and the magnetic flux creates a heating effect in the
article. UMH has been used to create a uniform heating effect in
articles.
[0009] The C-shaped laminate consists of a fixed rear and bottom
section and an upper section that can move vertically to alter the
distance between the jaws of the C-core. The use of more than one
core has been proposed where greater heating power is required
and/or multi-phase power supplies are available.
[0010] It is one object of the invention to provide a method for
use in the densification of a porous body which is fast in use and
simple to operate. The invention is based on the appreciation that
the UMH method of heating can be used to focus heating in a porous
preform where deposition could then be initiated to densify the
composite using a thermal gradient effect. A more specific object
of the invention is to provide an improved method for the
densification of a porous structure, and a porous structure
densified by the method of the invention and a densified structure
formed by the invention.
[0011] According to the invention in one aspect there is provided a
method of depositing a carbon-containing substance in the pores of
a porous body, the method comprising the steps of:
[0012] establishing a varying magnetic field flux loop;
[0013] placing the porous body within the loop such that only a
localised portion thereof is heated by direct exposure to the
varying magnetic field flux generated by said varying magnetic
field flux loop;
[0014] creating a thermal gradient across the porous body;
[0015] exposing the heated localised portion of the porous body to
a thermally decomposable carbon-containing gas to deposit the
carbon-containing substance in the pores of the localised
portion.
[0016] According to the invention, in a further aspect, there is
provided a method of depositing carbon in the pores of a porous
body, the method comprising:
[0017] mounting the body in a gap defined by jaws of a frame
containing a coil and defining an open magnetic field flux loop,
such that a varying magnetic flux passes in a centre region of the
body and induces heat therein;
[0018] locating the assembly so formed in a deposition chamber;
[0019] applying an alternating electric current to the coil to
generate the magnetic field flux from said open magnetic field flux
loop whereby to heat the body;
[0020] causing or allowing the passage of heat from the periphery
of the body whereby to create a thermal gradient across the
body;
[0021] bringing a carbon-containing gas into contact with the
heated part(s) of the porous body; and
[0022] continuing the supply of gas for a time period until
sufficient elemental carbon has deposited in the pores of the
porous body.
[0023] Preferably the support comprises a coil which contain a high
magnetic permeability low reluctance laminate material. In one
embodiment the coils are present at the ends of a generally
C-shaped member, the mouth of which defines the gap to receive the
body to be treated.
[0024] In one preferred step, the method includes the step of
locating a stack of porous bodies to be densified in the gap
defined by the jaws of the support.
[0025] The infiltrating gas may be of any suitable type and may be
derived from a gas, e.g. methane, or from a solid or liquid.
Preferably the material deposited is carbon which may be derived
from a hydrocarbon gas, e.g. natural gas or methane. Other
materials include silicon carbide which may be derived from e.g.
methyltrichlorsilane and which will form a ceramic matrix.
[0026] The porous bodies may be used to form carbon-carbon
composite brake discs of the type used in aircraft disc brake
assembly. Such bodies include fibrous preforms, partially densified
composites; and the like.
[0027] One or more different techniques may be used to create a
thermal gradient across the body, typically from the centre to the
periphery. Such techniques include:
[0028] passing gas about the body so as to cool selected
portions;
[0029] incorporating fibres of selected thermal conductivity in
selected zones of the body;
[0030] externally cooling the carbon deposition chamber, e.g. water
cooling or otherwise cooling the body itself.
[0031] When the carbon-containing substance, such as a
carbon-containing gas comes into contact with heated areas of the
porous body the gas is cracked to deposit carbon. The level of
heating of the body and the slope of the gradient will be
determined in known manner.
[0032] One preferred method includes mounting a number of the
frames about the body (or stack of bodies) to concentrate the heat
generation in selected areas.
[0033] A further aspect of the invention resides in a method of
forming a densified structure comprising the steps of;
[0034] establishing a varying magnetic field flux loop;
[0035] placing a porous body within the loop such that only a
localised portion thereof is heated by direct exposure to a varying
magnetic field flux generated by the magnetic field flux loop;
[0036] exposing the body to a decomposable carbon-containing gas;
and
[0037] propagating a deposition front through the body to deposit a
carbon-containing substance in the pores of the porous body.
[0038] In order that the invention may be well understood it will
now be described by way of example only with reference to the
accompanying diagrammatic drawings, in which:
[0039] FIG. 1 is a perspective view of one assembly of the
invention
[0040] FIG. 2 is a top plan view of another assembly of the
invention.
[0041] The heating apparatus shown in FIG. 1 comprises a generally
C shaped frame 1, having an open mouth 2 between the jaws 6 at the
free ends. The apparatus comprises a laminated assembly of
generally parallel metal strips formed into a magnetic loop and
separated by insulating layers. A coil 3 is wound about each end of
the core. As shown they are generally square blocks but can be of
any suitable shape, so long as the magnetic flux loop lines will
pass from one to the other via a workpiece in the mouth 2. The
distance between the coils 3 may be varied both vertically and
horizontally.
[0042] A stack of porous carbon bodies 4 is located between the
coils 3, two are shown.
[0043] The apparatus is connected to an AC supply, not shown. When
the alternating current is applied a magnetic field flux is created
in the coils 3 and flows between them via the stack as shown by the
dotted lines. Heat is generated quickly and spreads from the inside
of the bodies 4 to their periphery, the heat being focussed in a
localised region 40 by virtue of the interaction of the magnetic
field with the body 4. To create the thermal gradient for carbon
vapour infiltration, a cooling gas 7 may be passed by the stack.
When a carbon containing gas 7 contacts the heated body 4 the gas 7
will be cracked and carbon will be deposited in the pores in
graphitisable form. For this the assembly is, or just the preform
porous bodies 4 are, located in a suitable carbon deposition
chamber 5 provided with a gas inlet port 8. Carbon-containing gas 7
is introduced via inlet port 8 and heated by contact with the
heated porous bodies 4 to cause infiltration of carbon into the
porous body 4.
[0044] Additionally or alternatively, fibres 41 of high thermal
conductivity may be incorporated within the bodies 4, such fibres
facilitating the flow of heat within the bodies 4.
[0045] As previously stated, the temperature gradient across the
bodies 4 is a consequence of the centre region thereof 40 at
commencement of the process, being maintained at a higher
temperature than the cooled periphery. The current in the coils is
controlled such that the temperature at the centre region of the
bodies 4 reaches the deposition temperature and carbon is deposited
in the pores of that region after subjection of the bodies 4 to the
carbon-containing gas 7. As carbon is deposited the thermal
conductivity of the densified region increases thereby increasing
the heat flow toward the periphery of the bodies 4.
[0046] The increase in heat flow through the bodies 4, from the
centre region 40 to the periphery, causes those regions toward the
periphery to reach the necessary temperature for carbon deposition.
In essence, a so-called "deposition front" propagates through the
volume defined by the discs, ensuring carbon deposition throughout
the bodies 4 and consequent densification thereof.
[0047] The method is quick, simple and reliable.
[0048] In the embodiment of FIG. 2 three C shaped frames are used
for 3 phase heating and/or to heat selected areas only of a disc 4'
or stack of discs 4'.
[0049] The invention is not limited to the embodiment shown. For
example, the porous body may have an aperture; it may be partially
densified; the article need not be a brake disc or pad.
* * * * *