U.S. patent application number 11/786277 was filed with the patent office on 2008-07-24 for process for producing shaped bodies of carbon fiber reinforced carbon and shaped body produced by the process.
This patent application is currently assigned to SGL Carbon AG. Invention is credited to Andreas Kienzle, Ingrid Kratschmer.
Application Number | 20080176067 11/786277 |
Document ID | / |
Family ID | 36913096 |
Filed Date | 2008-07-24 |
United States Patent
Application |
20080176067 |
Kind Code |
A1 |
Kienzle; Andreas ; et
al. |
July 24, 2008 |
Process for producing shaped bodies of carbon fiber reinforced
carbon and shaped body produced by the process
Abstract
A process is provided for producing shaped bodies including
carbon fiber reinforced carbon in which the fibers are present in
the form of bundles having a defined length, width and thickness.
The defined configuration of the fibers in the bundles allows a
targeted configuration of the reinforcing fibers in the carbon
matrix and thus a structure of the reinforcement which matches the
stress of shaped bodies including carbon fiber reinforced carbon,
for example brake disks. A shaped body produced according to the
invention is also provided.
Inventors: |
Kienzle; Andreas;
(Mottingen/Balgheim, DE) ; Kratschmer; Ingrid;
(Biberbach, DE) |
Correspondence
Address: |
LERNER GREENBERG STEMER LLP
P O BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Assignee: |
SGL Carbon AG
|
Family ID: |
36913096 |
Appl. No.: |
11/786277 |
Filed: |
April 11, 2007 |
Current U.S.
Class: |
428/338 ;
156/250; 427/249.4 |
Current CPC
Class: |
C04B 2235/5248 20130101;
C04B 2235/5268 20130101; C04B 2235/614 20130101; Y10T 428/268
20150115; C04B 2235/48 20130101; C04B 35/83 20130101; C04B
2235/3813 20130101; C04B 2235/3826 20130101; C04B 35/64 20130101;
F16D 69/023 20130101; C23C 16/045 20130101; C23C 16/26 20130101;
C04B 2235/3839 20130101; C04B 2235/77 20130101; C04B 2235/602
20130101; Y10T 156/1052 20150115; C04B 2235/422 20130101 |
Class at
Publication: |
428/338 ;
427/249.4; 156/250 |
International
Class: |
B32B 5/02 20060101
B32B005/02; C23C 16/26 20060101 C23C016/26; B32B 38/00 20060101
B32B038/00; B32B 5/08 20060101 B32B005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 11, 2006 |
EP |
06 007 572.8 |
Claims
1. A process for producing shaped bodies having a carbon matrix
reinforced with carbon fiber bundles, the process comprising the
following steps: producing or providing bundles of parallel carbon
fibers held together by a dimensionally stable cured, carbonizable
binder, with the bundles having a specifically set, defined uniform
length, width and thickness; producing a molding composition by
mixing the fiber bundles, a carbonizable matrix former and optional
auxiliaries; producing a green body being close to a final shape by
pressing the molding composition in a mold being close to the final
shape at elevated temperature with curing of the carbonizable
matrix former, and subsequent demolding; carbonizing the green body
to form a carbonized shaped body; mechanically re-working the
carbonized shaped body, if necessary; and densifying the carbonized
shaped body by deposition of a carbon matrix in a chemical vapor
infiltration (CVI) process.
2. The process according to claim 1, which further comprises:
carbonizing the binder in the fiber bundles produced or provided in
the step of producing or providing bundles of parallel carbon
fibers; and re-impregnating the fiber bundles with a carbonizable
matrix former in a mechanically generated fluidized bed before the
step of producing the molding composition.
3. The process according to claim 1, wherein the production of the
fiber bundles includes the following steps: impregnating at least
one roving, including a plurality of parallel carbon fiber
filaments, with a carbonizable binder to yield a prepreg; pressing
at least one impregnated roving or a plurality of parallel
impregnated rovings to form a laminate sheet including parallel
filaments (UD laminate) and having a defined thickness, combined
with curing of the binder by heat treatment to yield a
dimensionally stable laminate sheet of defined thickness; and
cutting the (UD) laminate sheet, which may have been separated into
individual bands, to yield segments of fiber bundles of defined
width and length.
4. The process according to claim 1, which further comprises
setting a thickness of the fiber bundles to a value in a range of
from 0.15 to 0.4 mm, setting a length of the fiber bundles to a
value in a range of from 6 to 15 mm, and setting a width of the
fiber bundles to a value in a range of from 0.5 to 3.5 mm.
5. The process according to claim 4, which further comprises
setting the width of the fiber bundles at 1 mm.
6. The process according to claim 1, wherein the carbonizable
matrix former is a phenolic resin.
7. The process according to claim 1, wherein a proportion by mass
of the fiber bundles in the molding composition is from 70 to
80%.
8. The process according to claim 1, wherein the molding
composition contains not more than 10% by mass of the
auxiliaries.
9. The process according to claim 8, which further comprises
providing at least one of the following auxiliaries: tribological
auxiliaries such as silicon carbide, oxidation-inhibiting
auxiliaries, such as zirconium carbide, tantalum carbide or
tantalum boride.
10. The process according to claim 1, which further comprises
introducing the molding composition into the mold through a
charging grate causing the fiber bundles to assume an alignment
determined by the charging grate.
11. The process according to claim 1, which further comprises
producing the green body with a mold being close to the final shape
at a pressure in a range of from 1.5 to 5 N/mm.sup.2 and a
temperature of from 120 to 200.degree. C. in a hot molding
press.
12. The process according to claim 1, wherein the carbonized shaped
body is re-impregnated with a carbonizable matrix former and then
carbonized again before the chemical vapor infiltration (CVI)
process.
13. The process according to claim 12, which further comprises
using a resin or pitch as the carbonizable matrix former for the
re-impregnation of the carbonized shaped body.
14. The process according to claim 1, which further comprises using
methane as a carbon donor in the chemical vapor infiltration (CVI)
process.
15. The process according to claim 1, wherein the shaped body is a
brake disk.
16. The process according to claim 15, which further comprises
introducing the molding composition into the mold through a
charging grate containing a plurality of concentric rings causing
the fiber bundles to assume a tangential alignment.
17. A shaped body, comprising: carbon reinforced with carbon fiber
bundles and a carbon matrix including a pyrolysis residue of a
carbonizable matrix former and carbon deposited by chemical vapor
infiltration (CVI); said carbon fiber bundles having specifically
set, defined uniform dimensions, with a thickness of said bundles
being set to a value in a range of from 0.15 to 0.4 mm, a length of
said bundles being set to a value in a range of from 6 to 15 mm,
and a width of said bundles being set to a value in a range of from
0.5 to 3.5 mm, and said carbon fiber bundles having carbon fibers
aligned parallel to one another.
18. The shaped body according to claim 17, wherein said width of
said fiber bundles is 1 mm.
19. The shaped body according to claim 17, wherein said fiber
bundles have a random orientation.
20. The shaped body according to claim 17, wherein said fiber
bundles are aligned according to a loading direction of the shaped
body.
21. The shaped body according to claim 17, wherein the shaped body
is a brake disk in which said fiber bundles are disposed
tangentially.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority, under 35 U.S.C.
.sctn.119, of European application EP 06 007 572.8, filed Apr. 11,
2006; the prior application is herewith incorporated by reference
in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to a process for producing
shaped bodies, e.g. brakes disks, of carbon reinforced with short
carbon fibers. The invention also relates to a shaped body produced
by the process.
[0003] Composites including a carbon matrix reinforced with carbon
fibers (CFRC materials), which are also referred to as
carbon/carbon materials (C/C materials), have a high mechanical
strength and heat resistance. Those materials are therefore, inter
alia, suitable materials for high-performance brakes, e.g. for
aircraft or in racing.
[0004] The reinforcing carbon fibers are often present in the form
of flat or three-dimensional textiles, for example as woven fabrics
or needled preforms. However, both variants are relatively
expensive to produce and can only be fitted to complex geometries
to a limited extent.
[0005] An alternative is to make up the fiber reinforcement from
loose short fibers or/and short fiber bundles.
[0006] U.S. Pat. No. 5,242,746 discloses a friction element which
includes carbon fiber reinforced carbon and is composed of a
plurality of different functional layers. The friction element
includes at least one structural layer which typically has a
thickness of from 10 to 20 mm and has a high mechanical strength
and at least one friction layer which typically has a thickness of
from 3 to 7 mm and has advantageous tribological properties and a
high abrasion resistance.
[0007] The fiber reinforcement of the structural layer has a
relatively coarse texture and is formed by bundle-like segments of
rovings. The segments have a mean length of from 5 to 60 mm and
include from 1,000 to 320,000 virtually parallel individual
filaments. The rovings, which are cut to form bundles, can be
pre-impregnated to avoid disintegration of the bundles.
[0008] The fiber reinforcement of the friction layer has a fine
texture and is formed by broken up individual filaments or bundles
of less than 100 individual filaments having a mean length of from
0.05 to 60 mm, preferably from 0.2 to 2 mm. The fiber bundles in
the structural layer are randomly distributed, like the individual
fibers in the friction layer. There is a continuous transition in
the texture of the fiber reinforcement and of the carbon matrix
between the two layers, so that the layers form a one-piece
component.
[0009] If the fiber bundles are produced, as proposed in U.S. Pat.
No. 5,242,746, by cutting of rovings, the disintegration of the
bundles can be reduced by pre-impregnation of the rovings, but the
bundles obtained in that way are defined only by the mean length
and the number of individual filaments, i.e. they do not have a
defined width (dimension perpendicular to the longitudinal
extension of the fibers, dependent on the number of fibers located
side by side to one another) and thickness (dimension perpendicular
to the length and width, dependent on the number of fibers located
above one another) which can be set in a predetermined manner. That
is because the individual filaments in the rovings can be both
disposed side by side to one another and above one another and
their configuration depends greatly on the external conditions
(pressure, tension, shear force during mixing, etc.) to which the
roving or the segments cut therefrom are subjected until the
impregnation has cured sufficiently for it to fix the filaments in
their configuration present at that point in time.
SUMMARY OF THE INVENTION
[0010] It is accordingly an object of the invention to provide a
process for producing shaped bodies having a carbon matrix
reinforced with carbon fibers (C/C shaped bodies) and a shaped body
produced by the process, which overcome the hereinafore-mentioned
disadvantages of the heretofore-known processes and products of
this general type, and which make it possible for the reinforcement
to be formed of fiber bundles having defined dimensions, with a
cohesion and parallel spatial configuration of the fibers in the
bundles being retained upon mixing into a molding composition. The
defined configuration of the fibers in the bundles allows a
targeted configuration of the reinforcing fibers in the carbon
matrix and thus a structure of the reinforcement which matches the
stress.
[0011] With the foregoing and other objects in view there is
provided, in accordance with the invention, a process for producing
shaped bodies, especially brake disks, including a carbon matrix
reinforced with short carbon fiber bundles. The process comprises
the following steps: [0012] production or provision of bundles of
parallel carbon fibers held together by a dimensionally stable
cured, carbonizable binder, with the bundles having a specifically
set, defined length, width and thickness, [0013] production of a
molding composition by mixing of the fiber bundles, a carbonizable
matrix former and optional auxiliaries, [0014] production of a
green body which is close to the final shape by pressing of the
molding composition in a mold which is close to the final shape at
elevated temperature with curing of the carbonizable matrix former,
and subsequent demolding, [0015] carbonization of the green body to
form a carbonized shaped body, [0016] mechanical re-working of the
carbonized shaped body, if necessary, [0017] optional
re-impregnation of the carbonized shaped body with a carbonizable
matrix former and carbonization, and [0018] densification of the
carbonized shaped body by deposition of a carbon matrix through the
use of a CVI process.
[0019] With the objects of the invention in view, there is also
provided a second process variant of the invention. The second
process comprises the following steps: [0020] production or
provision of bundles of parallel carbon fibers held together by a
carbonized binder, with the bundles having a specifically set,
defined length, width and thickness, [0021] impregnation of the
fiber bundles with a carbonizable matrix former in a mechanically
generated fluidized bed, [0022] production of a molding composition
by mixing of the impregnated fiber bundles, a carbonizable matrix
former and optional auxiliaries, [0023] production of a green body
which is close to the final shape by pressing of the molding
composition in a mold which is close to the final shape at elevated
temperature with curing of the carbonizable matrix former, and
subsequent demolding, [0024] carbonization of the green body to
form a carbonized shaped body, [0025] mechanical re-working of the
carbonized shaped body, if necessary, [0026] optional
re-impregnation of the carbonized shaped body with a carbonizable
matrix former and carbonization, and [0027] densification of the
carbonized shaped body by deposition of a carbon matrix through the
use of a CVI process.
[0028] In this process variant, the fiber bundles are densified by
impregnation in a fluidized bed. The green body including the
molding composition having impregnated fiber bundles and the
carbonized shaped body obtained therefrom are therefore more highly
densified than in the first process variant. The time required for
densification of the carbonized shaped body through the use of CVI
is therefore lower in the second process variant.
[0029] With the objects of the invention in view, there is also
provided a shaped body, comprising carbon reinforced with carbon
fiber bundles and a carbon matrix including a pyrolysis residue of
a carbonizable matrix former and carbon deposited by chemical vapor
infiltration (CVI). The carbon fiber bundles have specifically set,
defined dimensions, with a thickness of the bundles being set to a
value in a range of from 0.15 to 0.4 mm, a length of the bundles
being set to a value in a range of from 6 to 15 mm, and a width of
the bundles being set to a value in a range of from 0.5 to 3.5 mm,
and the carbon fiber bundles having carbon fibers aligned parallel
to one another.
[0030] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0031] Although the invention is illustrated and described herein
as embodied in a process for producing shaped bodies of carbon
fiber reinforced carbon and a shaped body produced by the process,
it is nevertheless not intended to be limited to the details shown,
since various modifications and structural changes may be made
therein without departing from the spirit of the invention and
within the scope and range of equivalents of the claims.
[0032] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a diagrammatic, perspective view illustrating
charging of a tool for producing a brake disk having tangential
alignment of fiber bundles;
[0034] FIG. 2 is a top-plan view of the tool with a charging grate;
and
[0035] FIG. 3 is an enlarged, fragmentary, top-plan view showing a
tangential configuration of the fibers in the tool brought about by
the charging grate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] For the purposes of the present invention, carbon fibers are
all types of carbon fibers regardless of the starting material, but
with polyacrylonitrile, pitch and cellulose fibers being the most
widely used starting materials.
[0037] A process for producing fiber bundles which have a defined
length, width and thickness and include parallel carbon fibers and
a dimensionally stable cured polymeric binder is disclosed in
European Patent Application EP 1 645 671, corresponding to U.S.
Patent Application Publication No. US 2006/0076699. In a variant
adapted for the present invention, that process includes the
following steps: [0038] impregnation of at least one roving,
including a plurality of parallel carbon fiber filaments, with a
carbonizable binder so as to yield a prepreg, [0039] pressing of at
least one impregnated roving or a plurality of parallel impregnated
rovings to form a laminate sheet including parallel filaments
(unidirectional laminate, hereinafter referred to as UD laminate)
and having a defined thickness, combined with curing of the binder
by heat treatment so as to yield a dimensionally stable laminate
sheet of defined thickness, and [0040] cutting of the UD laminate
sheet, which may have been separated into individual bands, to
yield segments (fiber bundles) of defined width and length.
[0041] The rovings are preferably fanned out before impregnation in
order to aid in the parallel configuration of the fibers next to
one another in the plane.
[0042] The binder content of the prepreg is from 25 to 48% by mass
and depends on the impregnation conditions selected. The prepreg
has a mass per unit area of from 200 to 500 g/m.sup.2.
[0043] The prepreg in the form of one or more impregnated rovings
disposed side by side is passed through rollers, a calender, a belt
press or another suitable continuous pressing apparatus. In this
pressing apparatus, the excess binder is preferably squeezed out of
the rovings through the use of a plurality of gaps between rollers
which are disposed in series with decreasing gap width and the
rovings are pressed flat to such an extent that each roving
includes no more than three superposed layers of fibers, preferably
only one single layer of fibers, having substantially parallel
filaments. The pressing of the prepreg is carried out in the hot
state (a temperature up to 200.degree. C.), so that the
carbonizable binder either cures fully or cures to at least such an
extent that dimensionally stable rovings in which the individual
filaments are fixed in their parallel configuration next to and
above one another, are obtained.
[0044] Cooling of the now flat rovings bonded by the cured binder
is also preferably carried out in the pressing apparatus. After
leaving the continuous pressing apparatus, a flat laminate sheet
including parallel filaments (unidirectional laminate, hereinafter
referred to as "UD laminate") and having a thickness of from 0.15
to 0.4 mm is obtained. The laminate sheet can, if necessary to
assist handling, be divided up into bands having a width of from 20
to 60 mm.
[0045] The UD laminate sheets or bands are then cut longitudinally
into strips having a width which corresponds to the desired width
of the fiber bundles. This is preferably effected through the use
of a cutting roller or a plurality of cutting rollers disposed side
by side. It is also possible to cut the laminate sheet or the bands
in the not yet fully cured state into strips through the use of
wires stretched across the path of the band.
[0046] The strips are fed directly to a preferably continuously
operated apparatus for cutting to length and cut into segments
(fiber bundles) of the desired length. However, it is also possible
to carry out the cutting to length in a process which is separate
from the cutting of the strips and operates at a different speed.
For this purpose, the strips which have been cut to the chosen
width are wound up onto spools and transported to the apparatus for
cutting to length. The continuous cutting of the strips to the
desired length is preferably carried out through the use of a blade
roller.
[0047] The fiber bundles obtained in this way have a defined,
uniform length, width and thickness. The bundle thickness, i.e. the
number of superposed layers of fibers, was set during pressing of
the roving to form the laminate sheet. The bundle width, i.e. the
dimension which is perpendicular to the fiber direction and is
determined by the number of parallel fibers disposed side by side
to one another in a layer of fibers, is set in the longitudinal
cutting of the laminate sheet or the bands to yield strips. The
bundle length, i.e. the dimension in the fiber direction, is set by
the cutting to length of the strips to yield segments (fiber
bundles).
[0048] At least 90% of the fiber bundles produced in this way have
a length which is in the range of from 90 to 110% of the mean
length and a width which is in the range of from 90 to 110% of the
mean width.
[0049] The fiber bundles obtained in this way are very easy to
handle, they are free-flowing and can be poured and can easily be
mixed with other components to yield relatively homogeneous molding
compositions. Within the bundles, the fibers are held together by
the dimensionally stable cured binder, so that the bundles cannot
disintegrate during further processing and the fibers are fixed in
their parallel spatial configuration within the bundles.
[0050] Fiber bundles having a thickness of from 0.15 to 0.4 mm, a
length of from 6 to 15 mm and a width of from 0.5 to 3.5 mm are
particularly suitable for the process of the present invention.
Fine fiber bundles, i.e. fiber bundles having a low thickness
(preferably only one layer of fibers) and a low width (not more
than 1 mm) are preferred since a particularly homogeneous
distribution of the fibers in the molding composition and thus a
fairly uniform density of the molding composition and a
particularly homogeneous microstructure of the shaped body can be
achieved therewith. The more homogeneous the microstructure of the
shaped body, the fewer the opportunities for failure under
load.
[0051] The fiber bundles are mixed with a carbonizable matrix
former and, if appropriate, auxiliaries, to yield a molding
composition.
[0052] For the purposes of the present invention, a carbonizable
matrix former is a carbon-containing polymeric material, for
example a resin, which upon heating in a nonoxidizing atmosphere
forms a pyrolysis residue consisting essentially of carbon. The
carbonizable matrix former can be present as a pulverulent dry
resin or as a wet resin. Phenolic resins are particularly suitable
as matrix formers. The proportion by mass of the fiber bundles in
the molding composition is from 70 to 80%. If a dry resin is used
as a matrix former, mixing can be carried out in a tumble mixer.
When a wet resin is used, more intensive mixing is necessary, which
can be achieved, for example, through the use of an Eirich
mixer.
[0053] Due to the dimensionally stable cured binder in the fiber
bundles which holds the parallel fibers together, the fiber bundles
do not break up during mixing with the matrix former. This ensures
that the fiber bundles have a largely uniform defined length, width
and thickness in the molding composition. If required, auxiliaries
such as silicon carbide for improving the tribological properties
and oxidation inhibitors such as zirconium carbide, tantalum
carbide or tantalum boride which inhibit oxidative attack upon
exposure to oxygen by glass formation, can be mixed into the
molding composition. The total proportion by mass of auxiliaries in
the molding composition is not more than 10%.
[0054] In an advantageous embodiment of the process of the
invention, the carbonizable binder present in the fiber bundles is
firstly carbonized before production of the molding composition or,
as an alternative, the binder in the UD laminate is carbonized
before cutting of the bundles. The bundles obtained in this way
include parallel carbon fibers held together with a carbonized
binder. Due to the volume shrinkage of the binder occurring upon
carbonization, these bundles are relatively open-pored and can
therefore directly take up further carbonizable matrix former. For
the purposes of the present invention, a carbonizable matrix former
is a carbon-containing polymeric material, for example a phenolic
resin, which upon heating in a nonoxidizing atmosphere forms a
pyrolysis residue consisting essentially of carbon.
[0055] In order to prevent the impregnated fiber bundles from
sticking together as a result of the resin adhering to their
surfaces, impregnation is advantageously carried out in a
mechanically generated fluidized bed. This can be generated through
the use of a blade mixer. In this case, the carbon fiber bundles
are firstly preheated to a temperature sufficient for curing or
drying of the resin while mixing at a Froude number of less than 1.
The resin is subsequently introduced while briefly increasing the
Froude number to values in the range of from 1.5 to 4, preferably
not more than 2.5, and after the resin has been mixed into the
fluidized bed is maintained at a Froude number of less than 1 until
the resin has cured or dried completely so that the bundles can no
longer stick together.
[0056] In this impregnation, the bundles including parallel carbon
fibers held together by a carbonized binder can take up to 35% of
their own mass of carbonizable matrix former.
[0057] Further details of the impregnation process may be found in
European Patent Application EP 06 007 562.9, filed Apr. 11, 2006,
corresponding to U.S. Patent Application No. (Attorney Docket No.
SGL 06/09) entitled Process for the Impregnation of Carbon Fiber
Bundles, Resin-Impregnated Carbon Fiber Bundle, Shaped Body and
Intermediate Body for Silicization, filed on the same day as the
instant application and assigned to the same assignee as the
instant application.
[0058] A molding composition is produced in the above-described way
from the impregnated fiber bundles, a carbonizable matrix former
and, if appropriate, auxiliaries.
[0059] A green body having the desired shape, for example in the
form of a brake disk, is produced from the molding composition
through the use of a mold which is close to the final shape.
Pressing is typically carried out at a pressure in the range of
from 1.5 to 5 N/mm.sup.2 and a temperature in the range of from 120
to 200.degree. C. Preference is given to using a hot molding press.
After curing, the tool is opened and the green body which is close
to the final shape is taken out.
[0060] In the next step, the carbonizable matrix former in the
green body is converted into a carbon matrix so as to yield a
carbonized shaped body. For this purpose, the green body is heated
slowly in a protective gas atmosphere, i.e. under nonoxidizing
conditions, to a temperature at which pyrolysis of the matrix
former to yield a residue consisting essentially of carbon occurs
and is maintained at this temperature for a particular time.
Heating has to be carried out sufficiently slowly to avoid
formation of cracks in the shaped body due to sudden release of
gaseous pyrolysis products. Heating is typically carried out at a
rate of 1 K/min to a temperature of 900.degree. C., which is then
maintained for about one hour. The body is subsequently slowly
cooled down to room temperature again. During carbonization, the
shaped body experiences a decrease in mass and correspondingly an
increase in porosity as a result of the elimination of gaseous
pyrolysis products from the matrix former. The density of the
carbonized shaped body is typically from about 1.3 to 1.45
g/cm.sup.3.
[0061] In order to compensate for the decrease in mass, the
carbonized shaped body can be re-impregnated with a carbonizable
matrix former (resin or pitch) and then carbonized again.
[0062] The carbonized shaped body can be subjected to further
mechanical working if necessary. For example, in the case of a
brake disk, cooling channels can be cut out or holes can be
introduced.
[0063] However, it is also possible to carry out such shaping
through the use of lost cores during production of the green body.
The production of shaped bodies containing hollow spaces in a
pressing process through the use of lost cores having external
dimensions which correspond to those of the hollow space to be
produced and are introduced into the molding composition at
positions of the hollow spaces to be produced, is prior art. The
cores are formed of a material which at the pressing temperature
decomposes thermally leaving virtually no residue and thus leaves
behind the desired hollow space.
[0064] The porous carbonized shaped body is re-densified by
deposition of a carbon matrix through the use of chemical vapor
infiltration (CVI), so that its density increases to values in the
ranges from 1.6 to 1.8 g/cm.sup.3. The deposition of carbon through
the use of chemical vapor infiltration is prior art. A suitable
carbon-donating gas is methane.
[0065] The time required for the re-densification through the use
of CVI can be reduced by about 10-30% if the fiber bundles are
impregnated with a carbonizable matrix former before being mixed
into the molding composition, so as to yield a denser green body.
Re-impregnation of the carbonized shaped body with a carbonizable
matrix former which is subsequently carbonized, also effects a
comparable shortening of the time required for CVI.
[0066] The orientation of the fiber bundles in the shaped bodies
produced according to the invention can be random, i.e.
statistically distributed. This is preferred when the body is
subjected to an approximately uniform load in all spatial
directions.
[0067] However, in the case of shaped bodies which are subject to a
particular load in a particular direction, orientation of the fiber
bundles according to stress is desirable. This can be achieved by
simple measures during introduction of the molding composition
containing the fiber bundles into the mold, for example by use of a
charging grate.
[0068] In the case of brake disks, preference is given to aligning
the fiber bundles in a tangential direction corresponding to the
tensile stress which occurs. A charging grate which has a plurality
of concentric rings is used for this purpose.
EXAMPLE
[0069] A variant of the process of the invention is described below
for the example of the production of a brake disk.
Production of the Fiber Bundles
[0070] Carbon fiber rovings each including 50,000 virtually
parallel individual filaments are impregnated with a phenolic resin
(Norsophen 1203 from the firm Hexion) so as to form a prepreg
having a resin content of 35% by mass and a weight per unit area of
320 g/m.sup.2.
[0071] This prepreg is continuously compacted at a speed of 1 m/min
and a pressure of 1 MPa (10 bar) on a belt press at a temperature
of 180.degree. C. to form a laminate sheet having a thickness of
200 .mu.m and is at the same time cured so as to yield a
dimensionally stable laminate sheet.
[0072] The UD laminate sheet is subsequently divided into
individual bands having a width of 50 mm each. These are cut as
described above to yield segments (fiber bundles) having a length
of 9.4 mm and a width of 1 mm.
Production of the Molding Composition
[0073] 2400 g of the fiber bundles are transferred to a tumble
mixer, 600 g of powder resin (phenolic resin SP 227 from the firm
Hexion as a carbonizable matrix former) are poured over them, and
the fiber bundles and the resin are mixed for 5 minutes.
Production of the Green Body
[0074] Referring now to the figures of the drawings in detail and
first, particularly, to FIG. 1 thereof, there is seen a
diagrammatic illustration of the charging of the mold. A mold 1,
having a cavity which corresponds to the geometry of the brake disk
to be produced, is charged with the molding composition containing
the fiber bundles 3. In order to achieve a preferred tangential
alignment of the fiber bundles 3, a charging grate 2 which has a
plurality of concentric rings having a spacing of less than or
equal to the length of the fiber bundles 3, is used. FIG. 2
diagrammatically shows how the charging grate 2 is disposed on the
mold 1. During filling, the molding composition containing the
fiber bundles 3 falls through the intermediate spaces between the
concentric rings of the charging grate 2 and the fiber bundles 3
take on the substantially tangential configuration shown
diagrammatically in FIG. 3. The charged mold is subjected to a
pressure of 4.0 N/mm.sup.2 and a temperature of 160.degree. C. on a
hot molding press for 30 minutes and subsequently opened. During
pressing, the phenolic resin cures. A green body which is close to
the final shape in the form of a brake disk is obtained.
Carbonization
[0075] The green body is heated at a heating rate of 1 K/min to a
temperature of 900.degree. C. under a nitrogen atmosphere in a
protective gas furnace. In this case, the phenolic resins are
decomposed to leave a residue consisting essentially of carbon.
This temperature is maintained for one hour. The carbonized shaped
body is then cooled to room temperature.
Re-densification Through the Use of CVI
[0076] A carbon matrix is deposited in the porous carbonized shaped
body through the use of chemical vapor infiltration (CVI). The CVI
is carried out at 1100.degree. C. using methane as a carbon donor.
As a result of the deposition of carbon, the density of the
carbonized shaped body increases from 1.3 to 1.8 g/cm.sup.3.
Characterization of the Brake Disk
[0077] A coefficient of friction of .mu.=0.5 to 0.6 was determined
on a pendulum test rig.
[0078] Due to the tangential orientation of the fiber bundles, the
strength of the brake disks determined in a bending test increased
by 12-20% as compared to brake disks having a random configuration
of the fiber bundles.
* * * * *