Note: Descriptions are shown in the official language in which they were submitted.
-- 1321869
RD-18,769
FIsER-CoN~AINING COMPo8I~E
U.S. Patent No. 5,01tii,540, issued May 14, 1991
for Fiber-Containing Composit6!, Borom et al, assigned to
the assignee hereof, discloses~ a process where fibrous
material is coated with boron nitride and a
silicon-wettabl~ material, the coated fibrous material is
admixed with an infiltration-E)romoting material which is
at least partly elemental carbon and the mixture is
-~ B ~ormed into a ~ ~ rm which is infiltrated with molten
silicon producing a composite containin~ boron nitride
coated fibrous material.
This invention relates to the production of a
composite containing boron nitride-coated fibrous
material in a matrix containing silicon carbide and/or
boron-containing silicon carbide phase and a phase of a
solution of boron and silicon.
UOS. Patent No~. 4,120,731; 4,141,948;
~,148,89~; 4,110,455; 4,238,433; 4,240,~35; 4,242,106;
4,247,304; 4,353,953 and 4,626,516, assigned to the
assignee hereof, disclose silicon in~iltration of
materials which include carbon, molybdenum, carbon-
: coated diamond and/or cubic boron nitride, and blends of
carbon with silicon carbide, ~oron nitride, silicon
nitride, aluminum oxide, magnesium oxide and zirconium
oxide.
Many effcrts have been extended to produce fiber
reinforced, high temperature materiaIs. Structures of
carbon f iber reinforced carbon matrices (carbon-carbon or
C/C composites) have been used in aircra~t construction
but they have the disadvantaye of poor to no oxidation.
." ,. .
, . ~, ' .!
.
RD-18,76g
~L~21~9
resistance (i.e. they burn). High ~trength carbon fibers
were infiltrated with molten silicon with the hope that the
silicon matrix would protect the carbon filaments. However,
the carbon filaments converted instead into relatively weaX,
irregulAr columns of Si~ crystals resulting in composites
with low toughness and relatlvely modeRt strength.
As an alternative approach, attempts have been
made to incorporate SiC t~pe fibrous material in a silicon
matrix by the prooess of silicon iniltration. There are a
numb~r o~ problems when silicon carbide fibrou~ material i~
infiltrated with silicon. Even ~hough SiC has limited
solubility in molten 6ilicon, thi8 801ubility leadæ to
transport and recry~tallization o~ SiC thereby cau~ing the
SiC fibers to lose sub~tantial strength. Al o, silicon
carbide forms a strong bond with silicon which results in
brittle fracture of ~he composite.
The present prooess utilizes a molten solution of
boron and ~ilicon to i~filtrate a preform containing a
carbon-containing fibrouc material suçh as, for example,
carbon or silicon carbide ~ibrous material to produce a
composite in which the fibrous matPrial ha~ not been
affected, or has not been significantly deleteriously
affected by processing co~dition~. In the present proceY~, :
boron nitride, whioh i~ coated on the ~ibrous material, bars
25 any signiicant contact of the fibrous material with the
infiltrant. Since boron nitride is not wettable by ~ilicon,
a coating sf a silicon-wett~ble material is deposited on the
boron nitride coating. Material~, which include elemental
carbon, are admixed with the coated fibrou~ material
preferably to strengthen ~he preform, enhance i~filtration
and provide disper~io~ ~trengthening for the matrix. Tho
mixtur~ orm~d into a pre~orm, ~nd a molten 301ution of
-2-
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` 132186~ RD-18,769
boron and silicon is in~iltrated into the preform to produce
the present c~mposite.
Th~se killed in the ar~: will gain a further and
better understanding of the present invention from the
detailed description ~et forth below, considered in
conjunction with the figures ac~ompanying and ~orming a part
of the sp~cification, in which:
FIGURE 1 i~ a Rcanning electron micrograph of an
as-fractured cross-s0ction of the prese~t composite which
19 was produced with coated carbon fabric and which displayed
fiber pullout on fra~ture;
FIGURE 2 is a ~canning electron micrograph of a~
a~-fractured cross-section o~ a composite which was produced
; with uncoated carbon fabric and which displayed brittle
fracture, i.e. no fi~er pullout; and
FIGURE 3 i~ a scanning electro~ micrograph of an
as-fractured cross-~ection o~ the pre~ent composite which
w~s produced with coated bundles of carbon fiber and which
: sh~ws fiber pullout, the boron Aitride coating intact around
th~ fibers and in~iltrant penetration between th~ fibers,
~: i.e. ~ backgro~nd pha~e containing a ~oIution of ~lemental
boron and elemental ~ilicon and boron-~ilicon precipitates.
Briefly tated, th~ pre~ent proces~ for produci~g :
a composite with a porosity of le~s than abouk 20% by volume
comprised of, based sn ~he volume of the composite, a coated
fibrous material of W~i5~ the ~i~Qrou~ materi~l component
eomprises at least abQut 5% by vol~me, at lea~t about 5% ~y
volume of a phase formed in situ of ~ilicon carbide and/or
: boron-conta~nin~ ~illcon c~r~ide and at l~a~t about 1% by
volume of a pha~e o~ a solution o elem~ntal ~oron ~nd
elemental ~ilico~, compris~ the ~ollowin~ step~: :
(a~ depositing bsron nitrid~ on a
carbon-~ontaining ~ibrou~ ~a~erial producing a coating
-3-
"~
~ .
.: , ~ . - .. , .: : : : . . . . . . ~ .
~ . : . : .. .. : ; : . ::: . ::: : .:.. :: . .:::: , : :: . ,:. .: ~:
1321869 RD-18,769
thereon which leaves no significant portion of said fibrous
material exposed;
(b) depositin~ a silicon-wsttable material on said
boron nitride-coated fibrous material producing a coatinq
thereon which leaves no fiignificant portion of said boron
nitride exposed, said silicon~wettable material adh~ring to
boron nitride sufficiently to ~orm said coatin~ thereon and
being wetted by silicon sufficiently to produc~ said
composite;
(c) admixing an infiltration-promoting material
containing elemental carbon with the resulting coated
fibrous material producing a mixture wherein the fibrous
material component of ~aid coated fibrous material comprises
at least about 5% b~ volume of said mixture,
(d) forming said mixture into a preorm having an
open porosity ranging ~rom about 25% by volume to about 90%
by volume of the preorm;
(e) providing an infiltrant comprised o~ boron and
silico~ containing elemental ~oron i~ solution in silico~ in
an amount of at le~st about 0.1% by weight of elemental
silicon;
(~ contaoting said preform wi~h infiltrant-asso-
ciated infiltrating ~eans whereby said infiltrant is infil~
tra~.ed into said proorm;
(g) heating the resulting structur~ to a
temperature at which said in~iltrant is molten and anfil-
tratin~ said molten in~iltrant into ~aid preform to produce
an infiltrated product having the compositio~ of said com-
posite, ~aid pre~orm co~taining ~ufficie~t ~lemental carbon
to react with ~id infiltran~ to fo~m said compo~i~e; and
(h) cooling said product to produG~ ~aid compos-
ite.
. ~
.. .
~ - , - ~ , .,, : :.
~ 32 ~ 8 69 RD-1~ 76g
As u ed herein "elemental carbon" or "carbon" in-
clude~ all ~orms of elemental carbDn including graphite.
As used herein "fibrous~material" in11udes fibers,
filaments, str nd , bundles, whisk~ers, cloth, felt and a
combination th~reof.
Reference herein to a ibrous material of silicon
carbide, includes, among others, presently available
materials wherein silicon carbide envelops a core or
substrate, and which generally are produced by chemical
vapor deposition of silicon carbide on a core or substrate
such as, for example, elemental carbon or tung~ten.
In the present invention, the fibrous material to
be coated with boron nitride can be amorphou~, crystalline
or a mixture thereo~. The crystalline ~ibrous material can
be sinqle crystal and/or polycrystalline. The ~ibrous
material i8 a carbon-containing material which generally
contains carbon in an amount of at least about 1% by weight,
frequently ~t least about 5% by weight, of th2 ~ibrous
material. Gener~lly, the fibrou~ material to be coated with
boron nitride i8 selected from the group con~isting o~
elemental carbon, a SiC~containing material and a
combination ~ereof. The SiC-containing material, excluding
any core or substrate material, contains at le~st about 50%
by weight of silicon and at l~ast about 25% ~y weight of
carbon, ba~ed on the we~ght of the material. Examples of
SiC-containing materials ~re ~ilicon carbide, Si-C-0,
Si-C-0-N, Si-C-0-Metal and Si-C-0-N-~etal where the Metal
c~mponent can vary but fre~ue~tly is Ti or Zr. There are
processes known in the art whieh u~e organ~c precursor~ to
producs Si-C containing ~ibers which ~ay introduc~ a wide
variety of elemen~s i~to the fiberQ! ~
The fibro~ material to be coa~ed with boron
nitride a~ stable at ~he temperatur~ of ~he pre3~nt process.
:
-5-
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. . .~, ~
1~21869 RD-18,769
Also, this fibrous material preferi~bly has ~t ro~m
temperature, i.e. about 22C, in air a minimum tensile
strength of about 100,0~0 psi and a minimum tensile modulus
of ab~ut 25 million p6i. Preferably, the carbon fiber is a
high strength, high modulus fiber ,such a~ derived from ~he
pyrolysis of rayon, polyacrylonikrile or pitch.
The present fibrous material can be used as
; continuous ~ilament. Alternatively, it can be used a
discontinuous fibers, which frequently have an aspect ratio
of at least 10, and in o~e embodiment it i5 higher than 50,
and yet in another embodiment it is higher than 1000.
Generally, in a random mixing mode, low aspect ratio fibers
are preferred since they pack better and produce high
density preforms. On the o~her hand, generally in an
ordered array, high aspect ratio fibers are pre~err~d sin~e
they produce composites with the highe~t density of
rein~orcement and the best mechanical properties.
Generally, th* present ibers range from about 0.3 micron to
about 150 miorons in diameter, and ~rom about lO microns to
about 10 centimeters in length or longer. Fre~uently, ~he
fiber is continuous and as long as desired.
Continuous fiber~ can be ~ilamont-wound to form a
- cylindrical tube. They can als~ be formed into sh~ts by
plaeing long lengths of fiber next to ~d parallel to one
another. Such sheets can consist of single or multiple
layers of filament~. Continusu~ filaments can also be
woven, braided, or otherwi~e arrayed into desired
configuratio~s. When fiber~ are ontinuou~ or very long the
use of th~ term "asp~ct ratio" i~ no lo~ger u~e~ul.
I~ one em~odiment, fibers ~roquently have a
diameter great~r than about S ~icron~ or ~reat~r than dbout
10 micron~, and are as long a~ de~ired for producing the
-6-
1 3 2 1 8 6 9 RD-la 769
preform. Frequently, each fiber is longer than ab~ut 1000
microns or longer than about 2000lmicron~.
In carrying out the presen~ process, boron nitride
is coated on the fibrous material to produce a coating
thereon which leaves at lea~t no significant portion o~ the
fibrous material expos~d, and preferably, the entire
material is coated with boron nitride. Preferably the
entire wall o~ each iAdividual fiber is totally coated with
~oron nitride leaving none of the wall exposed. The ends of
the ~iber may be exps~ed but su~h exposure i5 ~ot considered
significant. Most preforably, the entire ~iber i5 totally
env~loped, i.e. encapsulated, with a coating of boron
nitride. The boron nitride coating ~hould be continuou~,
free of any ~ignificant porosity and preferably it is pore-
free. Preferably, the boron nitride c~ating is uni~orm or
at least significantly uniform.
The boron nitride coating can be deposited on the
fibrous material by a number of known technigue~ under
: conditions which have no significant deleterious effect on
the material. Generally, the boron ~itride coating can be
deposited by chemical vapor deposition by reactions such as:
(g) ~ 3~N( )+3~2(~) (1)
B3N3~3C13(g~ ' 3BN(æ)~3~Cl(g~ (2)
:
BCl3(g)~N~3(g) ~ ~N(8)~3HCl(~) (3~ ~:
Generally, the che~ical vapor deposition of boron
nitride i~ carried out at temp~rature~ ranglng from about
: 900C to 1800C in a partial vacuum, with:~he particular
processing cQndition~ ~eing knQWn in the art or determinable
empirically.
,~
:
~ 3~ 869 RD-18,769
The boron nitride coating should be at lea6t
su~ficiently thick to be continuous and free of ~ignificant
porosity. Generally, its thickness ranges from about 0.3
micron~ to about 5 microns, and txpically it is ~bouk 0.5
microns. The p rticular ~hick~ess ~ tho c~ating is deter-
minable empirically, i.e. it should be su~ficient to prevent
reaction, or prevent significant reaction, between the
fibrous ~aterial and the infiltrant, i.e. it~ elem~ntal
silicon component, under th~ particular proce~sing
conditions used. During the infiltration proce~s, the boro~
nitride coating may or may not react with or di~solve in the
molten in~iltrant depending on the amount of elemental boron
in solution in elemental silicon. When a ~atu~ated solution
o~ boron and ~ilicon is used a~ an infiltrant, the boron
nitride coating will not react with or dissolv~ in the
molten infiltrant. However, when an unsaturated solution of
boron and ~ilicon is used as infiltrant, ~he boron nitride
coating may or may not react with or dis~olve in the molten
infiltrant and this is determi~able empirically depending
largely on time, temperature and concentration of boron in
solutio~. For example, for a given unsaturated olution,
the boron nitride coating will survive b~tter at lower
temperatures and/or shorter times. Generally, infiltration
time increase~ wi~h the size of the prefo~m. Larger-sized
preorms, there~ore, may require thicker boron ~itride
coatings when the infiltrant i~ an unsaturated solution.
However, for a given infiltration time and temperature, a~
the concentration o~ boron in ~olution is increased, the
tendency o~ the boron ~itride coating to react wi~h or
dissolve in ~he ~olten infiltrant usually decrea e~.
A ~umber of tech~igue~ can ~e u~ed to determine i~
tho boron nitricle coating ~urvived. For example, i~ the
composite exhi~it~ fiber pull-out on ~racture, ~hen the
,.. ~
-- ....
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- :: ::,: : . , , "
` 13~869 RD-1~,769
boron nitride coating has ~urvived. Also, ~canning electron
microscopy of a cross~section of the pr~s~nt composite can
detect a boron nitride coating on the fibrous material.
Th~ boron nitride-coated material is then coated
with a silicon-wettable material leaving no si~nificant
portion of the boron ~itride expoæed, and preferably leaving
none of the boron nitride coating exposed. Most preferably,
the coatin~ of ~ilicon-wettable mat~rial totally envelops,
i.e., encapsulatesO the boron nitrido-coated material.
Specifically, the coating of ~ilicon-wettable material
should be free of ~ignificant porosity and pre~erably i~
pore-free. Also, preferably, the coating i~ uniform or at
least significantly uniform. Gonerally, the thickne~s o~
the coating o~ salicon-w~ttable material ranges from about
500 An~stroms to about 3 microns, and typically it is about
O.5 microns. The particular thickness of the coating is :~
determinable empirically and dapends largely on the rate of
consumption of the coating, if any, and the particular
composite desired.
The silicon-wettabl~ material i5 a ~olid which
covars the boron nitride ~nd adheres ~uffici~ntly to form
the present coatin~ thereon. AI~o, throughout the proqe~t
process it r~mains a ~olid. The ~ilicon-wettable material
~: : should be ~uf~iciently wettod by the i~iltrant to enable
the productisn of th~ pr~sent composit~ ~aving a porosity o~
less th n about 20% by volume. The $nfiltrant should have a
contact or wetting ~ngle against ~he silicon-wettabl~
material o~ le~ than 90 degr~es to allow the infiltration
to occur by capillarity.
Representative of u~ful silicon-wettable mate-
rials i~ elemental carbos~, metal earbide, a metal which
reacts wi~h silicon to form a ~illcide, a metal nitricile ~uc~
as ~ilicon nitride, and a metal silicido. Elome2lt~1 carbon
o9_
.'~1'`~
,'1
r '~ ."~,
RD-18,76g
.~3218~
is preferred and usually, it i~ deposited on the boron
nitride-coated material in the ~o~m of pyrolytic carbon.
Generally, the metal carbide is a carbide of ~ilicon,
tantalum, titanium or tungsten. G,enerally, the metal
silicide is a ~ilicide o~ chro~ium, molybdenum, tantalum,
titanium, tungst0n and zirconium.
The metal which reacts w.ith ~ilicon to ~orm a
silicide thereof as well as the silicide must have melting
point higher than the melting point o~ 8ilicon and
preferAbly higher than about 1450C. Generally, ~he metal
and silicide thereof are solid in ~he pres~nt proces~.
Repre~entative of such metals i8 chromium, mol~bdenum,
tantalum, titanium and tungsten.
Rnown technigues can b~ used to deposit the
coating of silicon-wettable material which generally i8
deposited by chemical vapor deposition using low preasure
technigues.
The metal carbide or metal silicide coating can be
directly deposited from the vapor thereof. Alternatively,
the metAl carbide coating can be formed in situ by initially
depositing car~on o~ ~he boron nitride coati~g ~ollowed by
depositing metal thereon under conditions which form the
metal carbide. If desired, metal silicide coating ean be
produced ~y initially depositing the mstal on the boron
nitride coatlng followed by deposition o~ silicon ~nder
conditions ~hich ~orm the met~l ilicide.
An infiltration-promotin~ material i~ Admixed with
the re~ulting coat~d fibrou~ material to produ~e the de~ired
~ mixture. The infiltration-promoti~g ~te~ial i~ a matexial 30 which i~ wett~d by ~olten silicon a~d ~herefore by the
present iniltrant. The infiltration~promoting ~atorial as
well as ~ny reaction product thereof produced in the present
proce~s should not flow to any ~ignificant ext~nt snd
--10-
~, ~
. . .
~ . ; , : . - : .
RD-18,769
1321869
pref~rably is solid in the present: process. Also, the
infiltration-promoting material should have no ~igni~ica~t
deleterious effect on the present process or ~he resulting
composite. The partic~-lar composition of the
infiltration-promoting material ia; determinable emplrically
and depends largely on the particular composite de~ired,
i.e. the particular pr~perties des;ired in the composite.
However, the infiltration-promoti~lg material alway~ c~ntains
sufficient elemental carbon to enable th~ production of the
present composite. Speci~ically, the pr~orm should contain
sufficient elemental carbon, generally mo~t or all of which
may be provided by the iniltration-promoting material and
some of which may be provided a~ a coatanq on tha boron
nitride-coated m~terlal, to react with ~he infiltrant to
:. 15 produce the present composite containing silicon carbide
: and/or boro~-containing 8i licon carbide formed in situ in an
amount of at least about 5~ by volume of the composite.
Generally, elemental carbon ranges from about 5% by volume,
or ~rom ~bout 10% or 20% by volu~e, to about 100% by volume,
of the in~iltration-prumoting material.
Th~ infiltration-promoting material also may
:; include a metal,.g~nerally in an amount o~ at least about 1%
by volume of tho i~iltration-promoting mat~rial, which
reacts with the infiltr~nt i~ the prese~t proce~s to form a
phase of a met~l silicide and/or boron-containing m~tal
silicide. Representative of such a metal i~ chromium,
molybdenum, tantalum, titanium, tungsten and zirconium.
Th~ in~iltration-promoting material may also
include a ceramic material, generally in an ~mount of ~t
~0 least about 1% by volume o~ the in~iltration-promotin~
material, which may or m~y ~ot react with th~ in~ltrant,
such a a ceramic carbide, ~ cer~mic nit~ide or a ceramic
: ~ilicide. Th@ cerami~ carbide i~ ~elected from the group
s: ,
; ~
. . -
RD-lB,769
1~21869
consiQti~g of b~ron carbide, molybdenum carbide, niobium
carbide, silicon carbide and titan;ium carbide. The ceramic
nitride is selected ~rom th~ group consisting of aluminum
nitride, ni~bium nitrid~, silicon nitride, titanium nitride
and 2irconium nitride. The ~eramic silicide is selected
from the ~roup ccnsisting o~ chromium silicide, molybdenum
silicide, tantalum silicide, titanium ~ilicide, tungsten
silicide and zir~onium silicide.
The infiltration-promoting material can be in ~he
form of a powder, a fibrou~ material or a combination
ther~of. When the in~iltration~promoting material is in the
form o a powder, it preferably has an average particle size
of less than about 50 microns, more prefera~ly less than
about 10 microns. The amount and type o~
infiltration-promoting material depends largely on the
particular composite desired and i8 det~rminable
empirically.
The in~iltration-promoti~g material should be
admixed with the coated fibrous material in a manner which
will not have a significantly deleterious effect on the
coatings o~ ~ilicon-wettable material and boron nitride.
Mixing can be carried out in a k~own an~ conventional
manner. In on~ embodiment, a ~lurry o~ the
infiltration-promoting mat~rial can b~ deposited ~hrough the
coated material to form a mixture. The ælurry ca~ be an
organic sl~rry containing kno~n bonding maan~, such as for
example ~poxy resin, to aid i~ forming the prefor~.
The mixture can b~ formed or shaped into a preform
or compact by a n ~ er o~ known tech~ique~. For example, it
can be extruded, injectio~ mold~d, die-pre3~ed, iso~tatic-
ally pres3ed or 81ip ca~t to produce the pre~orm of desired
size and shape. ~referably, the preform is o~ ~he size E~d
~hape desired o~ the eompo6ite. Generally, thero i6 no
-12-
: ~
RD- 18 , 769
1321~
significant difference in dimension between the preorm and
the resulting composite. Any lubri~ants, binders, or Y
similar materials used in shaping the mixture ~hould have no
si~nificant deleterious effect in the present process. Such
5 materials are of the type which evaporate on h~ati~g at
temperatures below th~ present in~iltration temperature,
preerably below 500C, leaving no deleteriou~ residue.
Generally, the present pre~orm ha~ an open porosi-
ty rangi~g from about 25% by volume to about 90% by volume
10 of the preform, and the parti~ular amount of ~uch open
porosity depends largely on the particular composite
desired. Frequently, ~he pre~orm has an open porosity
ranging from about 35% by volume to about 80% by ~olume, or
from about 40% by volume to about 60% by volwme, of the
15 preform. By ope~ porosity of the preform, it i8 meant
herein pores, voids or channels which are open to the
surface of the preform thereby making the interior surfaces
; accessible to the ambi~nt atmosphare or the in~iltrant.
Gener~lly, the preform has no closed poro~ity. By
20 closed porosity it i~ ~eant herein closed pores or void~,
i.~. pores ~ot open to tho ~ur~ace of the ~reform and
therefore not in contact with the ambient atmosphere.
Void or pore content, i.e. bo~h open and closed
porosity, can be determined by standard physical and
25 metallographic techni~ue~.
Pra~erably, the pores i~ the pre~orm are small,
preferably rangi~ from about 0.1 mirron to abDUt 5~
microns, a~d at least æignificantly or ~ubstantially
uni~ormly distributed through the proform thereby e~ab}ing
30 the production o a composi~e wher~in the matrix pha~e i8 at
least significantly or 8tlb3tantially uniforrnly di~tri~uted
through the composite. Al~o, thl~ would pr~duce ~ composite
--13-
RD-lB,769
132186g
wherein the matrix phase has a thicknes~ between the fibers
ranging from about O.l micron to about 50 microns.
The present boron-contai.ning infiltrant is
comprised of boron and silicon wherein boron range~
S generally from about 0.1% by weight to about 10% by weit3ht,
frequently from about l~ by weighl; to about 10% by weight,
and preferably from about 1% b~ weight to about 3% by
weight, o~ silicon. Boron rangint~ from about 0.1% ~y weight
to about l.6% by weight of silicon is in olution in
sili~on, and at about l.6% by weight it ~orms a saturated
~ solution. In excess of about l.6% by weight of silicon,
- boron forms a compound therewith whi~h precipitate~ as a
finely dispersed solid. Amounts of boron ~n excesæ of about
10% by weight of silicon provide no advantage. When tho
infiltrant is molten, the pre~ipitate u6ually is SiB6. When
the infiltrant is solid, the precipitate can b~ SiB3, Si~6
or a mixture thereof. The compounds of boron and ~ilicon
have no significant effect on the pre~ent process, i.e. they
are substAntially inert her~in. Preferably, the infiltrant
is a saturated solution.
The infiltrant can be ~ormed in a known ~anner.
For example, a Rolid particulate mixture of boron and
silicon can be heated in an atmosphere non-oxidizing with
r~spect to silicon to a temperature at which silicon is
molten and boron will dissolve ~herein.
In carrying out the pre ent proces~, th~ preform
~: is contacted with infiltrant-associated infiltrating means
: whereby the i~filtrant is infiltrated into the ~reform. The
infiltrating m~ans allow the molt~ in~iltrant to be
in~iltrated into the preform. For example, a ~tru~ture or
a~embly is formed compri~ed of the preform in cont3ct with
means that are in contact with the ~olid in~iltrant and
: which permit infiltration o~ the infiltrant, when ~olten,
-14-
. ~
` 1321~69
RD-18,769
into the preform. In one infiltration technique, the
preform is placed on a woven cloth of elemenk~l carbon, a
piece o~ infiltrant is also placed on the cloth, and the
resulting structure is heated to infiltration
temperature. At infiltration temperature, the molten
infiltrant migrates along the cloth and wicks into the
preform. After infiltration, the wicking carbon cloth
may be removed from the composite by diamond grinding.
In another technique, the infiltration procedure
can be carried out as set forth in U.S. Patent 4,626,516
which discloses an assembly that includes a mold with
infiltration holes and a reservoir holding elemental
silicon. The preform is placed within the mold and
carbon wicks are provided in the infiltrating holes. The
wicks are in contact with the preform and also with the
silicon and at infiltration temperature the molten
silicon migrates along the wicks into the preform.
U.S. Patent Nou 4,737,328 to C.R. Morelock ~or
INFILTRATION OF MATERIAL WITH SILICON, assigned to the
assignee hereof, discloses another infiltration technique
which comprises contacting the preform with a powder
mixture composed of silicon and hexagonal boron nitride,
heating the resulting structure to a temperature at which
the silicon is fluid and infiltrating the ~luid silicon
into the preform. After in~iltration, the resulting
porous hexagonal boron nitride powder is brushed off the
composite.
The present structure or assembly is heated to
infiltration temperature in a gaseous atmosphere in which
the molten silicon in~iltrant is inert or substantially
inert, i.e. the gaseous atmosphere should not significantly
oxi~ize ~he silicon. Suitable gaseous atmospheres include
argon, helium and hydrogen~ The gaseous atmosphere can be
. . . .
~321869 RD-18,769
at about atmospheric pressure but preferably it'is below
atmospheric pressure, i.e. preferably a partial vacuum is
used .
In a preferred embodimen~, the pres~nt . ~ruc~ure
S or assembly is heated to irlfiltration temperature in a
nonoxidizing partial vacuum wherein the residual gases have
~o significantly deleterious efect on ~aid ~t~ucture or
assembly and the present infiltrakion is carried out in such
nonoxidizing partial va~uum. Preforably, su~h nonoxidizing
partial vacuum is provided before heating i~ initiated. The
partial va~uum ~hould be at lea3t sufficien1: ~o avoid the
entrapment of poc~et~ of ~as which would lead to ~xce~sive
porosity, i.e. it ~hould be sufficient to produce the
present composite. Generally, such a partial Vacuum ran~es
from about O.Ol torr to about 2 torr, and usually from about
O.01 torr to about 1 korr to in~ure removal o entrapped gas
in the pr~form being infiltrated.
Ordinarily and as a practical matter, the furnace
used is a carbon ~urnace, i . e . a furna~e fabricatee~ from
20 elemental carbon. Such a furnace acts as an oxy~en getter
for the atmosphere within the furnace reacting with oxygen
to produce CO or C0 ~ and thereby provides a no~oxidizing
~ atmosphere, i.e. the residual ga~es have no significantly
: deleterious effect on the infiltr2nt. The present
infiltration cannot ~e carried out in air because the molten
: silicon would oxidize to form a denYe silica coatin~ bofore
any significant in~usion by the iniltru~t occurred. In
such i~a~kance where a carbon furnace is not us~d, it i~
pre~er~le to have asl oxygen ~etter pre~en~ in the ~u~nace
30 chamber, such as elem~ntal carbon, in order to in~ure the
maintenance of a nonoxidizir~g atmospher~. Alternativoly,
other nonoxidizirlg at~o-phere3 which have no ~ ant
:
'
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RD-18,769
1321869
deleterious effect on the structur,e within the urnace can
be used at partial vacuum~ of about 10 2 torr to 2 torr.
The present iniltration is carried ~ut at a tem-
perature at which the in~ ran~ is molten, which in this
instance is a temperature ~t whioh silicon is m~lten, and
which has no signi~icant deleterious effect o~ th~ preform
being infiltrated. The present infiltration temperature
ranges ~rom a t~mperature at which the ~ilicon i~ molten to
a temperature at which there is no 6ignificant vaporization
10 of the silicon. Molten silicon has a low viscosity. The
melting point o~ the silicon can vary depending largely on
the particular impurities which may be pre~ent. Gener~lly,
the pre~ent infiltration temperature ranges from ~reater
than about 1400C to about 1550C, and preferably ~rom about
1450C to about 1500C. The ra~e of penetration of the
infiltrant into the preform depend~ on the wetting of the
preform by the in~iltrant melt and the ~luidity o~ ~he melt.
With increase in temperature, the ability of the molte~
infiltr3~t to wet the preform improves.
In the present proces~, su~icient infiltrant is
infiltrated into the preform to produce the pr~sent compos-
ite. Specifically, the molten infiltra~t is mo~ile and
highly reactive with element~l carbon, i.~. it has an affin~
: ity for eleme~tal carbon, wetting it and reacting with it to
orm silicon carbide and~or boron-co~taining silicon
carbide. The molten infiltrant also bas an affinity for any
metal with which it reacts to for~ the ~ilicide ~hereo~. In
addition, suffi~ient infiltrant i~ in~iltrated into the
prefor~ to ~ill pores or void~ which may remain to produce
the pr~sent composite.
The period o~ ti~e required ~or i~filtration i~
det~rminable e~pirically ~nd depend larg~ly o~ ~he ~ize of
~he pre~orm and extent of i~iltration reg~ir~d. Generally,
-17-
:,`
,,.:
.~,, .~, . .
:. .
~32~g69 R~-18,769
it is completed in less than about 20 minutes, and o~ten in
less than about 10 minutes.
The resulting infiltrate~l body is co~led i~ an
atmosphore and at a rate which has no æigni~icant deleter
ious effect on it. Preferably it is furnac2 cooled in the
nonoxidizing partial vacuum to about room temperatur~, and
th~ resulting comp~site i~ recovercd.
The present composite has a porosity o less than
about 20% by ~olume, pre-ferably less tha~ about 10% or 5% by
volume, and more preferably less than about 1~ by volume, of
the composite. Most preferably, the compo~it~ is void- or
pore-free or has no significant or no detectable porosity.
Preferably, any voids or pores i~ the composite are small,
preerab1y less ~han about 50 microns or le85 than about 10
microns, and significantly or substantially uniformly
distributed in the compo~ite. Specifically, any voids or
pores are sufficiently uniformly distributed ~hroughout the
composite so that th~y have no significant deleteriou~
effect on its mechanical propextie3.
The present composite i~ comprised o~ boron
nitride-coated fibrous material a~d a matr~x phase. The
matrix phase is distributed through the bvro3l nitride-coated
fibrous material a~d generally it is substantially
completely space ~illing and usually it is interconnecting.
Generally, th@ boron nitride-coated ~ibrou~ material i~
totally enveloped by the matrix pha~e. The fi~rous material
component of the }ioro~ nitride-coated ~ibrou~ material
comprises at least abou~ 5% by volumo, or at lea~t about 10%
by volum~, or al: least about 30% by volume of th2 comFaosite.
Th~ matrix p~a3e c~ntains a phase or phase~ fonmed in ~itu
of silico~ carbide and/or boron-containing ilicon carbide
i~ an amount o~ at lea~t about 5% by ~olume or at lea~t
about 10~ by volume, or at lea~t about 30% by volume, or at
;
~18-
: '
~ .. .....
~32~ 869 RD-18,769
least about ~5% by volume, of the composite, and a phase in
an amount of at least about 1% by volume of the composite of
a solution of boron and ~ilicon wherein boron i8 at least
about 0.1% by weight of silicon.
The coated ~ibrou material in the composite is at
least coated with boron nitride which i6 at least dstectable
by scanning electron microscopy and gener~lly rang~s in
thickness from such detectable amount to about 5 microns,
freguently from about 0.5 microns to about 1.5 microns. The
particular amount of boron nitride in the compo~ite provided
by the boron nitride coating depends largely on the amount
of coat~d ibrous material pre~ent, the thicknes~ of the
boron nitride coati~g and th~ diameter of the fiber. There-
fore, the volume ~raction of boron nitride provided by the
coating is the balance of the volume fraction of all other
components of the composite. However, i~ one embodiment,
the boron nitride coati~g on the fibrous material in the
composite generally ranges ~rom less than about 1% by volume
to about 30% by volume, or ~rom about 1% by volume to about
10% by volume, of the total volume o~ boron nitride-coated
fibrous material. Al~o, in another embodiment, the boro~
: nitride coating on the fibrous material ga~erally ranges
from les~ than 1% by volu~e to ~bout 20% by volume, or from
: about 1% by volume to about 5% by volume, of th2 composite.
Generally, the fibrous material oomponent of the
boron nitride-coated ~ibrous material ran~o~ from about 5%
by volume to less ~ an about 75% by volume, or from about
10% by volume to about 70% by volum2, or from hbou~ i5% by
volume to le~s ~han about 65~ by volume, or from about 30%
by volume to about 60~ by volume, of ~he composite.
Generally, ~he boron nitride-coated material i~ di~tributed
throu~h the composite, and most often, it i8 di~tributed
~ignificantly uni~or~ly through the compo~ite. However, in
-19-
.
,
1321869 ~D-18,769
some cases it is desirable to have higher packing fractions
of the boron nitride-coat~d material in reyions of the
composite wher~ higher local strength or stiffness may be
desired. For example, in a structure having a long thin
part, such as a valve stem, it is advantageous to strengthen
the stem by increasing the volume fraction o~ the boron
nitride-coated mster~al in the stem region of the structure.
Generally, ~he phase formed in situ of ilicon
carbide and/or boron-containing ~ilicon carbide ranges from
about 5% by volume to about 89% by volume, or ~rom about 10%
by volume to about 79% by volume, or from about 30~ by
volume to about 59% by volume, or from about 45% by volume
to about 55% by volume, of the composite. Generally, the in
situ-formed carbide phase is distributed through the
composite, and preferably, it is distributed signiicantly
uniformly.
Generally, the phase comprised of a ~olution of
~lemental boron and eleme~tal silicon ranges from about 1%
by volume to about 30% by volume, or to about 10% by ~olume,
or to about 5% by volume, or to about 2% by volume of the
composite. In this phase, boron range~ from about 0.1% by
weight to about 1.6% by weight of sqlico~. More sensitiv~
techniques such as microprobe analysis or Auger electron
: spectroscopy may be re~uired to detect or determine the ;~
: 25 amount of boron dissolved in silicon. Generally, this phase
: of a solution of boron and silicon is di tributed through
the composite, and preferably, it is distributed
significantly uni~ormly.
The presen~ composite may contain a phase of a
compound of ~oron and Qilicon usually sel~cted ~rom the
group con~istins~ of SiB3, SiB6 and;a mixture thereo~ which
generally is di~stributed ~rough the oomposite. The
: ~ compound o~ boron a~d cilico~ ~sually ran~e~ ~rom an amount
-20~
,, ~
, .. .
- : ; . :~ - :,, .: : : ::: ~ :
-, : ": .... ;:. . - .. , ,.: :., , : .
:. ,,: .: ~ , -. , : .
132~69 RD-18,769
detectable by microprobe analysis up to about 30% by volume,
or up to about 5% by volume, or up to about 1% by volume, of
the composite.
The present composite may contain a phase of a
ceramic material di~closed as an infiltration-promoting
material herein, as well BS a boron-co~taining metal
silicide phase ~ormed in situ, generally ra~ging up to about
50% by volume, frequently from about 1% by volume to about
30% by volume, of the composite. Generally, the ceramic
material is distribute~ through the composite, and
preferably, it is distributed at least significantly
uniformly.
The present composite may contain a pha3e of a
metal which forms a silicide but which had not reacted with
the infiltrant silicon. I~ such instance, it would be
encapsulated by a metal silicide pha~e and/or a
boron-containing metal silicide phase. Such metal ~enerally
can range from about 0.5% by volume to about 5% by volume,
of the composite. Generally, such metal is distributed
through the composite, and preferably, it is distributed at
least signi~icantly u~i~ormly.
The matrix of the present composite may co~tai~ a
phase of eleme~tal carbon which has a significant amount of
gr~phitic structure, i.e. a les~ reactive type of carbon,
: 25 which had not ~ompletely reacted with ~he i~filtrant. In
such in~tance, this type of carbon would be totally encapsu-
lated by a phase of ~ilicon carbide andjor boron~co~taining
~ilicon carbide formed in situ. 5uch graphitic
structure-containing elemental car~on ge~erally ca~ range
30 from about 0.5~ by volume to about 10~ by volume, r~9uently
from about 1% by volume to about 5% by volume, of the
composit~. ~enerally, ~uch graphitic structure~containing
~lem~ntal carbon i8 distributed ~hrough the composite, and
-21-
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- ~ : ... .~ .: :: . ; . . ,
1~218 69 RD 18,769
pr~ferably, it i distributed at least significantly uni-
formly.
The present composite i9; at least bonded by sili-
con carbide and/or boron-containing silicon carbide phase
formed in situ. It may also be bonded by a metal silicide
phase and/or boron-containing metall 8i licid~ phase which
formed in situ. It may also be bonded by a pha~e formed by
the present in~iltrant comprised of a solution of boron and
silicon or a bond formed in situ between ~uch in~iltrant and
a cerami~ material.
The bonding of the boron nitride-coated fibrous
material in the present composite e~ables ~uch fibrous
material to impart significant toughness to the composite.
Specifically, the bonding of the boron nitride-coated
lS fibrous material is of a type which prevents brittle frac-
ture of the composite at room temperature, i.e. 25~C. By
brittle fracture of a eomposite it is meant herein that the
entire composite cracks apart at the plane of fracture. In
contrast to a brittle ~racture, the present compo~ite
exhibits fiber pull-out on fractur~ at room temperature.
Specifically, as tha present composite ~racks open,
generally at least about 10% by voluma, frequently at le~st
about 50% by volume and preferably all of the boron
nitride-coat d fibrou~ material pull out and does not ~reak
at the plane of fracture at room temperature.
One particular advanta~e of ~his invention i~ that
~ the present composite ca~ be produced directly i~ a wide
- range of sizes and h~pes which heretofore may ~ot have been
~ able to be manufactured or which may have reguired expen~ive
;~ 30 and tedious machining. For example, the pr~ ent composite
can be as short a~ about an inch or le~æ, or a~ long ~s
desired. It can be o ~imple, complax a~d/or hollow
geometry. For example, it can be produced i~ ~he ~orm o~ a
-22-
.~,
"' `
.~321~6.~ RD-18,769
tube or a hollow cylinder, a ring, a sphere or a bar having
a sharp point at one end. Also, since the preRent preform
usually does not differ significantly in dimension from the
resulting composite, i.e. since the present composite can be
produced in a predetermined configuration c~ prodetermined
dim~nsions, it requires little or no machining.
The present composite has a wide range of applica-
tions depending lar~ely on its particular composition. It
can be used, for example, a~ a wear r~sistant part, bearing
or tool insert, acou~tical part and high-temperature
structural components.
The invention i5 further illustrated by the
following example~ where, unless otherwise stated, the
procedure was ~s follows:
The in~iltrant was produced by forming a mixture
of boron and silicon powders wherein boron was pres0nt in an
amount of about 3% by w~ight of silicon. The mixture was
heated in a vacuum non-oxidizing with respect to 8ilico~ to
about 1450C a~d boron dissolved in the molten silicon
forming a saturated solution as well as a finely divided
:: precipitate of a compound of boron and silicon. The melt
was then cooled to room temperature in the same vacuum. The
resulting solid was then broken in~o small chunks.
; Comm~rcially ~v~ilable ~trands of elemental
carbon, i.e. fiber bundles, sold under the trademark
~agnamite AS4 were used. Each fiber bundle consisted cf
: ~ about 3000 fiber~ and waæ about 2 inche long and had a
: diameter of about 7 micron~. In air at room temperature the
fiber bundle has a ten~ile ~trength of about 550 ~housa~d
psi and a ~ensile modulu~ of about 34 millio~ p8i.
~ oven cloth with:a plain weave structure o~
elemental carbo~, i.e. fiber bundles, was u~od. The fiber
bundles are ~oIcl under the trademark ~agnamite AS4.
.
-~3-
,
. . ,- .~ . . .: , .;
~`" 1~21~69
RD-18,769
The binder was comprised o~ Epon 828TM and a
curing agent. Epon 828 is a resin formed from the reaction
of epichlorohydrin and Bisphenol A, which is a liquid at
room temperature and which ha~s an epoxide equivalent of
185~192. The curing agent was diethylenetriamine, a liquid
commonly called D~A which curles Epon 828 thereby
solidifying it. It was used in an amount o~ about 10% by
weight of Epon 828. The binder decomposes completely below
1300C.
The carbon resistance ~urnace used to form the
composite was contained in a vacuum belljar system.
The composite was fractured using a standard three
point bend test.
EXAMPL~ 1
A layer o~ carbon ~iber bundles was placed on-a
molybdenum screen and coated with boron nitride by the
following low pressure chemical vapor deposition process
utilizing the reaction B3N3H3C13 ~ 3BN + 3HCl.
The molybdenum screen containing the carbon
; 20 bundles was positioned at about the midpoint o~ the hot
zone of a pyrexTM/quartz/pyrexTM furnace tube.
Commercial trichloroborazine ~B3N3H3C13) was used.
A 1.00 gram sample of this solid was transferred in an
argon-filled glove box to a pyrex end-section which
contained a thermocouple vacuum gauge, a cold trap and a
vacuum stopcock.
The closed pyrex end-section was then taken out of
the glove box and attached to an end of the furnace tube
and to a vacuum system. The end-section containing the
trichloroborazine was then cooled using liquid nitrogen and
the furnace tube was opened to the vacuum system via the
- ~4 -
.,
r ~:
RD-18,769
~321g6~
stopcock of the pyrex end~section. After th~ 6ystem reached
a pressure lower than 0.020 torr, the furnace was heated t~
about 1050C. When the pressure h~d again dropped below
O.020 torr and the furnace temperature had stabilized, the
end-section containing the trichlornborazene was warmed by
an oil bath maintained at 60C, whereupon the ~olid began tD
vaporize, depositing BN and liberating gaseous HCl in the
hot zone of the urnace tube and producing an increase in
pressure.
The pressure was observed to reach as high a6
about 200 torr before stabilizing at about 50 torr. A~ter
two hours, the pr~ssure was found to have ~ecrea~ed to about
0.020 torr, whereupon the furnace wa~ ~hut down and the
system allowed to cool to room temperature be~ore opening
the tube and removing the sample.
Identification o~ he chemically vapor deposited
layer as BN was accompli~hed by means of electrical
resistance measurement and a guantitative ESCA analysis of a
film deposited in ~ubstantially the same manner on a SiC
disk surface. This film was amorphous to x-rays in the
as-deposited condition and appeared ~ully dense and smoo~h
at high ma~nification in the SEM.
Scanning electron micro~copy observation of the
ends of the coa ed bundle revealed that the coating was
continuous and smooth and about 1.5 ~m thick and le~t no
signi~icant portion of the ~i~er bundle~ exposed.
The boro~ nitride-coated ~iber bundles were then
coated in a standard manner with pyrolytic carbon derived
from the cracking of methane ga~ in a heated furnaceO The
carbon coating wa~ ~igniicantly uni~orm with a ~hickness of
about 0.5 micro~ and left no ~igni~icant portion o~ ~e
boron nitride coating.expo ed.
-2~-
RD~1~,769
1321~
A layer of ~oated caxbon fiber bundles were
aligned in a mold and a slurry compri~ed of 1 part (by
weight) crushed carbon felt, 1 part of binder, and 1 part
methyl-ethyl-ketone was poured around the ali~ned f~ber
bundles. The house vacuum was the~n applied to the mold
which produced a vacuum~cast preform containing coated
fibers submerged in the slurry of carbon flbers and binder.
This pre~orm was cured overnight in ~he mold At r~om
temp~rature and subsequently for an hour at ~bout 100C. At
this point the pre~orm had sufficient strength and could be
shaped by machining. The çrushed ear~on in the preorm
provided the channels and optimum pore ~ize for rapid
infiltration of the molten infiltrant by way of Si-C
reaction and wicking. Th~ preform wa~ di~mond cut into the
shap~ o~ a bar about 1.5 i~ches long, 0.3 inch wide and 0.1
inch thick and had an open porosity of about 50% by volume.
The carbon ~iber bundles comprised more than 5% by volume of
~he preform.
The preform a~d solid pieces of infiltrant were
placed on a woven carbon fabric, i.e. the infiltrating
means, which was contained i~ a BN-sprayed ~raphite tray.
This tray was then placed in a carbon r~sistance he ted
belljar ~urnace and 810wly heat~d at a rate of about lO~C
per minute to about 400~C in a vacuum of about 0.05 tors.
The slow haating at this stage assured ~}ow decomposition o~
; : ~he bindar which otherwi e may lead to di integration of ~he
preform. Sub3equent tD: this, the preform was rapidly hested
: : to about 1420C a~ which point th~ infiltrant was ~luid and
reacted with the carbon cloth and wicked i~to the preform.
:~ 30 A co~siderable amount of heat which was det~ct~d by a
: ~ thermocouple placed:on top of the preform was ~erated due
to the exothermic reactio~ of the in~iltr~t wi~h carbon
fiber~ in ~he matrix. The preform wa~ held for 5 mi~ute~
-260
t
RD-18,769
a32~8~9
under these conditions during which temperatures reached
about 1500~C. After this thP furnace power was turned off
and the infiltrated sampl~ was cooled to room temperature in
the vacuum of ~elljar.
The resulting composite had a poro~ity of le88
than about 1% by volu~e. It wa~ estima~2d to be comprised
of, based on the volume o~ the compo~ite, o~ about 70% by
volume o~ silicon car~id~ and/or boron-containing silicon
carbide phase, almost about 10~ by volume of a phase
comprised of a solution o~ elemental boron and elem~ntal
silicon wharein boron wa~ present in an amount o~ about 1.6%
by weight of silicon, a minor amount of a compound of boron
: and silicon, and about 20% by volume of boron nitride coated
carbon ~iber bundles of which the carbon fiber bundles
comprised about 18% by volume.
On fracture, the composite showed toughened
ceramic-like behavior. It exhibited fiber pull out with at
least about 50% ~y ~olume o the boron ni~ride-coated fiber
bundles pulled out. Th~ fractured cross-section is
illustrated in Figure 3 and show~ that the carbon fibers
were protected from reaction with the molten iniltrant.
All of the components of the composite wer~ distri~uted
through the composite.
This composite would ~e useful a~ a high
temperature ~txuctural material.
EXAMPLE 2
Unles~ otherwise stated herein, thi~ Example wa~
carried out in ~ubstantially the same manner as ~et forth in
Example 1.
Carbon cloth in~tead of carbon fiber bundle~ was
used to form the composite. Each piece o~ carbon cloth wa~
-27-
.
,., , .: , , , . : .,
RD-18,76~
~21~69
about 2 inches long, about ~ inch wide and had a thickness
of about 0.012 inch.
Four pieces of the carbon cloth were ooated with
boron nitride leaving no significant portion ther~of
exposed. The boron nitride coated cloth was then coated
with carbon leaving no cignificant portion of boron nitride
exposed.
All pi~ces of the coated carbon cloth, as well as
four pieces of uncoated carbon cloth, were dipped into the
slurry totally and then laid in the mold, one against th~
other ~orming a sandwich of eight alternating lay~r~ of
coated and uncoated carbon cloth. Some lurry was then
poured on top of th~ sandwieh which wa~ then vacuum-cast and
cured. The carbon cloth component of the coated carbon
cloth comprised more than 5% by volume of the resulti~g
preform.
The preform was cut and ground into the shape o~ a
bar about 0.3 inch wide, about 2 inche~ long and about 0.1
inches thicX.
Th~ preform was then infiltrated to form the
composite.
The resulti~g composite had a porosity o~ less
than about 1~ by volume. It was e~timated to be comprised
of, based on the volum~ of the composite, of about 70% by
volume of silico~ carbide and/or boron-conta~nin~ silicon
carbide, almo~t about 15% by volume of a phase comprised of
a solution of boron and silicon wherein boron i8 present in
an amount of about 1.6% by weight of silicon, a minor a~ount
of a compound of ~oron and ~ilicont and about 15% by volume
of boron nitride coated car~on cloth wherein the carbon
: cloth component compriæed about 13~ by volume.
On fracture, the compo ite ~xhibited fiber
pull-out, i.e. at least about 50% by volume o ~he boron
-28-
, .. ~
.~,, .~
~ ~21 g 69 RD-1~,769
nitride coated cloth pulled out. The fractured
cross-section is illustrated in Figure 1. ~ll o the
components of the composite wer~ distributed through the
composite.
This composite would be useful as a high
temperature structural material.
EXAMPLE 3
This Example was carried out in substantially the
same manner as 6et forth in Example 2 except that nona of
the carbon cloth was coated.
On fracture, the composite displayed brittle
fracture and no fi~er pull-out. The fractured Gross-section
is shown in Figure 2.
EXAMPLE 4
This Example was carried out in substantially tho-
same manner as disclosed in Exa~ple 2 except that every
layer o~ carbon cloth was coated with boron nitride and
carbon.
The resulting GompOSit~ had a porosity of le~s
than about 1% by volume. It was estima~ed to be compri ed
of, based on the volum~ of the composite, of about 60~ by
volume of silicon carbid~ and/or ~oron-conta~ silicon
carbide, almost about 10% by volume of ~ phase comprised of
~ : a solution of boron and silicon wherain boron is pre~ent in; 25 an amount of about 1.6% by weight oP silicon, a minor amount
of a compound of boron and ilicon, and about 30% ~y volume
of boron nitride c:oated car~on cloth wherein the carbon
cloth component comprised about 26% by volume.
.
-29
~'
1 3218 69 RD-lB, ~69
Thi~ eomposite would be useful as a high
temperatus~e structurial material.
.
~30-- .
~ .. .....
. . , :, :: . , : ~ : :, , i : . i -; . ::
