CA1321869C - Fiber-containing composite - Google Patents

Abstract

RD-18,769 FIBER-CONTAINING COMPOSITE
ABSTRACT OF THE DISCLOSURE
Fibrous material is coated with boron nitride and a silicon-wettable material, the coated fibrous material is admixed with an infiltration-promoting material which is at least partly elemental carbon and the mixture is formed into A preform which is infiltrated with a molten solution of boron and silicon producing a composite containing boron nitride coated fibrous material.

Description

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.

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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 .~ . ~ , ,.

` 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 "~
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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.

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~ 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.
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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 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.

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~ 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'`~
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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 ~, ~
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RD-18,769 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 : ~

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 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, . ~

` 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 :
' " ~ :

RD-18,769 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, :,`
,,.:
.~,, .~, . .

:. .

~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 .

, 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~

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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 ,. ~

- ~ : ... .~ .: :: . ; . . , 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 .~, "' `

.~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~

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.

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~

.

,., , .: , , , . : ., 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 , .. ~
.~,, .~

~ ~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.

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.

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.

.

~' 1 3218 69 RD-lB, ~69 Thi~ eomposite would be useful as a high temperatus~e structurial material.

.

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Claims (44)

1. A process for producing a composite with a porosity of less than about 20% by volume comprised of boron nitride-coated fibrous material and a matrix phase, said fibrous material component of said coated fibrous material comprising at least about 5% by volume of the composite, said matrix phase containing silicon carbide and/or boron-containing silicon carbide formed in situ in an amount of at least about 5% by volume of the composite and a solution of elemental boron and silicon in an amount of at least about 1% by volume of the composite, which comprises the following steps:
(a) depositing boron nitride on a carbon-containing fibrous material producing a coating thereon which leaves no significant portion of said fibrous material exposed;
(b) depositing a silicon-wettable material on said boron nitride-coated fibrous material producing a coating thereon which leaves no significant portion of said boron nitride exposed;
(c) admixing an infiltration-promoting material containing elemental carbon with the resulting coated fibrous material producing a mixture wherein the fibrous material component of said coated fibrous material comprises at least about 5% by volume of said mixture;
(d) forming said mixture into a preform having an open porosity ranging from about 25% by volume to about 90%
by volume of the preform;
(e) providing an infiltrant comprised of boron and silicon containing elemental boron in solution in silicon in an amount of at least about 0.1 % by weight of elemental silicon;

RD-18,769 (f) contacting said preform with infiltrant-associated infiltrating means whereby said infiltrant is infiltrated into said preform;
(g) heating the resulting structure to a temperature at which said infiltrant is molten and infiltrating said molten infiltrant into said preform to produce an infiltrated product having the composition of said composite, said preform containing sufficient elemental carbon to react with said infiltrant to form said composite; and (h) cooling said product to produce said composite.

2. The process according to claim 1 wherein said silicon-wettable material is selected from the group consisting of elemental carbon, metal carbide, a metal which reacts with silicon to form a silicide thereof, a metal nitride and a metal silicide.

3. The process according to claim 1 wherein said preform has an open porosity ranging from about 35% by volume to about 80% by volume.

4. The process according to claim 1 wherein said infiltration-promoting material is comprised of elemental carbon.

5. The process according to claim 1 wherein said fibrous material component is elemental carbon.

6. The process according to claim l wherein said fibrous material component is silicon carbide.

7. The process according to claim 1 wherein said matrix phase is comprised of said silicon carbide and/or boron-containing silicon carbide formed in situ and said infiltrant phase.

8. The process according to claim 1 wherein said fibrous component of said coated fibrous material comprises from about 10% by volume to about 70% by volume of said composite.

RD-18,769

9. The process according to claim 1, wherein in step (e) said infiltrant is comprises of boron and silicon wherein boron ranges from about 1% by weight to about 3% by weight of silicon containing boron in solution in said silicon ranging from about 1% by weight of silicon to an amount which forms a saturated solution with silicon.

10. A process for producing a composite with a porosity of less than about 20% by volume comprised of boron nitride-coated fibrous material and a matrix phase, said fibrous material component of said coated fibrous material comprising at least about 5% by volume of the composite, said matrix phase containing silicon carbide and/or boron-containing silicon carbide formed in situ in an amount of at least about 5% by volume of the composite and a solution of elemental boron and silicon in an amount of at least about 1% by volume of the composite, which comprises the following steps:
(a) depositing boron nitride on a carbon-containing fibrous material producing a coating thereon which leaves no significant portion of said fibrous material exposed;
(b) depositing a silicon-wettable material on said boron nitride-coated fibrous material producing a coating thereon which leaves no significant portion of said boron nitride exposed;
(c) admixing an infiltration-promoting material containing elemental carbon with the resulting coated fibrous material producing a mixture wherein the fibrous material component of said coated fibrous material comprises at least about 5% by volume of said mixture;
(d) forming said mixture into a preform having an open porosity ranging from about 25% by volume to about 90%
by volume of the preform;

RD-18,769 (e) providing an infiltrant composition comprised of boron and silicon wherein boron ranges from about 0.1%
by weight to about 10% by weight of elemental silicon;
(f) contacting said preform and said infiltrant composition with infiltrating means whereby molten infil-trant is infiltrated into said preform;
(g) heating the resulting structure to a temperature at which said infiltrant composition is molten resulting in a molten infiltrant containing elemental boron in solution in an amount ranging from about 0.1% by weight of silicon to an amount which forms a saturated solution with silicon and infiltrating said molten infiltrant into said preform to produce an infiltrated product having the composition of said composite, said preform containing sufficient elemental carbon to react with said infiltrant to form said composite; and (h) cooling said product to produce said composite.

11. The process according to claim 10, wherein in step (e) said infiltrant composition is comprised of boron and silicon wherein boron ranges from about 1% by weight to about 3% by weight of silicon and wherein in step (g) said molten infiltrant contains elemental boron in solution in an amount ranging from about 1% by weight of silicon to an amount which forms a saturated solution with silicon.

12. A process for producing a composite with a porosity of less than about 10% by volume comprised of boron nitride-coated fibrous material and a matrix phase, said fibrous material component of said coated fibrous material comprising at least about 10% by volume of the composite, said matrix phase containing silicon carbide and/or boron-containing silicon carbide formed in situ in an amount of at least about 5% by volume of the composite and a solution of elemental boron and silicon in an amount RD-18,769 of at least about 1% by volume of the composite, which comprises the following steps:
(a) depositing boron nitride on a carbon-containing fibrous material producing a coating thereon which leaves no significant portion of said fibrous material exposed;
(b) depositing a silicon-wettable material on said boron nitride-coated fibrous material producing a coating thereon which leaves no significant portion of said boron nitride exposed;
(c) admixing an infiltration-promoting material containing elemental carbon with the resulting coated fibrous material producing a mixture wherein the fibrous material component of said coated fibrous material comprises at least about 5% by volume of said mixture and is sufficient to produce said composite;
(d) forming said mixture into a preform having an open porosity ranging from about 35% by volume to about 80%
by volume of the preform;
(e) providing an infiltrant comprises of boron and silicon wherein boron ranges from about 1% by weight to about 3% by weight of elemental silicon and contains boron in solution in an amount ranging from about 1% by weight of silicon to an amount which forms a saturated solution with silicon;
(f) contacting said preform with infiltrant-associated infiltrating means whereby said infiltrant is infiltrated into said preform;
(g) heating the resulting structure to a temperature at which said infiltrant is molten and infiltrating said molten infiltrant into said preform to produce an infiltrated product having the composition of said composite, said preform containing sufficient elemental carbon to react with said infiltrant to form said composite; and RD-18,769 (h) cooling said product to produce said composite.

13. The process according to claim 12, wherein said fibrous material component is elemental carbon.

14. The process according to claim 12, wherein said fibrous material component is a SiC-containing fibrous material.

15. The process according to claim 12, wherein said infiltration-promoting material is selected from the group consisting of a powder, a fibrous material and combinations thereof.

16. A composite comprised of boron nitride-coated carbon-containing fibrous material and a matrix phase, said matrix phase containing silicon carbide phase and/or boron-containing silicon carbide phase and a solution phase comprised of a solution of boron and silicon wherein boron is present in an amount of at least about 0.1% by weight of silicon, said carbide phase being present in an amount of at least about 5% by volume of said composite, said solution phase being present in an amount of at least about 1% by volume of said composite, said fibrous material of said boron nitride-coated fibrous material being present in an amount of at least about 5% by volume of said composite, said boron nitride coating being at least detectable by scanning electron microscopy, said composite having a porosity of less than about 20% by volume.

17. The composite according to claim 16 wherein there is also present a compound of boron and silicon is present ranging up to about 10% by volume of said composite.

18. The composite according to claim 16 wherein said carbide phase ranges from about 15% by volume to about 79% by volume of the composite.

RD-18,769

19. The composite according to claim 16 wherein said fibrous material is comprised of elemental carbon.

20. The composite according to claim 16 wherein said fibrous material is comprised of silicon carbide.,

21. The composite according to claim 16 wherein said matrix phase is comprised of said carbide phase and said solution phase.

22. The composite according to claim 16 wherein said fibrous material ranges from about 10% by volume to about 70% by volume of said composite.

23. The composite according to claim 16 having a porosity of less than about 10% by volume.

24. The composite according to claim 16 wherein said fibrous material is in the form of continuous filaments.

25. A composite comprised of boron nitride-coated fibrous material and a matrix phase, said fibrous material of said boron nitride-coated fibrous material being present in an amount ranging from about 10% by volume to about 70% by volume of the composite and being selected from the group consisting of elemental carbon, a SiC-containing material containing by weight of the SiC-containing material at least about 50% by weight of silicon and at least about 25% by weight of carbon, and a combination thereof, said boron nitride coating being at least detectable by scanning electron microscopy, said matrix phase being comprised of a phase of silicon carbide and/or boron-containing silicon carbide and a solution phase, said solution phase being comprised of a solution of boron and silicon wherein boron is present in an amount of at least about 1% by weight of silicon, said carbide phase being present in an amount of at least about 10% by volume of said composite, said solution phase being present in an amount of at least about 1% by volume of said composite, RD-18,769 said matrix phase totally enveloping said boron nitride-coated fibrous material, said composite having a porosity of less than about 10% by volume.

26. The composite according to claim 25 wherein there is also present a compound of boron and silicon is present ranging up to about 10% by volume of said composite.

27. The composite according to claim 25 wherein said carbide phase ranges from about 10% by volume to about 79% by volume of said composite.

28. The composite according to claim 25 wherein said fibrous material ranges from about 15% by volume to less than about 65% by volume of said composite.

29. The composite according to claim 25 wherein said fibrous material is comprised of elemental carbon.

30. The composite according to claim 25 wherein said fibrous material is comprised of silicon carbide.

31. The composite according to claim 25 wherein said fibrous material is comprised of continuous filaments.

32. The composite according to claim 16, wherein said matrix phase contains a ceramic carbide selected from the group consisting of boron carbide, molybdenum carbide, niobium carbide, and titanium carbide.

33. The composite according to claim 16, wherein said matrix phase contains a ceramic nitride selected from the group consisting of aluminum nitride, niobium nitride, silicon nitride, titanium nitride, and zirconium nitride.

34. The composite according to claim 16, wherein said matrix phase contains a ceramic silicide selected from the group consisting of chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide, and zirconium silicide.

RD-18,769

35. A composite comprised of boron nitride coated fibrous material and a matrix phase, said fibrous material being comprised of elemental carbon, said matrix phase containing silicon carbide phase and/or boron-containing silicon carbide phase and a solution phase comprised of a solution of boron and silicon wherein boron is present in an amount of at least about 0.1% by weight of silicon, said carbide phase being present in an amount of at least about 10% by volume of said composite, said solution phase being present in an amount of at least about 1% by volume of said composite, said fibrous material of said boron nitride-coated fibrous material being present in an amount of at least about 10% by volume of said composite, said boron nitride coating being at least detectable by scanning electron microscopy, said composite having a porosity of less than about 10% by volume.

36. The composite according to claim 35, wherein said matrix phase contains a ceramic carbide selected from the group consisting of boron carbide, molybdenum carbide, niobium carbide, and titanium carbide.

37. The composite according to claim 35, wherein said matrix phase contains a ceramic nitride selected from the group consisting of aluminum nitride, niobium nitride, silicon nitride, titanium nitride and zirconium nitride.

38. The composite according to claim 35, wherein said matrix phase contains a ceramic silicide selected from the group consisting of chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide and zirconium silicide.

39. A composite comprised of boron nitride-coated fibrous material and a matrix phase, said fibrous material being comprised of a SiC-containing material containing by weight of said SiC-containing material at least about 50% by weight of silicon and at least about 25%

RD-18,769 by weight of carbon, said matrix phase containing silicon carbide phase and/or boron-containing silicon carbide phase and a solution phase comprised of a solution of boron and silicon wherein boron is present in an amount of at least about 0.1% by weight of silicon, said carbide phase being present in an amount of at least about 10% by volume of said composite, said solution phase being present in an amount of at least about 1% by volume of said composite, said fibrous material of said boron nitride coated fibrous material being present in an amount of at least about 10%
by volume of said composite, said boron nitride coating being at least detectable by scanning electron microscopy, said composite having a porosity of less than about 10% by volume.

40. The composite according to claim 39, wherein said matrix phase contains a ceramic carbide selected from the group consisting of boron carbide, molybdenum carbide, niobium carbide, and titanium carbide.

41. The composite according to claim 39, wherein said matrix phase contains a ceramic nitride selected from the group consisting of aluminum nitride, niobium nitride, silicon nitride, titanium nitride and zirconium nitride.

42. The composite according to claim 39, wherein said matrix phase contains a ceramic silicide selected from the group consisting of chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide and zirconium silicide.

43. The composite according to claim 39, wherein said SiC-containing fibrous material envelops a core.

44. The composite according to claim 25, wherein said SiC-containing fibrous material envelops a core.