CA1302060C

CA1302060C - Fiber-reinforced silicon nitride composite ceramics

Info

Publication number: CA1302060C
Application number: CA 542543
Authority: CA
Inventors: Craig A. Willkens; Normand D. Corbin
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1986-08-06
Filing date: 1987-07-20
Publication date: 1992-06-02
Anticipated expiration: 2009-06-02
Also published as: US4885199A

Abstract

ABSTRACT OF THE DISCLOSURE
Tough composites of polymer derived silicon car-bide fibers in silicon nitride matrices, especially re-action bonded silicon nitride matrices, can be made by precoating the fibers with pyrolytic carbon and con-trolling the nitridation or other process which forms the silicon nitride matrix so that a thickness of at least 5 nanometers of carbon remains in the composite after it is formed. Failure of such composites is non-catastrophic. Alternatively, with at least some spe-cific types of polymer derived silicon carbide fibers, composites with non-catastrophic failure can he made by controlling the nitriding conditions to produce an es-sentially void space around the fibers in the final composites. As still another alternative, the space around the fibers may be partially filled with silicon nitride whiskers generated during the nitridation pro-cess.

Description

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FIBER REINFORCED SILICON NITRIDE COMPOSITE CERAMICS

RiGHTS OF THE UNITED STATES GOVERNMEN'r HEREIN

The government of the United States of America has certain rights to the invention described in this application pursuant to Contract No. F33615-83-C-5006iP00004 awarded by the United States Air Force.
BACKGROUND OF THE I~VENTION
Field of the Invention This invention relates to the field of ceramic composites which comprise a continuous phase, also in-terchangeably called matrix, and a discontinuous phase, also interchangeably called reinforcement. The discon-tinuous phase is at least predominantly in the form of elongated fibers. Such materials are generally denoted in the art as fiber reinforced composites. This in-vention relates more particularly to composites with a matrix comprised predominantly of silicon nitride and reinforclng fibers predominantly of silicon carbide.
Technical Background Like almost all other ceramics, silicon nitride inherently has little ~uctility, extensibility, or oth-er capacity for stress relief, so that when subjected , ~L3~
for even a short tlme to mechanical stresses in excess of its capacity, it normally breaks. Practical uses for ceramic objects genexally expose them to discontin-uous and non-uniform mechanical loads, so that the me-chanical stress in small areas of the ceramic can easi-ly exceed the capacity of the ceramic even when the overall stress is well below a value which would lead to fracture in laboratory testing. High stresses in small areas cause cracks to form, and because cracks themselves concentrate stress at their tips, a single initial crack can propagate entirely across a ceramic object, causing its catastrophic failure.
Although the term "catastrophic" is often used loosely to describe the failure of materials, for pur poses of this application it is useful to give it a more precise definition, with reference to a conven-tional measurement o~ the stress induced in a materialb~ mechanical strain. For most materials, including ceramics, the relation between stress and strain is linear at low strains. Increased strain leads even-tually to a value, called the yield strain, at which the rate of increase of stress with increasing strain begins to fall below the value it had at very low strains. For typical unreinforced ceramics, the yield strain coincides with fracture of the ceramic, so that the stress falls essentially to zero. Failure of a body is defined as catastrophic for purposes of this application if the stress on the body at a strain 10%
higher tha~ the yield strain is less than 20% of the stress on the body at a strain 2% less than the yield strain.
A methoa well known in general terms in the art for improving the mechanical stability of typically brittle ceramics such as silicon nitride is reinforcing the ceramic with inclusions of other material, often another ceramic. Small ceramic fibexs or other parti~
cles, hecause of more nearly perfect crystallinity, are ~3~
usually stronger and sometimes more shock resistant than bulk bodies, even of the same nominal ceramic com-position, which are made by conventional practical pro-cesses such as powder sintering or reaction ~onding.
Reinforcement, of course, need not be limited to parti-cles of the same composition as the matrix, and often it is advantageous to utilize a different composition for some particular property in which it is superior to the matrix.
In some but far from all cases, reinforcement, especially with elongated strong fibers, will prevent catastrophic fracture of a composite, even under con-ditions expected to cause fracture of the matrix of the cornposite alone. This improvement in fracture resis-tance from fiber reinforcement is believed to result primarily from three mechanisms generally recognized in general terms in the prior art: load transfer, crack bridging, and debonding.
Until the recent past, most new types o~ fiber ` 20 reinforced composites were made by workers trying to improve strength or rigidity. For such puxposes a strong bond between the matrix ana the reinforcing fi-bers is needed, so that strong bonding has usually been a goal. For example, an improvement in modulus of rup-ture for composites having a silicon nitriae matrix formed by sintering was disclosed by Yajima et al. in U. S. Patent 4,158,687. Continuous silicon carbide fi-bers formed by a special process described in U. S.
Patent 4,100,233 were used as the reinforcement, and "polycarbosilane" powder was added to the silicon ni-tride powder to improve the bonding between the matrix and the fibers. By these ~eans a composite body con-taining unidirectionally oriented fibers with a modulus of rupture (denominated in this instance as "flexural strength") of 610 MPa was achieved. Good o~idation re-sistance, corrosion resistance, heat resistance, and strength at high temperatures were asserted as :

~3~ 0 properties of the composites formed, but nothing was stated about the nature o~ the rupture of the compos-ite.
In fiber reinforced composites with such tight bonding as illustrated by this Yajima patent, cracks resulting from concentrated mechanical stresses in the matrix tend to propagate into the fibers and crack them as well. Recent workers have discovered that such un-desirable crack propagation can be avoided by surround-ing the reinforcing fibers with a crack deflectionzone~ The crack deflection zone should have mechanical properties which will cause most cracks which propagate into the zone from the matrix either to be arrested or to follow a path which will keep them away from the re-inforcing fibers.
~ One of the earlier workers to recognize the pos-- sible value of coating fibers with weakly bonded coat-; ings appears to have heen Warren, as exemplified by EPO
- Application 0 121 797 published Oct. 17, 1984. On page ~ lines 23-25, this application states, "[Ploor fiber to matrix bonds produce tough composites while good fi-ber to matri~ bonds result in brittle, flaw sensitive materials." In the embodiment believed most relevant to the present applicatior, the Warren application teaches forming an array of carbon fibers, coating them while they are in the array with a layer of pyrolitic carbon, machining the resulting porous body to the de-sired final shape, overcoating with a layer of chami-cally vapor desposited silicon carbide, heating to about 2700F "to effect dimensional stability between the silicon carbide/pyrolitic carbon and the sub-strate", and finally overcoating again with a chemical-ly vapor deposited silicon nitriae.
Because the carbon fibers which formed the origi-nal substrate were arrayed in a fabric, felt, or simi-lar structure before coating, the coating was not uni-form around the fibers, as clearly illustrated in .

~ 4 .

Figure 3 of the Warren application drawings: where two of the original fibers touched in the original array, the coating apparently could not penetrate between them.
The pyrolytic carbon layer deposited according to the teachings of Warren was so weakly bonded that the "fibers were free to move at a different rate from the carbon and/or silicon carbide matrix systems."
Because of its low c~efficient of thermal expan-sion, silicon nitride has long been regarded as one of the most attractive ceramics for use in conditions re-quiring resistance to thermal stresses. Nevertheless, the low mechanical shock resistance of unreinforced silicon carbide at almost any practical service temper-atures and its low creep s~rength at high temperatures seriously limit its practical uses.
One of the early attempts to improve the proper-ties o~ silicon nitride by inclusion of other materials in it was disclosed by Parr et al. in U. S. Patent 3,222,438. This taught the inclusion o~ 5-10% of sili-con carbide powder among silicon metal powder whi.ch was to be converted to a solid ceramic body by treatment with nitrogen gas at a sufficiently high temperature to promote the conversion of the silicon to its nitride.
This process, termed reaction bonding, produced coher-ent silicon nitride ceramic bodies with creep resis-tance significantly improved over those made without the silicon carbide powder additions. The bodies to be fired were formed from powders by cold pressing in a die set, and the addition of cetyl alcohol as a binder and lubricant for the powder before pressing was recom-mended. The disclosure of this patent strongly recom-mended, and the claims all required, that the reaction bonding temperature exceed la20C, the melting point of silicon, during par~ of the bonding cycle. The modulus of rupture for the composite bodies formed was not -~ given, being described merely as comparing "favourably .

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[sic] with those already published by others".
The use of relatively short silicon carbide fi-bers for reinforcing ceramics was disclosed by Hough in U. S. Patent 3,462,340. Orientation of the fibers by mechanical or electrostatic forces was taught as an ad-vantage in this patent, but no quantitative information about the mechanical properties of the resulting com-posites was given. Moreover, the matrix of the compos-ites taught by this patent was limited to "pyrolitic"
materials. The term "pyrolitic" was not particularly clearly defined in the patent specification~ but it was apparently restricted to materials having all their chemical constituent elements derived from a gas phase in contact with the hot reinforcing filaments and a mold-like substrate which determined the inner shape of the body to be formed. ~o method was taught or sug-gested in the patent or obtaining silicon nitride as a "pyrolitic" product within this definition.
A use of very short fibers of silicon carbide to reinforce ceramic composites having a silicon nitride matrix was tau~ht by Komeya et al. in U. S. Patent 3,833r389. According to the teachings of this patent, the matrix was formed by sintering silicon nitride pow-der rather than by nitriding silicon metal powder, and the maximum length of the silicon carbide fiber incIu-sions was 40 microns. A rare earth component was re-quired in the matrix in addition to silicon nitride, and the highest modulus of rupture (denominated as "breaking strength") was 375 megapascals (hereinafter MPa). A much more recent publication, P. Shalek et al., "Hot-Pressed SiC Whisker/Si3N4 Matrix Composites", 65 American Ceramic Society Bulletin 351 (1986), also utilized hot pressed silicon nitride powder with elon-gated "whiskers" of silicon carbide as reinforcement, but these whiskers still are no more ~han 0.5 mm in length. Use of silicon carbide whiskers in still other matrices is taught in U. S. Patent 4,543,345 o~ Sep.
:

~3~J~6() 24, 1985 to Wei (alumina, mullite, or boron carbide ma-trices) and U. S. Patent 4,463,058 of July 31, 1984 to Hood e-t al. (predominantly metal matrices).
A composite with oriented continuous fiber sili-con carbide reinforcement was taught by Brennan et al.in U. S. Patent 4,324,843. The matrix specified by Brennan was a crystalline ceramic prepared by heating a ~- glassy, non~crystalline powder of the same chemical composition as the matrix desired in the composite.
This description of the matrix appears to exclude sili-con nitride, which was not taught in the patent as a matrix material. In fact, the broadest claim of this patent required a matrix of metal aluminosilicates or mixtures thereof. Perhaps for this reason, the highest modulus of rupture noted in this patent for any of its product was less than 100 MPa.
Still another microstructural variation for sili-con nitride~silicon carbide composites was disclo~ed by Hatta et al. in U. S. Patent 4,335,217. According to this teaching, neither ~ibers nor powder of silicon - carbide or silicon nitride is used as an initial con-`~ stituent of the composite. Instead, a powdery polymer containing both silicon and carbon is mixed with sili-con metal powder, pressed, and then heated in a nitro-- 25 gen atmosphere. The polymer gradually decomposes under heat to yield silicon carbide, while the silicon powder reacts with nitrogen to yield silicon nitride. The composition of the final composite is described as "comprising crystals of beta-silicon carbide, alpha-silicon nitride, and beta-silicon nitride ... forming interwoven textures of beta-silicon carbide among said alpha-silicon nitride and beta-silicon nitride crystals without chemical bonding to provide micro gaps ... for absorption of thermal stresses.-l The highest reported modulus of rupture for these composites was 265 MPa.
In this Hatta patent there was also a casual ref-erence to "Conventional SlC-Si3N~ composit~ systems ..~
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~ 7 ~3~Z~0 fabricated by Xiring a mixture of silicon powder with ... SiC fiber~ in a nitrogen gas at~osphere at a tem-perature above 1~20C." No further details about how to make such allegedly conventional composites were given in the specification, however.
Much of the non-pztent literature in the field of silicon nitride-silicon carhide composites, which in general terms covers the same ground 2S the patents referenced above, was reviewed by Fischbach et al. in their final report to the Department of Energy un2er Grants ET-78-G-01-3320 and DE-FG-01-78-ET-13389. These investigators found that the types of fibers reported as very successfully used by Yajima in U. S. Patent 4,158,687 were not satisfactory for their bonding be-cause of a tendency for the interior of these fibers todebond from the sheath layer of the fibers during nitridation.
Metal coatings for ceramic reinforcing ~ibers are taught in U. S. Patent 3,B69,335 of ~ar. 4, 1975 to Siefert. Such coatings are presumably effective be-cause the ductility of metals allows absorption of the energy of propagating cracks by distortion of the met-al~ Composites with me`tal coa~ed fibers are satisfac-tory for service at relatively low temperatures, but at ~5 elevated temperatures the metal coatings can melt and thereby seriously weaken the composite. The matrices taught by Siefert were glasses, which have lower tem-perature service capability than ceramics. Thus for ceramics, temperature limitation is a serious disad~an-tage for metal coatings on the reinforcement.
U.S. Patent No. 4,642,271 of February lD, 1987,Rice teaches the use of boron nitride as a coating for ceramic fibers to produce a crack daflection zone when the coated fibers are incorporated into composites.
Silicon carbide, alumina, and graphite fibers and silica, silicon carbide, cordierite, mullite, and zirconia matrices are specifically taught~

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Results were highly variable. The toughness of compos-ites of silicon carbide fibers in silica matrices was dramatically increased by a coating of about 0.1 micron of boron nitride, but the same type of coated fibers in zirconia or cordierite produced little improvement in composite toughness compared with composites of un-coated fibers.
A reason for the effectiveness of boron nitride ~ was suggested, and there was additional relevant infor-mation, in European Patent Application No. 0 172 082 by Societe Europeenne de Propulsion, published Feb. 19, 1986. This teaches that boron nitride coated onto fi-bers by gas phase reactions between boron and nltrogen containing gases, as well as carbon coatings produced - ~ 15 by certain kinds of pyrolysis, is deposited on fibers in laminar form, with relatively weak bonds between laminae. Thus a crack which enters the coating will ~ normally have its direction of propagation changed if i ; necessary so that the crack will propagate along an in-;; 20 terface between laminae of the coating. These inter-faces are parallel to the fiber surface, so that the crack is usually prevented from entering the fiber.
` Carbon and silicon carbide fibers in silicon carbide - matrices are specifically taught by this application, `~"- 25 and other matrices, such as alumina formed by decompo-sition of aluminum butylate, are suggested.
The advantages of a crack deflection zone around reinforcing fibers in a different matrix is illustrated by John J. Brennan, "Interfacial Characterization of Glass and Glass-Ceramic Matrix/NICALON SiC Fiber Com-posites'i, a paper presented at the Conference on Tai-loring Multiphase and Composite Ceramics, held at Penn-sylvania State University, July 17-l9, 1985. This teaches that certain processing conditions lead to com-- 35 posites in which a carbon rich layer forms around SiC
- ~ reinforcing fibers, and the carbon rich layer acts as a crack deflection zone. Similarly, advantages for boron ~L3~
nitride coatings on reinforcing fibers are taught by s.
Bender et al., "Effect of Fiber Coatings and Composite Processing on Properties of Zirconia-Based Matri~ SiC
Fiber Composites'l, 65 American Ceramic Society Bulletin 363 ~1986). As expected from the titles of these re-ports, silicon nitride is not taught as a matrix by ei-- ther of them.
J. W. Lucek et alO, "Stability of Continuous Si-C(-O) Reinforcing Elements in Reaction Bonded Silicon Nitride Process Environments", Metal Matrix, Carbon, and Ceramic Matrix Composites, NASA Conference Publi-cation #2406, p. 27-38 ~1985), described silicon ni-tride matrices reinforced with silicon carbide fibers about 10-25 microns in diameter. These SiC fibers were -- 15 derived Erom organo-silicon polymer starting materials.
`~ High strength, non-brittle composites were not achieved with these silicon carbide fibers. Lucek et al. re-ported, on the basis of in~ormation supplied by others, that some of the fibers they used had been precoated with boron nitride. Whether the`fibers actually were coated has been subjected to some doubt since the orig-inal report. Lucek et al., because OL government secu-rity restrictions imposed on them as a condition of the ; supply of the allegedly coated fibers, did not attempt to characterize the composites they had prepared to a sufficient extent to determine whether boron nitride, or any other material, actually was present around the polymer derived (PD) silicon carbide fibers they used after their composites had been made ~ith the allegedly coated fibers.
The PD SiC fibers are known to be subject to some recrystallization, with accompanying volume shrinkage, and to partial volatilization, probably preceded by chemical reaction to give volatile products, upon heat-ing in the temperature range required for formation ofreaction bonded silicon nitride IRBSN). In contrast, - chemically vapor deposited ~CVD) silicon carbide 10`

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fibers, also briefly studied by Lucek et al., are much less subject to detrimental changes while nitriding.
Lucek et al. determined that the tensile strength of their allegedly boron nitride-coated PD SiC
fibers was degraded substantially less by e~posure of the fibers to the temperature and atmosphere of nitrid-ing than was the tensile strength of similar uncoated ~- fibers. ~evertheless, they further determined, by - flexural testing of com~ sites made with various SiC
fibers, that (1) the strengths of RBSN composites rein-forced with both types of PD SiC fibers were substan-tially less than those of composites reinforced with CVD SiC fibers, ~2) the strength of such composites made with the allegedly coated PD fibers was even less ~-` 15 than that of similar composites with uncoated PD fi-;` bers, and ~3) the tensile failure of the composites with both types of PD fibers was essentially cata-;; strophic. Presumably some unascertained part of the process of making the fibers into composites destroyed and/or changed the properties of whatevèr coating was ``` on them, so that when bonded into the RBSN matrix, the coating no longer functioned effectively for crack de-flection.
., .
-- At present, both CVD and PD silicon carbide fi-bers are very expensive, but it is believed that if significant volume demand developed, PD fibers could be made at much lower costs than CVD ones. There are also fundamental advantages to the smaller diameter of the PD fibers: smaller fibers are more flexible and versa-tile, especially in reinforcing complex shapes whichrequire strength in more than one direction and which have thin sections. It is practically difficult to ar-range a single thickness of fibers in a plane in an - array which will give substantially isotropic reinforce-ment. It is thereforè more co~mon to use fibers within a single layer in nearly parallel array and to superim-pose layers of such fibers with different orientations : , 11, . .

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in order to obtain substantially isotropic mechanical properties. Obviously, if an object with a thickness little more than that of one layer of CVD fibers is de---sired, such an arrangement i5 impossible with such fi-bers, but it could be accomplished with the PD fibers, which can be obtained with less than one tenth the di-~ameter of the CVD fibers. The smaller and more flexi-; ~ble PD fibers also can more easily be accommodated in -~sharp curves of the desired composite. On the other hand, the fundamentally greater thermal stability of the CVD type fibers should make their use safer in com-posites intended for sustained hlgh temperature service.
-EFor these reasons, it is advantageous to provide :~15 strong, tough RsSN composites with PD or other small diameter SiC fiber reinforcements as well as with larg-`er fiber reinforcements, and composites of both types made be made according to this invention.
One generalization which appears clear from the background information recited above is that the prop-erties of composites of silicon nitride and silicon carbide, like those of composites generally, are very sensitive to the details of microstructure of the com-posite. (A similar conclusion was stated in the Fisch--~-25 bach reference already cited.) Microstructural details -in turn are sensitive to the chemical and physical characteristics of the starting materials ana the pro-cesses used to convert the starting materials into a coherent composite body. Little predictability about the mechanical toughness of new and different composite microstructures has been possible heretofore.
SUMMP~RY OF THE INVE2~TI ON
Silicon carbide fibers at least one millimeter in length can be used more advantageously than short fi--35 bers to reinforce composites with a silicon nitride ma-trix, especially one formed by reaction bonaing~ The terms "silicon carbide fibers" or "SiC filers" or gram-~3~ O
matical variations of these terms should be understood for the purposes of this application to include any material in fibrous form with at least 55% of its con-stituent atoms comprised of silicon and carbon. This specifically includes the PD type of fibers already de-scribed above, which are known to be amorphous under some conditions and to include substantial amounts of oxygen~and nitrogen atoms, and fibers with a core of some other material such as carbon. The moduli of the composite products reinforced with silicon carbide ~ fibers are particularly high if substantial regions of : the composite produced contain long fibers that are substantially straight and mutually parallel, with an orientation transverse to the direction(s) of the greatest strain(s) exerted on the co~posite during its service life.
A-t a minimum, the strength and number of the in-: dividual fibers in the matrix should be hlgh enough so tha-t the fibers collectively are capable of bearing the load on the composite after matrix failure. Thus if necessary a complete load transfer from the matrix to the fibers can occur without mechanical failure due to , tensile overloading. In mathematical terms, if lc is the maximum load which the composite can sustain with-- 25 out matrix failure, Vf is the fraction of fiber area in - a cross section of the composite transverse to applica--- tion of force, and t~ is the tensile strength per unit area of the fibers, then lc/vf should be less than or equal to t~. For safety it is preferable that lc/vf should be substantially less than tf. As is apparent from the relation given above, if the total strength of the composite is increased and fibers of the same ten-sile strength are used, the fiber fraction may need to be increased to meet this criterion. For example, if - 35 the total composite has a matrix failure in tension at 552 MPa and the fiber fraction is only 30%, a fiber tensile strength of at least 1.84 GPa is needed. If ' ~ . , :
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the fiber fraction is raised to 60~, fibers with t.en-sile strength of 0.94 GPa would be adequate, The frac-: tion of fibers will normally be between 20 and 80% by -,, volume in the composites.
The resistance of the composites of this inven-, tion to catastrophic failure can be made particularly `.- high by assuring the presence in the final composite of crack deflection zones surrounding substantially all - the reinforcing fibers. A crack deflection zone is a region with significantly different mechanical prop-erties from either the matrix or the reinforcing fi-bers. In general crac~ deflection occurs most effec-tively in materials with some potential new surfaces ~-, that can be formed with smaller inputs of energy than '''-~ 15 are required to form other possible new surfaces by ~, fracturing the material of the deflection zone. Sur-'; faces that require such relatively low energy to form ~, can result from surface energy anisotropy within crys-;,, tals, grain boundaries in polycrystalline materials, or '; 20 other similar phenonena.
'~' The crack deflection zone will often differ in ,~ chemical composition from both the reinforcing fibers , and the matrix, but a difference in morphology could ";~ also be sufficient. A highly preferred characteristic i''';~'''`' 25 of a crack deflection zone is the presence of at least - one favored slippage surface running approximately par-, allel with the surfaces of the reinforcing fibers. The favored slippage surface may be, and often is, at or near the interface of the crack deflection zone with either the fibers or the matrix, as a result of such ' ~ phenomena as compositional changes at the interface, thermal expansion coefficient mismatches, or morpho-logical differences. ~Such an interfacial slippage surface is considered to be part of the crack deflec--~ ~ 35 tion zone for purposes of this application.
-~ Examples of appropriate deflection zone mate-~ rials include carbon that is chemically vapor deposited :' ~L3~

(also called pyrolytic carbon), boron nitride, and pol-ytypes 2~(d), 27R, 16 H, 21R, 12H, and 32H of the alu-minum-nitrogen-silicon-oxygen syste~.. Pyroly-ic carbon is particularly preferred.
In U.S. Patent No. 4,369,943 of September 26, 1989, composites with reinforcing fibers greater than 100 microns in diameter were described in detail. This application concentrates on composites with smaller fiber reinforcements.
According to a broad aspect the invention relates to a fiber reinforced composite, comprising (a) from 20 80% by volume of reinforcing silicon carbide ceramic fibers at least one millimeter in average length, said fibers collectively having sufficient tensile strength to bear the load on said composite at the point of matrix failure without fiber tensile failure, said fibers having a diameter between 8 and 25 microns; (b) crack deflection zones, having mechanical properties substantially different from those of both the matrix and the reinforcing fibers of the composite, occupying a predominant portion of the space in the order of 1 micron thick around said reinforcing fibers; a~d (c) a matrix comprising predominantly reaction bonded silicon nitride, said composite having non-catastrophic failure under mechanical stress, wherein said reinforcing fibers consist predominantly of silicon carbide derived from decomposition of organosilicon polymers, and wherein said crack deflection zones are comprised predominantly of a material with its most probable direction of slip under mechanical stress substantially parallel to the surfaces of said reinforcing fibers.

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BRIEF DESCRIPTION OF T~E DRAWINGS
Figure 1 shows the time and temperature progxam for short nitriding according to this invention. Fig-ure 2 depicts the stress-strain relationship for cer-tain composites of silicon carbide fiber, uncoated orwith various coatings, in an RBSN matrix. ~igure 3 de-picts the stress strain relationship for a composite made according to this invenLion, with non-catastrophic failure. Figure 4 depicts the stress-strain relation-ship for another composite according to this invention and compares it with one made with the same initial components but different nitriding conditions, which gives a different result. Figures 5 and 6 are photo-micrographs of a cross section through composites of the type with stress-strain curves depicted in Figure 4.
DESCRIPTION OF THE PREFERR D EMBODIMENTS
Reinforcin Fibers g_ _ Two effectives type of PD silicon carbide fibers for making composites according to this invention are available co~mercially under the trade name~Nicalon fi-bers from Nippon Carbon Ltd., Tokyo, Japan. These fi-bers, described by the supplier as derived from poly-carbosilane precursors, have diameters between 8 and 20 microns, and are available in two grades: s~andard and ceramic. The latter, which has a higher bulk densityand less oxyyen impurity, is generally preferred over TRADEMARK
15a - i, .

the standard grade for making composites according to this invention. Another effective type of fiber is one supplied by Dow Corning (not yet commercially3 under the designation MPDZ, Lot No. 33050-22-1. These fibers are reported to be derived from methyl poly(disilyl azane). MPDZ fibers made composites with greater toughness when used according to this invention than did the Nicalon fibers. Other fibers not specifically tested may well be equally effective.
Crack Deflection Zones Crack deflection zones may be advantageously pro-vided by at least two different methods: ~1) coating the reinforcing fibers with a suitable material to serve as a crack deflection zone and then preserving and/or improving the properties of the coating during nitridation, or (2) generating the crack deflection zone during nitridation from the materials o~ the fi-ber, the matrix, and/or the nitriding atmosphere. Ei-ther method is capable o~ giving good results; which one is preferable depends on the desired results and on the type of fibers used.
Coatinq: A suitable coating is pyrolytic carbon, -~ applied according to a process taught by J. V. Marzik, "CVD Fibers", Proceedings of the Metal and Ceramic Ma-- 25 trix Composite Processing Conference, Vol. II, p. 39-65 --; (Conference held at Battelle's Columbus Laboratories ` ~ from 13-15 November 1984). An initial coating thick-ness of at least 1 micron is normally preferred, but the most important characteristic of the coating is its crack deflection ability after processing, not the ini-tial coating thickness.
In-Situ Generation: It has been found that PD
SiC fibers normally decrease in diameter during nitri-~- dation. The mechanism for the size reduction is not clear, but surface mass loss, reduction in fiber void volume, and chemical reaction are all reasonable pos-sibilities. The decrease ln diameter may produce an ~' , ~3~
empty or almost empty space around the fibers after some nitridation cycles, as in illustrated in so~le of the subsequent examples~ In some other examples, the space between the dense part of the reinforcing fiber and the bulk of the matrix is filled with a variable amount of material presumably derived from reaction among the fiber, the matrix, and/or the nitriding atmo-sphere during nitridation. In at least one case~ the zone around the fibers contained silicon nitride whisker, although not enough to fill the space. Either a thin gap or a zone filled with porous material such as randomly oriented whiskers can provide an effective crack deflection zone.
Green Body Assembly~ Debinderizin~, and Sintering The placement of fibers within the shape of the desired final composite may be accomplished by any `~ ~ means conventional in the art. When the final product is expected to be subject to stresses in use primarily along a single direction, the fibers should be arranged : 20 as much as practicable transverse to that direction, so ; that the expected stress will have to bend the fibers in order to distort the body they are reinforcing. For applications with stresses applied in various direc-- ~ tions, it may be advantageous to use several layers, with parallel orientation of fibers within each layer, but different directions between layers. In many cases, however, it will be adequate to utilize the fi-~ . ~
ber in lengths of as little as one millimeter with rel-atively random orientation.
Many final product structures can be effectively assembled from thin flat "tapesl' containing oriented fibers. To make such tapes, a sufficient number of fi-bers or fiber tows to cover the desired width are sup-ported by any appropriate mechanical means in a mono-- 35 layer with the fibers substantially straight and copar-.
allel. This fiber array is supported in some appropri-ate fashion so that it can be coated with a slurry of - .

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silicon powder and at least one polymeric binder in a suitable solvent.
The preferred silicon po~der was a technical grade, nominally 99~ pure, with a mean pzrticle size of about 3 microns. (A suitable material was obtained from Elkem Metals Co., Marietta, Ohio.). Although many polymeric materials, natural or synthetic, such as pol-yvinyl acetate, vegetable gums, etc. could be used as the polymeric binder, the preferred one was a plasti-cized poly(vinyl butyral), marketed as Butvar 891 byMonsanto Chemical Co., Springfield, Massachusetts.
About 26 parts by weight o~ silicon, 8.6 parts by weight of polymer ~including plasticizer~, 2 parts by weight of acetone, and 63.~ parts by weight of a suit-able solvent such as isopropyl alcohol are mixed ~o-gether. ~he mixture is coated by any appropriate means, such as hand application, spraying, painting, a curtain coater, etc. over the prepared array of silicon carbide fibers to a sufficient depth so as to cover the fibers after drying. The combination of fibers and slurry is dried at about 20C for about 2 hours in the ambient atmosphere, resulting in a flexible coherent tape from which the solvent has been substantially ex-pelled.
The tapes thus prepared may be laid up by conven-tional means to fit any desired final shape. To make samples of composites for testing, suitab~e lengths of the tape thus made were cut, stacked one atop another while preserving a common direction of orientation of the fibers within the cut lengths of tape, and mechani-cally pressed perpenaicular to the planes of the tape segments in the stack, preferably under a pressure o~`
at least 0~4 but not more than 0.7 Megapascals and at a temperature of a~out 100C. In a typical example, com-pressed blorks fifty millimeters in bo~h width andlength and 6-8 mm in thickness were thus prepared.
The compressed blocks, or other bodies of any * TRADEMARK

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:~3`fJ2~i~il3 shape, are then treated to remove the polymer binder constituent in the bodies~ Preferably, this is accom-plished by heating the bodies in an inert gas atmo-sphere at a rate of temperature increase of about 1150C per hour to a final temperature of about 1150C, holding at that temperature for about fifteen minutes, - and cooling by natural convection at a rate estimated - to be between 100 and 200C per hour. During the heat-ing process, the flow rate of inert gas should be main-tained at a sufficient volume to sweep away any signif-icant gaseous decomposition proclucts formed, and the bodies should be maintained under pressure. By this process the original content of polymer binder is al-most totally removed from the bodies, but because of ~ 15 sintering of the silicon powder particles, the bodies : remain coherent.
Nitriding The debinderized bodles are converted to their final ceramic form by heating the bodles in an atmo-sphere of nitroyen gas with chemical purity of at least99.998%. Preferably, the nitriding is continued long : enough to convert substantially all the elemental sili-con in the body to silicon nitride. Because silicon ... ..
- has a much lower melting point than silicon nitride, substantial residual elemental silicon can limit high ~; temperature serviceability of the final composites.
Complete absence of silicon in an X ray diffraction analysis, which could detect as little as 0.1 atomic percent, is preferred.
In the long established process of forming reac-tion bonded silicon nitr~de (RBSN) without reinforce-ment, it has become customary to use a lengthy nitrida--~ tion cycle to maximize the amount of the alpha crystal form of silicon nitride in the final product. Crystals of the alpha form were believed to form a stronger ~- product than those of the beta form, which predominate ~- in products prepared at higher nitroyen gas pressure -~ . .
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and correspondingly shor-ter times. Processing cycles 100 hours long with a maximum temperature of 1410 C
are co~mon for making RBS~ monolithic bodies.
Such long nitridation cycles as used for mono-lithic RBSN can sometimes be effective for making com-posites according to our invention, but they are rareIy if ever preferred. Instead much shorter cycles, as little as 8 hours at a maximum temperature of 1350 C, are generally preferred. If development of a whisker-containing crack deflection zone is desired, an inter-mediate length cycle such as 48 hours length may be preferred. Specific modes of practicing the invention are given in the e~amples below.
Nitridation for the preparation of composites ac-- 15 cording to this invention is normally done in a cold-wall vacuum furnace, by either a flow-controlled or a pressure-controlled method.
For a flow-controlled nitridation, which is pre-ferred for in situ generation of crack de1ection zones, the debinderized composite samples are initially heated in vacuo to a temperature of about 1100C. Ni-trogen gas is then admitted to the furnace chamber un-til the total furnace pressure has reached a desired initial value. Gas flow is then interruptea and the temperature increased at a rate of about 100C per hour. As the temperature rises, the pressure in the , .
furnace initially rises but later falls as nitrogen is - coverted to non-volatile silicon nitride. The pressure drop is monitored by a sensor, and when the pressure has dropped to a pre-deter~ined trigger value, a solenoid valve controlled by the sensor allows additional nitrogen gas into the furnace at a constant - flow rate for the remainder of the nitridation.
A pressure controlled nitriding cycle is pre-ferred for use with fibers coated with carbon coatings.
In this method, the nitrogen gas pressure is controlled throughout nitridation at a fixed pre-determined value.
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Initial nitrogen gas pressures between 0.55 and 200 atmospheres, times between 6 and 48 hours, and final temperatures between 1325 and 1400 C could be used for such nitriding cycles in the p~actice of this inven-tion. The only necessary conditions are that suffi-cient tensile strength be maintained in the fibers to meet the strength criterion already given above and that the properties of the interface between the fibers and the matri~, including any crack deflection zone present, are adequate to prevent catastrophic failure of the composites produced after nitriding. A pressure controlled nitriding cycle especially preferred for composites with coated Nicalon fibers is one with an initial nitrogen gas pressure of at least one atmo-sphere, a time of not more than fifteen hours betweenfirst heating above room temperature and the beginning of cooling, a ma~imum temperature not more than 1375C, and no more than ten hours above 1~00C.
After completion of the nitriding treatment, the composite ceramic bodies are preferably cooled at a rate of not greater than 200 C per hour. The final result is a ceramic body resistant to thermal and me-chanical shocks and suitable for long term service at temperatures up to about 1200C, at least in non-oxi-dizing atmospheres. These composites contain 20-50 volume percent silicon carbide filaments, and the den-- sity of the silicon nitride matrix component of the composites is believed to be about 1.8-2.0 gm/cm3.
The mechanism of initial failure of elasticity of composites made according to this invention is non-catastrophic. With all prio~ art SiC reinforced RBS~
composites known to the applicants, the first reduction of load-bearing capacity under stress usually results in complete rupture, with obvious adverse consequences -35 for the integrity of any structure composed of such ~la-terials.

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The scope and variety of the invention can be ~urther appreciated from the following examples.
Examples 1-5 Ceramic grade PD SiC fibers from ~ippon Carbon were used for all these examples~ Uncoated fibers were designated as Example 1; the coatings and corresponding - example numbers are shown in Table 1 below.
Table l COATING AND FIBER CHAR~CTERISTICS
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10 No Coatin~ Characteristics _ Fiber Composit on Thick- Morphol- Tensile ness, o~y Strength, Microns Gigapas-cals 15 1none -- - 2.3+0.5 2Alumina, stoichio- 0.1-0.15 Fiber 2.3+0.5 metric bridging, rough 3 Silicon carbide, 0.5-0.6 Fiber Fragile 20 carbo~ rich bridging 4 Carbon, pyrolitic l.oContinuous 2.0~0.6 5 Silicon nitride,0.2+0.1Cracked 2.0 stoichiometric .
Coating thicknesses shown in Table 1 were esti-~; 25 mated by scanning electron microscopy (SEM).
Composites with fibers of each of the types shownin Table 1 were prepared by first winding, or laying up by hand, the fiber tow unidirectionally to form an ar-ray with essentially parallel fibers one tow thick.
The fiber ends were secured with tape to maintain fiber alignment during coating and other processing.
A slurry coating material was prepared, having the following composition in parts by weight:

2-Propanol 63.4 parts Poly[vinyI butyral) 4.3 parts Butyl benzyl phthalate (plasticizer) 4.3 parts ` Acetone 2 parts Silicon metal powder 26 parts ., ' ' , ,'.

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The silicon metal powder used had a mean particle size of about 3 microns and about 99% purity, with 0~7~ iron as the principal impurity. The slurry was sprayed onto one side of the previously pr pared array of SiC fihers to give a minimum coating thickness of about 5 microns.
The slurry had sufficient viscosity to remain on the fibers. After application of the slurry on the first side, the coated composite was d:ried at about 20C for about two hours. The fiber array was now coherent, so that it could be turned over without disturbing the alignment of the fibers. The opposite side of the ar-ray was coated with the same slurry to a minimum coat-ing thickness of about five microns, and again dried by the same conditions as after the first coating. The dried, doubly coated composite was flexible and is called a "tape".
Squares 75 mm on each side were cut from the tape. ~pproximatel~ ei~ht o~ these squares were stacked with ~iber directions in all squares the same, and the stack was then pressed at 100 C in a steel die at a pressure of about 21 MPa. The polymer bonded com-posite formed by this first pressing was then trans-ferred to a graphite hot pressing die and compacted further at 1150C and about 21 MPa pressure in an atmo-sphere of flowing argon gas for about fifteen minutes.This secondr hot pressing served to sinter the si-icon matrix and pyrolize and expel the fugitive organic binder components: poly(vinyl butyral) and butyl benzyl phthalate. The result of this process was a coherent composite with a silicon metal matrix and silicon carbide fibers.
The sintered silicon bodies were then nitrided under the conditions shown in Figure 1 to form the fina~ composites. As a result of this treatment, the silicon metal matrix was substantially quantitatively converted to silicon nitride, with a 66.5~ weight in-crease and a 22~ volume increase. The volume increase, ~3 ~3~
however, is accommodated within the pores of the sili-con matrix composite, so that no change in its external : dimensions occurs during the transformation into sili-con nitrideO
The silicon nitride composite was sliced into test samples along two sets of perpendicular planes, each of which was parallel to the direction of the in-cluded silicon carbide fibers. These samples were used for strength tests, made in a three point bena config-uration with a span of 62 mm, at a constant displace-ment rate of 0.,127 mm/min. Span to depth ratios varied fro~ 16:1 to 46:1, depending on sample thick-ness. The as-nitrided surfaces of the samples were not disturbed, except to the extent necessary to cut the samples to the size needed for the strength testing.
The samples were used for both quantitative strength evaluations, as shown in Table 2, and for qualitative assessments of failure hehavior.
The short nitridin~ cycle produced composites ~20 with non-catastrophic failure with carbon coated fibers -~(Example 4). Using 48 flow controlled nitriding condi-for composites containing the same coated fibers pro-duced composites with catastrophic failure. All other types of coatea or uncoated fiber, i.e., Examples 1-3 ~-25 and 5, yielded composites with catastrophic failure, even with short nitriding cycles. Examination of the interfaces in the composites by Auger spectroscopy and the SEM indicated the presence of a carbon layer at least 60 nanometers thick around the SiC fibers in the non-brittle composite of Example 4.
Thè yield strains, maximum sustainable strains, and the stresses at the yield strains for the compos-ites of Examples 1-5 ~all with nitriding conditions as shown in Figure 1, except for Example 3 which was ni-trided for 48 hours at a maximum nitrogen pressure of .7 atmospheres and maximum temperature of 1380 C, are ~3~ 6~;3 shown in Table 2 below. The figures in the table weredetermined at a displacement rate of 0.12 mm/min~
" Table 2 Example Fiber Yield Yield Maximum Number Volume Strain, % Stress, MPa Sustainable _ Fraction Stress, MPa 1 0027 0.187 252 252 2 0.20 0.191 376 376 3 0.07 0.110 194 194 - 4 0.21 0.214 285 390 - 5 0.18 0.260 180 180 Unreinforced RBSN 0.00 0.140 315 315 ~, _____________ :` 15 Stress-strain curves for the composites produced ~` by Examples 1-5 are shown in Figures 2 and 3. The ., .
non-catastrophic failure of the composite of Example 4 allowed it to reach a maximum sustainable stress at 0.396~ strain, almost twice its yield strain, while for ~` 20 all the other composites shown in Figure 2, the yield , i ~- stress was the maximum sustainable stress.
All the composites made in Examples 1-5 exhibited ` a significant degree of load transfer from the matrix to the fiber during testing, but in all of them except Example 4, there was apparently an insufficiently ef~-; fective crack deflection zone around the fibers to re-sult in non-catastrophic failure. This indicates that fibers which retain strength after processing, load transfer, and effective crack deflection zones around the fibers are all important in achieving the toughest composites.
Example 6 For this example, uncoated ceramic grade Nicalon fibers as in Example 1 above were used, but the nitrid-ing cycle was increased to 48 hours in length. The composite produced in this example had improved tough- :
ness, compared to the product from Example 1. Micro-~3~2q~
scopic e~aminatlon o~ the composite indicated develop-ment of a zone, partially filled with a material dis-tinct from either the fibers or the matrix, around the fibers in this composite, while no such zone was ob-served in the product from Example 1. This appears toaccount for the difference in toughness. The general physical properties of this composite were not as good as that from Example 4, however. It is believed that this difference results from degradation of the tensile strength of the fibers during the relatively long ni-tridation cycle.
Example 7 This was the same as Example 6, except that stan-dard rather than ceramic grade Nicalon fibers were -~15 used. The zone around the fihers after nitridation in this case had randomly oriented silicon nitride whis~-ers which were only very poorly if at all consolidated with the silicon nitride matrix, Qualitatively, the ~composites from this example were evaluated as slightly - 20 tougher than those of Example 6 and considerably tough-er than those of Example 1, but they still sufferred catastrophic failure when stressed.
- Examples 8 and 9 For these examples, uncoated MPDZ fiber was used instead of Nicalon fiber. A 48 hour flow controlled nitriding cycle was used for Example 8 and an 8 hour pressure controlled cycle for Example 9. Other pro-cessing was the same as for Example 4. Stress-strain curves of the resulting products are shown in Figure 4.
The composite with the longer nitriding cycle had non-catastrophic failure. The difference from Example 6 is believed to be due to the greater resistance of MDPZ
fibers to tensile strength loss under nitriding condi-tions. ~ith MDPZ fiber, short nitridation cycles such as used for Example 4 give composites subject to cata-strophic failure. This is believea to be due to the fact that the shorter nitridation time does not result 13~
in the generation of a gap around the fibers in the composite, as the longer time does. The difference is apparent in cross~sections of the composites shown in Figures 5 and 6 ~Ihile the examples involve unidirectional fila-ment or fiber arrays, it should be emphasized that this - is not a limitation of the invention.
Composites prepared according to this invention are useful for any of the uses now served by silicon nitride, including but not limited to: thermocouple sheaths, riser stalks for low pressure die casting, crucibles, and furnace tapping seals and plugs for foundries for non-ferrous metals, particularly alumi-num; degassing tubes and lining plates for primary alu-minum smelters; precision jigs and fixtures for solder-ing, brazing, and heat treatment processes in the man-ufacture of electronic and semiconductor goods, ~ewel-ry, or any other metal or glass object requirin~ heat treating; wear resistant fixtures for optical devices, nose guides and electrode holders for electrodischarge - machining, or guides and templates for electrochemical machining; welding nozzles and insulators; components of pumps, valves, or vessels for handling corrosive ~-; chemicals and abrasive mixtures; artificial teeth and ~- 25 dental bridges; low signature struc-tural fairings and similar structures for aircraft and other vehicles which will be unusually ~ifficult to detect by radar;
and components for engines which can operate at higher temperatures than engines with all metal combustion containment chambers.
Composites with reaction bonded silicon nitride are particularly useful as structural ceramics, because of their combination of light weight with high stif-ness and a low coefficient of thermal e~pansion. This combination of properties is particularly valuable for structures to be used in space, where (1) the absence of oxygen avoids one of the major limitatlons of RBSN

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in terrestrial environments, the susceptibility of RBSN
to oxidative degradation at high temperatures, (2) the low weight is especially valuable because of the cost of launching weight into space, and (3) a low coeffi-cient of thermal expansion is particularly valuable forstructures exposed to sunlight on only one side.
~ ~ RBSN composites are also useful for electrical applications because of their strength and dielectric properties, and as biological replacement materials, - 10 because of corrosion resistancel absence of toxicity, and ability to bond well to anirnal tissue.
The greater strength and toughness of composite bodies made according to this invention will make them useful in additional applications now avoided with sil~
icon nitride bodies subject to catastrophic failure.

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Claims

1. A fiber reinforced composite, comprising:
(a) from 20-80% by volume of reinforcing silicon carbide ceramic fibers at least one millimeter in average length, said fibers collectively having sufficient tensile strength to bear the load on said composite at the point of matrix failure without fiber tensile failure, said fibers having a diameter between 8 and 25 microns;
(b) crack deflection zones, having mechanical properties substantially different from those of both the matrix and the reinforcing fibers of the composite, occupying a predominant portion of the space in the order of l micron thick around said reinforcing fibers; and (c) a matrix comprising predominantly reaction bonded silicon nitride, said composite having non-catastrophic failure under mechanical stress, wherein said reinforcing fibers consist predominantly of silicon carbide derived from decomposition of organosilicon polymers, and wherein said crack deflection zones are comprised predominantly of a material with its most probable direction of slip under mechanical stress substantially parallel to the surfaces of said reinforcing fibers.

2. A composite according to claim 1, wherein said silicon carbide is derived predominantly from the decomposition of methyl poly(disilyl azane).

3. A composite according to claim l, wherein said crack deflection zones are comprised predominantly of a material selected from the group consisting of carbon, boron nitride, and poly-type 2H(d), 27R, 16H, 21R, 12H, and 32H of the aluminum-nitrogen-silicon-oxygen system.

4. A composite according to claim 3, wherein said crack deflection zones are composed predominantly of laminar deposited pyrolytic carbon.

5. A composite according to claim 2, wherein said crack deflection zones have at least half their volume as empty space.

6. A composite according to claim 1, wherein said crack deflection zones comprise silicon nitride whiskers.

7. A composite according to claim 6 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

8. A composite according to claim 5 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

9. A composite according to claim 4 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

10. A composite according to claim 3 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

11. A composite according to claim 1 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

12. A composite according to claim 2 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

13. A composite according to claim 3 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.

14. A composite according to claim 4 having a maximum load bearing capacity of at least 350 mega-pascals at a fiber strain of at least 0.3%.