WO1993014040A1

WO1993014040A1 - Maintaining high flexural strength in ceramic fiber reinforced silicon carboxide composites

Info

Publication number: WO1993014040A1
Application number: PCT/US1992/011058
Authority: WO
Inventors: Stephen Thomas Gonczy; Roger Yu-Kwan Leung
Original assignee: Allied-Signal Inc.
Priority date: 1992-01-10
Filing date: 1992-12-22
Publication date: 1993-07-22

Abstract

An improved fiber reinforced glass composite includes a carbon-coated refractory fiber in a matrix of a black glass ceramic having the empirical formula SiCxOy where x ranges from above zero up to about 2.0, preferably 0.9 to 1.6 and y ranges from above zero up to about 3.0, preferably 0.7 to 1.8. Preferably the black glass ceramic is derived from cyclosiloxane monomers containing a vinyl group attached to silicon and/or a hydride-silicon group. By selecting the proper carbon thickness, the advantages of carbon in providing fibrous fracture of the composites can be retained even when the carbon has been lost by oxidation at high temperature.

Description

MAINTAINING HIGH FLEXURAL STRENGTH IN CERAMIC FIBER

REINFORCED SILICON CARBOXIDE COMPOSITES

Prior Art The invention relates generally to composites in which a matrix material is reinforced with fibers. Composites having a polymer matrix are widely used for various purposes, but they are generally limited to applications where temperatures are expected to be below about 300*C. The present invention relates to ceramic fiber reinforced-glass matrix composites which can be used at temperatures which would destroy conventional polymeric materials.

Matrix materials having enhanced performance have been suggested for use with fibers having high strength at elevated temperatures. Examples of such matrix materials are the glass and glass ceramics (Prewo et al.. Ceramic Bulletin, Vol. 65, No. 2, 1986).

In USSN 07/002,049 a ceramic composition designated "black glass" is disclosed which has an empirical formula Sico where x ranges from 0.5 to about 2.0 and y ranges from about 0.5 to about 3.0, preferably x ranges from 0.9 to 1.6 and y ranges from 0.7 - 1.8. Such a ceramic material has a higher carbon content than prior art materials and is very resistant to high temperatures - up to about 1400*C. This black glass material is produced by reacting in the presence of a hydrosilylation catalyst a cyclosiloxane having a vinyl group with a cyclosiloxane having a hydrogen group to form a polymer, which is subsequently pyrolyzed to black glass. The present invention involves the application of such black glass to reinforcing fibers to form composites very useful in high temperature applications.

In U.S. Patent 4,460,638 a fiber-reinforced glass composite is disclosed which employs high modulus fibers in a matrix of a pyrolyzed silazane polymer. Another possible matrix material is the resin sol of an organosilsesquioxane, as described in U.S. 4,460,639. However, such materials are hydrolyzed, and since they release alcohols and contain excess water, they must be carefully dried to avoid fissures in the curing possess.

_.Another patent, U.S. 4,460,640, disclosed related fiber reinforced glass composites using organopoly-siloxane resins of U.S. Patent 3,944,519 and U.S. Patent 4,234,713 which employ crosslinking by the reaction of silicon hydride (≡SiH) groups to silicon-vinyl (CH₂=CHSi≡) groups. These later two patents have in common the use of organosilsesquioxanes having C₆H_sSi0_3/2 units, which have been considered necessary by the patentees to achieve a flowable resin capable of forming a laminate. A disadvantage of such C₆H₅Si0_3/2 units is their tendency to produce free carbon when pyrolyzed. The black* glass of USSN 07/002,049 requires no such C₆H₅Si0_3/2 units and still provides a flowable resin, and does not produce easily oxidized carbon. Another disadvantage of the organopolysiloxanes used in the '640 patent is their sensitivity to water as indicated in the requirement that the solvent used be essentially water-free. The resins contain silanol groups and when these are hydrolyzed they form an infusible and insoluble gel. The black glass of USSN 07/002,049 requires no such silanol groups and is thus insensitive to the presence of water. In addition, the organopolysiloxanes of the *640 patent may not have a long shelf life while those of USSN 07/002,049 remain stable for extended periods. Still another disadvantage for the organopolysiloxanes disclosed in the '640 patent is that they require a partial curing step before pressing and final curing. This operation is difficult to carry out and may prevent satisfactory lamination if the polymer is over cured. The present invention requires no pre-curing step. In co-pending U.S. patent application SN 07/426,820 composites of refractory fibers with a black glass matrix were disclosed. Such composites have good physical properties but tend to exhibit brittle fracture with little evidence of fiber pullout. The composites reported in U.S. Pat. Nos. 4,460,639 and 4,460,640 also exhibit brittle fracture with a flexural strength of less than 308 MPa.

The type of failure is to large extent determined by the nature of the interface between the reinforcement fiber and the surrounding matrix. If a strong interfacial bond exists, the crack will pass through the fiber, resulting in a fracture behavior not much different from unreinforced monolithic ceramics. High toughness results when energy is absorbed as fibers pull out from the matrix as the composite cracks. Thus, a low interfacial stress or friction is needed to ensure fibrous fracture. In co- pending U.S. patent applications 07/464,470 and 07/523,620 the use of a carbon interface in a silicon carboxide 'black' glass matrix was shown to produce a composite having a high strain-to-failure and exhibiting fibrous fracture. Such carbon interfaces are subject to oxidation at high temperature and the ceramic composites consequently lose strength and may revert to a brittle failure mode. Also, if the reinforcing fibers can be oxidized, an oxide layer can be formed between the fiber and the matrix which may induce brittle failure. The present invention relates to a method for avoiding brittle fracture even after exposure of a ceramic composite to high temperature in an oxidizing atmosphere.

Summary of the Invention

The invention may be broadly defined as an improved composite comprising a ceramic matrix reinforced with ceramic fibers having a higher strength than the matrix and characterized by having an optimized spacing between the ibers and the matrix which maximizes strength and elastic modulus of the composite while providing fibrous, rather than brittle fracture under load. The optimized spacing may be provided by carbon coating the reinforcing fibers and, subsequently, removing the carbon by high temperature oxidation. Thus, the advantages of using carbon coated fibers may be obtained even after the carbon has been removed.

The refractory fibers are coated with a carbon layer of a preselected thickness prior to fabrication and pyrolysis of the cyclosiloxanes to form the black glass matrix. Preferred methods of forming such carbon coatings are chemical vapor deposition, solution coating, and pyrolysis of organic polymers such as carbon pitch and phenolics.

In one preferred embodiment an improved fiber reinforced glass composite comprises (a) at least one carbon-coated refractory fiber selected from the group consisting of boron, silicon carbide, graphite, silica, quartz, S-glass, E-glass, alumina, aluminosilicate, aluminoborosilicate, boron nitride, silicon nitride, silicon oxynitride, boron carbide, titanium boride, titanium carbide, zirconium oxide, and zirconia toughened alumina and, (b) a carbon-containing black glass ceramic composition having the empirical formula SiC_χO_y where x ranges from greater than zero up to about 2.0, preferably from 0.9 to 1.6, and y ranges from greater than zero up to about 3.0, preferably from 0.7 to 1.8. The carbon coating is deposited with a thickness selected to provide fibrous fracture of the composite even after the carbon has been oxidized by exposure to high temperatures. In a preferred embodiment fibers are aluminoborosilicate and the thickness of the carbon coating is 50 to 200 ran.

In another embodiment, the black glass ceramic composition is the pyrolyzed reaction product of a polymer prepared from (a) a cyclosiloxane monomer having the formula

where n is an integer from 3 to about 30, R is hydrogen, and R¹ is an alkene of from 2 to about 20 carbon atoms in which one vinyl carbon atom is directly bonded to silicon or (b) two or more different cyclosiloxane monomers having the formula of (a) where for at least one monomer R is hydrogen and R' is an alkyl group having from 1 to about 20 carbon atoms and for the other monomers R is an alkene from about 2 to about 20 carbon atoms in which one vinyl carbon is directly bonded to silicon and R¹ is an alkyl group of from 1 to about 20 carbon atoms, or (c) cyclosiloxane monomers having the formula of (a) where R and R' are independently selected from hydrogen, an alkene of from 2 to about 20 carbon atoms in which one vinyl carbon atom is directly bonded to silicon, or an alkyl group of from 1 to about 20 carbon atoms and at least one of said monomers contains each of said hydrogen, alkene, and alkyl moieties, said polymerization reaction taking place in the presence of an effective amount of hydrosilylation catalyst. The polymer product is pyrolyzed in a non-oxidizing atmosphere to a temperature in the range of about 800*C to about 1400*C or in an oxidizing atmosphere at a rate exceeding 5*C/min to the same temperature range to produce the black glass ceramic.

The polymer precursors may also be described as containing the following moieties R" R"

R" R°CH, R"

Si - CH - Si /

/

where R° is the unreacted residue of an alkene having 2 to 20 carbon atoms R" is H, an alkyl group having 1 to 20 carbon atoms, or an alkene having 2 to 20 carbon atoms

R° will be the residue of R and R* described above. Thus, R° may be hydrogen or an alkyl group of 1 to 18 carbon atoms or an alkene group of 1 to 18 carbon atoms containing unsaturation in addition to the reactive vinyl group. In another embodiment the invention comprises a method of preparing a fiber reinforced glass composite wherein the cyclosiloxane reaction product described above is combined with carbon-coated refractory fibers. The thickness of the carbon coating is selected to maximize strength and elastic modulus while retaining fibrous fracture, even when the carbon is removed by oxidation. Plies of the coated fibers may be 1aid-up to form a green laminate and thereafter pyrolyzed in a non-oxidizing atmosphere at a temperature between about 800^*C and about 1400*C, preferably about 850*C, or in an oxidizing atmosphere at a rate exceeding 5*C/min to the same temperature range to form the black glass composite. The laminate may be reimpregnated with polymer solution and repyrolyzed in order to increase density. Alternatively, a resin transfer technique may be used in which fibers are

SUBSTITUTESHEET placed in a mold and the black glass matrix precursor is added to fill the mold before curing to form a green molded product.

Brief Description of the Drawings

Figure l is a graph comparing the flexural strengths of composites of uncoated and coated Nextel* 440 fibers in black glass matrices. Figure 2 is a graph comparing the flexural strength of composites including coated Nextel* 440 fibers in black glass matrices in which the composites have been heated to 1000 ' C in air for 100 hours. Figure 3 is a series of photographs of the composites of Figure 1 after testing. Figure 4 is a series of photographs of the composites of Figure 2 after testing.

Description of the Preferred Embodiments Black Glass Ceramic

The black glass ceramic used as the matrix has an empirical formula SiC A„O_uy wherein x ranges from greater than zero up to about 2.0, preferably 0.9 - 1.6, and y ranges from greater than zero up to about 3.0, preferably 0.7 - 1.8, whereby the carbon content ranges up to about 40% by weight. The black glass ceramic is the product of the pyrolysis in a non-oxidizing atmosphere (or in an oxidizing atmosphere at a rate exceeding 5'C/min) at temperatures between about 800*C and about 1400*C of a polymer made from certain siloxane monomers.

The polymer precursor of the black glass ceramic may be prepared by subjecting a mixture containing cyclosiloxanes of from 3 to 30 silicon atoms to a temperature in the range of from about 10"C to about 300*C in the presence of 1-200 wt. ppm of a platinum hydrosilylation catalyst for a time in the range of from about 1 minute to about 600 minutes. When the polymer is placed in a nonoxidizing atmosphere, such as nitrogen, and

SUBSTITUTE pyrolyzed at a temperature in the range from about 800*C to about 1400*C for a time in the range of from about 1 hour to about 300 hours, black glass results. If the polymer is heated rapidly, at least 5°C/min, preferably 50* to lOOO'C/ in, an oxidizing atmosphere may be used. The polymer formation takes advantage of the fact that a silicon-hydride will react with a silicon-vinyl group to form a silicon-carbon-carbonsilicon bonded chain, thereby forming a network polymer. For this reason, each monomer cyclosiloxane must contain either a silicon-hydride bond or a silicon-vinyl bond or both. A silicon-hydride bond refers to a.silicon atom bonded directly to a hydrogen atom and a silicon-vinyl bond refers to a silicon atom bonded directly to an alkene carbon, i.e., it is connected to another carbon atom by a double bond.

The polymer precursor for the black glass ceramic may be defined generally as the reaction product of (a) a cyclosiloxane monomer having the formula

where n is an integer from 3 to 30, R is hydrogen, and R' is an alkene of from 2 to 20 carbon atoms in which one vinyl carbon atom is directly bonded to silicon or (b) two or more different cyclosiloxane monomers having the formula of (a) where for at least one monomer R is hydrogen and R* is an alkyl group having from 1 to 20 carbon atoms and for the other monomers R is an alkene from about 2 to 20 carbon atoms in which one vinyl carbon is directly bonded to silicon and R' is an alkyl group of from 1 to 20 carbon atoms, or (c) cyclosiloxane monomers having the formula of (a) where R and R' are independently selected from hydrogen, an alkene of from 2 to about 20 carbon atoms in which one vinyl carbon atom is directly bonded to silicon, or an alkyl group of from 1 to about 20 carbon atoms and at least one of said monomers contains each of said hydrogen, alkene, and alkyl moieties, said reaction taking place in the presence of an effective amount of hydrosilylation catalyst.

The polymer precursors may also be described as containing the following moieties

R" R° R"

\

Si - CH - CH -2, - Si

R" R°CH₂ R"

0 I I l - o Si - CH - Si

where R° is the unreacted residue of an alkene having 2 to 20 carbon atoms R" is H, an alkyl group having 1 to 20 carbon atoms, or an alkene having 2 to 20 carbon atoms Since the cyclosiloxane molecules are linked by the reaction of a hydrogen atom from one molecule and an alkene from the other, the residual group R° is derived from species of R and R¹ previously described. Thus, R° could be hydrogen or an alkyl group of 1 to 18 carbon atoms or an alkene group of 1 to 18 carbon atoms containing unsaturation in addition to the reactive vinyl group.

SUBSTITUTESHEET The black glass ceramic may be prepared from a cyclosiloxane polymer precursor wherein both the requisite silicon-hydride bond and the silicon-vinyl bond are present i n o ne m o l e c u l e , f o r e x am p l e , 1,3,5,7-tetravinyl-l,3,5,7-tetrahydrocyclotetrasiloxane. Alternatively, two or more cyclosiloxane monomers may be polymerized. Such monomers would contain either a silicon hydride bond or a silicon-vinyl bond and the ratio of the two types of bonds should be about 1:1, more broadly about 1:9 to 9:1.

Examples of such cyclosiloxanes include, but are not limited to:

1,3,5,7-tetramethyltetrahydrocyclotetrasiloxane, 1,3,5,7-tetravinyltetrahydrocyclotetrasiloxane, 1,3,5,7-tetravinyltetraethylcyclotetrasiloxane, 1,3,5,7-tetravinyltetramethylcyclotetrasiloxane, 1,3,5-trimethyltrivinylcyclotrisiloxane, 1,3,5-trivinyltrihydrocyclotrisiloxane, 1,3,5-trimethyltrihydrocyclotrisiloxane, 1,3,5,7,9-pentavinylpentahydrocyclopentasiloxane, 1,3,5,7,9-pentavinylpentamethylcyclopentasiloxane, 1,1,3,3,5,5,7,7-octavinylcyclotetrasiloxane, 1,1,3,3,5,5,7,7-octahydrocyclotetrasiloxane, 1,3,5,7,9,11-hexavinylhexamethylcyclohexasiloxane, 1,3,5,7,9,11-hexamethylhexahydrocyclohexasiloxane, 1,3,5,7,9,11,13,15,17,19-decavinyldecahydrocyclo- decasiloxane,

1,3,5,7,9,11,13,15,17,19,21,23,25,27,29-pentadecavinyl- pentadecahydrocyclopentadecasiloxane 1,3,5,7-tetrapropenyltetrahydrocyclotetrasiloxane, 1,3,5,7-tetrapentenyltetrapentylcyclotetrasiloxane, 1,3,5,7,9-pentadecenylpentapropylcyclopentasiloxane, 1,3,5,7,9-pentahydropentamethylcyclopentasiloxane, 1,1,3,3,5,5,7,7-octahydrocyclotetrasiloxane, 1,1,3,3,5,5,7,7,9,9-decahydrocyclopentasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11-dodecahydrocyclohexasiloxane.

It will be understood by those skilled in the ar that while the siloxane monomers may be pure species, i will be frequently desirable to use mixtures of suc monomers, in which a single species is predominant. Mixtures in which the tetramers predominate have been found particularly useful.

While the reaction works best if platinum is the hydrosilylation catalyst, other catalysts such as cobalt and manganese carbonyl will perform adequately. The catalyst can be dispersed as a solid or can be used as a solution when added to the cyclosiloxane monomer. With platinum, about 1 to 200 wt. ppm, preferably 1 to 30 wt. ppm will be employed as the catalyst. Black glass precursor polymer may be prepared from either bulk or solution polymerization. In bulk polymerization, neat monomer liquid, i.e., without the presence of solvents reacts to form oligomers or high molecular weight polymers. In bulk polymerization, a solid gel can be formed without entrapping solvent. It is particularly useful for impregnating porous composites to increase density. Solution polymerization refers to polymerizing monomers in the presence of an unreactive solvent. The resin used in impregnating fibers to form prepreg in our invention preferably is prepared by solution polymerization. The advantage of solution polymerization is the ease of controlling resin characteristics. It is possible but very difficult to produce B-stage resin suitable for prepregs with consistent characteristics by bulk polymerization. In the present invention, soluble resin with the desirable viscosity, tackiness, and flowability suitable for prepregging and laminating can be obtained consistently using solution polymerization process. The production of easily handleable and consistent resin is very critical in composite fabrication. Fibers

Reinforcing fibers useful in the composites of the invention are refractory fibers which are of interest for applications where superior physical properties are needed. They will generally be stronger than the matrix with which they are used. They will include such materials as boron, silicon carbide, graphite, silica, quartz, S-glass, E-glass, alumina, aluminosilicates, aluminoborosilicate, boron nitride, silicon nitride, boron carbide, titanium boride, titanium carbide, zirconium oxide, silicon oxynitride, and zirconia-toughened alumina. The reinforcing fibers should be resistant to oxidation at the service temperature since oxidation may produce a bonding of the fibers to the matrix, leading to brittle fracture rather than the desired fibrous fracture.

The fibers may have various sizes and forms. They may be monofilaments from 1 μm to 200 μm diameter or tows of 200 to 2000 filaments. When used in composites of the invention they may be woven into fabrics, pressed into mats, or unidirectionally aligned with the fibers oriented as desired to obtain the needed physical properties.

An important factor in the performance of the black glass composites is the bond between the fibers and the black glass matrix. Consequently, where improved strength is desired, the fibers have been provided heretofore with a carbon coating which reduces the bonding between the fibers and the black glass matrix as disclosed in co- pending applications 07/464,470 and 07/523,620. The surface sizings found on fibers as received or produced may be removed by solvent washing or heat treatment and the carbon coating applied. Various methods may be used, including chemical vapor deposition, solution coating, and pyrolysis of organic polymers such as carbon pitch and phenolics. One preferred technique is chemical vapor deposition using decomposition of methane or other hydrocarbons. Another method is pyrolysis of an organi polymer coating such as phenolformaldehyde polymer cross-linked with such agents as the monohydrate or sodiu salt of paratoluenesulfonic acid. Still another metho uses toluene-soluble and toluene-insoluble carbon pitch t coat the fibers. After pyrolysis, a uniform carbon coating is present. Multiple applications may be used to increase the coating thickness. A weakness of carbon coatings is that oxidation of the carbon at high temperatures causes the composite to lose strength and possibly to revert to brittle fracture. We have now found that by selection of the proper carbon thickness that a significant portion of the benefits of the carbon can be retained even after it has been lost to high temperature oxidation. More specifically, we have found in one embodiment of the invention that if the carbon coating is 50 to 200 nm that significant strength, elastic modulus and fibrous fracture can be retained, as will be seen in the examples below.

Processing

The black glass precursor is a polymer. It may be shaped into fibers and combined with reinforcing fibers or the black glass precursor may be used in solution for coating or impregnating reinforcing fibers. Various methods will suggest themselves to those skilled in the art for combining the black glass precursor with carbon-coated reinforcing fibers.

In one method, a continuous fiber is coated with a solution of the black glass precursor polymer and then wound on a rotating drum covered with a release film which is easily separated from the coated fibers. After sufficient fiber has been built up on the drum, the process is stopped and the unidirectional fiber mat removed from the drum and dried. The resulting mat (i.e., "prepreg") then may be cut and laminated into the desired shapes.

SUBSTITUT In a second method, a woven or pressed fabric of the reinforcing fibers is coated with a solution of the black glass precursor polymer and then dried, after which it may be formed into the desired shapes by procedures which are familiar to those skilled in the art of fabricating structures with the prepreg sheets.

A third method for fabricating the polymer composite is by resin transfer molding. In resin transfer molding a mold with the required shape is filled with the desired reinforcement material. The filled mold is injected, preferably under vacuum, with the neat monomer solution with an appropriate amount of catalyst. The filled mold is then heated to about 30^βC-150^βC for about 1/2-30 hours as required to cure the monomer solutions to a fully polymerized state.

Solvents for the black glass precursor polymers include hydrocarbons, such as toluene, benzene, isooctane, and xylene, and ethers, such as tetrahydrofuran, etc. Concentration of the prepregging solution may vary from about 10% to about 70% of resin by weight. Precursor polymer used in impregnating the fibers is usually prepared from solution polymerization of the respective monomers.

Since the precursor polymers do not contain any hydrolyzable functional groups, such as silanol, chlorosilane, or alkoxysilane, the precursor polymer is not water sensitive. No particular precaution is needed to exclude water from the solvent or to control relative humidity during processing.

The resin ages very slowly when stored at or below room temperatures as is evident from their shelf life of more than three months at these temperatures. The resin is stable both in the solution or in the prepreg. Prepregs stored in a refrigerator for three months have been used to make laminates without any difficulty. Also, resin solutions stored for months have been used for making prepregs successfully.

Large and complex shape components can be fabricated from laminating prepregs. One method is hand lay-up which involves placing the resin-impregnated prepregs manually in an open mold. Several plies of prepregs cut to the desired shape are laid-up to achieve the required thickness of the component. Fiber orientation can be tailored to give maximum strength in the preferred direction. Fibers can be oriented unidirectionally [0], at 90° angles [0/90], at 45° angles [0/45 or 45/90], and in other combinations as desired. The laid-up plies are then bonded by vacuum compaction before autoclave curing. Another fabrication method is tape laying which uses pre-impregnated ribbons in forming composites. The resins can be controlled to provide the desired tackiness and viscosity in the prepreg for the lay-up procedures.

After the initial forming, the composites may be consolidated and cured by heating to temperatures up to about 250"C under pressure. In one method, the composited prepreg is placed in a bag, which is then evacuated and the outside of the bag placed under a pressure sufficient to bond the layered prepreg, say up to about 1482 kPa. The resin can flow into and fill up any voids between the fibers, forming a void-free green laminate. The resulting polymer-fiber composite is dense and is ready for conversion of the polymer to black glass ceramic.

Heating the composite to temperatures from about 800*C up to about 1400"C in an inert atmosphere (pyrolysis) converts the polymer into a black glass ceramic containing essentially only carbon, silicon, and oxygen. Alternatively, the heating may be done in the presence of oxygen if the rate of heating is above about 5'C/min, preferably 50 to 1000*C /min. It is characteristic of the black glass prepared by pyrolyzing the cyclosiloxanes

SUBSTITUTE SHEET described above that the resulting black glass has a large carbon content, but is able to withstand exposure to temperatures up to about 1400°C in air without oxidizing to a significant degree. Pyrolysis is usually carried out with a heating to the maximum temperature selected, holding at that temperature for a period of time determined.by the size of the structure, and then cooling to room temperature. Little bulk shrinkage is observed for the black glass composites and the resulting structure typically has about 70-80% of its theoretical density.

Conversion of the polymer to black glass takes place between 430*C and 950*C. Three major pyrolysis steps have been identified by thermogravimetric analysis at

430*C-700'C, 680^βC-800*C and 780*C-950^βC. The yield of the polymer-glass conversion up to 800'C is about 83%; the third pyrolysis mechanism occurring between 780"C and 950*C contributes a final 2.5% weight loss to the final product.

Since the pyrolyzed composite structure still retains voids, the structure may be increased in density by impregnating with a neat monomer liquid or solution of the black glass precursor polymer. The solution is then gelled by heating to about 50*C-120*C for a sufficient period of time. Following gelation, the polymer is pyrolyzed as described above. Repeating these steps, it is feasible to increase the density up to about 95% of the theoretical.

It has been shown previously in co-pending application USSN 07/523,620 that the high temperature strength of composites can be substantially improved by continued impregnation of the composite with black glass solutions, which is believed to seal off micropores in the black glass coating making it possible to defend the carbon coating against destructive oxidation. We have now found that by proper selection of the thickness of the carbon coating that even if the carbon has been lost by high temperature oxidation that fibrous fracture can still be obtained and significant strength and elastic modulus retained.

A ceramic matrix reinforced with ceramic fibers need not fail in a brittle manner, but may achieve the graceful type of failure called fibrous fracture. In such failures the ceramic composite is able to sustain a load (i.e., stress) with a greater elongation (i.e. strain) and thus the composite can avoid a catastrophic brittle failure. Previously, thin coatings of about 50 nm have been used on fibers to provide a fibrous failure, however, these coatings have not been entirely successful since at high temperatures the effect of the coatings has been found to be lost. For example, carbon coatings can be oxidized and thus are no longer available to assist in the transfer of the load from the ceramic to the fiber. In the case of boron nitride, another coating used to provide fibrous fracture, the coating is converted to boron oxide at high temperature, which is believed to bond the fibers to the ceramic matrix, thus resulting in a reversion to brittle fracture. One might conclude that what is needed is a fiber coating which is able to stand exposure to high temperatures. However, the present inventors have discovered that instead of a coating an optimum spacing between the fibers and the matrix can serve to provide fibrous fracture and useful strength and elastic modulus after exposure to higher temperatures than have been heretofore possible.

As will be seen in the examples below, the width of the space between the reinforcing fibers and the matrix appears to be a critical factor in determining whether the failure mode is brittle or fibrous and what the strength and elastic modulus of the composite may be. When a series of fibers having carbon coatings of different thickness are used to form ceramic composites and then heated in air at temperatures which will oxidize the carbon, the performance of the composites are significantly affected. It will be shown that, when the spacing created by oxidation of the carbon coating is too small brittle fracture is found while, when the spacing is too large, the strength at failure is poorer than with thinner coatings. Therefore, the inventors have concluded that an optimum spacing should be found between the reinforcing fibers and the matrix. That optimum spacing which will be determined by a number of factors. It is believed that to achieve fibrous fracture the fibers must be free to move within the matrix material, but not so freely that a load cannot be transferred to the fiber. Thus, a fiber which cannot move at all would be expected to be loaded fully and to break on the plane on which the matrix breaks. This will be seen in Figures 3 and 4 where no coating is used or where a 50 nm coating has been applied. It should be noted that the high performance fibers used in ceramic matrices are generally brittle, although they will have higher strength than the matrix. Therefore, transfer of a load from the matrix to the fibers is desirable, but brittle fracture is not. However, a fiber which can move freely would not receive a transmitted load, and might pull out from the matrix. Consequently, the strength of the composite is low and the failure is fibrous. In between these two extremes, a fiber should move under load but accept some of the load, thus increasing the strength while avoiding brittle fracture. Once this behavior is recognized, it should follow that an optimum interaction between the fibers and the matrix should exist. This optimum should be determined by a number of factors, such as the surface roughness of the fiber, its physical properties, the spacing between the fibers and matrix, and any fiber coating which fills that space. A fiber coating acts as a means to transfer the load between the matrix and the fiber. Carbon is an attractive coating since it is inexpensive and easil deposited, but is removed by oxidation. The optimu spacing between the fibers and the ceramic matrix i considered to be about 50 to 200 nm in the Nextel*- black glass composite shown in the examples below. Th inventors expect that similar spacing would be effective for other composites where carbon is used as a coating, but some variation in the optimum spacing would be expected, depending upon the factors mentioned above. Example 1

Polymer Precursor Preparation The cyclosiloxane having silicon-vinyl bond was poly(vinylmethylcyclosiloxane) (ViSi) . The cyclosiloxane with a silicon-hydride bond was poly(methylhydrocyclo- siloxane) (HSi) . Both cyclosiloxanes were mixtures of oligomers, about 85% by weight being the cyclotetramer with the remainder being principally the cyclopentamer and cyclohexamer. A volume ratio of 59 ViSi/41 HSi was dissolved in isooctane to make a 10 vol. percent solution. Twenty-two (22) wt. ppm of platinum as a platinum- -cyclovinylmethylcyclosiloxane complex was added as a catalyst. The solution was heated to reflux conditions (about 100*C) and refluxed for about 2 hours. Then, the solution was concentrated in a rotary evaporator at 50*C to a 25-35% concentration suitable for use in prepregging. The resin produced was poly(methylmethylenecyclosiloxane) (PMMCS) . It was tacky at room temperature and was flowable at temperatures of about 60*C or higher under pressure and thus was suitable for use as a B stage resin. Example 2

Five panels of Nextel* 440 fabric (silica-alu ina- boria fibers in a plain weave, BF-18 from 3-M, Inc.) measuring 12.5" x 36" (317 mm x 914 mm) and having an areal weight of 372 gm/m² were heated at 800^*C in air to remove the sizing. Four of the panels of Nextel* 440 fabric were coated with carbon using a pyrolytic chemical vapor deposition process to provide carbon thicknesses of 500A (50 nm) , lOOOA (100 mm), 2000A (200 nm) and 3000A (300 nm) . The carbon-coated panels were fabricated into composites having a black glass matrix derived from pyrolysis of a polymer prepared as in Example 1.

Sections measuring 12" x 24" (304.8 mm x 609.6 mm) were cut from each fabric panel for coating with the polymer precursor. Each panel was coated with a 3Q wt.% solution in isooctane of a polymer prepared as described in Example 1. The fabric was dried for 2 hours in a flowing stream of air and a resin content of about 45 wt.% was obtained, based on the dried polymer and fabric. The resulting five prepreg panels were then cut into eight 6" x 6" (152.4 mm x 152.4 mm) squares which were laid-up to form five composite panels. The fabric was laid-up with the weave alternating in orientation as follows 0/90/0/90/90/0/90/0 i.e. the fabric in each layer contacted at least one and generally two layers oriented 90* from the first layer. The composite panels were placed under vacuum (13 kPa absolute) in an autoclave and then the panels were heated to 60*C under vacuum over 1 hour and then held at temperature for 30 minutes. The pressure was raised to 100 psig (689 kPa gauge) and the panels were heated to 150*C over 45 minutes and then held at those conditions for 1 hour. The temperature was reduced to 70'C over 45 minutes while holding the pressure at 100 psig (689 kPa) . Then, the pressure was released and the panels were free cooled to ambient conditions. Each cured composite panel was cut into 3" x 1/4" (76.2 mm x 6.35 mm) test bars, which were pyrolyzed to convert the polymer to a black glass. The pyrolysis procedure heated the test bars to 900*C at a rate of 90'C/hr in flowing nitrogen, held that temperature for 10 minutes and then cooled to room temperature at 180*C/hr. The density of the test bars was increased b infiltration with a neat polymer solution (59 vol. % ViS and 41 vol. % HSi and 22 wt. ppm Pt catalyst) followed b pyrolysis. The infiltration was done by placing the tes bars under a vacuum (about 13 kPa absolute) for 5 minutes, adding the polymer, and evacuating again for 5-10 minutes. The infiltrated bars were cured at 50"C overnight in ai and then pyrolyzed using the procedure outlined above. Th procedure was repeated until the final weight gain, in a single infiltration-pyrolysis cycle was 1.5% or less. Typically, the test bars were infiltrated and pyrolyzed five times.

The fully infiltrated test bars were tested on an Instron 1114 Universal Testing Machine in a three-point flexure mode with a 2.8" (71.1 mm) outer span. The crosshead speed was 0.5 mm/min and the load measured with a CCT (100#) load cell. The load was perpendicular to the plane of the fabric plies. Thus, the tensile stresses were in the plane of the fabric. Stress values were calculated from standard elastic stress formulas — σ (3-point) = 1.5 P L / (bd²) where P = load

L = outer span b = width of test bar d = thickness of test bar Strain values were calculated from ram displacement using e (3-point) = 6 y d / L² where y = ram displacement (calculated from crosshead speed and elapsed time from point of initial load)

A stress-strain curve was plotted for each flexure test and these are shown in Figures 1 and 2.

Fracture stress (σ_f) was calculated from maximum load. Fracture strain (e_f) was calculated at the point of maximum load, correcting for compliance observed in the stress-strain curve. The linear elastic strain (e_e) was determined from the point where the stress-strain curve deviated from linearity. The elastic modulus (E) was calculated from the stress-strain curve by drawing a straight line along the major elastic portion of the curve and calculating the slope of stress versus strain.

The maximum shear stress was calculated from the equation: ^τ ax (3-point) = σ_f / (2 S) where S = span-to-depth ratio

(for these test bars S = 2.8/0.1 = 28) The samples were exposed to 1000^βC for 4, 48 and 100 hours in air to determine the effect of oxidation on the carbon coating and the resulting effect on the strength of the samples. The results of these tests are given in the following table, which reports the mean values for a total of more than sixty test bars.

Coating Thickness

As-Prepared No Coat 50 nm 100 nm 200 nm 300 nm

Fracture Strength (kPa) 0.288x10^s 0.98x10^s 0.887x10' 1.11x10' 1.5x10' (KSI )* 4.17 14.21 12.87 16.08 21.7 Fracture Strain (%) 0.06% 0.18% 0.20% 0.25% 0.44%

Elastic Modulus (kPa) 52.4x10^s 55.4x10^s 48.3x10^s 46.9x10^s 38.2x10^s

(MSI)** 7.6 8.04 7.00 6.80 5.54

Fai l ure Mode Brittl e Brittl e PartFi b PartFi b Fi brous

1000°C-4 hrs Fracture Strength (kPa) 0.354x10' 0.723x10' 0.62x10' 0.535x10' 0.436x10'

(KSI) 5.14 10.48 9.00 7.76 6.33

Fracture Strain (%) 0.08% 0.20% 0.20% 0.25% 0.56%

Elasti c Modul us (kPa) 47.6x10' 36.5x10' 33. 1x10' 26.2x10' 16.6x10' (MSI) 6.9 5.3 4.8 3.8 2.4 Fai l ure Mode Bri ttl e Bri ttl e Fi brous Fibrous Fibrous

1000^-48 hrs Fracture Strength (kPa) 0.339x10' 0.723x10' 0.628x10^s 0.482x10' 0.437x10'

(KSI) 4.91 10.49 9.11 6.99 6.34

Fracture Strain (%) 0.07% 0.17% 0.19% 0.25% 0.32% El astic Modul us (kPa) 50.3x10' 41.4x10' 33.8x10' 22.8x10' 17.2x10'

(MSI) 7.3 6.0 4.9 3.3 2.5

Failure Mode Brittle Brittle Fibrous Fibrous Fibrous

1000°C-100 hrs

Fracture Strength (kPa) 0.303x10' 0.697x10' 0.606x10' 0.491x10' 0.45x10' (KSI) 4.40 10.11 8.79 7.12 6.53

Fracture Strain (%) 0.06% 0.17% 0.18% 0.23% 0.28%

Elastic Modulus (kPa) 47.6x10' 40x10^s 33.1x10' 23.4x10' 19.3x10'

(MSI) 6.9 5.8 4.8 3.4 2.8

Fai l ure Mode Brittle Bri ttl e Fi brous Fibrous Fi brous * KSI ■ Kilopounds per square inch

** MSI « Mil l ion pounds per square inch

The data obtained in the tests are plotted in Figures 1 and 2. When a test bar has broken in brittle manner, the stress vs. strain curve drops vertically from the point of maximum stress to the strain axis. This is seen in the curves for fibers having no coating and 0.05 microns (50 n ) coating of Figure l. However, fibrous fracture has occurred when the vertical line at the maximum 'stress does not reach the strain axis but shows that a reduced load is still being carried by the test bar as the strain increases. This is seen in Figure 1 for fibers having coatings of 0.1, 0.2, and 0.3 microns (100, 100, 300 nm) . The composite increases as the carbon coating becomes thicker. It will be seen that the carbon coating provides fibrous failure when the thickness is above about 100 nm. However, when the carbon coating is oxidized by heating (Figure 2) the strength is decreased, presumably due to the loss of carbon by oxidation. The elastic modulus is also reduced which suggests that load transfer to the fiber is reduced when oxidation removes the carbon. Thicker coatings result in a wider gap after oxidation, leading to reduced load transfer. An initial carbon thickness of about 0.1 microns (100 nm) appears to provide an optimum thickness which gives improved strength while retaining a high elastic modulus and fibrous failure, although carbon has been removed by high temperature oxidation. Thus, it is concluded that the spacing of 100 nm between the fibers and the matrix provides an optimum strength and elastic modulus and retains fibrous fracture. Larger spacings provide lower strength and elastic modulus although fibrous fracture is retained.

Figures 3 and 4 illustrate the transition from brittle to fibrous fracture with increasing carbon thickness (Figure 3) or spacing (Figure 4). The reinforcing fibers are ceramic in nature and, while stronger than the matrix, will fail by brittle fracture when under tension. When no coating has been provided and the fibers are locked to the matrix a crack will proceed through the fibers without increasing the ability to accept a load even after oxidation. However, when a carbon coating has been applied to the fibers,they pull out of the matrix and a greater loading is possible (see Figure 1) When the coating is thick enough, the failure mode change from brittle to fibrous. In the case where the carbon laye has been lost by high temperature oxidation (Figure 4) th failure mode remains fibrous, even through the carbon is n longer available. It is concluded that by providing a optimum spacing between the fibers and the matrix that th advantages of a carbon coating can be retained, whic effectively extends the temperature range over- whic fibrous fracture can be obtained with a fiber-reinforce black glass composite.

Claims

26 CLAIMS

1. In a ceramic matrix reinforced with ceramic fibers having higher tensile strength than said matrix, the improvement comprising an optimized spacing between said fibers and said matrix

.whereby strength and elastic modulus are maximized and fibrous failure is achieved under load.

2. A reinforced ceramic composite comprising

(a) a silicon carboxide matrix having the nominal formula Si^Oy where x is greater than zero and up to 2.0 and y is greater than zero and up to 3.0;

(b) reinforcing fibers having a diameter of 1 to 200 μm and selected from the group consisting of boron, silicon carbide, graphite, silica, quartz, S-glass, E-glass, a l um i na , a l um i n o s i l i c at e , aluminoborosilicate, boron nitride, silicon nitride, silicon oxynitride, boron carbide, titanium boride, titanium carbide, zirconium oxide, and zirconia toughened alumina; (c) a carbon coating deposited on the reinforcing fibers of (b) having a thickness of 50 to

200 μm; wherein said coating of (c) has a thickness selected to maximize the strength of said composite in the absence of said coating.

3. A composite of Claim 2 wherein said silicon carboxide matrix is the pyrolyzed reaction product of a polymer prepared from (a) a cyclosiloxane monomer having the formula

R'

(Si-O)

B where n is an integer from 3 to about 30, R is hydrogen, and R' is an alkene of from -2 to about 20 carbon atoms in which one vinyl carbon atom is directly bonded to silicon or (b) two or more different cyclosiloxane monomers having the formula of (a) where for at least one monomer R is hydrogen and R' is an alkyl group having from 1 to about 20 carbon atoms and for the other monomers R is an alkene from about 2 to about 20 carbon atoms in which one vinyl carbon is directly bonded to silicon and R' is an alkyl group of from 1 to about 20 carbon atoms, or (c) cyclosiloxane monomers having the formula of (a) where R and R' are independently selected from hydrogen, an alkene of from 2 to about 20 carbon atoms in which one vinyl carbon atom is directly bonded to silicon, or an alkyl group of from 1 to about 20 carbon atoms and at least one of said monomers contains each of said hydrogen, alkene, and alkyl moieties, said polymerization reaction taking place in the presence of an effective amount of hydrosilylation catalyst.

4. The composite of Claim 3 wherein said polymer is pyrolyzed in a non-oxidizing atmosphere to a temperature in the range of about 800*C to about 1400'C or in an oxidizing atmosphere at a rate exceeding 5*C/min to the same temperature range.

5. The composite of Claim 10 wherein said polymer contains the moieties R^* R° R"

0

\

Si - CH - CH? - Si

SUBSTITUTESHEET R* R°CH, R*

Si - CH - Si

where R° is the unreacted residue of an alkene having 2 to 20 carbon atoms R* is H, an alkyl group having 1 to 20 carbon atoms, or an alkene having 2 to 20 carbon atoms

6. In the method of preparing fiber-reinforced glass composites comprising reacting cyclosiloxane monomers having silicon-vinyl bonds with cyclosiloxane monomers having silicon-hydride bonds to form a polymer, applying said polymer to refractory fibers having a carbon coating to form a green structure, and pyrolyzing said green structure to form a fiber-reinforced glass composite, the improvement comprising optimizing said carbon coating thickness to maximize strength and elastic modulus to obtain fibrous fracture and removing said carbon coating by oxidation.