Ceramic Composite Containing Coated Fibrous Material

This application is related to Ser. No. 056,516, filed Jun. 1, 1987 for Borom et al., and Ser. No. 100,806, filed Sep. 24, 1987; Ser. No. 216,488, filed Jul. 8, 1988; Ser. No. 216,471, filed Jul. 8, 1988; Ser. No. 135,858, filed Dec. 21, 1987; Ser. No. 132,753, filed Dec. 14, 1987, now U.S. Pat. No. 4,886,682; all for Singh et al. The referenced applications are directed to the production of fiber containing ceramic composites. They are assigned to the assignee hereof and are incorporated herein by reference.

U.S. Pat. Nos. 4,120,731; 4,141,948; 4,148,894; 4,220,455; 4,238,433; 4,240,835; 4,242,106; 4,247,304, 4,353,953, 4,626,516, and 4,737,328; assigned to the assignee hereof and incorporated herein by reference, disclose silicon infiltration of materials which include carbon, molybdenum, carbon-coated diamond and/or cubic boron nitride, and blends of carbon with silicon carbide, boron nitride, silicon nitride, aluminum oxide, magnesium oxide, and zirconium oxide.

This invention relates to the production of a composite containing boron nitride-coated fibrous material in a matrix containing a phase of elemental silicon.

Fiber reinforcement of brittle ceramic materials offers significant opportunities for toughening of the brittle matrix. Many efforts have been extended to produce fiber reinforced, high temperature materials. Structures of carbon fiber reinforced carbon matrices have been used in aircraft construction but they have the disadvantage of poor to no oxidation resistance (i.e. they burn). High strength carbon fibers were infiltrated with molten silicon with the hope that the silicon matrix would protect the carbon filaments. However, the carbon filaments converted instead into relatively weak, irregular columns of SiC crystals resulting in composites with low toughness and relatively modest strength.

As an alternative approach, attempts have been made to incorporate SiC type fibrous material in a silicon matrix by the process of silicon infiltration. There are a number of problems when silicon carbide fibrous material is infiltrated with silicon. Even though SiC has limited solubility in molten silicon, this solubility leads to transport and recrystallization of SiC thereby causing the SiC fibers to lose strength. Also, silicon carbide forms a strong bond with silicon which results in brittle fracture of the composite.

The present invention utilizes silicon infiltration of a porous body comprised of boron nitride coated fibrous material and infiltration-promoting material to produce a composite in which the fibrous material has not been affected, or has not been significantly deleteriously affected, by processing conditions.

In one embodiment, since boron nitride is not wettable by silicon, a coating of a silicon-wettable material is deposited on the boron nitride coating.

In another embodiment, boron nitride coated filaments of a sufficiently large diameter are used which do not require a silicon-wettable coating. Generally, these filaments or boron nitride coated filaments have a diameter of at least abut 50 microns and a minimum length of at least about 10 times their diameter.

In one embodiment of the present process, a slurry of infiltration-promoting material and organic binder is cast onto a layer of the coated fibrous material to form a tape therewith, the tape is fired to burn out binder and then infiltrated with molten silicon.

In another embodiment of the present process, a plurality of the tapes are formed into a layered structure, then laminated, fired to burn out binder and then infiltrated with molten silicon.

The present invention permits control of the structure of the final composite. It enables production of any desired spacing and alignment within a layer and between layers of coated fibrous material in the matrix.

Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with the accompanying figure which forms a part of the specification, in which:

FIG. 1 shows a graph illustrating the load-deflection behavior of one embodiment of the present composite comprised of spaced boron nitride coated filaments in a matrix, and another graph illustrating the load-deflection behavior of a monolithic body; and

FIG. 2 is an optical micrograph of a polished cross-section of the composite whose load-deflection behavior is shown in FIG. 1.

Briefly stated, one embodiment of the present process for producing a composite containing at least about 100% by volume of boron nitride coated fibrous phase and having a porosity of less than about 20% by volume comprises:

(a) forming a slurry of infiltration-promoting material and organic binding material in a liquid medium;

(b) depositing a coating of boron nitride on fibrous material leaving no significant portion thereof exposed;

(c) depositing a silicon-wettable coating on said boron nitride-coated fibrous material leaving no significant portion of said boron nitride exposed;

(d) providing the resulting coated fibrous material substantially as a layer;

(e) casting said slurry onto said coated fibrous material in an amount sufficient to form a tape therewith;

(f) evaporating said liquid medium forming a tape;

(g) firing said tape to remove said organic binding material producing a porous body;

(h) contacting said porous body with silicon-associated infiltrating means whereby said silicon is infiltrated into said porous body;

(i) heating the resulting assembly in a partial vacuum to a temperature at which said silicon is molten and infiltrating said molten silicon into said porous body to produce an infiltrated product; and

(j) cooling said product producing said composite.

In a preferred embodiment, a plurality of the tapes are assembled to form a layered structure, the layered structure is laminated, and the laminated structure is fired to remove the organic binding material to produce a porous body which is then infiltrated with molten silicon.

As used herein "fibrous material" includes fibers, filaments, continuous filaments, strands, bundles, whiskers, cloth, felt and any combination thereof. The fibrous material can be amorphous, crystalline or a mixture thereof. The crystalline fibrous material can be single crystal or polycrystalline.

In one embodiment, the fibrous material is selected from the group consisting of aluminum oxide, mullite, a carbon-containing material, elemental carbon, a SiC-containing material, silicon carbide, a silicon nitride-containing material, silicon nitride and mixtures thereof.

The carbon-containing material preferably contains carbon in an amount of at least about 1% by weight, frequently at least about 5% by weight, of the fibrous material.

As used herein, "elemental carbon" includes all forms of elemental carbon including graphite.

Reference herein to a fibrous material of silicon carbide, includes, among others, presently available materials wherein silicon carbide envelops a core or substrate, and which generally are produced by chemical vapor deposition of silicon carbide on a core or substrate such as, for example, elemental carbon or tungsten.

The SiC-containing fibrous material generally contains at least about 50% by weight of silicon and at least about 25% by weight of carbon, based on the weight of the fibrous material. Examples of SiC-containing materials are silicon carbide, Si--C--O, Si--C--O--N, Si--C--O--Metal and Si--C--O--N--Metal, where the Metal component can vary but frequently is Ti or Zr and wherein O, N and Metal are present generally in an amount of at least about 1% by weight of the fibrous material.

The silicon nitride-containing fibrous material generally contains at least about 50% by weight of silicon and at least about 25% by weight of nitrogen based on the weight of said fibrous material and is selected from the group consisting of Si--N--O, Si--C--O--N, Si--N--O--Metal, and Si--C--O--Metal, wherein said 0, C, and Metal are each present in an amount of at least about 1% by weight of said fibrous material.

There are processes known in the art which use organic precursors to produce SiC--and silicon nitride-containing fibrous materials which may introduce a wide variety of elements into the fibrous material.

The fibrous material is stable at the temperature of the present process. Preferably, the fibrous material has in air at ambient or room temperature, i.e. from about 20.degree. C. to about 30.degree. C., a minimum tensile strength of about 100,000 psi and a minimum tensile modulus of about 25 million psi.

In carrying out the present process, boron nitride is coated on the fibrous material to produce a coating thereon which leaves no significant portion of the fibrous material exposed. The boron nitride coating should be continuous, free of any significant porosity and preferably it is pore-free. Preferably, the boron nitride coating is of uniform or at least significantly uniform thickness.

The boron nitride coating can be deposited on the fibrous material by a number of known techniques under conditions which have no significant deleterious effect on the fibrous material. Generally, the boron nitride coating can be deposited by chemical vapor deposition by reactions such as:

Generally, the chemical vapor deposition of boron nitride is carried out at temperatures ranging from about 900.degree. C. to 1800.degree. C. in a partial vacuum, with the particular processing conditions being known in the art or determinable empirically.

The boron nitride coating should be at least sufficiently thick to be continuous. Generally, the coating is sufficiently thin so that the thermal expansion coefficient of the boron nitride coated fibrous material is the same as, or not significantly different from, that of the uncoated fibrous material. Generally, the thickness of the coating ranges from about 0.3 microns to about 5 microns, and typically it is about 0.5 microns. The particular thickness of the coating is determinable empirically, i.e. it should be sufficient to prevent reaction, or prevent significant reaction, between the fibrous material and the infiltrating silicon under the particular processing conditions used. The boron nitride coating should leave none or no significant portion of the fibrous material exposed. In the present invention, the boron nitride coating bars contact, or bars significant contact, between the fibrous material and the infiltrating silicon. During the infiltration process, the boron nitride coating may or may not react with or dissolve in the molten silicon depending on time and temperature, i.e. the boron nitride coating will survive better at lower temperatures and/or shorter times. Generally, silicon infiltration time increases with the size of the structure, i.e. preform. Larger-sized preforms, therefore, are more likely to require thicker boron nitride coatings.

A number of techniques can be used to determine if the boron nitride coating survived. For example, if the composite exhibits fiber pull-out on fracture, then the boron nitride coating has survived. Also, scanning electron microscopy of a cross-section of the present composite can detect a boron nitride coating on the fibrous material.

In one embodiment, a coating of a silicon-wettable material is deposited on the boron nitride coating substantially covering it. Preferably, the silicon-wettable coating leaves no significant portion or none of the boron nitride coating exposed. Also, preferably, the silicon-wettable coating is uniform or at least significantly uniform. Generally, the thickness of the silicon-wettable coating ranges from about 500 Angstroms to about 3 microns, and typically it is about 0.5 microns. Its particular thickness is determined empirically and depends largely on its rate of consumption, if any, and the particular composite desired.

The silicon-wettable coating is a solid in the present process. Its particular composition depends largely on the particular composite desired and is determined empirically. It should be sufficiently wetted by the infiltrating silicon to enable production of the present composite having a porosity of less than about 10% by volume. Preferably, the infiltrating silicon has a contact or wetting angle against the silicon-wettable material of less than 90 degrees to allow the infiltration to occur by capillarity. The silicon-wettable coating should have no significant deleterious effect herein.

Representative of useful silicon-wettable materials is elemental carbon, metal carbide, a metal which reacts with silicon to form a silicide, a metal nitride such as silicon nitride, and a metal silicide. Elemental carbon is preferred and usually, it is deposited on the boron nitride-coated material in the form of pyrolytic carbon. Generally, the metal carbide is a carbide of silicon, tantalum, titanium or tungsten. Generally, the metal silicide is a silicide of chromium, molybdenum, tantalum, titanium, tungsten and zirconium.

The metal which reacts with silicon to form a silicide thereof as well as the silicide must have melting points higher than the melting point of silicon and preferably higher than about 1450.degree. C. Representative of such metals is chromium, molybdenum, tantalum, titanium, tungsten and zirconium.

Known techniques can be used to deposit the coating of silicon-wettable material which generally is deposited by chemical vapor deposition using low pressure techniques.

The metal carbide or metal silicide coating can be directly deposited from the vapor thereof. Alternatively, the metal carbide coating can be formed in situ by initially depositing carbon followed by deposition of metal thereon under conditions which form the metal carbide. If desired, metal silicide coating can be produced by initially depositing the metal followed by deposition of silicon under conditions which form the metal silicide.

In carrying out the present process, the coated fibrous material, i.e., the fibrous material coated with boron nitride alone or with boron nitride and silicon-wettable material, can be provided substantially as a layer by a number of conventional techniques. For example, a layer of the coated fibrous material can be deposited on a flat surface. The amount of coated fibrous material is determined empirically and should be sufficient to produce the final composite containing at least about 10% by volume of boron nitride coated fibrous material. The layer of coated fibrous material can be continuous or discontinuous and it contains sufficient spacing to permit production of the present composite. There is sufficient spacing between the coated fibers, filaments, strands, bundles, or whiskers to permit penetration thereof by the infiltration-promoting material sufficiently to produce the present composite. The extent of spacing in the layer of coated fibrous material is determined empirically and depends largely on the size of the infiltration-promoting material and the particular composite desired.

The infiltration-promoting material is also a silicon-wettable material which is sufficiently wetted by the infiltrating silicon to enable the production of the present composite having a porosity of less than about 10% by volume. Preferably, the infiltrating silicon has a contact or wetting angle against the infiltration-promoting material of less than 90.degree. to allow the infiltration to occur by capillarity. The infiltration-promoting material should have no significant deleterious effect in the present process or on the resulting composite. The particular composition of the infiltration-promoting material is determined empirically and depends largely on the particular composite desired, i.e. the particular properties desired in the composite.

Representative of useful infiltration-promoting materials is elemental carbon, a metal carbide, a metal which reacts with silicon to form a silicide, a metal nitride, a metal silicide and mixtures thereof.

Elemental carbon is a preferred infiltration-promoting material. Preferably, elemental carbon comprises from about 5% to about 100%, or from about 10% to about 90%, by volume of the infiltration-promoting material.

Representative of a useful infiltration promoting metal carbide is boron carbide, molybdenum carbide, niobium carbide, silicon carbide, titanium carbide and mixtures thereof. Silicon carbide is preferred.

In a preferred embodiment, the infiltration-promoting material is comprised of a mixture of elemental carbon and silicon carbide, each being present in an amount of at least about 10% by volume of the infiltration-promoting material.

Representative of the infiltration-promoting metal nitrides is aluminum nitride, niobium nitride, silicon nitride, titanium nitride, zirconium nitride and mixtures thereof.

Representative of a metal which reacts with silicon to form a silicide is chromium, molybdenum, tantalum, titanium, tungsten, zirconium and mixtures thereof. Likewise, representative of infiltration-promoting metal silicides is chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide, zirconium silicide and mixtures thereof. The metal which reacts with silicon to form a silicide thereof as well as the silicide must have melting points higher than the melting point of silicon and preferably higher than about 1450.degree. C.

Generally, the infiltration-promoting material is a solid in the present process. However, in one embodiment, elemental silicon forms part of the infiltration-promoting material and ranges up to about 30%, preferably from about 1% to about 10% by volume of the infiltration-promoting material. Elemental silicon, as part of the infiltration-promoting material, is molten when silicon is infiltrated into the porous structure and promotes its infiltration.

The infiltration-promoting material is in a form useful for forming a slurry therewith and for forming the desired tape with the layer of coated fibrous material. A sufficient amount of the infiltration-promoting material is of a size which can penetrate the spaces in the layer of coated fibrous material sufficiently to produce the present composite. Preferably, the infiltration-promoting material is comprised of a size which can penetrate the spaces in the layer of coated fibrous material sufficiently to produce the present composite.

The infiltration-promoting material can be in the form of a powder, a fibrous material and any combination thereof. When the infiltration-promoting material is in the form of particles, generally the particles have a specific surface area ranging from about 0.2 to about 10 meters.sup.2 per gram, and frequently, ranging from about 2 to about 4 meters.sup.2 per gram. When the infiltration-promoting material is in the form of a fibrous material, it generally is in the form of whiskers, chopped fibers and mixtures thereof. Generally, the whiskers are less than about 50 microns in length and less than about 10 microns in diameter. Preferably, the whiskers are crystalline and are comprised of silicon carbide, silicon nitride and mixtures thereof. Generally, the chopped fibers are less than about 50 microns in length and less than about 10 microns in width. Preferably, the chopped fibers are comprised of graphite. The amount and type of infiltration-promoting material depends largely on the particular composite desired and is determined empirically.

The organic binding material used in the present process bonds the infiltration-promoting material together and enables formation of tape of desired thickness and solids content. By solids content, it is meant herein the content of infiltration-promoting material and coated fibrous material. The organic binding material, i.e. that component of the tape other than its solids content, thermally decomposes at an elevated temperature ranging to below about 800.degree. C., generally from about 50.degree. C. to below about 800.degree. C., and preferably from about 100.degree. C. to about 500.degree. C., to gaseous product of decomposition which vaporizes away leaving no significant deleterious residue.

The organic binding material is a thermoplastic material with a composition which can vary widely and which is well known in the art or can be determined empirically. Besides an organic polymeric binder it can include an organic plasticizer therefor to impart flexibility. The amount of plasticizer can vary widely depending largely on the particular binder used and the flexibility desired, but typically, it ranges up to about 50% by weight of the total organic content. Preferably the organic binding material is soluble in a volatile solvent.

Representative of useful organic binders are polyvinyl acetates, polyamides, polyvinyl acrylates, polymethacrylates, polyvinyl alcohols, polyvinyl butyrals, and polystyrenes. The useful molecular weight of the binder is known in the art or can be determined empirically. Ordinarily, the organic binder has an average molecular weight at least sufficient to make it retain its shape at room temperature and generally such an average molecular weight ranges from about 20,000 to about 200,000, frequently from about 30,000 to about 100,000.

Representative of useful plasticizers are dioctyl phthalate, dibutyl phthalate, diisodecyl glutarate, polyethylene glycol and glycerol trioleate.

In carrying out the present process, the infiltration-promoting material and organic binding material are admixed with a liquid medium to form a suspension or slurry which preferably is uniform or at least substantially uniform. A number of conventional techniques can be used to form the slurry. Generally, the components are milled in an organic solvent in which the organic material is soluble or at least partially soluble to produce a castable suspension or slurry, i.e. a slurry suitable for depositing on the layer of coated fibrous material to form a tape therewith. Examples of suitable solvents are methyl ethyl ketone, toluene and alcohol. The particular composition of the slurry is determined empirically. Generally, at least about 10% by volume of the slurry is comprised of infiltration-promoting material.

The tape can be cast by a number of conventional techniques. Preferably, the layer of coated fibrous material is deposited on a carrier from which the resulting tape can be easily released such as Teflon.RTM.. The slurry can be deposited on the layer of coated fibrous material to form a tape therewith of desired thickness and solids content which is determined empirically. Frequently, the slurry is cast on the layer of coated fibrous material by doctor blading. The cast tape is dried to evaporate the solvent therefrom to produce the present tape which is then removed from the carrier.

The particular amount of organic binding material used in forming the slurry is determined empirically and depends largely on the amount and distribution of solids desired in the resulting tape. Generally, the organic binding material ranges from about 25% by volume to about 80% by volume, frequently from about 25% to about 50% by volume, of the solids content of the tape.

The present tape or sheet can be as long and as wide as desired, and generally it is of uniform or substantially uniform thickness. Its thickness depends largely on the volume fraction of coated fibrous material which must be accommodated and the particular composite desired and is determinable empirically. The tape should be at least sufficiently thick to contain an amount of infiltration-promoting material and coated fibrous material to produce the desired composite. Generally, with increasing volume fractions of coated fibrous material, correspondingly smaller amounts of infiltration-promoting material would be required. Generally, the tape has a thickness ranging from about 25 microns (0.001 inch) to about 1300 microns (0.052 inch), frequently ranging from about 125 microns (0.005 inch) to about 1000 microns (0.040 inch), and more frequently ranging from about 250 microns (0.01 inch) to about 500 microns (0.02 inch).

Generally, the tape is flexible or may be made flexible by the application of heat, and can be bent to form a desired shape.

In one embodiment of the present process, the coated fibrous material is comprised of boron nitride coated filaments preferably with a diameter ranging from about 50 microns to about 250 microns, frequently from about 70 microns to about 200 microns, or from about 100 microns to about 150 microns. This boron nitride coated filament is continuous and can be as long as desired. It has a minimum length of at least about 10 times its diameter, and generally, it is longer than about 1000 microns, or it is longer than about 2000 microns. The minimum diameter of the coated filament depends largely on the minimum spacing required between the coated filaments through which the infiltration-promoting material must penetrate and is determined empirically. For a given volume fraction of coated filaments, as the diameter of the coated filament decreases, the total amount of space between coated filaments decreases making it more difficult for the infiltration-promoting material to penetrate the space.

Preferably, in one embodiment, a preform comprised of a layer of a plurality of continuous boron nitride coated filaments which are spaced from each other and which are parallel, or at least substantially parallel, to each other is used. The minimum space between the coated filaments is at least sufficient to enable the infiltration-promoting material to penetrate therebetween, and generally, it is at least about 50 microns, and frequently at least about 100 microns. Generally, the spacing between coated filaments in a single layer is substantially equivalent, or if desired, it can vary. Filament loading in the composite can be varied by changing the spacing between the coated filaments and/or tape thickness. In a preferred embodiment, the filaments are comprised of silicon carbide or elemental carbon and the slurry contains whiskers of silicon carbide. The present invention enables the production of a final composite with a high volume fraction of uniaxially aligned boron nitride coated continuous filaments.

The preform of coated filaments can be produced by a number of conventional techniques. For example, the coated filaments can be uniaxially aligned and spaced by placing them in a suitable device provided with grooves and the desired spacing. The layer of coated filaments can be lifted off the device with adhesive tape placed across both ends of the filaments. The slurry can then be deposited on the layer of coated filaments to produce a tape therewith.

In a preferred embodiment of carrying out the present process, a plurality of the tapes are assembled into a layered structure. The number of tapes used can vary widely depending largely on the particular composite desired. Preferably, the tapes in the layered structure are at least substantially coextensive with each other, i.e. substantially a sandwich-type structure.

In one embodiment, before assembly of the layered structure, a solution of the present organic binder in organic solvent is deposited, generally sprayed, on the faces of the tapes to be contacted with each other, dried to evaporate the solvent and leave a sticky film of organic binder to enhance adhesion. The concentration of organic binder in solution can vary widely and generally ranges from about 1% by weight to about 10% by weight of the solution. The solution is sprayed on the face of the tape for a period of time, determinable empirically, so that on evaporation of the solvent sufficient sticky binder remains to significantly enhance adhesion or facilitate bonding of the tapes. Preferably, drying is carried out in air at ambient temperature in less than a minute, and typically, in a few seconds. The deposited binder can be a continuous or a discontinuous coating, and typically, 0.2 milligrams of sticky binder per square centimeter of surface is adequate.

The layered structure is then laminated under a pressure and temperature determined empirically depending largely on the particular composition of the organic binding material to form a laminated structure. Lamination can be carried out in a conventional manner. Laminating temperature should be below the temperature at which there is decomposition, or significant decomposition, of organic binding material and generally, an elevated temperature below 150.degree. C. is useful and there is no significant advantage in using higher temperatures. Typically, the lamination temperature ranges from about 35.degree. C. to about 95.degree. C. and the pressure ranges from about 500 psi to about 3000 psi. Generally, lamination time ranges from about 1/2 to about 5 minutes. Also, generally, lamination is carried out in air.

If desired, the laminated structure can be cut to desired dimensions by suitable means such as a diamond saw. Frequently, the laminated structure is flexible and can be bent to form a desired shape depending largely on fiber diameter and thickness of the laminated structure and may require some heat to impart flexibility thereto.

The laminated structure, or in another embodiment individual tape, is heated to thermally decompose the organic binding material therein producing a porous body comprised of the coated fibrous material and infiltration-promoting material. The rate of heating depends largely on the thickness of the sample and on furnace characteristics. At a firing temperature ranging up to about 500.degree. C., a slower heating rate is desirable because of the larger amounts of gas generated at these temperatures by the decomposition of the organic binding material. Typically, the heating rate for a sample of less than about 6 millimeters (6000 microns) in thickness can range from about 15.degree. C. per hour to about 30.degree. C. per hour. At a temperature of less than about 800.degree. C., thermal decomposition is completed leaving no significant deleterious residue.

Thermal decomposition can be carried out in any atmosphere, preferably at about or below atmospheric pressure, which has no significant deleterious effect on the sample such as, for example, argon. Preferably, thermal decomposition is carried out in a partial vacuum to aid in removal of gases.

The resulting porous body is comprised of coated fibrous material and infiltration-promoting material. The porous body has a structure and an open porosity which permit production of the present composite. Generally, the open porosity is distributed in or through the body. By open porosity it is meant herein pores, voids or channels which are open to the surface of the body thereby making the interior surfaces accessible to the ambient atmosphere or the infiltrant. The particular amount of open porosity is determined empirically and depends largely on the particular final composite desired. Generally, the porous body has an open porosity ranging from about 25% by volume to about 80% by volume of the body. Frequently, the porous body has an open porosity ranging from about 35% by volume to about 75% by volume or from about 40% by volume to about 50% by volume, of the body. Preferably, the open porosity enables the molten silicon to infiltrate by capillarity.

Preferably, the open porosity is comprised of small pores, preferably ranging from about 0.1 micron and about 50 microns, and preferably at least significantly or substantially uniformly distributed in or through the porous body thereby enabling the production of a composite wherein the matrix phase is at least significantly or substantially uniformly distributed in or through the composite. Also, this would produce a composite wherein the matrix phase has a thickness between the boron nitride coated fibers or fibrous material ranging from about 0.1 micron to about 50 microns.

Generally, the porous body has no closed porosity. By closed porosity it is meant herein closed pores or voids, i.e. pores not open to the surface of the body and therefore not in contact with the ambient atmosphere.

Void or pore content, i.e. both open and closed porosity, can be determined by standard physical and metallographic techniques.

In carrying out the present process, the porous body is contacted with silicon-associated infiltrating means whereby silicon is infiltrated into the body. The infiltrating means allows silicon to be infiltrated into the body. For example, an assembly is formed comprised of the porous body in contact with means that are in contact with silicon and which permit infiltration of molten silicon into the porous body. In one infiltration technique, the porous body is placed on a woven cloth of elemental carbon, a piece of silicon is also placed on the cloth, and the resulting assembly is heated to infiltration temperature. At infiltration temperature, the molten silicon migrates along the cloth and wicks into the porous body. After infiltration, the wicking carbon cloth may be removed from the composite by diamond grinding.

In another technique, the silicon infiltration procedure can be carried out as set forth in U.S. Pat. No. 4,626,516 which discloses an assembly that includes a mold with infiltration holes and a reservoir holding elemental silicon. A preform is placed within the mold and carbon wicks are provided in the infiltrating holes. The wicks are in contact with the preform and also with the silicon and at infiltration temperature the molten silicon migrates along the wicks into the preform.

U.S. Pat. No. 4,737,328 discloses another infiltration technique which comprises contacting a preform with a powder mixture composed of silicon and hexagonal boron nitride, heating the resulting structure to a temperature at which the silicon is fluid and infiltrating the fluid silicon into the preform. After infiltration, the hexagonal boron nitride powder is brushed off the composite.

The present assembly is heated to infiltration temperature in a nonoxidizing partial vacuum wherein the residual gases have no significantly deleterious effect on said assembly and the present infiltration is carried out in such nonoxidizing partial vacuum. Preferably, such nonoxidizing partial vacuum is provided before heating is initiated. The partial vacuum should be at least sufficient to avoid the entrapment of pockets of gas which would lead to excessive porosity, i.e. it should be sufficient to produce the present composite. Generally, such a partial vacuum ranges from about 0.01 torr to about 2 torr, and usually from about 0.01 tort to about 1 torr to insure removal of entrapped gas in the body being infiltrated.

Ordinarily and as a practical matter, the furnace used is a carbon furnace, i.e. a furnace fabricated from elemental carbon. Such a furnace acts as an oxygen getter for the atmosphere within the furnace reacting with oxygen to produce CO or CO.sub.2 and thereby provides a nonoxidizing atmosphere, i.e. the residual gases have no significantly deleterious effect on the infiltrating silicon. The present infiltration cannot be carried out in air because the liquid silicon would oxidize and form a dense silica coating before any significant infusion by silicon occurred. In such instance where a carbon furnace is not used, it is preferable to have an oxygen getter present in the furnace chamber, such as elemental carbon, in order to insure the maintenance of a nonoxidizing atmosphere. Alternatively, other nonoxidizing atmospheres which have no significant deleterious effect on the structure within the furnace can be used at partial vacuums of about 10.sup.-2 torr to 2 torr.

The present infiltration is carried out at a temperature at which silicon is molten and which has no significant deleterious effect on the porous body being infiltrated. The present infiltration temperature ranges from a temperature at which the silicon is molten to a temperature at which there is no significant vaporization of the silicon. Molten silicon has a low viscosity. The melting point of the silicon can vary depending largely on the particular impurities which may be present. Generally, the present infiltration temperature ranges from greater than about 1400.degree. C. to about 1550.degree. C., and preferably from about 1450.degree. C. to about 1500.degree. C. The rate of penetration of the silicon into the porous body depends on the wetting of the body by the silicon melt and the fluidity of the melt. With increase in temperature, the ability of the molten silicon to wet the porous body improves.

In the present process, sufficient silicon is infiltrated into the porous body to produce the present composite. The molten silicon is mobile and highly reactive. It has an affinity for silicon-wettable materials wetting them or wetting and reacting with them. It has a strong affinity for elemental carbon, wetting it and reacting with it to form silicon carbide. The molten silicon also has an affinity for any metal with which it reacts to form the silicide thereof. In addition, sufficient silicon is infiltrated into the porous body to fill pores or voids which may remain to produce the present composite.

The period of time required for infiltration by the silicon is determinable empirically and depends largely on the size of the porous body and extent of infiltration required. Generally, it is completed in less than about 20 minutes, and often in less than about 10 minutes.

The resulting infiltrated body is cooled in an atmosphere and at a rate which has no significant deleterious effect on it. Preferably, it is furnace cooled in the nonoxidizing partial vacuum to about room temperature, and the resulting composite is recovered.

In the present process, there is no loss, or no significant loss, of the coated fibrous material and infiltration-promoting material.

The present composite is comprised of boron nitride-coated fibrous material and a matrix phase. The matrix phase is distributed in or through the boron nitride-coated fibrous material and generally it is substantially completely space filling. Usually, the matrix phase is continuous and interconnecting. Generally, the matrix in the final composite has a thermal expansion coefficient ranging from lower than that of the boron nitride coated fibrous material to less than about 15% higher than that of the boron nitride coated fibrous material. The production of such a composite can be determined empirically. More preferably, for optimum mechanical properties of the composite, the matrix has a thermal expansion coefficient ranging from less than to about the same as that of the boron nitride coated fibrous material.

Generally, the matrix is in direct contact with more than 70% of the surface area of the boron nitride coated fibrous material. Frequently, the matrix coats or envelops each coated fiber, filament, strand, bundle or whisker of the boron nitride coated fibrous material sufficiently to be in direct contact with more than 80% preferably more than 90%, more preferably more than 99%, of the surface area of the boron nitride coated fibrous material in the composite. Most preferably, the boron nitride-coated fibrous material is substantially or totally enveloped by the matrix phase.

Generally, the boron nitride coated fibrous material ranges from about 10% to about 70%, frequently from about 20% to about 60% or from about 30% to about 50%, by volume of the composite.

The boron nitride coating on the fibrous material in the composite is detectable by scanning electron microscopy and generally ranges in thickness from about 0.5 microns to about 1.5 microns. The particular amount of boron nitride in the composite provided by the boron nitride coating depends largely on the amount of coated fibrous material present, the thickness of the boron nitride coating and the diameter of the fiber, filament, or whisker. Therefore, the volume fraction of boron nitride provided by the coating is the balance of the volume fraction of all other components of the composite. Frequently, however, the boron nitride coating on the fibrous material in the composite generally ranges from less than about 1% by volume to about 20% by volume, or from about 1% by volume to about 10% by volume or from about 1% by volume to about 5% by volume, of the total volume of boron nitride coated fibrous material. The boron nitride coating can be amorphous, crystalline or a combination thereof.

Generally, the boron nitride coating optimizes interfacial shear stress between the fibrous material and matrix resulting in a composite with a toughness significantly higher than that of a composite wherein the fibrous material is not coated with boron nitride.

Generally, there is no significant amount of reaction product formed, and preferably no reaction product detectable by scanning electron microscopy, directly between the matrix and the fibrous material coated with boron nitride.

The matrix comprises at least about 30% by volume of the composite. The composition of the matrix can vary and depends largely on the composition of the infiltration-promoting material and any silicon-wettable coating. Preferably, at least about 70% or at least 90%, or at least 95%, or more than 99%, by volume of the matrix is crystalline. In one embodiment, the matrix has an average grain size of less than about 100 microns, or less than about 50 microns, or less than about 20 microns, and most preferably less than about 10 microns. In another embodiment, part of the matrix is comprised of crystalline whiskers ranging up to about 50% by volume of the matrix. Preferably, the whiskers are comprised of silicon carbide, silicon nitride and mixtures thereof.

In one embodiment, at least about 70% by volume, or at least about 95% by volume or more than 99% by volume of the matrix is a crystalline ceramic selected from the group consisting of metal carbide, metal nitride, metal silicide and mixtures thereof.

Preferably, the metal carbide in the matrix is selected from the group consisting of boron carbide, molybdenum carbide, niobium carbide, silicon carbide, titanium carbide and mixtures thereof. Silicon carbide is most preferred.

Preferably, the metal nitride in the matrix is selected from the group consisting of aluminum nitride, niobium nitride, silicon nitride, titanium nitride, zirconium nitride and mixtures thereof.

Preferably, the metal silicide in the matrix is selected from the group consisting of chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, tungsten silicide, zirconium silicide and mixtures thereof.

Generally, the matrix contains elemental silicon in at least a detectable amount, i.e. an amount detectable by microprobe analysis or Auger electron spectroscopy. In one embodiment, elemental silicon ranges from a detectable amount to about 50% by volume, or from about 1% by volume to about 5% by volume, or to about 2% by volume, of the matrix. Generally, the elemental silicon phase is distributed in the composite, and preferably, it is distributed through the composite significantly uniformly.

The matrix may contain a phase of a metal which forms a silicide but which had not reacted with the infiltrating silicon. In such instance, it would be encapsulated by a metal silicide phase. Generally, such metal may range from an amount detectable by microprobe analysis, or from about 0.5% by volume to about 10% by volume, of the matrix.

The matrix of the present composite may contain a phase of elemental carbon which had not completely reacted with the infiltrating silicon. In such instance, the carbon would be totally encapsulated by a phase of silicon carbide formed in situ. Generally, this elemental carbon may range from an amount detectable by microprobe analysis, or from about 0.5% by volume, to about 10% by volume, of the matrix.

In one embodiment, the present composite is comprised of matrix containing a layer of boron nitride coated fibrous material.

In another embodiment, the present composite is comprised of a plurality of layers of boron nitride coated fibrous material in the matrix wherein the coated fibrous layers preferably are substantially parallel to each other and separated from each other by matrix. Preferably, the matrix is distributed in each layer of coated fibrous material generally significantly or substantially uniformly.

In another embodiment, the composite is comprised of a plurality of layers of boron nitride coated filaments in the matrix wherein the layers preferably are substantially parallel to each other and are separated from each other by matrix. In each layer, preferably more than 95% by volume, or more than 99% by volume, of the coated filaments, and preferably all or substantially all of the coated filaments, are spaced from each other and parallel or at least substantially parallel, to each other. Preferably, more than 95% by volume or more than 99% by volume or substantially all, of the coated filaments in each layer are aligned, or substantially aligned, in a single plane. Any misalignment of the coated filaments should not significantly degrade the mechanical properties of the composite. Also, preferably more than 95%, or more than 99%, or substantially all, of the surface area of the coated filaments is in direct contact with the matrix. Also, crystalline whiskers may be present up to about 50% by volume of the matrix.

The boron nitride coating optimizes interfacial shear stress between the filaments and matrix resulting in a composite with a toughness significantly or substantially higher than that of a composite wherein the filaments are uncoated. Specifically, if the matrix and filaments were in direct contact, even a slight reaction therebetween would increase interfacial bonding thereby requiring a higher stress to pull out the filaments making the composite less tough. If the interfacial bonding were too high, then the composite would fail in a brittle manner. In contrast, the present boron nitride coating provides an interfacial shear stress which is significantly lower than that produced with uncoated filaments thereby allowing the coated filaments to pull out more easily and gives the composite more toughness. The coated filaments prevent brittle fracture of the composite at room temperature. By brittle fracture of a composite it is meant herein that the entire composite cracks apart at the plane of fracture. In contrast to a brittle fracture, this embodiment of the composite exhibits filament pull-out on fracture at room temperature. Specifically, as this composite cracks open, generally at least about 10% by volume, frequently at least about 30% or 50 % by volume, of the coated filaments, and preferably all of the coated filaments, pull out and do not break at the plane of fracture at room temperature.

One particular advantage of this invention is that the present composite can be produced directly in a wide range of sizes. For example, it can be as long or as thick as desired.

Also, since the tape, sheet or laminated structure used to produce the present composite frequently is flexible, or may be made flexible by application of heat thereto, the present composite can be comprised of a variety of shapes such as tape, sheet, bent tape, bent sheet or any combination thereof. Therefore, the present composite can be of a form which is simple, complex, hollow or any combination thereof.

The present composite has a porosity of less than about 20%, preferably less than about 10%, or less than about 5%, more preferably less than about 1%, by volume of the composite. Most preferably, the composite is void- or pore-free, or has no significant porosity, or has no porosity detectable by scanning electron microscopy. Preferably, any voids or pores in the composites are less than about 70 microns, more preferably less than about 50 microns or less than about 10 microns, and generally they are distributed in the composite.

The present composite has a wide range of applications depending largely on its particular composition. For example, it is useful as a wear resistant part, acoustical part or high-temperature structural component.

The invention is further illustrated by the following examples where, unless otherwise stated, the procedure was as follows:

Commercially available continuous filaments of silicon carbide produced by a chemical vapor deposition process and sold under the trademark AVCO SCS-6 were used. These filaments had a 35 micron carbon core on which silicon carbide was deposited to an overall diameter of about 145 microns. The outside surface of the filaments consisted of two layers of pyrolytic carbon and carbon-silicon, with overall thickness of about 3 microns. In air at room temperature these filaments have a tensile strength of about 500 thousand psi and a tensile modulus of about 60 million psi. These filaments have an average thermal expansion coefficient of less than about 5.0.times.10.sup.-6 in/in-.degree. C.

The filaments were cut to a length of about 2 inches and were coated with boron nitride by the following low pressure chemical vapor deposition process utilizing the reaction B.sub.3 N.sub.3 H.sub.3 Cl.sub.3 .fwdarw.3BN+3HCl. Specifically, the filaments were placed on a molybdenum screen which was then positioned at about the midpoint of the hot zone of a pyrex/quartz/pyrex furnace tube. A 1.00 gram sample of commercial trichloroborazine (B.sub.3 N.sub.3 H.sub.3 C.sub.13) was transferred in an argon-filled glove box to a pyrex end-section which contained a thermocouple vacuum gauge, a cold trap and a vacuum stopcock. The closed pyrex end-section was then taken out of the glove box and attached to an end of the furnace tube and to a vacuum system. The end-section containing the trichloroborazine was then cooled using liquid nitrogen and the furnace tube was opened to the vacuum system via the stopcock of the pyrex end-section. After the system reached a pressure lower than 0.020 torr, the furnace was heated to about 1050.degree. C. When the pressure had again dropped below 0.020 torr and the furnace temperature had stabilized, the end-section containing the trichloroborazine was warmed by an oil bath maintained at 60.degree. C., whereupon the solid began to vaporize, depositing BN and liberating gaseous HCl in the hot zone of the furnace tube and producing an increase in pressure. The pressure was observed to reach as high as about 200 torr before stabilizing at about 50 torr. After two hours, the pressure was found to have decreased to about 0,020 torr, whereupon the furnace was shut down and the system allowed to cool to room temperature before opening the tube and removing the sample. Identification of the chemically vapor deposited layer as BN was accomplished by means of electrical resistance measurement and a quantitative ESCA analysis of a film deposited in substantially the same manner on a SiC disk surface. This film was amorphous to x-rays in the as-deposited condition and appeared fully dense and smooth at high magnification in the SEM. Scanning electron microscopy observation of the ends of coated and broken filaments revealed that the coating was continuous and smooth and about 1.5 microns thick on the filament and left no significant portion of the filament exposed.

The boron nitride coated filaments were uniaxially aligned by placing them in a device for aligning filaments and maintaining the required spacing between them. This device was made from a copper foil laminated on a printed circuit board which was etched by the photolithographic technique in such a way as to produce parallel grooves about 0.006 inch diameter, 0.004 inch deep, and 0.008 inch apart (center-to-center). The coated filaments were placed on this device and a simple scoop of the filaments using a straight edge led to filling of each of the grooves with a filament. This resulted in a single layer of uniformly spaced coated filaments which was lifted off the board by putting adhesive tapes across each end portion of the filament layer. The adhesive tapes were sufficient to maintain the alignment and spacing between the coated filaments in the layer. Several such pre-formed layers of coated filaments were produced in which the coated filaments were substantially parallel and spaced about 100 microns from each other.

The silicon carbide powder had an average surface area of about 5 m.sup.2 /g.

The crushed carbon felt was comprised of chopped fibers sufficiently small to form a slurry.

By ambient temperature herein it is meant room temperature, i.e. from about 20.degree. C. to about 30.degree. C.

The organic binding material was comprised of commercially available organic binder comprised of polyvinylbutyral (average molecular weight of about 32,000) and commercially available liquid plasticizer comprised of polyunsaturated hydroxylated low-molecular weight organic polymers. Specifically, the organic binding material was comprised of 6.0 grams of polyvinylbutyral and 4.5 grams of liquid plasticizer.

Standard techniques were used to characterize the hot pressed composite for density, microstructure and mechanical properties.

72 grams of a crushed carbon felt and 48 grams of silicon carbide powder were dry mixed with 1.5 grams of organic binder. To this, 70 grams of a mixture consisting of toluene and methyl isobutyl ketone in the ratio 3:1 was added. This mixture was mixed for 5 minutes in a paint shaker. Then, 4.5 grams of the organic binder and 4.5 grams of liquid plasticizer was added to the mixture. To this, 0.1 gram of a commercially available organic silicone oil (which can be considered a part of the organic binding material) was added. This mixture was ball milled for about 30 minutes at room temperature to form a slurry. The slurry was then deaired in a vacuum. Each pre-formed layer of boron nitride coated filaments was deposited on a Kodacel sheet, the slurry was deposited on the filaments using a doctor blade, the cast tape was then dried in air at room temperature and atmospheric pressure to remove the solvent, and the resulting tape was stripped from the Kodacel sheet.

The tape was about 6 inches wide and had a substantially uniform thickness of about 0.012 inch.

The tape was cut to the length and width of the aligned layer of filaments. A number of such tapes were produced.

A layered sandwich-type structure was formed comprised of four layers of tape. Before assembly, to enhance adherence, the faces of the tapes which were to be contacted with each other were sprayed with an organic solution of binder, and dried for a few seconds in air at room temperature leaving a coating of sticky organic binder. Specifically, a solution comprised of 3 weight % of commercially available polyvinylbutyral (average molecular weight of about 32,000) 39 weight % toluene 9.5 weight % acetone, 39 weight % xylene and 9.5 weight % ethanol was used. The solution was sprayed on the faces of the tapes for a sufficient time so that upon evaporation of the solvent there remained about 0.2 milligrams of sticky organic binder per square centimeter of surface.

The resulting layered structure was laminated in air in a laminating press at about 93.degree. C. under a pressure of about 1000 psi for about one minute.

The laminated structure was sliced perpendicular to the filament axis into a bar-shaped sample (1.50 inch.times.0.3 inch.times.0.10 inch) using a diamond saw. Examination of a cross-section showed uniform spacing between the filaments as well as between the layers of filaments.

The sample was placed in a vacuum oven for removing the organic binding material wherein the vacuum was typically about 20 millitorr. The burn-out cycle was comprised of heating the furnace at a rate of 30.degree. C. per hour to 500.degree. C., a five-hour hold at 500.degree. C. and a cool-down to room temperature at a rate of 200.degree. C. per hour. This led to complete removal of the organic binding material from the laminated structure which resulted in a porous body comprised of elemental carbon, silicon carbide and boron nitride coated filaments. From other work, it was known that the porous body had an open porosity of about 50% by volume.

The porous body and solid pieces of silicon infiltrant were placed on a woven carbon fabric which was contained in a BN-sprayed graphite tray. This tray was then placed in a carbon resistance heated bell jar furnace and slowly heated at a rate of about 10.degree. C. per minute to about 400.degree. C. in a vacuum of about 0.05 torr. Subsequent to this, the porous body was rapidly heated to 1420.degree. C. at which point the silicon was molten and reacted with the carbon cloth and got wicked into the preform. A considerable amount of heat which was detected by a thermocouple placed on top of the porous body was generated due to the exothermic reaction of silicon with carbon fibers in the matrix. The sample was held for 5 minutes under these conditions and then slowly cooled to below the melting point of silicon after which the furnace power was turned off and the infiltrated sample was cooled to room temperature in the vacuum of the bell jar.

The resulting composite was broken at room temperature in a three-point-bend configuration to determine fracture strength and load-elongation characteristics. It exhibited filament pullout, i.e. more than 10% by volume of the filaments pulled out and did not break at the plane of fracture.

FIG. 1 shows a load deflection curve for this composite. It can be seen that this composite showed toughened ceramic-like behavior. The load-deflection curve shows that at the onset of matrix cracking, the load carrying capability of the composite was maintained for a while reaching an ultimate strength of 60,605 psi (418 MPa) beyond which the composite showed substantial but not complete failure. This composite would be useful as a structural or wear-resistant part.

A polished cross-section (perpendicular to filament axis) of the composite, which is illustrated in FIG. 2, was examined. It showed uniform spacing between the filaments. It also showed that each layer of filaments was maintained in a substantially single plane. In addition, it showed a fully dense silicon-silicon carbide matrix surrounding each individual boron nitride coated filament and in direct contact therewith. No porosity was detected by microscopy.

It was estimated that this composite was comprised of about 65% by volume of silicon carbide phase, about 15% by volume of elemental silicon phase and about 20% by volume of boron nitride coated filaments.

From other work, it was known that the average thermal expansion coefficient of the matrix was lower than that of the boron nitride coated filaments.

This example was carried out in substantially the same manner as Example 1, except that no filaments were used.

For comparison, four tapes comprised of silicon carbide powder, crushed carbon felt, and organic binding material were produced, formed into a layered sandwich-type structure, laminated, heated to remove organic binding material, and melt infiltrated in substantially the same manner as the sample of Example 1 to produce a monolithic body of substantially the same size which was broken in substantially the same manner. Its load deflection curve is also shown in FIG. 1. It fractured in a brittle manner at 41,000 psi (283 MPa).