Materials and methods for making ceramic matrix composites

Abstract

Ceramic matrix composites and fiber reinforced ceramic matrix composite components of brake systems and other friction tolerant composite articles of this invention are made by providing a fiber preform, coating the fibers with an interface layer of carbon or ceramic, infiltrating the coated fiber preform with a composition comprising a liquid ceramic forming polymer including friction controlling additives, and pyrolyzing the polymer in the infiltrated preform to form a ceramic matrix around the fibers of the preform.

Description

This invention is directed to materials and methods for manufacture of ceramic matrix composites, including fiber reinforced ceramic matrix composites, for use in high temperature and high friction energy applications such as brake components for aircraft, heavy vehicles, racing vehicles, sports utility vehicles, and mechanical power transmission equipment. More particularly the invention is directed to ceramic composite materials and fiber reinforced composite materials having optimized friction coefficients for high energy applications and uses. The invention provides a major innovation in the ability to regulate and adapt the ceramic composition to changing friction requirements by choice or selection of the preceramic polymer and the type and amount of additive powders.

BACKGROUND

Vehicles are generally provided with braking systems for speed and for slowing moving vehicles. Brake systems generally comprise rotating components associated with the vehicle wheels and stationary components which are forced against the moving part to frictionally slow, control, and stop movement of the wheels. In high energy stopping, significant quantities of heat are generated by friction between the moving and fixed brake components. Friction is necessary to slow and stop the vehicle. Without friction there is no stopping force. When there is a high level of friction the heat generated can be sufficient to damage brake components, including parts other than the moving parts. Excess wear, surface erosion, seizing, and fire are possible results of undissipated frictional heat. In emergency stopping situations involving heavy equipment such as trains, trucks, and heavy aircraft, friction heat can completely destroy the brake system rendering it useless for control and necessitating extensive repair or total replacement. Other components such as rubber wheels, hoses, hydraulic fluids, fuel tanks, and the like can ignite and burn.

The frictional heat problem in heavy equipment can be illustrated and understood by considering emergency stopping of a large heavy commercial or military aircraft in an aborted take-off situation. As previously noted, effective stopping requires good friction in the moving and stationary parts of the brake system. Aircraft braking systems generally include brake stacks comprising rotors carried on the wheel shaft and which rotate with the wheels and adjacent stators that are fixed and do not rotate. The brakes are activated by compressing the brake stack thereby squeezing the stators against the rotors. In a normal landing stop and runway taxiing the brakes are applied gradually and intermittently to reduce ground speed. The rotors and stators are not continuously engaged. This allows for dissipation of heat from the contact surfaces by air flow during the slow down period. In the emergency circumstances of an aborted take-off, the ground speed of the aircraft is approaching take-off speed and the only means to stop the aircraft is the wheel brakes. The extreme braking condition requires full and continuous compression of the brake stack and frictional contact between rotors and stators. The large amount of heat generated by friction can not be reduced or dissipated by air flow around the contact surfaces. Fire, tire blowout, and brake seizure are common before and after complete stop.

Carbon rotor and carbon stator brakes were developed to solve the above-described problems. However, carbon/carbon brakes exhibit low friction characteristics until the contact surfaces get hot. Should the need for an emergency quick stop arise before sufficient friction has built up, the airplane cannot be stopped. They are porous and absorb moisture in humid environments leading to decreased performance. The porous materials have been shown to be subject to contamination and property loss by de-icing and other fluids. These materials oxidize at a temperature similar to that experienced during certain taxiing conditions, they generate corrosive dust, and they are very expensive to make. The carbon rotors and carbon stators are formed by an infiltration process that is very expensive and literally takes weeks to accomplish. Because the problem of brake seizure is eliminated, many airlines and the military presently use the carbon/carbon brakes despite their shortcomings.

BRIEF DESCRIPTION OF THE INVENTION

Ceramic matrix composites and fiber reinforced ceramic matrix composite components of brake systems and other friction tolerant composite articles of this invention are made by providing a fiber preform, coating the fibers with an interface layer of carbon or ceramic, infiltrating the coated fiber preform with a composition comprising a liquid ceramic forming polymer including friction controlling additives, and pyrolyzing the polymer in the infiltrated preform to form a ceramic matrix around the fibers of the preform. Important features of the invention include the use of near-stoichiometric silicon carbide forming matrix polymer formulations comprising particulate friction controlling additive materials that produce a transfer film, sometimes called a third body film that permits adjusting the friction coefficient and the mechanical wear properties of the articles. This film is a material formed during application of braking pressure and which is different than the fiber reinforced composite material which constitutes the brake material. The processes disclosed include the use of ceramic matrix forming polymers doped with third body film forming reactive particulates, and interface coated fibers to densify, strengthen, control the friction behavior, and harden fiber performs for use as braking materials and in other friction applications.

A purpose of the invention is to produce brake articles and materials optimized for aircraft applications, including emergency braking situations, as well as for heavy truck, train, and racing applications. Another object of the invention is to provide durable, seizure resistant friction articles and materials characterized by an environment and temperature stable friction coefficient for a stack type aircraft brake system. The stable friction coefficient is achieved by controlling the rotor-stator or pad-rotor third body friction transfer layer and minimizing the formation and smearing of silica during light braking. An important feature of the invention is prevention of seizing of a brake stack after an aborted take off by keeping the amount of oxygen in the component matrix below the critical threshold needed to form a silica “weld” after the aircraft ground speed is greatly reduced or it has come to a complete stop subsequent to an aborted take off. This is accomplished in the invention by utilizing as the matrix material substantially stoichiometric silicon carbide which has excellent high and low temperature wear performance. The friction transfer film that is formed is essentially silicon carbide. A further aspect of this invention is the composition of matrix polymer and ratios of friction controlling additives which provide an optimum combination of erosion resistance and friction. Friction controlling additives provide a composition of silicon dioxide, or siliconoxycarbide, or silicon carbide transition films which form an effective friction transfer layer. Another aspect of the invention is to provide techniques and materials to produce a light weight, long life brake material system for trains, heavy trucks, and racing applications as well as aircraft.

DESCRIPTION OF THE INVENTION

The present invention will be more readily understood by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings. The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. “Optional” or “optionally” means that the subsequently described event, circumstance, material or composition may or may not occur, and that the description includes instances where it does and instances where it does not.

The invention is applicable to a broad spectrum of friction tolerant surfaces and articles such as brake systems for aircraft, heavy trucks, sports utility vehicles, luxury automobiles, racing vehicles, trains, power transfer systems such as clutch mechanisms, and the like. The invention provides a major innovation in the ability to regulate or adapt the ceramic compositions to changing friction requirements by choice or selection of the preceramic polymer and the type and amount of additive powders.

For convenience, the following description is expressed in terms of one method of making ceramic matrix composite aircraft brake components such as stators, rotors and pads in which a carbon fiber, graphite fiber, silicon carbide fiber, siliconoxycarbide fiber, silicon carbonitride fiber, or oxide fiber preform is provided. The preform can be any of the following types: needled felt, needled or stitched fabric, two-dimensional fabric, or three-dimensional woven fiber. The fibers of the preform are then provided with an interface layer of ceramic or carbon by coating the fibers with a liquid resin that is converted to ceramic or carbon by pyrolysis in air or inert gas to temperatures between 800° C. and 1000° C. The interface layer controls bonding of the ceramic matrix to the fibers, thereby imparting toughness to the final material or component. The interface layer coating can be one or more of the following materials: silicon carbide, carbon, siliconoxycarbide, or carbon-rich silicon carbide. The fiber coatings can be applied to the fibers by chemical vapor deposition or in a preferred embodiment by dip or spray coating, with or without diluting the coating material with solvents.

The matrix of the composite article is provided by infusing or infiltrating the preform by liquid ceramic forming polymers which are cured and pyrolyzed at between 800° C. and 1500° C. to form a ceramic matrix around the interface coated fibers in the preform. A number of infiltration and pyrolysis cycles may be required to fill in the spaces between the fibers in the preform. Optionally, a number of powdered ceramic materials can be added to the liquid ceramic forming polymers to assist in densification and to provide the optimized friction coefficient embodied in this invention. The partially densified preform can be machined to near-net shape, followed by further infiltrations and pyrolysis cycles with one or more ceramic forming polymers until an open porosity level of less than about 12% by volume is attained. In a preferred method about 10% to about 35% by volume of a combination of two or more of the following materials, alumina, mullite, iron, iron oxide, silicon carbide, siliconoxycarbide, silica, carbon, is distributed throughout the preform during one or more densification cycles by admixture, generally as a powder, with the ceramic polymer precursor. These additives should be at least at the contacting braking surfaces. The materials, methods, and resulting articles of this invention may include the step of disposing fibers which are compatible with the preform fibers and structure of the preform type adjacent the contacting braking surface parallel to or along circular arc segments and radial lines with respect to a center of rotation of a brake component to contact the braking surfaces. The friction components or parts of an aircraft brake stack can be made from preforms composed of needled or stitched carbon or graphite fiber felt. Other felts or fibers can be utilized in the practice of this invention. Silicon Carbide fiber such as the Tyranno™ fibers supplied by UBE; the Nicalon™ type fibers supplied in the U.S. by COI Ceramics, Inc.; the Nextel™ family of fibers supplied by 3M, and high modulus steel fiber from Hardwire, LLC. The preforms can be machined, pressed, cut, or otherwise shaped for the specific article, part, or brake stack during the fabrication process. The fibers of the preform are coated with an interfacial layer to decrease the extent of bonding between the fiber and the ceramic matrix subsequently formed the preform. This layer imparts toughness and strength to the ceramic matrix composite material enhancing its suitability for desired applications. In a preferred embodiment applicable to friction and wear resistant articles, the layer comprises about 0.01 micron to about 1.5 micron thick layer comprising one or more materials selected from the group consisting of siliconoxycarbide, carbon enriched silicon carbide, boron nitride, and carbon. This interfacial layer coating can be derived from one or more ceramic forming precursor resins or polymers resins. Suitable ceramic forming compositions include sp-500, spcs, sp-matrix polymer, sp oxycarbide a, or sp oxycarbide c, supplied by Starfire Systems Inc.; or carbon pitch supplied BP-Amoco; the boron nitride coating can be applied using polyborazaline or by heat treating boron doped fiber in ammonia. The resin coatings which form the interfacial layer can be applied by melt infiltration, direct immersion, direct spraying, or vapor deposition. When necessary, the resins can be diluted with suitable solvents to enhance the uniformity of the fiber coating. Once applied, the coatings are pyrolyzed at between 800° C. and 1200° C., preferably about 850° C. to 950° C., in an inert gas such as nitrogen, argon, helium or a mixture of the above-mentioned gases with each other or with up to 5% hydrogen to form the interfacial ceramic layer. After completion of the coating and pyrolysis procedures, the interfacial layer carrying preforms can be infused or infiltrated by, for example, immersion in ceramic forming polymer resin. The resin can be a meltable type such as the SP oxycarbide-a or sp oxycarbide-c, or a liquid resin such as the polycarbosilane supplied by Starfire Systems, Inc. of Malta, N.Y., as SP Matrix Polymer. Selection of the resin is determined by the end use of the friction material (ie. for large commercial aircraft or for smaller regional or personal aircraft).

The ceramic forming polymer resin can be one or more of the following: stoichiometric or near stoichiometric silicon carbide forming polymers such as the ahpcs or sp matrix polymers (allylhydridopolycarbosilane polymers) developed and supplied by Starfire Systems, Inc.; silicon carbonitride forming polymers such as ceraset supplied by Kion corporation, or hpz (sylramic) resin supplied by COI ceramics; a non-cyclic silicon oxycarbide precursors such as the SP-500, SP oxycarbide-a, or SP oxycarbid-c, also supplied by Starfire Systems, Inc. The resin can be infused into the preform by itself, i.e., neat with no fillers or in admixture with one or more ceramic or metal fillers. In an illustrative embodiment, the fillers comprise one or more of the following: alumina, silicon carbide, carbon, iron, oxides of iron, oxides of magnesium, zirconium metal or oxides of zirconium, silica, siliconoxycarbide, or titanium oxide. The fillers can be in the form of powders or added as substituents to the ceramic forming polymer itself and comprise between 5 and 35 volume percent of the matrix between the fibers of the preform. In one embodiment the fillers comprise about 10 to about 15 volume percent of the matrix and are comprise one or more of the group consisting of iron, alumina, silica, carbon, and silicon carbide. After infusion into the preform, the resin or resin slurry saturated preform is cured by heating under inert gas to 200° C.-400° C. at a heating rate of 1-2° C. per minute with 1 to 4 hour hold. The cured preform is then heated under inert gas such as nitrogen, argon, helium or a mixture of the above-mentioned gases with each other or with up to 5% hydrogen at between 1 and 4 degrees per minute up to between 800° C. and 1650° C., with the preferred heating rate determined by the mass of the component and the preferred maximum temperature between 850° C. and 11650.00° C. After cooling, the partially or semi-rigid preform is placed in a liquid tight container and the container is placed into an apparatus designed to evacuate the preform to remove air or other gas contained in the pores of the preform. After evacuation, the appropriate resin is allowed into the vacuum chamber, while still under vacuum, until the preform is fully immersed in the resin. The resin can be neat or in the form of a slurry of the resin and selected appropriate fillers as described herein. After vacuum infusion, the container with the preform is cured and pyrolyzed. The preform is cured by heating under inert gas to about 200° C. to about 400° C. at a heating rate of 1-2 degrees per minute with a hold of about 1 to about 4 hours after reaching the final temperature. The cured preform is then heated under inert gas such as nitrogen, argon, helium or a mixture of the above-mentioned gases with each other or with up to 5% hydrogen at between 1 and 4 degrees per minute up to about 800° C. to about 1500° C. The heating rate is determined by the mass of the component and the preferred maximum temperature is between about 850° C. and about 1100° C.

The vacuum infusion-cure-pyrolysis cycle can be repeated, as necessary, to achieve an open porosity level of below about 12 volume percent and preferably from about 4 to about 10 volume percent. The preform can be machined to a near net shape, within about 5 to about 10 volume percent of desired dimensions, between selected cycles. Final machining of the article e.g., the friction surfaces of brake parts, is generally accomplished just before or after the final pyrolysis cycle. The foregoing describes one of the preferred embodiments and applications of the disclosed invention applying the novel features of the invention and should not be considered as limiting application or use of the invention. The examples are set forth to provide those of ordinary skill in the art with a detailed description of how the invention claimed herein are evaluated, and are not intended to limit the scope of what the inventors regard as their invention. Unless indicated otherwise, parts are by weight and temperature is in degrees centigrade (° C.).

EXAMPLE 1

Brake Rotors and Stators for Aircraft

Needled carbon or graphite fiber felt with 20 to 50 percent fiber volume fraction, either pitch fiber or PAN fiber graphitized by heating to over 1400° C. using heat cycles commonly know in the carbon processing industry. The preform is cut to about 10%-15% larger than the desired dimensions of the end product. For an F-16 aircraft brake rotor, this is about 13.25″ outer diameter by 5.2″ ID by 0.9″ thick. A mixture of additives selected from the group comprising iron oxide, alumina, and silica powder having a size of size less than about 40 microns, and preferably less than about 10 microns is added during the process of forming the fiber felt preform. This is to provide uniform distribution of the additives throughout the rotor and stator. The addition of the additives during preform manufacturing also simplifies and lowers the cost of the follow-on densification process. A typical composition for an F-16 rotor size preform would be about 200 to about 260 grams of iron oxide, and about 30 to about 60 grams of silica, for certain applications about 20 to about 50 grams of alumina is also added. Other additives such as carbon, silicon carbide, and boron nitride powders can also be added to adjust the friction coefficient and wear properties to the applications. Successful tests have been conducted on rotors with no additives and on rotors with up to 4 times the typical loading. The friction performance and wear characteristics are altered by both the quantity of the various additives as well as the combination of additives used. When higher loadings of iron oxide are used (more than double the typical loading) carbon powder must also be included in the combination of additives to assure the correct third body friction film formation on the rotor surface. The additives can be incorporated in the preform as described above by other methods such as grafting the appropriate elements or compounds onto the ceramic forming polymer materials. The additive powders can be admixed with the ceramic forming polymers to form a slurry that can be painted onto each face of the rotor or stator as described below.

After making the preforms, each stator or rotor is treated with a 15 to 25 weight percent, based on the weight of the preform, solution of a ceramic forming resin such as Starfire Systems SR 350 resin in ethanol. Ethanol is used as a solvent to disperse the ceramic forming resin uniformly throughout the preform. The amount of ethanol used is sufficient to wet out the felt perform without causing the preform to become soggy. The ethanol is allowed to evaporate off in a hood before air curing. The air cure process includes raising the temperature of polymer fiber combination to about 93° C. at about 30 per minute, then to about 120° C. at about 10 per minute, then to about 200° C. at about 20 per minute. The piece is held about 225° C. for two hours and then allowed to cool to room temperature. The air cure was immediately followed by pyrolysis under nitrogen according to the following schedule. The temperature was raised to about 225° C. at 40 per minute, then to about 850° C. at about 20 per minute then held at 850° C. for one hour, the cooled to room temperature. The cure and pyrolysis treatments form the fiber interface coating that provides the toughness for the desired applications. The rotor or stator preform containing the proper amount and type of additives, is placed in a liquid tight pan or container that can withstand at least 1000° C. Silicon carbide forming polymer such as the Starfire Systems, Inc. matrix polymer disclosed in U.S. Pat. No. 5,153,295 to Whitmarsh et al. is poured into the container in sufficient quantity to completely saturate the preform. The container holding the preform is then placed in an inert gas atmosphere furnace and heated under flowing nitrogen or other inert gas at a rate of 2 degrees centigrade per minute to a temperature range of 850° C. to 1100° C. and held for 1 hour.

Alternatively, once cut to near net shape, the rotor or stator preform, containing the proper amount and type of additives can be placed in a liquid tight pan or container filled with silicon carbide forming polymer covering the surface of the rotor to depth at least about 0.25 inch. The container holding the rotor or stator preform is then placed in a vacuum chamber and held under vacuum less than about 50 to about 250 millitorrs of mercury for about 15 minutes. The saturated rotor or stator can then be removed from the container and allowed to drain for several minutes before being placed in a liquid tight container that can withstand at least 1000° C. The saturated perform and container is then placed into the inert gas atmosphere furnace and heated under flowing nitrogen or other inert gas at a rate of 2 degrees centigrade per minute to a temperature range about of 850° C. to 1100° C. and held for about 1 hour. After cool down, the preform can be machined to near final dimensions allowing a margin of about 5% to 10% on the thickness for final grinding of the wear surfaces.

Upon completion of machining, the preform is vacuum infiltrated with silicon carbide forming polymer by placing the preform into a liquid-tight steel tray that is then placed into a vacuum chamber. The chamber is evacuated for a minimum of 15 minutes per half inch of section thickness to an absolute pressure below 5 inches of mercury (Hg) and preferably below 250 millitorr. Once chamber is evacuated, sufficient polymer is drawn into the chamber to completely immerse the preform. The preform is permitted to remain under vacuum while immersed for an additional 15 minutes per half inch of preform thickness. The vacuum is then broken and the immersed component placed into an inert gas furnace. Alternatively, the excess polymer can be drained off and the component pyrolyzed (fired) in the inert gas furnace without being immersed. One or more of the additives mentioned above can be added to the polymer prior to its introduction into the vacuum chamber. This would allow a larger concentration of the additives to be placed into the preform to further alter the friction and wear properties of the rotor or stator. The polymer infiltrated into the preform is converted to ceramic by heating the infiltrated preform under inert gas. Heat the preform under inert gas at about 2 degrees C. per minute up to 150° C., followed by heating at 0.5-1 degree C. per minute from 150° C. to 400° C. with a 1 hour hold at 400° C. Heat at 2° C. per minute from 400° C. to 850° C.-1000° C. and hold for 1 hour. Cool at less than 5° C. per minute to room temperature. The rotor or stator can be further densified by repeated vacuum infiltration and pyrolysis cycles until the desired density of about 2.05 to about 2.3 grams per cubic centimeter and porosity in the range of about 4 to about 12 volume percent are attained.

EXAMPLE 2

Brake Rotors and Pads for Heavy Trucks and Heavy Equipment

Needled carbon or graphite fiber felt with 20 to 50 percent fiber volume fraction either PAN fiber graphitized by heating to over 1400° C. using heat cycles commonly known in the carbon processing industry or pitch fiber. The perform is cut to roughly 10%-15% larger than the desired dimensions of the end product. For example a truck rotor might be 10″ OD with a 5″ ID and 0.625″ (⅝) thickness. Each rotor preform is treated with a 15-25 wt % (based on the weight of the preform) solution of SOC A500B in hexane. Once the solvent is evaporated off, the coated preform is heated in inert gas at a rate of 2 deg. C. per minute up to 850-1000° C. and held for one hour before being allowed to cool to room temperature.

A mixture of iron oxide, alumina, and silica powders of size less than 40 microns, and preferably less than 10 microns of typical composition for a 10 inch truck rotor size preform would be about 90 to 120 grams of iron oxide, and about 11 to 13 grams of silica, for certain applications about 16 to 18 grams of alumina can also be added. It should be noted that other additives such as carbon and silicon carbide and/or boron nitride powders can also be added to adjust the friction coefficient and wear properties to the applications. Successful tests have been conducted on rotors with no additives and on rotors with up to 4 times the typical loading. The friction performance and wear characteristics are altered by both the quantity of the additives as well as the combination. When higher loadings of iron oxide are used (more than double the typical loading) the addition of carbon powder must also be included to assure the correct third body friction film formation on the rotor surface. As an alternative practice of the above section of the disclosed art; the above additives can be added during the process of making the carbon felt preform. The powder mixture is then be added to approximately 750 grams of Starfire Matrix Polymer and 150 grams of Hexane and 1.5 grams of tetrahydrofuran to produce a slurry to be applied to each rotor or stator. The solvents are used for convenience and are not critical to the performance of the brake rotor or stator. A rotor preform is placed into a liquid tight tray. One half of the mixture would be evenly distributed over the top surface of the rotor or stator and allowed to soak into the preform. The preform is then turned over and the remaining half of the slurry distributed evenly on the top surface of the rotor. In this manner the slurry is applied to both contact surfaces of the rotor or stator and be absorbed into the porous preform. The coated preform is then placed in an inert gas atmosphere furnace capable of reaching 1000° C. and heated under flowing nitrogen or other inert gas at a rate of about 2 degrees centigrade per minute to a temperature range o aboutt 850° C. to 1000° C. and held for 1 hour. After cool down, the rotor can be machined to final dimensions. Upon completion of machining, the preform is vacuum infiltrated with silicon carbide-forming polymer by placing the preform into a liquid-tight steel tray that is then placed into a vacuum chamber. The chamber is evacuated for a minimum of 15 minutes per half inch of section thickness to an absolute pressure less than 1 torr and preferably below about 250 millitorr of mercury. Once the part is evacuated, sufficient polymer is drawn into the chamber to completely immerse the preform. The preform is permitted to remain under vacuum while immersed for an additional 15 minutes per half inch of preform thickness. The vacuum is then broken and the immersed component placed into an inert gas furnace. Alternatively, the excess polymer can be drained off and the component pyrolyzed (or fired) in the inert gas furnace without being immersed. The rotor can be further densified by repeated vacuum infiltration and pyrolysis cycles until the desired density of 2.05 g/cc to 2.3 grams per cubic centimeter and porosity in the range of about 4% to 12% are attained.

A brake pad which is pressed against the rotor to apply the frictional stopping force is constructed in a similar manner to a rotor, but made from different materials. The pad is made from a preform of chopped fibers mixed with one or more of the ceramic forming polymers and additives described above for manufacturing of the aircraft brake rotors and stators. Brake pads require higher concentrations of carbon to help prevent noise and to create a pad that will wear out more quickly than the rotor. Higher pad wear is typical in automotive applications and is different from aircraft applications where the rotor and pad are designed to wear at the same rate. In making a brake pad for a heavy duty truck an admixture of chopped heat treated (as for the aircraft brake material) carbon fiber (20-30 volume percent) with 10-20% by mass of iron oxide and silica based additives and 10-20% by mass SiC or carbon fillers is prepared. A typical pad is 25 volume percent chopped carbon fiber, with 30 volume percent filler and additives. The fibers, additives, and fillers are compounded during the fabrication of the pad preform (typically a “wet lay-up” process similar to making paper or felt). Alternative fibers such as aluminosilacate, alumina, boron nitride, silicon nitride or silicon carbide can also be used either alone or in conjunction with the carbon fibers. The pad is densified by infiltration with ceramic forming polymers, typically an oxycarbide ceramic forming polymer followed by a number of infiltrations with a silicon carbide forming polymer such as the Starfire Matrix Polymer optionally in admixture with a carbon rich silicon carbide forming polymers. The pad preform is infiltrated with methylsesquioxime, a meltable, thermally curing, oxycarbide ceramic forming, by placing the preform into a pan containing a mass of the powder roughly equivalent to 3 times the mass of the preform (or aggregate masses of all performs in the pan). The pan would be heated in air, or preferably, under vacuum to 125° C. and held for ½ hour, to melt-infiltrate the performs with the resin. The pan is then be placed into an oven and the resin cured as follows: (↑93° C. @ 3°/min, ↑120° C. @ 1°/min, ↑200° C. @ 2°/min, hold at 225° C. for two hours, ↓ room temperature). Alternatively, The mixture of methylsesquioxime powder, fillers, additives, and chopped fiber could be thoroughly mixed together and loaded into a heated mold (250° C.) and press cured. The part would be ejected from the mold and heat treat cured as described below.

The air cure is followed by pyrolysis under nitrogen (room temperature to 225° C. @ 4°/min, ↑850° C. to 1000° C. @ 2°/min, hold for one hour, then cooling to room temperature). Upon cooling, the performs can be reinfiltrated with methylsesquioxime one or more time as described above in the melt infiltration method. The pads can be be further densified to a porosity range of 4% to 20% with the preferred range of 10% by infiltration and pyrolysis silicon carbide forming forming polymers such as the Starfire Matrix Polymer. The processing would be as follows: the preform is vacuum infiltratrated with silicon carbide forming polymer by placing the preform into a liquid-tight steel tray that is then placed into a vacuum chamber. The chamber is evacuated for a minimum of 15 minutes per ½″ of section thickness to an absolute pressure below about 5 and preferably below about 2 inches of mercury. Once the part is evacuated, sufficient polymer is drawn into the chamber to completely immerse the preform. The preform is permitted to remain under vacuum while immersed for an additional 15 minutes half inch of preform thickness. The vacuum is then broken and the immersed component placed into an inert gas furnace. Alternatively, the excess polymer can be drained off and the component pyrolyzed (or fired) in the inert gas furnace without being immersed. The component is then pyrolyzed by heating up to 850° C.-1000° C. @ 2°/min, hold for one hour and then cooled to room temperature. The vacuum infiltration and pyrolysis are repeated until the desired porosity is achieved.

EXAMPLE 3

Brake rotors and pads for racing vehicles, high end automobiles, and sport utility vehicles require temperature and friction resistance similar to the requirements for aircraft and heavy trucks. Brake rotors, pads can be formed in the same manner as described in Example 2. Alternatively, more traditional brake pad materials could be used. Specific examples include carbon loaded sponge iron, carbon rich semi-metallic pads, and sintered metal pads.

The broad range of operational environments covered in automotive applications make it impossible to describe each alternative. The key factors in selecting a pad will be desired wear life, desired friction coefficient, and noise vibration and harshness (NVH) requirements. Luxury suvs and passenger cars are designed to be quiet and smooth. For these reasons the selected pad would probably be a carbon loaded sponge iron like the FERODO DS3000 material. These pads are extremely quiet and smooth but have only moderately improved wear versus conventional pads. SUV's or SUT's used for heavy duty towing applications may be equipped with a ceramic pad or a sintered metal pad. These pads provide higher friction coefficients and longer wear life with a small penalty in NVH levels. The Improved friction materials of this invention are applicable to high energy uses such as clutch plates, clutch plate segments, and mechanical power transmission equipment. Such articles can be manufactured as described above. The fillers and additives play an important role. Heat dissipation and strength are less critical as the component is usual mounted onto a metal support. A typical clutch or other friction material component could be made using the disclosed art as follows. The component would be made from a preform of chopped fibers or layered fabric mixed with one or more of the ceramic forming polymers and additives described above for manufacturing of the aircraft brake rotors and stators. For example a clutch component can be formed as follows. Mix chopped heat treated (as for the aircraft brake material) carbon fiber (20-30 volume percent) with 10-20% by mass of additives and 10-20% by mass SiC or carbon fillers. A typical clutch preform would be 25 volume percent chopped carbon fiber, 20 volume percent additives and 10 volume percent silicon carbide or boron nitride filler materials. The fibers, additives, and fillers would be added during the fabrication of the preform, typically a wet lay-up process similar to making paper or felt. Alternative fibers such as aluminosilacate, alumina, boron nitride, silicon nitride or silicon carbide could also be used either alone or in conjunction with the carbon fibers. The component can be densified by infiltration with ceramic forming polymers, typically an oxycarbide ceramic forming polymer such as Starfire SOC 500 or SOC 35A, followed by a number of infiltrations with a silicon carbide forming polymer such as the Starfire Matrix Polymer. The preform can be infiltrated with a meltable, thermally curing, oxycarbide ceramic forming polymer such as methylsesquioxime by placing the preform(s) into a pan containing a mass of the meltable powder roughly equivalent to 3 times the mass of the preform (or aggregate masses of all performs in the pan). The pan would be heated in air, or preferably, under vacuum to 125° C. and held for ½ hour, to melt-infiltrate the performs with the resin. The pan would then be placed into an oven and the resin cured as according to the following schedule: ↑93° C. @ 3°/min, ↑120° C. @ 1°/min, ↑200° C. @ 2°/min, hold at 225° C. for two hours, ↓ room temperature. Air cure is followed by pyrolysis under nitrogen as follows: room temperature to 225° C. @ 4°/min, ↑850° C. to 1000° C. @ 2°/min, hold for one hour, then cooling to room temperature). Upon cooling, the performs can be reinfiltrated with resin as described above. The preforms can be further densified to a porosity range of 4% to 20% with the preferred range of 10% by infiltration and pyrolysis using silicon carbide forming polymers. The preform can be vacuum infiltrated with silicon carbide forming-polymer by placing the preform into a liquid-tight steel tray that is then placed into a vacuum chamber. The chamber is evacuated for a minimum of 15 minutes per ½″ of section thickness to an absolute pressure below 5 inches of Hg and preferably below 2 inches of Hg. Once the part is evacuated, sufficient polymer is drawn into the chamber to completely immerse the preform. The preform is permitted to remain under vacuum while immersed for an additional 15 minutes per ½″ of preform thickness. The vacuum is then broken and the immersed component placed into an inert gas furnace. Alternatively, the excess polymer can be drained off and the component pyrolyzed (or fired) in the inert gas furnace without being immersed. The component would then be pyrolyzed by heating up to 850° C.-1000° C. @ 2°/min, hold for one hour, then cooling to room temperature). Alternatively, to reduce flywheel and pressure plate wear in automotive type clutch assemblies the ceramic clutch disk can be partially densified with the Starfire ceramic forming Matrix Polymers and subsequently infiltrated with organic resins such as (but not limited to) polyamide resin, phenolic resin, or furfural alcohol. The processing would be as follows: After 2 infiltration and pyrolysis cycles using the ceramic forming polymers, the component would be further infiltrated with a carbon-forming resin such as phenolic resin to alter the friction properties. Alternatively, the article could be infiltrated with polyamide resin to improve toughness of the component. Finally, sequential densifications with a carbon-forming resin such as phenolic resin, followed by infiltrating with a non-carbon forming resin such as polyamide can be performed.

Experiment 4

Objective: To densify, with additives, needled PAN carbon fiber felt preforms, with Starfire Matrix Polymer (allylhydridopolycarbosilane).

Materials: 1 Rotor (rough cut into shape)
           2 Stators (rough cut into shape)
           1 large disk to be cut into smaller disks

The large solid disk was cut crosswise into ½″ slices then cut again into 8 disks each about 3″ in diameter. These disks were fiber coated with methylsesquioxime, SR 350, at a rate of 20 wt %, based upon the mass of the individual fiber preform(s). Four of the small disks were vacuum infiltrated with Starfire Matrix Polymer and pyrolyzed according to the following brake pyrolysis program: ↑850° C. at 2′/min, hold 60 min, ↓room temp at 3°/min.

The process used to apply the SR 350 was as follows: Mix ETOH and resin in a large flask by stirring ETOH and adding SR 350 to the ETOH. The solution was then poured over the brake preforms, one at a time. These three main pieces were air cured as suggested by the SR 350 spec sheet according to the following program: ↑78° C. @ 1/min, hold 30 min, ↑100° C. at 1°/min, hold 60 min, ↑120° C. at 1°/min, ↑200° C. at 1°/min, hold 60 min, ↓room temp @ 3°/min. Due to the large volume of ETOH used to solubilize the SR 350, an additional cure cycle was used to insure that the crosslinking step was complete. This program was slightly faster than the previous one due to the presence of significantly less ETOH. Second cure program: ↑100° C. at 2°/min, hold 30 min, ↑120° C. at 1°/min, hold 30 min, ↑200° C. at 2°/min, hold 60 min, ↓room temp at 3°/min. These pieces were then pyrolyzed under nitrogen according to the following program: ↑850° C. at 2°/min, hold 60 min, ↓room temp at 3°/min.

The remaining four disks, rotor and two stators, were treated with a mixture of Starfire Matrix Polymer, Fe₂O₃, and SiO₂ (rust and sand), with 10-20 mL hexane and 0.5-1.0 mL THF as a wetting agent. The Fe₂O₃ was heat treated by pyrolyzing under nitrogen ↑850° C.@ 3°/min with a 60 min hold, ↓room temp@ 3°/min. The heat treated Fe₂O₃ was added to the SiO₂ and ground in a mortar and pestle to a fine powder. Use of this finer powder mixture plus Starfire Matrix Polymer (˜80-90 cp) facilitated the movement of this mixture into the disks. The rust and sand was applied in the following manner. In an attempt to get as much rust and sand into the individual parts, a thick slurry was prepared with ½ of the mixture being applied to one side of the brake part. The part was then flipped over and the other ½ was applied to the other side, allowed to set for a short period of time, then vacuum infiltrated. The small disks were treated, agitated, then vacuum infiltrated in small coffee cans to reduce the mess factor. The large disks were subjected to the same treatment except that the vacuum infiltration and subsequent pyrolysis was done in aluminum foil. All of these rust and sand parts were pyrolyzed as follows: ↑850° C. @ 2′/min, hold 60 min, ↓room temp @ 3°/min. The four disks containing just Starfire Matrix Polymer each received one coating cycle followed by three cycles of polymer before being sent out to Chand for machining. The other four disks each received one coating cycle followed by two rust/sand/polymer infiltrations before being sent out to Chand for machining. Following the machining process, the rotor, stators, and the eight small disks all received a total of eight polymer infiltration cycles.

Results and Discussion

For this set of brakes, the desired Fe₂O₃ content was 6%. While not sure if this method of introducing Fe₂O₃ and SiO₂ into the brake preform was most effective, we do know that the mass gain from the first Fe₂O₃/SiO₂/Matrix polymer treatment was larger than those that contained none of these additives. There was no appreciable difference in masses on the second Fe₂O₃/SiO₂/Matrix polymer infiltration leading one to believe that any additives must be added at the fabrication stage. In other words, there may not be any second chances.

The process by which these parts were processed is standard procedure. Brake preforms were obtained, interface coated with an siliconoxycarbide resin to produce a very thin coating in the range of 5-10 microns and subsequently vacuum infiltrated with Starfire Matrix Polymer until the desired density and/or open porosity is met. In this case our target open porosity was 10% or less. This varies from our previous brake work in that prior brake work involved a target open porosity of 5% or less. A softer brake with additives has an increased friction coefficient, compared to 5% open porosity, improving the performance of the brake material.

The small disks were processed to an open porosity of about 6% (5.91-7.37%) and an average apparent density of 2.17 g/cc (2.14-2.21 g/cc range). The target mass for the rotor and stators was 1876 g-1922 g. At the end of the 6^th polymer infiltration cycle these parts were about the 1870 g mark. During the course of this experiment it was determined that it is not necessary to pyrolyze our brake preforms submersed in polymer. There was no significant uptake of polymer into the part by pyrolyzing submersed. To the contrary, It was determined that a brake preform, pyrolyzed free standing took on significantly more mass than the brake preforms pyrolyzed submersed. It is preferred that the additives be incorporated into the preform for maximum effectiveness. Other additives, such as ground garnet, can be incorporated into the preforms at the manufacturing stage. It is also preferred that the polymer loaded preforms be pyrolyzed free standing instead of submersed in Matrix polymer.

The foregoing are just examples of techniques utilizing the disclosed art. They are not intended to be limiting or the only possible examples.

The ceramic precursor polymer identified as SP 500 herein is poly[chloromethylmethoxychlorosilane-CO-bis(chloromethyl) tetramethylsisiloxane. This compound which is useful as a fiber coating polymer for the herein disclosed composites can be linear or branched.

The SP 500 composition can be made by the following procedure. Process for preparation of poly[chloromethylmethoxychlorosilane-co-bis(chloromethyl)tetramethyldisiloxane] (SP-500)

1) Preparation of Chloromethylmethoxychlorosilane

• • To a 2-L three-necked round bottom flask, 1472 g of chloromethyltrichlorosilane was charged. This material was stirred managetically and purged with dry nitrogen gas. Then 435.2 g of absolute methanol was added dropwise within 3 h. By-product HCl generated from this reaction was purged off by flowing nitrogen gas. After stirred at room temperature for another 3 h, the resultant compounds were employed directly as starting materials in next step without purification. The obtained compounds were a mixture of chloromethylmethoxydichlorosilane, chloromethyldimethoxychlorosilane, and chloromethyltrimethoxysilane in about a ratio of 1:6:1. An average formula from this three compounds was Cl_1.3(MeO)_1.7SiCH₂Cl.
    2) Preparation of poly[chloromethylmethoxychlorosilane-co-bis(chloromethyl)tetramethyldisiloxane]
  • To a 12-L three-necked round bottom flask equipped with a mechanical stirrer and a condenser, 665 g of magnesium powder and 600 ml of anhydrous tetrahydrofuran (THF) were charged. Then, a solution of 1410.8 g of Cl_1.3(MeO)_1.7SiCH₂Cl and 30.4 g of allylchloride in 3 L THF was added dropwise to the magnesium mixture. The Grignard reaction could be initialized smoothly in 2-5 minutes. Once the reaction was fully started, cold water was employed to cool down the reaction. The silane solution was added in 2 h. A large amount of magnesium chloride was formed as by-product during the Grignard reaction. At this stage, a polymer with a [Si(MeO)₂CH₂]_0.7n[Si(MeO)(allyl)CH₂]_0.05n[Si(MeO)ClCH₂]_0.25n formula was formed as an intermediate. This intermediate was not isolated, instead, 1790 g of bis(chloromethyl)tetramethyldisiloxane in 2.4 L THF was added to restart the Grignard reaction. This silane was added within 3 h. The Grignard reaction was cooled by cold water during the addition of silane. The resultant mixture was stirred overnight at 50° C., and then poured into a mixture of 1.2 L concentrated HCl, 1 L hexane, and 12 kg ice with vigorous agitation. The yellow organic phase was separated from the aqueous phase and washed by 1 L saturated NaHCO₃ solution. The organic phase was separated again from the NaHCO₃ phase and dried over anhydrous sodium sulfate. After removing hexane and THF by rotor-vapor distillation under the conditions of 60° C. at 20 mmHg, a viscous yellow oil was obtained in 1600 g. This polymer should have an average formula of [Si(CH₂SiMe₂O_0.5)₂CH₂]_0.95n[Si(CH₂SiMe₂O_0.5)(CH₂CH═CH₂)CH₂]_0.05n, although its structure is very complicated due to the branched chains.

Many other uses of the disclosed compositions and methods could be made by a skilled practitioner in the friction materials and related fields. The invention has been described in detail with particular reference to preferred embodiments of the invention, but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention.

The invention, in various embodiments thereof, includes the polymer and friction modifying powder slurry admixtures, optionally including solvents, for infusion or infiltration into the reinforcing fiber preform; the interface coated fiber preform after infusion or infiltration with the polymer powder slurry; the final fiber reinforced silicon carbide ceramic matrix composite after one or more infusion or infiltrations each followed by pyrolysis to the desired density; and the methods for making the fiber reinforced silicon carbide matrix composites and each of the intermediate combinations of materials described herein.

Claims

1. A fiber reinforced ceramic matrix composite comprising a polymer derived silicon carbide matrix, reinforcing fibers incorporated within the matrix, and an amorphous glass interface coating on the surface of the reinforcing fibers.

2. Friction materials for high energy applications characterized by enhanced frictional properties and high temperature stability comprised of a structural fiber reinforced silicon carbide composite having an amorphous glass coated reinforcing fiber system disposed throughout a polymer derived silicon carbide matrix.

3. The friction materials of claim 2 wherein the silicon carbide ceramic matrix is derived from a stoichiometeric or near stoichiometeric silicon carbide polymeric precursor infused into a fiber preform by polymer infusion and pyrolyzed to for a stoichiometric or near stoichiometric silicon carbide matrix

4. The friction materials of claim 3 wherein the silicon carbide forming polymers are one or more of the following polymer compositions polycarbosilanes, hydridopolycarbosilanes, polyhydridosiloxanes, polymethylsiloxanes, polyphenylsiloxanes, and polyhydridosilanes.

5. The friction materials of claim 3 wherein the fiber has an interface coating of amorphous glass coating of about 0.01 to about 1.5 micron s in thickness.

6. The friction materials according to claim 3 wherein the fiber is selected from the group consisting of PAN based carbon fibers pitch based carbon or graphite fibers, boron doped fibers, silicon carbide fibers, silicon carbonitride fibers, silicon oxycarbide fibers, alumina fibers, and oxide based fibers.

7. The friction materials of claim 3 wherein the polymer precursor infused into the preform contains one or more friction modifying powder additives selected from the group consisting of aluminum oxide, iron, iron oxide, iron silicide, iron silicate, magnesium oxide, titanium oxide, zirconium oxide, carbon, and mixtures thereof.

8. A silicon carbide precursor composition for infusion into a fiber preform comprising a silicon carbide forming polymer resin selected from the group consisting of polycarbosilanes, hydridopolycarbosilanes, polyhydridosiloxanes, polymethylsiloxanes, polyphenylsiloxanes, and polyhydridosilanes and a friction modifying powder additive selected from the group consisting of aluminum oxide, iron, iron oxide, iron silicide, iron silicate, magnesium oxide, titanium oxide, zirconium oxide, carbon, and mixtures thereof.

9. A composition according to claim 8 wherein the powder additive are of the size range from about 10 nanometers to about 100 micrometers.

10. A composition according to claim 9 wherein the powder additive are of the size range from about 0.6 micrometers to about 45 micrometers.

11. A composition according to claim 10 in which the ratio of polymer to additive is from about 25 percent to about 150 percent by mass.

12. A composition according to claim 10 in which the ratio of polymer to additive is from about 50 percent to about 95 percent by mass.

13. A composition according to claim 8 also comprising a solvent.

14. A composition according to claim 8 wherein the fiber is selected from the group consisting of PAN based carbon fibers pitch based carbon or graphite fibers, boron doped fibers, silicon carbide fibers, silicon carbonitride fibers, silicon oxycarbide fibers, alumina fibers, and oxide based fibers.

15. A composition according to claim 14 in which the fibers are provided with an interface coating.

16. A composition according to claim 15 in which the interface coating has a thickness of about 0.01 to about 1.5 microns.

17. A process for making fiber reinforced silicon carbide matrix structures which comprises providing a carbon or ceramic fiber preform, applying a fiber interface coating on the surfaces of the preform fibers, infusing the preform with a silicon carbide forming resin composition, and pyrolyzing the resin to a silicon carbide matrix.

18. A process according to claim 17 in which the fiber preform is infused with a stoichiometric or near stoichiometric silicon carbide forming resin composition selected from the group consisting of polycarbosilanes, hydridopolycarbosilanes, polyhydridosiloxanes, polymethylsiloxanes, polyphenylsiloxanes, and polyhydridosilanes, a friction modifying powder additive selected from the group consisting of aluminum oxide, iron, iron oxide, iron silicide, iron silicate, magnesium oxide, titanium oxide, zirconium oxide, carbon, and mixtures thereof and optionally a solvent.

19. A process according to claim 18 in which multiple infusions each followed by pyrolysis are carried out until the open porosity of the silicon carbide matrix composite is less than 12 percent by volume.

20. A process according to claim 19 in which the final porosity is from about 4 to about 10 percent by volume.

21. The process according to claim 18 in which the preform is infused with a composition in which the ratio of polymer to powder additive is from about 50 percent to about 95 percent by mass.

22. A process according to claim 19 in which the silicon carbide matrix is formed by firing the resin at between 850° C. and about 16500° C. in an inert gas.

23. A process according to claim 22 in which the inert gas is nitrogen, argon, or helium and mixtures thereof optionally mix with up to about 5 volume percent hydrogen.