Patent Number: 
Section: description

According to the present invention, there are provided a novel class of debond coated non-oxide ceramic reinforcing fibers comprising a non-oxide continuous ceramic fiber, preferably carbon fiber or silicon carbide fiber, preferably, but not necessarily, first surface coated with a layer of pyrolytic carbon and then overcoated with one or more layers of a non-hygroscopic, oxidation resistant, protective material. According to various preferred embodiments of the instant invention, the non-hygroscopic oxidation resistant, protective layer(s) may comprise a monolithic layer of Ti3SiC2, Ti5Si3, TiSi2, or TiSi, one or more layers of SiC/TiC or the oxidation product thereof, or one or more layers of SiO2/TiO2. Referring now to FIG. 1 that depicts a cross-sectional view, according a first preferred embodiment of the present invention, the enhanced continuous reinforcing fiber of the present invention 10 comprises a continuous, non-oxide fiber core 12 having a thin, from about 0.1 xcexcm to about 0.2 xcexcm, layer 14 of pyrolytic carbon annularly applied about the surface thereof and a layer 16 of a non-hygroscopic, oxidation resistant material annularly applied thereover. In the embodiment depicted in FIG. 1, layer 16 is preferably Ti3SiC2 that is applied as described hereinafter. FIG. 2 depicts an alternative preferred embodiment wherein non-hygroscopic, oxidation resistant layer 16 is applied annularly directly over the surface of continuous, non-oxide core 12 with no pyrolytic carbon layer 14 therebetween. In the description and examples that follow, reference will be made to and description will be provided primarily of embodiments of the present invention that include pyrolytic carbon layer 14 as part of the structure or as a step in the fabrication process. It should be specifically noted that all such structures and the processes for preparing them can be identically prepared and performed without the presence of the pyrolytic carbon layer and both such structures and methods for their preparation are clearly intended and contemplated as within the scope of the appended claims and the herein described invention. An alternate preferred embodiment of the present invention is depicted in FIG. 2. According to this embodiment, the debond coated reinforcing fiber 10 comprises a non-oxide continuous fiber core 12 having a similarly thin layer 14 of pyrolytic carbon about the surface thereof and a pair of non-hygroscopic, oxidation resistant layers 18 and 20 applied sequentially thereover as described hereinafter. According to a specifically preferred embodiment of the present invention, layers 18 and 20 are SiC and TiC, respectively. In an alternate preferred embodiment, the sequentially applied SiC and TiC layers are oxidized to SiO2 and TiO2. Oxidation resistant layers 18 and 20 can, of course, be coated directly over the surface of fiber core 12. In the third alternative preferred embodiment of the present invention depicted in cross section in FIG. 3, the debond coated reinforcing fiber 10 of the present invention comprises a continuous fiber core 12 having a similarly thin layer of pyrolytic carbon 14 applied annularly thereabout that is subsequently coated as described hereinafter with alternating layers 22, 24, 26 and 28 that are respectively either SiC and TiC, or SiO2 and TiO2. This structure may be expanded to include a further plurality of such alternating layers. Again, alternating layers 22, 24, 26, and 28 can be coated directly onto the surface of fiber core 12 in the absence of pyrolytic carbon layer 14. In each of the foregoing structures, the thickness of the pyrolytic carbon layer, when present, is preferably between about 0.1 xcexcm and about 0.2 xcexcm. Each of the other non-hygroscopic, oxidation resistant layers 16 through 28 and any additional protective layers are preferably between about 0.2 xcexcm and about 0.5 xcexcm thick in total, although thicker layers may of course be used in those applications where layer thickness does not affect the functionality of the coatings in the final composite structure. For example, the entire matrix could consist of a multi-layer structure. While not wishing to be bound in any way by any specific mechanism that describes the effectiveness or functional operation of the improved reinforcing fibers described and claimed herein, it is postulated from the TiO2xe2x80x94SiO2 phase diagram that on oxidation of fibers with or without an inner carbon layer at the surface and one of the protective layers described herein coated thereover separate SiO2 and TiO2 layers are formed. TiO2 is known to be a lubricious, low shear strength oxide that is ideal for an interface coating and that SiO2 will provide oxidation resistance for both carbon and silicon carbide fibers. The effect of adsorbed water in these coatings appears to be negligible at temperatures of 700xc2x0 C. and above. The immiscibility of these two materials even at temperatures up to about 1550xc2x0 C. provides that they will each retain their inherent lubricious and antioxidant characteristics even at these temperatures. Hence, since TiO2 and SiO2 are the oxidation products of titanium suicides (TixSiy), Ti3SiC2 and SiC/TiC layers of these materials, upon oxidative attack they will provide SiO2 and TiO2 that will impart their respective needed properties to the reinforcing fiber at temperatures well in excess of 1200xc2x0 C. The preferred methods for the application of the non-hygroscopic, oxidation resistant coatings of the present invention to continuous non-oxide reinforcing fibers to yield the improved fibers of the present invention are presented schematically in FIGS. 4 and 5. Referring now to FIG. 5, the continuous non-oxide reinforcing fiber is first, preferably, coated with a thin layer of pyrolytic carbon preferably applied by chemical vapor deposition (CVD) or chemical vapor infiltration (CVI) depending upon whether the fiber to be coated is in the form of a single fiber, fiber cloth or a preform shape. Deposition is accomplished by placement of the fiber, fiber cloth or preform into an appropriate reaction chamber of the type well known in the art and decomposing, for example, CH4 or C3H8 at temperatures between about 1000 and 1300xc2x0 C. and pressures of 10 Torr or less. This procedure is common to all of the fabrication processes described herein that apply pyrolytic carbon layer 14 regardless of the nature of the coatings(s) applied over pyrolytic carbon layer 14. Selection of the pyrolytic coating process as with all of the other coating processes described hereinafter will depend largely upon the form of the fiber being coated, i.e. whether it is in the form of a single continuous fiber, a fiber cloth (tow) or a preform. CVD coating is preferred for single fiber or fiber tow coating while CVI is preferred for coating of fibers as a preform. In the case of the formation of the single phase Ti3SiC2 coatings described hereinabove, coating is accomplished through the introduction of: 1) the continuous fiber, cloth or preform along with; 2) SiCl4, TiCl4, and CCl4 in relative concentrations according to the following reaction: 3TiCl+SiCl4+3CCl4, (as specified further below) and 3) hydrogen and or hydrogen and argon as a carrier gas, into a suitable reaction chamber. Reaction is accomplished within the temperature range of from about 1000xc2x0 C. and about 1600xc2x0 C., preferably between about 1100xc2x0 and about 1400xc2x0 C. and most preferably between about 1100xc2x0 C. and about 1200xc2x0 C., at a pressure preferably below about 760 Torr, more preferably below about 400 Torr and most preferably below about 250 Torr and preferably for a period of from about 3 to about 240 minutes, more preferably from about 6 to about 60 minutes and most preferably from about 9 to about 30 minutes or until a thickness of from about 0.2 to about 0.5 xcexcm of Ti3SiC2 has been deposited on the fibers. The carrier gas preferably comprises from about 32% to about 99% by weight hydrogen and from about 0% to about 69% by weight of argon, more preferably from about 48 to about 98% by weight of hydrogen and from about 0 to about 50% by weight of argon and most preferably from about 58 to about 98% by weight of hydrogen and from about 10 to about 40% by weight of argon. TiCl4 is introduced preferably at a concentration of between about 0.06% and about 18% by weight, more preferably between about 0.2% and about 3% by weight and most preferably between about 0.4% by weight and about 2.2% by weight. SiCl4 is preferably introduced at a concentration of between about 0.04% by weight and about 16% by weight, more preferably between about 0.15% and about 1.4% by weight and most preferably between about 0.2% and about 1.2% by weight. The concentration of CCl4 introduced preferably ranges from about 0.02% to about 8% by weight, more preferably between about 0.15% and about 1.4% by weight and most preferably between about 0.2% and about 1.2% by weight. The deposited Ti3SiC2 coating may then optionally be converted to produce in situ a dual phase coating of SiO2/TiO2 by heating the coated fiber structure at a temperature of from about 1000xc2x0 C. to about 1600xc2x0 C. for a period of from about 1 minute to about 120 minutes. Most preferably, oxidation is accomplished by heating in air at a temperature of between about 1300xc2x0 C. and about 1400xc2x0 C. for a period of from about 10 minutes to about 20 minutes. As noted hereinabove, a similar process can be performed to provide the debond coatings of the present invention directly on the surface of fiber core 12 in the absence of any pyrolytic carbon layer 14 by the omission of the carbon application step. The titanium silicide (TixSi5 wherein x=1 or 5 and y=1,2, or 3) layers(s) that can be subsequently oxidized according to the procedures described hereinabove are formed by the reaction between TiCl4 and SiCl4 described immediately hereinabove, but in the absence of the carbon contributing CCl4. Referring now to FIG. 6, the two layered coatings of SiC/TiC are formed by first forming the pyrolytic coating on the fibers either as individual fibers, fiber cloth or a preform as described above, and then sequentially forming layers of SiC and TiC thereover through CVD) or CVI (depending upon the form of the fiber i.e. continuous single fiber, fiber cloth or preform) by: A) decomposing trichloromethyl silane (CH3SiCl3) with hydrogen or hydrogen and argon as a carrier gas at a temperature of from about 800xc2x0 C. to about 1600xc2x0 C. and a pressure of from about 0 Torr to about 760 Torr for a period of from about 3 minutes to about 240 minutes and then B) reacting TiCl4 with C3H8 in a concentration of from about 0.08% to about 1.5% TiCl4 in C3H8 at a temperature of from about 1000xc2x0 C. to about 1600xc2x0 C. and a pressure of from about 0 Torr to about 760 Torr for a period of from about 15 seconds to about 30 minutes. The layered coating will consist of alternating 0.3 xcexcm layers of SiC and TiC for a total coating thickness of 0.4 xcexcm. Optionally, the coated ceramic fibers, cloth or preform may then be oxidized in air at a temperature as described hereinabove to produce a single layered SiO2/TiO2 structure prior to further processing. The identical process may, of course be performed in the absence of the pyrolytic carbon application step to obtain an equally useful product. Referring now to FIG. 7, multi-layered SiO2/TiO2 structures may be produced by first applying the pyrolytic coating as described hereinabove to the ceramic reinforcing fiber, and then repeating the process described in connection with FIG. 6 several times, each time building alternating SiC/TiC layers only 0.05 xcexcm thick to a total coating thickness of 0.5 xcexcm. Oxidation of the coated ceramic fibers, cloth or preform by heating in air as described hereinabove yields a multi-layered SiO2/TiO2 structure prior to further processing. Again, the application of the pyrolytic carbon layer may be omitted to obtain a similarly useful product. Coatings of TiC and SiC may also be applied according the procedures and under the reaction conditions described immediately hereinafter for the production of SiO2/TiO2 coatings, except that the carrier gas contains no water or CO2. Two layered and multi-layer oxide coatings can also be produced by sequential application of SiO2 and TiO2 over pyrolytic carbon layer 14 or directly to fiber core 12 as described hereinabove but in a hydrogen or hydrogen and argon plus CO2 and water atmosphere comprised of from about 32% to about 99% by weight hydrogen, from about 0 to about 60% by weight argon, from about 0% to about 32% by weight of water and from about 0 to about 16% by weight of CO2 at a temperature of from about 1000xc2x0 C. to about 1600xc2x0 C. and a pressure below about 760 Torr for a period of from about 15 minutes up to about 120 minutes. It is preferred that the reaction be performed at a temperature of between about 1100xc2x0 C. and about 1400xc2x0 C. for a period of from about 15 seconds up to about 30 minutes and at a pressure below about 400 Torr. Most preferably the reaction conditions are at a temperature of between about 1200xc2x0 C. and about 1300xc2x0 C. for a period of from about 1 to about 15 minutes and at a pressure below about 250 Torr. The concentrations of TiO2 and SiO2 preferably range from about 0.08% and about 16% by weight, more preferably these concentrations range from about 0.12% and about 2.7% by weight and most preferably between about 0.18% and about 1.5% by weight. The layered coatings consists of alternating 0.2 xcexcm layers of SiO2 and TiO2 and 0.05 xcexcm layers of SiO2 and TiO2 to a total coating thickness of 0.4 xcexcm. As will be apparent to the skilled artisan, although CVD and CVI processes are preferred as the means to produce the coated ceramic reinforcing materials of the present invention, any number or alternative processes can be envisioned for obtaining similar results. For example, physical vapor deposition (PVD) processes, sputtering, laser ablation, cathodic arc and even electrophoretic deposition processes among others can be used to produce the novel coated, non-oxide, fibrous ceramic structures of the present invention. Colloidal sol suspensions of, for example, mixed sols of TiO2 and SiO2 can also be used to coat individual fibers or to infiltrate fiber cloths or preforms. In such instances, the impregnated cloth commonly referred to as tow or preform, is heated to a temperature of about 1400xc2x0 C. in an inert gas such as nitrogen or argon for about 2 hours to achieve phase separation and consolidation and subsequently fired at from about 1100xc2x0 C. to about 1600xc2x0 C. for from about one minute up to about 120 minutes prior to incorporation into a ceramic matrix composite structure. All of the structures produced as just described demonstrate significant oxidation resistance at temperatures above 1200xc2x0 C. for extended lifetimes. Incorporation of the coated reinforcing fibers described herein into ceramic matrix composite structures or parts is accomplished by further processing according to well known conventional procedures that involve impregnation of a fiber cloth xe2x80x9clay upxe2x80x9d or preform with an appropriate ceramic matrix and firing of the structure thus produced to yield a ceramic matrix composite part of a desired configuration. There have thus been described a novel class of coated, non-oxide, ceramic reinforcing fibers comprising a core of continuous non-oxide ceramic fiber coated sequentially with, optionally, a pyrolytic carbon layer and a variety of non-hygroscopic, oxidation resistant layer(s). Non-hygroscopic and oxidation resistant compounds of silicon and titanium form the preferred basis of these improved structures. Incorporation of the coated reinforcing fibers of the present invention into ceramic matrix composite structures and parts provides more oxidation resistant and therefore longer lived such parts that are used in or exposed to oxidizing atmospheres especially at high temperatures in excess of 1200xc2x0 C. As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.