Patent Publication Number: US-2018037512-A1

Title: Fiber unwinding system and methods of unwinding a fiber from a bobbin

Description:
FIELD OF THE INVENTION 
     The described subject matter relates generally to composite materials and more specifically to methods for manufacturing composite materials. 
     BACKGROUND OF THE INVENTION 
     Due to high thermal and mechanical performance, coupled with relatively low density, numerous components could benefit from the use of Ceramic Matrix Composites (CMCs) in place of metals or intermetallics. During the manufacturing processes of CMC, the fibers need to be coated in order to survive the processes as well as for mechanical properties in service. Currently, two of the primary cost-effective methods of processing ceramic matrix composite (CMC) components are chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP). Another process is glass transfer molding, which is faster than CVI and PIP, but is also much more expensive and resource intensive. Each of these processes uses a filament handling device using various forms of tension control on fiber movement during processing. 
     In the fiber coating process, fibers are typically unwound from a spindle to begin processing. During the unwind process, tension of the filaments is carefully controlled, since too much tension could destroy the filaments while not enough tension can allow the tow to jump off rollers and mis-track. In a fiber coating process, tension can also affect filament spacing which, in turn, can affect coating thickness uniformity and mechanical properties. In a conventional filament handling apparatus, the fiber bundles often break in midstream at any place along the fiber path length and breakage often occurs due to a failure in a process of unwinding the fiber bundles from fiber bundle feeding packages. The breakage of the fiber bundle typically occurs when friction exceeds the fiber strength or one or more of a plurality of single fibers of the fiber bundle is snarled or tangled at the time of unwinding process. 
     Thus, a need exists for an automated device that is constantly correcting, adjusting and maintaining the unwinding process of the tow during fiber processing. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     Methods are generally provided for coating a fiber. In one embodiment, the method includes unwinding a silicon carbide-containing fibrous material from a bobbin rotatably mounted around an axle and forming a boron nitride coating onto the silicon carbide-containing fibrous material. The bobbin is moved along the axial direction such that the silicon carbide-containing fibrous material defines an unwind angle with the axial direction, with the unwind angle being maintained between about 80° to about 100°. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figs., in which: 
         FIG. 1  shows a schematic of an exemplary unwinding system for unwinding a fiber from a bobbin; 
         FIG. 2  shows a schematic of a portion of the exemplary unwinding system of  FIG. 1  from another angle; 
         FIG. 3  shows a perspective view of an exemplary bobbin apparatus, such as for use with the exemplary unwinding system of  FIG. 1 ; and 
         FIG. 4  shows an exemplary method of intelligently unwinding a fiber from a bobbin. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. 
     An intelligent unwind system is generally provided, along with methods of its use. In particular embodiments, a unwinding system uses at least one sensor (e.g., an optical sensor) to assess the fiber position, and a system of motors and/or drivers that align the fiber tow unwinding from the bobbin into the downstream receivers (e.g., a pulley) so as to minimize processing damage of the fiber as it leaves the surface of the wound fibers on the bobbin and enters the pulley. In particular, any scraping as the fiber unwinds from the bobbin, either with adjacent fibers on the bobbin and/or the bobbin surface, can be minimized by keeping the payoff angle (i.e., the first angle described below) near 90°. In one embodiment, the at least one sensor (e.g., a light sensor) is utilized to establish the position of the fiber as it is payed off of the bobbin. The bobbin can then be constantly aligned, in real-time, such that fiber is centered into the pulley. As such, the intelligent unwind system manages all aspects of the fiber handling, particularly when utilized within a vacuum chamber. The intelligent unwind system improves fiber quality, tow coating quality, thereby allowing the CMC raw material supply chain to reach industrial supply levels. 
     Referring to the drawings,  FIG. 1  shows an exemplary unwinding system  10  for unwinding a fiber  12  from a bobbin  14  rotatably mounted around an axle  16 . The axle  16  defining a first axis  18  extending an axial direction  20 , as shown in  FIG. 2 , such that the bobbin  14  is rotatable around the first axis  18 . Additionally, the bobbin  14  is controllably movable along the axial direction  20  to control the angle of the fiber  12  coming off of the bobbin  14 . Consequentially, the angle of the fiber  12  going into the pulley  22  is controlled. As shown, the fiber  12  extends tangentially from a surface  15  of the bobbin  14 , and into a pulley  22  positioned to receive the fiber  12  from the bobbin  14 . The pulley  22  is rotatable around a second axis  24 . In one embodiment, the pulley  22  is in a fixed location along the second axis  24 . 
     As more particularly shown in  FIG. 2 , a sensor  26  is positioned between the bobbin  14  and the pulley  22 . The sensor  26  is configured to determine the position of the fiber  12  with respect to the pulley  22  along at least one point of the length of the fiber  12 . As stated, the fiber  12  extends a length from the bobbin  14  to the pulley  22 . When a tension is applied on the fiber  12 , the fiber length extends tangentially from the surface  15  of the bobbin  14  and tangentially into the pulley  22 . Thus, the length of the fiber  12  between the bobbin  14  and the pulley  22  is substantially the same as the length L between the first axis  18  and the second axis  24 . 
     The fiber  12  defines a first angle  19  with the first axis  18  as it is unwound from the surface  15  of the bobbin  14 . Similarly, the fiber  12  defines a second angle  25  with the second axis  24  as it is received into the pulley  22 . The unwinding system  10  is utilized to move the bobbin  14  along the axial direction  20  of the first axis  18  (e.g., moving the bobbin  14  along the axial direction  20  of the axle  16 ) such that the first angle and the second angle are kept as close to 90° as possible. For example, each of the first angle  19  and the second angle  25  can be maintained between about 80° to about 100°, such as about 85° to about 95° (e.g., about 88° to about 92°). Thus, any fraying of the fiber  12  is minimized as it enters the pulley  22 , since the fiber  12  moves into the pulley such that the fiber  12  avoids contact with the pulley sides  23  and scraping against other fibers as it leaves the surface of the wound bobbin. 
     Referring again to  FIG. 1 , the unwinding system  10  is shown encased within a vacuum chamber  5 . A pump  102  is fluidly connected to the vacuum chamber so as to adjust the pressure within the vacuum chamber  5 . As such, the environment  101  within the vacuum chamber  5  can be controlled as desired. In particular embodiments, the environment  101  within the vacuum chamber  5  can be evacuated to an unwinding pressure of about 1 torr to about 5 torr (e.g., about 2 torr to about 3 torr) during the unwinding process. However, it should be noted that the presently described system can be used in any vacuum level, any pressure, or even in a chemical environment. The presently described system is particularly suitable for such processes due to the space saving design in a chamber. 
     Controlling of the first angle  19  and the second angle  25  through lateral movement of the bobbin  14  is particularly useful when the length L between the first axis  18  and the second axis  24  is relatively small with respect to the width W of the bobbin  14  (e.g., within a vacuum chamber). Since the fiber is wound around the bobbin  14  along most of its width W, the fiber  12  is unwound from the bobbin  14  from a changing point along its width. The closer the bobbin  14  is to the pulley, the more exaggerated the first angle  19  and the second angle  25  can become, if the bobbin  14  is not moved laterally in the axial direction  20 . For example, the length L of the fiber  12  from the bobbin  14  to the pulley  22  can be about 50% to about 1000% of the width of the bobbin  14  along the first axis  18 . 
     In one embodiment, the sensor  26  is a light sensor having a light emitter  28  (e.g., via a LED array) and a receiver  29  (e.g., a camera) that detects the location of the fiber  12  between the bobbin  14  and the pulley  22 . The sensor  26  can then generate a signal that is received at a controller  30 . The can move the bobbin  14  laterally in the axial direction  20  along the axle  16 . The controller  30  is configured to move the bobbin  14  laterally in the axial direction  20  along the first axis  18 . 
     The controller  30  may include a discrete processor and memory unit (not pictured). The processor may include a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed and programmed to perform or cause the performance of the functions described herein. The processor may also include a microprocessor, or a combination of the aforementioned devices (e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration). 
     Additionally, the memory device(s) may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD), and/or other suitable memory elements. The memory can store information accessible by processor(s), including instructions that can be executed by processor(s). For example, the instructions can be software or any set of instructions that when executed by the processor(s), cause the processor(s) to perform operations. For the embodiment depicted, the instructions include a software package configured to operate the controller  30  to, e.g., execute the exemplary method  400  described below with reference to  FIG. 4 . 
     Referring now to  FIG. 3 , an exemplary bobbin apparatus  100  is generally shown that may be utilized with the unwinding system  10 . The bobbin apparatus  100  includes the bobbin  14 , the controller  30 , and a motor  32  attached to the bobbin  14  and configured to move the bobbin  14  in the axial direction  20 . The motor  32  can actuate the bobbin  14  laterally in the axial direction  18  as controlled by the controller  30  in response to real-time signals received at the controller  30  from the sensor  26  regarding the position of the fiber  12  between the bobbin  14  and the pulley  22 . The bobbin apparatus  100  may also include a magnetic drive mechanism for moving the bobbin  14  along the first axis  18 . 
     As more particularly shown in  FIG. 1 , the fiber  12  exits the pulley  22  and is received into an idler pulley  34 . Then, the fiber  12  can be received from the idler pulley  34  into a dancer pulley  36  that can be connected to a tension controller  38 . The tension controller  38  is generally configured to maintain a desired tension on the fiber  12  as it is processed through the unwinding system  10 . In certain embodiments, the tension controller  38  senses the load on the dancer pulley  36  (i.e. tension on the fiber) and then responds to change the tension on the fiber  12  by moving the dancer pulley  36  and/or accelerates/decelerates the rotation of the bobbin  14 . 
     The fiber  12  is, in one embodiment, a ceramic fiber such as silicon carbide for forming a fiber reinforced ceramic matrix composites (CMCs). The resulting CMC can be a continuous uniaxial or woven fibers of ceramic material embedded in a ceramic matrix. These materials are designed to have a relatively weak fiber-matrix bond strength compared to the matrix strength so as to increase overall composite strength and toughness. When the CMC is loaded above a stress that initiates cracks in the matrix, the fibers debond from the matrix allowing fiber/matrix sliding without fiber fracture. The fibers can then bridge a matrix crack and transfer load to the surrounding matrix by transferring tensile stresses to frictional interfacial shear forces. Such fiber reinforced CMCs have great potential for use in aircraft and gas turbine engines due to their excellent properties at high temperatures. 
       FIG. 4  shows a diagram of exemplary method  400  of intelligently unwinding a fiber from a bobbin. At  402 , a fiber is unwound from a bobbin rotating around a first axis extending an axial direction. The fiber is received into a pulley rotatable around a second axis at  404 . The fiber extends a length from the bobbin to the pulley, and defines a first angle with the first axis and a second angle with the second axis. At  406 , the location of the fiber is sensed along at least one point of the length of the fiber between the bobbin and the pulley. The bobbin is moved laterally (i.e., in the axial direction) along its rotational axis (i.e., the first axis) to maintain a desired angle of the fiber leaving the surface of the wound bobbin (e.g. the first angle) and entering into the pulley (e.g., the second angle). For example, the first angle can be maintained between about 80° to about 100°, such as about 85° to about 95° (e.g., about 88° to about 92°), and the second angle can be maintained between about 80° to about 100°, such as about 85° to about 95° (e.g., about 88° to about 92°). 
     Through the exemplary unwinding system  10  described herein, the fibers, usually in the form of long fiber tows, can be unwound from a bobbin (i.e., the fiber source) to begin further processing, such as coating and/or saturating with a slurry of matrix powder in suitable solvents and binders, are then can be wound onto a mandrel to form cylinders or sheets of matrix containing aligned fibers. 
     In one particular embodiment, a process is generally provided for producing silicon carbide-containing fiber reinforced dense silicon-silicon carbide matrix composites, where the fibers are coated with at least a silicon-doped boron nitride coating. For example, the matrix material is molten silicon infiltrated silicon-silicon carbide which possesses net shape processing capability and ease of fabrication. 
     As such, a dense ceramic matrix composite, such as generally having a porosity of less than about 20% by volume, can be formed according to the methods described herein. The composite comprises, in one embodiment, a fibrous material of which the fibrous material component comprises at least about 5% by volume of the composite and has at least a silicon-doped boron nitride coating {B(Si)N} with a weight ratio of silicon to total weight of the {B(Si)N} coating between about 5 weight percent to about 40 weight percent; and a composite matrix having at least about 1% by volume of a phase of elemental silicon comprising substantially silicon. The elemental silicon phase comprises substantially silicon, but may have other dissolved elements, such as boron. 
     A method is also generally provided for making a silicon-silicon carbide matrix composite capable of improved properties in oxidative and wet environments via depositing at least a silicon-doped boron nitride coating on a silicon carbide-containing fibrous material such that the coating substantially covers an outer surface of said fibrous material. A matrix constituent material may be admixed to include particles (e.g., carbon, silicon carbide, and mixtures thereof) with the fibrous material, and the admixture may be formed into a preform. 
     The preform may then be impregnated with an infiltrant comprising substantially molten silicon; and cooling said infiltrated preform to produce the silicon-silicon carbide matrix composite, where a ratio of silicon weight to total weight of said B(Si)N coating is between about 5 weight percent to about 40 weight percent. 
     As used herein, “carbon” includes all forms of elemental carbon including graphite, particles, flakes, whiskers, or fibers of amorphous, single crystal, or polycrystalline carbon, carbonized plant fibers, lamp black, finely divided coal, charcoal, and carbonized polymer fibers or felt such as rayon, polyacrylonitrile, and polyacetylene. “Fibrous material” includes fibers, filaments, strands, bundles, whiskers, cloth, felt, and a combination thereof. The fibers may be continuous or discontinuous. Reference to silicon carbide-containing fiber or fibrous material includes presently available materials where silicon carbide envelops a core or substrate, or where silicon carbide is a core or substrate. Other core materials which may be enveloped by silicon carbide include carbon and tungsten. The fibrous material can be amorphous, crystalline, or a mixture thereof. The crystalline material may be single crystal or polycrystalline. Examples of silicon carbide-containing fibrous materials are silicon carbide, Si—C—O, Si—C—O—N, Si—C—B, and Si—C—O-Metal where the Metal component can vary, but frequently is titanium, zirconium, or boron. There are processes known in the art which use organic precursors to produce silicon carbide-containing fibers which may introduce a wide variety of elements into the fibers. Examples of these fibers include NICALON™ HI-NICALON™, and HI-NICALON S™, registered trademarks of Nippon Carbon Company, Ltd., Yokohama, Japan; TYRANNO™ fibers, a registered trademark of Ube Industries, Ltd., Ube City, Yamaguchi, Japan; and SYLRANIC™ fibers, a registered trademark of Dow Corning Corporation, Midland, Mich. 
     In carrying out the present process, a coating system is deposited on the fibrous material which leaves at least no significant portion of the fibrous material exposed, and preferably, the entire material is coated. The coating system may contain one coating or a series of coatings. If there is only one coating, it is a silicon-doped boron nitride {B(Si)N} coating or a graded coating of boron nitride to silicon doped boron nitride. The coating should be continuous, free of any significant porosity and preferably it is pore-free and significantly uniform. The silicon-containing compound in the coating is present in a sufficient amount to have a weight ratio of silicon to total weight of the B(Si)N coating between about 5 weight percent to about 40 weight percent. The preferred range is about 10 to 25 weight percent, and the most preferred range is about 11 to 19 weight percent. 
     The B(Si)N coating can be thought of chemically as an atomic mixture of boron nitride (BN) and silicon nitride (Si 3 N 4 ), which can be amorphous or crystalline in nature. Different levels of silicon doping would correspond to different ratios of BN to Si 3 N 4 , and a complete range of B(Si)N compositions can be envisioned from pure BN to pure Si 3 N 4 . At one extreme of this range, pure BN gives good fiber-matrix debonding characteristics for a ceramic matrix composite, but the oxidation/volatilization resistance is poor. At the other extreme, pure Si 3 N 4  has very good oxidation/volatilization resistance, but does not provide a weak fiber-matrix interface for fiber debonding during composite failure. At intermediate compositions, there exists a range of silicon contents where the B(Si)N provides both good fiber-matrix debonding characteristics and has good environmental stability. A range of silicon weight percent in the B(Si)N coating is about 5 to about 40 weight percent, and preferably about 10 to about 25 weight percent, and most preferably about 11 to about 19 weight percent silicon. 
     In addition to at least a B(Si)N coating, other configurations containing B(Si)N can also be used, such as multiple layers of B(Si)N with initial and/or intermediate carbon layers, or an initial layer of B(Si)N followed by further coatings of silicon carbide or Si 3 N 4 , or with additional layers of a silicon-wettable coating over the B(Si)N, such as carbon, or combinations of the above. 
     Still further examples of coating systems used in any combination with a B(Si)N coating on the fibers or fibrous material are: boron nitride and silicon carbide; boron nitride, silicon nitride; boron nitride, carbon, silicon nitride, etc. Examples of further coatings on the fibrous material that can be utilized include but are not limited to nitrides, borides, carbides, oxides, silicides, or other similar ceramic refractory material. Representative of ceramic carbide coatings are carbides of boron, chromium, hafnium, niobium, silicon, tantalum, titanium, vanadium, zirconium, and mixtures thereof. Representative of the ceramic nitrides useful in the present process are the nitrides of hafnium, niobium, silicon, tantalum, titanium, vanadium, zirconium, and mixtures thereof. Examples of ceramic borides are the borides of hafnium, niobium, tantalum, titanium, vanadium, zirconium, and mixtures thereof. Examples of oxide coatings are oxides of aluminum, yttrium, titanium, zirconium, beryllium, silicon, and the rare earths. The thickness of the coatings may range between about 0.3 to 5 micrometers. 
     As stated, the fibrous material may have more than one coating. An additional protective coating may be wettable with silicon and be about 500 Angstroms to about 3 micrometers. Representative of useful silicon-wettable materials is elemental carbon, metal carbide, a metal coating which later reacts with molten silicon to form a silicide, a metal nitride such as silicon nitride, and a metal silicide. Elemental carbon is preferred and is usually deposited on the underlying coating 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 later reacts with molten silicon to form a silicide must have a melting point higher than the melting point of silicon and preferably higher than about 1450° C. Usually, the metal and silicide thereof are solid in the present process. Representative of such metals is chromium, molybdenum, tantalum, titanium, and tungsten. 
     Known techniques can be used to deposit the coatings which generally is deposited by chemical vapor deposition using low pressure techniques. 
     In this process, fibers may be bundled in tows and coated with a coating or combination of coatings. The tows are formed into a structure, which is then infiltrated with molten silicon. In these methods, a boron nitride coating on the fiber is often used to protect the fiber from attack by molten silicon or for debonding. The silicon-doped boron nitride coating would then be in addition to or in place of the undoped boron nitride coating. The coatings in this invention can be graded from an undoped boron nitride to a silicon doped boron nitride coating. Non-graded coatings are also contemplated for use in this invention. 
     Another method used to make silicon carbide-silicon composites uses fibers in the form of cloth or 3-D structure, which are layered into the desired shape. Boron nitride coating is deposited on the cloth layers by chemical vapor infiltration as mentioned above, and a silicon-doped boron nitride coating would then be in addition to or in place of the undoped boron nitride coating. Additional coatings of silicon carbide or silicon nitride may be present on the boron nitride coating. The coatings can be graded from an undoped boron nitride to a silicon doped boron nitride coating. However, non-graded coatings are also contemplated for use with these processes. The structure is then processed in a slurry and melt infiltrated with molten silicon. The molten silicon may contain minute amounts of other elements, such as boron and molybdenum. 
     As stated above, the coated fibrous material is admixed with a matrix constituent material which comprises at least a carbon or silicon carbide or mixture of carbon and silicon carbide material. Other elements or compounds may be added to the admixture to give different composite properties or structure. The particular composition of the admixture is determinable empirically and depends largely on the particular composition desired, i.e., the particular properties desired in the composite. However, the admixture always contains sufficient elemental carbon, or silicon carbide, or mixtures of carbon and silicon carbide, to enable the production of the present silicon-silicon carbide matrix composite. Specifically, the preform should contain sufficient elemental carbon or silicon carbide or mixtures of carbon and silicon carbide, generally most or all of which may be provided by the admixture and some of which may be provided as a sacrificial coating on the fibrous material, to react with the molten silicon infiltrant to produce the present composite, containing silicon carbide and silicon. Generally, elemental carbon ranges from about 0% by volume, or from about 10% or 20% by volume, to almost about 100% by volume of the admixture. 
     The mixture of carbon or silicon carbide or carbon and silicon carbide in the preform can be in the form of a powder and may have an average particle size of less than about 50 microns, more preferably less than about 10 microns. The molten silicon that infiltrates the preform is comprised substantially of silicon, but may also contain elemental boron, which has limited solubility in the molten silicon. The silicon infiltrant may also contain boron-containing compounds or other elements or compounds. 
     The admixture in the preform containing the carbon or silicon carbide or mixture of silicon carbide and carbon, is wetted by the molten silicon infiltrant. In carrying out the present process, the preform is contacted with the silicon infiltrant by an infiltrating means. The infiltrating means allow the molten silicon infiltrant to be infiltrated into the preform. In the present process, sufficient molten silicon infiltrant is infiltrated into the preform to produce the present composite. Specifically, the molten silicon infiltrant is mobile and highly reactive with any carbon present in the preform to form silicon carbide. Pockets of a silicon phase also form in the matrix. 
     The period of time required for infiltration is determinable empirically and depends largely on the size of the preform and extent of infiltration required. Generally, it is complete in less than about 60 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. 
     The present composite then is comprised of coated fibrous material and a matrix phase. The matrix phase is distributed through the coated fibrous material and generally it is substantially space filling and usually it is interconnecting. Generally, the coated fibrous material is totally enveloped by the matrix phase. The matrix phase contains a phase mixture of silicon carbide and silicon. The fibrous material comprises at least about 5% by volume, or at least about 10% by volume of the composite. The matrix contains a silicon carbide phase in an amount of about 5% to 95% by volume, or about 10% to 80% by volume, or about 20% to 60% by volume of the composite. The matrix may contain an elemental silicon phase in an amount of about 1% to 30% by volume of the composite. 
     The impregnated shapes made therefrom are at this stage of the process commonly termed “prepregs.” A prepreg can be reshaped as desired and ultimately formed into a preform for a composite article. The preform is subjected to a burn-out step to remove organic or other fugitive coating components. The preform is finally consolidated into a dense composite material by reaction with molten silicon at high temperature. 
     The fibers are coated for several purposes such as to protect them during composite processing, to modify fiber-matrix interface strength and to promote or prevent mechanical and/or chemical bonding of the fiber and matrix. A number of different techniques have been developed for applying fiber coatings, such as slurry-dipping, sol-gel, sputtering and chemical vapor deposition (CVD). Of these, CVD has been most successful in producing impervious coatings of uniform thickness and controlled composition. In a typical CVD process, fibers and reactants are heated to some elevated temperature where coating precursors decompose and deposit as a coating. CVD coatings can be applied either in a batch or continuous mode. In a batch mode, a length of fiber is introduced into a reactor and kept stationary throughout the coating process while reactants are passed through the reactor. In a continuous process, fibers and coating precursors are continuously passed through a reactor. Continuous fiber coating processes are preferred for composites processed by filament winding. As such, the exemplary unwinding system  10  described herein is particularly suitable for providing a continuous fiber into such a process. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.