Patent Publication Number: US-2007099527-A1

Title: Method and reactor to coat fiber tows and article

Description:
This invention was first conceived or reduced to practice in the performance of work under contract DE-FC02-92CE41000 with the United States Department of Energy. The United States of America may have certain rights to this invention. 
    
    
     BACKGROUND OF THE INVENTION  
      The invention relates to a method and reactor for applying a coating to a fiber by chemical vapor deposition (CVD). Also, the invention relates to an article comprising a multiplicity of fiber tows.  
      Fiber reinforced ceramic matrix composites (CMCs) are formed of 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. The fiber reinforced CMCs have great potential for use in aircraft and gas turbine engines due to their excellent properties at high temperatures.  
      The CMCs can be manufactured by filament winding. In this process, fibers, usually in the form of long fiber tows, are saturated with a slurry of matrix powder in suitable solvents and binders and are then wound onto a mandrel to form cylinders or sheets of matrix containing aligned fibers. 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.  
      One common continuous fiber coating process involves running a single tow or fiber into the reactor at a time. The coating is conducted at low pressure to insure uniform coating. The tow or fiber is transported through the reactor only at a slow speed. Although current processes provide useful results, there remains a need for further improvements to CVD processes for uniformly coating fiber tows with higher productivity.  
     BRIEF SUMMARY OF THE INVENTION  
      The invention provides a method and reactor to uniformly coat fibers at improved throughput. According to the invention, a method of coating fiber comprises simultaneously passing a multiplicity of fiber tows aligned as a ribbon through a reactor and passing a flow of fiber coating reactant though the reactor to coat the tow fibers.  
      In an embodiment, a coating system comprises a reactor chamber to accommodate a multiplicity of fiber tows passing along a path substantially parallel to a longitudinal axis of the chamber and a flow of fiber coating reactant and an aligning structure at an end of the chamber to maintain the multiplicity of fiber tows in a narrow, elongated ribbon configuration.  
      In another embodiment, an article comprises a multiplicity of fiber tows aligned in a longitudinal planar array in the form of a ribbon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of a reactor to simultaneously coat a multiplicity of fiber tows;  
       FIG. 2  is a schematic representation of a multiplicity of fiber tows aligned with a spacer tape for loading to a take-up spool;  
       FIG. 3  is a schematic representation of aligning and tensioning structures with a reactor; and  
       FIG. 4  is a schematic representation of a disrupter reactor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Ceramics have excellent heat resistance, corrosion resistance and heat insulating properties in comparison to metal materials. Hence, ceramics can be used as structural materials in place of metals in harsh high temperature or corrosive environments. However, ceramics cannot be deformed in the same way as metal materials. In a ceramic, stress becomes concentrated on defects in the material and flaws on the material surface. Fibers can be dispersed in a matrix of the ceramic to improve its toughness. The present invention generally relates to fibers, including those that are adapted for use as a reinforcement phase in a composite ceramic. Such fibers, when disposed in a CMC, prevent catastrophic failure of a CMC by debonding and bridging cracks as matrix fracture occurs.  
      Typically, the fibers are provided with a continuous coating to control interfacial shear strength between the fibers and the ceramic material. The coating can be applied by CVD. For example, Corman, U.S. Pat. No. 6,168,827 discloses a method for applying a CVD coating to reinforcing fibers arranged into a tow or cloth. The method comprises aligning the tow or cloth with an adjacent separation layer of a porous non-woven chopped fiber material. The chopped fiber separation layer has a porosity of at least about 85%. The tow or cloth and the separation layer are wound onto a support structure. The tow or cloth and layer are wound in an interleaved manner so that the layer separates a wrap of the tow or cloth from an adjacent wrap of the tow or cloth. The support structure with wound materials is then placed into a CVD deposition furnace where the tow or cloth is coated.  
      Dietrich et al., U.S. Pat. No. 4,657,776 teaches another process in which fiber tows are fanned out into carrier ribbons comprising strands of single fibers. The ribbons are passed through the center of the furnace chamber while a reactant gas is injected either counter to or in the same direction as movement of the ribbons and substantially parallel to the ribbons.  
      According to the invention, fiber tows are arranged into ribbons of tows for continuous processing in a CVD furnace. The term “tow” means “[a] large number of continuous filaments collected in ropelike form . . . .” McGraw-Hill dictionary of Scientific and Technical Terms, 5 th  Ed. page 2048, 1994. In this application, “tow” means a plurality of fiber filaments collected in a ropelike form and “plurality of fiber filaments” means at least more than a single fiber filament and “ribbon” means a longitudinal planar array of multiple tows.  
      In the inventive process, the tows are separated by sufficient spacing to assure uniform coating of fibers in each tow. Spacing of the fiber tows in the ribbon can be the same as a desired spacing in the composite preform. For example, the spacing is about 0.5 mm to about 25 mm in some embodiments, about 0.625 mm to about 10 mm in certain embodiments, and about 0.9 mm to about 5 mm in particular embodiments.  
      Because a large number of fiber tows can be passed through the coating furnace together, throughput of the process is greatly improved. Improved throughput results in lower coating cost. The process of the invention differs from standard coating practice in that it passes a planar array of aligned fiber tows through the furnace. The process requires only a single drive mechanism. By providing high density packing, the invention makes it possible to coat a large number of fiber tows in a furnace of relatively small cross section.  
      The tows can be arranged in the form of a ribbon by first winding them together onto a common spool. The degree to which precursor gas can penetrate fiber tow can depend on the tension applied to the tow. Uniformly tensioned tows promote the fabrication of uniformly coated individual fibers. Tensioning devices can be controlled during winding to provide uniformly tensioned tows on a common spool. For example, the tows can be fed through an aligning device as shown in  FIG. 3  to provide a parallel ribbon array with controlled spacing and uniform tension.  
      A “fugitive” binder can be used to hold the tows together in ribbon form for ease of handling. Suitable binders include polymeric binders, such as epoxies, polyvinyl alcohol, etc. Preferably, a binder is selected that will decompose “cleanly” upon heating, meaning that it will not leave significant amount of residue on the fiber. Suitable decomposing binders include acrylate polymers such as polymethylmethacrylate. These polymers “unzip” when heated—they depolymerize back to monomers, which readily volatilize, leaving no residue on the tow fibers. The binder can be applied to the tows as solution in a suitable solvent. It can be applied to individual tows or to the aligned tow array by passing the fibers through a binder solution, or by spraying binder solution onto the tows.  
      In another embodiment, the fiber tows can be held in ribbon form by weaving, stitching or braiding to impart strength in an orthogonal or substantially orthogonal direction. The weaving or stitching can be temporary and can be removed subsequent to the coating process. Weaving can be accomplished by running a set of fibers (secondary fibers or tows) substantially orthogonal to the longitudinal tows at periodic intervals along the ribbon. The secondary fiber or tow must be stable under coating conditions, meaning that it should not decompose or react with the longitudinal tow and should not react with reactants involved in the coating step. The secondary fiber can be the same material as the longitudinal tows or the fiber can be different. For example, a carbon fiber or tow can be used to hold together silicon carbide tows. Other examples of suitable secondary fibers include fibers made from thermodynamically stable oxides, carbides and nitrides.  
      After coating, the ribbon tows can be processed into a composite preform. The ribbons can be separated into individual tows, which can be processed by filament winding or lay-up techniques. Alternatively, the ribbons can be processed directly into a preform by ribbon winding. Ribbon winding is similar to single tow winding. The winding can be either wet or dry winding. In wet winding, the ribbon is run through a bath of matrix slurry until saturated. Matrix slurry is a suspension of matrix particles in a solution of organic binders in a suitable solvent. The ribbon can then be wound onto a preform mandrel as a unit. Winding pitch (the length by which the preform mandrel is moved per each winding revolution of the mandrel) is chosen to allow the ribbons to butt against adjacent ribbons without overlapping. Overlapping of tows can result in thickening of the piece at the area of overlap. Generally, pitch is selected to be very close to the width of the ribbon.  
      Ribbon winding is much faster than the winding of single fiber filaments. If for example, a ribbon consists of 10 tows, it can be wound at 10 times greater pitch than a single tow or fiber. Consequently, a wound piece can be produced in a tenth of the time necessary for single tow or filament winding. Otherwise, the winding parameters for ribbon tow winding, such as fiber speed, tension etc do not significantly differ from parameters of single tow winding.  
      The tow ribbons can be processed into composite preforms by prepregging of individual ribbons. The ribbons are saturated with matrix slurry by passing through a slurry or by spraying slurry onto the ribbons. The ribbons can be dried for handling and formed into desirable shapes by hand- or machine-placing partially or fully dried ribbons into a preform shape. The preforms can then be consolidated by autoclaving.  
      Fibers that are available in tows can have a diameter of 4-25 microns. A fiber tow can contain from 50 to 12000 fibers, depending on fiber type, size and intended use. Numerous fibers are available in the form of a tow. These fibers range in composition from glasses to polycrystalline materials. Suitable fibers include oxide, silicon carbide and silicon nitride types. Other suitable types include glass fibers (fiberglass composites) and carbon fibers. Several types of polycrystalline oxide fibers are suitable and are commercially available.  
      Silicon carbide fiber tows are produced by spinning fibers from a polymer precursor, curing and converting the fibers to nearly pure silicon carbide by subsequent heat treatment. Commercially available silicon carbide fibers typically range in size from about 8 to 14 microns and are available in tows containing from 400 to 800 fibers. The silicon carbide-based fibers are of particular interest for composites described in present invention. However, the invention can be used to coat any fiber tow.  
      The coating method of the invention can be a continuous coating process. In the continuous process, fibers are passed through a furnace, which is held at an elevated temperature. A constant flow of precursor gas is maintained through the furnace. Gas decomposes at about 700° C. to about 1800° C. temperature and reacts and deposits to form a coating on the fiber tows. Desirably, the temperature is about 1000° C. to about 1650° C. and preferably about 1250° C. to about 1550° C.  
      The number of fiber tows being coated simultaneously varies with the size of the CVD chamber. For example, a typical furnace can accommodate from one to about 25 fiber tows. Tow fiber spacing, as used herein, means the distance from the center of a fiber in a tow to the center of its nearest neighboring fiber. This tow fiber spacing is about 0.5 mm to about 25 mm in some embodiments, about 0.625 mm to about 10 mm in certain embodiments, and about 0.9 mm to about 5 mm in particular embodiments.  
      The speed at which fibers are transported through the furnace is dependent on the type of coating being applied, desired thickness and the length of deposition zone in the furnace. A typical rate, in some embodiments, is from about 25 to about 5000 mm/minute. In certain embodiments, the rate is from about 125 to about 4000 mm/min, and in particular embodiments the rate is from about 150 to about 2500 mm/minute.  
      The reactant gases that are used are determined by the desired coating. Those skilled in the art are familiar with the selection of precursors and processing parameters for depositing coatings onto substrates using various chemical vapor deposition processes. Some coatings are produced by decomposition of a single reactant gas. For example, deposition of carbon is typically accomplished by decomposition of a hydrocarbon, such as methane. The decomposition reaction can be described by the following: 
 
CH 4 -&gt;C+2H 2  
 
      A two-gas reaction can be used for other coatings, such as a boron trichloride and ammonia reaction to form boron nitride: 
 
BCl 3 +NH 3 →BN+3HCl 
 
      A three-gas reaction can be used, such as a boron trichloride, ammonia and a silicon precursor to produce silicon-doped boron nitride. Silicon precursors include dichlorosilane, trichlorosilane, silicon tetrachloride and silane. For example, trichlorosilane and ammonia may be suitable precursors for silicon nitride coatings. Hydrogen or nitrogen, which may not be directly involved in the chemical deposition reaction, can be used to dilute precursor gases to control reaction speed and temperature. Moreover, coatings of multiple layers (same or different compositions) may be deposited on the fiber by, for instance, performing multiple passes through a reactor or passing the fiber through multiple reactors. Various combinations of coating layers may be applied to the fiber to control interfacial properties and to improve protection of the fiber during molten silicon infiltration. These combinations may consist of two, three, four, or more layers. Examples of four-layer combinations include boron nitride/silicon-doped boron nitride/silicon nitride/carbon; and boron nitride/carbon/silicon nitride/carbon. Examples of three-layer combinations include boron nitride/silicon nitride/carbon; and boron nitride/silicon-doped boron nitride/carbon. An exemplary two-layer combination is boron nitride/carbon. None of these exemplary combinations should be read as limiting the invention.  
      The thickness of coating deposited on a fiber is dependent on a number of factors such as tow speed, reactor pressure and precursor gas flow rate. The gas flow rate has to be sufficient to provide desired coating thickness on the fiber. The rate depends on the cross section of the reactor, number of tows being coated and their transport rate. The rate can vary from 0.1 to 100 standard liters per minute (slpm). Standard liters per minute means that the gas flow rate is referenced to a measurement made at standard conditions (pressure of 1 atmosphere, temperature of 298° K). This convention is necessary because gas volume changes with pressure and temperature. For a specific reactor having a cross section of about 30 cm 2 , a gas flow rate can be about 0.5 to about 20 slpm.  
      Reactor pressure determines how fast the precursor gas decomposes. It also has an effect on the mean free path of gas molecules, so it may affect fiber coating thickness uniformity. Operation at a reduced pressure (below atmospheric) is preferred, although not required. Typically, deposition rates decrease with pressure; however, coating thickness tends to be more uniform through a tow or bundle. Reactor pressure can be from about 0.05 Torr to atmospheric pressure (760 Torr). Desirably, the pressure for the process is about 0.1 to about 50 Torr and preferably about 0.3 to about 10 Torr.  
      These and other features will become apparent from the drawings and following detailed discussion, which by way of example without limitation describe preferred embodiments of the invention.  
      A schematic cross-section drawing of a reactor  10  for a continuous CVD coating process is shown in  FIG. 1 . The reactor  10  includes a reactor chamber  12 , each end of which is terminated by one of a pair of slots  14  and  16 . The reactor chamber  12  has a reactant inlet  18  and an exhaust port  20 . Each slot,  14 ,  16  is openable to admit fiber tow ribbon  26 .  
      The tow ribbon  26  comprises a flat, narrow, elongated arrangement of fiber tows  24 . The tows  24  are aligned in a spaced apart ribbon configuration; that is, the tows  24  are arranged with parallel longitudinal axis aligned into a narrow horizontal plane. Substantially evenly spaced fiber tows promote substantially uniform coating of the fiber tows.  
      Fiber tows in a ribbon according to the invention tend to remain well separated if the tows are stabilized such that they remain substantially equally tensioned and aligned. One way of stabilizing the tows is by weaving. In weaving, the alignment of primary fiber is maintained by introducing stabilizing secondary fiber at a right angle to the axis of the primary tows at periodic intervals along the ribbon. As the stabilizing fiber will mask the primary fiber wherever the primary and stabilizing fibers are in contact, thereby inhibiting the formation of the coating at these locations, it is desirable that the selected periodic interval along the ribbon between stabilizing fibers is sufficiently large to provide adequate lengths of uniformly coated primary fiber. In some embodiments, the selected interval is in the range from about 1 mm to about 25 mm.  
      According to another embodiment shown in  FIG. 2 , the tows  24  are aligned onto a supporting co-aligned spacer base tape  28 , either before coating the tows  24  (in which case the tape is removed prior to coating), after coating, or both before and after coating. The spacer base tape  28  can be a polymer film, a paper strip or a metal foil. The spacer base tape  28  prevents fiber tows from being abraded by or tangled with subsequent windings of tows on the spool.  
      In  FIG. 1 , ribbon  26  is pulled on-line through reactor chamber  12  for chemical vapor deposition of a coating. The ribbon is made up of multiple fiber tows  24  that are uniaxially aligned as shown. The ribbon  26  is fed from feed spool  30  over guide pulley  32  and through slot  14  and into chamber  12 . Take up spool  36  receives coated ribbon  26  as it leaves chamber  12  via slot  16 . Spool  44  takes up spacer ribbon  26  as tow ribbon  26  is unwound and fed into reactor chamber  12 . Spool  46  includes spacer ribbon, which is fed into the take up spool  36  along with the coated ribbon  26 . The reactor chamber  12  is heated by heater  42 . The reactor chamber  12  is oriented vertically so that weight of the fiber tow  24  does not produce sag (as it could in a horizontal deposition process). Flow  38  of reactant gas is designated by arrows. The reactor chamber  12  is relatively narrow to ensure that reactant gas flow  38  passes through the reactor chamber  12  in close proximity to the fiber tow ribbon  26  for effective coating.  
      Tension during coating can be applied by a frictional brake and by friction in the bearings of the fiber transport mechanism (not shown). The tension can be about 5 to about 200 grams per tow, about 5 to about 100 grams per tow in some embodiments, and about 10 to about 100 grams per tow in particular embodiments. In a single tow coating process, one tow has to overcome frictional forces of the transport mechanism. Certain embodiments of the invention reduce tension required for uniform coating by utilizing a single transport mechanism for the multiple tows of a ribbon. With the ribbon configuration described herein, frictional forces are divided over the multiplicity of the ribbon tows.  
       FIG. 1  shows ribbon  26  being transported from slot  14  through the reactor chamber  12  to exit through slot  16 . The particular form of slot  14  and slot  16  permits the feed spool  30 , guide pulleys  32  and  34  and take-up spool  36  to operate to draw the ribbon  26  through the reactor chamber. The slots  14  and  16  maintain the fiber tows within a horizontal place in the ribbon  26  form. In this embodiment, the tows are maintained in a ribbon form without the necessity of a backing or spacing. Advantageously then, each tow of the ribbon is completely exposed to reactant gas around its complete circumference.  
      In the process, continuous lengths of tows  24  in ribbon  26  form are passed through a reactor chamber at a predetermined speed. Reactant gas flow  38  is passed through the chamber. Decomposition of the gaseous reactant results in coating of the fibers of the tow ribbon  26 . As reactant in an immediate vicinity of the fiber tow is consumed, additional reactant diffuses from a bulk gas phase. At the same time, gaseous reaction product diffuses away from each fiber tow  24  to create a gradient of reactant and product.  
      According to the invention illustrated in  FIG. 1 , improved coating of each fiber tow is obtained since each tow  24  is exposed in its entirety to the reactant gas as the tow passes through the reactor chamber. With some tow material however, it is difficult to retain a tension equilibrium across the entire ribbon and along the ribbon as it is drawn through the length of the reactor chamber. The ribbon  26  may sag and the tows  24  may contact one another to cause spots of incomplete coating. However, stabilization of the ribbon with periodic intervals of stabilizing secondary fibers, as described above, aids in maintaining even spacing between tows, and further assists in the maintenance of uniform tension along the length of the stabilized ribbon. In doing so, stabilization assists in achieving substantially uniform coatings among the individual tows  24 .  
       FIG. 3  is a schematic top view of an aligning structure  50  placed at the charge end of a furnace  48 .  FIG. 3  shows a single ribbon  26  of twelve fiber tows  24 . Individual tows  24  are fed from respective spools  62  through the aligning structure  50  to configure ribbon  26 . The respective spools  62  are fed from a tensioning structure  52 , which is a common axis for the spools  62 . Structure  52  maintains an equal tension on each tow  24 . Each tow  24  is threaded around two axially mounted guide pulleys  54  and  56 . Inside pulleys  56  are mounted equally spaced on respective rails  58  and  60 . The distance between pulleys  56  on rails  58  and  60  and the angle “a” between the rails  58  and  60  can both be adjusted to determine a spacing between the tows  24  making up the ribbon  26 . The resulting configured planar parallel array making up the ribbon  26  can then be wound under equal tension onto ribbon spool  64 . While pulleys  54  and  56  are shown in  FIG. 3 , other structures such as stationary fiber guides can be used for aligning the tows  24 . Pulleys are preferred structures because they cause less friction and damage to the fiber tows during winding.  
      The ribbon  26  of tows  24  can be wound on pick-up spool  64  and unwound into reactor chamber  12  as shown in  FIG. 1 . In the embodiment shown in  FIG. 3 , furnace  48  is interposed between the aligning structure  50  and the ribbon pick-up spool  64 . This embodiment provides for pretreatment of the tows prior to coating. For example, pretreatment can involve removal of fiber sizing (commonly referred to as de-sizing). Sizing is a polymeric coating applied by a fiber tow manufacturer to protect the tows during handling. Removing sizing during pretreatment avoids production of gaseous polymer products inside the CVD reactor. The fiber tows tend to relax as sizing is removed. The extent of relaxation can vary from tow to tow. Winding the tows on pick-up spool  64  under tension and immediately after de-sizing imparts a uniform tension to the ribbon tows—another advantage of the embodiment shown in  FIG. 3 .  
       FIG. 4  shows a schematic of a reactor according to one embodiment of the invention. In  FIG. 4 , the reactor  130  includes a reactor chamber  132 , each end of which is terminated by one of a pair of slots  134  and  136 . The reactor chamber  132  has a reactant inlet  138  and an exhaust port  140 . The slots  134  and  136  enable a fiber tow ribbon  146  to be pulled on-line through the reactor chamber  132  for chemical vapor deposition of a coating. Chamber  132  is equipped with a set of gas flow disrupters  150  positioned along its length. Flow of gas in the chamber is designated  148 . The disrupters  150  force gas to flow  148  though the reactor chamber in a convoluted pathway, rather than parallel to the tube axis. In  FIG. 4 , gas flow disrupters  150  have deflecting faces  152  shown set at an angle of 45° to the furnace axis, thus causing gas to change direction of travel by 90° at each disrupter. The change in direction facilitates uniform coating of the tows in ribbon form.  
      The following Examples are illustrative and should not be construed as a limitation on the scope of the claims unless a limitation is specifically recited.  
     EXAMPLE 1  
      Twenty-two tows of Nicalon™, a silicon carbide fiber from Nippon Carbon Company, were wound on a spool and installed into a spool box on one end of a CVD reactor. The tows were passed through the CVD reactor as multiple parallel tows under tension and arranged in ribbon form as shown in  FIG. 1 . Tow spacing was 0.045 inches. The tows were attached to a pick up spool in a receiving box on the opposite end of the CVD reactor. An end box was connected to the reaction chamber by narrow slots, just large enough to pass the tow ribbon. The deposition chamber was a graphite tube of uniform cross section. The chamber had a total of six disrupters in the form of ⅝-inch long wedges attached alternately to opposite sides of the chamber wall as shown in  FIG. 4 .  
      The reactor was evacuated and a flow of nitrogen gas was introduced into the reactor chamber through each end box as a seal against reactant leakage. The reactor chamber was heated in accordance with the description above. Reactant gases, boron trichloride and ammonia, were injected into the bottom of the reactor chamber for flow through the chamber in the same direction as the tow bundle. A vacuum pumping system at the top of the chamber expelled spent gas.  
      Reactant gases were passed at a constant flow rate while the tow ribbon was transported through the chamber in the same direction at constant speed. Boron trichloride and ammonia reacted to form boron nitride, which deposited on the tow (and on furnace wall). Once an entire length of tows of the ribbon was coated, gas flow was stopped and equipment was cooled to room temperature, still under vacuum.  
      Coated tows were examined in a scanning electron microscope. Examination showed that all fibers in the bundle were coated with boron nitride. The coating thickness of the tows on the periphery of the ribbon was compared to coating thickness on the tows in the center of the ribbon. It was found that average coating thickness of fibers at a tow periphery was about a factor of two greater than the coating thickness in the center of the tow bundle. This result showed that there was good flow of reactant gases through the fiber tows.  
     EXAMPLE 2  
      Eleven 100-meter long tows of Hi-Nicalon™, a silicon carbide fiber from Nippon Carbon Company, were wound on a spool in a parallel arrangement. The tows were then passed through a CVD reactor as multiple parallel fiber tows arranged in ribbon form as described in Example 1. The tows were coated with boron nitride in the manner described in Example 1. After the furnace was cooled to room temperature, a spool with coated tow was transferred into the entrance box of the coated system and an empty take-up spool was installed in the exit box. The tows were passed as a ribbon through the reactor and attached to the take-up spool. The furnace was heated back into the coating temperature range and the tow ribbon was passed through the furnace at a constant rate while a mixture of boron trichloride, trichlorosilane and ammonia was passed through the reactor at the same time. The original boron nitride coating on fibers was thus covered with a coating of boron nitride doped with silicon. The fiber tows were again transferred to the entrance box of the reactor and passed through the furnace again as a ribbon with a mixture of trichlorosilane and ammonia. This step deposited a third coating, silicon nitride, on top of the first two coatings. A final coating, carbon, was deposited on the tow ribbon by passing it through the heated reactor while passing methane gas through the reactor.  
      The resulting ribbon of coated fiber tows was separated into individual tows and the tows were wound on separate spools. The fiber tows were then processed into composites by first filament winding them to produce sheets containing parallel fibers imbedded in a matrix. The sheets were stacked to yield an 8-ply, 0/90° preform. Binder was burned out and the preform was infiltrated with liquid silicon. The resulting composites were tested under tension. The composites exhibited excellent composite properties. All samples had an ultimate strength in excess of 45 kpsi and a strain-to-failure exceeding 0.8%. By comparison, composite made with improperly coated fibers will typically have strengths below 30 kpsi and strain-to-failure of less than 0.1%. Extensive fiber pullout indicated that the coatings successfully protected fiber during the silicon infiltration step and provided a debond interface between the fibers and the matrix.  
     EXAMPLE 3  
      A ribbon consisting of tows of Hi-Nicalon™ fibers was formed and coated with boron nitride, silicon-doped boron nitride and carbon in the manner described in Example 2. The ribbon containing 5 fiber tows was used for ribbon winding. The ribbon was passed through a bath containing matrix slurry and wound onto a mandrel using a pitch 5 times larger than used for a single tow winding. The ribbons were wound longitudinally butting against each other to form a uniform sheet containing aligned fibers in a matrix. The sheet was cut and stacked into an 8-ply, 0/90° composite preform. The preform was autoclaved to burn out binder. Then the preform was infiltrated with molten silicon. The resulting samples exhibited good composite properties when tested in tension, indicating that ribbon processing produced sound composites and did not damage fiber coatings.  
      While preferred embodiments of the invention have been described, the present invention is capable of variation and modification and therefore should not be limited to the precise details of the Examples. The invention includes changes and alterations that fall within the purview of the following claims.