Patent Publication Number: US-2016229758-A1

Title: Continuous chemical vapor deposition/infiltration coater

Description:
BACKGROUND 
     This invention relates to chemical vapor deposition, chemical vapor infiltration and ceramic matrix composite materials. In particular the invention relates to continuous production of interface coated ceramic matrix composite precursor fabrics and/or unidirectional tow sheets. 
     Ceramic matrix composite (CMC) materials are finding increased utility in gas turbine engines because of their high temperature applicability and low density. A common CMC structure consist of woven ceramic fibers in a sheet form such as a cloth infiltrated with the same or different ceramic to form a ceramic matrix composite with densities up to 100 percent. In most cases, the ceramic matrix is formed by either chemical vapor infiltration (CVI), polymer infiltration pyrolysis (PIP) or reactive melt infiltration (RMI). Regardless of matrix the mechanical integrity of all CMCs depends on a fiber interface coating to provide proper bonding/debonding behavior between the fiber and matrix that provides the toughening behavior that is required for successful application. 
     In general CVD is a process whereby a solid ceramic is deposited from the vapor phase, usually at an elevated temperature and reduced pressure. The high temperature provides the activation energy needed to make or break the precursor bonds of a reactant gas. Low pressure provides for a more efficient method to defuse the reactive and byproduct gases to and from the substrate. The deposition rate is directly proportional to the precursor gas concentration, partial pressure and temperature. CVI is a process whereby a pore filling solid is deposited from a reactant gas vapor phase. This process is similar to CVD in that it uses both elevated temperatures and low pressures, but the temperatures and pressures are generally lower to reduce the deposition rate and provide the required increased time for reactant gases to defuse into a porous substrate before reacting. The lower pressure allows for a greater “mean free path” for the reactant vapors to travel before contacting a nucleation cite and reacting. 
     Increased throughput in CVD or CVI processing is a continuing goal in the art. 
     SUMMARY 
     A method to form a ceramic interface coating on ceramic matrix composite (CMC) precursor tape by a continuous process includes passing a ceramic fiber woven cloth tape or unidirectional tape of a first ceramic with a first and second surface through at least one zone of a continuous vacuum chemical vapor deposition (CVD) or chemical vapor infiltration (CVI) reactor heated to a reaction temperature. The method further includes directing a flow of CVD OR CVI reactant gas of a second ceramic at the first surface of the tape in a direction perpendicular to the tape such that the reactant gas passes through the tape in a forced flow process depositing the second ceramic on the fibers of the first ceramic thereby coating the fibers of the first ceramic tape with the second ceramic to form a coated fiber CMC precursor tape product. 
     An apparatus to continuously form ceramic interface coatings on ceramic matrix composite (CMC) precursor tape includes a ceramic fiber woven cloth tape or unidirectional tape of a first ceramic with a first and second surface mounted on a supply spool in a storage chamber. The storage chamber also contains a tension roller, a purge gas inlet and a tape outlet seal. The apparatus further includes at least one chemical vapor deposition (CVD) or chemical vapor infiltration (CVI) deposition zone. The deposition zone consists of a vacuum chamber, containing a tape inlet seal, a reactive gas dispensing system directed at the first side of the tape in a direction perpendicular to the tape for forming a CVD or CVI coating of a second ceramic on the fibers to form an interface coating, a purge gas inlet, at least one heating element for heating the tape to a CVD or CVI reaction temperature, a vacuum system, high temperature insulation, and a tape outlet seal. The apparatus further includes a collection chamber containing a tape collection spool mounted on a drive shaft, a tension roller, a purge gas inlet and a tape inlet seal. The apparatus also includes a variable speed drive attached to the collection spool to gather the coated CMC precursor tape product on the collection spool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a forced flow isothermal continuous CVI reactor of the invention. 
         FIG. 2  is a schematic representation of a forced flow thermal gradient continuous CVI reactor of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes the development of a continuous coater to be used to form interface coatings on CMC precursor woven porous fiber cloth or unidirectional tape. As discussed above, a CMC may be a woven ceramic fiber cloth or unidirectional tape that is infiltrated with a matrix by any suitable infiltration process including CVI, PIP, RMI and others known in the art of the same or different ceramic such that the surfaces of the fibers are coated with a matrix bond enhancing interface. As known in the art, the bond strength between the fibers and the matrix may be optimized with an interface coating to maximize the mechanical properties of the composite matrix composite material. The density of the CMC may approach 100 percent depending on the application. 
     CVD is a process whereby a solid is deposited from the vapor phase, usually at elevated temperatures and low pressures. The high temperature provides the activation energy needed to make or break the precursor bonds in a reactant gas. The low pressure provides for an efficient method to defuse the reactant and byproduct gases to and from the substrate. The deposition rate is directly proportional to the precursor gas concentration, partial pressure and temperature. 
     CVI is a process whereby a pore filing solid is deposited from a vapor phase. The process is similar to CVD in that it uses both elevated temperatures and low pressures but the temperatures and pressures are generally lower to reduce the deposition rate and provide greater time for precursor reactant gases to defuse into the porous substrate before reacting. The lower pressure allows for a greater “mean free path” for the precursor vapors to travel before impacting a nucleation cite and reacting. If a CVI process is run like a CVD process, i.e. with a high deposition rate induced by high temperatures and/or high precursor partial pressure, the deposition rate through the thickness of the sample will vary greatly, with the deposition rate at the surface being much higher than the deposition rate inside the substrate. Carefully controlling deposition parameters allows deposition rates within the substrate to be equal to or higher than deposition rates at the surface in order to allow even deposition throughout the entire substrate. 
     There are several CVI processes that attempt to accomplish the effect of controlling the deposition rate throughout the thickness of a woven fiber substrate. Exemplary processes include isothermal/isobaric, forced flow/isothermal, and forced flow/thermal gradient procedures. By far the most common process is isothermal/isobaric. In an isothermal/isobaric CVI process, the substrates that are intended to be infiltrated are loaded into a reactor in which the pressure and temperature are held constant throughout the volume of the reactor and the reactor is allowed to run for an extended period of time. Deposition (or infiltration) is governed by the diffusion of reactant gases into the substrate and the diffusion of byproduct effluent gases out of the substrate. The distribution of precursor reactant gases through the volume of the reactor is controlled to maintain a constant deposition rate throughout the reactor from location to location. Deposition rates throughout the volume of a given sample are dictated by reactant partial pressures and are higher at the surface than at internal locations. Minimizing this deposition rate gradient demands that the process be run at very low pressures and low deposition rates. The process is extremely slow but very simplistic. 
     A forced flow/isothermal CVI process eliminates diffusion control by forcing precursor reactants to fill a woven fiber preform and decompose or react to form the desired product. This forced flow is accomplished by creating a pressure differential across the substrate with the precursor reactant gas supplied at a higher pressure on the supply side than on the vacuum exhaust side. The forced flow process ensures a high concentration of precursor reactant gas through the porosity of the substrate thereby increasing the reaction rate through the entire thickness of a woven fiber substrate. This increases infiltration rate by up to several orders of magnitude. The forced flow/isothermal process is typically used for thin cross-section materials where the reduction in precursor concentration through the thickness due to reaction is minimal and the reaction rate through the bulk of the sample constant or nearly constant. 
     A forced flow thermal gradient process is a CVI process that combines forced flow with an engineered thermal gradient through the thickness of the substrate being infiltrated. The thermal gradient is set up such that the reactant gas precursor feed side of the substrate is at a lower temperature than the exhaust side. In this arrangement, the thermal gradient offsets the reduction in deposition rate through the thickness of the substrate due to reactant depletion and results in a constant deposition rate through the thickness of the substrate. On the precursor feed side, the reactant concentration is high, but the temperature is low. On the vacuum exhaust side of the substrate, the reactant concentration is low due to depletion, but the temperature is high encouraging diffusion. By controlling the thermal gradient through the thickness of the substrate, it is possible to optimize a forced flow thermal gradient CVI process to achieve constant or nearly constant deposition rate through the entire thickness of the sample. This process is typically used for thick cross-section infiltrations that would be difficult or impossible to perform with isothermal/isobaric or forced flow isothermal CVI processes. 
     The deposition of an interface coating onto the fibers of a woven fabric substrate or a unidirectional tape requires a process that is fundamentally more of a CVI process than a CVD process. In order to apply an even thickness, thin multi-layered coating onto each individual filament within the tows of fiber that make up a fabric or unidirectional tape, a process that is analogous to the infiltration of pores within a porous substrate may be performed. The precursors must diffuse around all sides of all the filaments and react at approximately the same rate to achieve an even coating thickness distribution throughout the fabric. 
     In the present disclosure, methods and apparatus to continuously produce interface coating ceramic fabric and unidirectional tape by two processes are described. 
     In one embodiment of the invention, interface coated fabric may be continuously produced by forced flow isothermal chemical vapor infiltration (CVI). Forced flow isothermal continuous CVI reactor  10  of the invention is shown in  FIG. 1 . In reactor  10 , the starting material may be woven ceramic fiber tape, or fabric cloth or parallel ceramic fiber tape. Unfilled precursor ceramic tape  14  in continuous CVI reactor  10  is housed in metal storage chamber  12  on spool  16 . The width of tape  14  may be from 6 inches (15.24 cm) to 60 inches (152.4 cm) and the thickness may be from 0.006 inches (152 microns) to 0.25 inches (6350 microns). During processing, tape  14  in storage chamber  12  travels from left to right as indicated by arrow  15  under tension roller  18  into deposition chamber  28  through tape exit seal  26 . Tape storage chamber  12  is maintained under a controlled positive pressure atmosphere by purge gas entering chamber  12  through purge gas inlet  22  as indicated by arrow  24 . In the embodiment shown, deposition chamber  28  has two insulated deposition zones  28 A and  28 B. Insulated deposition zones  28 A and  28 B are contained in metal shell  30  lined with thermal insulation  34 . After tape  14  passes over tension roller  18 , it passes into metal deposition zone  28 A through storage chamber tape outlet seal  26 . 
     In the exemplary embodiment shown in  FIG. 1 , reactor  10  has two deposition zones. In other embodiments, reactor  10  may have any number of deposition zones, depending on the nature and requirements of the deposited interface layer or layers. Formation of multilayer interface coatings of the invention may be carried out with multiple deposition zones or with multiple passes through a single deposition zone. Each deposition zone may be configured to deposit the same chemistry in a given fabric pass or each zone may deposit a different coating chemistry in a given fabric pass. 
     Deposition zone  28 A is protected with purge gas  24 A entering deposition zone  28 A through purge gas inlet  22 A as indicated by arrow  24 A. Tape  14  is heated by resistance heating elements  32  positioned on both sides of tape  14  in deposition zone  28 A. Heating elements  32  may be resistance heated graphite rods or plates. 
     Reactant gas  28  enters reaction zone  28 A through gas inlet  36  as indicated by arrow  38 . In the embodiment shown, reactant gas  38  is directed at tape  14  in a direction perpendicular to the surface of tape  14  to ensure maximum penetration of reactant gas  38  into tape  14 . Reaction zone  28 A is evacuated through vacuum outlet  40  as indicated by arrow  42 . Outlet  40  is connected to a vacuum system, traps and vacuum controls (not shown.) The vacuum system removes unreacted reactant gas and reacted gaseous effluents from reaction zone  28 A during a run. 
     Heating elements  32  positioned on both sides of tape  14  in reaction zone  28 A provide an isothermal coating environment for tape  14  as it travels under reactant gas inlet  36  dispensing reactant gas  38  in a direction perpendicular to the surface of tape  14 . Vacuum  42  creates a pressure differential in reaction zone  38 A that pulls reactant gas  38  through tape  14  enhancing penetration of reactant gas  38  into tape  14 . 
     After tape  14  passes through reaction zone  28 A, it exits the reaction zone through tape outlet seal  44  and enters reaction zone  28 B. Reaction zone  28 B is identical to reaction zone  28 A except that the direction of reactant gas flow  38 A in gas inlet  36 A is directed at the bottom of tape  14  in vacuum outlet  40 A for the purpose of improving coating homogeneity through the thickness of tape  14 . Vacuum outlet  40 A is connected to vacuum system  42 A and evacuates the top side of tape  14 . In this zone the flow of reactant gas  38 A is directed at the bottom of tape  24  in a direction perpendicular to tape  14 . Resistance heaters  32 A maintain an isothermal temperature in reaction zone  28 B and the vacuum system indicated by arrow  42 A in outlet  40 A creates a pressure differential in reaction zone  28 B that draws reactant gas  38 A into the bottom side of tape  14  and evacuates unreacted reactant gas  38 A and reacted gas effluent from the top surface of tape  14  from reaction chamber  28 B. In this way, fibers at the bottom side of tape  14  that were not completely coated in reaction zone  28 A may be further coated to increase the homogeneity of the interface coating on the fibers of CMC precursor porous tape  14  of the invention. 
     Infiltrated, coated ceramic fiber tape  14  exits reaction zone  28  and enters metal collection chamber  48  through tape inlet seal  46 . Collection chamber  48  is maintained under a controlled atmosphere by purge gas  24 C entering purge gas inlet  22 C. Coated ceramic fiber tape  14  passes under tension roller  52  and is wound on collection spool  50  in the direction indicated by arrow  15 . By passing coated fiber tape  14  under tension roller  52 , the deformation imparted to the tape will tend to separate filtration fiber contact points (i.e. “bridges”) and increase the flexibility of coated fiber tape  14 . Collection spool  50  is driven by a variable speed drive (not shown) to gather infiltrated ceramic fiber tape  14  for further processing. 
     In another embodiment of the invention, CMC precursor interface coated fabric or unidirectional tape is continuously produced by forced flow thermal gradient chemical vapor infiltration (CVI). Forced flow thermal gradient continuous CVI reactor  100  is shown in  FIG. 2 . The starting material for the fiber coating process may be parallel ceramic fiber tape, fabric or cloth woven ceramic fiber tape. Ceramic fiber tape  114  in continuous CVI reactor  100  is housed in metal storage chamber  112  on spool  116 . The width of tape  114  may be from 6 inches (15.24 cm) to 60 inches (125.4 cm) and the thickness may be from 0.006 inches (152 microns) to 0.25 inches (6350 microns). During processing, ceramic fiber tape  114  in storage chamber  112  travels from left to right as indicated by arrow  115  under tension roller  118  into deposition chamber  128  through tape exit seal  118 . Tape storage chamber  112  is maintained under a controlled positive pressure atmosphere by purge gas entering chamber  112  through purge gas inlet  122  as indicated by arrow  124 . The embodiment showing metal deposition chamber  128  has two insulated deposition zones,  128 A and  128 B. Insulated deposition zones  128 A and  128 B are contained in metal shell  130  lined with thermal insulation  134 . After tape  114  passes under tension roller  118 , it passes into metal deposition zone  128 A through storage chamber tape outlet seal  126 . 
     Deposition zone  128 A is protected with purge gas  124 A entering deposition zone  128 A through purge gas inlet  122 A as indicated by arrow  124 A. In the embodiment shown, the bottom of tape  114  is heated by resistance heating elements  132  positioned under tape  114  to form a temperature gradient across tape  114 . 
     Reactant gas enters reaction zone  128 A through gas inlet  136  as indicated by arrow  138 . In the embodiment of the invention, reactant gas  138  is directed at tape  114  in a direction perpendicular to the surface of tape  114  to ensure penetration of reactant gas  138  into tape  114 . In the absence of heating elements above tape  14 , the top side of tape  14  is at a lower temperature than the bottom side and the rate of formation of reaction product at the top side of tape  114  is low. However, since the bottom of the tape is at a higher temperature than the top side, the reaction product in that region is formed at a higher rate whereby the deposition rate of coating infiltrant in the lower half of the tape balances that forming in the reactant gas feed side and a constant deposition rate may be achieved throughout the thickness of the tape. Reaction zone  128 A is evacuated through vacuum outlet  140  as indicated by arrow  142 . Outlet  140  is connected to a vacuum system, traps and vacuum controls not shown. The vacuum system provides a pressure differential that assists in drawing reactant gas  138  through the thickness of tape  114  during the CVI process. The vacuum system removes unreacted reactant gas and reacted gaseous effluents from reaction zone  128 A during a run. 
     In the exemplary embodiment shown in  FIG. 2 , reactor  110  has two deposition zones. In other embodiments, reactor  110  may have any number of deposition zones, depending on the nature and requirements of the deposited interface layer or layers. Formation of multilayer interface coatings of the invention may be carried out with multiple deposition zones or with multiple passes through a single deposition zone. Each deposition zone may be configured to deposit the same chemistry in a given fabric pass or each zone may deposit a different chemistry in a given fabric pass 
     After tape  114  passes through reaction zone  128 A, it exits the reaction zone through tape outlet seal  144  and enters reaction zone  128 B. Reaction zone  128 B is identical to reaction zone  128 A except that the direction of reactant gas flow  138 A from gas flow inlet  136 A is opposite to that in reaction zone  128 A for the purpose of improving coating homogeneity through the thickness of tape  114 . In addition, heating elements  132 A are positioned to heat only the top side of tape  124  opposite the reactant gas inlet side and to create a temperature gradient in reaction zone  128 B. Vacuum system  142 A is also on the heated side in reaction zone  128 B and creates a pressure differential that draws reactant gas  138 A into the bottom of tape  114  and evacuates unreacted reactant gas  138 A and reacted gas effluent from the top side of tape  114  in reaction chamber  128 B. Since the reactant gas inlet side of tape  114  is not heated, the infiltration rate of reactant gas  114  is low in this region. However, the top side of tape  114  is at a higher temperature and the reaction product in that region forms at a higher rate. As a result, the deposition rate of coating infiltrant in the upper half of the tape balances that forming in the reactant gas feed side and a constant coating deposition rate may be achieved through the thickness of the tape coating any uncoated fiber left after deposition in reaction zone  128 A. 
     Infiltrated coated ceramic fiber tape  114  exits reaction chamber  128 A and enters collection chamber  148  through tape inlet seal  146 . Collection chamber  148  is maintained under a positive controlled atmosphere by purge gas  124 C entering purge gas inlet  122 C. Coated ceramic fiber tape  114  passes under tension roller  152  and is wound on collection spool  150  in direction indicated by arrow  115 . By passing coated fiber tape  114  under tension roller  152 , the deformation imparted to the tape will tend to separate fiber-to-fiber contact points (i.e. “bridges”) and increase the flexibility of coated fiber tape  114 . Collection spool  150  is driven by a variable speed drive (not shown) to gather completed CMC precursor tape  114  for further processing. Collection chamber  148  is maintained under a positive controlled atmosphere by purge gas  124 C entering purge gas inlet  122 C. 
     Fiber interface coating systems of the present disclosure are focused at high temperature ceramic matrix composite (CMC) precursor materials for applications in an oxidizing environment such as a gas turbine or other form of a combustion engine. Primary fibers may include carbon, silicon carbide, boron carbide, aluminum oxide and others known in the art. 
     Fiber interface coatings are typically duplex coatings consisting of an initial layer deposited directly on the fiber to tailor the adhesive strength of the fiber/matrix interface. Secondary ceramic layers in duplex interface coatings may be oxidation resistant or hydrolysis resistant coatings. Non-limiting examples of fiber interface coating systems include carbon, doped carbon, silicon carbide, silicon nitride, boron nitride, silicon doped boron nitride, boron carbide and others known in the art. Multi-layer interfaced coating systems of the disclosure may include alternating layers of duplex coating systems. Non-limiting examples include (carbon/silicon carbide)×n, (boron nitride/silicon nitride)×n, (boron nitride/silicon carbide)×n, and others known in the art. In these examples n may be up to 5 iterations. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A method to form a ceramic interface coating on ceramic matric composite (CMC) precursor tape by a continuous process may include: passing a ceramic fiber woven cloth tape or unidirectional tape of a first ceramic with a first and second surface through at least one zone of a continuous vacuum chemical vapor deposition (CVD) or chemical vapor infiltration (CVI) reactor heated to a reaction temperature; and directing a flow of CVD or CVI reactant gas of a second ceramic at the first surface of the tape in a direction perpendicular to the tape such that the reactant gas passes through the tape in a forced flow process depositing the second ceramic on the fibers of the first ceramic thereby coating the fibers of the first ceramic tape with the second ceramic interface coating to form a coated fiber CMC precursor tape product. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively any, one or more of the following features, configurations and/or additional components: 
     A temperature of the first surface of the tape may be equal to a temperature of the second surface of the tape. 
     The temperature of the first surface of the tape may be lower than the temperature of the second surface of the tape. 
     The reactant gas pressure is higher at the first surface of the tape than at the second surface of the tape. 
     The method may comprise at least two reaction zones. 
     The reaction temperature may be from about 575° F. (302° C.) to about 2730° F. (1499° C.). 
     The width of the tape may be from about 6 inches (15.24 cm) to about 60 inches 125.4 cm). 
     The thickness of the cloth tape or unidirectional tape may be from about 0.006 inches (152 microns) to about 0.25 inches (6352 microns). 
     The first ceramic may be carbon, silicon carbide, boron carbide or aluminum oxide. 
     The second ceramic may be carbon, doped carbon, silicon carbide, silicon nitride, boron nitride, silicon doped boron nitride or boron carbide. 
     An apparatus to continuously form ceramic interface coatings on ceramic matrix composite (CMC) precursor tape may include: a ceramic fiber woven cloth tape or unidirectional tape of a first ceramic with a first and second surface mounted on a supply spool in a storage chamber; a tension roller and a purge gas inlet in the storage chamber; a storage chamber tape outlet seal; at least one chemical vapor deposition (CVD) or chemical vapor infiltration (CVI) deposition zone comprising; a vacuum chamber; a tape inlet seal; a reactive gas dispensing system directed at the first side of the tape in a direction perpendicular to the tape for forming a CVD or CVI coating of a second ceramic on the fibers to form an interface coating; a purge gas inlet; at least one heating element for heating the tape to a CVD or CVI reaction temperature; a vacuum system; high temperature insulation; and a tape outlet seal; a tape collection spool mounted on a drive shaft in a collection chamber; a tension roller, a purge gas inlet and tape inlet seal in the collection chamber; and a variable speed drive attached to the collection spool to gather the coated CMC precursor tape product on the collection spool. 
     The apparatus of the preceding paragraph can optionally include, additionally and/or alternatively any, one or more of the following features, configurations and/or additional components: 
     The heating elements provide a temperature of the second side of the tape equal to a temperature of the first side of the tape. 
     The temperature of the second side of the tape may be lower than the temperature of the first side. 
     The reactive gas pressure at the first side of the tape may be higher than at the second side of the tape. 
     The heating elements may be resistance heated graphite rods or plates positioned on one or both sides of the tape. 
     The reaction temperature may be from about 575° F. (302° C.) to about 2730° F. (1499° C.). 
     The apparatus may comprise at least two deposition zones. 
     The first ceramic may be carbon, silicon carbide, boron carbide, or aluminum oxide. 
     The second ceramic may be carbon, doped carbon, silicon carbide, silicon nitride, boron nitride, silicon doped boron nitride, or boron carbide. 
     The width of the tape may be from about 6 inches (15.24 cm) to about 60 inches (125.4 cm). 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.