Abstract:
The invention relates to the field of high temperature composites made by the chemical vapor infiltration and deposition of a binding matrix within a porous structure. More particularly, the invention relates to pressure gradient processes for forcing infiltration of a reactant gas into a porous structure, apparatus for carrying out those processes, and the resulting products. The invention is particularly suited for the simultaneous CVI/CVD processing of large quantities (hundreds) of aircraft brake disks.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to the field of high temperature composites made by the chemical vapor infiltration and deposition of a binding matrix within a porous structure. More particularly, the invention relates to pressure gradient processes for forcing infiltration of a reactant gas into a porous structure, apparatus for carrying out those processes, and the resulting products.  
           [0002]    Chemical vapor infiltration and deposition (CVI/CVD) is a well known process for depositing a binding matrix within a porous structure. The term “chemical vapor deposition” (CVD) generally implies deposition of a surface coating, but the term is also used to refer to infiltration and deposition of a matrix within a porous structure. As used herein, the term CVI/CVD is intended to refer to infiltration and deposition of a matrix within a porous structure. The technique is particularly suitable for fabricating high a temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure resulting in very useful structures such as carbon/carbon aircraft brake disks, and ceramic combustor or turbine components. The generally known CVI/CVD processes may be classified into four general categories: isothermal, thermal gradient, pressure gradient, and pulsed flow. See W. V. Kotlensky,  Deposition of Pyrolytic Carbon in Porous Solids,  8 Chemistry and Physics of Carbon, 173, 190-203 (1973); W. J. Lackey,  Review, Status, and Future of the Chemical Vapor Infiltration Process for Fabrication of Fiber - Reinforced Ceramic Composites,  Ceram. Eng. Sci. Proc. 10[7-8] 577, 577-81 (1989) (W. J. Lackey refers to the pressure gradient process as “isothermal forced flow”). In an isothermal CVI/CVD process, a reactant gas passes around a heated porous structure at absolute pressures as low as a few millitorr. The gas diffuses into the porous structure driven by concentration gradients and cracks to deposit a binding matrix. This process is also known as “conventional” CVI/CVD. The porous structure is heated to a more or less uniform temperature, hence the term “isothermal,” but this is actually a misnomer. Some variations in temperature within the porous structure are inevitable due to uneven heating (essentially unavoidable in most furnaces), cooling of some portions due to reactant gas flow, and heating or cooling of other portions due to heat of reaction effects. In essence, “isothermal” means that there is no attempt to induce a thermal gradient that preferentially affects deposition of a binding matrix. This process is well suited for simultaneously densifying large quantities of porous articles and is particularly suited for making carbon/carbon brake disks. With appropriate processing conditions, a matrix with desirable physical properties can be deposited. However, conventional CVI/CVD may require weeks of continual processing in order to achieve a useful density, and the surface tends to densify first resulting in “seal-coating” that prevents further infiltration of reactant gas into inner regions of the porous structure. Thus, this technique generally requires several surface machining operations that interrupt the densification process.  
           [0003]    In a thermal gradient CVI/CVD process, a porous structure is heated in a manner that generates steep thermal gradients that induce deposition in a desired portion of the porous structure. The thermal gradients may be induced by heating only one surface of a porous structure, for example by placing a porous structure surface against a susceptor wall, and may be enhanced by cooling an opposing surface, for example by placing the opposing surface of the porous structure against a liquid cooled wall. Deposition of the binding matrix progresses from the hot surface to the cold surface. The fixturing for a thermal gradient process tends to be complex, expensive, and difficult to implement for densifying relatively large quantities of porous structures.  
           [0004]    In a pressure gradient CVI/CVD process, the reactant gas is forced to flow through the porous structure by inducing a pressure gradient from one surface of the porous structure to an opposing surface of the porous structure. Flow rate of the reactant gas is greatly increased relative to the isothermal and thermal gradient processes which results in increased deposition rate of the binding matrix. This process is also known as “forced-flow” CVI/CVD. Prior fixturing for pressure gradient CVI/CVD tends to be complex, expensive, and difficult to implement for densifying large quantities of porous structures. An example of a process that generates a longitudinal pressure gradient along the lengths of a bundle of unidirectional fibers is provided in S. Kamura, N. Takase, S. Kasuya, and E. Yasuda,  Fracture Behaviour of C Fiber/CVD C Composite,  Carbon &#39;80 (German Ceramic Society) (1980). An example of a process that develops a pure radial pressure gradient for densifying an annular porous wall is provided in U.S. Pat. Nos. 4,212,906 and 4,134,360. The annular porous wall disclosed by these patents may be formed from a multitude of stacked annular disks (for making brake disks) or as a unitary tubular structure. For thick-walled structural composites, a pure radial pressure gradient process generates a very large, undesirable density gradient from the inside cylindrical surface to the outside cylindrical surface of the annular porous wall. Also, the surface subjected to the high pressure tends to densify very rapidly causing that surface to seal and prevent infiltration of the reactant gas to low density regions. This behavior seriously limits the utility of the pure radial pressure gradient process.  
           [0005]    Finally, pulsed flow involves rapidly and cyclically filling and evacuating a chamber containing the heated porous structure with the reactant gas. The cyclical action forces the reactant gas to infiltrate the porous structure and also forces removal-of the cracked reactant gas by-products from the porous structure. The equipment to implement such a process is complex, expensive, and difficult to maintain. This process is very difficult to implement for densifying large numbers of porous structures.  
           [0006]    Many workers in the art have combined the thermal gradient and pressure gradient processes resulting in a “thermal gradient-forced flow” process. Combining the processes appears to overcome the shortcomings of each of the individual processes and results in very rapid densification of porous structures. However, combining the processes also results in twice the complexity since fixturing and equipment must be provided to induce both thermal and pressure gradients with some degree of control. A process for densifying small disks and tubes according to a thermal gradient-forced flow process is disclosed by U.S. Pat. No. 4,580,524; and by A. J. Caputo and W. J. Lackey,  Fabrication of Fiber - Reinforced Ceramic Composites by Chemical Vapor Infiltration,  Prepared by the OAK RIDGE NATIONAL LABORATORY for the U.S. DEPARTMENT OF ENERGY under Contract No. DE-AD05-840R21400 (1984). According to this process, a fibrous preform is disposed within a water cooled jacket. The top of the preform is heated and a gas is forced to flow through the preform to the heated portion where it cracks and deposits a matrix. A process for depositing a matrix within a tubular porous structure is disclosed by U.S. Pat. No. 4,895,108. According to this process, the outer cylindrical surface of the tubular porous structure is heated and the inner cylindrical surface is cooled by a water jacket. The reactant gas is introduced to the inner cylindrical surface. Similar forced flow-thermal gradient processes for forming various articles are disclosed by T. Hunh, C. V. Burkland, and B. Bustamante,  Densification of a Thick Disk Preform with Silicon Carbide Matrix by a CVI Process,  Ceram. Eng. Sci. Proc 12[9-10] pp. 2005-2014 (1991); T. M. Besmann, R. A. Lowden, D. P. Stinton, and T. L. Starr,  A Method for Rapid Chemical Vapor Infiltration of Ceramic Composites,  Journal De Physique, Colloque C5, supplement au n&#39;5, Tome 50 (1989); T. D. Gulden, J. L. Kaae, and K. P. Norton,  Forced - Flow Thermal - Gradient Chemical Vapor Infiltration  ( CVI )  of Ceramic Matrix Composites,  Proc.-Electrochemical Society (1990), 90-12 (Proc. Int. Conf. Chem. Vap. Deposition, 11th, 1990) 546-52. Each of these disclosures describes processes for densifying only one porous article at a time, which is impractical for simultaneously processing large numbers of composite articles such as carbon/carbon brake disks.  
           [0007]    In spite of these advances, a CVI/CVD process and an apparatus for implementing that process are desired that rapidly and uniformly densifies porous structures while minimizing cost and complexity. Such a process would preferably be capable of simultaneously densifying large numbers (as many as hundreds) of individual porous structures. In particular, a process is desired for rapidly and economically densifying large numbers of annular fibrous preform structures for aircraft brake disks having desirable physical properties.  
         SUMMARY OF THE INVENTION  
         [0008]    According to an aspect of the invention, a CVI/CVD process is provided, comprising the steps of:  
           [0009]    partially densifying a porous structure within a CVI/CVD furnace by depositing a first matrix within the porous structure with a pressure gradient CVI/CVD process in which a first portion of the porous structure is subjected to a greater pressure than a second portion of the porous structure and the first portion has a greater bulk density gain than the second portion; and,  
           [0010]    subsequently densifying the porous structure by depositing a second matrix within the porous structure with at least one additional densification process in which the second portion has a greater bulk density gain than the first portion.  
           [0011]    According to another aspect of the invention, a CVI/CVD process is provided, comprising the steps of:  
           [0012]    partially densifying a multitude of annular fibrous carbon structures within a CVI/CVD furnace by depositing a first carbon matrix within the annular fibrous carbon structure with a pressure gradient CVI/CVD process in which a first portion of each annular fibrous carbon structure is subjected to a greater pressure than a second portion of each annular fibrous carbon structure and the first portion has a greater bulk density gain than the second portion; and,  
           [0013]    subsequently densifying the multitude of annular fibrous carbon structures by depositing a second carbonaceous matrix within each annular fibrous carbon structure with at least one additional densification process in which the second portion has a greater bulk density gain than the first portion.  
           [0014]    According to yet another aspect of the invention, a friction disk is provided, comprising:  
           [0015]    a densified annular porous structure having a first carbon matrix deposited within the annular porous structure and a second carbon matrix deposited within the annular porous structure overlying the first carbon matrix, the densified annular porous structure having two generally parallel planar surfaces bounded by an inside circumferential surface and an outside circumferential surface spaced from and encircling the inside circumferential surface, a first circumferential portion adjacent the inside circumferential surface, and a second circumferential portion adjacent the outside circumferential surface, wherein the first and second circumferential portions are bounded by the two generally parallel planar surfaces, the second circumferential portion having at least 10% less of the first carbon matrix per unit volume relative to the first circumferential portion, the first carbon matrix and the second carbon matrix having a substantially rough laminar microstructure, and the first carbon matrix being more graphitized than the second carbon matrix.  
           [0016]    According to still another aspect of the invention, a CVI/CVD process in a CVI/CVD furnace is provided, comprising the steps of:  
           [0017]    introducing a reactant gas into a sealed preheater disposed within the CVI/CVD furnace, the sealed preheater having a preheater inlet and a preheater outlet, the reactant gas being introduced into the preheater inlet and exiting the sealed preheater through the preheater outlet and infiltrating at least one porous structure disposed within the CVI/CVD furnace;  
           [0018]    heating the at least one porous structure;  
           [0019]    heating the sealed preheater to a preheater temperature greater than the reactant gas temperature;  
           [0020]    sensing a gas temperature of the reactant gas proximate the outlet;  
           [0021]    adjusting the preheater temperature to achieve a desired gas temperature; and,  
           [0022]    exhausting the reactant gas from the CVI/CVD furnace.  
           [0023]    According to still another aspect of the invention, an apparatus is provided for introducing a first reactant gas into a CVI/CVD furnace, comprising:  
           [0024]    a first main gas line for supplying the first reactant gas;  
           [0025]    a plurality of furnace supply lines in fluid communication with the first main gas line and the CVI/CVD furnace;  
           [0026]    a plurality of first flow meters that measure a quantity of first reactant gas flow through each furnace supply line; and,  
           [0027]    a plurality of first control valves configured to control the quantity of flow of the first reactant gas through each furnace supply line.  
           [0028]    According to still another aspect of the invention, a CVI/CVD densification process is provided, comprising the steps of:  
           [0029]    densifying a first porous wall within a CVI/CVD furnace by a pressure gradient CVI/CVD process wherein a first flow of reactant gas is forced to disperse through the first porous wall;  
           [0030]    densifying a second porous wall by a pressure gradient CVI/CVD process wherein a second flow of reactant gas is forced to disperse through the second porous wall; and,  
           [0031]    independently controlling the first flow of the reactant gas and the second flow of the reactant gas.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 presents a schematic sectional view of a CVI/CVD furnace according to an aspect of the invention.  
         [0033]    [0033]FIG. 2 presents a sectional view of a fixture according for a pressure gradient CVI/CVD process according to an aspect of the invention.  
         [0034]    [0034]FIG. 3 presents a sectional view of a fixture according to an aspect of the invention.  
         [0035]    [0035]FIG. 4 presents a sectional view of a fixture according to an aspect of the invention.  
         [0036]    [0036]FIG. 5 presents a sectional view of a fixture according to an aspect of the invention.  
         [0037]    [0037]FIG. 6 presents a sectional view of a fixture according to an aspect of the invention.  
         [0038]    [0038]FIG. 7 presents a sectional view of a fixture according to an aspect of the invention.  
         [0039]    [0039]FIG. 8 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0040]    [0040]FIG. 9 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0041]    [0041]FIG. 10 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0042]    [0042]FIG. 11 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0043]    [0043]FIG. 12 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0044]    [0044]FIG. 13 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0045]    [0045]FIG. 14 presents a sectional schematic view of a furnace for a conventional CVI/CVD process.  
         [0046]    [0046]FIG. 15 presents a sectional schematic view of a furnace for simultaneously densifying a large number of porous structures by a pressure gradient CVI/CVD process according to an aspect of the invention.  
         [0047]    [0047]FIG. 16 presents a perspective view of a preheater according to an aspect of the invention.  
         [0048]    [0048]FIG. 17 presents a fixture with porous structures according to an aspect of the invention.  
         [0049]    [0049]FIG. 18 presents a fixture with porous structures according to an aspect of the invention.  
         [0050]    [0050]FIG. 19 presents a process according to an aspect of the invention.  
         [0051]    [0051]FIG. 20 presents a process according to an aspect of the invention.  
         [0052]    [0052]FIG. 21 presents a process according to an aspect of the invention.  
         [0053]    [0053]FIG. 22 presents an alternate cover plate for use with the preheater of FIG. 16.  
         [0054]    [0054]FIG. 23 presents a sectional view of a densified structure according to an aspect of the invention.  
         [0055]    [0055]FIG. 24 presents a graph showing bulk density gain versus time for a variety of processes according to the invention.  
         [0056]    [0056]FIG. 25 presents a graph showing average deposition rate versus normalized reactant gas flow for a variety of processes according to the invention.  
         [0057]    [0057]FIG. 26 presents a graph showing average deposition rate versus normalized reactant gas flow for a variety of reactor volume pressures according to an aspect of the invention.  
         [0058]    [0058]FIG. 27 presents a graph showing change in pressure across the porous wall versus average bulk density for a variety of reactant gas flow rates and reactor volume pressures according to an aspect of the invention.  
         [0059]    [0059]FIG. 28 presents a fixture for holding porous structures having alternating “OD” and “ID” ring-like spacers.  
         [0060]    [0060]FIG. 29 presents a fixture for holding porous structures having all “ID” ring-like spacers.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0061]    The invention and various embodiments thereof are presented in FIGS. 1 through 29 and the accompanying descriptions wherein like numbered items are identical. As used herein, the term “conventional CVI/CVD” is intended to refer to the previously described isothermal CVI/CVD process. The term “pressure gradient CVI/CVD” is intended to refer to the previously described pressure gradient CVI/CVD or forced-flow process and is intended to specifically exclude the previously described thermal gradient and thermal gradient-forced flow processes to the extent that these processes utilize an intentionally induced thermal gradient that affects the deposition process.  
         [0062]    Referring now to FIG. 1, a schematic depiction is presented of a CVI/CVD furnace  10  adapted to deposit a matrix within a porous structure  22  by a pressure gradient CVI/CVD process according to an aspect of the invention. The furnace  10  has a shell  13  with an inner surface  12  that defines a furnace volume  14 , and a gas inlet  16  for introducing a gas into the furnace  10 . A susceptor  30  is disposed around the reactor volume  35  and is induction heated by an induction coil  20  according to methods well known in the art. Other methods of heating may also be utilized such as resistance heating and microwave heating, any of which are considered to fall within the purview of the invention. An insulation barrier  31  is disposed between the susceptor  30  and the induction coil  20 . The susceptor  30  has an inner surface  33  that defines a reactor volume  35  which is included within the furnace volume  14 . The porous structure  22  is disposed within a fixture  2  in the reactor volume  35  and is predominantly heated by radiation from the susceptor  30 . A vacuum apparatus  58  comprising a vacuum pump or steam vacuum system is in fluid communication with an exhaust  32  and evacuates the furnace volume  14  to a pressure below atmospheric pressure. A reactant gas is introduced into the reactor volume  35  through the gas inlet  16  that receives the reactant gas from a furnace supply line  26 . The reactant gas infiltrates the porous structure  22  where it cracks and deposits a matrix within the porous structure  22 . A single type of gas or mixtures of multiple types of gases may be supplied to the gas inlet  16 .  
         [0063]    According to a preferred embodiment, the reactant gas comprises a mixture of two reactant gases that are introduced through a first main gas line  42  and a second main gas line  44 . The furnace supply line  26  is in fluid communication with the first and second main gas lines  42  and  44  and the inlet  16  thereby serving to transfer the reactant gases to the furnace  10 . A first flow meter  46  measures the quantity of flow of a first gas (indicated by arrow  50 ) introduced into the furnace supply line  26  through the first main supply line  42 , and a second flow meter  48  measures the quantity of flow of a second gas (indicated by arrow  52 ) introduced into the furnace supply line  26  through the second main gas line  44 . The flow of gas into furnace supply line  26  is controlled by a first control valve  54  which controls the flow of the first reactant gas from the first main gas line  42 , and by a second control valve  56  which controls the flow of the second reactant gas from the second main gas line  44 .  
         [0064]    The porous structure  22  includes a porous structure aperture  23 . A tube  60  is in fluid communication with fixture  2  and the inlet  16  thereby serving to transfer the reactant gas to the fixture  2 . The fixture  2  comprises a pair of plates  38  and  40 , and the tube  60  is sealed to the gas inlet  16  and to the plate  38 . The porous structure  22  is sealed between the plates by ring-like spacers  62  and  64 , and the plates  38  and  40  are held together by tie-rods  66 . The porous structure  22  forms a porous wall  68  disposed between the inlet  16  and the exhaust  32 . The furnace volume  14  and reactor volume  35  are reduced to a pressure below atmospheric pressure, and the gas is introduced to the porous structure aperture  23  at a greater pressure than the reactor volume pressure which develops a pressure gradient through the porous wall  68  and forces dispersion of the gas through porous structure  22  before being withdrawn from the reactor volume  35  and the furnace volume  14  by the vacuum apparatus  58  as indicated by arrows  34 ,  36 , and  28 .  
         [0065]    Pressure inside the furnace volume is measured by an exhaust pressure sensor  72 , and pressure inside the porous structure aperture  23  is measured by an inlet pressure sensor  70 . An approximate reactant gas temperature inside the porous structure aperture  23  is measured by a flow temperature sensor  74 , and porous structure temperature is approximated by a structure temperature sensor  76  which is placed in close proximity to the plate  40 . As will be discussed in more detail, the temperature and pressure conditions are chosen to cause the gas to crack and deposit a matrix having certain desired properties within the porous structure  22 . The various aspects of the invention may be used to deposit any type of CVI/CVD deposited matrix including, but not limited to, carbon or ceramic matrix deposited within carbon or ceramic based porous structures  22 . The invention is particularly useful for depositing a carbon matrix within a carbon-based porous structure, and especially for making carbon/carbon composite structures such as aircraft brake disks.  
         [0066]    Referring now to FIG. 2, a detailed view of the fixture  2  for holding porous structure  22  is presented. According to a preferred embodiment, the porous structure is annular and has two opposing generally planar surfaces  78  and  80  that are bounded by an inside circumferential surface  82  and an outside circumferential surface  84 . An “OD” (outside diameter) type ring-like spacer  64  having a mean diameter less than the outside circumferential surface  84  is placed between the porous structure  22  and the plate  38 . An “ID” (inside diameter) type ring-like spacer  62  having a mean diameter slightly greater than the inside circumferential surface  82  is placed between the porous structure  22  and the plate  40 . The ring-like spacers  62  and  64  also serve as spacers to permit gas flow between porous structure  22  and the plates  38  and  40 , and also seal the porous structure  22  to the plates  38  and  40 . The tie-rods  66  may be threaded on one or both ends and include nuts  67  in threaded engagement. Washers  69  may be used to distribute the load to the plates  38  and  40 .  
         [0067]    As discussed previously, the furnace volume is subjected to a vacuum and reactant gas is introduced into the tube  60  at a greater pressure than the furnace volume. Thus, a first portion  86  (indicated by fine crosshatching) of the fibrous structure  22  is subjected to a greater pressure than a second portion  88  (indicated by fine crosshatching) of the fibrous structure  22  which induces dispersion of the reactant gas through the porous structure  22  as indicated by the arrows  90 . As gas disperses through the porous structure, additional gas flows through the tube  60  and toward the porous structure  22  as indicated by arrows  92 . Thus, reactant gas is continuously supplied and forced to disperse through the porous structure  22 . In this example, the first portion  86  includes one surface  78  of the two opposing surfaces  78  and  80 , and the second portion  88  includes the other surface  80  of the two opposing surfaces  78  and  80 . The first portion  86  also includes the inside circumferential surface  82 , and the second portion  88  includes the outside circumferential surface  84 .  
         [0068]    Referring now to FIG. 3, an alternative fixture  4  that may be used in place of fixture  2  is depicted wherein two porous structures  22  are stacked and simultaneously densified. Two ring-like spacers  64  are utilized and tie-rods  65  are longer versions of the tie-rods  66  of FIG. 2. A pressure gradient is applied to the porous structure (as previously described in relation to FIG. 2) resulting in dispersion of the reactant gas through the porous structures  22  as indicated by arrows  90 . Other features of fixture  4  are identical to fixture  2 .  
         [0069]    The reactant gas tends to crack and preferentially deposit the matrix within the portions of the porous structure  22  subjected to a pressure relatively greater than the pressure in other portions. For example, FIG. 8 presents a densified structure  300  that results from the FIGS. 2 and 3 processes beginning with a porous structure  22 . The degree of crosshatching is intended to generally indicate relative density: finely crosshatched areas represent greater density relative to coarsely crosshatched areas. The density monotonically decreases from a greatest density zone  302  to a least density zone  308  with density zones  304  and  306  representing intermediate density ranges. The densified structure  300  has an average bulk density, and density zone  302  is typically 110%-140% of the average bulk density, and density zone  308  is typically 60%-90% of the average bulk density. Note that the highest density zone  302  generally corresponds to the first portion  86  and the lowest density zone  308  generally corresponds to the second portion  88 . Thus, the first portion  86  has a greater bulk density gain than the second portion  88  during the pressure gradient CVI/CVD process depicted in FIGS. 2 and 3.  
         [0070]    The density gradient depicted in FIG. 8 is unacceptable for many applications. The density gradient may be reduced by depositing a first matrix within the porous structure with a pressure gradient CVI/CVD process, as shown in FIGS. 2 and 3. In this first process, the first portion  86  has a greater bulk density gain than the second portion  88 , as shown in FIG. 8. Subsequently, the porous structure  22  may be further densified by depositing a second matrix with at least one additional densification process in which the second portion  88  has a greater bulk density gain than the first portion  86 . For example, the partially densified structure  300  of FIG. 8 could be flipped and subjected to the pressure gradient CVI/CVD process depicted in FIGS. 2 and 3. The second portion  88  is subjected to a greater pressure than the first portion  86 , which results in the second portion  88  having a greater bulk density gain than the first portion  86 . FIG. 9 depicts a densified structure  310  resulting from this two-step/flip process. The density monotonically decreases from a greatest density zone  312  to a least density zone  316  with density zone  314  representing an intermediate density range. The densified structure  310  has an average bulk density, and density zone  312  is typically 105%-115% of the average bulk density, and density zone  316  is typically 85%-95% of the average bulk density. The density gradient is now generally symmetrical through the thickness of the porous structure  22  which is desirable for brake disk applications. The density gradient is also less than the density gradient of the densified structure  300  depicted in FIG. 8. The second or additional densification processes may include pressure gradient CVI/CVD, conventional CVI/CVD, and resin impregnation followed by charring. In addition, a porous structure partially densified with a carbon matrix may be heat treated at a temperature greater than the processing temperatures of previous CVI/CVD processes to increase graphitization of the carbon matrix before further depositing additional matrix.  
         [0071]    Referring now to FIG. 4, another alternative fixture  6  that may be used in place of fixture  2  for an alternative pressure gradient CVI/CVD process is presented. The fixture  6  utilizes all “ID” ring-like spacers  62  resulting in only the inner circumferential surface  82  of each porous structure being subjected to a greater pressure than the reactor volume  35 . Thus, a first portion  87  of porous structure  22  is subjected to a greater pressure than a second portion  89  resulting in pressure driven flow of the reactant gas through the porous structures  22  as indicated by arrows  91 . In this example, the first portion  87  includes the inside circumferential surface  82 , and the second portion  89  includes the outside circumferential surface  84  and two opposing surfaces  78  and  80 . The reactant gas tends to quickly flow through the porous structure  22  and exit near the ring-like spacer  62 . Thus, reactant gas is not forced to disperse through all of the porous structure  22 . FIG. 10 presents a densified structure  320  generated by the FIG. 4 process. The densified structure  320  comprises a zone  322  of greatest density adjacent the inside circumferential surface  82  which drops off to a zone  328  of least density at the core. The density monotonically increases from the least density zone  328  to the greatest density zone  322  with density zones  324  and  326  representing intermediate density ranges. The densified structure  320  has an average bulk density, and density zone  322  is typically about 140% of the average bulk density, and density zone  324  is typically about 115% of the average bulk density. Density zone  328  is typically about 80% of the average bulk density. The zone  322  of greatest density generally corresponds with the first portion  87  of FIG. 4. The region of intermediate density  324  adjacent the outside circumferential surface  320  is generated by a conventional CVI/CVD process induced by reactant gas flow that has not fully cracked exhausting from adjacent porous structures. The densified structure  320  may be further densified by second or additional densification processes which include pressure gradient CVI/CVD, conventional CVI/CVD, and resin impregnation followed by charring.  
         [0072]    Referring now to FIG. 5, an alternative fixture  8  that may be used in place of fixture  2  for an alternative pressure gradient CVI/CVD process is presented. The fixture  8  utilizes all “OD” ring-like spacers  64  resulting in the inside circumferential surface  82  and the opposing surfaces  78  and  80  of each porous structure being subjected to a greater pressure than reactor volume  35 . The outside circumferential surface  84  is subjected to the pressure of the reactor volume  35 . Thus, a first portion  94  of porous structure  22  is subjected to a greater pressure than a second portion  96  resulting in pressure driven flow of the reactant gas through the porous structures  22  as indicated by arrows  98 . In this example, the first portion  94  includes the inside circumferential surface  82  and the opposing surfaces  78  and  80 , and the second portion  96  includes the outside circumferential surface  84 . As depicted, the reactant gas is forced to disperse through all of the porous structure  22 . FIG. 11 presents a densified structure  330  generated by the FIG. 5 process. The densified structure  330  comprises a zone  332  of greatest density adjacent the inside circumferential surface  82  and part of the two opposing surfaces  78  and  80 . The zone  332  sometimes extends all the way to the outside circumferential surface  84  and includes essentially all of the opposing surfaces  78  and  80 . The density monotonically decreases from the greatest density zone  332  to a least density zone  338  with density zones  334  and  336  representing intermediate density ranges. The densified structure  330  has an average bulk density, and density zone  332  is typically 110%-125% of the average bulk density, and density zone  338  is typically 80%-90% of the average bulk density. The FIG. 5 process generates a densified structure  330  that has a symmetric density gradient through the structure thickness. However, the density gradient may be skewed toward one of the surfaces  78  or  80  in some densified structures  330  due to process variations. Note that the zones  332  and  334  generally correspond to the first portion  94  of FIG. 5, and the second portion  96  experiences a relatively less bulk density gain as indicated by zones  336  and  338 . The densified structure  330  may be further densified by second or additional densification processes which may comprise pressure gradient CVI/CVD, conventional CVI/CVD, or resin impregnation followed by charring.  
         [0073]    Referring now to FIG. 12, a densified structure  340  is presented that is generated by further densifying the porous structure  330  of FIG. 11 by a conventional CVI/CVD process. As shown, the greatest density appears in a zone  342  adjacent the inside circumferential surface  82 , which is residual from zone  332  of FIG. 11. The subsequent conventional CVI/CVD process decreases the radial density gradient. This is indicated by a zone  344  of intermediate density adjacent the outside circumferential surface  84 . A zone of lesser density  346  encircles a core zone  348  of least density. The subsequent process fills the lower density portions remaining in the densified structure  330  of FIG. 11. Thus, the second portion  96  from the FIG. 5 process experiences a greater bulk density gain than the first portion  94  during the subsequent conventional CVI/CVD process. In addition, the pressure gradient CVI/CVD process generated by the FIG. 5 process produces a desirable porosity distribution in densified structure  330  that renders structure  330  extremely susceptible to subsequent densification by conventional CVI/CVD processes. Densified structure  330  reaches final density quicker and has minimal tendency to seal-coat during subsequent conventional CVI/CVD processes than a structure having the same bulk density that was previously densified by only conventional CVI/CVD processes. This greatly minimizes the need for surface machining operations during the subsequent processes, which greatly simplifies and expedites the entire densification process. This synergistic effect was a surprising discovery.  
         [0074]    Referring now to FIG. 6, an alternative fixture  9  that may be used in place of fixture  2  for an alternative pressure gradient CVI/CVD process is presented. The process presented in FIG. 6 is a “reverse flow” process wherein the reactant gas enters the porous structure  22  from the outside rather than the inside of the porous structure  22 . This is accomplished by disposing the porous structure  22  between plates  38  and  41 . Plate  41  is essentially identical to plate  40  except that plate  41  includes an aperture  43 . A cylindrical barrier structure  350  is disposed between and sealed to plates  38  and  41 . The barrier structure  350  encircles the porous structure  22 . The outside diameter of surface  80  is spaced from and sealed to the plate  41  by an “OD” ring-like spacer  64 . The outside diameter of surface  78  is spaced from and sealed by an “OD” ring-like spacer  64  to a seal plate  352 , which is disposed between the porous structure  22  and plate  38 . A plurality of spacing blocks  353  space the seal plate  352  from the plate  38  thereby forming a plurality of apertures  354 . Reactant gas is introduced into fixture  9  the direction of arrow  92 . The seal plate  352  forces the gas to flow radially outward and through the apertures  354 . The barrier structure  350  then forces the gas to flow upward as indicated by arrows  356  toward the outside circumferential surface  84  of porous structure  22 . The aperture  43  in plate  41  subjects the inside of the fixture to the furnace volume pressure which is less than the gas supply pressure in tube  60 . Thus, a first portion  95  is subjected to a greater pressure than a second portion  97  which forces the gas to disperse through the porous structure  22  as indicated by arrows  99 . The gas exhausts from fixture  9  to the reactor volume  35  through the aperture  43  as indicated by arrow  358 . In this example, the first portion  95  includes the outside circumferential surface  84 , and the second portion  97  includes the inside circumferential surface  82  and the opposing surfaces  78  and  80 . The densified structure may be further densified by second or additional densification processes including pressure gradient CVI/CVD, conventional CVI/CVD, or resin impregnation followed by charring.  
         [0075]    Referring now to FIG. 7, an alternative fixture  7  that may be used in place of fixture  2  for an alternative pressure gradient CVI/CVD process is presented. FIG. 7 presents a reverse flow process which is very similar to the FIG. 6 process. Fixture  7  is essentially identical to fixture  9 , except that fixture  7  comprises “ID” ring-like spacers  62  rather than “OD” ring like spacers  64 . The flow of reactant gas enters the opposing surfaces  78  and  80  and the outside circumferential surface  84 , and exits the inside circumferential surface  82  of porous structure  22  as indicated by arrows  101 . The inside circumferential surface  82  is subjected to the pressure of the reactor volume  35 , and the outside circumferential surface  84  and the opposing surfaces  78  and  80  are subjected to the reactant gas supply pressure. Thus, a first portion  552  of porous structure  22  is subjected to a greater pressure than a second portion  550 . In this example, the first portion  552  includes the inside circumferential surface  82 , and the second portion  550  includes the outside circumferential surface  84  and the opposing surface  78  and  80 . FIG. 13 presents a densified structure  341  generated by the FIG. 7 process. The densified structure  341  comprises a zone  343  of greatest density adjacent the outside circumferential surface  84  and part of the two opposing surfaces  78  and  80 . The density monotonically decreases from the greatest density zone  343  to a least density zone  349  with density zones  345  and  347  representing intermediate density ranges. The densified structure  341  has an average bulk density, and density zone  343  is typically about 120% of the average bulk density, and density zone  349  is typically about 80% of the average bulk density. The densified structure  341  may be further densified by second or additional densification processes including pressure gradient CVI/CVD, conventional CVI/CVD, or resin impregnation followed by charring.  
         [0076]    The various components of fixtures  2 ,  4 ,  6 ,  7 ,  8  and  9  are preferably formed from graphite, but any suitable high temperature resistant material may be used in the practice of the invention. The various joints may be sealed using compliant gaskets and/or liquid adhesives such as a graphite cement. The porous structures may be pressed against the ring-like spacers to form appropriate seals if the porous structures are compliant before densification. Suitable compliant gaskets may be formed from a flexible graphite such as EGC Thermafoil® brand flexible graphite sheet and ribbon-pack available from EGC Enterprises Incorporated, Mentor, Ohio, U.S.A. Comparable materials are available from UCAR Carbon Company Inc., Cleveland, Ohio, U.S.A.  
         [0077]    A conventional CVI/CVD process may be carried out using a CVI/CVD furnace  11  as depicted in FIG. 14. Furnace  11  is very similar to Furnace  10  (see FIG. 1). However the fixture  2  is eliminated and replaced with a fixture  360 . Fixture  360  comprises a support plate  362  disposed on a plurality of support posts  364 . The porous structure is disposed on a plurality of spacers  368  that separate the porous structure  22  from the plate  362  permitting dispersion of the reactant gas between the plate  362  and the porous structure  22 . The support plate  362  has a multitude of perforations (not shown) to permit dispersion of reactant gas through the plate and around the porous structure  22 . The support posts  364 , spacers  368 , and perforated support plate  362  are preferably formed from graphite. Tube  60  of FIG. 1 is replaced by a larger diameter tube  366 . Gas enters the furnace volume and freely expands as indicated by arrows  370 . The gas passes over the porous structure as indicated by arrows  34  and exhausts from the furnace volume  14  to the vacuum device  58  as indicated by arrows  36  and  28 . Normally, only one temperature sensor  76  is used which generally senses the temperature of porous structure  22 . The pressure measured by pressure sensor  70  is only slightly greater than the pressure measured by pressure sensor  72  during a conventional CVI/CVD process. A mixture of reactant gases may be introduced from main supply lines  42  and  44 , as previously described in relation to FIG. 1.  
         [0078]    With each of the FIG. 2 through FIG. 7 fixtures, each annular porous structure  22  has a surface area with a majority (more than 50%) of the surface area of each annular porous structure being exposed to the reactant gas as it enters or leaves the porous structure  22 . Establishing a high level of exposure reduces the pressure gradient required to force dispersion of the gas through each porous structure. As much of the porous structure surface area as possible is preferably exposed to the reactant gas. Preferably, at least 80% of the porous structure surface area is exposed.  
         [0079]    Referring now to FIG. 15, a CVI/CVD furnace  400  and an apparatus  402  for introducing a first reactant gas into the furnace  400  is presented. Furnace  400  and apparatus  402  are particularly suited for simultaneously densifying large quantities of porous articles, for example five hundred to one thousand annular preforms for manufacturing aircraft brake disks. A first main gas line  404  supplies the first reactant gas as indicated by arrow  406 . A plurality of furnace supply lines  408  are in fluid communication with the first main gas line  404  and the CVI/CVD furnace  400 . A plurality of first flow meters  410  measures a quantity of first reactant gas flow through each furnace supply line  408 . A plurality of first control valves  412  are configured to control the quantity of flow of the first reactant gas through each furnace supply line  408 . Apparatus  402  comprises four supply lines  408 , four control valves  412 , and four flow meters  410 , but the invention is not limited to four of each component, since the number may be increased or decreased as required.  
         [0080]    According to a preferred embodiment, the furnace  400  and reactant gas supply apparatus  402  are controlled by a controller  414 . Each flow meter  410  may communicate the measured quantity of flow to the controller  414  via a first flow sensor line  416 , and the controller  414  may control each control valve  412  via a first valve control line  418 . Thus, the quantity of flow of the first reactant gas into the furnace  400  may be independently set and controlled for each supply line  408 . The controller  414  is preferably micro-processor based and comprises a screen  415  for monitoring the various conditions and control states in the reactant gas supply apparatus  402  and the furnace  400 . According to a certain embodiment, each furnace supply line  408  comprises one first flow meter  410  and one first control valve  412 , as shown in FIG. 15, and a first main control valve  420  disposed within the first main gas line  404 . The first main control valve  420  preferably controls pressure in the first main gas line  404 . A first main flow meter  422  may also be disposed within the first main gas line  404 .  
         [0081]    A mixture of gases may be supplied to furnace  400  by providing at least a second main gas supply line  424  for supplying a second reactant gas as indicated by arrow  426 . A plurality of second flow meters  430  are provided that measure a quantity of second reactant gas flow through each furnace supply line  408  with a plurality of second control valves  432  configured to control the quantity of flow of the second reactant gas through each furnace supply line  408 . Each second flow meter  430  may communicate the measured quantity of flow to the controller  414  via a second flow sensor line  436 , and the controller  414  may control each second control valve  432  via a second valve control line  438 . According to a certain embodiment, the second main gas line  424  comprises a second main control valve  440  disposed within the second main gas line  424 . A second main flow meter  442  may also be disposed within the second main gas line  424 . The second main control valve  440  preferably controls pressure in the second main gas line  424 .  
         [0082]    The furnace  400  comprises a furnace shell  444  that defines a furnace volume  446 . A reactor volume  447  is included within the furnace volume  446 . The furnace supply lines  408  are in fluid communication with the reactor volume  447 . A vacuum apparatus  448  is in fluid communication with the furnace volume  446  and reactor volume  447  via exhaust stacks  450 . The vacuum apparatus  448  reduces the pressure in furnace volume  446  to a vacuum pressure (below atmospheric) and may comprise any suitable device such as a vacuum pump or steam vacuum system with appropriate filters and scrubbers that remove undesirable by-products from the spent reactant gas. The reactant gas from a given furnace supply line  408  is introduced into a corresponding preheater  458 . A first preheater  458  is disposed within the reactor volume  447  and has an inlet  460  and an outlet  461 . The first preheater  458  is sealed such that substantially all of the reactant gas introduced into the inlet  460  from a corresponding furnace supply line  408  is heated and leaves the preheater through the corresponding outlet  461  where it infiltrates at least one porous structure disposed within the furnace. The term “substantially all of the gas” is intended to allow for a small amount of leakage. The first preheater  458  is heated to a preheater temperature greater than the reactant gas temperature from the corresponding furnace supply line  408 . The porous structure is also heated. In this example the porous structure comprises a first porous wall  452  disposed within the reactor volume  447 . The first porous wall  452  is preferably annular and comprises a first top plate  454  that seals the upper open end of the first porous wall  452 , thereby defining a first enclosed cavity  456 . The other end of the first porous wall  452  is sealed against the first preheater  458 , with the first preheater outlet  461  in fluid communication with the first enclosed cavity  456 .  
         [0083]    A first flow of reactant gas is introduced into the first preheater  458 , and then passes into the first enclosed cavity  456  at a pressure greater than the pressure within the reactor volume  447 . Thus, one side of the first porous wall  452  is subjected to a greater reactant gas pressure than the other side of the first porous wall. In the example shown in FIG. 15, the inner side of the porous wall  452  (the enclosed cavity  456 ) is subjected to a greater reactant gas pressure than the outer side of porous wall  452 . The pressure difference forces the first flow of reactant gas to disperse through the first porous wall  452  where the heated gas cracks and deposits a binding matrix within the heated first porous wall  452 . The remaining gas and any by-products then exit the first porous wall  452  and are exhausted from the reactor volume  447  through exhaust stacks  450  by vacuum apparatus  448 . Thus, the reactant gas is forced to disperse through the annular porous wall by introducing the reactant gas to the CVI/CVD furnace and exhausting the reactant gas from the CVI/CVD furnace on opposite sides of the annular porous wall. At least one exhaust stack  450  is preferably provided between each pair of porous walls. Also, each preheater  458  may supply reactant gas to more than one annular porous wall  452 . Furnace  400  may be heated by any method known in the art for heating a CVI/CVD furnace, including resistance heating and induction heating.  
         [0084]    According to a preferred embodiment, the preheater  458  and porous wall  452  are radiation heated by a susceptor  462  that encloses the first preheater  458  and porous wall  452  on all sides. The susceptor  462  defines the reactor volume  447  and a floor  463  upon which the first preheater  458  rests. The susceptor  462  preferably comprises a circumferential portion  464  and the furnace  400  comprises a first induction coil  466 , a second induction coil  468 , and a third induction coil  470  that encircle the circumferential portion  464 . The susceptor  462  is inductively coupled with the induction coils  466 ,  468 , and  470  which transfer energy to the susceptor  462 , where it is transformed into heat in a manner well known in the art. Maintaining a uniform temperature from the bottom to the top of a CVI/CVD furnace during densification of a large number of porous structures (hundreds) may be difficult. The rate at which the gas cracks and deposits the binding matrix is largely determined by temperature assuming the reactant gas concentration is sufficient. Thus, variations in porous structure temperature throughout the furnace cause corresponding variations in bulk density gain which can reduce yield during a given CVI/CVD run. Incorporating multiple induction coils, as depicted in FIG. 15, permits application of differing amounts of heat along the length of the furnace. A more uniform porous structure temperature profile along the length of the furnace (in direction of gas flow) may thus be obtained.  
         [0085]    According to a further embodiment, a first gas temperature of the first flow of reactant gas is sensed proximate the first preheater outlet  461  by a first temperature sensor  490 . Temperature sensor  490  may comprise a Type K thermocouple with appropriate protective sheathing. The preheater temperature may be adjusted to achieve a desired gas temperature. Measuring the preheater temperature directly is not necessary since the preheater temperature is convectively related to the gas temperature at the outlet  461 . The preheater temperature is adjusted by increasing or decreasing the heating of the first preheater  458 . In FIG. 15, the susceptor wall  464  is comprised of a first susceptor wall portion  467 , a second susceptor wall portion  469 , and a third susceptor wall portion  471 . As previously described, the first induction coil  466  is inductively coupled to the first susceptor wall portion  467  in a manner that transforms electrical energy from the first induction coil  466  to heat energy in the first susceptor wall portion  467 . The same applies to the second susceptor wall portion  469  and the second induction coil  468 , and the third susceptor wall portion  471  and third induction coil  470 . The first preheater  458  is predominantly heated by radiation heat energy from the first susceptor wall portion  467  which is adjacent the first induction coil  466 . Thus, the first preheater temperature may be adjusted by adjusting electrical power to the first induction coil  466 . The electrical power to the second induction coil  468  and  470  may be adjusted as necessary to maintain a desirable porous structure temperature profile along the length of the furnace. The first preheater  458  is preferably disposed proximate the first susceptor wall portion  467  which improves the transfer of heat energy by radiation. The temperature sensed by first temperature sensor  490  may be transmitted to the controller  414  via a first temperature sensor line  494 . The controller may process the temperature sensor information and automatically adjust electrical power to the first induction coil  466  as necessary to achieve a desired temperature of the first gas flow as it leaves the first preheater outlet  461 . In certain furnace arrangements, a preheater may be disposed proximate the center of the furnace and surrounded by adjacent preheaters that are proximate the susceptor wall and block transfer of heat energy by radiation to the center preheater. In such a case, the center preheater is heated predominantly by conduction from the adjacent preheaters that are heated by radiation. Thus, the center preheater is indirectly heated by radiation from the susceptor wall and the center preheater temperature may be controlled by varying power to the first induction coil  466 . Also, the preheaters could be resistance heated which would permit direct control of the heat energy supplied to each preheater. Any such variations are considered to be within the purview of the invention.  
         [0086]    A second porous wall  472  may be sealed to a second preheater  478  with the second porous wall having a second top plate  474 . The second preheater  478  has a second preheater inlet  480  and a second preheater outlet  481 . A second temperature sensor  492  may be provided for sensing the temperature of the second flow of reactant gas as it exits the second preheater outlet  481 . The second porous wall  472  defines a second enclosed cavity  476  that is in fluid communication with the second preheater outlet  481 . A second flow of gas is introduced to the second preheater through a corresponding furnace supply line  408  and is forced to disperse through the second porous wall  472  and exit the furnace volume  446  in the same manner as described in relation to the first porous wall  452 . Thus, one side of the second porous wall  472  is subjected to a greater pressure than the other side of the second porous wall. According to a certain embodiment, the second preheater  478  and second porous wall  472  are heated predominantly by radiation from the susceptor wall  464 . The second preheater  478  is heated to a preheater temperature greater than the reactant gas temperature from the corresponding furnace supply line  408 . The heated gas infiltrates the second porous wall  472  where it cracks and deposits a binding matrix. The remaining gas and any by-products then exit the second porous wall  472  and are drawn out of the furnace volume  446  by vacuum apparatus  448 . A second temperature sensor  492  may be provided proximate the second preheater outlet  481 . The temperature sensed by second temperature sensor  492  may be transmitted to the controller  414  via a second temperature sensor line  496 . The controller  414  may process the temperature sensor information and automatically adjust electrical power to the first induction coil  466  as necessary to achieve a desired temperature of the second gas flow as it leaves the second preheater outlet  481 . Electrical power to the first induction coil  466  may also be manually adjusted as necessary in order to achieve the desired gas flow temperature. At least a third porous wall may be densified by a similar pressure gradient CVI/CVD process wherein at least a third flow of reactant gas is forced to disperse through at least the third porous wall by subjecting one side of at least the third porous wall to a greater pressure than the other side of at least the third porous wall, and the third flow of gas may be independently controlled. Additional porous walls may be added and densified in an identical manner using additional furnace supply lines  408  and additional preheaters. Additional preheaters and temperature sensors for sensing temperature of the gas flow proximate the outlet of each additional preheater may be provided as required. Thus, the invention permits simultaneous densification of large numbers of porous walls.  
         [0087]    A porous wall temperature sensor  498  may be provided in close proximity to the first porous wall  452  for sensing a first porous wall temperature. The first porous wall temperature may be increased or decreased by increasing or decreasing the first flow of reactant gas that passes through the first porous wall  452 . For example, the first flow of reactant gas may be at a lesser temperature than the porous structure as it exits the first preheater outlet  461 . Increasing the first flow of reactant gas at this lesser temperature tends to decrease the porous wall temperature and decreasing the flow tends to increase the porous wall temperature. The reverse would apply if the first flow of reactant gas was at a greater temperature than the first porous wall  452 . The first porous wall temperature sensor  498  may communicate with the controller  414  via a first porous wall temperature sensor line  502  which permits automatic or manual control of the first gas flow as necessary to achieve a desired first porous wall temperature. A second porous wall temperature may be similarly sensed by a second porous wall temperature sensor  500 . The second porous wall temperature sensor  500  may communicate with the controller  414  via a second porous wall temperature sensor line  504  which permits automatic or manual control of the second gas flow as necessary to achieve a desired second porous wall temperature by increasing or decreasing the second gas flow. Temperature of third and additional porous walls may be sensed and controlled in similar manner. Each individual flow of gas from the furnace supply lines  408  may be independently controlled in order to influence the CVI/CVD deposition process by virtue of the reactant gas supply apparatus  402 . The porous wall temperature sensors may also be inserted directly in to the porous walls as indicated by temperature sensor  506 . A thermocouple may be placed between an adjacent pair of annular porous structures if the porous wall is formed from a stack of porous structures. Porous wall temperature may also be sensed by an optical pyrometer  548  focused through a window  546  on an optical target  544  disposed between an adjacent pair of porous walls  452  and  472 .  
         [0088]    According to a preferred embodiment, the furnace volume  446  is maintained at a constant vacuum pressure. The pressure inside the first enclosed cavity  456 , second enclosed cavity  476 , and any third or additional enclosed cavities is determined by the flow of reactant gas introduced into that cavity and the porosity of the corresponding porous wall. For example, the flow into the first enclosed cavity  456  may be maintained at a constant value. At the beginning of the densification process, the pressure inside the first enclosed cavity may be only slightly higher than the furnace volume pressure outside the enclosed cavity. The pressure inside the first enclosed cavity  456  increases as matrix is deposited within the first porous wall  452  because porosity decreases and the quantity of first flow of reactant gas is constant. The pressure inside the first enclosed cavity  456  may be controlled by increasing or decreasing the flow of reactant gas into the first enclosed cavity. Increasing flow increases pressure and decreasing the flow decreases pressure. A first pressure sensor  508  may be provided for sensing the pressure inside the first enclosed cavity  456 . The first pressure sensor  508  may communicate via first pressure sensor line  512  with the controller  414  which allows automatic or manual control of the quantity of flow introduced into the first enclosed cavity  456  as necessary to achieve a desired pressure. A second pressure sensor  510  and second pressure sensor line  514  may be provided for controlling the flow and pressure inside the second enclosed cavity  476  in like manner. Third and additional pressure sensors and pressure sensor lines may be provided as required. The quantity of gas flow into a given enclosed cavity is preferably fixed and the pressure allowed to naturally rise as the porous wall densities unless the pressure rises too rapidly or exceeds a maximum desired pressure, in which case the flow may be reduced or completely stopped. The reactant gas supply apparatus  402  allows independent control of the flow to each porous wall. Monitoring the pressure inside the porous cavity also provides a real time indication of the degree of densification of each porous wall. The lack of a pressure rise, or an unusually slow pressure rise, indicates the presence of a leak in the preheater and/or the porous wall. The process may be terminated and subsequently restarted once the leak is located and fixed. An unusually rapid pressure may indicate sooting or tarring of one or more of the annular porous walls.  
         [0089]    Referring now to FIG. 16, a preheater  100  is presented which is a preferred embodiment for the preheaters  458  and  478  of FIG. 15. The preheater  100  is described in more detail in a copending United States patent application entitled APPARATUS FOR USE WITH CVI/CVD PROCESSES, filed the same day as the present application naming James W. Rudolph, Mark J. Purdy, and Lowell D. Bok as inventors, and which is fully incorporated herein by reference. The preheater  100  comprises a sealed duct structure  102  disposed within the furnace  10  and resting on the susceptor floor  463 . The preheater  100  receives gas from the gas inlet  460  (FIG. 15). The gas inlet  460  may be connected to one or more perforated tubes  19  which promote dispersion of the gas throughout the sealed duct structure  102 . Preheater  100  comprises a sealed baffle structure  108  that rests upon a sealed duct structure  102 . The sealed baffle structure  108  comprises an array of spaced perforated plates  128  and  129  with a bottom perforated plate comprising a baffle structure inlet  104  and a top perforated plate comprising a baffle structure outlet  106 . The sealed duct structure  102  and sealed baffle structure  108  are sealed to each other, and the sealed duct structure  102  is sealed to the susceptor floor  463  at joint  118  so that gas cannot avoid flowing through the sealed baffle structure  108 . The sealed duct structure  102  comprises at least two pieces  119 ,  120 , and  121 , upper ring  122  and lower ring  123  which together form several sealed joints  124 ,  125 ,  166 ,  168 ,  170 ,  172 , and  174 . The support bars  119 ,  120 , and  121 , and lower ring  123  support the weight of the sealed baffle structure  108 . A cover plate  110  preferably adjoins the sealed duct structure  102  disposed over the baffle structure outlet  106 . The cover plate  110  serves to support the porous structure fixtures. Cover plate  110  is adapted for use with a pressure gradient CVI/CVD process and comprises a plurality of apertures  114  and  116  with each aperture providing reactant gas to an annular porous wall. The cover plate  110  is sealed to the sealed duct structure  102  by a compliant gasket placed in the joint between the sealed duct structure  102  and the cover plate  110 . The perforated plates  128  and  129  are coterminous and arranged in a stack that defines a baffle structure perimeter  132 . Each sealed baffle structure plate  128  comprises an array of perforations  130 , with the array of perforations  130  of one susceptor plate  128  being misaligned with the array of perforations  131  of an adjacent susceptor plate  129 . This arrangement greatly facilitates transfer of heat by radiation from the susceptor wall  464  directly to the perforated plates  128  and  129 . The heat is transferred by conduction along plates  128  and  129  and to the gas by forced convection. The baffle structure perimeter  132  is sealed by compliant gaskets  134  and comprises the outer plane-wise limit of each susceptor plate  128  and  129  and is disposed in close proximity to the susceptor wall  464 . The compliant gaskets  134  also serve to space the perforated plates  128  and  129  from each other. The sealed duct structure  102  preferably defines a ledge  136  upon which said sealed baffle structure  108  rests. In the embodiment presented, the support bars  119 ,  120 , and  121  define the ledge in combination with lower ring  123 . A plurality of posts  140  may be provided that facilitate loading the baffle structure  108  into the furnace and also further support the sealed baffle structure  108  and cover plate  110 . Each post  140  comprises an enlarged portion that defines a seat (not shown) which rests on the susceptor floor  463 . The sealed baffle structure  108  rests upon the seat. The various components of preheater  100  are preferably formed from monolithic graphite. The various sealed joints are preferably formed using compliant gaskets and/or graphite cement. Suitable compliant gaskets may be formed from a flexible graphite such as EGC Thermafoil® and Thermabraid® brand flexible graphite sheet and ribbon-pack available from EGC Enterprises Incorporated, Mentor, Ohio, U.S.A. Comparable materials are available from UCAR Carbon Company Inc., Cleveland, Ohio, U.S.A.  
         [0090]    The porous walls  452  and  472  of FIG. 15 may be formed from stacks of annular porous structures, which is particularly preferred for manufacturing aircraft brake disks. Referring now to FIG. 17, a preferred fixture  200  is presented for densifying a stack of annular porous structures  22  by a pressure gradient CVI/CVD process. The fixture  200  is described in more detail in a copending United States patent application entitled APPARATUS FOR USE WITH CVI/CVD PROCESSES, filed the same day as the present application naming James W. Rudolph, Mark J. Purdy, and Lowell D. Bok as inventors. Fixture  200  is preferably used with the preheater  100  of FIG. 16. The porous structures  22  are arranged in a stack  202 . The fixture comprises a base plate  204 , a spacing structure  206 , and a top plate  208 . The top plate  208  optionally has an aperture  210  which is sealed by a cover plate  212 , compliant gasket  213 , and weight  214 . The base plate  204  is adapted to engage the cover plate  110  and has a base plate aperture (item  216  in FIG. 18) that aligns with one of the cover plate apertures  114  or  116 . The base plate  204  is preferably located by a plurality of conical pins  226 . The base plate  204  has mating conical base plate holes that are aligned with and receive the conical pins  226 . This arrangement facilitates aligning the base plate aperture with a corresponding cover plate aperture. The base plate  204  is preferably sealed to the cover plate  110  by use of a compliant gasket.  
         [0091]    The top plate  208  is spaced from and faces the base plate  204 . The spacing structure  206  is disposed between and engages the base plate  204  and the top plate  208 . In the embodiment presented, the spacing structure comprises a plurality of spacing posts  218  disposed about the stack of porous structures and extending between the base plate  204  and the top plate  208 . Each post  218  has pins  220  at either end that are received in mating holes  224  in base plate  204  and top plate  208 . The spacing structure  206  preferably comprises at least three posts  218 . The spacing structure  206  could also be formed as a single piece, and other arrangements for engaging the base plate  204  and top plate  208  are possible, any of which are considered to be within the purview of the invention. At least one ring-like spacer  234  is disposed within the stack  202  of porous structures  22  between each pair of neighboring porous structures  22 . The ring-like spacer  234  encircles the neighboring porous structure apertures  23 . At least one of the ring-like spacers  234  is preferably disposed between the base plate  204  and porous structure  22  adjacent the base plate  204 , and between the top plate  208  and porous structure  22  adjacent the top plate  208 . The base plate  204 , the stack of porous structures  202 , and the at least one ring-like spacer  234  define an enclosed cavity  236  extending from the base plate aperture (item  216  in FIG. 18), including each porous structure aperture  23 , and terminating proximate the top plate  208 . According to a certain embodiment, the outside diameter of ring-like spacer  234  is about  21 . 9  inches and the spacer inside diameter is about  19 . 9  inches for processing annular porous structures  22  having an outside diameter of about  21  inches. The ring-like spacers are preferably at least  0 . 25  inch thick.  
         [0092]    Referring to FIG. 18, a preferred fixture  201  is presented for pressure gradient CVI/CVD densifying simultaneously a large number of porous structures  22 . The spacing structure  207  comprises at least one intermediate plate  272  disposed between the base plate  204  and the top plate  208  that divides the stack of porous structures  203 . The posts  218  extend between the top plate  208  and one of the intermediate plates  272 , between the base plate  204  and another of the intermediate plates  272 , and between adjacent pairs of intermediate plates  272 . In other respects, fixture  201  is essentially identical to fixture  200 . Each intermediate plate  272  has an intermediate plate aperture  274  therethrough is sandwiched between a pair of the porous structures  22 . The enclosed cavity  236  further includes each intermediate plate aperture  274 . At least one of the ring-like spacers  234  is disposed on either side of and sealed to the intermediate plate  272  between the intermediate plate  272  and the porous structures  22 . Multiple fixtures  201  may be stacked. In such case, the base plate  204  from one fixture  201  engages the top plate  208  of a lower fixture  201  with the upper fixture base plate aperture  216  in fluid communication with the lower fixture top plate aperture  210 . Thus, the enclosed cavity extends from one fixture  201  to the next until being terminated by the cover plate  212  disposed over the uppermost top plate aperture  210 . As shown more clearly in this view, the base plate  204  is provided with conical holes  230  that receive a conical portion of the conical pins  226 , and the cover plate  110  is provided with holes  228  that receive a cylindrical portion of the conical pins  226 .  
         [0093]    Referring now to FIG. 28, an alternative fixture  300  for pressure gradient densifying a stack of porous structures  302  is presented. Fixture  299  is essentially identical to fixture  200 , except stack  302  comprises “OD” (outside diameter) ring-like spacers  234  disposed around the outside diameter of each porous structure  22  alternated with “ID” (inside diameter) ring-like spacers  284  disposed around the inside diameter of each porous structure. The OD ring-like spacers  234  preferably have an inside diameter  233  slightly less than the porous structure outside diameter  608 , and an outside diameter  235  that is generally coterminous with the porous structure outside diameter  608 . The ID ring-like spacers  284  preferably have an outside diameter  286  slightly greater than the porous structure inside diameter  610 , and an inside diameter  288  that is generally coterminous with the porous structure inside diameter  610 . With ID ring-like spacers  284 , the porous structure outside diameter  608  is greater than the outside diameter  286  of the ring like spacer  284 . The wall thickness of each ring-like spacer  234  and  284  is preferably minimized in order to maximize exposure of the porous structure surface area to the reactant gas as it enters or leaves each porous structure  22 . Referring to FIG. 29, an alternative fixture  301  for pressure gradient densifying a stack of porous structures  303  is presented. Fixture  301  is essentially identical to fixture  200 , except stack  303  comprises all “ID” ring-like spacers  284  disposed around the inside diameter of each porous structure.  
         [0094]    The various components of fixtures  200 ,  201 ,  299  and  301  are preferably formed from graphite. The various joints comprised within the fixtures are preferably sealed using compressible ring-like gaskets from a flexible graphite material, as previously disclosed. If the porous structures  22  are compressible, they may be compressed directly against the ring-like spacers  234  to provide a sufficient seal and eliminate the need for compressible gaskets between the porous structures  22  and ring-like spacers  234 . The ring-like spacers prior to use are preferably seal-coated with a surface deposition of pyrolytic carbon which facilitates removal of the ring-like spacer from a densified porous structure following deposition of the matrix.  
         [0095]    Fixtures similar to fixtures  200  and  201  may be used in a conventional CVI/CVD process in which the ring-like spacers  234  are replaced by spacer blocks that separate the porous structures and permit the reactant gas to freely pass through, over, and around the porous structures  22 . In such case, cover plate  110  may be replaced by cover plate  152  of FIG. 22 in order to promote dispersion of the reactant gas throughout the furnace volume. Cover plate  152  comprises an array of perforations  153 . Sealing the various joints comprised within a fixture adapted for a conventional CVI/CVD process is not necessary or desirable.  
         [0096]    Referring now to FIG. 19, a CVI/CVD process is presented according to an aspect of the invention. According to a preferred embodiment, a multitude of annular porous carbon structures are disposed within a CVI/CVD furnace such as furnace  400  (FIG. 15) using multiple fixtures such as fixture  201  (FIG. 18) which are sealed to multiple preheaters such as preheater  100 . Reactant gas is supplied to the furnace using an apparatus such as the gas supply apparatus  402  (FIG. 15). The furnace is heated until conditions are stabilized, and a first carbon matrix is deposited within the porous structures by a pressure gradient CVI/CVD process. More support for the porous structures than depicted in FIGS. 17 and 18 during the pressure gradient CVI/CVD process is not necessary since the porous structures do not sag during the pressure gradient CVI/CVD process. The porous structures are then subjected to a heat treatment process without removing the porous structures from the furnace or from the fixtures. Alternatively, the porous structures may be removed from the furnace and pressure gradient CVI/CVD fixtures before the heat treatment process. The heat treatment process is conducted at a higher temperature than the previous deposition process temperatures which increases graphitization of the first carbon matrix. Following heat treatment, the porous structures are then removed from the furnace and surface machined in order to derive an accurate bulk density measurement. Machining the surface may also increase open porosity at the surface. A second carbon matrix is then deposited within the porous structures by a conventional CVI/CVD process. Thus, the second matrix overlies the first matrix. After reaching final density, the densified structures are machined into final parts. In certain embodiment, the pressure gradient CVI/CVD process and conventional CVI/CVD process are conducted at about  1750 - 1900  ° F., and heat treatment is conducted at about  3300 - 4000  F. Thus, the first matrix has a greater degree of graphitization than the second matrix due to the intermediate heat treatment process.  
         [0097]    Referring now to FIG. 20, an alternative process is presented that begins with a pressure gradient CVI/CVD process in which a first carbon matrix is deposited within the porous structures. The porous structures are then subjected to a heat treatment process without removing the porous structures from the furnace or from the fixtures. A second carbon matrix is then deposited in another pressure gradient CVI/CVD process that immediately follows the heat treatment process without removing the porous structures from the furnace or the fixtures. Alternatively, the porous structures may be removed from the furnace and pressure gradient CVI/CVD fixtures before the heat treatment process, and replaced in the pressure gradient CVI/CVD fixtures before the second pressure gradient CVI/CVD process. The porous structures are then subjected to a surface machining operation. Further second carbon matrix is then deposited in a conventional CVI/CVD process and the porous structures are machined into final parts. Leaving the porous structures in the same furnace and fixtures during the first and second pressure gradient processes and the heat treatment process results in a “continuous” process. Additional support blocks between adjacent pairs of porous structures in the pressure gradient CVI/CVD fixtures may be necessary in order to prevent sagging during the heat treatment process.  
         [0098]    Referring now to FIG. 21, an alternative process is presented that begins with a pressure gradient CVI/CVD process in which a first carbon matrix is deposited within the porous structures. The porous structures are surface machined and a second carbon matrix is then deposited in a conventional CVI/CVD process followed by a heat treatment process. After heat treatment, the fully densified porous structures are then machined into final parts. It is evident that the sequences of the FIGS.  19 - 21  processes may be rearranged, and additional steps inserted, without departing from the invention.  
         [0099]    The first carbon matrix and second carbon matrix preferably comprise a substantially rough laminar microstructure. A rough laminar microstructure has a greater density (about 2.1 g/cc), greater thermal conductivity, and lesser hardness than smooth laminar microstructure (1.9-2.0 g/cc or less). Rough laminar microstructure is particularly preferred in certain carbon/carbon aircraft brake disks. Microstructure may be optically characterized as described by M. L. Lieberman and H. O. Pierson,  Effect of Gas Phase Conditions on Resultant Matrix Pyrocarbons in Carbon/Carbon Composites,  12 Carbon 233-41 (1974).  
         [0100]    Referring now to FIG. 23, a densified porous structure  600  manufactured according to either the FIG. 19, 20 or  21  process is presented. The densified porous structure  600  comprises a first circumferential zone  512  adjacent the inside circumferential surface  82 , and a second circumferential zone  514  adjacent the outside circumferential surface  84 . The first and second circumferential zones  512  and  514  extend all the way through the thickness of the densified porous structure  600  and are bounded by the opposing surfaces  78  and  80 . Densified porous structure  510  comprises a first carbon matrix deposited within a porous structure comprised of carbon fibers according to a pressure gradient CVI/CVD process. According to a preferred embodiment, the first carbon matrix is deposited by a process using fixtures  200  and/or  201  having all “OD” ring-like spacers  234  (FIGS. 17 and 18) which is similar to the process described in relation to FIG. 5, resulting in the first carbon matrix being deposited unevenly in a density distribution similar to densified porous structure  330  of FIG. 11. The first circumferential zone  512  is subjected to a greater reactant gas pressure than the second circumferential zone  514  during the pressure gradient CVI/CVD densification process which causes the first circumferential zone  512  to experience a greater bulk density gain than the second circumferential zone  514 . According to a certain embodiment, the second circumferential zone  514  has about 15% less of the first carbon matrix per unit volume relative to the first circumferential zone  512 , and the first carbon matrix preferably has a substantially rough laminar microstructure. The second circumferential zone  514  generally has at least 10% less of the first carbon matrix per unit volume relative to the first circumferential zone  512 , and may have 20%, 30%, 40% or less of the first carbon matrix. Densified porous structure  510  also comprises a second carbon matrix overlying the first carbon matrix that is deposited by a conventional CVI/CVD process resulting in the densified porous structure  600  having a final density distribution similar to densified porous structure  340  of FIG. 12. The second carbon matrix also preferably has a substantially rough laminar microstructure. The first and second carbon matrices preferably have at least  90 % rough laminar microstructure, more preferably at least 95% rough laminar microstructure, and in certain preferred embodiments 100% rough laminar microstructure.  
         [0101]    The first carbon matrix may be heat treated which causes the first carbon matrix to be more graphitized than the second carbon matrix. Increasing graphitization increases the apparent density and thermal conductivity. Thus, the original density gradient from the pressure gradient CVI/CVD process may be identified in the densified porous structure  600  after deposition of the second carbon matrix. If the first carbon matrix has a distribution as shown in FIG. 11, the first circumferential portion  512  has a generally greater thermal conductivity than the second circumferential portion  514 , and a generally greater apparent density than the second circumferential portion  514  even after the second carbon matrix is deposited. Closed porosity remaining within the densified porous structure  600  affects the measurement of apparent density. Porosity effects may be minimized by measuring apparent density of crushed samples which will be referred to herein as crushed apparent density. According to a certain technique, crushed apparent density is measured by cutting a specimen from a densified porous structure and fracturing the specimen between parallel steel platens of a load testing machine. The specimen is preferably fractured in a manner that maintains the specimen in one piece. This may be accomplished by compressing the sample past the yield point without fragmentation. Apparent density is then measured according to the Archimedes technique using mineral spirits such as Isopar M (synthetic isoparaffinic hydrocarbon) available from Exxon Chemical Americas, Houston, Tex., U.S.A. Vacuum is used to force the mineral spirits into the structure. Apparent density is a measurement of the density of the material that is impervious to penetration by the mineral spirits. Fracturing the specimen opens previously closed porosity that was impervious to penetration by the mineral spirits and minimizes porosity effects. Alternatively, crushed apparent density of a pulverized sample may be measured using a helium pyconometer. Measurements of densified porous structures processed similar to densified porous structure  600  demonstrated that the crushed impervious density adjacent the inside circumferential surface  82  was consistently at least 0.2% greater, and may be as much as 0.4% and 0.5% greater, than adjacent the outside circumferential surface  84 . Thus, crushed apparent density tends to generally decrease from the inside surface  82  to the outside surface  84 .  
         [0102]    Thermal conductivity of densified porous structures similar to densified porous structure  600  (as described in the immediately preceding paragraph) was measured in two directions: normal to the opposing surfaces  78  and  80  which will be referred to as “thermal flat conductivity”, and normal to the circumferential surfaces  82  and  84  (in the radial direction) which will be referred to as “thermal edge conductivity.” Thermal flat conductivity of circumferential portion  514  was at least 5% less than circumferential portion  512  when measured at the opposing surfaces  78  and  80 . Thermal flat conductivity of circumferential portion  514  was at least 12% less than circumferential portion  512  at one-half of the distance between opposing surfaces  78  and  80 . Thermal edge conductivity of circumferential portion  514  was at least 5% less than circumferential portion  512  when measured at the opposing surfaces  78  and  80 . Thermal edge conductivity of circumferential portion  514  was at least 4% less than circumferential portion  512  when measured at one-half of the distance between opposing surfaces  78  and  80 . Thus, thermal conductivity tends to generally decrease from the inside circumferential portion  512  to the outside circumferential portion  514 . This trend is induced by the first matrix being more graphitized than the second matrix.  
         [0103]    The following examples further illustrate various aspects of the invention.  
       EXAMPLE 1  
       [0104]    A base-line was established for a conventional CVI/CVD process as follows. A fibrous textile structure about 1.5 inch thick was manufactured according to FIGS. 1 through 4 of U.S. Pat. No. 4,790,052 starting with a 320K tow of unidirectional polyacrylonitrile fiber. An annular porous structure was then cut from the textile structure having an outside diameter of about 7.5 inch, an inside diameter of about 2.5 inch. The annular porous structure was then pyrolyzed to transform the fibers to carbon. The pyrolyzed porous structure, having a bulk density of 0.49 g/cc, was then placed in a furnace similar to furnace  11  of FIG. 14. Pressure was reduced to 10 torr inside the furnace volume and the furnace was heated and stabilized at a temperature of about 1860° F. when measured by a temperature sensor positioned as temperature sensor  76  of FIG. 14. A reactant gas mixture was introduced as described in relation to FIG. 14 and allowed to freely disperse over and around the porous structure in a manner typical of a conventional CVI/CVD process. The reactant gas mixture was comprised of 87% (volume percent) natural gas and 13% propane at a flow rate of 4000 sccm (standard cubic centimeters per minute) and a residence time of about 1 second in the reactor volume. The natural gas had a composition of 96.4% methane (volume percent), 1.80% ethane, 0.50% propane, 0.15% butane, 0.05% pentane, 0.70% carbon dioxide, and 0.40% nitrogen. The process was stopped three times to measure bulk density gain of the porous structure. Total deposition process time was 306 hours. An average rate of deposition was calculated for each of the three densification runs. The test conditions and data from this example are presented in Table 1, including cumulative deposition time (Cum. Time) and total density gain (Density Gain) at each cumulative time noted. The carbon matrix deposited within the densified porous structure at the end of the process comprised nearly all rough laminar microstructure with minimal deposits of smooth laminar microstructure at the surface of the porous structure.  
                                   TABLE 1                                   Cum.   Gas Flow   Part   Density           Time   Rate   Temp.   Gain           (hour)   (sccm)   (F)   (g/cc)                            41   4000   1857   0.310           166   4000   1860   0.886           306   4000   1855   1.101                      
 
       EXAMPLE 2  
       [0105]    An annular porous structure having a thickness of 1.6 inch, an outside diameter of 6.2 inch, and an inside diameter of 1.4 inch was cut from a fibrous textile structure and processed according to Example 1 by a conventional CVI/CVD process. The test conditions and data from this example are presented in Table 2.  
                                   TABLE 2                                   Cum.   Gas Flow   Part   Density           Time   Rate   Temp.   Gain           (hour)   (sccm)   (° F.)   (g/cc)                           92   4000   1858   0.370                      
 
       EXAMPLE 3  
       [0106]    Two annular porous structures (Disks A and B), prepared from a fibrous textile structure and having the same dimensions as described in Example 1, were densified by a pressure gradient CVI/CVD process using a furnace similar to furnace  10  of FIG. 1, a fixture similar to fixture  2  of FIG. 2 having ID/OD spacers, and the reactant gas mixture of Example 1. The test conditions and data from this example are presented in Table 3. Furnace pressure was 10 torr. Temperature of the gas stream was estimated to be 1740° F. when measured by a temperature sensor such as temperature sensor 74 of FIG. 1. The gas was forced to flow through the porous structure, as previously described in relation to FIG. 2, at a flow rate of 4000 sccm. The carbon matrix deposited within Disk A comprised all rough laminar microstructure. The microstructure of Disk B was not evaluated. Disk A was cut into smaller samples and the bulk density measurements of these samples were determined using the Archimedes method, and demonstrated a density profile similar to FIG. 8.  
                               TABLE 3                           Run   Gas Flow   Part   Density           Time   Rate   Temp.   Gain       Disk   (Hour)   (sccm)   (° F.)   (g/cc)                   A   165   4000   1861   1.106       B   123   4000   1859   0.928                  
 
       EXAMPLE 4  
       [0107]    Three annular porous structures (Disks A, B and C) were prepared and individually densified by a pressure gradient CVI/CVD process according to Example 3 except that the porous structures were flipped part way through the process in order to obtain a more uniform final density distribution. Temperature of the gas stream was approximately 1740° F. when-measured by a temperature sensor such as temperature sensor  74  of FIG. 1. The test conditions and data from this example are presented in Table 4. The carbon matrix deposited within Disks A and C was all rough laminar before the flip, and essentially smooth laminar after the flip. The microstructure of Disk B was not determined. The final densified porous structures had density profiles similar to FIG. 9.  
                               TABLE 4                           Cum.   Gas Flow   Part   Density           Time   Rate   Temp.   Gain       Disk   (hour)   (sccm)   (° F.)   (g/cc)                   A   72   4000   1859   0.743           Flip           96   4000   1859   0.853           111    4000   1855   1.034       B   49   4000   1854   0.619           Flip           74   4000   1849   0.898       C   49   4000   1858   0.625           Flip           75   4000   1853   0.915                  
 
       EXAMPLE 5  
       [0108]    Two annular porous structures, prepared from a fibrous textile structure and having the same dimensions as described in Example 1, were simultaneously densified by a pressure gradient CVI/CVD process with a fixture similar to fixture  6  of FIG. 4 having all “ID” spacers, and the reactant gas mixture of Example 1. Temperature of the gas stream was estimated 1745 ° F. when measured by a temperature sensor such as temperature sensor  74  of FIG. 1. The test conditions and data from this example are presented in Table 5. Density gain on Table 5 is an average for the two disks. The carbon matrix deposited within the densified porous structure at the end of the process comprised all rough laminar microstructure. Computed tomagraphy scans of the disk demonstrated density profiles similar to FIG. 10.  
                                   TABLE 5                                   Cum.   Gas Flow   Part   Density           Time   Rate   Temp.   Gain           (hour)   (sccm)   (° F.)   (g/cc)                           24.4   8000   1860   0.262           70.7   8000   1856   0.593                      
 
       EXAMPLE 6  
       [0109]    Four annular porous structures, prepared from a fibrous textile structure and having the same dimensions as described in Example 1, were densified by a pressure gradient CVI/CVD process using a fixture similar to fixture  8  of FIG. 5 having all “OD” spacers, and the reactant gas mixture of Example 1. Two disks were simultaneously densified (Disk Pair A and B) and reactant gas flow rate was doubled to maintain a flow rate of 4000 sccm per disk. Temperature of the gas stream was approximately 1750° F. when measured by a temperature sensor such as temperature sensor  74  of FIG. 1. The test conditions and data from this example are presented in Table 6. The density gain on Table 6 is an average for each disk pair. The carbon matrix deposited within the densified porous structure at the end of the process comprised all rough laminar microstructure. Computed tomagraphy scans of Disk Pair B demonstrated density profiles similar to FIG. 11.  
                               TABLE 6                           Run   Gas Flow   Part   Density       Disk   Time   Rate   Temp.   Gain       Pair   (hour)   (sccm)   (° F.)   (g/cc)                   A   70   8000   1860   0.951       B   70   8000   1855   0.861                  
 
       EXAMPLE 7  
       [0110]    An annular porous structure was prepared from a fibrous textile structure, having the same dimensions as described in Example 2, and densified by a pressure gradient CVI/CVD process using a fixture similar to fixture 7 of FIG. 7 having all “ID” seals with a reverse flow process, and the reactant gas mixture of Example 1. Temperature of the gas stream was estimated 1730° F. when measured by a temperature sensor such as temperature sensor  74  of FIG. 1. The reactant gas was forced to flow through the porous structure as previously described in relation to FIG. 7 at a flow rate of 3000 sccm (the flow was lowered since the disk was smaller than the disks used in Examples 3-6). The test conditions and data from this example are presented in Table 7. The carbon matrix deposited within the densified porous structure at the end of the process comprised mostly smooth laminar microstructure.  
                                   TABLE 7                                   Cum.   Gas Flow   Part   Density           Time   Rate   Temp.   Gain           (hour)   (sccm)   (° F.)   (g/cc)                           50   3000   1854   0.987                      
 
         [0111]    Referring now to FIG. 24, the data presented on Tables 1 through 7 is depicted in graphical form. The data from Tables 1 and 2 is presented as a single smoothed curve  516  representing conventional CVI/CVD. The data from Tables 3 and 4 is presented as a single smoothed curve  518  representing pressure gradient CVI/CVD using “ID/OD” spacers. The data from Table 5 is presented as a single smoothed curve  520  representing pressure gradient CVI/CVD using all “ID” spacers. The data from Table 6 is presented as a single smoothed curve  522  representing pressure gradient CVI/CVD using all “OD” spacers. The data from Table 7 is presented as curve  524  representing reverse flow pressure gradient CVI/CVD with all “ID” spacers. Densification rates increased by factors from about one and one-half to five times conventional CVI/CVD densification rates. Time to achieve a bulk density increase of 1 g/cc was reduced by about 25% to 80% relative to conventional CVI/CVD time. The importance of eliminating as many leaks as possible is apparent from FIG. 24. Any leakage tends to decrease the densification rate from the maximum attainable rate. Increased densification rates may be achieved even with a small amount of leakage. Thus, some leakage may occur while remaining within the purview of the invention.  
         [0112]    Referring now to FIG. 25, curves representing densification rate versus normalized flow are presented. The normalized flow is indicated as F* and represents a quantity of flow per unit of disk volume (for example, 4000 sccm per 1000 cc disk volume=4 min −1 ). Additional tests were run according to Example 6 and 7 above except flow rates of reactant gas were varied from one test to the next. The data from tests conducted according to Example 6 with varying flow are presented on Tables 8, and the data from tests conducted according to Example 7 with varying flow are presented on Table 9. A curve  526  represents conventional CVI/CVD. Data from Table 8 is presented as curve  528  which represents pressure gradient CVI/CVD with all “OD” spacers (FIG. 5). Data from Table 9 is presented as curve  530  which represents reverse flow pressure gradient CVI/CVD with all “ID” spacers (FIG. 7).  
                               TABLE 8                       Cum.   Gas Flow   Part   Density   Average       Time   Rate   Temp.   Gain   Deposition Rate       (hour)   (sccm)   (° F.)   (g/cc)   (g/cc/h)                   50   1000   1853   0.232   0.0046       50   2000   1856   0.414   0.0083       50   4000   1851   0.547   0.0109       70   8000   1858   0.906   0.0129                  
 
         [0113]    [0113]                               TABLE 9                       Cum.   Gas Flow   Part   Density   Average       Time   Rate   Temp.   Gain   Deposition Rate       (hour)   (sccm)   (° F.)   (g/cc)   (g/cc/h)                   50    500   1852   0.323   0.0065       50   1000   1853   0.498   0.0100       56   2000   1855   0.920   0.0164       46   3000   1854   0.987   0.0215       38   4000   1852   0.919   0.0242                    
         [0114]    Referring now to FIG. 26, curves representing densification rate versus normalized flow are presented. Additional tests were run according to Example 6 (pressure gradient with all “OD” spacers) above except the furnace volume pressure and flow rates of reactant gas were varied from one test to the next. The data from these tests is presented in Table 10. Data from Table 10 is presented as three curves  532 ,  534 , and  536 . Curve  532  represents data at a furnace volume pressure of 10 torr when measured by a pressure sensor such as sensor 72 of FIG. 1. Curve  534  represents data at a furnace volume pressure of 25 torr when measured by a pressure sensor such as sensor 72 of FIG. 1. Curve  532  represents data at a furnace volume pressure of 50 torr when measured by a pressure sensor such as sensor  72  of FIG. 1. The matrix deposited in all of these tests comprised all rough laminar microstructure. As demonstrated by FIG. 26, additional gains in densification rate may be realized by increasing the furnace volume pressure (Reactor Pressure) while maintaining a desired rough laminar microstructure. This was a surprising discovery.  
                                   TABLE 10                                           Average       Cum.   Reactor   Gas Flow   Part   Density   Deposition       Time   Pressure   Rate   Temp.   Gain   Rate       (hour)   (torr)   (sccm)   (° F.)   (g/cc)   (g/cc/h)                   50   10   2000   1856   0.414   0.0083       50   10   4000   1851   0.547   0.0109       70   10   8000   1858   0.906   0.0129       50   25   2000   1853   0.449   0.0090       50   25   4000   1853   0.611   0.0122       50   50   2000   1853   0.493   0.0099       50   50   4000   1852   0.683   0.0137                  
 
         [0115]    Referring now to FIG. 27, pressure differential across the porous structure versus bulk density is presented for several reactant gas flow rates. Additional tests were run according to Example 6 with varying flow rates. Data from these tests is presented in Table 11. The data from Table 11 is presented in FIG. 27 as a first set of curves  538  for a flow rate of 1000 sccm per disk, a second set of curves  540  for a flow rate of 2000 sccm per disk, and a third set of curves  542  for a flow rate of 4000 sccm per disk. The matrix deposited in all of these tests comprised all rough laminar microstructure. Table 11 includes the initial pressure differential across the porous structures (Init. Delta P), final pressure differential across the porous structures (Final Delta P), and furnace volume pressure (Reactor Pressure) which was maintained constant. As demonstrated by FIG. 27, the pressure gradient across the porous structure may be at least as high as 80 torr (which indicates 90 torr on the high pressure side of the porous structure) while maintaining a desired rough laminar microstructure.  
                                                                         TABLE 11                           Gas           Average   Init.   Final           Cum.   Flow       Density   Deposit.   Delta   Delta   Reactor       Time   Rate   Temp   Gain   Rate   P   P   Pressure       (h.)   (sccm)   (° F.)   (g/cc)   (g/cc/h)   (torr)   (torr)   (torr)                                50   2000   1856   0.414   0.0083    5   1210       50   2000   1853   0.449   0.0090   10   025        50   2000   1853   0.493   0.0099    6   1650       50   4000   1851   0.547   0.0109   14   3210       50   4000   1853   0.611   0.0122   15   3925       50   4000   1852   0.683   0.0137   15   4250       70   8000   1860   0.951   0.0136   38   8110       70   8000   1855   0.861   0.0123   32   5610                  
 
         [0116]    Tests have demonstrated that the pressure gradient CVI/CVD process according to the invention may be conducted with a part temperature in the range of 1800-2000° F., a reactor pressure in the range of 10-150 torr, normalized reactant gas flow rate (F*) in the range of 0.4-10 min-2, and with a hydrocarbon reactant gas mixture of natural gas and 0-40% (volume percent) propane. Conducting the process within these ranges generally produces a rough laminar and/or smooth laminar microstructure. Conducting the process with all of these process parameters at or near the high extreme of each of these ranges may result in tarring or sooting. Other carbon bearing gases, pressures, and temperatures known in the art for CVI/CVD processes may be substituted without departing from the invention.  
         [0117]    Densifying a porous structure by a pressure gradient CVI/CVD process according to the invention followed by a conventional CVI/CVD process produces a densified porous structure having a more uniform density distribution than a comparable porous structure densified only by a conventional CVI/CVD process. According to a certain embodiment, for example, an annular porous carbon structure having an inside diameter of about 10.5 inches (indicated as 602 in FIG. 23), a web (indicated as 604 in FIG. 23) of about 5.25 inches, and a thickness (indicated as 606 in FIG. 23) of about 1.25 inches, is densified first with a carbon matrix deposited by a pressure gradient CVI/CVD process (Example 6 conditions) using a fixture such as fixture 201 (FIG. 18) in furnace such as furnace 400 (FIG. 15) resulting in a density distribution similar to densified structure 330 of FIG. 11. Carbon matrix is further deposited by a conventional CVI/CVD process (Example 1 conditions) resulting in a density distribution similar to densified structure 340 of FIG. 12, and having a mean bulk density of about 1.77 g/cc. According to standard statistical practice, the standard deviation of the bulk density throughout the densified structure is about 0.06 g/cc. The standard deviation of the bulk density throughout a comparable porous carbon structure densified to an equivalent mean bulk density by only conventional CVI/CVD processes is about 0.09 g/cc. Thus, a porous structure densified by a pressure gradient CVI/CVD process followed by a conventional CVI/CVD process is more uniform than a porous structure densified by only conventional CVI/CVD processes. Circumferential as well as total variation is reduced. Uniformity is desirable for carbon/carbon aircraft brake disks.  
         [0118]    The standard deviation of the bulk density throughout a carbon/carbon structure manufactured according to the invention is preferably less than or equal to 0.07 g/cc, is more preferably less than or equal to 0.06 g/cc or 0.05 g/cc, and is most preferably less than or equal to 0.04 or 0.03 g/cc. Coefficient of variation of bulk density in any densified porous structure is preferably less than or equal to 4%, more preferably less than or equal 3.5% or 3%, and most preferably less than or equal to 2.3% or 1.8%.  
         [0119]    It is evident that many variations are possible without departing from the scope of the invention as defined by the claims that follow.