Patent Publication Number: US-2004047990-A1

Title: Hardware assembly for CVI/CVD processes

Description:
BACKGROUND  
       [0001] The present invention relates generally to chemical vapor infiltration and deposition (“CVI/CVD”) processes, and more particularly, to a hardware assembly for supporting porous structures during a CVI/CVD process.  
       [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 depositions (“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 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.  
       [0003] Generally speaking, manufacturing carbon parts using a CVI/CVD process involves placing preformed porous structures in a furnace and introducing a high temperature reactant gas to the porous structures. A variety of porous structures and reactant gases may be used, but typically, a fibrous carbon porous structure is used with a reactant gas mixture of natural gas and/or propane gas when carbon/carbon aircraft brake disks are manufactured. As well understood by those in the art, when the hydrocarbon gas mixture flows around and through the porous structures, some of the carbon atoms separate from the hydrocarbon molecules, thereby depositing the carbon atoms within the interior and onto the surface of the porous structures. As a result, the porous structures become more dense over time as more and more of the carbon atoms are deposited onto the structures. This process is sometimes referred to as densification because the open spaces in the porous structures are eventually filled with a carbon matrix until generally solid carbon parts are formed. U.S. Pat. Nos. 5,480,578 and 5,853,485 to Rudolph et al., hereby incorporated by reference, also describe in detail additional aspects of CVI/CVD processes.  
       [0004] Densification processes for annular brake disks may be characterized as either conventional densification processes or rapid densification processes or variants thereof. In conventional densification, annular brake disks are arranged in stacks with adjacent brake disks stacked on top of each other. A center opening region is thus formed through the center of each stack. Typically, spacers are placed between adjacent brake disks to form open passages between the center opening region and the outer region. Thus, the reactant gas flows randomly around the stack and may flow through the open passages from the center opening region to the outer region or vice versa. As a result, the pressure differential between the inlet and outlet ducts of the furnace is usually relatively low in conventional processes. On the other hand, in rapid densification, the open passages between the center opening region and the outer region are sealed to constrict the flow of the reactant gas between the center opening region and the outer region. Therefore, the pressure differential between the inlet and outlet ducts of the furnace is higher than the pressure used in conventional densification. As a result, the high pressure differential forces the reactant gas to flow through the interior of the porous brake disk structures, thereby increasing the rate of densification compared to conventional processes. Conventional and rapid densification processes may also be combined to achieve optimum densification. For example, a rapid densification process may be used in a first densification to decrease densification time, and a conventional densification may be used in a second densification to improve densification quality.  
       [0005] One area of concern during densification is the distribution of the reactant gas flow through and around the porous structures. Gas flow distribution can have a significant impact on the quality of the densified carbon parts and also can affect the cost of production. For example, in one method disclosed in U.S. Pat. No. 5,904,957 to Christin et al., stacks of annular preforms are placed in a furnace with spacer elements placed between each of the preforms and between the last performs in the stacks and the screens at the top end. Thus, leakage passages are formed between adjacent preforms. The gas is then channeled towards only (i.e., exclusively) the interior passage of each annular stack at the bottom end. The top end of the stacks are closed by solid screens. One disadvantage with this method is that the outer surfaces of the brake disks near the bottom of the stacks may become starved for gas, thereby producing an undesirable densification of the bottom brake disks and nonuniformity between the bottom and top brake disks. Another disadvantage is that the closed top end of the stacks blocks the gas flow out of the top end, thus causing gas stagnation problems as described in detail below.  
       [0006] Another problem that often occurs during densification is soot and tar accumulations on surfaces of the brake disks and/or on the furnace equipment. As is known to those in the art, soot usually refers to undesirable accumulations of carbon particles on the furnace equipment, while tar usually refers to undesirable accumulations of large hydrocarbon molecules on the brake disks and the furnace equipment. Typically, accumulations of soot and tar form when the reactant gas stagnates for a period of time in an area or comes into contact with cooler furnace surfaces. Stagnation typically occurs in areas where the gas flow is blocked or where the gas flow is moving more slowly than the surrounding gas flow. Sudden temperature changes also contribute to the formation of oily substances, such as tar.  
       [0007] Accumulations of soot and tar can cause a number of problems which affect both the quality of the carbon parts and the costs of manufacturing. Seal-coating is one typical problem that can result from soot and tar accumulations, although seal-coating can also be caused by other conditions that are described below. Seal-coating can occur when soot and tar accumulates early in the densification process on surfaces of the porous structure. As the carbon and large hydrocarbon molecules accumulate on the surfaces of the porous structure, the surface pores eventually become blocked, or sealed, thus preventing the flow of reactant gas from further permeating the porous structure. As a result, densification of the interior region around the seal-coated surface prematurely stops, thereby leaving interior porous defects in the finished carbon part.  
       [0008] Maintenance costs also increase due to soot and tar accumulations on the furnace equipment. During the densification process, accumulations of soot and tar often form throughout the furnace equipment. As a result, an extensive manual cleaning process may be periodically required after each production run to remove all the accumulations and prepare the furnace for the next production run. This cleaning job can be very time consuming and can result in significant delays between production runs. The accumulations can also make disassembly of close fitting parts especially difficult since the accumulations tend to bind the parts tightly together. As a result, furnace equipment sometimes becomes damaged during disassembly due to the difficulty of separating the parts. Additionally, the furnace vacuum lines sometimes become constricted by soot and tar. As those in the art are familiar, the vacuum lines are used to generate the desired gas flow through the furnace. However, soot and tar accumulations sometimes build up in these lines and reduce the performance of the vacuum. Therefore, the vacuum lines must be regularly cleaned, which is a time consuming and expensive task.  
       [0009] In order to produce high quality, low cost parts, carbon deposition should be as uniform as possible around and through the porous structures. One way to achieve this desired uniformity is to optimize the residence time of the gas in the furnace. Residence time typically refers to the amount of time required for a gas to travel through the furnace or other designated area. Typically, a low residence time is associated with an unobstructed flow path and is generally preferred.  
       [0010] Gas flow obstructions often cause additional problems during densification. As previously explained, seal-coating is a common problem that causes porous defects within the interior region of the completed carbon parts. However, in addition to the causes previously described, seal-coating also can occur due to nonuniform carbon deposition. This typically occurs when a nonuniform gas flow accelerates carbon deposition at the surface of a part, thereby sealing the surface with carbon deposits and blocking gas diffusion into the interior of the carbon structure. Usually this type of seal-coating occurs later in the densification process when the density of the porous structures are higher.  
       [0011] Another problem associated with nonuniform carbon deposition is the formation of undesirable carbon microstructures. For example, in the case of high performance carbon/carbon brake disks, a rough laminar carbon microstructure is preferred because of the friction and thermal characteristics of this microstructure. However, when the residence time of the gas flow is prolonged or the gas flow stagnates in obstructed areas, smooth laminar and dark laminar carbon microstructures may form instead. As known by those in the art, smooth and dark laminar microstructures are generally undesirable because brake disk performance is reduced unless the outer surfaces containing the undesirable microstructures are machined off in subsequent operations.  
       BRIEF SUMMARY  
       [0012] Accordingly, a hardware assembly is provided for controlling the flow of gas through and around a stack of annular porous structures in a furnace during CVI/CVD processes. In one embodiment, a distributor is provided which separates the gas into a first portion and a second portion. The first portion of gas passes to a center opening region formed by the stack of annular porous structures. The second portion of gas passes to the outer region around the stack of annular porous structures. Most of the first portion of gas is blocked from completely passing out the top end of the center opening region by a plate and a cap. Some of the first portion of gas, however, is allowed to pass out the top end of the center opening region through holes in the cap and through an open passage between the top annular porous structure and the plate. In another embodiment, the first portion passes to the center opening region and the second portion passes to the outer region without the use of a separate distributor.  
       [0013] In another embodiment, an open space is provided between a floor plate and a bottom base plate. Most of the gas flows from the open space as a second portion to the outer region, while some of the gas flows from the open space as a first portion to the center opening region. Most of the second portion flows through open passages between adjacent porous structures thereby commingling with the first portion. The commingled first and second portions pass out of the center opening region through an exit hole adjacent one end of the stack of porous structures.  
       [0014] A method of densification is also provided with two densification processes. The flow of gas between the center opening region and the outer region is reversed in the two densification processes. In one embodiment, one of the densification processes is a rapid densification process. 
     
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
     [0015] The invention, including its construction and method of operation, is illustrated more or less diagrammatically in the drawings, in which:  
     [0016]FIG. 1 is a side cross sectional view of a furnace, showing a hardware assembly;  
     [0017]FIG. 2 is a perspective view of the furnace, showing the top of the furnace open and a portion of the furnace wall broken away to show the hardware assembly;  
     [0018]FIG. 3 is a perspective view of a hardware assembly similar to the hardware assembly shown in FIG. 2;  
     [0019]FIG. 4 is an exploded perspective view of a furnace floor plate, a number of distributors and a hardware assembly base plate;  
     [0020]FIG. 5 is a side view of one of the distributors;  
     [0021]FIG. 6 is a top plan view of the distributor;  
     [0022]FIG. 7 is a side cross section view of a cap;  
     [0023]FIG. 8 is a top plan view of the cap;  
     [0024]FIG. 9 is a side cross sectional view of a furnace, showing an alternative hardware assembly;  
     [0025]FIG. 10 is a side cross sectional view of a furnace, showing another alternative hardware assembly; and  
     [0026]FIG. 11 is a side cross sectional view of a hardware assembly for a rapid densification process. 
    
    
     DETAILED DESCRIPTION  
     [0027] Referring now to the drawings, and particularly to FIGS. 1 through 4, a hardware assembly  32  with gas distributors  24  is provided for chemical vapor infiltration and deposition (“CVI/CVD”) processes. The hardware assembly  32  is especially useful for carbon densification of annular porous structures  2  used for high performance brake disks  2  but is likely to be useful in many other applications for composite structures as well. Typically, the hardware assembly  32  supports and positions a number of brake disks  2  which are stacked on top of each other in a number of stacks  4 . During the densification process, the hardware assembly  32  and stacks  4  of disks  2  are enclosed in a furnace  10  and hot hydrocarbon gases flow around and through the stacks  4  of brake disks  2 , thereby depositing a carbon matrix within the interior regions and on the surface of the porous brake disk structures  2 . A variety of processing parameters may be used to densify the porous structures  2 , but an absolute gas pressure for the furnace  10  between 1 torr and 20 torr, a temperature range between 1,700° F. and 1,950° F., and a densification time between 150 hours and 500 hours is especially suited for manufacturing aircraft brake disks  2 . A variety of gas mixtures may also be used, but a gas mixture of 87% natural gas and 13% propane or a range of 80% to 100% natural gas and 20% to 0% propane is especially suited for manufacturing aircraft brake disks.  
     [0028] A number of different types of furnaces  10  may be used for CVIICVD processes. Commonly, an induction furnace  10  is used that includes furnace walls  12  that enclose the hardware assembly  32  and the stacks  4  of brake disks  2 . The furnace  10  also includes inlet ducts  14  and outlet ducts  16  for introducing and exhausting the gas mixture into and out of the furnace  10 . A preheater  18  is also commonly provided within the furnace  10  to heat the gas before the gas is directed to the porous structures  2 . Typically, the preheater  18  is sealed and the incoming gas from the inlet ducts  14  is received by the preheater  18  before being introduced to the hardware assembly  32 . The preheated gas is then discharged from the preheater  18  through discharge openings  20  in the furnace floor plate  22  of the preheater  18 .  
     [0029] At least one distributor  24  is provided at the preheater discharge openings  20  for controlling the flow of gas around the stacks  4  of brake disks  2 . Preferably, the distributors  24  are removably mounted between the floor plate  22  of the preheater  18  and the base plate  46  of the bottom hardware assembly modules  34 . To aid installation of the distributors  24 , recessed areas  19 ,  45  with guide diameters  21 ,  47  are provided in both the top surface of the floor plate  22  and the bottom surface of the hardware assembly base plate  46 . The recessed areas  19  in the floor plate  22  are generally concentric with each of the discharge openings  20 , and the recessed areas  45  in the hardware assembly base plate  46  are generally concentric with each of the inlet openings  53 . Therefore, the distributors  24  may be easily installed by inserting the outer diameter  26  of each distributor  24  into one of the guide diameters  21  in the floor plate  22  and one of the guide diameters  47  in the base plate  46 .  
     [0030] The distributor  24  (also shown in FIGS. 5 and 6) divides the gas from the preheater  18  into at least two different portions and directs the portions in different directions. Accordingly, the distributor  24  includes an axial hole  28  that extends longitudinally through the distributor  24 . Thus, a first portion of gas flows through the axial hole  28  from the preheater discharge opening  20  to the hardware assembly inlet opening  53 . The distributor  24  also includes a number of radial holes  30  that extend out from the axial hole  28  to the outer diameter  26  of the distributor  24 . Thus, a second portion of gas flows out of the distributor  24  through the radial holes  30  to the space between the floor plate  22  and the bottom base plate  46 . Other equivalent passageways, such as grooves or the like, formed into the floor plate  22 , the bottom base plate  46 , the distributor  24 , or other hardware member may also be used in place of the radial holes  30 . In one exemplary embodiment, the first portion that flows through the inlet opening  53  of the bottom base plate  46  represents about 76% of the gas mixture, and the second portion that flows out through the radial holes  30  represents about 24% of the gas mixture. In this embodiment, the first gas flow portion is restricted by the inlet opening  53 , which is about 5 inches in diameter, in the bottom base plate  46 . The second gas flow portion is then restricted by the radial holes  30 , which consist of eight holes about  1  inch in diameter. Other proportions for the first portion and second portion may also be advantageous, and other sizes and placement of the inlet opening  53 , axial hole  28  and radial holes  30  may be used. For example, the range of flow through the inlet opening may be as low as 20% to as much as 80%, while the range of flow into the space between the floor plate  22  and the bottom base plate  46  may be as high as 80% to as low as 20%. Typically, the preferred embodiment uses a proportion of about 80% for the first portion and about 20% for the second portion, but proportions between 70% and 90% for the first portion and 30% to 10% for the second portion and between 60% to 80% for the first portion and 40% to 20% for the second portion, or vice versa, may be used.  
     [0031] A typical hardware assembly  32  preferably consists of a number of separate modules  34 ,  36  to make assembly, disassembly, loading and unloading of the hardware assembly  32  easier. Accordingly, as shown in FIG. 1, the hardware assembly  32  includes a bottom set of modules  34  with three units  38 . A unit  38  usually refers to the area between an adjacent base plate  46 ,  48  and a support plate  50  or between adjacent support plates  50 ,  52  where one level of brake disk stacks  4  is supported. Support posts  40  separate the base plates  46 ,  48  and support plates  50 ,  52 , thereby forming each unit  38 . The hardware assembly  32  also includes a top set of modules  36  similar to the bottom set  34  with two units  38 . As shown in FIG. 2, the top and bottom sets of modules  34 ,  36  also include a center module  42  with typically four stacks  4  of brake disks  2  and a number of arc-shaped outer modules  44  with two or more stacks  4  of brake disks  2  each, although different configurations may be used. Accordingly, each of the modules  34 ,  36 ,  42 ,  44  may be loaded into the furnace  10  one at a time, leaving approximately 1 inch gaps  74  between the outer modules  44  and between the outer modules  44  and the center modules  42 . Typically, the base plates  46 ,  48  and support plates  50 ,  52  are usually referred to as single base plates  46 ,  48  and single support plates  50 ,  52  for simplicity even though the base plates  46 ,  48  include separate center plates  66  and outer plates  68  and the support plates  50 ,  52  include similar separate center plates  70  and outer plates  72 . Preferably, each of the components of the hardware assembly  32  and the distributor  24  are made from a graphite (e.g., HTM or HLM graphite) material that is compatible with typical CVI/CVD processes used for manufacturing carbon/carbon brake disks  2 .  
     [0032] The porous brake disk structures  2  are loaded into the hardware assembly  32  in stacks  4 , with each brake disk  2  being separated from adjacent brake disks  2  with spacers  6  about 0.125 to 1.0 inch thick. Therefore, open passages  8  are formed between adjacent brake disks  2 . Similarly, the top brake disk  3 ,  9  in each unit  38  is spaced about 0.125 to 1.0 inch from the bottom surface of the adjacent support plate  50 ,  52  to form another open passage  8 . The stacks  4  of porous brake disk structures  2  are also positioned within the hardware assembly  32  with the center openings  5  of the annular brake disks  2  coaxial with the inlet openings  53  in the bottom base plate  46  and with the transfer openings  54  in the support plates  50  and top base plate  48 .  
     [0033] Caps  56  (also shown in FIGS. 7 and 8) are installed into the transfer openings  54  of the top support plate  52  of the top module  36  in order to restrict gas flow through the top of the stacks  4 . Each of the caps  56  include an extended portion  58  that extends down into the center openings  5  of the top brake disks  9 . Four longitudinal holes  60  are also provided through the caps  56  to allow some gas flow to escape upward from the center openings  5  of the stacks  4 . Thermocouple wires  7  may also be routed through the holes  60  in the caps  56  and down through the center openings  5  in the stacks  4 . The thermocouple wires  7  are then connected to thermocouples embedded in sample brake disks (not indicated) at various heights in the stacks  4  to measure the representative temperature of the brake disks  2 .  
     [0034] It is now apparent that the gas flow through the hardware assembly  32  is more uniform and beneficial compared to other densification processes. Thus, higher quality parts (i.e., with a more uniform and more desirable microstructure) may be produced with lower manufacturing costs. Accordingly, a gas mixture is supplied to the inlet ducts  14 , while a vacuum is produced at the outlet ducts  16 . The gas is then drawn through the preheater  18 , thereby raising the temperature of the gas. Next, the gas exits the preheater  18  through the discharge openings  20  in the floor plate  22 , thereby passing into the axial hole  28  of each of the distributors  24 . The gas is then separated into a first portion of about 76% of the gas and a second portion of about 24% of the gas. The first portion passes through the axial hole  28  in the distributor  24  and through the inlet opening  53  in the hardware assembly base plate  46 . The second portion passes out through the radial holes  30 .  
     [0035] The first portion of gas passes up through the center opening region  5  in the stacks  4  of annular porous structures  2 . The gas passes to adjacent stacks  4  in the adjacent units  38  through the transfer openings  54  in the support plates  50  and the top base plate  48 . The gas also passes out from the center opening region  5  through the open passages  8  between the adjacent brake disks  2 . A controlled pressure is maintained in the center opening region  5  by the caps  56  which block and restrict the gas from completely flowing out from the center opening  5  in the top brake disk  9  of the hardware assembly  32 . However, some gas flow is permitted through the center opening  5  of the top brake disk  9  to avoid stagnation of the gas near the top of the stacks  4 . Accordingly, some gas flows out through the longitudinal holes  60  in each of the caps  56 , and some gas flows out the open passage  8  between the top brake disk  9  and the top support plate  52 .  
     [0036] The second portion of gas exits the radial holes  30  in the distributor  24  and passes to the open space  23  between the floor plate  22  and the hardware assembly base plate  46 . The gas then passes up into the hardware assembly  32  through passage holes  62  in the center plate  66  and the outer plates  68  of the bottom base plate  46 . The gas also passes up through the gaps  74  between the center plate  66  and the outer plates  68  and between each of the outer plates  68 . Thus, the gas passes up along the outer region  11  around the outer surfaces of the stacks  4 . The gas passes through the units  38  by passing through passage holes  62  and gaps  74  in the support plates  50  and the top base plate  48 . As the second portion of gas passes up through the hardware assembly  32 , it combines with the first portion of gas from the center opening region  5  as the gas passes out through the open passages  8 . When the gas reaches the top of the hardware assembly  32 , the gas passes out of the hardware assembly through passage holes  62  and gaps  74  in the top support plate  52 . Both portions of gas then exit the furnace  10  through the outlet ducts  16 . Thus, it is apparent that the hardware assembly  32  and distributor  24  minimize gas stagnation zones. Therefore, the related problems typically associated with gas stagnation zones are avoided, such as soot and other oily substance accumulations, seal-coating, nonuniform carbon deposition and undesirable microstructures.  
     [0037] As shown in FIG. 9, the flow of gas through the hardware assembly  80  may also be controlled between a first portion and a second portion without using the distributors  24  and caps  56 . In this alternative arrangement, the bottom base plate  82  rests directly on top of the furnace floor plate  22 . The inlet openings  84  include a lower, larger diameter hole  86 . The radial holes  90  extend through the base plate  82  from the lower, larger diameter holes  86  to the gaps  74  between the outer base plates  81  and between the outer base plates  81  and the center base plate  83  and to the outer edge of the outer base plates  81 . Small holes  94  are also provided through the top support plate  92 .  
     [0038] The gas flow through the alternative hardware assembly  80  is now apparent. Like the hardware assembly  32  previously described, the hot reactant gas enters through the inlet ducts  14  and passes through the preheater  18 . The gas then exits the preheater  18  through the discharge openings  20  and passes directly into the lower, larger diameter hole  86  of the inlet opening  84 . Next, a first portion of gas passes through the upper, smaller diameter hole  88  in the inlet opening  84 . A second portion of gas also passes through the radial holes  90 . Accordingly, as previously described with respect to the first hardware assembly  32 , the first portion of gas then passes up through the center opening region  5 , while the second portion of gas passes up along the outer region  11 . As the first portion of gas passes up through the center opening region  5 , most of the first portion passes out to the outer region  11  through the open passages  8  between adjacent brake disks  2  and commingles with the second portion. Some of the first portion, however, passes up through the entire center opening region  5  and exits the hardware assembly  80  through the small holes  94  in the top support plate  92 . The remaining commingled gas then exits the hardware assembly  80  through the gaps  74  between the plates  70 ,  72  and along the outside of the hardware assembly  80 .  
     [0039] Turning now to FIG. 10, another alternative hardware assembly  100  is shown for flowing most of the gas from the outer region  11  to the center opening region  5 . In this hardware assembly  100 , spacers  102  are provided between the floor plate  22  and the bottom base plate  104 . The spacers  102  may be round or square members and do not restrict gas flow through the space  106  between the floor plate  22  and the bottom base plate  104 . The inlet openings  108  in the bottom base plate  104  are also smaller in size than the discharge openings  20  in the floor plate  22  to restrict flow through the inlet openings  108 .  
     [0040] If desired, the top unit  38 , which is shown in the previous hardware assemblies  32 ,  80 , may be removed in this alternative hardware assembly  100 . The top stack  4  of brake disks  2  is then stacked so that the top brake disk  9  is spaced away from the bottom surface  112  of the susceptor lid  110  with an open passage  116  therebetween. Preferably, the open passage  116  is no more than  1  inch wide although larger widths may also be used. Spacer rings, well known to those in the art, may be used to achieve a desired width for the open passage  116 . Exit holes  118  are provided through the susceptor lid  110 , or comparable plate, directly above each of the stacks  4 . Small holes  120  through the susceptor lid  110  may also be provided away from the exit holes  118 . The susceptor lid  110  is supported by and sealed to the susceptor walls  114  (the susceptor walls  114  and susceptor lid  110  are not shown in FIGS. 1 and 9 for simplicity).  
     [0041] It is now apparent that in the alternative hardware assembly  100  most of the gas flows from the outer region  11  towards the center opening region  5  instead of from the center opening region  5  towards the outer region  11  as described in the previous hardware assemblies  32 ,  80 . Like the previous hardware assemblies  32 ,  80 , hot reactant gas enters through the inlet ducts  14  and passes through the preheater  18 . The gas then exits the preheater  18  through the discharge openings  20  and passes into the space  106  between the floor plate  22  and the bottom base plate  104 . Most of the gas flows as a second portion through the space  106  between the floor plate  22  and the bottom base plate  104 . The second portion then flows up through the hardware assembly  100  along the outer region  11 . The second portion passes through the units  38  by passing through the passage holes  62  (see FIG. 2) and the gaps  74  in the base plates  66 ,  68  and support plates  70 ,  72  as previously described. On the other hand, some of the gas flows as a first portion through the inlet openings  108 . The first portion then flows up through the center opening region  5 . As the first and second portions pass upwards, some of the second portion passes through the open passages  8  between adjacent brake disks  2  towards the center opening region  5 , thus commingling with the first portion of gas. The remaining second portion in the outer region  11  passes through the open passage  116  between the top brake disk  9  and the bottom surface  112  of the susceptor lid  110 . When the small holes  120  are not provided, the entire first and second portions then pass through the exit holes  118  in the susceptor lid  110  and out through the outlet ducts  16 . On the other hand, when the small holes  120  are provided, some of the second portion passes out through the small holes  120 , while the rest of the second portion passes through the exit holes  118 . A number of proportions for the first and second portions may be used, including 85% for the second portion and 15% for the first portion, a range of 65% to 85% for the second portion and 35% to 15% for the first portion, or other proportions previously described.  
     [0042] Accordingly, one advantage of the hardware assemblies  32 ,  80 ,  100  is that successive densification processes may flow reactant gas in opposite directions, that is, from the center opening region  5  to the outer region  11  in one densification process and from the outer region  11  to the center opening region  5  in another densification process. As is well understood by those in the art, densification of porous structures typically involves several successive densification processes. Sometimes, the processing parameters, such as the gas mixture, temperature and pressure, are varied during each of the successive processes in order to optimize the quality of the finished densified product. Machining operations may also be performed on the structures between densification processes.  
     [0043] One way that has been discovered to improve the quality of densification is to reverse the direction of gas flow from prior densification processes. This may be achieved, for example, by using one of the hardware assemblies  32 ,  80  shown in FIGS. 1 and 9 in a first densification process and then using the hardware assembly  100  shown in FIG. 10 in a second densification process. Thus, in the first process most of the gas flows from the center opening region  5  towards the outer region  11 , and in the second process most of the gas flows oppositely from the outer region  11  towards the center opening region  5 . The processes may also be reversed to flow the gas from the outer region  11  to the center opening region  5  in the first process and from the center opening region  5  to the outer region  11  in the second process. Additional benefits may be achieved by combining a densification process using one of the hardware assemblies  32 ,  80 ,  100  with a rapid densification process, or sometimes referred to as a pressure gradient CVI/CVD process, where the open passages  8  are sealed to restrict gas flow. As shown in FIG. 11, a typical hardware assembly  130  for a rapid densification process includes a base plate  132  with an inlet opening  134 . The top of the hardware assembly  130  is sealed with a top plate  136 . The open passages are then also sealed with spacer rings  138 . Therefore, the reactant gas is forced to flow through the interior region of the brake disks  2  from the center opening region  5  to the outer region  11  or vice versa. Accordingly, a rapid densification process that flows the gas from the center opening region  5  to the outer region  11  may be combined with the hardware assembly  100  shown in FIG. 10 which flows the gas in reverse from the outer region  11  to the center opening region  5 . This combination decreases the total time required to completely densify a product while providing a high quality of densification. As previously described, the reactant gas may also be separated into first and second portions that flow separately to the center opening region  5  and the outer region  11  in either one or both of the first and second densification processes. Moreover, substantially all of the reactant gas may flow from the center opening region  5  to outer region  11 , or vice versa, in either one or both of the first and second densification processes.  
     [0044] While a preferred embodiment of the invention has been described, it should be understood that the invention is not so limited, and modifications may be made without departing from the invention. The scope of the invention is defined by the appended claims, and all devices that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.