Abstract:
A method of densifying porous substrates by chemical vapor infiltration comprises loading porous substrates for densification in a loading zone of an enclosure ( 10 ), heating the internal volume of the enclosure, and introducing a reagent gas into the enclosure though an inlet situated at one end of the enclosure. Before coming into contact with substrates ( 20 ) situated in the loading zone, the reagent gas admitted into the enclosure is preheated, at least in part, by passing along a duct ( 30 ) connected to the gas inlet and extending through the loading zone, the duct being raised to the temperature inside the enclosure, and the preheated reagent gas is distributed in the loading zone through one or more openings ( 33 ) formed in the side wall ( 32 ) of the duct, along the duct.

Description:
This application is the entry of the U.S. national phase filing under 35 C.F.R. 371 of International Application No. PCT/FR03/00097 filed Jan. 14, 2003, and claims priority to French application 02 00412 filed Jan. 15, 2002. 
     BACKGROUND OF THE INVENTION 
     The invention relates to chemical vapor infiltration techniques. The field of application of the invention is densifying porous substrates, in particular making composite material parts by densifying fiber substrates by means of a matrix. 
     In conventional manner, a method of densifying substrates by chemical vapor infiltration comprises the steps of loading porous substrates to be densified into a loading zone of an enclosure, heating the internal volume of the enclosure, introducing a reagent gas in the enclosure through an inlet situated at one end thereof, and preheating the reagent gas after it has entered into the enclosure and before it comes into contact with the parts situated in the loading zone. 
     The temperature and the pressure that exist inside the enclosure are selected so as to enable the reagent gas to diffuse into the pores of the substrates and deposit therein the material for constituting the matrix, either by one or more components of the reagent gas decomposing, or else by a plurality of components reacting together. 
     The reagent gas is conventionally preheated by passing the gas through a preheater zone situated inside the enclosure and into which the reagent gas inlet opens out. A conventional preheater zone comprises a plurality of perforated plates disposed one above the other and raised to the temperature inside the enclosure. 
     The purpose of preheating the reagent gas is to ensure that when it enters into the loading zone it is at a temperature that is as close as possible to the temperature required for forming the desired matrix. When the reaction temperature is typically about 1000° C. in order to form a matrix of pyrolytic carbon or of ceramic, having the reagent gas at a temperature that is only a few tens of ° C. below the desired temperature can have a significant effect on the rate of densification and on the microstructure of the deposited matrix material. 
     This has been observed particularly in the case of densifying substrates disposed in stacks, in particular substrates of annular shape for making brake disks out of composite material. Methods and installations for densifying annular substrates in stacks are described in documents U.S. Pat. No. 5,904,957 and EP 0 792 385. The reagent gas coming from the preheater zone is admitted into the internal volumes of the stacks which are made up of superposed annular substrates and which extend vertically in the loading zone above the preheater zone, with the reagent gas inlet being situated at the bottom of the enclosure. A densification gradient is observed between the substrates situated at the bottoms of the stacks and the other substrates, which gradient becomes greater the more insufficient the preheating of the reagent gas. 
     The problem could be solved by increasing the volume of the preheater zone. However, for a given total enclosure volume, that would reduce the space available into which substrates can be loaded. Unfortunately, the processes of densification by chemical vapor infiltration are lengthy and expensive to implement, so installations need to have their loading capacities used to the full. 
     In addition, the reagent gas reaching the tops of the stacks has traveled through them along their full height and has matured, such that the substrates situated at the top of the stacks receive a reagent gas of composition that may be different from that of the reagent gas on entering into the loading zone. This also can give rise to densification characteristics that are different. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method enabling the distribution and the preheating of the reagent gas to be improved, and more generally enabling densification gradients between substrates situated at different locations in the loading zone to be reduced, and to achieve this without decreasing loading capacity, and possibly even while increasing it. 
     This object is achieved by a method as defined in the introduction to the description and in which the reagent gas admitted into the enclosure is preheated, at least in part, by passing along a duct connected to the gas inlet and extending through the loading zone, the duct being raised to the temperature inside the enclosure, and the preheated reagent gas is distributed into the loading zone through one or more openings formed in the side wall of the duct, along the length thereof. 
     Thus, the duct serves both to preheat the reagent gas and to distribute it in the loading zone. 
     The reagent gas may be distributed via one or more slots extending longitudinally through the side wall of the duct. 
     In a variant, the reagent gas may be distributed in the enclosure via a plurality of perforations formed through the side wall of the duct. 
     In order to enhance preheating, the reagent gas advantageously flows inside the duct while making contact with walls forming heat exchanger surfaces that extend into the inside of the duct. 
     When densifying annular substrates placed in the loading zone in at least one vertical stack, the reagent gas admitted into the enclosure is advantageously preheated and distributed by passing along a duct extending vertically inside the stack. 
     The reagent gas is then preferably distributed solely via openings formed in the side wall of the duct. 
     Another object of the invention is to provide an installation enabling the above-defined method to be implemented. 
     This object is achieved by an installation comprising an enclosure inside which there is a zone for loading substrates to be densified, a susceptor defining the enclosure and associated with means for heating the enclosure, a reagent gas inlet at one end of the enclosure, and means situated inside the enclosure for preheating the reagent gas, in which installation a duct is connected to the reagent gas inlet inside the enclosure and extends through the loading zone, the duct being provided along its length with lateral openings which open out into the loading zone in order to distribute the reagent gas therein. 
     In an embodiment, the openings are in the form of at least one longitudinal slot. The wall of the tube may then be formed by a plurality of panels leaving longitudinal gaps between one another. 
     In another embodiment, the openings are in the form of perforations distributed along the duct. 
     Advantageously, walls are disposed inside the duct. These internal walls can then be in the form of longitudinal panels that leave gaps between one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description given by way of non-limiting indication and made with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic elevation view in section showing an installation for densification by chemical vapor infiltration in an embodiment of the invention; 
         FIG. 2  is a fragmentary cross-section view on a larger scale showing more particularly the duct for preheating and distributing the reagent gas in the  FIG. 1  installation; 
         FIG. 3  is a cross-section view showing a variant embodiment of the duct for preheating and distributing the reagent gas; 
         FIG. 4  is a diagrammatic elevation view in section showing an embodiment of a prior art installation for densification by chemical vapor infiltration; 
         FIG. 5  is an elevation view showing another embodiment of a duct for preheating and distributing the reagent gas; 
         FIG. 6  is a cross-section view of the  FIG. 5  duct; 
         FIG. 7  is a diagrammatic elevation view in section showing an installation for densification by chemical vapor infiltration constituting another embodiment of the invention; and 
         FIGS. 8 and 9  are diagrammatic elevation views in section showing her applications of an installation of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a diagram of an enclosure  10  containing a load of porous substrates  20 . By way of example, the substrates  20  are carbon fiber preforms or blanks constituted by pre-densified preforms, which preforms or blanks are for use in making brake disks of carbon/carbon (C/C) composite material by being densified with a matrix of pyrolytic carbon. 
     The load is in the form of a stack of substrates defining an inside volume  21  formed by the central passages in the vertically-aligned substrates. The stack is carried by a bottom support plate  11  standing on legs  12   a . It may be made up of a plurality of superposed sections that are separated from one another by one or more intermediate support plates  13 . The plate  11  is provided with an opening  11   a  which is in axial alignment with the central passages through the substrates  20  and with openings  13   a  in the intermediate plates  13 . At its top, the stack of substrates is provided with a cover  22  closing the internal volume  21 . The plates  13  are supported by the support plate  11  via columns or posts  12   b.    
     Each substrate  20  is separated from an adjacent substrate, and where appropriate from an adjacent plate  11  or  13  or the cover  22  by one or more spacers  23  which define gaps  24  (see  FIGS. 1 and 2 ). The spacers  23 , which are disposed radially for example, are arranged to form passages that put the internal volume  21  into communication with an external volume  25  situated inside the enclosure and outside the stack. 
     The passages left between the spacers  23  may be dimensioned in such a manner as to balance pressures between the volumes  21  and  25 , as described in document U.S. Pat. No. 5,904,957. In a variant, they may constitute leakage passages providing a flow section that is small so as to allow a pressure gradient to exist between the volumes  21  and  25 , as described in French patent application No. 01/03004. 
     The enclosure is heated by means of a susceptor  14  which defines the sides of the enclosure. By way of example, the heater plate is constituted by an inductor inductively coupled with an induction coil  15 . The coil  15  surrounds the enclosure and is separated from the susceptor  14  by a wall  16  that provides thermal insulation. In a variant, the susceptor may be heated by means of electrical resistances thermally coupled therewith. 
     A reagent gas containing one or more constituents that are precursors of carbon is introduced into the enclosure through an opening  17   a  formed in the bottom  17  of the enclosure. The precursors are gaseous hydrocarbons, typically methane, propane, or a mixture thereof. In the gap between the bottom  17  and the plate  12 , the reagent gas is channeled by a cylindrical wall  18  interconnecting the openings  17   a  and  11   a.    
     A vertical tubular duct  30  has its bottom end connected to the opening  11   a  and extends vertically inside the volume  21  to the immediate vicinity of the top of the stack of substrates. At its top end, the duct  30  is closed by a cover  31 . The duct  30  may be made up of a plurality of sections connected end to end so as to enable it to be built up in modular manner. 
     In the example shown in  FIGS. 1 and 2 , the duct  30  has its side wall  32  provided with a plurality of openings  33  in the form of perforations which are distributed both along the length of the duct  30  and around the axis thereof. 
     Thus, the reagent gas admitted into the enclosure is distributed into the internal volume  21  by passing through the openings  33  in the duct  30  and passes from the volume  21  to the volume  25  by diffusing through the substrates  20  and passing through the passages left between the spacers  23 . The residual gas is extracted from the enclosure  10  via an opening  19   a  formed through the cover  19  of the enclosure and connected to suction means (not shown). 
     The duct  30  serves not only to distribute the reagent gas over the full height of the stack, but also to preheat this gas, the duct  30  being raised to the temperature that exists inside the enclosure. 
     In order to improve preheating, internal heat exchanger walls may be disposed inside the duct  30 . In the embodiment of  FIG. 3 , these inside walls are in the form of longitudinal panels  35  distributed around the axis of the duct and leaving gaps  36  between one another. 
     The duct  30 , the cover  31 , and any internal walls  35  are made of graphite, for example. Other materials could be used, for example a C/C composite material. The walls  14 ,  17 ,  19  of the enclosure  10  are advantageously made of graphite. The plates  11 ,  13 , the cover  22 , the spacers  23 , and the wall  18  are made, for example, out of graphite or out of C/C composite material. 
     In comparison with a prior art installation having a preheater zone  1  between the reagent gas inlet and the plate  11  on which the stack stands (see  FIG. 4 ), the installation of  FIGS. 1 and 2  does not have a preheater zone, thereby providing significantly increased loading capacity. The loading zone of the enclosure  10  which extends above the plate  11  is greater than the loading zone in the installation of  FIG. 4 , with the preheater zone and the perforated plates  2  situated one above another occupying a relatively large amount of space in that installation. 
     Nevertheless, it should be observed that it is possible for a preheater zone to be present in the context of the invention, which zone can be smaller than those of prior art installations. 
     The diameter of the duct  30  must be large enough to be capable of providing a large area for heat exchange, while nevertheless being spaced apart from the stack of substrates  20 . 
       FIGS. 5 and 6  show a variant embodiment of a duct  40  for preheating and distributing reagent gas that can take the place of the duct  30  in the installation of  FIGS. 1 and 2 . 
     The side wall  42  of the duct  40  has openings  43  in the form of longitudinal slots that extend over the entire length of the duct, the duct being closed by a cover  41  at its top end. In the example shown, the slots  43  are rectilinear and they are regularly distributed around the axis of the duct  40 . 
     The slots  43  are formed by gaps between longitudinal panels  44  that make up the side wall  42  of the duct  40 . Additional internal walls for heat exchange purposes are disposed inside the duct  40 . As in the embodiment of  FIG. 3 , these internal walls are in the form of longitudinal panels  45  distributed around the axis of the duct and leaving gaps  46  between one another. The panels  44  and  45  are disposed in a staggered configuration around the axis of the duct  40  so that each gap  46  opens out facing a panel  44  between two slots  43 . 
     Naturally, the slots could follow paths other than rectilinear paths, for example they could follow helical paths from the bottom to the top of the duct. 
     In general, it is possible to give any desired shape to the openings formed in the side wall of the duct, for example oblong shapes or elongate openings extending axially, circumferentially, or obliquely. 
     In the embodiment of  FIGS. 1 and 2 , a single stack of substrates  20  is shown. In a variant, a plurality of stacks of substrates could be placed side by side inside the enclosure. In which case, a respective duct for preheating and distributing reagent gas is placed inside each stack and is connected to a common inlet for the reagent gas, or preferably to a particular inlet in alignment with the duct. 
     It should also be observed that the flow direction of the reagent gas may be reversed, with a gas inlet being formed through the cover of the enclosure and an outlet formed in the bottom which is spaced apart from the plate supporting the stack, with the central passage of the stack then being closed at its bottom end. 
     As shown in  FIG. 7 , and in the same manner as shown in  FIG. 1 , the stack of annular substrates  120  is received in an enclosure  110  defined laterally by a susceptor  114  inductively coupled with an induction coil  115 , there being insulation  116  disposed between them. The stack of substrates  120  is formed by a plurality of sections that are superposed and separated from one another by one or more intermediate plates  113 , and standing on the bottom plate  111  which does not have a central opening so as to close the stack. 
     At its top end, the stack is surmounted by a cover  122  provided with a central opening  122   a  in axial alignment with the internal volume  121  of the stack. 
     Between its inlet into the enclosure  110  through the cover  119  and the central opening  122   a , the admitted reagent gas is channeled by a cylindrical wall  118  which may optionally surround a small gas preheater zone. 
     A vertical tubular duct  130  has its top end connected to the opening  122   a  and extends down to the plate  111  which closes the bottom end of the duct. The duct  130  may be similar to the duct  30  or the duct  40  described above. In the example shown, the duct  130  has a wall  132  provided with a plurality of openings  133  that are distributed along the length and around the axis of the duct. 
     The reagent gas admitted into the enclosure is distributed in the internal volume  121  of the stack of substrates by passing through the openings  133 . The gas passes from the volume  121  to the volume  125  outside the stack of substrates by diffusing through the substrates  120  and by passing through the passages left between spacers interposed between the substrates. The residual gas is extracted from the enclosure through the central opening  117   a  in the bottom  117  of the enclosure. 
     Otherwise, the installation is similar to that of  FIG. 1 . 
     The method and the installation of the invention can be used for densifying porous substrates other than brake disk preforms, for example for substrates constituting preforms  220  for the diverging portions of rocket engines, as shown in  FIG. 8 . 
     A plurality of substrates  220  are disposed in the same loading zone of an enclosure  210  with their axial passages in vertical alignment. The bottom substrate is carried by a plate  211  which stands on legs  212   a , while the other substrates stand on annular intermediate plates  213 . The plates  213  are supported by the support plate  211  via columns or posts  212   b.    
     With the central openings  213   a  in the plates  213  the internal volumes of the substrates  220  form the internal volume  221  of the stack of substrates. The volume  221  is closed by a cover  222  at its top end. Spacers  223  are interposed between the axial ends of the substrates  220  and the plates  211 ,  213 , thereby enabling passages to be left to put the volume  221  into communication with the volume  225  outside the substrates and inside the enclosure. 
     A duct  230  for preheating and distributing the reagent gas is connected at a bottom end to a central opening  211   a  of the plate  211 . The duct  230  extends vertically inside the volume  221  to the immediate vicinity of the top of the stack of substrates, where the duct  230  is closed by a cover  231 . 
     The side wall  232  of the duct  230  has openings  233 , e.g. in the form of perforations, the duct  230  being of the same type as the duct  30  in the embodiment of  FIGS. 1 and 2 . 
     Otherwise, the installation is identical to the embodiment of  FIGS. 1 and 2 . 
     The field of application of the invention is not limited to densifying substrates of annular shape or of hollow axially symmetrical shape. 
     Thus,  FIG. 9  shows an enclosure  310  having a bottom support plate  311  and a plurality of intermediate support plates  313  in a loading zone of the enclosure  310 . The plates  311  and  313  are provided with respective central openings  311   a  and  313   a  that are in alignment with an inlet for admitting reagent gas into the enclosure. 
     A vertical duct  330  for preheating and distributing the reagent gas has its bottom end connected to the opening  311   a  and extends vertically through the loading zone of the enclosure  310 , passing through the openings  313   a . At its top end situated in the vicinity of the top of the loading zone, the duct  330  is closed by a cover  331 . 
     The plates  311  and  313  are supported by legs  312   a  and by columns  312   b.    
     The plates  311 ,  313  support substrates for densifying  320  (not all of them are shown) which may be in a variety of shapes and sizes. 
     Otherwise, the installation is identical to that shown in  FIGS. 1 and 2 . 
     It should be observed that the method and the installation of the invention can be implemented for densifying porous substrates with matrices other than matrices of pyrolytic carbon, for example with ceramic matrices. Chemical vapor infiltration processes for ceramic matrices, e.g. made of silicon carbide (SiC), are well known. The composition of the reagent gas is selected as a function of the nature of the matrix that is to be deposited. 
     It should also be observed that the flow section offered by the openings passing through the side walls of the duct for preheating and distributing the gas may be distributed uniformly or otherwise along the height of the duct. A non-uniform distribution may be adopted specifically when the need for reagent gas is greater at certain levels of the tube than at other levels. This can be the case when the configuration of the load of substrates and/or the dimensions of the substrates vary along the height of the loading zone.