Abstract:
A gas preheater for a CVI furnace designed for the densification of annular porous substrates arranged in a plurality of vertical annular stacks of substrates, comprising: a sleeve made of heat conductive material resting upon the bottom wall of a susceptor and delimiting a gas preheating chamber, with a gas inlet opening in the gas preheating chamber; a heat exchange assembly located in the gas preheating chamber; a gas distribution plate resting upon the sleeve, covering the gas preheating chamber and provided with a plurality of passages for preheated gas; a load supporting plate for supporting stacks of annular substrates and provided with a plurality of passages in communication with respective passages of the gas distribution plate and registration with internal volumes of respective stacks of annular substrates; and nozzles inserted in passages communicating the gas preheating zone with the internal volumes of respective stacks of annular substrates for adjusting the flows of preheated gas respectively admitted in said internal volumes.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to densification of porous annular substrates by chemical vapor infiltration (CVI). 
     A particular field of application of the invention is the making of annular parts in a thermostructural composite material, such as carbon/carbon (C/C) composite brake discs for airplanes or land vehicles. 
     Thermostructural composite materials are remarkable because they possess mechanical properties that enable them to be used for making structural parts and have the ability to conserve these properties at high temperatures. Typical examples of thermostructural composite materials are C/C composite materials having a reinforcing fibrous texture of carbon fibers densified by a pyrolytic carbon matrix, and ceramic matrix composites (CMCs) having a reinforcing texture of refractory fibers (carbon or ceramic) densified by a ceramic matrix. 
     In a CVI process, substrates to be densified are placed in a reaction chamber of a furnace in which they are heated. A reactive gas containing one or more gaseous precursors of the material that is to constitute the matrix is introduced into the reaction chamber. The temperature and pressure inside the reaction chamber are adjusted to enable the reactive gas to diffuse within the pores of the substrate and deposit the matrix-constituting material therein by one or more components of the reactive gas decomposing or reacting together. The process is performed under low pressure in order to enhance diffusion of the reactive gas into the substrates. The temperature at which the precursor(s) is transformed to form the matrix material, such as pyrolytic carbon or ceramic, is usually greater than 900° C., and is typically close to 1000° C. 
     In order to enable substrates throughout the reaction chamber to be densified as uniformly as possible, whether in terms of increasing density or in terms of microstructure of the matrix material deposited, it would ideally be necessary to have a substantially uniform temperature within the reaction chamber and to allow the reactive gas to reach all substrates relatively uniformly. 
     CVI furnaces usually include a gas preheater situated inside the furnace between the reactive gas inlet into the furnace and the reaction chamber. Typically, a gas preheater zone comprises a heat exchange assembly in the form of a plurality of perforated plates through which the reactive gas passes before entering the reaction chamber. 
     The substrates, like the heat-exchange assembly, are heated because they are located in the furnace. The latter is generally heated by means of a susceptor, e.g. made of graphite. The susceptor defines the side of the wall of the reaction chamber and is heated by inductive coupling with an inductor surrounding the reaction chamber or by resistors surrounding the furnace. 
     The applicants have found that the efficiency of the gas preheater is not always as good as desired. A significant example is that of densifying porous substrates constituted by annular preforms of carbon fibers or pre-densified annular blanks for use in making C/C composite brake disks. 
     The annular substrates are loaded in vertical stacks in the reaction chamber above the gas preheater which is situated at the bottom of the furnace. In spite of the reactive gas being preheated, a temperature gradient is often observed in the reaction chamber, with the temperature close to substrates situated at the bottom of the stacks possible being several tens of ° C. lower than the temperature that applies in the remainder of the stacks. This may give rise to a large densification gradient between the substrates in a same stack, depending on the position of a substrate within the stack. 
     In order to solve that problem, it would be possible to increase the efficiency with which the reactive gas is preheated by increasing the size of the gas preheater. However, for a given volume of the furnace, that would reduce the loading capacity for the substrates. Since CVI processes require large amounts of industrial investment and long processing time, it is highly desirable for furnaces to have the highest possible productivity, and thus as high as possible a ratio of volume dedicated to the load of substrates over the volume dedicated to preheating the reactive gas. 
     Another problem resides in the fact that a temperature gradient is observed not only in the vertical direction, along the stacks of substrates, but also in the horizontal direction, between different stacks. In particular, it has been noted that stacks located in a central part of the reaction chamber may not benefit from the heat radiated by the susceptor in the same way as stacks located closer to the internal side wall of the susceptor. 
     This also results in a gradient of densification between substrates belonging to different stacks. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the invention is generally to provide means for achieving an efficient and cost effective substantially uniform densification of porous annular substrates in a CVI furnace. 
     A particular object of the invention is to provide a gas preheater which allows such a substantially uniform densification to be achieved without significantly affecting the productivity of the CVI substrate. 
     According to one aspect of the invention, in a CVI furnace for the densification of annular porous substrates arranged in a plurality of vertical annular stacks of substrates, comprising a susceptor having an internal side wall delimiting a gas preheating zone and a reaction chamber within the furnace and a bottom wall, and at least one gas inlet opening through the bottom wall of the susceptor, a gas preheater is provided which comprises: 
     a sleeve made of heat conductive material resting upon the susceptor bottom wall and delimiting a gas preheating chamber, with the at least one gas inlet opening in the gas preheating chamber, 
     a heat exchange assembly located in the gas preheating chamber, 
     a gas distribution plate resting upon the sleeve, covering the gas preheating chamber and provided with a plurality of passages for pre-heated gas, 
     a load supporting plate for supporting stacks of annular substrates to be loaded in the reaction chamber for densification and provided with a plurality of passages in communication with respective passages of the gas distribution plate and in registration with internal volumes of respective stacks of annular substrates, and 
     nozzles inserted in passages communicating the gas preheating zone with the internal volumes of respective stacks of annular substrates for adjusting the flows of preheated gas respectively admitted in said internal volumes. 
     The sleeve, which is preferably formed of a massive body made in one piece of heat conductive material, achieves different functions: 
     resting upon the susceptor bottom wall and being thus surrounded by the susceptor side wall, it enables an efficient heating of the preheating zone to be reached, 
     it encloses the preheating zone and contributes to the sealing thereof, avoiding a large fraction of the reactive gas admitted to reach the reaction chamber without having fully passed through the gas preheater, and 
     it supports the load of substrates through the gas distribution plate and load supporting plate and transfers the weight to the susceptor bottom wall without the need for a separate supporting structure for the load supporting plate. 
     The above contributes to the efficiency of the gas preheating and compactness of the structure located at the bottom of the furnace. 
     The provision of flow adjusting nozzles which may be inserted in the passages of the gas distribution plate, makes it possible to feed stacks of substrates with a larger flow of reactive gas compared to other stacks of substrates. It is thus possible to compensate for a gradient of temperature between different stacks of substrates in order to achieve a substantially uniform densification. Indeed, the deposition rate of the matrix material varies as a function of the temperature and of the flow of reactive gas. 
     According to a particular aspect of the invention, the heat exchange assembly of the gas preheater comprises a plurality of spaced apart plates surrounded by the sleeve and extending substantially horizontally between the susceptor bottom wall and the gas distribution plate, the plates of the heat exchange assembly being made of a heat conductive foil material. The use of foil material such as graphite foil material or of C/C composite material makes it possible to reduce the thickness of the plates, hence the bulk of the gas preheater. The plates, which may be of a substantially circular form, are then preferably spaced apart by means of radially extending spacers interposed therebetween. 
     According to another particular aspect of the invention, the plates of the heat exchange assembly include at least one pair of plates located one immediately above the other in which one plate has perforations only in a central part thereof and the other plate has perforations only in the peripheral part thereof. Thus, the gas is forced to follow a tortuous path, whereby an efficient preheating may be achieved within a limited volume. The gas distribution plate and the load supporting plate may be formed of one and same plate, or of two different plates located one above the other. In the latter case, a plurality of ducts are provided each for connecting a passage of the gas distribution plate to a corresponding passage of the load supporting plate. Each duct may be provided with an insert made of a heat conductive material for achieving heat exchange with reactive gas flowing in the duct and thus completing preheating of the gas. 
     According to a further aspect of the invention, a process is provided for controlling distribution of preheated reactive gas in a CVI furnace for densification of annular porous substrate loaded in a reaction chamber of the furnace in a plurality of vertical stacks, each stack comprising superposed substrates defining an internal volume of the stack, the reaction chamber being heated by a susceptor having an internal wall delimiting the reaction chamber, 
     said process comprising admitting the reactive gas into a preheating zone at the bottom of the furnace, preheating the reactive gas by passing it through the preheating zone, dividing the preheated reactive gas into a plurality of separate flows at respective outlets of the preheating zone, and directing the separate flows of reactive gas into respective internal volumes of the stacks of annular substrates, 
     wherein the separate flows of reactive gas are adjusted as a function of the location of the corresponding stacks of substrates within the reaction chamber. 
     Preferably, the separate flow of reactive gas directed into the internal volume of a stack of substrates located farther from the internal wall of the susceptor than another stack of substrates is larger than the separate flow of gas directed into the internal volume of said another stack of substrates. 
     The separate flows of reactive gas may be adjusted by inserting nozzles having different cross-sections into passages formed in a gas-distribution plate covering a gas preheating chamber in the gas preheating zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will appear on reading the following description given by way of non-limiting indication and with reference to the accompanying drawings in which: 
     FIG. 1 is a highly diagrammatic sectional view of a CVI furnace having a gas preheater according to the invention, the section being on plane I—I of FIG. 2; 
     FIG. 2 is a highly diagrammatic partial sectional view on plane II—II of FIG. 1; 
     FIG. 3 is an enlarged diagrammatic partial sectional view of the gas preheater of the furnace of FIG. 1; 
     FIGS. 4 to  7  are partial diagrammatic sectional views on planes IV—IV, V—V, VI—VI, and VII—VII of FIG. 3; and 
     FIG. 8 is a diagrammatic sectional view showing a variant embodiment of a gas preheater according to the invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     FIGS. 1 and 2 show a furnace  10  having a cylindrical side wall  12  formed by a susceptor, with a susceptor bottom wall  14  and a susceptor top wall  16 . The susceptor  12  constitutes a secondary transformer circuit which is inductively coupled with a primary transformer circuit in the form of at least one induction coil  18 . Insulation  20  is interposed between the induction coil  18  and the susceptor  12  and further insulation  22  is provided under the susceptor bottom wall  14 . The furnace  10  is heated by feeding electrical current to the induction coil  18 . As a variant, heating of the susceptor can be carried out by means of electrical resistors thermally coupled thereto. 
     The internal volume of the furnace  10  comprises a gas preheating zone  24  located at the bottom of the furnace and a reaction chamber or loading zone  26  where porous annular substrates  30  to be densified are loaded, the reaction chamber  26  being located above the preheating zone  24 . 
     The substrates  30  to be densified may constitute carbon fiber preforms or pre-densified blanks for making brake disks out of C/C composite materials, the pre-densified blanks being obtained by pre-densification of preforms by CVI or by liquid (resin) impregnation followed by carbonization. Such C/C brake disks are commonly used for aircraft landing gears and for racing cars. 
     The annular substrates  30  are arranged so as to form a plurality of annular vertical stacks  32  resting on a bottom load-supporting plate  40 . Each stack of substrates may be subdivided into a plurality of superposed sections that are separated by one or more intermediate plates  42 , the plates  40  and  42  may be made of graphite. They have passages  40   a ,  42   a  formed therethrough in alignment with the internal passages of the substrates. The intermediate plates  42  are supported by plate  40  by means of posts  44 . 
     In the example shown (FIG.  2 ), 12 stacks of substrates are provided, with 9 stacks forming a ring of regularly spaced stacks located in proximity to the susceptor  12  and 3 stacks located in the central part of the loading zone. Other arrangements may be provided, for instance including 7 stacks of substrates with 6 stacks forming a peripheral ring and 1 central stack. 
     Each annular stack  32  is closed at the top by a cover  34 , whereby the internal volume of the reaction chamber  26  is subdivided into a plurality of stack internal volumes  36  and a volume  28  outside the stacks. Each stack internal volume is formed by the aligned central passages of the substrates  30  and intermediate plates  42 . 
     Each substrate  30  in a stack  32  is spaced apart from an adjacent substrate, or where appropriate from a plate  40 ,  42  or cover  34  by spacers  38  which leave gaps  39  between substrates. The spacers  38  may be arranged to leave passages for gas between the volumes  36  and  28  via the gaps  39 . These passages can be provided in such a manner as to ensure pressures in volumes  36  and  28  are in equilibrium, as described in U.S. Pat. No. 5,904,957 or in such a manner as to constitute simple leakage passages for maintaining a pressure gradient between the volumes  36  and  28 . 
     The gas heating zone  24 , which is surrounded by the susceptor, like the reaction chamber  26 , encloses a gas preheater assembly  50  shown in detail by FIGS. 3 to  6 . 
     The gas preheater assembly comprises a ring or sleeve  52  which rests on the susceptor bottom wall  14  and extends close to the susceptor side wall  12 . The sleeve  52  is preferably formed of a massive body made in one piece of heat conductive material such as graphite. 
     The sleeve  52  delimits a gas preheating chamber  54 . A passage  56  formed through bottom insulation  22  and bottom susceptor wall  14  constitutes an inlet for reactive gas opening into the gas preheating chamber  54 . Gas inlet  56  is connected to a reactive gas source (not shown). As a variant, several gas inlets may be provided, all opening into the gas preheating zone. Gas inlet  56  may be provided with a screen  58  opposing radiation of heat from the gas preheating chamber. 
     The gas preheating chamber  54  is covered by a gas distribution plate  60  supported by the sleeve  52  by resting upon its upper edge. The gas distribution plate has passages  60   a  formed therethrough in registration with the passages  40   a  and the internal volumes  36  of the stacks  32 . As shown by FIGS. 3 and 4, each passage is provided with an insert  62  in form of a nozzle of calibrated cross-section (the locations of stacks  32  are shown in chain-dotted lines in FIG.  4 ). 
     Gas admitted through inlet  56  is preheated within the preheating chamber  54  before reaching passages  60   a . Preheating is performed by forcing the gas to flow along and through a plurality of spaced apart perforated plates  66  extending horizontally between the susceptor bottom wall  14  and the gas distribution plate  60 . 
     The perforated plates  66  may be made of a heat conductive foil material such as a graphite foil material. Use of such thin perforated plates makes it possible to reduce the bulk of the gas preheater compared with massive perforated graphite plates. As an alternative, plates  66  may be made from C/C composite material. 
     The perforated plates  66  are maintained in a spaced apart relationship by means of spacers  68 , preferably in the form of radially extending graphite bars (also shown in broken lines in FIGS.  5  and  6 ). 
     Advantageously also, plates  66  include one or several pairs of plates  66   1 ,  66   2  which are located one immediately above the other, with one plate  66   2  having perforations  67   2  only in a central part thereof and the other plate  66   1  having perforations  67   1  only in a peripheral part thereof. Thus, the flow of gas is forced to flow not only across but also along the plates. The lower perforated plate is then preferably the one provided with perforations only in its peripheral part. 
     In order to ensure a relatively uniform output of preheated gas at the upper part of the gas preheating chamber, at least the upper perforated plate or the two upper perforated plates  66   3  are provided with perforations regularly distributed over their surface (FIG.  3 ). 
     The plates  66  are maintained in their desired horizontal positions by means of vertical rods  70  passing through holes  71  formed in the plates. The rods  70  are carried by a bottom non-perforated plate  72  having a central passage in registration with the gas inlet  56  and resting on an internal edge  52   a  provided at the lower part of the sleeve  52 . The sleeve  52 , with plates  66 ,  72  and rods  70  may thus be pre-assembled before insertion into the furnace. 
     Gas exiting through passages  60   a  of the gas distribution plate  60  is channeled through ducts, or chimneys  74  which are inserted into passages  76  formed in a holding plate  78  in registration with passages  60   a . The chimneys  74  have upper flanges resting upon the plate  78 , around passages  76 . Inserts  80 , for example in the form of dihedrons (FIGS.  3  and  7 ), are provided inside the chimneys  74  for further heating the gas flowing therethrough. Chimneys  74  and inserts  80  are made of a heat conductive material, such as graphite, as well as plates  60  and  78 . Plate  78  is supported by gas distribution plate  60  by means of posts  82 . 
     The chimneys  74  communicate with the passages  40   a  of the load supporting plate  40 . Rings  84  are inserted in passages  40   a  and rest upon the upper edge of chimneys  74  for channeling the flow of gas between plates  78  and  40 . Plate  40  is supported by plate  78  by means of posts  86 . 
     In operation, stacks of substrates are loaded into the reaction chamber, over the gas preheater. The weight of the load is supported by the susceptor bottom wall through plates  40 ,  78 ,  60 , posts  86 ,  82 , and sleeve  52 . The susceptor bottom wall  14  rests upon posts (not shown) which support the whole furnace. 
     The furnace is heated by the susceptor in order to bring the substrates loaded in the reaction chamber to the required temperature. The elements of the load supporting structure and of the gas preheater are similarly heated. 
     After the desired temperature within the furnace has been reached, reactive gas is admitted through gas inlet  56 . The gas is preheated by flowing along and across the perforated plates  66  in the preheating chamber  54 . Use of a massive graphite sleeve  52  having high thermal inertia and made in one piece contributes to an efficient heating and sealing of the gas preheating chamber. 
     The preheated gas leaves the preheating chamber  54  through nozzles  62  and is further heated by heat exchange with the walls of chimneys  74  and inserts  80 , before reaching the internal volumes of the stacks of substrates. 
     An efficient preheating of the reactive gas is thus achieved, minimizing the temperature gradient between the lower part of each stack and the rest of the stack. 
     The gas admitted into the internal volume  36  of a stack of substrates reaches volume  28  of the reaction chamber by diffusing through the porosity of the substrates—and forming the desired matrix constituting deposit—and eventually passing through gaps  39 . The effluent gas is extracted from the volume  28  of the reaction chamber through a gas outlet  17  formed in the susceptor top wall  16  and connected to a pumping device (not shown). 
     Advantageously, the division of the flow of preheated reactive gas into individual flows feeding the internal volumes of the stacks of substrates is controlled as a function of the location of the stack in the reaction chamber. The control is performed in order to allow a larger flow of reactive gas to feed a stack which is remote from the internal wall  12  of the susceptor, in comparison with the flow of reactive gas feeding a stack located close to the internal wall of the susceptor. 
     Indeed, a stack of substrates located in the central part of the reaction chamber, like stack  32   1 , in FIG. 2, is slightly less efficiently heated by the susceptor compared with a stack of substrates located close to the internal wall of the susceptor, like stack  32   2 . Slightly increasing the flow of reactive gas feeding stack  32   1  makes it possible to compensate for the slightly less efficient heating and reduce the gradient of densification between different stacks. 
     The individual flows of reactive gas feeding the different stacks are controlled by selecting the cross-section of the passages defined by the nozzles  62 . As shown by FIG. 4, a nozzle  62   1  for a central stack (like stack  32   1 ) defines a passage having a cross-section slightly larger than the cross-section of a passage defined by a nozzle  62   2  for a peripheral stack (like stack  32   2 ). Different sets of nozzles  62  having the same outer diameter corresponding to the diameter of passages  60   a , but different internal calibrated cross-section may be provided to allow appropriate selection for adjusting the individual flows of gas as needed. 
     Another, simplified, embodiment of a gas preheater according to the invention is diagrammatically shown in FIG.  8 . 
     The embodiment of FIG. 8 differs from the one of FIG. 3 in that the stacks  32  of annular preforms  30  are supported directly by plate  60  which constitutes both a gas distribution plate and a load supporting plate. 
     This alternate embodiment may be used when the efficiency of the gas preheating chamber is sufficient to avoid having further preheating of the individual gas flows exiting therefrom. Efficiency of the gas preheating chamber  54  may be adjusted by selecting an appropriate number of perforated plates  66 .