Method for densifying porous annular substrates by chemical vapour infiltration

A method for densifying porous annular substrates by chemical vapor infiltration, includes providing a plurality of unit modules including a support tray on which substrates are stacked, the support tray including a gas intake opening extended by an injection tube disposed in an internal volume formed by the central passages of the stacked substrates, the injection tube including gas injection orifices opening into the internal volume, forming stacks of unit modules in the enclosure of a densification furnace and injecting, into the stacks of unit modules, a gas phase including a gas precursor of a matrix material to be deposited within the porosity of the substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of PCT/EP2019/071602, filed Aug. 12, 2019, which in turn claims priority to French patent application number 1857435 filed Aug. 10, 2018. The content of these applications are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to the general field of methods for densifying porous annular substrates by Chemical Vapor Infiltration (CVI).

To densify such substrates, methods are in particular known in which stacks of substrates are disposed in the heated enclosure of a densification installation, and a gas phase comprising a matrix material precursor is introduced inside the stacks of substrates so as to form the matrix in the porosity of the substrates. Document FR 2834713 describes such a method and an installation for its implementation.

When it is desired to densify a large number of stacks of substrates, a larger installation is generally used. A known example of loading that can be used in the enclosure1of a densification furnace is illustrated inFIG. 1. The enclosure1is cylindrical about an axis X. The loading comprises a plurality of stacks of porous annular substrates2carried by the same lower support tray3. Each stack is formed of several sections4of superimposed stacks separated by intermediate support trays5common to all the stacks. The trays3and5comprise openings aligned with central passages of the substrates2in order to circulate in each stack a reactive gas phase which will then pass through the substrates2to densify them. An upper tray6surmounts the loading and closes the internal volume of each stack. The intermediate support trays5are held with vertical pins7. Such a loading has several drawbacks: it is long and complex to make, and the sealing between the support trays and the stack sections are difficult to guarantee, which may alter the efficiency and uniformity of the densification. This loading also does not allow optimizing the volume occupied in the enclosure of the furnace, the support tray being generally thick. Because of these sealing problems, it is furthermore difficult to implement CVI technologies of the semi-forced flow type which allow reducing the duration of the densification cycles.

There is therefore a need for a densification method by chemical vapor infiltration which does not have the aforementioned drawbacks.

OBJECT AND SUMMARY OF THE INVENTION

The main aim of the present invention is therefore to overcome such drawbacks by proposing a method for densifying porous annular substrates by chemical vapor infiltration, the method comprising at least the following steps:providing a plurality of unit modules, each unit module comprising a support tray on which porous annular substrates are stacked, the support tray comprising a gas intake opening extended by an injection tube disposed in an internal volume formed by the central passages of the stacked substrates, the injection tube comprising a first end connected to the tray, a second free end and gas injection orifices opening into the internal volume,forming stacks of unit modules in the enclosure of a densification furnace, each stack comprising at least a second unit module stacked on a first unit module, the intake opening of the support tray of the second unit module communicating with the free end of the injection tube of the second unit module so as to allow the circulation of a gas between the first and the second module, andinjecting, into the stacks of unit modules, a gas phase comprising a gas precursor of a matrix material to be deposited within the porosity of the substrates.

The method according to the invention is remarkable in that it implements unit modules which allow the formation of stacks in the enclosure of the densification furnace in a simpler manner than in the methods of the prior art. Indeed, the unit modules can be prepared outside the enclosure of the furnace and then stacked in the enclosure. The formation of the stacks of unit modules can further be automated. The injection tube of each unit module also plays the role of thermal mass ensuring a gas preheating function and making it possible to reduce the preheating area in the loading and increase the rate of loading in the enclosure. The stacks of unit modules allow improving compactness because there is no longer any need for vertical pins and intermediate support trays, which also allows increasing the rate of loading in the enclosure. Furthermore, in such a method, it is no longer necessary to monitor the sealing between stack sections and intermediate support trays, but only between the stacked unit modules. The sealing between the different stacks are thus improved compared to the methods of the prior art, which allows a more uniform densification of the substrates and the implementation of CVI methods of the semi-forced flow type. The method according to the invention is therefore simpler to implement and more efficient than those implementing for example a loading such as the one described in the introduction.

In one exemplary embodiment, each stack can be surmounted by a cover closing a volume formed by joining the internal volumes of the unit modules of said stack. The cover can have a diameter equal to the diameter of a support tray.

In one exemplary embodiment, each unit module can comprise calibrated orifices providing a leak passage between the internal volume of the stacked substrates and a volume external to the unit module. Thus, the invention applies in particular to chemical vapor infiltration methods called forced flow methods, where the gas phase injected into a stack passes through the substrates using solid spacers positioned between the substrates, and to methods called semi-forced flow methods, where the calibrated orifices providing a leak passage are disposed in the loading to force only part of the gas phase to pass through the substrates. The calibrated orifices can be obtained by spacers positioned for example between the substrates providing a leak passage between the internal volume of the stacked substrates and a volume external to the unit volume.

In one exemplary embodiment, each stack of unit modules can be supported only by a bottom of the enclosure of the densification furnace. In other words, the stacks of unit modules can be independent of each other.

In one exemplary embodiment, each stack of unit modules can be supported by a graphite cylinder comprising a channel communicating with the intake opening of a unit module on the one hand, and with a gas inlet on the other hand. Such a cylinder allows taking up the forces of the entire stack it supports, and also ensures a preheating of the gas phase injected into the stack of unit modules through the gas inlet.

In one exemplary embodiment, the injection orifices can be holes in a wall of the injection tube.

In one exemplary embodiment, the injection orifices can be distributed around and along a wall of the injection tube.

In one exemplary embodiment, each injection orifice can be positioned facing a porous substrate.

In one exemplary embodiment, each support tray can be circular in shape and have a diameter comprised between 90% and 110% of the external diameter of a porous annular substrate, for example equal to the external diameter of a porous annular substrate.

In one exemplary embodiment, the enclosure of the densification furnace can be delimited by a susceptor coupled to an inductor.

In one exemplary embodiment, the enclosure can be cylindrical in shape about an axis. In this case, a stack can be centered along this axis and the other stacks can be distributed about this axis in the enclosure.

In one exemplary embodiment, each porous annular substrate can comprise carbon.

In one exemplary embodiment, each porous annular substrate can constitute a brake disc fiber preform.

DETAILED DESCRIPTION OF THE INVENTION

A densification method by chemical vapor infiltration according to the invention firstly comprises a step during which a plurality of unit modules10is provided (FIGS. 4 and 5).

A unit module10firstly comprises a structural part12(FIGS. 2 and 3) including a support tray14provided with a gas intake opening14aextended by an injection tube16. The support tray14, the opening14aand the injection tube16have a shape of revolution about the same axis Y and are centered on this same axis Y. The injection tube16here has the shape of a cylinder and the tray support14the shape a perforated disc. The injection tube16has a first end16aconnected to the support tray14and a second free end16b. The injection tube16and the support tray14here form one and the same part. As a variant, the injection tube16and the support tray14can be assembled. The injection tube16further comprises a plurality of holes forming gas injection orifices16cdistributed around and along the wall of the injection tube16. The structural part12can be made of graphite and/or of composite material for example of carbon-carbon C/C composite.

A unit module10is then produced by forming a stack of porous annular substrates18on the support tray14of the structural part12. Each porous annular substrate18has a central passage18a. The porous annular substrates18are stacked on the support tray14such that the injection tube16is present inside an internal volume17formed by the central passages18aof the stacked substrates18. The porous substrates18and the unit module10are centered on the same axis Y. It is advantageous that the gas injection orifices16care positioned along and around the wall of the injection tube16so as to be located facing the substrates18. In this example, the substrates18have an external diameter substantially equal to that of the support tray14for greater compactness. The diameter of an injection tube16is here smaller than the diameter of the central passage18aof a porous substrate18to leave a space ensuring good circulation of the gas phase in the internal volume17.

In the example illustrated, a porous substrate18is separated from an adjacent substrate or from the support tray14by one or more spacers20which define intervals20a. The spacers20are for example disposed radially with respect to the axis Y of a module10and are arranged to form passages communicating the internal volume17of the stacked substrates with an external volume22(FIG. 7) located outside the stacks, in the enclosure10.

The passages formed by the spacers20, constituting calibrated orifices in the loading, can offer a more or less restricted passage section so as to allow the existence of a pressure gradient between the volumes17and22, as described in the patent application FR 2821859, this then referred to as forced flow (zero passage section) or semi-forced (non-zero passage section) CVI densification. It will be noted that calibrated orifices forming such a leak passage can be provided at other locations, for example only at the bottom or at the top of a stack, within the cover surmounting the stack or a support at the base of a stack.

Thus, each unit module10comprises, in the illustrated example, a stack of substrates18between which spacers20are present, the stack of substrates18being placed on a spacer20in contact with the support tray14, and being surmounted by a spacer20intended to be in contact with another unit module10or with a cover24(FIGS. 6 and 7). The length of the injection tube16is here such that the free end16bthereof ends at the same level as an upper face of the last spacer20.

The substrates18are for example carbon fiber preforms or blanks formed of pre-densified preforms, intended for the production of brake discs made of carbon/carbon (C/C) composite material, by densification with a pyrolytic carbon matrix.

FIG. 5shows how the stack of two unit modules10is made. To make such a stack, the support tray14of the upper module10is laid on the last spacer20and the free end16bof the injection tube16of the lower module10by centering the modules10along the same axis Y. The injection tube16of the lower unit module10is thus in contact with the support tray14of the upper unit module10so as to take up the mass thereof. One or more seals can be present on the lower face of a support tray14intended to be in contact with a free end16bof an injection tube and/or on the free end of the injection tube16bintended to be in contact with a support tray14.

The method according to the invention comprises the formation of several stacks of unit modules10directly in the enclosure26of a densification furnace, either manually or automatically. It is for example possible to provide the unit modules10outside the enclosure26, then to form the stacks inside the enclosure26. It is also envisaged to load a stack of modules10all at once into the enclosure26.

FIG. 6schematically shows the loading of substrates18into the enclosure26of a densification furnace obtained after having formed stacks of unit modules10. The enclosure26is represented schematically in dotted lines, and only the two first and the last unit modules of each stack have been represented for greater readability. The loading of substrates18comprises a plurality of stacks of unit modules10, in this example nineteen stacks are present in the enclosure26. Each stack of unit modules10comprises, in this example, seven unit modules.

Each stack of unit modules10is surmounted by a cover24which closes the volume formed by joining the internal volumes17of the stack. At its base, each stack of unit modules10comprises a graphite cylinder30centered on the axis Y of the stacked unit modules10, comprising a central channel30acommunicating with the gas intake opening14aof the support tray14of a unit module10on the one hand, and with a gas inlet32provided in a bottom26bof the enclosure26on the other hand. The graphite cylinder30is fixed on the bottom26bof the enclosure26. There is, in this example, as many gas inlets32as there are stacks of unit modules10in the enclosure26. The graphite cylinder30allows taking up the forces of the stack acting as a thermal mass ensuring a preheating of the gas phase entering the stack.

The enclosure26comprises a heated wall26awhich here constitutes a susceptor laterally delimiting the enclosure26. More specifically, the wall26ais here an armature which is inductively coupled with an inductor28present around the enclosure26. In the illustrated example, the loading of porous substrates18is adapted to the cylindrical shape of the enclosure10about the axis X. Particularly, the stacks of unit modules10are distributed in the enclosure26about the axis X.

Once the loading has been made, a gas phase (or reactive gas) containing one or more carbon precursor constituents is introduced into the enclosure26through the gas inlets32. The introduced gas phase is conveyed, for each stack, by the cylinder30up to the gas intake opening14aof the first unit module10of the stack. The gas phase then arrives in the injection tubes16of each stack to be injected at the internal volume17formed by the central passages18aof the substrates18through the injection orifices16c. The more or less significant pressure difference between the internal volume of each stack and the external volume22ensures that the gas phase passes through the substrates18in order to densify them. Once the gas phase has passed through the substrates18, it reaches the external volume22and can finally be discharged through a discharge port (not represented) arranged in an upper wall of the enclosure26, which can be optionally associated with suction means.

Typically, the gas phase comprises a carrier gas and one or more gas matrix precursors. To form a carbonaceous matrix, methane, propane or a mixture of both can be used as a gas precursor. The carrier gas can be natural gas.