Method of densification by chemical vapor infiltration

Abstract

The present invention relates to a process for the densification of a plurality of fibrous preforms arranged in a chamber (12) of a densification furnace (11) by chemical vapor infiltration, the process comprising at least the following steps a) and b), each step being repeated more than once: a) introduction of a first gaseous phase, the thermal decomposition of which makes it possible to form a matrix in the pores of the plurality of fibrous preforms; b) introduction of a second gaseous phase, simultaneously depressurizing the furnace chamber, so that the introduction of the second gaseous phase is simultaneous with the evacuation of the first gaseous phase from the chamber, the temperature of the second gaseous phase being less than or equal to 25°C when it is introduced into the chamber of the furnace.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of composite materials and more particularly to the field of chemical vapor infiltration processes for obtaining these materials by densifying a fibrous preform. BACKGROUND

[0002] Composite parts are the focus of growing technological interest due to the good compromise they offer in terms of strength and weight.

[0003] It is known to obtain composite parts using a fibrous preform, by chemical vapor infiltration or by chemical vapor deposition. Such a process uses a reactive gas phase, the decomposition of which allows the formation of the desired matrix. This reactive gas phase is introduced into the pores of the fibrous preform, which itself is positioned in a densification furnace, and the conditions of the furnace are chosen so that the gas phase decomposes directly in the pores of the fibrous preform and forms the matrix of the composite part. The choice of the gas that constitutes the gas phase, as well as the temperature and pressure conditions that prevail inside the furnace, determine the nature of the matrix that is formed in the composite part.

[0004] One of the limitations of the chemical vapor infiltration process is the fact that it is difficult to densify the core of the preform. Indeed, as mentioned above, during the process, the matrix is formed directly in the pores of the substrate due to the decomposition of the reactive gas phase. This decomposition blocks the pores near the surface of the fibrous preform during the process, making it more difficult for the gas phase to access the pores at the core of the preform. In addition, the reactants of the gas phase that reach the core are less abundant because some of them have already decomposed at the surface. The further the process progresses, the more difficult it becomes to densify the core, until the surface pores are completely blocked and it becomes impossible to densify the core. The result is a high level of residual porosity in the resulting part.

[0005] To improve the densification of the core of the preform, it is generally proposed to take advantage of the kinetics of the reactions that form the matrix. For example, providing a temperature gradient between the core of the preform and its surface means that the matrix is deposited more quickly in the hotter core of the preform before being deposited in the pores closer to the surface, helping to improve the densification of the core of the preform.

[0006] When the preform is conductive, for example when the preform is made of carbon, it is relatively simple to generate the temperature gradient needed to implement the above-mentioned solution, because the gradient can then be generated by thermal induction coupling.

[0007] However, when a ceramic preform is used, it is not possible to implement thermal induction coupling because such preforms do not conduct an electric current. Therefore, there remains a need for a chemical vapor infiltration process that helps to improve the rate of densification achieved by a ceramic preform. SUMMARY

[0008] The inventors propose a method that provides a solution to the aforementioned problem.

[0009] To this end, they propose a method for the densification of a plurality of fibrous preforms arranged in the shell of a densification furnace by chemical vapor infiltration, which method comprises at least the following steps a) and b), each step being repeated more than once:

[0010] a) introduction of a first gaseous phase, the thermal decomposition of which allows the formation of a matrix in the pores of the plurality of fibrous preforms;

[0011] b) introduction of a second gaseous phase, the shell of the furnace being simultaneously placed under vacuum, so that the introduction of the second gaseous phase occurs simultaneously with the evacuation of the first gaseous phase from the shell, the temperature of the second gaseous phase being less than or equal to 25°C when it is introduced into the shell of the furnace.

[0012] The inventors have observed that such a method, and in particular the introduction of the second gaseous phase at the specified temperature, temporarily cools the surface of the fibrous preforms and thus creates a temperature gradient on the surface of the fibrous preforms. The kinetics of the reaction forming the matrix are then reduced at the surface of the preforms rather than in the core of the preforms, which remains hotter and densification becomes faster at the core of the preforms than at the surface. This leads to a densification method that makes it possible to obtain levels of residual porosity that cannot be achieved when using conventional isothermal and isobaric densification methods.

[0013] In addition, the purification of the reactive gas contained in the first gaseous phase, which is carried out simultaneously with the cooling of the preforms, ensures that the first gaseous phase is always rich in reactive gas. Finally, the alternating repetition of steps a) and b) helps to ensure that the desired temperature gradient between the core and the surface of the fibrous preforms is maintained throughout the duration of the method.

[0014] In one embodiment, the preforms comprise or consist of silicon carbide fibers.

[0015] The method is in fact particularly advantageous when preforms made of SiC fibers are used, since the latter are not able to generate a temperature gradient by inductive coupling. Indeed, such preforms do not conduct electricity.

[0016] In one embodiment, the first gaseous phase comprises methyltrichlorosilane and hydrogen. These substances make it possible to form a silicon carbide SiC matrix. In one embodiment, the preforms comprise or consist of silicon carbide fibers, the method then making it possible to form SiC-SiC composites with particularly low residual porosities compared to those that can be obtained using the methods of the prior art.

[0017] In one embodiment, the second gaseous phase comprises hydrogen, helium, nitrogen or a mixture of these substances. These substances are advantageous because they do not form undesirable by-products in the enclosure of the densification furnace. These substances also provide very good thermal conductivity, making it possible to form the desired temperature gradient at the surface of the preform while minimizing the duration of step b), which therefore reduces the duration of the densification process.

[0018] In one embodiment, the pressure applied in the enclosure of the furnace during step b) is less than or equal to 200 mbar. Having a low pressure in the enclosure of the furnace when the second gaseous phase is introduced makes it possible to purify the first gaseous phase.

[0019] In one embodiment, step b) is interrupted by a new step a) when the temperature at the surface of the fibrous preform is less than or equal to 800°C.

[0020] The temperature at the surface of the fibrous preform can be measured, for example, using an infrared pyrometer. Controlling the surface temperature of the fibrous preform helps to ensure that the temperature gradient between the core and the surface is the temperature gradient desired for the densification process. In particular, the inventors have observed that a surface temperature of the fibrous preform of less than or equal to 800°C, in addition to having densification conditions consistent with the prior art, represents an optimal temperature gradient that makes it possible to densify the preform while also ensuring that the cooling step is not too long.

[0021] In one embodiment, during step a), the set temperature of the enclosure of the densification furnace can be less than or equal to 1100°C, for example between 800°C and 1100°C.

[0022] In one embodiment, step a) can be interrupted by step b) when the temperature at the surface of the preform is greater than or equal to 1000°C.

[0023] Indeed, if the surface temperature of the preform becomes too high, the temperature gradient between the core and the surface is less than the desired gradient and optimal densification cannot be achieved.

[0024] In one embodiment, the duration of step b) is less than or equal to 5 minutes. The inventors have observed that this duration of the cooling step b) makes it possible to obtain a desired increase in temperature gradient sufficient to achieve densification at the end of the process.

[0025] In one embodiment, the two gaseous phases are circulated in parallel. In this embodiment, the two gaseous phases follow similar paths through the furnace, in particular through the fibrous preform.

[0026] In this case, the cooling induced by the introduction of the second gas phase during step b) acts more effectively on the zone of the reactor where the deposition kinetics is maximum during step a) due to the richness of the reactive gas.

[0027] In one embodiment, the two gas phases are circulated in counter-current with respect to each other. In this embodiment, the first and second gas phases follow opposite paths through the enclosure of the densification furnace and in particular through the fibrous preform.

[0028] In this case, during step b), the circulation of the second gas phase in the opposite direction will have the effect of destroying the boundary layer surrounding the fibrous preform, which will promote the renewal of the first gas phase during step a) in addition to cooling the part.

[0029] In one embodiment, a calibrated quantity of first gas phase is introduced during each step a). For example, the circuit for introducing the first gas phase comprises a supply chamber upstream of the densification furnace to ensure that the quantity of first gas phase delivered during each step a) is limited to the quantity contained in this supply chamber.

[0030] This embodiment makes it possible to know the precise quantity of first gas phase introduced into the enclosure of the densification furnace during each step a), and therefore to limit the duration of each step a) to the time required for the decomposition of this introduced quantity of first gas phase. This leads to a method in which the precise duration of each step a) can be known and relatively short.

[0031] For example, in this embodiment, the duration of each step a) can be less than or equal to 1 minute, or indeed less than or equal to 30 seconds.

[0032] In one embodiment, the flow rate of the first gas phase is constant during each step a).

[0033] This embodiment allows the first gas phase to be continuously renewed so that it is therefore continuously charged with reactive species, and therefore the duration of step a) is not limited by the depletion of the reactive species in the first gas phase. Thus, in this embodiment, the duration of step a) is limited by the time taken to heat the surface of the fibrous preform.

[0034] In this embodiment, the duration of each step a) can be between 1 minute and 30 minutes, for example between 3 minutes and 10 minutes. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is a schematic representation of the equipment for implementing the method of the application.

[0036] Figure 2 is a schematic representation showing how the characteristic quantities vary in the first embodiment of the method.

[0037] Figure 3 is a diagram showing how the characteristic quantity varies in the second embodiment of the method. DETAILED DESCRIPTION

[0038] The application will now be described with reference to the drawings, which are provided purely by way of better illustrating certain embodiments and should not be considered limiting.

[0039] Figure 1 is a diagram of an apparatus 10 for implementing the method described above. The apparatus 10 comprises a densification furnace 11 defining an outer shell 12. The outer shell 12 of the densification furnace 11 is loaded with a fibrous preform to be densified, which is not shown.

[0040] As mentioned above, the method then comprises repeating steps a) and b) described above.

[0041] For example, steps a) and b) can be repeated at least 5 times, or indeed at least 10 times. The number of repetitions depends on the quantity of ceramic to be deposited.

[0042] Each time step a) is implemented, the densification of the preform increases. Furthermore, as illustrated, step b) helps to ensure that there is a temperature gradient between the core and the surface of the preform, which means that the thermal decomposition of the first gas phase occurs more quickly in the core of the preform than at their surface.

[0043] For example, the apparatus 10 comprises a first gas phase source 20 and a second gas phase source 21. These gas phase sources 20, 21 are in fluid communication with the inlet 14 located at the bottom of the furnace 11, respectively through pipes 16 and 15.

[0044] All the supply paths from the sources 20, 21 to the outer shell 12 can comprise a valve, as illustrated.

[0045] In one embodiment, when supplying the outer shell 12, the temperature of the first gas phase is greater than or equal to 1000°C, or indeed greater than or equal to 1100°C. If necessary, heating means not shown in the figure can be provided. For example, the reactive gas can be pre-heated outside the outer shell. Figure 1

[0046] The apparatus 10 also comprises pumps and other means, not shown here, which make it possible to supply the outer shell with the first gas phase and the second gas phase, respectively from the sources 20 and 21.

[0047] ​In one embodiment shown, the path for supplying the gas phase from the sources 20, 21 to the enclosure 12 can further comprise a supply chamber 18, 19 respectively on the path of the second or first gas phase. Such a supply chamber can be useful in the embodiments described below, in which a specific amount of first or second gas phase is to be introduced.

[0048] In other embodiments, these supply chambers 18, 19 can be absent.

[0049] In the embodiment shown in solid line, the conduits 15 and 16 for supplying the first and second gas phase supply the furnace at the same end 14, in this case at the end located at the bottom of the furnace 11. Such conduits 15 and 16 thus allow the first and second gas phase to be supplied to the furnace in co-current circulation. Figure 1

[0050] In the alternative embodiment shown in dotted line, the supply conduit 17 supplying the first gas phase can be connected to the end of the furnace 11 opposite to the end supplying the second gas phase. In this case, the first gas phase is supplied to the enclosure 12 of the furnace 11 through the top end 13, and the second gas phase is supplied to the enclosure 12 of the furnace 11 through the bottom end 14, via the supply conduit 15. Figure 1

[0051] Such an embodiment allows the first and second gas phase to circulate through the enclosure 12 in counter-current with respect to each other.

[0052] Naturally, the first or second gas phase can be supplied to the enclosure in a different manner from that shown, without departing from the scope of the application. For example, the first and second gas phase can be supplied through the top end 13 of the furnace 11.

[0053] Similarly, the first and second gas phase can be supplied to the enclosure 12 through the bottom end 14 of the furnace 11. Figure 1 Exhaust circuits 24, 25 for exhausting the gas phase from the furnace are shown. Although Figure 1 Two circuits are shown, but only one of these two circuits can be present, without departing from the scope of the application.

[0054] These circuits allow the first and / or second gas phase to be exhausted from the enclosure 12 of the furnace 11.

[0055] As mentioned above, the step b) of supplying the second gas phase to the enclosure 12 must be carried out while simultaneously placing the enclosure 12 under vacuum, so that the introduction of the second gas phase occurs simultaneously with the exhaustion of the first gas phase from the enclosure 12.

[0056] For this purpose, for example, the exhaust circuits 24, 25 can be connected to a low-pressure chamber 22. Thus, when the enclosure 12 is to be exhausted, the low-pressure chamber is brought into fluid communication with the enclosure 12. The pressure of the low-pressure chamber 22 is lower than that of the enclosure 12, so that the gas phase present is exhausted.​​

[0057] For example, the low-pressure chamber 22 can be connected to a pump 23 which creates a vacuum in the low-pressure chamber 22.

[0058] In one embodiment not shown, the enclosure 12 can be directly purged by a pump if necessary.

[0059] As mentioned above, the method of the application comprises repeating steps a) and b).

[0060] Figure 2 The variations in partial pressure of the first gas phase 110, in the flow rate of the first gas phase 111 and in the partial pressure of the second gas phase 112 in the enclosure 12 during the implementation of the method are schematically described.

[0061] More particularly, Figure 2 Embodiments are described in which step a) is implemented by introducing a calibrated quantity of the first gas phase into the reaction enclosure 12.

[0062] For example, the first step a) can be implemented by bringing the supply chamber 19 having a known volume to a given pressure and then supplying the contents of this supply chamber 19 to the enclosure 12 without further supply of the first gas phase. The implementation of this first step a) over a duration 100 results in a variation in the flow rate of the first gas phase 111 between zero 303 and a maximum value 304, as Figure 2 shown.

[0063] Similarly, the partial pressure of the first gas phase 110 varies between zero 301 and a maximum value 302.

[0064] At the end of the chosen duration 100, step b) begins.

[0065] Alternatively, the duration 100 of step a) can be determined by following the temperature variations at the surface of the fibrous preform and step a) is interrupted when the surface temperature becomes too high, for example greater than or equal to 1000°C.

[0066] This embodiment helps to ensure that the temperature gradient between the core of the preform and their surface is always sufficient to ensure that the formation of the matrix takes place preferentially at the core of the preform.

[0067] Step b) corresponds to the introduction of the second gas phase resulting in an increase in the partial pressure 112 of the second gas phase between zero 305 and a maximum value 306.

[0068] This step also comprises placing the enclosure 12 under vacuum. Thus, while the partial pressure of the second gas phase 112 is increasing, a rapid decrease in the partial pressure of the first gas phase 110 is observed and the flow rate of the first gas phase 111 returns to zero.

[0069] At the end of the defined duration 200, step a) is started again.

[0070] Alternatively, step b) can be interrupted when the surface temperature of the fibrous preform falls again below the target value, for example less than or equal to 800°C. This makes it possible to ensure that the desired temperature gradient between the inside and the outside of the fibrous preform is sufficient in order to obtain preferential deposition of the matrix at the core of the fibrous preform, while making it possible to make step b) last for as little time as possible.

[0071] Figure 2 Four alternating steps a) 100, 101, 102, 103 and b) 200, 201, 202, 203 are shown, but a different number of steps a) and b) can be chosen without departing from the scope of the application.

[0072] Figure 3 The variations in partial pressure of the first gas phase 210, in flow rate of the first gas phase 211 and in partial pressure of the second gas phase 212 in the enclosure 12 during implementation of the method in another embodiment are shown schematically, in which the flow rate of the first gas phase is constant during step a).

[0073] In such an embodiment, unlike the embodiment shown, Figure 2 The enclosure 12 is supplied with a flow rate of the first gas phase 211 that varies between zero 307 during step b) and a constant value 308 during step a), unlike the embodiment shown.

[0074] This embodiment can be obtained in the absence of a supply chamber 19 for supplying the enclosure 12 with a calibrated quantity of the first gas phase. For example, in such an embodiment, during step a), the source 20 of the first gas phase can supply the enclosure 12 directly.

[0075] Figure 3 The first step a) is shown, during which the flow rate 211 of the first gas phase is constant and has a selected value 308. This step allows densification of the preform to proceed for a duration 100.

[0076] Next, after this step a), step b) can be implemented, during which the flow rate 211 of the first gas phase is zero 307. The partial pressure of the first gas phase 210 decreases, while the partial pressure of the second gas phase 212 increases in the enclosure 12.

[0077] This step b) makes it possible to purify the first gas phase of the enclosure 12 and to restore the necessary temperature gradient between the core and the surface of the fibrous preform.

[0078] Figure 3Four alternating steps a) 100, 101, 102, 103 and b) 200, 201, 202, 203 are shown, but a different number of steps a) and b) can be chosen without departing from the scope of the invention.

Claims

1. A process for the densification of a plurality of fibrous preforms arranged in a casing (12) of a densification furnace (11) by chemical vapor infiltration, the process comprising at least the following steps a) and b), each step being repeated more than once: a) introducing a first gaseous phase, the thermal decomposition of which allows the formation of a matrix in the pores of the plurality of fibrous preforms; b) introducing a second gaseous phase, the casing of the densification furnace being simultaneously placed under vacuum, so that the introduction of the second gaseous phase occurs simultaneously with the evacuation of the first gaseous phase from the casing, the temperature of the second gaseous phase being less than or equal to 25°C when it is introduced into the casing of the densification furnace; wherein the second gaseous phase comprising hydrogen, helium, nitrogen or a mixture of these substances; the first gaseous phase having a predetermined composition, and the process comprising the following sequence of steps: (i) performing step a); (ii) interrupting step a) to perform step b); (iii) performing step b); (iv) interrupting step b) to perform step a), and then (v) performing step a) so as to introduce the first gaseous phase into the casing, wherein the process comprises the interruption of step a) and the beginning of step b) when the temperature at the surface of the preforms is greater than a first predetermined temperature threshold.

2. The densification method of claim 1, wherein, the fibrous preforms comprise silicon carbide fibers.

3. The densification method of claim 1 or 2, wherein, the first gaseous phase comprises methyltrichlorosilane and hydrogen.

4. The densification method of claim 1, wherein, the pressure applied in the casing (12) of the densification furnace (11) during step b) is less than or equal to 200 mbar.

5. The densification method of claim 1, wherein, the duration of step b) is less than or equal to 5 minutes.

6. The densification method of claim 1, wherein, step b) is interrupted by a new step a) when the temperature at the surface of the fibrous preforms is less than or equal to 800°C.

7. The densification method of claim 1, wherein, the two gaseous phases are circulated in co-current.

8. The densification method of claim 1, wherein, the two gaseous phases are circulated in counter-current.

9. The densification method of claim 1, wherein, during each step a), a calibrated quantity of the first gaseous phase is introduced.

10. The densification method of claim 1, wherein, during each step a), the flow rate of the first gaseous phase is constant.

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