Heat treatment process for friction parts made of carbon / carbon composite material

Abstract

Heat treatment process for friction parts made of carbon / carbon composite material The invention relates to a heat treatment process for at least one friction part (120) made of carbon / carbon composite material extending along a thickness direction between a first external portion (122a) and a second external portion (122b) delimiting between them an internal portion (123), characterized in that it comprises a step of simultaneous heating of the first external portion (122a), the second external portion (122b) and the internal portion (123), the first external portion (122a) and the second external portion of each friction part (120) being heated to a first temperature (T1), while the internal portion of each friction part (120) is heated to a second temperature (T2) higher than the first temperature (T1) and in that the difference between the first temperature (T1) and the second temperature (T2) is at least 1%.Figure for the abridged version: Fig.3.

Description

Title of the invention: Heat treatment process for friction parts made of carbon / carbon composite material. Technical field

[0001] The present invention relates to the field of manufacturing friction parts made of carbon / carbon composite material, and more particularly to brake discs. Prior art

[0002] A common technique for manufacturing carbon / carbon or C / C composite brake discs involves producing annular fibrous preforms from carbon fibers or a precursor, which precursor may be, for example, oxidized polyacrylonitrile (PAN), pitch, rayon, or a phenolic compound, and densifying these preforms with a carbon matrix by chemical vapor infiltration (CVI) or any other densification process such as heat treatment, polymer impregnation, and pyrolysis (PIP). When the preform is made from carbon precursor fibers, the transformation of the precursor is carried out by carbonization, preferably after the preform has been produced and before its densification. The C / C discs thus obtained are generally subjected, after densification, to a heat treatment that generates partial or total graphitization of the carbon present in the matrix of the part.Heat treatment is generally carried out at temperatures between 1500°C and 3000°C and for periods of several hours.

[0003] Graphitizing carbon / carbon (C / C) brake discs improves their tribological and thermal properties. Increasing the level of graphitization of the carbon matrix enhances their thermal diffusivity and oxidation resistance. However, too high or too low a level of graphitization is detrimental to wear resistance.

[0004] Thus the optimal level of graphitization corresponds to a compromise between the minimum expected performance (braking efficiency) and the maximum expected durability (resistance to wear and oxidation) for the brake disc.

[0005] To achieve this compromise, it is necessary to limit the temperature and duration of the heat treatment in order not to degrade braking efficiency and wear resistance. These constraints result in a limitation of thermal diffusivity and oxidation resistance.

[0006] Existing processes propose to heat-treat several parts simultaneously in installations comprising isothermal or quasi-isothermal furnaces. For the implementation of these processes, it is necessary to carry out a Inerting the entire furnace volume and cooling the parts near the hot elements results in costly installations with complex and delicate maintenance. These installations generally have significant gas and electricity consumption because several areas in the tooling used are heated dead zones.

[0007] There is therefore a real need for a process that makes it possible to give the part optimal tribological and thermal properties in its thickness, without the disadvantages inherent in the known processes mentioned above. Description of the invention

[0008] To this end, the invention proposes a heat treatment method for at least one friction piece made of carbon / carbon composite material extending along a thickness direction between a first external portion and a second external portion delimiting between them an internal portion, characterized in that it comprises a step of simultaneous heating of the first external portion, the second external portion and the internal portion, the first external portion and the second external portion of each friction piece being heated to a first temperature, while the internal portion of each friction piece is heated to a second temperature higher than the first temperature and in that the difference between the first temperature and the second temperature is at least 1%.

[0009] The carbon in the carbon / carbon composite friction part is present in the form of pyrocarbon or pyrolytic carbon. During the process, the pyrocarbon in the friction part is at least partially transformed into graphite. In other words, such a process allows for at least partial graphitization of the pyrocarbon in the part. The higher the temperature, the greater the level of graphitization of the pyrocarbon. Thus, it is possible to obtain a part with different thermal and tribological properties between the core or inner portion and the outer or surface portions of the part. This is possible thanks to the differentiated and simultaneous heat treatment of the outer and inner portions, which allows for different modifications of the microstructure of the outer and inner portions of the part as required.

[0010] According to a particular characteristic, the difference between the first temperature and the second temperature can be at least 4%.

[0011] According to another particular feature, the difference between the first temperature and the second temperature can be at least 6%.

[0012] According to another particular feature, the difference between the first temperature and the second temperature can be at least 10%.

[0013] According to another particular characteristic, the difference between the first temperature and the second temperature can be at least 40%.

[0014] According to another particular feature, the difference between the first temperature and the second temperature can be at least 80%.

[0015] According to another particular characteristic, the difference between the first temperature and the second temperature can be at least 90%.

[0016] According to a particular feature of the process, during the simultaneous heating step, each friction piece can be placed between a first inductor and a second inductor, the first inductor can extend parallel to a first face of the friction piece and the second inductor can extend parallel to a second face of the friction piece, each friction piece being able to be heated individually.

[0017] Thus, it is possible to limit the Kelvin effect, commonly known as the skin effect, in the room and to promote even heat distribution. Furthermore, the choice of induction heating frequency and the choice of inductor further limits the Kelvin effect. The thickness of the room's skin can be less than one millimeter.

[0018] According to a particular feature of the process, the friction part can correspond to a brake disc, the first inductor and the second inductor each being able to have a planar spiral shape.

[0019] Indeed, the resonant frequency of an inductive heating system depends on several parameters: the current generator, the inductor, and the material to be heated. Regarding the inductor, its geometry impacts its inductance, which in turn impacts the resonant frequency. The inductance of the inductor is calculated based on the number and dimensions of its spirals. When calculating the inductance, the outer diameters and lengths of the spirals are taken into account. This type of inductor allows it to adapt to the annular shape of the brake disc and optimize the resonant frequency.

[0020] According to a particular feature of the process, during the simultaneous heating step, a heat transfer fluid may be present or circulate between the first face of the friction piece and the first inductor and between the second face of the friction piece and the second inductor.

[0021] Thus, it is possible to increase the difference between the first and second temperatures. Indeed, the presence of a fluid increases the convective losses of the external portions of the friction part. Consequently, the temperature of the external portions can be reduced simply and quickly, thereby causing a greater difference between the first and second temperatures.

[0022] According to a particular feature of the process, the heat transfer fluid can be a liquid, the liquid can be chosen from: liquid helium, liquid nitrogen, cyclohexane, toluene, petroleum carbon derivatives, supercritical carbon dioxide, liquid carbon dioxide, water or a mixture of these.

[0023] Cyclohexane, toluene, and petroleum carbon derivatives have boiling points that facilitate their handling and re-condensation. Furthermore, they have the advantage of being inexpensive.

[0024] Liquid helium and liquid nitrogen have the advantage of being chemically inert.

[0025] Liquid carbon dioxide and supercritical carbon dioxide have the advantage of being inexpensive.

[0026] According to a particular feature, the process may further include a step of transferring the heat recovered by the heat transfer fluid to a network. The network may be, for example, a cooling water network.

[0027] According to another particular feature of the process, the heat transfer fluid can be a gas, the gas can be chosen from: helium, nitrogen, carbon dioxide, air, argon, xenon, krypton, neon or a mixture of these.

[0028] Helium, argon and nitrogen have the advantage of being chemically inert with respect to the carbon / carbon composite friction part.

[0029] According to a particular feature, the method may further include a step of bringing a face of the carbon / carbon composite material friction piece into contact with a cooling piece, the cooling piece being at a temperature lower than the second temperature, the contacting step being carried out during the heating step.

[0030] Thus, it is possible to facilitate heat dissipation and limit radiation after the heating stage. When a heat transfer fluid is used, such a step prevents the presence of harmful chemical species from the heat transfer fluid on the carbon / carbon composite friction surface.

[0031] According to a particular characteristic, a heat transfer fluid can circulate in the cooling room.

[0032] According to a particular feature, the process may further include a step of regulating the temperature of the cooling room.

[0033] According to a particular feature of the process, during the simultaneous heating step, a thermally insulating material may be present between the first face of the friction piece and the first inductor and between the second face of the friction piece and the second inductor.

[0034] Such a thermally insulating material makes it possible to reduce the difference between the first temperature and the second temperature. Indeed, the presence of the thermally insulating material reduces heat loss from the external portions of the room of friction. Consequently, the temperature of the external portions undergoes only a very small decrease, causing a small difference between the first temperature and the second temperature.

[0035] According to a particular feature of the process, the thermally insulating material can be chosen from a refractory material comprising silica or silicates, alumina, quartz, mullite, refractory concretes, carbon (Papyex ®, Sigraflex ®) and carbonates, carbides (SiC, BC, TiC), or nitrides (BN, Si3N4, AIN, TiN), borides, chromium, or a mixture thereof.

[0036] According to a particular feature of the process, the second temperature can be greater than or equal to 1400°C.

[0037] Graphite comprises a stack of atomic planes, while pyrocarbon comprises a stack of defective atomic planes. Such a temperature allows for at least a partial transformation of pyrocarbon into graphite. Indeed, during the process, the interplanar and intraplanar defects of pyrocarbon are reduced, bringing it closer to graphite. Furthermore, the higher the temperature, the more the defects are corrected. Consequently, the closer the final structure is to graphite.

[0038] According to a particular feature of the process, the second temperature can be greater than or equal to 1600°C.

[0039] According to a particular feature of the process, the second temperature can be greater than or equal to 1650°C.

[0040] Such a temperature ensures an optimal degree of graphitization of the part.

[0041] According to a particular feature of the process, the second temperature can be greater than or equal to 2000°C.

[0042] Such a temperature allows for total graphitization of the internal portion.

[0043] According to a particular feature, the internal portion comprises between 80% and 96% of the friction part made of carbon / carbon composite material.

[0044] It is thus possible to optimally modify the microstructure of the external and internal portions of the part.

[0045] According to a particular feature of the process, the duration of the simultaneous heating step can be between 1 minute and 10 hours.

[0046] According to a particular feature, the duration of the simultaneous heating stage can be between 5 minutes and 60 minutes.

[0047] According to a particular feature, the duration of the simultaneous heating stage can be between 10 minutes and 60 minutes.

[0048] According to a particular feature, the duration of the simultaneous heating stage can be between 20 minutes and 60 minutes.

[0049] According to a particular feature, the duration of the simultaneous heating stage can be between 60 minutes and 10 hours.

[0050] According to another aspect of the invention, the invention proposes a heat treatment installation for at least one friction part made of carbon / carbon composite material extending along a thickness direction between a first external portion and a second external portion delimiting between them an internal portion, the installation comprising: - a cold-walled enclosure, - two inductors configured to heat the first external portion and the second external portion of the friction part to a first temperature, and to heat the internal portion of the friction part to a second temperature higher than the first temperature, - at least one means of regulating the temperature of the external portions of the friction part made of carbon / carbon composite material.

[0051] Such an installation makes it possible to obtain a part having different thermal and tribological properties between the core or internal portion and the external or surface portions of the part.

[0052] According to a particular feature, the cold wall enclosure may include means for circulating a heat transfer fluid.

[0053] Such a feature makes it possible to accentuate the difference between the first temperature and the second temperature.

[0054] According to a particular feature, the means for circulating a heat transfer fluid may include a first inlet port of the heat transfer fluid and a second outlet port of the heat transfer fluid.

[0055] According to a particular feature, each friction piece can be placed between a first inductor and a second inductor, and the installation can further include a first cooling piece fixed between a first face of the friction piece and the first inductor and a second cooling piece fixed between a second face of the friction piece and the second inductor.

[0056] These cooling parts make it easier to regulate the temperature of the external portions of the friction part made of carbon / carbon composite material.

[0057] According to a particular feature, each friction piece can be placed between a first inductor and a second inductor, and the installation can further include a first piece made of a thermally insulating material fixed between a first face of the friction piece and the first inductor and a second piece made of a thermally insulating material fixed between a second face of the friction piece and the second inductor. Brief description of the drawings

[0058] Other features and advantages of the present invention will become apparent from the description given below, with reference to the attached drawings which illustrate an example of an embodiment without any limiting character.

[0059] [Fig-1] The [Fig. 1] schematically represents an installation for implementing the process of the invention according to a first embodiment,

[0060] [Fig.2] Figure [Fig.2] schematically represents an inductor according to one embodiment,

[0061] [Fig.3] Fig.3 represents an installation for implementing the process of the invention according to another embodiment,

[0062] [Fig.4] The [Fig.4] represents an installation for implementing the process of the invention according to another embodiment. Description of the implementation methods

[0063] The invention applies generally to any friction part made of carbon / carbon (C / C) composite material.

[0064] The invention will be described below in the context of its application to a brake disc made of C / C material as a friction part.

[0065] In the example illustrated in [Fig. 1], the installation 100 comprises a treatment chamber 130 delimited by a cold-walled enclosure 103 and a pair of inductors 110a and 110b arranged opposite each other in the treatment chamber 130. A brake disc 120 made of C / C material is held between the two inductors 110a and 110b. In an embodiment other than the one shown, the installation 100 may comprise a plurality of discs 120 and a plurality of pairs of inductors 110a, 110b, one pair of inductors being associated with each disc.

[0066] For the sake of simplification, embodiments are described here which include only a disk 120 and a pair of inductors 110a and 110b.

[0067] The disk 120, as shown very schematically in [Fig. 1], comprises a central orifice 121. The disk 120 extends along a direction of thickness X between a first external portion 122a and a second external portion 122b, delimiting between them an internal portion 123. The first external portion 122a extends along a direction of thickness X between a first face 124a and the internal portion 123 of the disk 120. The second external portion 122b extends along a direction of thickness X between a second face 124b and the internal portion 123 of the disk 120.

[0068] In the example illustrated in [Fig. 1], the disk 120 and the pair of inductors 110a and 110b extend along a longitudinal direction Y corresponding to that of the height of the installation 100. The disk(s) and their associated pair of inductors can nevertheless be arranged in other directions in the chamber of treatment, in particular in order to optimize the loading according to the geometry of the treatment chamber.

[0069] In the example described here, the first inductor 110a extends parallel to the first face 124a of the disk 120 while the second inductor 110b extends parallel to the second face 124b of the disk 120.

[0070] According to a particular feature, each of the inductors 110a and 110b has a planar spiral shape, as illustrated in [Fig. 2]. The spiral shape allows for optimal coverage of each face of the disk, thus enabling better control of its heating throughout its entire volume. However, other inductor shapes can be considered, such as a single cylindrical coil or a double cylindrical coil.

[0071] The presence of an inductor 110a extending parallel to the first face 124a and a second inductor 110b extending parallel to the second face 124b reduces the skin effect when heating the disc 120. In addition, the use of a pair of inductors ensures and controls heating throughout the thickness of the disc 120.

[0072] The skin effect is an electromagnetic phenomenon whereby, at a high resonant frequency, the electric current tends to flow only on the surface of the inductor and the surface of the susceptor. The susceptor corresponds to the disk 120. This phenomenon causes a decrease in current density as one moves away from the periphery of the inductor and the outer portion of the heated part. Thus, the higher the resonant frequency, the shallower the penetration depth of the electric current into the part. Consequently, the current density in the portion of the part furthest from the inductor will be the lowest. Therefore, when using a single inductor, the outer portion furthest from the inductor is not heated, or not sufficiently heated.

[0073] During heating, the outer portions 122a and 122b of the disk are heated to a first temperature T1, while the inner portion 123 of the disk 120 is heated to a second temperature T2. The temperature T2 is higher than the temperature T1, and the difference between the first temperature T1 and the second temperature T2 is at least 1%. The heating of the outer portions 122a and 122b, as well as the heating of portion 123 of the disk, is carried out simultaneously, which ensures the modification of the microstructure of the part while streamlining the number of process steps. Indeed, the inventors have observed, contrary to expectations, that induction heating and the thermal conductivity of the carbon in the disk 120 generate a temperature difference between the outer portions 122a, 122b and the inner portion 123 of the disk 120.Indeed, at the first and second temperatures T1, T2 there is the presence of natural radiation from the disk 120 towards the walls of the . furnace or reactor in which the heating takes place. This is possible because the walls of the furnace or reactor are cooler than the 120 disc. Thus, it is possible to create high or very high heat losses, even under vacuum.

[0074] This type of process allows the external and internal portions to be treated simultaneously and separately in order to obtain different physical and chemical properties between the internal and external portions. The difference between the first temperature T1 and the second temperature T2 can be further increased or decreased by adjusting the heat losses in the external portions of the disk and / or by modifying the heating profile. Indeed, the temperature in any portion of the disk depends on the local balance between the heating power and the heat losses. Heat losses include radiation, convection on the surface of the disk, heat conduction within the disk to cooler areas, and heat conduction between the external portions of the disk and the disk support components in the heating system or any component in contact with the disk.Such heat losses can also be influenced by the room environment during heating. For example, a gas flow, a liquid, a vacuum, and / or a thermally insulating material near the disc can all affect these heat losses. Therefore, it is possible to generate a high, medium, and low temperature differential between the first and second temperatures by adapting the heating conditions and the room environment. The heating profile includes the total power delivered to the room during heating, the distribution of heated zones within the room, and the skin effect in the heating system. The zone distribution can be modified by changing the geometry of the inductor.

[0075] A "high difference" is understood to mean a difference between the first temperature T1 and the second temperature T2 greater than 50%.

[0076] The term "average deviation" means a difference between the first temperature T1 and the second temperature T2 greater than or equal to 20% and less than or equal to 50%.

[0077] A "small deviation" is understood to mean a difference between the first temperature T1 and the second temperature T2 greater than or equal to 1% and less than 20%. According to a particular characteristic of the process, the second temperature T2 may preferably be greater than 1600°C.

[0078] Simultaneous heating can be carried out over a period of time ranging from 1 minute to 10 hours.

[0079] According to a particular feature of the process, a heat transfer fluid may be present or circulate between the external portions 122a and 122b of the disk 120 and the inductors 110a and 110b. Such a fluid makes it possible to increase the heat losses at the external portions 122a, 122b of the disk 120 during heating by cooling of these latter. It is thus possible to increase the difference or gradient between the first temperature T1 and the second temperature T2. Such a heat transfer fluid is a means of regulating the temperature of the external portions 122a and 122b of the disk 120.

[0080] According to a particular characteristic of the process, the flow of the heat transfer fluid can be laminar or turbulent.

[0081] Turbulent flow promotes heat loss and is best suited to the geometry of the room to be heated.

[0082] According to a particular feature of the process, the heat transfer fluid can be a gas 140 as illustrated in [Fig. 1]. In this case, the installation 100 can include a port 101 for the inlet of the gas 140 and a port 102 for the outlet of the gas 140. Thus, it is possible to continuously renew the gas 140 circulating in the installation 100. This allows control of the temperature of the gas 140 near the external portions 122a and 122b of the disk, and consequently control of the difference between the first temperature T1 and the second temperature T2.

[0083] The gas 140 can be inert or non-inert with respect to the C / C composite material of the disc 120. When the gas 140 is non-inert, the process can include a step of removing a surface layer of the part, this removal being carried out after heating the part.

[0084] When the heat transfer fluid is a gas, the gas pressure can influence the difference between the first temperature T1 and the second temperature T2. Thus, the higher the gas pressure, the greater the difference between the first temperature T1 and the second temperature T2 will be. Conversely, a low-pressure gas allows for low convective losses and consequently creates a smaller average difference between the first temperature T1 and the second temperature T2, as illustrated in [Fig. 1]. "Low pressure" is defined as a pressure lower than ambient pressure and between 10 and 50 mbar.

[0085] Gas 140 can be chosen from helium, nitrogen, carbon dioxide, air, argon, xenon, krypton, neon or a mixture of these.

[0086] Such gases make it possible to reduce the risk of oxidation of the part during the process.

[0087] According to a particular feature of the process, a thermally insulating material can be disposed between the external portions 122a and 122b and the inductors 110a and 110b of the disk 120. An example of this embodiment is illustrated in [Fig.3].

[0088] In the exemplary embodiment illustrated in [Fig. 3], the thermally insulating material comprises three parts or layers 150a, 150b, and 150c. A first part 150a extends between the first inductor 110a and the first face 124a of the disk 120. A second part 150b extends between the second inductor 110b and the second face 124b of the disk 120. A third part 150c extends between the first part 150a and second part 150b of thermally insulating material, and around the outer perimeter of the disk 120. The third part thus forms a ring surrounding the disk 120 and extending between the first part 150a and the second part 150b of the thermally insulating material.

[0089] In one variant, the third part could extend both around the disk 120 and the first part 150a and the second part 150b of the thermally insulating material.

[0090] In another variant, we could have only the first part 150a and the second part 150b of the thermally insulating material.

[0091] Such a thermally insulating material makes it possible to reduce radiative and convective losses at the external portions 122a and 122b of the disk 120 and consequently generate a small difference between the first temperature T1 and the second temperature T2. Such a thermally insulating material is a means of regulating the temperature of the external portions 122a and 122b of the disk 120.

[0092] According to a particular feature of the process, a thermally insulating material may be present on one face of the inductors 110a and 110b opposite the external portion 122a and 122b. In this case, the thermally insulating material reduces radiative heat transfer between the disk 120 and the inductors 110a and 110b. This configuration thus reduces the difference between the first temperature T1 and the second temperature T2, and consequently creates a small difference between the temperatures T1 and T2. The presence of the thermally insulating material also protects the inductors and the tooling used during the implementation of the process. Furthermore, when an oxidizing heat transfer fluid is used, the thermally insulating material can protect the surface of the disk from oxidation. When a carbon-based heat transfer fluid is used, an unwanted deposit is formed during the process.Indeed, at high temperatures, the carbon-based heat transfer fluid degrades and eventually decomposes. Upon contact with hot areas, this heat transfer fluid forms heavier compounds such as benzene, toluene, ethylbenzene, xylenes (BTEX), polycyclic aromatic hydrocarbons (PAHs), tars, and / or solid carbon. When the component is a disc, and particularly a carbon-on-carbon disc, the unwanted deposits can concentrate on the disc's surfaces. Therefore, the presence of the thermally insulating material helps to concentrate these unwanted deposits away from the disc.

[0093] An additional thermally insulating material may further be disposed between the first face 124a of the disk 120 and the first inductor 110a and between the second face 124b of the disk 120 and the second inductor 110b. Such an additional thermally insulating material is a means of regulating the temperature of the external portions 122a and 122b of the disk 120.

[0094] The thermally insulating material and / or the additional thermally insulating material may be chosen from a refractory material comprising silica or silicates, alumina, quartz, mullite, refractory concretes, carbon (Papyex ®, Sigraflex ®) and carbonates, carbides (SiC, BC, TiC), or nitrides (BN, Si3N4, AIN, TiN), borides, chromium, or a mixture thereof.

[0095] In another variant, the process may include a machining step during which the parasitic deposit formed on the faces of the disc is removed.

[0096] In the embodiment illustrated in [Fig.3], the installation 100 combines both the presence of a heat transfer fluid in the form of a low-pressure gas 140 and the presence of a thermally insulating material comprising parts 150a, 150b, 150c. In this case, a small difference between the first temperature T1 and the second temperature T2 is obtained during heating.

[0097] When the heat transfer fluid is present in the form of a low-pressure gas, the first inductor 110a and the second inductor 110b preferably have a protective coating. Such a coating protects the inductors from the risk of electric arcing and from any chemical reaction between the low-pressure gas and the first and second inductors 110a, 110b.

[0098] According to another particular feature of the process, the heat transfer fluid can be a liquid 160, as illustrated in [Fig. 4]. Indeed, the liquid generates high convective losses at the external portions 122a and 122b. Thus, during heating, it is possible to cool the external portions 122a and 122b of the disk very efficiently. Consequently, the liquid 160 makes it possible to generate a large difference between the first temperature T1 and the second temperature T2.

[0099] When the heat transfer fluid is a liquid 160, the liquid particles may be transformed into a gas 170 during heating. In this case, the installation 100 may include a gas outlet port 102, as illustrated in [Fig. 4].

[0100] When the heat transfer fluid is a liquid 160, the installation 100 may include means for circulating the heat transfer fluid. Thus, the installation 100 may include a first port for the liquid inlet and a second port for the liquid outlet. This allows for the continuous renewal of the liquid 160 circulating in the installation 100. This makes it advantageous to control the temperature of the liquid near the external portions 122a and 122b of the disk 120, and consequently improves the control of the difference between the first temperature T1 and the second temperature T2.

[0101] According to a particular feature of the process, the temperature of the liquid may be lower than the second temperature T2. The lower the temperature of the heat transfer fluid, the more pronounced the temperature gradient in the room. The gradient of The temperature can also be accentuated when the heat transfer fluid has a high heat capacity.

[0102] Furthermore, the higher the temperature of the heat transfer fluid, the greater the evaporation and recondensation of the fluid. Conversely, recondensation will be lower when the enthalpy of vaporization of the heat transfer fluid is high.

[0103] The liquid can be chosen from liquid helium, liquid nitrogen, cyclohexane, toluene, petroleum carbon derivatives, supercritical carbon dioxide, liquid carbon dioxide, water or a mixture thereof.

[0104] When the liquid includes liquid nitrogen and / or liquid hydrogen, this gas evaporates or these gases evaporate and therefore generate a gaseous effluent containing dihydrogen and / or dinitrogen.

[0105] The dihydrogen present in the gaseous effluent is recovered and can be consumed by combustion and / or stored and / or recycled. When the dihydrogen is recycled, it can be used as a reducing agent in other industrial processes, reinjected into the heating system after being reliquefied, or used in the manufacturing process of brake discs prior to their heat treatment.

[0106] The nitrogen present in the gaseous effluent is recovered and can be used in the manufacturing process of the brake discs prior to their heat treatment.

[0107] According to a particular feature of the method, the disk 120 can be immersed in the liquid 160, as illustrated in the example of [Fig. 4]. In this case, in one embodiment, the first inductor 110a and the second inductor 110b can comprise a protective and inert layer with respect to the liquid 160. According to one embodiment, the protective and inert layer comprises an epoxy resin.

[0108] In another embodiment, the first inductor 110a and the second inductor 110b may comprise a material inert to the liquid 160. According to another particular feature of the process, the liquid 160 may circulate inside at least one heat transfer tube, which may be in contact with the external portions 122a and 122b of the disc 120. Such a configuration avoids direct contact between the external portions 122a and 122b of the disc 120 and the heat transfer fluid in its liquid state. This embodiment is advantageous when the liquid is not inert to the composite material of the disc. Indeed, contact between the disc and the non-inert liquid can cause degradation of a surface layer of the disc. A step to remove the surface layer would be necessary in this case after the heat treatment.Thus, thanks to the circulation of the liquid in at least one heat transfer tube, the step of removing the surface layer can be eliminated.

[0109] The process just described makes it possible to obtain carbon / carbon composite material parts comprising a modified microstructure and having tribological and thermal properties superior to those of the prior art, due to the treatment The thermal treatment is localized and differentiated between the external and internal portions of the part. As a result, the resulting part exhibits improved wear resistance and braking efficiency in its external portions, or portions subject to friction, compared to prior art. Similarly, the internal portion, or core, of the part exhibits better oxidation resistance and improved thermal diffusivity.

Claims

Demands

1. A heat treatment method for at least one friction piece (120) made of carbon / carbon composite material extending along a thickness direction between a first outer portion (122a) and a second outer portion (122b) delimiting between them an inner portion (123), characterized in that it comprises a step of simultaneously heating the first outer portion (122a), the second outer portion (122b) and the inner portion (123), the first outer portion (122a) and the second outer portion of each friction piece (120) being heated to a first temperature (Tl), while the inner portion of each friction piece (120) is heated to a second temperature (T2) higher than the first temperature (Tl) and in that the difference between the first temperature (Tl) and the second temperature (T2) is at least 1%.

2. A method according to claim 1, wherein, during the simultaneous heating step, each friction piece (120) is placed between a first inductor (110a) and a second inductor (110b), the first inductor (110a) extending parallel to a first face (124a) of the friction piece (120) and the second inductor (110b) extending parallel to a second face (124b) of the friction piece (120), each friction piece (120) being heated individually.

3. Method according to claim 2, wherein the friction piece (120) corresponds to a brake disc and wherein the first inductor (110a) and the second inductor (110b) each have a planar spiral shape.

4. A method according to any one of claims 2 or 3, wherein, during the simultaneous heating step, a heat transfer fluid is present or circulates between the first face (124a) of the friction piece (120) and the first inductor (110a) and between the second face (124b) of the friction piece (120) and the second inductor (110b).

5. A method according to claim 4, wherein the heat transfer fluid is a liquid (160), the liquid being selected from: liquid helium, liquid nitrogen, cyclohexane, toluene, petroleum carbon derivatives, supercritical carbon dioxide, liquid carbon dioxide, water or a mixture thereof.

6. A method according to claim 4, wherein the heat transfer fluid is a gas (140), the gas being selected from: helium, nitrogen, carbon dioxide, air, argon, xenon, krypton, neon or a mixture thereof.

7. A method according to any one of claims 4 to 6, wherein, during the simultaneous heating step, a thermally insulating material (150a, 150b, 150c) is present between the first face (124a) of the friction piece (120) and the first inductor (110a) and between the second face (124b) of the friction piece (120) and the second inductor (110b).

8. A method according to claim 7, wherein the thermally insulating material (150a, 150b, 150c) is selected from a refractory material comprising silica or silicates, alumina, quartz, mullite, refractory concretes, carbon (Papyex®, Sigraflex®) and carbonates, carbides (SiC, BC, TiC), or nitrides (BN, Si3N4, AIN, TiN), borides, chromium, or a mixture thereof.

9. A method according to any one of claims 1 to 8, further comprising a step of bringing a face of the carbon / carbon composite friction piece into contact with a cooling piece, the cooling piece being at a temperature lower than the second temperature, the contacting step being carried out during the heating step.

10. A method according to any one of claims 1 to 9, wherein the second temperature (T2) is greater than or equal to 1400°C.

11. A method according to any one of claims 1 to 10, wherein the duration of the simultaneous heating step is between 1 minute and 10 hours.

12. Heat treatment installation for at least one friction part (120) made of carbon / carbon composite material extending along a thickness direction between a first outer portion (122a) and a second outer portion (122b) delimiting between them an inner portion (123), the installation comprising: - a cold-walled enclosure (130), - two inductors (110a, 110b) configured to heat the first outer portion (122a) and the second outer portion (122b) of the friction part (120) to a first temperature (T1), and to heat the inner portion of the friction part (120) to a second temperature (T2) higher than the first temperature (Tl), - at least one means of regulating the temperature of the external portions of the friction part made of carbon / carbon composite material.

13. Installation according to claim 12, wherein the cold-walled enclosure (130) includes means for circulating a heat transfer fluid.

14. An installation according to any one of claims 12 to 14, wherein each friction piece (120) is placed between a first inductor (110a) and a second inductor (110b), and the installation further comprising a first cooling piece fixed between a first face (124a) of the friction piece (120) and the first inductor (110a) and a second cooling piece fixed between a second face (124b) of the friction piece (120) and the second inductor (110b).

15. Installation according to any one of claims 12 to 14, wherein each friction piece (120) is placed between a first inductor (110a) and a second inductor (110b), and the installation further comprising a first piece (150a) of a thermally insulating material fixed between a first face (124a) of the friction piece (120) and the first inductor (110a) and a second piece (150b) of a thermally insulating material fixed between a second face (124b) of the friction piece (120) and the second inductor (110b).

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