METHOD FOR JOINING A FIRST PART AND A SECOND PART BY DIRECT SOLDERING, INCLUDING STEPS FOR PREPARING THE SURFACE OF AT LEAST ONE OF THE PARTS

Description

[0001] The present invention relates to the general field of brazing and more particularly to a method of direct brazing of a first part and a second part, see claim 1.

[0002] The invention is particularly interesting since it allows direct assembly by soft soldering (typically for temperatures below 450°C) of two parts using non-active solders.

[0003] The invention finds applications in the field of implants, for example prostheses. STATE OF THE PRIOR ART

[0004] Conventionally, the brazing of a carbon part with a metal part is carried out at relatively high temperatures (in particular at temperatures above 800°C). As examples, the following heterogeneous assembly processes are known: a diamond part and a steel part joined with a Cu-10Sn-15Ti brazing at 925°C and 1050°C (Li et al. “Interfacial Segregation of Ti in the Brazing of Diamond Grits onto a Steel Substrate Using a Cu-Sn-Ti Brazing Alloy”, Metallurgical and Materials Transactions A 33A (2002) 2163-2172); a C / C composite and a titanium part assembled with solders marketed under the references Cu-ABA (92.8Cu-3Si-2Al-2.25Ti%), TiCuNi (70Ti-15Cu-15Ni) and TiCuSil (68.8Ag-26.7Cu-4.5Ti) at brazing temperatures between 920 and 1050°C under vacuum (Singh et al., “Active metal brazing and characterization of brazed joints in titanium to carbon-carbon composites”, Materials Science and Engineering A 412 (2005) 123-128); a C / C composite assembled to a Ti-6Al-4V metal part with an AgCuTi braze at 880°C (Liu et al."Brazing of CIC composite and Ti-6Al-4V with graphene strengthened AgCuTi filler: Effects of graphene on wettability; microstructure and mechanical properties", Chinese Journal of Acronautics, 31(7), (2018), 1602-1608); a C / C composite joined to a titanium-based metal part (TA6V) (WO 2007 / 066051 A2) using ceramic interlayers (e.g. tungsten or aluminum nitride) to reduce the stresses resulting from the differential expansion between the parts, and a brazing of the Ag-Mn type (whose melting point is higher than 961°C) or of the AgCuTi type (whose liquidus is higher than 815°C); considering the melting temperatures of the brazings, the brazing temperatures are certainly higher than 800°C.

[0005] However, to avoid the high thermomechanical stresses that appear in solid parts when assembling a carbon part and a metal part and not to damage the materials to be assembled, it is sometimes necessary to carry out the assembly at a lower temperature.

[0006] For example, in document US 2016 / 0144460 A1, tin-based alloys, such as Sn-Ag-Cu-Cr, Sn-Ag-Cu, or Sn-Ag-Cu-Ni are used for brazing carbon nanotubes to a copper element. To avoid damaging the carbon nanotubes by the liquid alloy, the process is carried out at temperatures below 700°C. This process is used to produce small devices, in this case electrical components. Such devices involve low deformations and therefore low thermomechanical stresses. Brazing is further facilitated by the use of copper, which is very ductile. As the tin-based alloy oxidizes easily in air and the presence of oxide leads to poor wetting of the brazing alloy, the process must be carried out using a flux (acids, solvents, etc.) to remove the oxides in air and / or in a protective atmosphere with a reduced oxygen content (inert atmosphere, reducing atmosphere, vacuum).However, such constraints complicate the implementation of the process.

[0007] In the article by Yu et al. (“Interface reaction in ultrasonic vibration-assisted brazing of aluminum to graphite using Sn-Ag-Ti solder foil,” Journal of Materials Processing Technology 221 (2015) 285-290), SnAgTi solders are used to braze aluminum with graphite. Brazing is carried out at 500°C in air, applying ultrasonic vibrations to destroy oxide films on the surface of the solder and aluminum and to improve wettability. The mechanical behavior of the joint is not studied. However, the use of ultrasound complicates the implementation of the process.

[0008] Another possible way to braze ceramics at low temperature is to achieve non-wettable metallization of the components with a coating whose process includes high-temperature annealing. This approach has been widely used for several decades to braze alumina.

[0009] For example, the most common alumina metallization technique involves applying a mixture of Mo and Mn powders and then performing a high-temperature heat treatment (>1400°C) to form a glassy phase. This coating is then covered by a 2 to 10 µm layer of Ni or Cu, which then allows conventional brazing using non-active brazing materials. This technique, known as "moly-manganese," is not applicable to carbon-based parts.

[0010] In document US 6528123 B1, the authors carry out the metallization of a ceramic (alumina or SiC) by depositing a submicron layer of an active element (Ti, Zr, Cr, V etc.) by PVD deposition then by depositing a thicker layer of a non-active element (Au, Cu, Ni, Ag, Pd, In or Mo). The parts thus metallized are then annealed at high temperature (>900°C) to ensure good adhesion of these layers to the ceramic part.

[0011] Concerning the metallization of carbon parts, current knowledge is very limited. However, we can cite the work of Song et al. (“A novel metallization process for soldering graphite to copper at low temperature”, Journal of Alloys and Compounds 696 (2007)1199-1204). Their process consists of applying a Sn0.3Ag0.7Cu-9%Cr powder of approximately 50 µm thickness on graphite parts and then carrying out a heat treatment at 950°C for 30 min to create a layer wettable by non-active solders. The assembly with copper was then carried out at 250°C using the Sn3Ag0.5Cu solder paste in air on small parts (20 mm × 10 mm × 3 mm). The disadvantage of this method is that the deposition step is carried out in two stages and it would require the metallized surface to be reworked by polishing.

[0012] Document FR 3 095 151 A1 (describing the preamble of claim 1) proposes a method for assembling a first carbon part and a second part based on titanium or a CrCo alloy, the method comprising the steps of (a) bringing a filler material into contact with the first part and the second part, (b) heating the assembly to an assembly temperature greater than or equal to 600°C and less than or equal to 800°C and (c) cooling the assembly so as to form a brazing joint between the two parts and to assemble them.

[0013] Document US 4,468,309 A describes a method for protecting a tubular part against damage by friction. The tubular part, which may be made of a nickel-based alloy, is, depending on the embodiment of the figure 2 , covered with a layer of chrome then with a layer of gold.

[0014] Document CN 103 352 222 A describes a method for preparing a tungsten coating on a carbon material part comprising the following steps: (1) pretreating the surface of the part; (2) plasma cleaning the part and depositing a layer of a transition element such as, for example, chromium by cathode sputtering and (c) gas-depositing a tungsten coating. STATEMENT OF THE INVENTION

[0015] An aim of the present invention is to propose a method for assembling two parts by brazing comprising a step of preparing the surface of a part made of carbon (in particular, pyrolytic carbon, graphite or C / C composite covered with pyrolytic carbon, or vitreous carbon) or titanium-based, nickel-based or CoCr which is simple and inexpensive to implement, this part being intended to be assembled by brazing with another part at temperatures below 450°C, using non-active brazing materials, without using interlayer material, flux or ultrasonic vibrations, the assembly obtained having to have good mechanical strength and a long service life.

[0016] For this, the present invention proposes a method of assembly by brazing as defined in the appended claim 1, comprising the preparation of the surface of a part intended to be assembled with another part by brazing comprising the following steps: a) providing a part intended to be brazed, the part being made of carbon, preferably pyrolytic carbon, graphite or C / C composite coated with pyrolytic carbon, or vitreous carbon, or titanium-based, nickel-based or CoCr, b) carrying out a plasma treatment of a neutral gas on the part, whereby the part is cleaned and an active surface is formed on the part, c) depositing a first layer of an active element (called a bonding layer) on the active surface of the part, the active element being a carbide-forming element; the surface activation carried out in step b) leading to better adhesion of the metallic bonding layer, d) depositing a second layer of gold on the first layer, whereby the first layer is protected from oxidation.

[0017] The invention fundamentally differs from the prior art by the implementation of specific steps in order to prepare the surface of the part to be assembled.

[0018] It is thus possible to then assemble the part with another part at an assembly temperature below 350°C, and preferably below 320°C.

[0019] The surface preparation of the part to be assembled in the proposed process allows to obtain good wetting at such temperatures, and a better mechanical response. The reduction of the thermal amplitude reduces the thermomechanical stresses.

[0020] The preparation method implemented in the invention makes it possible to durably assemble parts of different dimensions, and in particular solid parts resistant to mechanical stress. The parts thus assembled have strong interfaces and good mechanical strength despite the difference in thermomechanical behavior between, for example, a carbon part and a metal part based on titanium, nickel or CoCr.

[0021] Particular aspects of the present invention are defined in the dependent claims.

[0022] Advantageously, the active element is Ti, Cr or Zr.

[0023] Advantageously, the first layer has a thickness of 300 µm to 500 µm.

[0024] According to a first advantageous embodiment variant, the method comprises a step e), between step c) and step d), during which a third layer of a non-active element is deposited. This produces a stack of three layers comprising, from the active surface of the part: the first layer, the third layer and the second layer. The third layer acts as a diffusion barrier with respect to the first layer.

[0025] Advantageously, the non-active element is W or Nb.

[0026] Advantageously, according to this embodiment variant, the first layer has a thickness of 30 to 50 nm and the third layer has a thickness of 100 to 500 nm.

[0027] According to a second advantageous embodiment variant, the first layer comprises, in addition to the active element, a non-active element, preferably W or Nb, deposited simultaneously with the active element during step c). Advantageously, such a layer comprises at least 40% by volume, and preferably at least 60%, of the active element in order to form a continuous phase of active element.

[0028] Advantageously, according to this embodiment variant, the third layer has a thickness of 50 to 100 nm.

[0029] For these two advantageous embodiments, the non-active element acts as a diffusion barrier and protects the active element of the bonding layer from the solder. Indeed, the dissolution of the active element in the solder can lead to a loss of adhesion.

[0030] The first part and the second part can be the same or different.

[0031] According to a first particular embodiment, the second part provided in step ii) is advantageously prepared according to the preparation method as described previously. In other words, the second part is covered with a stack comprising a first layer containing a carbide-forming element and a second layer of gold. The stack may also comprise a third layer containing a non-active element, positioned between the first layer and the second layer. Alternatively, the first layer may comprise the non-active element. This first particular embodiment is particularly advantageous in the case of a second carbon part.

[0032] According to a second particular embodiment, preferably carried out in the case of a second metal part based on titanium, based on nickel or in CoCr, the second part can be prepared according to the following steps: a') carrying out a plasma treatment of a neutral gas, preferably argon, on the second metal part, whereby the second metal part is cleaned and an active surface is formed on the second metal part, b') covering the active surface of the second metal part with a layer of gold, preferably having a thickness of 50 to 100 nm, whereby the active surface is protected from oxidation and good wetting is ensured.

[0033] This second particular embodiment is particularly advantageous in the case of a second metal part, for example in CoCr, titanium-based or nickel-based.

[0034] Advantageously, the filler material is tin or a tin-based alloy. It may be a binary tin-based alloy such as SnAg, SnCu or SnIn. It may also be a ternary tin-based alloy.

[0035] Preferably, the assembly temperature is less than 350°C (for example between 250°C and 350°C), and even more preferably less than 320°C (for example between 250°C and 320°C). The assembly temperature depends on the brazing chosen.

[0036] Advantageously, the first part is a graphite substrate coated with pyrolytic carbon and the second part is made of CoCr alloy.

[0037] The process has many advantages: it does not require a high-temperature thermal annealing step of the layers deposited on the part before assembly, unlike the process for preparing oxide ceramic parts by PVD metallization, for which the annealing step is necessary to ensure good adhesion, it allows the assembly of carbon parts with a metal part at low temperature (<450°C) without applying ultrasonic vibrations or using flux during the brazing operation, the brazing temperatures are lower than 350°C, and preferably lower than 320°C, which makes it possible to assemble massive parts that must withstand mechanical forces, despite a difference in thermal expansion of the parts to be assembled;such a difference generally leads to high thermomechanical stresses when assembling a carbon part and a metal part at high temperature (>800°C), the assembly of such surfaces by brazing is direct and does not require the use of an interlayer material, such as copper or molybdenum, it allows the assembly of two carbon parts by direct brazing with a non-active brazing material, the joints formed and the final assembly have good mechanical strength. ;

[0038] Other characteristics and advantages of the invention will emerge from the additional description which follows.

[0039] It goes without saying that this additional description is given only as an illustration of the subject of the invention and must in no case be interpreted as a limitation of this subject. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will be better understood by reading the description of exemplary embodiments given purely for informational purposes and in no way limiting, with reference to the appended drawings in which: There figure 1 schematically represents different stages of a process for preparing a part for brazing. The figure 2 schematically represents, in a schematic manner, different stages of a method of preparing a part for brazing, according to another embodiment. The figure 3 schematically represents different stages of a process for preparing a part, preferably metallic (for example based on Ti, Ni or CoCr) for brazing, according to another embodiment: The figure 4 represents, schematically, in section, two parts to be brazed and a brazing supply in a so-called “sandwich” configuration before assembly, according to a particular embodiment of the invention. The Figure 5 is a photographic image of various graphite parts coated with PyC and metallized, and positioned in a holding tool, in a brazing furnace, according to a particular embodiment of the invention. The figure 6 is a photographic image of graphite parts coated with PyC and metallized by a TiAu bilayer formed by PVD, covered by brazing sheets, on which TA6V rods metallized by a TiWAu trilayer formed by PVD have been positioned, the assemblies thus formed being held in a tool, in a brazing furnace, according to another particular embodiment of the invention. figure 7is a photographic image of a torsion test specimen obtained by brazing a graphite part coated with PyC and metallized by a TiAu bilayer formed by PVD and a CoCr part metallized by a gold layer obtained by PVD, with a SnAg brazing, according to another particular embodiment of the method of the invention. figure 8 is an optical image of the joint of the torsion test piece obtained by brazing a graphite part coated with PyC and PVD metallized with TiAu and a CoCr part metallized with a gold layer formed by PVD after a mechanical torsion test, according to another particular embodiment of the method of the invention.

[0041] The different parts represented in the figures are not necessarily on a uniform scale, to make the figures more readable.

[0042] The different possibilities (variants and embodiments) must be understood as not being mutually exclusive and can be combined with each other.

[0043] Furthermore, in the following description, terms that depend on the orientation, such as "above", "below", etc. of a structure apply with the assumption that the structure is oriented as illustrated in the figures. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0044] We will now describe in more detail the method of assembling a first part 100 with a second part 200 by brazing at temperatures below 450°C, using non-active brazes, without using interlayer material, flux or ultrasonic vibrations.

[0045] The first part 100 and / or the second part 200 may be metal parts, for example titanium-based, nickel-based or CoCr-based or carbon parts.

[0046] The carbon part (or carbonaceous part) can be chosen from pyrolytic carbon parts, graphite parts or a C / C composite covered with pyrolytic carbon, or glassy carbon.

[0047] Titanium-based means that the part contains more than 50% titanium by mass. This can be titanium or a titanium alloy. The titanium-based part can be, for example, Ti-6Al-4V (TA6V), Ti-6Al-7Nb, Ti-5Al-2.5Fe, Ti-13Nb-13Zr, Ti-12Mo-6Zr-2Fe, Ti35Nb-7Zr-5Ta, Ti-29Nb-13Ta-4.6Zr, Ti-35Nb-5Ta-7Zr-0.40, Ti-15Mo-5Zr-3Al, Ti-Mo or T40.

[0048] Nickel-based means that the part contains more than 50% nickel by mass. This can be nickel or a nickel alloy. The nickel-based part can be made of an Inconel ®< , Monel ®< , or Hastelloy ®< alloy.

[0049] The CoCr part can be made of a UNS R30008 alloy.

[0050] The part(s) to be assembled may have dimensions of a few mm 2< to a few cm 2< .

[0051] The method of assembling the first part 100 and the second part 200 comprises the following successive steps: i) surface preparation of the first part 100, ii) surface preparation of the second part 200, iii) bringing a filler material 300 into contact with the first part 100 and with the second part 200, iv) heating the assembly obtained in step iii) to an assembly temperature higher than the melting temperature of the filler material 300, so as to melt the filler material 300, and maintaining the assembly temperature for a holding time, the assembly temperature being less than 450°C, and preferably less than 350°C, and even more preferably less than 320°C, v) cooling the assembly so as to form a brazing joint between the first part 100 and the second part 200, and to assemble them.

[0052] The method of preparing the first part 100 (step i) comprises the following steps ( figure 1 ) : providing the first part 100 intended to be brazed, possibly chemical cleaning of the first part 100, for example in an ultrasonic bath with acetone then ethanol, rinsing and drying, carrying out a surface treatment of the first part 100 by ionic etching by bombardment of neutral gas ions, preferably an argon plasma, to, if necessary, remove anything that remains after the chemical cleaning and activate the surface 101 of the first part 100 for better adhesion of the first layer on the active surface 101, depositing a first layer 110 (called the bonding layer) on the active surface 101 of the first part 100, the first layer 110 comprising a carburizing active element to create good adhesion with the carbonaceous active surface 101, depositing a second gold layer 120,preferably having a thickness of 50 to 100 nm to protect the first layer 110 from oxidation and ensure good wetting during assembly by soldering.

[0053] A carbide-forming element is an element that forms carbides. Examples of such elements are zirconium, molybdenum, chromium, titanium, niobium, and tungsten. Preferably, the bonding layer is made of Ti, Cr, or Zr. It could also be an alloy of these elements, such as TiZr.

[0054] The bonding layer 110 preferably has a thickness of 300 to 500 µm.

[0055] According to an embodiment variant shown in the figure 2 , a third layer 130 can be deposited between the bonding layer 110 and the gold layer 120. This layer acts as a diffusion barrier.

[0056] The barrier layer 130 is made of a non-active element. It can be, for example, made of W or Nb. It could also be made of nickel. According to this embodiment variant, the bonding layer 110 advantageously has a thickness of 30 to 50 nm and the diffusion barrier layer 130 advantageously has a thickness of 100 to 500 nm.

[0057] According to another embodiment variant, it is possible to simultaneously deposit the active carburizing element and the non-active element. The first layer 110 thus obtained advantageously has a thickness of 100 to 500 nm. The active carburizing element preferably represents at least 40% by volume of the first layer 100 in order to form a continuous phase within the first layer 110.

[0058] Layers 110, 120 and / or 130 can be deposited by physical vapor deposition (PVD), chemical vapor deposition (CVD), or by plasma spraying.

[0059] Preferably, a PVD deposit will be carried out.

[0060] Even more advantageously, plasma treatment can be carried out in situ in the deposit machine.

[0061] The neutral gas can be argon or nitrogen. Preferably, it is argon.

[0062] This surface preparation process is particularly advantageous in the case of a carbon part.

[0063] According to a first embodiment, the second part 200 is also prepared using the preparation method described above. For example, to assemble two carbon parts 100, 200, the surface of the two parts can be prepared using the preparation method described above.

[0064] According to another variant embodiment, the method for preparing the surface of the second part 200 may comprise the following steps: ( Figure 3 ) : carrying out a surface treatment of the second part 200 by ionic etching, so as to form an active surface, depositing a layer of gold 210 preferably having a thickness of 50 to 100 nm, on the active surface.

[0065] The 210 gold layer will quickly dissolve in the solder as soon as the latter is liquid.

[0066] This embodiment variant is advantageous in the case of a second metallic part 200 based on titanium, nickel or CoCr.

[0067] The metal part may be covered by a thin native oxide layer that does not allow good wetting by the solder at low temperatures (<450°C). However, the reactivity at the interface is good enough to ensure good solder adhesion.

[0068] The first part 100 and the second part 200 are then assembled ( figure 4 ).

[0069] The solder 30 (also called filler material or solder alloy) is advantageously a non-active solder. Preferably the solder is a tin solder or a solder made of a tin alloy. Preferably, the tin alloy is chosen from SnIn, SnAg (96.5Sn-3.5Ag) and SnAgCu (96.5%Sn 3% Ag 0.5%Cu).

[0070] The solder 30 is positioned between the two parts 100, 200 to be assembled, and more particularly between the two gold layers 120, 210 (step iii).

[0071] After heating (step iv) and cooling (step v), an assembly with good mechanical properties is obtained. The parts 100, 200 are assembled by a crack-free brazing joint.

[0072] Although this is in no way limiting, the invention finds particular applications in the medical field for manufacturing biocompatible implants. The implants can, for example, be formed from the following two parts (or portions): a first part made of carbon and in particular graphite coated with PyC or C / C composite coated with PyC, and a second metal part, for example made of titanium, TA6V or CrCo.

[0073] These implants (prostheses) can, for example, have brazing surfaces of the order of 20 cm 2< . Illustrative and non-limiting examples of an embodiment :

[0074] In this example, several torsion specimens were produced by joining a carbon part and a metal part using low-temperature brazing.

[0075] The metal part was machined into a 20 mm long, 10 mm diameter rod. It is made of Co-based alloy (UNS R30008), the composition of which is given in the following table: Element % by mass Co 39,0 à 42,0 Cr 18,5 à 21,5 Fe base Neither 15,0 à 18,0 Mo 6,5 à 7,5 Mn 1,0 à 2,0

[0076] The carbon part, denoted G / PyC, is a POCO graphite disc coated with PyC pyrolytic carbon (~200 µm) 21 mm in diameter and 3 mm in height.

[0077] Solder (or soldering alloy) is a tin-based alloy SnAg. Its composition is 96.5Sn-3.5Ag (by mass), its melting point is close to that of tin (232°C). Such an alloy is, for example, marketed in the form of a ribbon of various thicknesses by the Indium Corporation under the trade name Indalloy-121. In these examples, the ribbon has a thickness of 100 µm. Metallization of G / PyC parts and CoCr parts:

[0078] First, the carbon and metal parts are cleaned in an ultrasonic bath with acetone then ethanol and then dried with compressed air.

[0079] In the following examples, the parts are then covered with metal masks (preferably stainless steel) to limit the metallization area to the assembly surface.

[0080] Then, the parts covered by the masks are positioned in the enclosure of a PVD machine, then the system is pumped up to 5×10 -7< mbar.

[0081] The parts are then treated in the machine enclosure by Ar plasma in order to thoroughly clean their surfaces and form an active surface to improve the adhesion of the subsequent metal deposit (50W, 120 s). Then the parts are metallized. Example 1: Metallization of a G / PyC part by Ti / Au (figure 1):

[0082] 100 coins are in G / PyC.

[0083] A 500 nm active element (Ti) bonding layer 110 is deposited on the active surface 101 of the part 1000 at a speed of 0.3 nm / s in the PVD enclosure.

[0084] Then, a 100 nm thick non-active element (Au) coating layer 120 is deposited to protect the Ti thin film from oxidation and improve wetting. Example 2: Metallization of a G / PyC part by Ti / W / Au (figure 2):

[0085] 100 coins are in G / PyC.

[0086] A 30 nm Ti bonding layer 110 is deposited on the active surface 101 of the part 100 at a speed of 0.3 nm / s in the PVD enclosure.

[0087] Then a 100 nm thick W barrier layer 130 is deposited to block the diffusion and dissolution of the bonding layer 110 in the solder. This barrier layer 130 prevents the dissolution of the active element layer.

[0088] Finally, a final 120 layer of non-active element (Au) 100 nm thick is made to protect the deposit from oxidation and improve wetting. Example 3: Metallization of a CoCr part by Au (figure 3):

[0089] The 200 pieces are made of CoCr.

[0090] A 100 nm thick non-active element (Au) 210 layer is deposited to improve low temperature wetting. Assembly (figure 4):

[0091] The 100 metallized G / PyC parts and the 200 metallized CoCr parts are assembled without additional cleaning.

[0092] A 30 solder in sheet or ribbon form is cut with a die cutter and cleaned in an ultrasonic bath with acetone and then ethanol.

[0093] The 30 solder is then positioned on the 100 carbon part ( Figure 5 ), then the metal part 200 is brought into contact with the solder 30. The assembly is held in a tool ( figure 6 ), then placed in a secondary vacuum furnace. A mass is added to apply pressure (typically 15 to 20 g / cm 2< ) to the area to be brazed.

[0094] The assembly is heated to a holding temperature, higher than the melting point of the solder (350°C, in this case) for a holding time of a few tens of minutes (10 to 15 min).

[0095] After cooling, the assembly is removed from the oven. The pieces are joined together ( figure 7 ). Mechanical torsion tests :

[0096] The assemblies thus obtained by brazing are subjected to various mechanical torsion tests. The tests were carried out at room temperature (typically 20 to 25°C) with an electromechanical tensile testing machine marketed by MTS Systems Corporation. The rotation of the mandrel is actuated by the movement of the crosshead (0.5 mm / min).

[0097] The maximum tangential stresses (at break) are calculated by the following formula: with F is the maximum force applied; R is the radius of the torsion axis and C the radius of the torsion pin.

[0098] The following table lists the results of the mechanical torsional resistance for the different tests carried out. Example Substrate Metal part Solder Stress (MPa) 1a G / PyC (Ti, Au) CoCr (Au) Sn3.5Ag 29,7 1b G / PyC (Ti, Au) CoCr (Au) Sn3.5Ag 34,6 1c G / PyC (Ti, Au) CoCr (Au) Sn3.5Ag 38,9 1d G / PyC (Ti, Au) CoCr (Au) Sn3.5Ag 41,8 1e G / PyC (Ti, Au) CoCr (Au) Sn3.5Ag 31,1 2a G / PyC (Ti, W, Au) CoCr (Au) Sn3.5Ag 25,3 2b G / PyC (Ti, W, Au) CoCr (Au) Sn3.5Ag 24,2 2c G / PyC (Ti, W, Au) CoCr (Au) Sn3.5Ag 22,8 The maximum tangential stresses (at break) obtained for carbon parts with Ti / Au deposit (1a to 1e) are greater than or equal to 30 MPa and go up to 41.8 MPa, confirming the strength of the brazing joint obtained. The breakage is essentially cohesive and occurs in the brazing ( figure 8 ).

[0099] The metallization of the carbon part with the Ti / Au deposit ensures better mechanical strength due to excellent adhesion of the solder to the Ti layer. The rupture occurs in the solder with values ​​corresponding to the strength of the solder itself.

[0100] As for carbon parts with Ti / W / Au deposit (2a to 2c) the maximum tangential stresses (at rupture) are greater than or equal to 22.8 MPa. The rupture is essentially adhesive and occurs at the brazing / W layer interface.

Claims

1. A method for assembling a first part (100) and a second part (200) by brazing, comprising the following steps: i) preparing a first part (100) made of carbon, titanium-based, nickel-based or made of a CoCr alloy, ii) preparing a second part (200) made of carbon, titanium-based, nickel-based or made of a CoCr alloy, iii) bringing a filler material (300) into contact with the first part (100) and with the second part (200), iv) heating the assembly obtained at the step iii), v) cooling the assembly so as to form a solder joint between the part made of carbon (100) and the metal part (200), whereby the parts are assembled, characterised in that the preparation or the first part (100) during step i) and optionally the preparation or the second part (200) during step ii) is performed according to the following sub-steps a) to d) : a) providing the first part (100) made of carbon, titanium-based, nickel-based or made of a CoCr alloy, b) carrying out a plasma treatment with a neutral gas, on the first part (100) whereby the part is cleaned and an active surface (101) is formed on the part, c) depositing a first layer (110) comprising an active element over the active surface (101) of the first part (100), the active element being a carbide-forming element, and d) depositing a second layer (120) made of gold over the first layer (110), whereby the first layer (110) is protected from oxidation and good wetting is ensured and in that the heating at step iv) is performed up to an assembly temperature higher than the melting temperature of the filler material (300), so as to melt the filler material, and maintaining the assembly temperature for a hold time period, the assembly temperature being lower than 450°C.

2. The method according to claim 1, characterised in that the active element is Ti, Cr or Zr.

3. The method according to one of claims 1 and 2, characterised in that the first layer (110) has a thickness of 300 µm to 500 µm.

4. The method according to one of claims 1 and 2, characterised in that the method comprises a step e), between step c) and step d), during which a third layer (130) of a non-active element, preferably W or Nb, is deposited.

5. The method according to the preceding claim, characterised in that the first layer (110) has a thickness of 30 to 50 nm and in that the third layer (130) has a thickness of 100 to 500 nm.

6. The method according to one of claims 1 and 2, characterised in that the first layer also comprises a non-active element, preferably W or Nb, deposited simultaneously with the active element during step c).

7. The method according to claim 4, characterised in that the third layer (130) has a thickness of 50 to 100 nm.

8. The method according to any one of the preceding claims, characterised in that the filler material (300) is tin or a tin-based alloy such as SnAg, SnCu or SnIn.

9. The method according to claim 8, characterised in that the assembly temperature is comprised between 250°C and 320°C.

10. The method according to any one of the preceding claims, characterised in that the first part (100) is a graphite substrate covered with pyrolytic carbon and in that the second part (200) is made of a CoCr alloy.