Method for joining, by direct brazing, a first part and a second part, including steps of preparing the surface of at least one of the parts

Abstract

A method for joining, by brazing, a first part and a second part, the steps of preparing at least one of the parts including the following: a) providing a part intended to be brazed, the part being made of carbon or based on titanium, nickel or a CoCr alloy, b) performing inert gas plasma treatment on the part whereby the part is cleaned and an active surface is formed on the part, c) depositing a first layer comprising an active element on the active surface of the part, the active element being a carbide-forming element, d) depositing a second layer of gold on the first layer, whereby the first layer is protected from oxidation and good wetting is ensured.

Claims

1. A method for assembling a first part and a second part by brazing, comprising the following steps: i) preparing a first part made of carbon, a titanium-based material, a nickel- based material or a CoCr alloy according to the following sub-steps a) to d): a) providing the first part 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 whereby the part is cleaned and an active surface is formed on the part, c) depositing a first layer comprising an active element over the active surface of the first part, the active element being a carbide-forming element, d) depositing a second layer made of gold over the first layer, whereby the first layer is protected from oxidation and wetting of a filler material is improved during heating; ii) preparing a second part made of carbon, a titanium-based material, a nickel-based material or a CoCr alloy, the second part being, advantageously, prepared according to sub-steps a) to d) as defined in step i); iii) bringing the filler material into contact with the first part and with the second part whereby an assembly is obtained; iv) heating the assembly obtained at the step iii) up to an assembly temperature higher than the melting temperature of the filler material, 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.; v) cooling the assembly so as to form a solder joint between the first part and the second part, whereby the parts are assembled.

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

3. The method according to claim 1, wherein the first layer has a thickness of 300 m to 500 m.

4. The method according to claim 1, wherein the method comprises a step e), between step c) and step d), during which a third layer of a non-active element is deposited.

5. The method according to claim 4, wherein the first layer has a thickness of 30 to 50 nm and in that the third layer has a thickness of 100 to 500 nm.

6. The method according to claim 1, wherein the first layer also comprises a non-active element deposited simultaneously with the active element during step c).

7. The method according to claim 4, wherein the third layer has a thickness of 50 to 100 nm.

8. The method according to claim 1, wherein the filler material is tin or a tin-based alloy.

9. The method according to claim 8, wherein the assembly temperature is between 250 C. and 320 C.

10. The method according to claim 1, wherein the first part is a graphite substrate covered with pyrolytic carbon and in that the second part is made of a CoCr alloy.

11. The method according to claim 4, wherein the non-active element is W or Nb.

12. The method according to claim 6, wherein the non-active element is W or Nb.

13. The method according to claim 8, wherein the tin-based alloy is SnAg, SnCu or SnIn.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood upon reading the description of embodiments given merely for indication and without limitation with reference to the appended drawings wherein:

(2) FIG. 1 schematically shows different steps of a method for preparing a part for brazing, according to a particular embodiment of the invention.

(3) FIG. 2 schematically shows different steps of a method for preparing a part for brazing, according to another particular embodiment of the invention.

(4) FIG. 3 schematically shows different steps of a method for preparing a part, preferably metallic (for example based on Ti, Ni or CoCr) for brazing, according to another particular embodiment of the invention.

(5) FIG. 4 schematically shows, in section, two parts to be brazed and a supply of solder in a so-called sandwich configuration before assembly, according to a particular embodiment of the invention.

(6) FIG. 5 is a photograph of different parts made of graphite coated with PyC and metallised, and positioned in a holding tool, in a brazing furnace, according to a particular embodiment of the invention.

(7) FIG. 6 is a photograph of graphite parts coated with PyC and metallised with a TiAu bilayer formed by PVD, covered with solder foils, on which TA6V rods metallised with a TiWAu trilayer formed by PVD have been positioned, the sets thus formed being held in a tooling, in a brazing furnace, according to another particular embodiment of the invention.

(8) FIG. 7 is a photograph of a torsion specimen obtained by assembling by brazing a part made of graphite coated with PyC and metallised by a TiAu bilayer formed by PVD and a part made of CoCr metallised by a gold layer obtained by PVD, with a SnAg solder, according to another particular embodiment of the method of the invention.

(9) FIG. 8 is an optical photograph of the joint of the torsion specimen obtained by assembling by brazing a part made of graphite coated with PyC and metallised by PVD with TiAu and a part made of CoCr metallised with a gold layer formed by PVD after a mechanical torsion test, according to another particular embodiment of the method of the invention.

(10) The different portions shown in the figures are not necessarily plotted according to a uniform scale, to make the figures more readable.

(11) The different options (alternative embodiments and embodiments) should be understood as not being mutually exclusive and can be combined with one another.

(12) Furthermore, in the description hereinafter, terms that depend on the orientation, such as top, bottom, etc., of a structure apply while considering that the structure is oriented as illustrated in the figures.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

(13) We will now describe in more detail the method for assembling a first part 100 with a second part 200 by brazing at temperatures lower than 450 C., using non-active solders, without using any intermediate material, fluxing or ultrasonic vibrations.

(14) The first part 100 and/or the second part 200 may be metal parts, for example titanium-based, nickel-based or made of CoCr or parts made of carbon.

(15) The part made of carbon (or carbon part) may be selected from among parts made of pyrolytic carbon, of graphite or of a C/C composite covered with pyrolytic carbon, or with vitreous carbon.

(16) By titanium-based, it should be understood that the part comprises more than 50% by mass of titanium. It may consist of titanium or of a titanium alloy. For example, the titanium-based part may be made of 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, TiMo or T40.

(17) By nickel-based, it should be understood that the part comprises more than 50% by mass of nickel. It may consist of nickel or a nickel alloy. The nickel-based part may be made of an Inconel, Monel, or Hastelloy alloy.

(18) The part made of CoCr may be made of a UNS R30008 alloy.

(19) The part(s) to be assembled may have dimensions of a few mm.sup.2 to a few cm.sup.2.

(20) The method for assembling the first part 100 and the second part 200 comprises the following successive steps: preparing the surface of the first part 100, ii) preparing the surface of the second part 200, iii) bringing a filler material 300 in contact with the first part 100 and with the second part 200, iv) heating the assembly obtained in step iii) up to an assembly temperature higher than the melting temperature of the filler material 300, so as to melt the filler material 300, and keeping the assembly temperature for a hold time period, the assembly temperature being lower than 450 C., and preferably lower than 350 C., and still more preferably lower than 320 C., v) cooling the assembly so as to form a solder joint between the first part 100 and the second part 200, and to assemble them.

(21) The method for preparing the first part 100 (step i) comprises the following steps (FIG. 1): providing the first part 100 intended to be brazed, optionally chemically cleaning the first part 100, for example in an ultrasonic tank with acetone and then ethanol, rinsing and drying, carrying out a surface treatment of the first part 100 by ion etching by bombardment with neutral gas ions, preferably an argon plasma, where appropriate, to eliminate anything that persists after the chemical cleaning and activate the surface 101 of the first part 100 for a better adhesion of the first layer on the active surface 101, depositing a first layer 110 (called adhesion layer) over the active surface 101 of the first part 100, the first layer 110 comprising an active carbide-forming element to create a good adhesion with the active carbon surface 101, depositing a second layer of gold 120, preferably having a thickness of 50 to 100 nm to protect the first layer 110 from oxidation and ensure a good wetting during assembly by brazing.

(22) By carbide-forming element, it should be understood an element which forms carbides. For example, such elements are zirconium, molybdenum, chromium, titanium, niobium and tungsten. Preferably, the adhesion layer is made of Ti, Cr or Zr. It could also consist of an alloy of these elements such as TiZr.

(23) Preferably, the adhesion layer 110 has a thickness of 300 to 500 m.

(24) According to a variant shown in FIG. 2, a third layer 130 may be deposited between the adhesion layer 110 and the gold layer 120. This layer serves as a diffusion barrier.

(25) The barrier layer 130 is made of a non-active element. For example, it may be made of W or Nb. It could also be made of nickel. According to this variant, the adhesion 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.

(26) According to another variant, it is possible to simultaneously deposit the carbide-forming active element and the non-active element. Advantageously, the first layer 110 thus obtained has a thickness of 100 to 500 nm. Preferably, the carbide-forming active element forms at least 40% by volume of the first layer 100 in order to form a continuous phase within the first layer 110.

(27) The layers 110, 120 and/or 130 may be deposited by physical vapour deposition (or PVD), by chemical vapour deposition (or CVD), or else by plasma sputtering.

(28) Preferably, a deposition by PVD will be carried out.

(29) Still more advantageously, the plasma treatment may be carried out in situ in the deposition machine.

(30) The neutral gas may be argon or nitrogen. Preferably, it consists of argon.

(31) This process for preparing the surface is particularly advantageous in the case of a part made of carbon.

(32) According to a first variant, the second part 200 is also prepared with the previously-described preparation method. For example, to assemble two carbon parts 100, 200, the surface of the two parts may be prepared with the previously-described preparation method.

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

(34) The gold layer 210 will be rapidly dissolved in the solder as soon as the latter becomes liquid.

(35) This variant is advantageous in the case of a second metal part 200 titanium-based, nickel-based or made of CoCr.

(36) The metal part may be covered with a thin native oxide layer which does not allow for a good wetting by the solder at low temperature (<450 C.). Nonetheless, the reactivity at the interface is good enough to ensure a good adhesion of the solder.

(37) Afterwards, the first part 100 and the second part 200 are assembled (FIG. 4).

(38) Advantageously, the solder 30 (also called filler material or brazing alloy) is a non-active solder. Preferably, the solder is a tin solder or a solder made of a tin alloy. Preferably, the tin alloy is selected from among SnIn, SnAg (96.5Sn-3.5Ag) and SnAgCu (96.5% Sn 3% Ag 0.5% Cu).

(39) 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).

(40) After heating (step iv), and cooling (step v), an assembly is obtained having good mechanical properties. The parts 100, 200 are assembled by a solder joint devoid of cracks.

(41) Although this is in no way limiting, the invention finds particular applications in the medical field to manufacture biocompatible implants. For example, the implant may be formed of the following two parts (or portions): a first part made of carbon and in particular of graphite coated with PyC or of a C/C composite coated with PyC, and a second metal part, for example of titanium, of TA6V or of CrCo.

(42) For example, these implants (prostheses) may have surfaces to be brazed in the range of 20 cm.sup.2.

Illustrative and Non-Limiting Examples of One Embodiment

(43) In this example, several torsion specimens have been made by assembling a part made of carbon and a metal part by means of a low-temperature solder.

(44) The metal part has been machined in the form of a 20 mm long and 10 mm diameter rod. It is made of a Co based alloy (UNS R30008) whose composition is given in the following table:

(45) TABLE-US-00001 Element w % Co 39.0 to 42.0 Cr 18.5 to 21.5 Fe base Ni 15.0 to 18.0 Mo 6.5 to 7.5 Mn 1.0 to 2.0

(46) The carbon part, denoted G/PyC, is a POCO graphite disc coated with pyrolytic carbon PyC (200 m) with a diameter of 21 mm and a height of 3 mm.

(47) The solder (or brazing alloy) is a SnAg tin-based alloy. Its composition is 96.5Sn-3.5Ag (by mass), its melting point is close to that of tin (232 C.). For example, such an alloy is commercialized in the form of a strip of different thicknesses by the company Indium Corporation under the trade name Indalloy-121. In these examples, the strip has a thickness of 100 m.

(48) Metallisation of the Parts Made of G/PVC and of the Parts Made of CoCr:

(49) In a first step, the carbon parts and the metal parts are cleaned in an ultrasonic tank with acetone then ethanol and then dried with compressed air.

(50) In the following examples, the parts are covered afterwards by masks made of metal (preferably made of stainless steel) to limit the metallisation area to the assembly surface.

(51) Then, the parts covered with the masks are positioned in the enclosure of a PVD machine, then the system is pumped up to 510.sup.7 mbar.

(52) Afterwards, the parts are treated in the machine by Ar plasma in order to thoroughly clean their surfaces and form an active surface in order to improve the adhesion of the subsequent metal deposit (50 W, 120 s). Then the parts are metallised.

Example 1: Metallisation of a Part Made of G/PyC by Ti/Au (FIG. 1)

(53) The parts 100 are made of G/PyC.

(54) A 500 nm adhesion layer 110 made of an active element (Ti) is deposited over the active surface 101 of the part 1000 at a speed of 0.3 nm/s in the PVD enclosure.

(55) Then, a 100 nm thick coating layer 120 made of a non-active element (Au) is deposited to protect the thin film of Ti from oxidation and improve wetting.

Example 2: Metallisation of a Part Made of G/PyC by Ti/W/Au (FIG. 2)

(56) The parts 100 are made of G/PyC.

(57) A 30 nm adhesion layer 110 made of Ti is deposited over the active surface 101 of the part 100 at a speed of 0.3 nm/s in the PVD enclosure.

(58) Then, a 100 nm thick barrier layer 130 made of W is deposited to block the diffusion and the dissolution of the adhesion layer 110 in the solder. This barrier layer 130 avoids the dissolution of the active element layer.

(59) Finally, a 100 nm thick final layer 120 made of a non-active element (Au) is made to protect the deposit from oxidation and improve wetting.

Example 3: Metallisation of a Part Made of CoCr by Au (FIG. 3)

(60) The parts 200 are made of CoCr.

(61) A 100 nm thick layer 210 made of a non-active element (Au) is deposited to improve wetting at low temperature.

(62) Assembly (FIG. 4):

(63) The metallised parts 100 made of G/PyC and the metallised parts 200 made of CoCr are assembled without any additional cleaning.

(64) A solder 30 in the form of a foil or strip is cut out with a punch and cleaned in an ultrasonic bath with acetone and then ethanol.

(65) Afterwards, the solder 30 is positioned over the carbon part 100 (FIG. 5), then the metal part 200 is brought into contact with the solder 30. The assembly is held in a tool (FIG. 6), then placed in a furnace under secondary vacuum. A mass is added to apply a pressure (typically 15 to 20 g/cm.sup.2) over the area to be brazed.

(66) The assembly is heated up to a set temperature, higher than the melting point of the solder (350 C., in this case) for a hold time period of a few tens of minutes (10 to 15 min).

(67) After cooling, the assembly is taken out of the furnace. The parts are secured (FIG. 7).

(68) Mechanical Torsion Tests:

(69) The assemblies thus obtained by brazing are subjected to different mechanical torsion tests. The tests have been carried out at room temperature (typically 20 to 25 C.) with an electromechanical tension machine commercialized by the MTS Systems Corporation. The rotation of the chuck is actuated by the movement of the crosspiece (0.5 mm/min).

(70) The maximum tangential stresses (at break-up) are calculated by the following formula:

(71) ${}_{\max }=\frac{2}{R^S}*F*C$ wherein F is the applied maximum force; R is the radius of the torsion axis and C the radius of the torsion pin.

(72) The following table lists the results of the mechanical torsional strength for the different tests that have been carried out.

(73) TABLE-US-00002 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

(74) The maximum tangential stresses (at break-up) obtained for the carbon parts with the Ti/Au deposit (1a to 1e) are higher than or equal to 30 MPa and go up to 41.8 MPa, confirming the solidity of the obtained brazing joint. Break-up is essentially cohesive and occurs in the solder (FIG. 8).

(75) The metallisation of the carbon part with the Ti/Au deposit ensures better mechanical strength thanks to excellent adhesion of the solder to the Ti layer. Break-up occurs in the solder with values corresponding to the strength of the solder itself.

(76) As regards the carbon parts with the Ti/W/Au deposit (2a to 2c), the maximum tangential stresses (at break-up) are higher than or equal to 22.8 MPa. Break-up is essentially adhesive and occurs at the solder/W layer interface.