METHOD FOR ASSEMBLING A ZIRCONIA PART TO A TITANIUM ELEMENT

Abstract

A method for assembling a zirconia part to a titanium element with braze, the method comprising the following steps: coating a surface of the titanium element with a niobium layer, positioning a braze between the zirconia part and the niobium, the braze being of gold or a gold alloy, heating the whole to a temperature higher than the melting temperature of the braze, and then cooling the whole, whereby an assembly comprising the zirconia part and the titanium element assembled by a brazing joint comprising a first portion of gold or a gold alloy, a second portion formed by a reaction layer comprising intermetallics of the AuNbTi system, and a third portion formed by an oxide reaction layer is obtained.

Claims

1. A method for assembling a zirconia part to a titanium element with braze, the method comprising the following steps: coating a surface of the titanium element with a niobium layer, positioning a braze between the zirconia part and the niobium layer, the braze being of gold or a gold alloy, heating the whole to a temperature higher than the melting temperature of the braze and then cooling the whole, whereby an assembly comprising the zirconia part and the titanium element assembled by a brazing joint comprising a first portion of gold or a gold alloy, a second portion formed by a reaction layer comprising intermetallics of the AuNbTi system, and a third portion formed by an oxide reaction layer is obtained.

2. The method according to claim 1, wherein the braze is of gold.

3. The method according to claim 1, wherein the braze is a gold alloy comprising gold and titanium.

4. The method according to claim 1, wherein the braze is a gold alloy comprising gold, titanium and nickel.

5. The method according to claim 1, wherein the braze is a gold alloy comprising gold and zirconium.

6. The method according to claim 1, wherein the braze is a gold alloy comprising between 0.5 and 4% by mass of titanium or zirconium.

7. The method according to claim 6, wherein the braze is a braze having composition Au3Ni0.6Ti, AuxTi or AuxZr with x between 0.5 and 3% by mass.

8. The method according to claim 1, wherein the braze is a braze foil having a thickness of between 25 μm and 200 μm.

9. The method according to claim 1, wherein step d) is carried out under secondary vacuum, under reducing atmosphere or under neutral gas.

10. The method according to claim 1, wherein before coating the surface of the titanium element with a niobium layer and before positioning the braze, the method comprises a step of cleaning the zirconia part, the titanium element or the braze.

11. An assembly successively comprising: a zirconia part, a brazing joint comprising a first portion of gold or a gold alloy, a second portion formed by a reaction layer comprising intermetallics of the AuNbTi system, and a third portion formed by an oxide reaction layer, a titanium element.

12. The assembly according to claim 11, wherein the brazing joint comprises a first portion of a gold alloy AuNiTi, AuTi or AuTiZr, a second portion formed by a reaction layer comprising intermetallics of the AuNbTi system, and a third portion formed by an oxide reaction layer.

13. The assembly according to claim 11, wherein zirconia is an yttria-stabilised zirconia.

14. The assembly according to claim 11, wherein zirconia is an alumina-zirconia composite.

15. A cochlear implant comprising an assembly as defined in claim 11, the brazing joint of the assembly comprising a first portion of gold or a gold alloy AuTi or AuTiZr, a second portion formed by a reaction layer comprising intermetallics of the AuNbTi system, and a third portion formed by an oxide reaction layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The present invention will be better understood upon reading the description of exemplary embodiments given by way of illustrating and not in any way limiting purposes, with reference to the appended drawings in which:

[0068] FIGS. 1A, 1B and 1C schematically represent in cross-section views different steps of a method for manufacturing an assembly of a titanium element with a zirconia part, according to one particular embodiment of the invention.

[0069] FIG. 2 is a scanning electron microscope (SEM) picture of an assembly comprising a titanium member initially covered with a niobium layer, a brazing joint (formed from a gold braze) and a zirconia part, according to one particular embodiment of the invention; the presence of two zones in the joint formed is noticed: gold and the reaction layer of the AuNbTi ternary system.

[0070] FIG. 3 is an SEM picture of an assembly comprising a titanium element, a brazing joint (formed from a gold braze) and a zirconia part, according to a method of prior art; the presence of two zones in the joint formed is noticed: gold and the reaction layer with 4 intermetallics of the Au—Ti system.

[0071] FIG. 4 schematically represents in three dimensions a tensile test piece before brazing comprising two zirconia parts, a titanium element covered on either side with a niobium layer, and two brazes (gold rings), according to one particular embodiment of the invention, the insertion represents a zoom of the centre of the test piece.

[0072] FIG. 5 is a photograph of a test piece in a tooling according to one particular embodiment of the invention.

[0073] FIGS. 6A and 6B are SEM pictures of a zirconia/Au/titanium braze with double metallisation of the zirconia with Ti+Nb, before and after heat treatment respectively, according to a comparative example.

[0074] FIG. 7 is an SEM picture of a Ti+Nb metallised zirconia/gold braze interface, after one brazing cycle, according to a comparative example.

[0075] Different parts represented in the figures are not necessarily represented to scale, to make the figures more legible.

[0076] The different possibilities (alternatives and embodiments) should be understood as being not exclusive of each other and can be combined with each other.

[0077] In addition, in the following description, terms that depend on the orientation, such as “on”, of a structure are applied assuming that the structure is oriented the illustrated way in the figures.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0078] Although this is by no means limiting, the invention particularly finds applications in the biomedical field for the manufacture of hearing implants, and in particular for the manufacture of cochlear implants, for the manufacture of pacemakers, orthopaedic or dental implants, or in the field of watchmaking.

[0079] The method for assembling a titanium element 10 to a zirconia part 20 with a brazing material comprises the following steps:

[0080] preferably cleaning the titanium element 10 and/or the zirconia part 20, as well as the braze 40,

[0081] b) covering a surface of the titanium element 10 with a niobium layer 30 (FIG. 1A), titanium possibly being chemically etched beforehand,

[0082] c) positioning braze 40 between the zirconia part 20 and the niobium layer 30, the braze 40 being of gold or a gold alloy (FIG. 1B),

[0083] d) heating the whole to a temperature higher than the melting temperature of the braze 40 and then cooling the whole, whereby an assembly comprising the zirconia part 20 and the titanium element 10 assembled by a brazing joint comprising a first portion of gold or a gold alloy, a second portion formed by a reaction layer 35 comprising, or consisting of, intermetallics of the AuNbTi ternary system on the titanium side (FIG. 1C) is obtained. An oxide reaction layer (third portion) is also present in the joint on the zirconia side. It is detectable by transmission electron microscopy and, depending on the thickness and/or nature of the oxides, it can also be detected by scanning electron microscopy (SEM).

[0084] The zirconia part 20 is of zirconium oxide (ZrO.sub.2), or yttria-stabilised zirconia (ZrO.sub.2—Y.sub.2O.sub.3) or alumina-zirconia composite, such as ZTA (Zirconia Toughened Alumina) with an alumina matrix and ATZ (Alumina Toughened Zirconia) with a zirconia matrix.

[0085] The yttria-stabilised zirconia preferably contains between 1 and 6 mole % of Y.sub.2O.sub.3. The yttria-stabilised zirconia is, for example, a Y-TZP (“Yttrium-Tetragonal Zirconia Polycrystal”) zirconia stabilised with 3 mole % of Y.sub.2O.sub.3.

[0086] By “between X and Y”, it is understood that bounds X and Y are included.

[0087] In the following, the term zirconia will be used, but it is obvious that the term zirconia can be replaced with yttria-stabilised zirconia or alumina-zirconia composite.

[0088] For example, for step a), it is preferable to clean the zirconia part 20 to be brazed, the titanium element 10 and the braze 40 in a bath of acetone, and then ethanol with ultrasound.

[0089] The surface of the titanium element 10, which is to be metallised, may be subjected to a chemical etching step and/or an ion etching step.

[0090] In addition, the zirconia may be heat treated at 1350° C., preferably under air (the so-called roasting method), prior to step c).

[0091] During step b), the titanium element 10 to be brazed is first coated with a niobium deposit. This metallisation layer 30 makes it possible to reinforce the Ti/Au interface by modifying, during the brazing method, the morphology, thickness and nature of the intermetallics in the Ti—Au system.

[0092] Deposition of the niobium layer 30 is, for example, performed by physical vapour deposition (PVD) or by sputtering. The thickness of the niobium deposited ranges, for example, from a few microns to 20 μm, typically 5 to 10 μm.

[0093] The zirconia part 20 is not metallised, that is the braze 40 is in direct contact with the surface of the zirconia part 20 to be assembled.

[0094] It is obvious that it is possible to assemble one or more zirconia parts with one or more titanium elements. For example, it is possible to assemble two zirconia parts with one titanium part. In this case, two brazes are implemented and the two faces of the titanium part to be in contact with both brazes are metallised.

[0095] Braze 40 is a gold-rich braze such as pure gold or a gold alloy.

[0096] By pure gold, it is understood gold with a purity of more than 99% by mass, preferably more than 99.5% by mass.

[0097] The gold alloy is, for example, a braze with the composition Au3Ni0.6Ti (% by mass), for example marketed under the reference GoIdABA, or an AuxTi or AuxZr braze with x between 0.5 and 3% by mass.

[0098] The melting temperature of the braze is lower than the melting temperature of the titanium element.

[0099] The braze 40 is advantageously in the form of a foil.

[0100] The thickness of the braze foil is, for example, between 25 and 200 μm, preferably between 50 and 150 μm.

[0101] The braze 40 can also be in the form of a powder.

[0102] During step c) and during step d), tooling for holding the zirconia part 20, the titanium element 10 and the braze 30 may be used.

[0103] During the brazing method (step d), the zirconia part 20 is assembled to the titanium part 10 by melting the braze 40. The whole is heated so that the braze melts. The braze plateau temperature depends on the braze composition. The braze plateau temperature is higher than the melting temperature of the braze 40. Typically, the braze plateau temperature is higher than 1064° C. in the case of pure gold and preferably lower than 1120° C. In the case of commercial braze GoIdABA, the liquidus and solidus are 1030° C. and 1003° C. respectively. The brazing temperature is preferably higher than 1030° C. and lower than 1120° C.

[0104] The brazing plateau duration is for example between 0 and 15 minutes, preferably between 30 seconds and 10 minutes.

[0105] After the brazing plateau, the whole is cooled to room temperature. The braze solidifies as it cools and the parts are then bonded by the braze solidified.

[0106] Metallisation of titanium element 10 by niobium does not lead to the formation of porosities at the Nb metallised titanium/Au brazing interface during heating associated with the brazing cycle, contrary to the case of the metallisation of zirconia (Ti+Nb metallised zirconia/Au interface weakened by the formation of porosities upon heating).

[0107] This brazing method can be carried out under a secondary vacuum or under a protective atmosphere (neutral gas or reducing atmosphere). It is preferably carried out in a secondary vacuum furnace.

[0108] The assembly obtained by the method described above successively comprises:

[0109] a zirconia part 20,

[0110] a brazing joint comprising a first portion 40 made of gold or a gold alloy (on the side of the zirconia part 20), a second portion formed by a reaction layer 35 comprising or consisting of intermetallics of the AuNbTi ternary system on the titanium side (layer formed by reaction of the metallisation layer 30 with the braze 40 and with the titanium element 10), and finally a third portion (not represented) formed by an oxide layer on the zirconia side, and

[0111] a titanium element 10.

[0112] The first portion has the same composition as the braze or a composition corresponding to the composition of the braze slightly enriched with titanium. The titanium content is between 0.1% and 5% by mass of titanium, preferably between 0.1% and 2% Ti.

[0113] The reaction layer 35 of intermetallics is positioned between the first gold or gold alloy portion 40 and the titanium element. The reaction layer 35 of intermetallics has, for example, a thickness of 5 μm to 30 μm.

[0114] The thin oxide layer is positioned between the first gold or gold alloy part 40 and the zirconia part. The thin oxide layer has, for example, a thickness between 100 nm and 2 μm.

[0115] Advantageously, the brazing joint comprises a gold alloy AuNiTi, AuTi or AuTiZr, a reaction layer 35 comprising or consisting of intermetallics of the AuNbTi ternary system, and possibly a thin oxide layer on the zirconia side.

[0116] According to one advantageous alternative embodiment, the zirconia is yttria-stabilised zirconia.

[0117] According to another advantageous alternative embodiment, the zirconia is an alumina-zirconia composite.

[0118] The invention is particularly interesting for forming hearing implants.

[0119] In particular, a cochlear implant comprising an assembly as defined above could be manufactured, the brazing joint of the assembly comprising, on the one hand, gold or a gold alloy AuTi or AuTiZr and, on the other hand, a reaction layer 35 comprising or consisting of intermetallics of the AuNbTi ternary system. A thin oxide reaction layer is also present on the zirconia side.

Illustrative and Non-Limiting Examples of One Embodiment

[0120] Several non-limiting exemplary embodiments as well as comparative tests will now be described.

[0121] In the following examples and tests, zirconia is 3 mole % yttria-stabilised zirconia.

EXAMPLE 1

Making Assemblies by Zirconia/Au/Titanium Brazing with Nb Deposits having Thickness X μm (X=5, 10, 15 μm) on Titanium

[0122] Three assemblies «zirconia disc 20 (ϕ15 mm, h 2 mm)/titanium disc 10 (ϕ15 mm, h 2 mm)» have been made by brazing with a pure gold foil 40. The titanium discs are previously metallised with a niobium layer 30. Three different thicknesses have been tested: 5 μm, 10 μm and 15 μm.

[0123] The gold foil 40 is cut to form a disc with a diameter of 15 mm and a thickness of 100 μm.

[0124] The following steps have been performed to make these assemblies:

[0125] Step 1: Cleaning the zirconia, titanium and gold parts to be brazed in a bath of acetone, and then ethanol with ultrasound, wherein titanium has been chemically etched beforehand.

[0126] Step 2: Metallising the surfaces to be brazed of the three titanium parts by performing ion etching of the titanium followed by a niobium deposition by PVD. The thickness of niobium is respectively 5 μm for assembly n°1, 10 μm for assembly n°2 and 15 μm for assembly n°3.

[0127] Step 3: Depositing the pure gold brazing foil between the zirconia and titanium parts to be assembled. The niobium-coated side of titanium is that positioned towards the braze.

[0128] Step 4: Placing the three assemblies formed by the parts and brazes in a secondary vacuum brazing furnace. A small 23 g W mass has been placed on each assembly to hold the assembly in the furnace.

[0129] Step 5: Heating the mixture to 1080° C. for a few minutes.

[0130] Step 6: After the brazing plateau, the assembly is cooled to room temperature and the parts are then bonded with the braze solidified.

[0131] The parts have been cut, coated with epoxy resin and polished. The joints are properly filled with braze, there is no lack of braze, that is there are no holes in the braze. The interfaces are not cracked.

[0132] It can be noted that the titanium/gold interface (FIG. 2) is not made up of successive layers of Ti—Au intermetallics, unlike the case of direct brazing without titanium metallisation by niobium.

[0133] Indeed, by way of comparison, an assembly with a direct brazing of zirconia/Au/titanium without titanium metallisation by niobium has been manufactured and then characterised. The presence of 4 parallel and successive layers of intermetallic Ti.sub.3Au, TiAu, TiAu.sub.2, TiAu.sub.4 (FIG. 3) is observed.

EXAMPLE 2

Manufacture of Tensile Test Pieces by Zirconia/Au/Titanium Brazing with a 5 μm Thick Nb Deposit, and Mechanical Tests

[0134] Tensile test pieces have been brazed based on the ASME F19-64 standard used for characterising ceramic/metal assemblies. A diagram sets forth this type of test piece in the case of zirconia/Au/titanium brazing (FIG. 4). In this diagram, the titanium ring 10 can be seen in the centre of the part. On either side of the ring, gold brazing rings 40, 41, and finally the zirconia parts 20, 21 are placed. Each side of the titanium ring 10 is covered with a niobium layer 30, 31. The whole is held in a tooling during the brazing operation so that there is no misalignment between the parts (FIG. 5).

[0135] The brazing has been carried out as follows:

[0136] Step 1: Cleaning the zirconia, titanium and gold parts to be brazed in a bath of acetone, and then ethanol with ultrasound, wherein titanium has been chemically etched beforehand.

[0137] Step 2: Metallising the 2 surfaces to be brazed of the titanium rings by performing ion etching of titanium followed by a niobium deposition by PVD. The niobium thickness is 5 μm.

[0138] Step 3: Depositing 2 rings of pure gold braze between the zirconia and titanium parts to be assembled (zirconia/Au/titanium/Au/zirconia).

[0139] Step 4: Placing each sample to be brazed in a hold tooling and install the whole in a secondary vacuum brazing furnace.

[0140] Step 5: Heating the mixture to 1080° C. for a few minutes.

[0141] Step 6: After the brazing plateau, the whole is cooled to room temperature and the test pieces are then bonded by the braze solidified.

[0142] Four test pieces have thus been produced. They have been subjected to tensile tests. The rupture stress values are measured. An average rupture stress of 179 MPa has been obtained.

COMPARATIVE EXAMPLE 3

Making Zirconia/Titanium Assemblies and Tensile Test Pieces by Zirconia/Au/Titanium Brazing without any Metallisation of the Substrates to be Brazed, and Mechanical Tests

[0143] Tensile test pieces have been brazed according to the same protocol as for example 2, but without making titanium metallisation nor zirconia metallisation. These test pieces have been subjected to the tensile test and the rupture stress values have been measured. After brazing at 1080° C. and a few minutes of a plateau, an average rupture stress of 97 MPa is obtained, which is significantly lower than the stress obtained with titanium metallisation by niobium.

[0144] Rupture occurs between the intermetallics TiAu.sub.2 and TiAu.sub.4.

COMPARATIVE EXAMPLE 4

Making Zirconia/Titanium Assemblies and Tensile Test Pieces by Zirconia/Au/Titanium Brazing with Double Metallisation of Zirconia by Ti+Nb, Mechanical Tests+Effect of Temperature Alone on Deposition

[0145] Tensile test pieces have been brazed according to the same protocol as for example 2, but by making double metallisation of zirconia with titanium and then niobium. However, the titanium ring is not metallised. These test pieces are subjected to the tensile test and the rupture stress values are measured. For 6 test pieces, an average rupture stress of 42 MPa is obtained, which is significantly lower than the rupture stresses obtained with the test pieces in example 2.

[0146] To understand this result, a heat treatment has been applied, identical to that of the brazing cycle, to the Ti+Nb metallised zirconia (without performing brazing). This treatment leads to a degradation of the deposit with the formation of porosities, which weakens the interface on the zirconia side. Scanning electron microscope pictures obtained before and after heat treatment are represented in FIGS. 6A and 6B respectively.

[0147] After brazing, these porosities are found at the zirconia side interface (FIG. 7), confirming that this interface represents the weak point of the assembly, namely it is the rupture place during mechanical tests.

[0148] The degradation of metallisation (presence of porosities) during the brazing cycle takes place even before the gold braze is melted. This leads to poorer interfaces than brazing without zirconia metallisation. Indeed, it seems that it is zirconia itself that releases oxygen which degrades the metallisation, the latter thereby becoming the weak point of the assembly.

[0149] This shows that the metallisation of zirconia does not result in an assembly with a good mechanical strength.

REFERENCES

[0150] 1 Agathopoulos, S. et al, ‘Interactions at Zirconia-Au—Ti Interfaces at High Temperatures’. Key Eng. Mater. 206-213, 487-490 (2001).

[0151] 2 Agathopoulos, S. et al, ‘A review of recent investigations on zirconia joining for biomedical applications’. Advances in Joining of Ceramics (2003) 135-147.

[0152] 3 Lei, Y. et al, ‘Evaluation of Biomedical Ti/ZrO.sub.2 Joint Brazed with Pure Au Filler: Microstructure and Mechanical Properties’. Metals 10, 526 (2020).

[0153] 4 Fischer M. et al, ‘Wetting and reactivity of zirconia by Au—Ti active alloys in view of zirconia/titanium brazing with pure gold’. Proceedings of the International Brazing and Soldering Conference 2021. 3-6 Oct. 2021.

[0154] 5 FR3051131-A1 ‘Procédé de brasage d′un élément métallique sur une piéce de zircone et dispositif implantable brasé’