ITEM MADE OF PRECIOUS CERMET

Abstract

An item produced in a cermet material including by weight between 85 and 94% of a ceramic phase and between 6 and 15% of a metal binder phase, the ceramic phase mostly including a tungsten carbide phase and optionally one or more phases of one or more secondary carbides selected from the elements Ti, Zr, Hf, V, Nb, Ta, Cr and Mo, the metal binder comprising Ag, Pd, Ru and Co. The invention generally relates to the method for manufacturing said item.

Claims

1. An item produced in a cermet material comprising by weight between 85 and 94% of a ceramic phase and between 6 and 15% of a metal binder phase, the ceramic phase mostly including a tungsten carbide phase and optionally one or more phases of one or more carbides selected from the elements Ti, Zr, Hf, V, Nb, Ta, Cr and Mo, the metal binder comprising Ag, Pd, Ru and Co.

2. The item according to claim 1, wherein the metal binder phase is present in a percentage by weight between 7 and 14% and in that the ceramic phase is present in a percentage by weight between 86 and 93%.

3. The item according to claim 1, wherein the metal binder phase is present in a percentage by weight between 9 and 11% and wherein the ceramic phase is present in a percentage by weight between 89 and 91%.

4. The item according to claim 1, wherein the carbide phase only includes tungsten carbide.

5. The item according to claim 1, wherein the metal binder phase consists, apart from the impurities, of Ag, Pd, Ru and Co.

6. The item according to claim 1, wherein Ag has a content between 5 and 14%.

7. The item according to claim 1, wherein Pd has a content between 0.5 and 4%.

8. The item according to claim 1, wherein Co and Ru are each present in a percentage between 0.02 and 4%.

9. The item according to claim 1, wherein it has a Vickers hardness, HV30, between 500 and 1,600.

10. The item according to claim 1, wherein it has a tenacity KiC greater than or equal to 3.0 MPa.Math.m½.

11. The item according to claim 1, wherein it has, in a CIELAB colour space, an L* component of minimum 60.

12. The item according to claim 1, wherein the cermet material is non-magnetic with a magnetic susceptibility χm per unit of volume less than 5×10-5.

13. The item according to claim 1, wherein it concerns an external timepiece component selected from the list comprising a middle, a back, a bezel, a push-piece, a bracelet link, a dial, a hand and a dial index.

14. The item according to claim 1, wherein it concerns a timepiece component of the movement selected from the list comprising a toothed wheel, an arbour, a pinion, a spring, a bridge, a plate, a screw and a balance.

15. The item according to claim 1, wherein it concerns a balance arbour.

16. A method for manufacturing an item comprising the following successive steps of: a) producing a mixture with a ceramic powder mostly comprising tungsten carbide and a powder of a metal binder consisting, apart from the impurities, of silver, of palladium, of ruthenium and of cobalt, b) forming an ébauche by giving to said mixture the shape of the item, c) sintering the ébauche at a temperature between 950 and 1,600° C., during a period between 15 minutes and 8 hours, wherein the ceramic powder is present in a percentage by weight between 85 and 94%, and wherein the powder of the metal binder is present in a percentage by weight between 6 and 15%.

17. The method according to claim 16, wherein step b) is performed by pressing, by injection, by extrusion or by additive manufacturing methods.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0020] FIG. 1 shows two images respectively in optical and electron microscopy of the cermet-type material according to the invention with a composition by weight of 90% of WC and of 10% of a precious binder consisting of 89% of Ag, of 10% of Pd, of 0.5% of Ru and of 0.5% of Co, i.e. in relation to the total weight 8.9% of Ag, 1% of Pd, 0.05% of Ru and 0.05% of Co.

[0021] FIG. 2 shows the paramagnetic behaviour of this same cermet-type material having a magnetic susceptibility χ.sub.m of 1.2×10.sup.−5.

DETAILED DESCRIPTION

[0022] The present invention relates to an item produced in a cermet-type material including a ceramic phase consisting of carbides and of a binder phase consisting of a plurality of metals of which precious metals. The cermet includes by weight between 85 and 94% of the ceramic phase and between 6 and 15% of the metal binder phase. Preferably, the cermet includes by weight between 86 and 93% of the ceramic phase and between 7 and 14% of the precious metal binder phase. Preferably, the cermet includes by weight between 87 and 92% of the ceramic phase and between 8 and 13% of the precious metal binder phase. Even more preferably, the cermet includes by weight between 89 and 91% of the ceramic phase and between 9 and 11% of the precious metal binder phase.

[0023] The metal binder comprises silver, palladium, ruthenium and cobalt. Preferably, the metal binder consists, apart from the impurities, of silver, of palladium, of ruthenium and of cobalt. In relation to the total weight, cobalt and ruthenium are each present in a percentage between 0.02 and 4% by weight. Preferably, cobalt and ruthenium are each present in a percentage between 0.02 and 3%, more preferably between 0.03 and 2% by weight, even more preferably between 0.03 and 1%, even more preferably between 0.03 and 0.5% and in a particularly preferred manner between 0.03 and 0.4%. In relation to the total weight, palladium is between 0.5 and 4% and silver between 5 and 14%. Preferably, palladium is between 0.5 and 3%, more preferably, palladium is between 0.5 and 2%, and even more preferably between 0.5 and 1.5%. Preferably, silver is between 7 and 13.4% and more preferably between 7 and 10% by weight.

[0024] The ceramic phase comprises a phase mostly of tungsten carbide and optionally one or more carbide phases of one or more elements selected from Ti, Zr, Hf, V, Nb, Ta, Cr and Mo. In other words, the ceramic phase consists either of a single tungsten carbide phase, or of a phase mostly of tungsten carbide added with one or more phases respectively of one or more carbides of Ti, Zr, Hf, V, Nb, Ta, Cr and Mo, this or these phases of carbides of Ti, Zr, Hf, V, Nb, Ta, Cr and Mo being present in a minor proportion. Mostly means a percentage greater than 50% by weight, preferably greater than or equal to 65%, more preferably greater than or equal to 75%. In short, within the ceramic phase, the tungsten carbide phase is present in a percentage by weight greater than 50% and less than or equal to 100% (50<WC≤100%), preferably greater than or equal to 65% and less than or equal to 100% (65≤WC≤100%), more preferably greater than or equal to 75% and less than or equal to 100% (75≤WC≤100%) with the complement formed of one or other carbides of Ti, Zr, Hf, V, Nb, Ta, Cr and Mo.

[0025] According to the invention, this cermet-type material is non-magnetic. For the remainder of this description, the mention of “non-magnetic” for this cermet-type material refers to a material that is only paramagnetic and having a very low magnetic susceptibility. It is well known that in a phase diagram, the Curie temperature indicates the transition temperature from a ferromagnetic state to a paramagnetic state, at a given temperature and for a fixed composition. Thus, for the Cobalt-Ruthenium phase diagram, this magnetic transition occurs at the ambient temperature for a Ruthenium mass concentration located between 45% and 50%. In the same way, it is possible to determine in the Cobalt-Palladium phase diagram a paramagnetic transition located between 4% and 6% by weight of cobalt, at the same ambient temperature. By respecting the two limits mentioned above, it is then possible to have a paramagnetic transition in the Palladium-Cobalt-Ruthenium ternary system. The cermet-type material with precious binder consists of the elements palladium, cobalt, ruthenium according to the limits of the compositions defined and silver is therefore paramagnetic.

[0026] The item may be a constituent element of watches, jewellery, bracelets, etc. In the field of horology, this item may be an external part such as a middle, a back, a bezel, a push-piece, a bracelet link, a dial, a hand, a dial index, etc. It may also concern a component of the movement selected from the non-exhaustive list comprising a toothed wheel, an arbour, a pinion, a spring, a bridge, a plate, a screw and a balance. Advantageously, within components of the movement, this concerns a balance arbour.

[0027] The cermet item may be shaped by conventional powder metallurgy methods such as pressing or injection or extrusion or by various additive manufacturing methods dedicated to the manufacture of three-dimensional parts such as selective laser melting (SLM), selective electron beam melting (SEBM), such as 3D printing (3DP), binder jet 3D printing (BJAD), 3D gel printing (3DGP) and fused filament fabrication (FFF).

[0028] The cermet item is preferably produced by sintering starting from a mixture of ceramic and metal powders. The manufacturing method includes the following steps of: [0029] a) Producing a mixture with the various powders and this possibly in a wet environment. The starting powders have preferably a d50 less than 10 μm, and more preferably between 0.8 and 5 μm. The mixture may possibly be produced in a mill, which reduces the d50 of the particles of the powder to a size in the order of the micron, or even less than the micron after milling. This mixture includes by weight between 85 and 94%, preferably between 86 and 93%, more preferably between 89 and 91%, of the ceramic powder and between 6 and 15%, preferably between 7 and 14%, more preferably between 9 and 11% of the metal powder. The ceramic powder includes tungsten carbide and optionally one or more other carbides. The metal powder only consists of the elements silver, palladium, ruthenium and cobalt. The cermet thus produced therefore includes by total weight 5 to 14% of silver, preferably between 7 and 13.4% of silver, more preferably between 7 and 10% of silver. It also includes by total weight 0.5 to 4% of palladium, preferably between 0.5 and 3% of palladium, more preferably between 0.5 and 2% of palladium, and even more preferably between 0.5 and 1.5% of palladium. This cermet also includes ruthenium and cobalt with a content by weight for each between 0.02 to 4%, preferably between 0.02 and 3%, more preferably between 0.03 and 2%, and even more preferably between 0.03 and 1%, between 0.03 and 0.5% and between 0.03 and 0.4%. It will be specified for this step that the metal phase may also be manufactured by arc melting then be sprayed in the form of pre-alloyed powders to finally be mixed with the carbide powders. [0030] b) Possibly, a second mixture comprising the aforementioned mixture and an organic binder system (paraffin, polyethylene, etc.) may be produced. [0031] c) Forming an ébauche by giving to the mixture the shape of the desired item, for example, by injection, by pressing, by extrusion or by 3D printing. [0032] d) Sintering the ébauche under inert atmosphere or under nitrogen or under vacuum at a temperature between 950° C. and 1,600° C., preferably between 1,000° C. and 1,500° C., and more preferably between 1,100 and 1,500° C., during a period between 15 minutes and 8 hours, preferably between 30 minutes and 4 hours, and more preferably between 30 minutes and 2 hours. This step may be preceded by a step of debinding in a range of temperatures between 200 and 800° C. if the mixture includes an organic binder system.

[0033] The ébauche thus obtained is cooled and polished. It may also be machined before polishing to obtain the desired item.

[0034] The item from the manufacturing method includes the ceramic phase and the metal phase in percentages by weight close to those of the starting powders. However, small variations of compositions and of percentages between the base powders and the materials from the sintering cannot be excluded, depending particularly on the various vapour pressures of the constituent elements of the metal binder with the temperature.

[0035] The item has a CIELAB colour space (in accordance with the standards CIE no. 15, ISO 7724/1, DIN 5033 Teil 7, ASTM E-1164) with a luminance L* component, representative of the manner in which the material reflects the light, of minimum 60, preferably of minimum 65 and more preferably of minimum 70.

[0036] The cermet material has a Vickers hardness measured under a load of 30 kg (HV30) between 500 and 1,600, preferably between 700 and 1,400, depending on the types and on the percentages of the constituents. Advantageously, it has a hardness greater than 1,000 Vickers for the external parts requiring a high scratch resistance. It has a tenacity Kc of minimum 3 MPa.Math.m.sup.1/2, preferably of minimum 4.0 MPa.Math.m.sup.1/2, the tenacity being determined based on measurements of the lengths of cracks at the four ends of the diagonals of the hardness indentation according to the formula:

[00001]$K_{1C}=0.0319\frac{P}{{\mathrm{al}}^{1/2}}$

with P that is the applied load (N), a that is the semi-diagonal (m) and/that is the length of the measured crack (m).

[0037] For the magnetic properties, the M(H) hysteresis curve has been characterised at ambient temperature and by varying the applied field on the samples with a vibrating sample magnetometer (VSM) of the MicroSense EZ9 type. The magnetic susceptibility (χm) per unit of volume of the cermet according to the invention is less than 5×10.sup.−5, preferably less than or equal to 4×10.sup.−5.

[0038] Table 1 below repeats a plurality of examples of cermet-type materials with a comparative example without cobalt (sample 01) and two examples according to the invention (samples 02 and 03). In the examples, the ceramic phase is a tungsten carbide phase with a percentage by weight of 90%. The metal binder is therefore present in a percentage by weight of 10% with the compositions given in the table.

[0039] The samples have all been manufactured by powder metallurgy. The mixtures of powders of distinct compositions have been prepared in a mill in the presence of a solvent. The mixtures have been produced without adding organic binders. After drying, they have been shaped by uniaxial pressure and sintered under argon and at the same temperature. After sintering, the samples have been flat polished in order to accurately measure the mechanical properties and the colour indices.

[0040] For the sample 01, a low hardness value less than 350 HV30 is obtained. It is attributed to the significant presence of porosity in the sample in the absence of cobalt. The addition of cobalt in the sample 02 makes it possible to improve the densification during the sintering and thereby to drastically increase the hardness with a value greater than or equal to 800 HV30. The reduction of cobalt and ruthenium contents in the sample 03 makes it possible to reduce the magnetic susceptibility value per unit of volume to a value of 1.2×10.sup.−5 with the hysteresis curve visible in FIG. 2. This makes it a cermet of choice for a timepiece component of the movement such as a balance arbour. The sample 03 is moreover well densified with very few porosities having sizes of approximately 1 micron as shown in the photos taken during optical microscopy and during electron microscopy of FIG. 1 with on the optical microscopy the black spots that are porosities. This results in an increase of the hardness with a value reaching 1,208 HV30. All of the samples according to the invention fall within the criteria set for a timepiece component having no ferromagnetic properties and no diamagnetic properties, with a tenacity greater than 4.0 MPa.Math.m.sup.1/2, with a hardness greater than 700 HV30 and having a high metallic lustre with an L* value greater than

TABLE-US-00001 TABLE 1 Ag Wt Ru Co χ.sub.m Hardness K1c (% wt) (% wt) (% wt) (% wt) (unit/Vol.) type (HV30) (Mpa .Math. m.sup.1/2) L* 01 Comp 87.1 10.0 2.9 — 4.0 × 10.sup.−5 Para 309 4.8 68.6 02 Inv 85.0 10.0 2.5 2.5 3.4 × 10.sup.−5 Para 830 5.4 72.6 03 Inv 89.0 10.0 0.5 0.5 1.2 × 10.sup.−5 Para 1208 4.8 74.5