CH712838A2 - Process for obtaining a carbo-nitrided zirconia article - Google Patents

Abstract

The present invention relates to a process for obtaining a finished or semi-finished article (1) based on zirconia, the surface of the article (2) having a metallic external appearance and a non-zero electrical conductivity, characterized in that it comprises the steps of: providing at least one zirconia article previously shaped in its finished or semi-finished form; disposing said article in an enclosure in which a mixture of gas based on hydrogen, carbon and nitrogen is heated; heating said article and the gas mixture by means of refractory metal resistive elements traversed by an electric current or with the aid of a plasma until a dissociation of the molecules of the hydrogen-based gases is obtained, carbon and nitrogen and an increase in the temperature of said article; and maintaining said article in the reactive atmosphere thus created to have a diffusion of carbon and nitrogen atoms in the outer surface of said article. The treatment results in a color of the surface of the piece going from the platinum to the gold, while passing by the bronze and the pink gold, according to the relative concentrations of carbon and nitrogen in the gaseous mixture injected into the pregnant during the process.

Description

Description: [0001] The present invention relates to an article based on zirconia, in particular sintered, and in particular such an article whose chemical structure is transformed over part of its thickness to give this article an outer surface having a metallic appearance as well as a non-zero electrical conductivity. The invention also relates to a use of such an article as an element of clothing of an article such as a wristwatch, a jewel or any other item of the luxury industry that can use this kind of product, in particular for producing construction elements of watch cases or bracelets. The present invention finally relates to a method for obtaining such an article.

PRIOR ART

Among all the known materials, metals are very widely used because of some very interesting properties: high mechanical strength, high tenacity, electrical conductivity, etc. In addition, their characteristic luster is highly sought after for decorative applications. Their hardness is however lower than that of ceramics, which can lead to significant wear and corrosion in many common conditions of use, for example, when these materials are used to make watchmaking pieces such as watch boxes or bracelets.

It is known processes for shaping and polishing ceramics to obtain trim parts whose hardness and resistance to wear and corrosion are much higher than those of metals.

It is known a method for modifying such a ceramic piece based on zirconia (Zr02) in order to give it metallic properties including a gray color and a brightness close to those of platinum and a surface conductivity non-zero, while maintaining its toughness and resistance to wear and corrosion superior to those of metals. This method makes it possible to obtain a finished or semi-finished zirconia article having a partially reduced ZrC 4 core and an external metallic appearance. It comprises the steps of: - providing at least one zirconia article previously shaped in its finished or semi-finished form: - arranging said article in an enclosure in which a plasma is created from a mixture of hydrogen, a neutral gas, and traces of carbon, and - maintain said article near the plasma for a time of about 15 to 240 minutes, the average temperature of the article being established in these conditions between about 600 and 1300 ° C to obtain a diffusion of carbon atoms in said article.

Such a method thus makes it possible to produce a zirconia piece having an outer layer of zirconium carbide ZrC with a metallic appearance and properties, ie a shiny and conductive surface, while retaining its advantageous properties with respect to a piece of metal. metal.

Also, advantageously with respect to methods of depositing decorative thin layers of metallic appearance by PVD or CVD or other deposition methods, the outer layer of metallic appearance produced by this carburizing process is integral with the core. of the piece, resulting in a perfect grip of the layer with a risk of zero delamination.

Finally, this method makes it possible to obtain finished or semi-finished parts with the advantageous properties of zirconium carbide ZrC on its surface without having to produce solid ZrC parts, which would imply high-temperature sintering processes. possibly under pressure. Also, the ZrC is more than 10% denser than the ZrO 2, so that ZrO 2 parts carbureted by the above process are lighter than parts of the same ZrC solid geometry. Finally, since ZrC is much harder (typically 25 GPa) than ZrO 2 (typically 13 GPa), solid ZrC parts would be much harder to machine and polish after sintering than the same ZrO 2 parts. The aforementioned method overcomes this problem by allowing to postpone the transformation of the workpiece surface ZrC after machining and polishing the Zr02 part.

This method can be adapted to obtain a zirconium nitride layer also having a metallic appearance and a non-zero electrical conductivity and a perfect grip, but yellow color close to gold. For this, the plasma is obtained from a mixture of hydrogen, a neutral gas, and traces of nitrogen.

However, it is found that the possible color spectrum for these carburizing / nitriding processes is quite limited. In fact, the carburation makes it possible to obtain a metallic effect with a platinum-type coloration, whereas the nitriding makes it possible to obtain a metallic effect with a typically golden coloration. However, there is a need to increase the level of personalization of items such as phones or watches or jewelry and this customization requires the increase in available colors.

SUMMARY OF THE INVENTION

The invention aims to overcome the disadvantages of the prior art by providing a method of obtaining a zirconia article for the formation of carbo-nitride zirconium layers on the thickness of its surface with the same aesthetic, electrical and mechanical and chemical resistance as the layers obtained by plasma process, but with a range of colors ranging from platinum for pure carburation to gold for a pure nitriding, through bronze and pink gold as a function of the relative stoichiometry (or concentration) of the carbon C and nitrogen N which have diffused into the zirconia, said relative stoichiometry being in turn dependent on the relative concentration of carbon and nitrogen in the atmosphere of the enclosure .

For this purpose, the subject of the invention is a process for obtaining a finished or semi-finished article based on zirconia, the article having a metallic external appearance and a non-zero electrical conductivity at the surface, characterized in that it comprises the steps of: - providing at least one zirconia article previously shaped in its finished or semi-finished form; placing said article in an enclosure in which a gaseous mixture is injected, this gaseous mixture comprising at least a first hydrogen and carbon-based gas compound in a first concentration and a second hydrogen-based gas compound and nitrogen in a second concentration, and - heating said gaseous mixture until a dissociation of the molecules of the first and second compounds and maintain said article in the chamber containing the heated gases to heat it also and obtain a diffusion of the atoms of carbon and nitrogen in the outer surface of said article.

The method according to the invention advantageously provides another color option for coloring a ceramic element, this color can be adjusted according to the concentration of the first compound and the second compound in the gas mixture.

In a first advantageous embodiment, the gaseous mixture is heated and its molecules are dissociated by at least one refractory metal resistive element through which an electric current flows and disposed in the chamber so as to heat the gas mixture and the items to be treated in a homogeneous way.

In a second advantageous embodiment, the gas mixture is heated and its molecules are dissociated by a plasma produced in the chamber so as to heat the gas mixture and the articles to be treated in a homogeneous manner.

BRIEF DESCRIPTION OF THE FIGURES

The objects, advantages and features of the device according to the present invention will appear more clearly in the following detailed description of at least one embodiment of the invention given solely by way of non-limiting example and illustrated by the accompanying drawings. in which: FIG. 1 schematically represents the article according to the invention; fig. 2 schematically represents the enclosure for operating the method according to the invention.

DETAILED DESCRIPTION

By way of example in FIG. 1, it is provided with a starting object formed by a zirconia article 1 having a tetragonal crystallographic configuration (zirconium oxide, ZrO 2), for example, of white color and which is produced according to the usual techniques of manufacturing articles. ceramic, for example by sintering.

This article may be a finished product having the final shape in which it will be used, for example a piece of generally parallelepiped shape having already undergone a mirror polishing and intended to constitute an element of the dressing of a watch, for example a bracelet link. Of course, where appropriate, the article may be a semi-finished product on which subsequent machining operations may be performed in order to adapt this article to its end use. Section 1 may be a part of a watch or jewel or any item related to the luxury industry that may use this type of clothing element.

This article is then introduced into a chamber 10 in which it will be treated as visible in FIG. 2. The treatment that will be applied is a simultaneous treatment of carburation and nitriding, also called carbo-nitriding. An essential point of the process is that it is a transformation of the surface of the article to a small thickness, of the order of 10 to 1000 nm, zirconia (zirconium oxide) in the outer region 2 visible in fig. 1 being converted to zirconium carbo-nitride ZrCxNy having a metallic appearance. It is therefore a superficial modification of the structure of zirconia in a new crystallographic structure corresponding to that of a carbo-nitride of zirconium and not a reported coating likely to be torn or to separate from the surface of the article, especially when it is subject to significant wear conditions. More particularly, the outer region of the surface layer which has the structure of zirconium carbo-nitride extends from the surface to a depth of between 20 and 200 nm.

To perform a carbonitriding treatment, it is necessary that the enclosure 10 contains an atmosphere A charged with gas containing hydrogen, carbon and nitrogen, and is provided with heating means 20, these heating means for activating the treatment.

According to the invention, the enclosure contains a gaseous mixture comprising at least a first compound and a second compound. The first compound is a gas based on hydrogen and carbon of the CH4 type found in a first concentration C1 in said enclosure. The second compound is an NH3-type hydrogen and nitrogen gas in the chamber in a second concentration C2.

The temperature rise in the chamber allows the treatment of the zirconia article can take place. Indeed, this rise in the temperature of the atmosphere of the chamber causes a dissociation of the gases so that the hydrogen atoms H, carbon C and nitrogen N are free, and a rise in the temperature of the chamber. piece of zirconia. Under the combined effect of the heat and the hydrogen atmosphere, part of the oxygen contained in the zirconia diffuses towards its surface to leave the zirconia.

This partial reduction of the zirconia by diffusion of a portion of its oxygen to its surface is concomitant with a diffusion of carbon and nitrogen atoms from the surface to the core of the article. Indeed, under the effect of heat, the dissociated carbon and nitrogen atoms of the gas mixture of the enclosure will diffuse into the partially reduced zirconia. This diffusion of the carbon and nitrogen atoms causes the appearance of a surface layer of these recombined atoms with the zirconia partially reduced to obtain a zirconium carbo-nitride layer.

According to a feature of the article according to the invention, the surface layer comprises a transition region between the partially reduced zirconia core and the outer treated carburized and nitrided (carbo-nitride). Indeed, since the gaseous mixture of the atmosphere comprises a first carbon-based compound and a second nitrogen-based compound, the article is carburized and nitrided simultaneously and the transition region simultaneously comprises oxycarbides and oxynitrides of zirconium. It is thus noted that the chemical composition of the surface layer varies as a function of the depth measured from the surface of the covering element and continuously passes from stoichiometric zirconium carbo-nitride (ZrCx-Ny) to a transition region comprising oxycarbides and zirconium oxynitrides, and finally at the heart of the sub-stoichiometric zirconium oxide part (partially reduced zirconia).

According to another preferred feature of the invention, the carbon-nitrogen content of oxycarbide oxynitride zirconium in the transition region decreases with depth while their oxygen content increases with depth. The transition region therefore comprises zirconium oxycarbide-oxynitrides, the carbon / nitrogen content of which gradually decreases when progressing towards the core of the article while the presence of oxygen increases gradually in the form of ZrOzCx-type compounds. -ZrOzNy, to progressively reach the core of the article which consists essentially of partially reduced zirconia type Zr02.x. It will of course be understood that the transition between these various regions is gradual.

Of course, the degree of carburetion-nitriding of the article will depend on the concentrations of the first compound and the second compound in the gas mixture injected into the chamber. Indeed, these are not necessarily identical so that, depending on the relationship between the concentration of the first compound and the concentration of the second compound in the gas mixture, the article will be more fuel than nitride or vice versa. However, this mixture of carburation and nitriding allows said article to have an unusual outer color resulting from the mixture between the platinum color due to carburation and the golden color due to nitriding. The color spectrum obtained will then range from platinum color to yellow gold color, passing through the bronze color and the color of the pink gold. The final color then depends on the initial concentration of the first and second compounds so that the higher the C1 concentration of the carbon compound is raised relative to the C2 concentration of the nitrogen-based compound, the more the final color of the piece will tend to towards bronze, even platinum. Conversely, the higher the C2 concentration of the nitrogen-based compound relative to the C1 concentration of the carbon-based compound, the more the final color of the piece will tend towards the pink gold or even the yellow gold. Preferentially, the concentration C1 is between 5 and 100 sccm (cubic standard centimeter per minute) while the concentration C2 is between 250 and 5000 sccm.

The articles obtained by this process acquire certain new properties, including a non-zero electrical conductivity surface, a color close to bronze or pink gold and a metallic luster while retaining the properties of ceramics, particularly their very high hardness as well as their resistance to wear and corrosion.

In a first embodiment, the heating means for activating the treatment comprise at least one resistive element made of a refractory metal. Such a resistive element uses an electric current passing through it to produce heat via the Joule effect. The heating means 20 may thus comprise a single resistive element arranged in said enclosure for heating the gases and the parts to be treated homogeneously or a multitude of independent resistive elements distributed to ensure a homogeneous heat distribution, as can be seen in FIGS. 3 and 4 respectively.

In a second embodiment, the heating means are a plasma. Such a plasma is obtained from the ionization of the gas mixture based on hydrogen, carbon and nitrogen and optionally a neutral gas. This plasma is obtained for example by means of an electric discharge. Of course, according to variants of the method of the invention, other means for generating the plasma can be envisaged. For example, the plasma can be obtained by radio frequency (RF) or by microwave.

According to the method used to obtain the plasma, the use of argon as a neutral gas is advantageous. Of course, the use of other neutral gases such as neon is possible.

Claims (8)

In a variant, it may be provided a preliminary zirconia reduction step. This reduction step is carried out before the carbonitriding step and consists in placing said article in an enclosure in which hydrogen H 2 is injected. The atmosphere in the chamber is heated so that the article has a rise in temperature causing diffusion of the oxygen contained in the zirconia to its surface to leave the zirconia. This reduction step may be performed in a specific chamber or in the same chamber as the carbon nitriding operation. In this case, the enclosure comprises means for modifying the nature of the atmosphere in said enclosure. It will be understood that various modifications and / or improvements and / or combinations obvious to those skilled in the art can be made to the various embodiments of the invention described above without departing from the scope of the invention defined by the appended claims. claims 1. Process for obtaining a finished or semi-finished article (1) based on zirconia, the article having a metallic external appearance and a non-zero electrical conductivity at the surface, characterized in that it comprises the steps consisting of: - providing at least one zirconia article previously shaped in its finished or semi-finished form; placing said article in an enclosure (10) in which a gaseous mixture is arranged, this gaseous mixture comprising at least a first compound of gas based on hydrogen and carbon in a first concentration (C1) and a second gas compound based on hydrogen and nitrogen in a second concentration (C2), and - heating the gas mixture until a dissociation of the molecules of the first and second compounds and maintaining said article in the reactive atmosphere thus created to have a diffusion of the carbon and nitrogen atoms in the outer surface (2) of said article. 2. Method according to claim 1, characterized in that it further comprises, during the step of heating the gases contained in the chamber, a diffusion of oxygen contained in said zirconia article to its surface to leave it. 3. Method according to claim 1, characterized in that it further comprises, prior to the step of heating the process gases contained in the chamber, a reduction step of placing said article in a chamber in which the dihydrogen is injected and heated this dihydrogen allowing diffusion of the oxygen contained in said zirconia article to its surface to leave it. 4. Method according to one of the preceding claims, characterized in that the thickness of the layer converted on the outer surface of said article is between 10 and 1000 nm. 5. Method according to claim 4, characterized in that the thickness of the layer transformed on the outer surface of said article is between 20 and 200 nm. 6. Method according to one of the preceding claims, characterized in that the concentration of the second compound based on hydrogen and nitrogen is greater than that of the first compound based on hydrogen and carbon. 7. Method according to one of the preceding claims, characterized in that the concentration of the first compound is between 5 and 100 sccm. 8. Method according to one of the preceding claims, characterized in that the concentration of the second compound is between 250 and 5000 sccm.