The present invention concerns a zirconia based article, particularly a sintered article, and in particular an article of this type whose chemical structure is transformed over part of its thickness to give it an external surface having a gold-plated metal appearance. The invention also concerns a use of such an article, as an exterior part for a wristwatch, in particular for making construction parts for watch cases or wristbands. The present invention concerns, finally, a method for obtaining such an article.
Among all known materials, metals are very widely used because of certain advantageous properties: a high level of mechanical resistance, a high level of toughness, electrical conductivity, etc. Moreover, the characteristic brilliance or lustre of such metals is very much sought after for decorative applications such as for making watch cases or such. The hardness of solid gold or gold coloured coatings is however inferior to that of ceramics, which may result in significant wear in numerous common conditions of use, for example when these materials are used to make exterior parts used in watchmaking, such as watch cases or wristbands.
In order to overcome these drawbacks, it has long been sought to increase the hardness of articles made of solid gold or including a gold coloured coating. However, conventional metallurgic methods do not allow very high levels of hardness to be attained, i.e., levels of hardness greater than 1,000 HV (Vickers hardness), which would allow scratchproof decorative parts having a gold-plated metal appearance to be made.
This is why one has proposed increasing the hardness of metals by adding very hard particles. Two-phase composites have thus been made formed of a metallic phase coating hard particles, which frequently take up most of the volume of the material. A certain number of materials of this type having a gold-plated metal appearance have been developed and are today commonly used in industry. They are generally called xe2x80x9ccermetxe2x80x9d when they are made of a ceramic material, for example titanium nitride.
All these materials have in common however the fact that they include a distinct metallic phase which may limit their resistance to corrosion and their mechanical properties at high temperatures.
Moreover, the metallic phase used as binding agent always contains nickel or cobalt, all attempts to substitute other metals for these elements having failed until now. However, the use of nickel and cobalt, in applications which result in prolonged contact with the skin, such as, for example, for watchmaking and jewelry applications, must be avoided since these elements are known to frequently cause allergies.
Finally, these materials are generally difficult to shape. This constitutes a significant limitation for the manufacture of exterior parts for watchmaking or pieces of jewelry which often have complex shapes with resultant high manufacturing costs.
An object of the invention is thus to overcome the drawbacks of the prior art by providing a zirconia based article, in particular a sintered article, having all the advantageous properties of conventional ceramic parts (hardness, shapeability, lack of allergenic effect, etc.) while having the brilliance or lustre of gold-plated metal parts.
Another object of the invention is to use in an advantageous manner certain properties of the zirconium carbide obtained by converting the zirconia at the surface of a part for aesthetic and decorative purposes within the scope of applications in the watchmaking industry.
Another object of the present invention is to provide a method for obtaining such a zirconia article which may easily be industrialized and which allows these articles to be obtained at cycle rates and costs compatible with market requirements.
The invention thus concerns a zirconia based article having a ZrO2 and/or partially reduced ZrO2 core characterized in that it includes, over at least part of its surface, a superficial layer integral with said article, said superficial layer including a plurality of regions of which one external region is formed of zirconium nitride having a gold, metallic brilliance.
According to a preferred feature of the article according to the invention, the superficial layer includes a transition region situated between the core and the external region, said transition region including sub-stoichiometric zirconium nitride and zirconium oxynitrides.
It is to be noted that the chemical composition of the superficial layer varies as a function of the depth measured from the surface of the exterior part and passes continuously, i.e., without interruption of continuity, from stoichiometric zirconium nitride (ZrN) to a transition region including sub-stoichiometric zirconium nitride and zirconium oxynitrides.
According to another preferred feature of the invention, the nitrogen content of the sub-stoichiometric zirconium nitride in the transition region decreases with the depth while the oxygen content of the oxynitrides increases with the depth.
The transition region thus includes sub-stoichiometric zirconium nitride whose nitrogen content gradually decreases as one progresses towards the core of the article whereas its oxygen content progressively increases in the form of zirconium oxynitride (ZrOxNy) type compounds, to progressively reach the core of the article which is essentially formed of partially reduced zirconia of the ZrO2xe2x88x92x type and/or zirconia (ZrO2). It will of course be understood that the transition between these various regions happens gradually, the quantity of a compound of an upper region decreasing in favour of a compound of a lower layer.
Chemical analysis of the superficial layer by RBS-ERDA measuring techniques has demonstrated the relative concentrations of zirconium, nitrogen and oxygen corresponding to the aforecited chemical compounds as a function of the depth from the surface of the article.
The invention also concerns a method for obtaining a finished or semi-finished zirconia article having a ZrO2 and/or partially reduced ZrO2 core, having a gold-plated metallic external appearance, characterized in that it includes the steps of:
providing at least one zirconia article previously shaped into its finished or semi-finished shape;
placing said article in a reaction vessel in which a plasma obtained from a gaseous mixture of ammonia and an inert gas, a mixture of nitrogen, hydrogen and an inert gas or a combination of such two mixtures is generated; and
maintaining said article in the plasma for a period of time of approximately 15 to 240 minutes, the average temperature of the article being in these conditions between approximately 600 and 1,300xc2x0 C.
In order to make such articles, one started by making, in accordance with conventional techniques, a conventional zirconia ceramic part having all the required properties, but with no metallic features. This part was then subjected to the method according to the invention in order to transform the crystallographic structure of its surface thereby forming a superficial layer including, in particular, an external region formed of zirconium nitride.
The external region of the superficial layer of the zirconia exterior part transformed according to the method of the invention thus gives such part the optical characteristics of a zirconium nitride of gold metallic appearance.
Moreover, grazing X ray diffraction analysis has shown that the structure of the superficial layer transformed at the surface has the crystallographic structure corresponding to that of zirconium nitride.
Standard X ray diffraction analysis has shown that the crystallographic structure of the core of the part corresponds to that of tetragonal zirconia.
The articles obtained from this method acquire certain new properties, in particular electrical conductivity and a gold metallic brilliance or lustre while keeping the properties of ceramics, in particular their very high level of hardness.
Quite unlike known xe2x80x9chard metalsxe2x80x9d and xe2x80x9ccermetsxe2x80x9d, the zirconia articles according to the invention do not contain any metallic phase containing Ni or Co.
The invention also concerns the use of a ceramic material having a ZrO2 and/or partially reduced ZrO2 core as a wear resistant exterior part for a wristwatch, such part including, over at least part of its surface, a superficial layer integral with said part, the thickness of said superficial layer including a plurality of regions of which one external region is formed of zirconium nitride having a gold metallic brilliance.
Other features and advantages of the invention will be better understood hereinafter with the aid of the following description of an embodiment example of a zirconia article having a gold metallic external appearance, such description being given solely by way of illustrative example and made with reference to the annexed drawings.