Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Producing or treating inorganic material – not as pigments,...
Reexamination Certificate
2001-05-08
2002-03-12
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Producing or treating inorganic material, not as pigments,...
C264S483000
Reexamination Certificate
active
06355202
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a method for obtaining a zirconia-based article, particularly a sintered article, and in particular an article whose chemical structure is transformed over part of its thickness in order to give it an external surface having a gold metallic appearance.
BACKGROUND OF THE INVENTION
European Patent No. EP 0 947 490 in the name of the Applicant discloses in particular a method for obtaining a zirconia-based article having a gold metallic appearance. According to this method, a stabilised zirconia article in tetragonal phase is placed into a reaction vessel in which a plasma is created obtained from a gaseous mixture of ammonia and a neutral gas and the article is kept in the plasma for a period of approximately 15 to 240 minutes, adjusting the conditions so that the mean temperature of the article is settled between 600 and 1300° C.
During the treatment, the nitrogen contained in the plasma diffuses in the surface of the ceramic and transforms the tetragonal phase of the zirconia into cubic phase. This phenomenon has already been disclosed in a publication by MM. J. Wrba & L. Lerch in the journal entitled J. Euro-Ceram. Soc. 18 (1998) at pages 1787 to 1793. Although this superficial transformation does not cause any cracking of the zirconia insofar as the cubic phase has the same density as the tetragonal phase, this transformation causes a reduction in its toughness at the surface. Since a certain quantity of nitrogen has to be incorporated in the zirconia to form the zirconium nitride which will provide the desired golden appearance, the detrimental effect of this structural transformation can only be limited by reducing the extent of the space over which it occurs. In order to do this, it is necessary to limit the diffusion of nitrogen in the zirconia. In other words, this means in practice reducing the temperature of the plasma treatment, for example, in the case of a discharge reactor, decreasing the relative quantity of hydrogen in the plasma, by reducing the intensity of the discharge or by moving the parts away from the plasma. By way of illustration, the transformed thickness typically passes from 300 &mgr;m for a treatment temperature of 950° C. to 10 &mgr;m for a treatment temperature of 600° C.
However, this decrease in temperature has a drawback in that the thickness of the layers of zirconium nitride (ZrN) obtained are very thin and allow the reduced zirconium oxide (ZrO
2−x
) to appear, which detracts considerably from the gold metallic appearance which it is sought to obtain and consequently makes the application of these transformed zirconia for aesthetic and decorative purposes virtually impossible.
SUMMARY OF THE INVENTION
The object of the invention is to overcome the drawbacks of the prior art by providing a method for obtaining a zirconia article allowing layers of ZrN to be formed over a large thickness, typically 300 &mgr;m, having an intense gold metallic appearance, as well as good mechanical properties.
The invention therefore concerns a method for obtaining a finished or semi-finished zirconia-based article, the article having a gold metallic external appearance, characterised in that it includes the steps of:
providing at least one zirconia article previously shaped into its finished or semi-finished shape;
partially reducing the zirconia forming said article;
placing said article in a reaction vessel in which a plasma is generated from ammonia, or a mixture of nitrogen and hydrogen, or a combination of this gas and this mixture; and
maintaining said article in the plasma for a period of at least 5 minutes, while adjusting the conditions so that the average temperature of the article is settled between 500 and 900° C.
The method of the invention thus allows a sufficiently thick superficial layer of zirconium nitride to be formed, while avoiding the transformation of the zirconia into cubic phase over too large a thickness, as a result of the combination of the prior reduction step and the relatively low temperature plasma treatment (between 500 and 900° C.), this latter treatment being permitted by the prior reduction step.
According to a preferred embodiment of the invention, an inert gas is added to the ammonia, or respectively to the nitrogen and hydrogen mixture.
The addition of an inert gas allows a lower voltage electric arc to be generated favorise in the plasma reactor, which encourages the plasma to arc at a steady mode, in particular in the case of an electric discharge reactor.
According to an advantageous embodiment of the invention, the prior reduction step of said article is performed in a hydrogen plasma.
Thus, it is possible to perform the prior zirconia reduction step in the same reactor as that used to perform the subsequent nitration step, which greatly simplifies the method and decreases the cost thereof.
According to another feature of the invention, the prior reduction step of said article is performed under atmospheric hydrogen pressure, at a temperature comprised between 1100° C. and 1300° C. for approximately 1 to 2 hours.
Other features and advantages of the invention will be better understood with reference to the following description of an implementation example of the method for obtaining a zirconia-based article having a gold metallic external appearance.
DESCRIPTION OF THE INVENTION
By way of example, a zirconia article having a tetragonal crystallographic configuration (zirconium oxide ZrO
2
), which is white in colour and which is made according to conventional manufacturing techniques for ceramic articles, for example by sintering, is provided as the starting article.
This article may be a finished product having the final shape in which it will be used, for example a part having already undergone mirror polishing and intended to form an external watch part, such as a link of a bracelet.
Of course, if required, the article may be a semi-finished product on which subsequent machining operations could be performed in order to adapt such article to its final use.
The method according to the invention consists first of all in partially reducing the zirconia forming the article in order to obtain by controlled reduction a reduction state gradient between the surface of the article and the core. Within the scope of the description “partial reduction” means that an oxygen sub-stoichiometry of the zirconia is created, namely a compound of the ZrO
2−x
type.
This partial reduction step is preferably achieved up to the core of the article. This step is for example achieved by a prolonged treatment of the order of one or two hours at a temperature comprised between 1100° C. and 1300° C., typically at 1200° C., and under hydrogen atmospheric pressure. It goes without saying that any other reduction means allowing reduction up to the core of the article may be envisaged by those skilled in the art. The duration of the reduction step will of course depend on the dimensions and the shape of the article or articles to be treated.
This article is then placed into a reaction vessel in which a plasma is generated from the ionisation an ammonia gaseous mixture and if required an inert gas, or a mixture of nitrogen, hydrogen and if required an inert gas, or a combination of these two mixtures. This plasma is obtained for example using an electric discharge. Of course, according to variants of the method of the invention, other means for generating the plasma may be envisaged. By way of example, the plasma may be obtained by radiofrequency (RF) or by microwaves.
According to the method used to obtain the plasma, the use of argon is advantageous. Of course, the use of other inert gases such as neon may also be envisaged.
The article is maintained in the plasma for at least 5 minutes, and preferably between 15 and 240 minutes. The average temperature of the article during treatment is settled between 500 and 900° C. according to the implementation parameters (time, composition of the gaseous mixture, flow rates, etc.) of the method. It will also be noted in this regard that i
Asulab S.A.
Fiorilla Christopher A.
Griffin & Szipl, P.C.
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