U.S. patent application number 09/850005 was filed with the patent office on 2001-11-15 for method for obtaining a zirconia-based article having a gold metallic appearance.
Invention is credited to Verdon, Christian.
Application Number | 20010040315 09/850005 |
Document ID | / |
Family ID | 4546512 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040315 |
Kind Code |
A1 |
Verdon, Christian |
November 15, 2001 |
Method for obtaining a zirconia-based article having a gold
metallic appearance
Abstract
The invention 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.degree. C.
Inventors: |
Verdon, Christian;
(Boussens, CH) |
Correspondence
Address: |
B. Franklin Griffin, Jr.
Griffin & Szipl, P.C.
Suite PH-1
2300 Ninth Street, South
Arlington
VA
22204-2320
US
|
Family ID: |
4546512 |
Appl. No.: |
09/850005 |
Filed: |
May 8, 2001 |
Current U.S.
Class: |
264/430 |
Current CPC
Class: |
C04B 41/5062 20130101;
G04B 37/225 20130101; G04B 37/1493 20130101; C04B 41/52 20130101;
A44C 5/00 20130101; C04B 41/009 20130101; C04B 41/87 20130101; C04B
41/89 20130101; A44C 27/001 20130101; C04B 41/52 20130101; C04B
41/0072 20130101; C04B 41/4519 20130101; C04B 41/455 20130101; C04B
41/5042 20130101; C04B 41/52 20130101; C04B 41/0054 20130101; C04B
41/0072 20130101; C04B 41/4529 20130101; C04B 41/4556 20130101;
C04B 41/5062 20130101; C04B 41/009 20130101; C04B 35/48
20130101 |
Class at
Publication: |
264/430 |
International
Class: |
H05B 006/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 9, 2000 |
CH |
0900/00 |
Claims
What is claimed is
1. A method for obtaining a finished or semi-finished
zirconia-based article, the article having a gold metallic external
appearance, including 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.degree. C.
2. A method according to claim 1, wherein an inert gas is added to
the ammonia, or respectively to the mixture of nitrogen and
hydrogen.
3. A method according to claim 1, wherein the prior reduction step
of said article is performed in a hydrogen plasma or a plasma of a
mixture of hydrogen and an inert gas.
4. A method according to claim 1, wherein the prior reduction step
of said article is performed in the presence of hydrogen at a
temperature comprised between 1100.degree. C. and 1300.degree. C.
for a period of time comprised between 1 and 2 hours.
5. A method according to claim 1, wherein the inert gas is
argon.
6. A method according to claim 1, wherein said plasma is generated
by a DC discharge.
7. A method according to claim 1, wherein the adjustment of the
conditions of said plasma includes the use of auxiliary heating
means.
8. A method according to claim 1, wherein the partial reduction
step is performed to reduce said article up to the core.
9. A method according to claim 1, wherein the article is maintained
in said plasma for a period of time comprised between 15 and 240
minutes.
Description
[0001] 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.
[0002] 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.degree.
C.
[0003] 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 .mu.m for a treatment temperature of
950.degree. C. to 10 .mu.m for a treatment temperature of
600.degree. C.
[0004] 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.sub.2 ,) 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.
[0005] 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 nm, having an intense gold metallic appearance, as
well as good mechanical properties.
[0006] 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:
[0007] providing at least one zirconia article previously shaped
into its finished or semi-finished shape;
[0008] partially reducing the zirconia forming said article;
[0009] 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
[0010] 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.degree.
C.
[0011] 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.degree. C.), this latter treatment being permitted by
the prior reduction step.
[0012] According to a preferred embodiment of the invention, an
inert gas is added to the ammonia, or respectively to the nitrogen
and hydrogen mixture.
[0013] 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.
[0014] According to an advantageous embodiment of the invention,
the prior reduction step of said article is performed in a hydrogen
plasma.
[0015] 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.
[0016] 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.degree.
C. and 1300.degree. C. for approximately 1 to 2 hours.
[0017] 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.
[0018] By way of example, a zirconia article having a tetragonal
crystallographic configuration (zirconium oxide ZrO.sub.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.
[0019] 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.
[0020] 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.
[0021] 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.sub.2-x type.
[0022] 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.degree. C. and 1300.degree. C.,
typically at 1200.degree. 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.
[0023] 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.
[0024] 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.
[0025] 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.degree. 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 in the event that
the energy generated by the plasma is not sufficient for the
article to reach a temperature comprised between 500 and
900.degree. C., auxiliary heating means will be used.
[0026] After this latter operation, the article has the gold
metallic brilliance of zirconium nitride, and its conduction and
very high superficial hardness, which is indispensable for
obtaining an article which is resistant to wear in normal
conditions of use. It will also be noted that this superficial
transformation does not significantly affect the toughness of the
transformed zirconia. the crystallographic structure of the
zirconia is thus superficially transformed into a new
crystallographic structure corresponding to that of zirconium
nitride, and not an added coating liable to be torn off or to
become detached from the surface of the article, in particular when
the latter is subjected to significant conditions of wear.
[0027] An essential point of the invention is the combination of
the plasma reduction step of the article prior to the plasma
treatment and the plasma treatment of the article at a relatively
low temperature. Indeed, it was observed that forming a superficial
layer of ZrN of sufficient thickness and obtaining the desired
optical, physical and mechanical properties strictly depended on
the state of reduction of the surface zirconium prior to the
treatment. In the absence of a prior reduction step, the layers of
ZrN which result from the low temperature nitriding of a
stoichiometric zirconia (ZrO2) are thus always of very low
thickness (several tens of nanometers), which allows the grey
colour of the subjacent partially reduced ceramic to appear, as was
mentioned hereinbefore. This is why, in accordance with the
invention, the method includes an additional step prior to the
nitriding step, during which the zirconia is partially reduced. The
prior reduction step allows a reduction state gradient to be
obtained which allows the layer of zirconium nitride to be formed
over a larger thickness, for example over a thickness of the order
of 300 nm. Such a zirconia thus has the colour and mechanical
resistance properties and in particular satisfactory toughness for
articles such as decorative articles.
EXAMPLE 1
[0028] Several watch bracelet links, made of white tetragonal
zirconia (ZrO2), having a length of 20 mm, a width of 7 mm, and a
thickness of 3 mm, are placed in a plasma reaction vessel having a
diameter of 700 mm. The zirconia used is a 100% tetragonal
zirconia, which is stabilised at the surrounding temperature by
adding yttrium oxide (3% mol) and which has toughness typically
higher than 5 Mpa.m.sup.1/2. A mixture of gas including 55% argon
(Ar) and 45% hydrogen is injected into the reaction vessel. The
flow rate of argon is 1200 sccm and the flow rate of hydrogen is
1000 sccm. The steady mode of the discharge current is fixed at
200A under 70V. Further, the pressure in the reaction vessel is
fixed at 1 millibar. A plasma is generated in a conventional manner
between the cathode and the anode which are located in the reaction
vessel. The atomic hydrogen then comes into contact with the
surface of the links and thus partially reduces the zirconia.
During this step of treating the links, the temperature of the
latter is settled at approximately 910.degree. C. The treatment
lasts for one hour.
[0029] A mixture of gas including 75% argon (Ar) and 25% ammonia
(NH3) is injected into the reaction vessel. The flow rate of argon
is 1200 sccm and the flow rate of ammonia is 410 sccm. The steady
mode of the discharge current is fixed at 80A under 82V. Further,
the pressure in the reaction vessel is fixed at 1 millibar. A
plasma is generated in a conventional manner between the cathode
and the anode which are located in the reaction vessel. The ionised
gas mixture then comes into contact with the already partially
reduced links and the atomic nitrogen of the plasma is substituted
for the oxygen of the zirconia. The hydrogen present in the
reaction vessel acts as a catalyst for this reaction and prevents
re-oxidisation of the zirconia. During this treatment step of the
links, the temperature in the reaction vessel is settled at
approximately 770.degree. C. The treatment lasts for two hours. The
links are then removed from the reaction vessel and subjected to
analysis. The observation of cross-sections of these links under an
electronic transmission microscope shows that part of the surface
of the links has been transformed into zirconium nitride (ZrN) over
a depth of approximately 250 nm. The hardness of the treated links
has also been measured. The Vickers hardness values obtained are of
the order of 12 Gpa. The links obtained have a gold metallic
appearance very close to that of gold. It will also be noted that
the thickness of the cubic phase zirconia transformation after
treatment is of the order of 18 .mu.m, the thickness of this phase
being directly linked to the temperature of the ceramic during
treatment.
EXAMPLE 2
[0030] A white tetragonal zirconia watch case is arranged as
described in example 1 in a plasma reaction vessel. The case is of
generally rectangular shape and has a length of 30 mm, a width of
24 mm and a height of 5 mm. This case is hollowed at its centre.
The operating process is identical to that described in example 1,
with the difference that the prior zirconia reduction step is
performed in a controlled atmosphere furnace in which a hydrogen
flow of 100 sccm flows at 1200.degree. C. for two hours and during
the nitriding step the gas mixture used includes 55% nitrogen, 34%
hydrogen and 11% argon. The flow rate of argon is 1200 sccm, the
flow rate of nitrogen is 250 sccm and the flow rate of hydrogen is
750 sccm. The steady mode of the discharge current is fixed at 50A
under 90V. In this example, the temperature in the reaction vessel
is settled at approximately 670.degree. C. during the treatment.
The treatment lasts two hours. The watch cases obtained have a gold
metallic appearance similar to that of the links described
previously, and have the same transformation and hardness features
as those described in example 1.
[0031] As a result of the foregoing, the method according to the
invention allows articles, particularly decorative articles, with a
gold metallic appearance, to be made, combining certain features of
ceramics and metals.
[0032] It is thus possible to obtain decorative articles which have
substantially the mechanical properties of tetragonal zirconia and
in particular its toughness, while having a particularly pleasing
appearance and metallic brilliance. These articles are perfectly
suited to use as decorative articles and in particular as
construction elements for wristbands or cases for timepieces.
* * * * *