Method for obtaining a zirconia based item

A method for obtaining a finished or semi-finished item in sintered zirconia having a metallic appearance comprises the steps of providing at least one previously shaped and sintered zirconia item in its finished or semi-finished shape, placing the item in an oven in which there is a plasma containing hydrogen, bringing the item to a temperature of around 800.degree. C. or more, and maintaining the item at this temperature for a period of time from around a quarter of an hour to three hours, preferably two hours.

FIELD OF THE INVENTION 
The present invention concerns a method for obtaining a zirconia based item 
and, in particular, an item of this type having a metallic appearance 
intended notably to be used in decorative applications. 
BACKGROUND OF THE INVENTION 
Among all known materials, metals are very widely used because of their 
very advantageous properties: high levels of mechanical resistance and 
toughness, electrical conductivity, etc. Furthermore, their characteristic 
lustre is very much sought after for decorative applications. Their 
hardness is however inferior to that of ceramic materials, which may lead 
to 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. 
For this reason, one has for a long time sought to increase the hardness of 
metals. Conventional metallurgic methods (quenching, structural hardening) 
do not enable the high levels of hardness, greater than 1000 HV (Vickers 
hardness) required for certain applications, such as those for cutting 
tools or scratchproof decorative items, to be obtained. 
For these applications, one has attempted further to increase the hardness 
of metals by the addition of very hard particles, by making two-phase 
composites formed of a metallic phase coating the hard particles, which 
frequently take up most of the volume of the material. A certain number of 
materials of this type have been developed and are today commonly used in 
the industry. They are generally called "hard metal" when the hard 
particles used are formed of tungsten carbide, and "cermet" when they are 
titanium carbonitride. 
However, all these materials have in common the fact that they comprise a 
distinct metallic phase, which may limit their resistance to corrosion and 
their mechanical properties at high temperatures. 
Further, this metallic phase always contains nickel or cobalt, all attempts 
to substitute other metals for these materials having failed until now. 
However, the use of this metallic phase with nickel and cobalt, in 
applications which lead to prolonged contact with the skin, such as for 
example watchmaking or jewellery applications, must be avoided for these 
elements are known to frequently cause allergies. 
Finally, these materials are generally difficult to shape, which 
constitutes a significant limitation during manufacturing of exterior 
parts for watchmaking or parts for jewellery--which are often complex 
shapes--and makes their cost very high. 
SUMMARY OF THE INVENTION 
An object of the invention is thus to overcome all these disadvantages by 
providing a method for obtaining a zirconia based item having all the 
conventional properties of ceramic parts (hardness, ability to be shaped 
easily, lack of allergenic reaction etc.) while having the lustre of 
metallic parts. 
The invention concerns a method for obtaining a finished or semi-finished 
item in sintered zirconia having a metallic appearance, characterised in 
that it comprises steps consisting of: 
providing at least one previously shaped zirconia item in its finished or 
semi-finished shape; 
placing said item in an oven in which there is a plasma containing 
hydrogen; 
bringing said item to a temperature of around 800.degree. C. or more; 
maintaining said item at this temperature for a period of time from around 
a quarter of an hour to three hours and preferably two hours. 
The approach for making items of this type is fundamentally new. The 
starting point was to make, in accordance with conventional techniques, a 
conventional 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 give it a metallic appearance. 
The items obtained by this method acquire certain properties of metals, 
notably their electrical conductivity and their metallic lustre while 
keeping their ceramic properties, in particular their hardness. 
In complete contrast to existing "hard metals" and "cermet", the zirconia 
items according to the invention have only certain characteristics of 
metals, but they do not contain any metallic phases.

DESCRIPTION OF PREFERRED EMBODIMENT 
An embodiment example of a sintered zirconia item having a metallic 
appearance with the method for obtaining it according to the invention 
will now be described below. 
By way of example, one provides a white zirconia item having a tetragonal 
crystallographic structure (zirconium oxide, ZrO.sub.2) which is made in 
accordance with conventional manufacturing techniques for ceramic items, 
for example by sintering. 
This item may be a finished product exhibiting the final shape in which it 
will be used, for example a part of generally parallelepiped shape 
intended to form a watch bracelet element. 
Of course, if necessary the item may be a semi-finished product which may 
subsequently undergo machining operations in order to adapt it to its 
final use. 
This item is then heated to 800.degree. C. or more and placed, for a period 
of time from a quarter of an hour to three hours, and preferably for two 
hours, in a hydrogen plasma, obtained for example with the aid of an 
electric discharge or a microwave generator. 
After the latter operation, the item exhibits an attractive appearance and 
a metallic lustre. 
The method according to the invention enables an item combining certain of 
the characteristics of ceramics and metals, without the presence of any 
metallic phase. 
It is thus possible to obtain items of great hardness having a particularly 
aesthetic metallic appearance and lustre, easy to shape and not liable to 
cause allergic reactions. Consequently, these items are perfectly suited 
to use as decorative items and in particular as construction element of 
bracelets or cases for watchmaking.