Alumina-based ceramic.

[0001] l'an alumina based ceramic, similar to the ruby and with high toughness. Such a ceramic can find applications, in particular, in the jewelry, the jewelry or the timepiece.

[0002] le is the ruby red variety of corundum family which consists of alumina having a particular crystal structure. The red color of the ruby is due to the presence of chromium in the corundum.

[0003] il being commercially available polycrystalline rubies are obtained by mixing alumina and chromia and somewhat MgO as sintering additive, then forming the mixture is shaped and in the sintering under hydrogen atmosphere or in a vacuum of at least 10"¹ Torrs. The ruby polycrystalline obtained by these lanes have however unsatisfactory mechanical properties due to a low homogeneity microstructures and grain size far too high since often exceeds the 10 micrometers. In addition, translucent appearance is not desirable during certain situations where it would be preferable to be able to employ a ceramic high tenacity dyed in the mass and opaque.

[0004] le major objective of the invention is to provide an opaque ceramic alumina-based, similar to the ruby and having mechanical properties, especially toughness, good enough to allow it to be used in the timepiece, for example to make clothing pieces constituting a timepiece, such as a telescope, a housing or a bracelet, which are exposed to the elements and impact.

[0005] ce aim is achieved by means of an alumina-based ceramic having the following composition, in percentages by weight:

0.3 to 3.0% - of at least one metal selected from chromium, cobalt, nickel, manganese, vanadium, titanium and iron;

- 0.0005 to 0.3% of magnesium; and

0.05 to 2% - of at least one rare earth element.

[0006] this ceramic, may be termed "ruby polycrystalline an opaque" thus has both good homogeneity, high density, a fine microstructure, a pore content acceptable carrier, high toughness, an opacity satisfactory and good polishability.

[0007] de more, its sintering takes place easily.

[0008] l'invention also relates to a method for preparing the ceramic which has reactors provided that it comprises a step during which is added at least one oxide of a rare earth element to a mixture of alumina, magnesium oxide and at least one oxide of at least one metal selected from chromium, cobalt, nickel, manganese, vanadium, titanium and iron.

[0009] d'other features and advantages of the invention will now be described in detail in the disclosure which follows and which is completed by the single figure appended showing the variation of the reflectance R [%] as a function of the wavelength λ in for a ceramic according to the invention, a commercially available polycrystalline ruby and other ceramics tested in comparison.

[0010] according to the invention, to present the properties mentioned previously, the ceramic must include the above constituents in their respective ranges of percentages.

[0011] according to a preferred embodiment of the invention, the alumina-based ceramic comprises, in percentages by weight:

0.3 to 3.0% - of chromium;

- of 0,005 to 0.3% magnesium;

0.1 to 1.0% - of at least one rare earth element. More preferably, the ceramic according to the invention comprises:

- 1.0 à2.5% of chromium;

- of 0,005 to 0.2% magnesium;

0.1 to 0.6% - of at least one rare earth element. As rare earth oxide, can be used any rare earth oxide. The ceramic according to the invention may also include various rare earth oxides.

[0012] de preferably employed lanthanum oxide or erbium oxide, particularly the latter.

[0013] of course, it is possible to provide a doping of the order of 0.01 to 0.2% by weight of other elements for example, yttrium.

[0014] si use other metals, such as iron, titanium, cobalt, manganese or vanadium instead of or in addition to the chromium, there can be obtained a color other than red. With cobalt or iron, for example, it is thus possible to obtain a blue-green according to the degree of oxidation. The degree of oxidation is determined by the heat treatment conditions, in particular the atmosphere in the furnace.

[0015] The ceramic according to the invention is preferably free of silicon because its presence induces a loss of toughness. In addition, silica and magnesia form in the presence of an aluminosilicate alumina magnesia color opaque white which causes stains in the ceramic.

[0016] The preparation of the ceramic according to the invention can be carried out according to known methods followed by adding a step of introducing the oxide or rare earth oxide.

[0017] This step may for example be heating the rare earth oxide in the presence of concentrated nitric acid until complete dissolution of the oxide. After cooling, the resulting solution may then be added to the other components.

[0018] Thus, to prepare the ceramic according to the invention, it is contemplated the following steps:

- mixture adequate amounts of alumina, magnesium oxide and at least one oxide of at least one metal selected from chromium, cobalt, nickel, manganese, vanadium, titanium and iron;

- grinding the mixture obtained;

- preparation of a solution (e) rare earth;

- addition of the oxide solution (e) rare earth obtained at ground mixture;

- attrited and then stirring of the assembly and atomization;

- shaping (e.g. pellets);

- high-temperature heat treatment for sintering.

[0019] The sintering temperature is generally between 1400 and 1700 °c, preferably between 1600 and 1700 °c. [0020] The resulting ceramic can be implemented in many areas, which include the jewelry, jewelry and timepieces.

[0021] Is carried out in a slurry by mixing bowl attrition:

98.0 g of alumina BAIKOWSKIBMA15,

2.9248 g of chromium Cr oxide₂ 0₃ Time Sigma-,

0.1009 g of the MgO Fluka,

a mixture of 3, 0277 containing g (40%) 1.2111 grams of PVA Fluka 1.8166 grams (60%) and PEG 20o00 in solution at 50% in water d=1.09 gm/cm.³Fluka,

200 ml of distilled water

a mixture of 1 kg of zirconia beads containing 20% of beads of diameter 5 mm, 3 mm diameter and 40% 40% of diameter 2 mm, these logs to serve only to grinding.

[0022] In parallel, 0.5609 g of erbium oxide Aldrich is poured into a Teflon beaker to which is added 250 ml of 1.5 ml of concentrated nitric acid 65% for Merck. The beaker is then placed on a hot plate with sand bath set at 150 °c until complete dissolution of the oxide. This solution is then poured into the bowl attrition containing the above mixture. Distilled water for rinsing the beaker is also added in the bowl attrition. The contents of the bowl is subjected to attrition to 400 rpm/min over 30 minutes.

[0023] The slip obtained is recovered in a second container which is added 450 ml of isopropyl alcohol rinsing beads and the container zirconia.

[0024] Sprayed the contents of the second vessel while stirring at 500 rpm/min. magnetically to avoid flocculation. on press the granules obtained with a hand press to gauge.

[0026] Pellets are carried out in a mold of 40 mm in diameter. The binders and plasticizers contained in the pellets are thermally removed during the pellets are then sintered in a furnace Nabertherm ht4 on air at 1650 °c.

[0027] The pellets are then polished at laboratory after being ground to a tower Schaublin 102n d91 equipped with a diamond grinding wheel.

[0028] Each pellet obtained thus consists of, in weight percentages based on the total weight of the pellet, about:

- 1.98% of chromium Cr,

- 0.06% magnesium mg and

nC - 0.44% erbium;

the balance to 100% - being provided by aluminum and oxygen.

[0029] Proceeding as in the example 1 except that instead of using erbium oxide, used 0,561 g of lanthanum oxide.

[0030] The resulting pellets are thus constituted, in mass percentages based on the total weight of the pellet, about:

- 1.98% of chromium Cr,

- 0.06% magnesium mg and

the lanthanum - 0.43%;

the balance to 100% - being provided by aluminum and oxygen.

Example 3: preparation of a ceramic outside the scope of the invention (based on prior to JP 09,208 295) Proceeding as in the example 1 except that is not added rare earth. The concentration of Cr is greater than that recommended in the document to JP 09,208 295 (0,005 - 0,1%) to obtain an opaque ceramic with satisfactory coloring, and not a translucent ceramic.

Example 4: preparation of a ceramic outside the scope of the invention (based on prior to JP 09,208 295) Proceeding as in the example 1 except that is not added rare earth and added 0.1 g of silica (0.1%).

Example 5

[0033] on has determined the color and fracture toughness of the ceramic prepared as described in examples 1 to 4.

[0034] The color was measured according to ISO 7724 established by the International according to the CIELAB system, a CIE being the abbreviation of the International and LABs the three coordinate axes, the axis L measuring the component white-black (black=0 and white=100), the a-axis measuring the component RGB (red=positive values, negative values=green) and the b-axis measuring the component yellow-blue (yellow=positive values, negative values=blue).

[0035] The toughness was calculated using the formula of Anstis-to-Chantikul from measurements made by a Vickers indenter using a load of 1000 gm force (9.80665 n-) on a microduromètre by Leica VMHT word.

[0036] the results are shown in the following table, wherein c * represents the saturation and H the hue.

[0037] as can be seen, the ceramics according to the invention (examples 1 and 2) have the best tenacities while having parameters as L *, has *, d *, the c * and H very close to those of the examples 3 and 4. further, the ceramics according to the examples 1 and 2 are opaque and non-translucent as the ruby poly crystalline according to JP the monocrystalline synthetic ruby 09,208 295 or commercially available.

Example 6

[0038] in this example, we compared the reflectance curves ceramics according to the examples 1.3 and 4 to that of a synthetic ruby crystal translucent (noted "single" Figure appended, provider berthed Sapphire of Ag, ruby red reference no. 8).

[0039] the results are visible in Figure appended. It is found that the curves ceramics examples 1.3 and 4 are substantially superposed. Thus, although the addition of rare earth (erbium in this case) has an impact substantially positive on the tenacity, it is not detrimental to the coloration of the ceramic.