Glass-ceramic article and enamel suitable for the coating thereof

Abstract

The present invention relates to a glass-ceramic article at least partly coated with at least one layer of an enamel formed from a glass frit having the following composition, the proportions being expressed as weight percentages: TABLE-US-00001 SiO.sub.2 50-66% and preferably 50 < SiO.sub.2 65% MgO 3-8% and preferably 3-6% Na.sub.2O 7-15% K.sub.2O 3% Li.sub.2O 3%, in particular 2% CaO 1% BaO >0-15% and preferably 5-15% Al.sub.2O.sub.3 3-20% and preferably 3 < Al.sub.2O.sub.3 < 20% ZrO.sub.2 0-4%, preferably 0 < ZrO.sub.2 < 4%, in particular 0.5-2% ZnO >0-5% B.sub.2O.sub.3 >0-6% and preferably 0 < B.sub.2O.sub.3 5% the sum of the alkaline-earth metal oxides CaO+BaO moreover being between 8 and 15%, and the sum of the alkali metal oxides Na.sub.2O+K.sub.2O+Li.sub.2O moreover being between 7 and 20%. The invention also relates to the reinforced glass-ceramics obtained.

Claims

1. A glass-ceramic article at least partly coated with at least one layer of an enamel formed from a glass frit having the following composition: TABLE-US-00005 SiO.sub.2 50-66% by weight; MgO 3-8% by weight; Na.sub.2O 7-15% by weight; K.sub.2O 3% by weight; Li.sub.2O 3% by weight; CaO 1% by weight; BaO 7-15% by weight; Al.sub.2O.sub.3 >3-20% by weight; ZrO.sub.2 0-4% by weight; ZnO >0-5% by weight; and B.sub.2O.sub.3 >0-6% by weight, wherein: a sum of alkaline-earth metal oxides CaO+BaO is between 8 and 15% by weight; and a sum of alkali metal oxides Na.sub.2O+K.sub.2O+Li.sub.2O is between 7 and 20% by weight.

2. The glass-ceramic article of claim 1, wherein the glass frit comprises more than 4% by weight of alumina Al.sub.2O.sub.3.

3. The glass-ceramic article of claim 1, wherein the glass frit comprises at least one of more than 7% by weight of Na.sub.2O, and less than 13% by weight of BaO.

4. The glass-ceramic article of claim 1, wherein the glass frit comprises at most 2% by weight of Li.sub.2O.

5. The glass-ceramic article of claim 1, wherein the article is formed from a glass-ceramic comprising less than 0.2% by weight of arsenic oxides.

6. The glass-ceramic article of claim 1, wherein the article is coated with the at least one layer of enamel on at least one part of its upper face.

7. A device for baking or for maintaining a high temperature, the device comprising the glass-ceramic article of claim 1 and one or more heating elements.

8. The glass-ceramic article of claim 1, wherein the glass frit comprises more than 5% by weight of alumina Al.sub.2O.sub.3.

9. The glass-ceramic article of claim 1, wherein the glass frit comprises more than 7% by weight of alumina Al.sub.2O.sub.3.

Description

EXAMPLE 1

(1) In this first example according to the invention, the procedure was the same as in reference example 3, the frit of the enamel being replaced with a frit according to the invention having the following composition: SiO.sub.2: 54.5%; MgO: 4%; Na.sub.2O: 9.5%; Li.sub.2O: 2%; BaO: 10%; ZrO.sub.2: 1%; ZnO: 4%; B.sub.2O.sub.3: 5%; Al.sub.2O.sub.3: 10%. The scale factor obtained was of around 85 MPa, the Weibull modulus being equal to 12.3. The delamination strength grade was 0. The expansion coefficient of the glass frit/enamel was 8410.sup.7 K.sup.1.

(2) It was observed that the mechanical strength of the glass-ceramic not refined with arsenic and coated with the enamel according to the invention is at least maintained at the level of that normally obtained with the conventional glass-ceramics refined with arsenic, whereas it falls much more strongly when the conventional enamels are used, the delamination strength being at the same time improved compared with that obtained using compositions which make it possible to retain a good mechanical strength.

EXAMPLE 2

(3) In this second example according to the invention, the procedure was the same as in example 1, the frit of the enamel being replaced with the frit according to the invention having the following composition: SiO.sub.2: 56.5%; MgO: 4%; Na.sub.2O: 9.5%; Li.sub.2O: 2%; BaO: 10%; ZrO.sub.2: 2%; ZnO: 4%; B.sub.2O.sub.3: 5%; Al.sub.2O.sub.3: 7%. The scale factor obtained was of around 85 MPa, the Weibull modulus being equal to 11.5. The delamination strength grade was 0. The expansion coefficient of the glass frit/enamel was 8510.sup.7 K.sup.1.

(4) It is here again observed that the mechanical strength of the glass-ceramic not refined with arsenic and coated with the enamel according to the invention is at least maintained at the level of that normally obtained with the conventional glass-ceramics refined with arsenic, whereas it falls much more strongly when the conventional enamels are used, the delamination strength being at the same time improved compared with that obtained using compositions which make it possible to retain a good mechanical strength.

EXAMPLE 3

(5) In this third example according to the invention, the procedure was the same as in example 1, the frit of the enamel being replaced with a frit according to the invention having the following composition: SiO.sub.2: 57.5%; MgO: 4%; Na.sub.2O: 9.5%; Li.sub.2O: 2%; BaO: 10%; ZrO.sub.2: 1%; ZnO: 4%; B.sub.2O.sub.3: 5%; Al.sub.2O.sub.3: 7%. The scale factor obtained was of around 93 MPa, the Weibull modulus being equal to 15. The delamination strength grade was 0. The expansion coefficient of the glass frit/enamel was 8510.sup.7 K.sup.1.

(6) It is here again observed that the mechanical strength of the glass-ceramic not refined with arsenic and coated with the enamel according to the invention is at least maintained at the level of that (or even greater than that, as in the present case) normally obtained with the conventional glass-ceramics refined with arsenic, whereas it falls much more strongly when the conventional enamels are used, the delamination strength being at the same time improved compared with that obtained using compositions which make it possible to retain a good mechanical strength.

EXAMPLE 4

(7) In this fourth example according to the invention, the procedure was the same as in example 2 but this time including an additional step of milling the frit/pigments assembly so as to achieve an average diameter D90 of the particles of the enamel of around 4.7 m. The scale factor of the enameled plates obtained after application of the enamel to and baking of the enamel on the glass-ceramic was of around 120 MPa, the Weibull modulus being equal to 9.7. The delamination strength grade was 1. The expansion coefficient of the glass frit/enamel was 8510.sup.7 K.sup.1.

(8) An additional improvement is observed in the mechanical strength of the glass-ceramic not refined with arsenic obtained according to the present example compared with that observed above, the delamination strength at the same time remaining improved compared with that obtained by using other compositions which make it possible to obtain a good mechanical strength.

(9) The plates according to the invention may especially be used advantageously to produce a new range of hobs for cookers or cooktops, or for producing elements of a wall or walls (for example doors) for ovens, or for producing chimney inserts or fire screens, etc.