GB2287933A

GB2287933A - Glazing ceramic materials

Info

Publication number: GB2287933A
Application number: GB9405586A
Authority: GB
Inventors: Christopher Patrick Hyndman
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1994-03-22
Filing date: 1994-03-22
Publication date: 1995-10-04
Also published as: GB9405586D0

Abstract

A vitreous glaze is formed on ceramic materials, eg reaction-bonded silicon nitride. The glaze is formed from a glazing mixture comprising a powder in a suitable binder, wherein the powder comprises: 15 to 40% by weight of silica, 10 to 35% by weight of alumina, and a balance comprising manganese dioxide and ferric oxide, the manganese dioxide forming 15 to 80% by weight of said balance.

Description

GLAZING CERAMIC MATERIALS This invention is concerned with glazing ceramic materials and, in particular, with glazing mixtures and with methods of forming vitreous glaze on a surface of an article made of ceramic material.

Ceramic materials have an adherent porosity which can be disadvantageous when the ceramic material is used in contact with liquid and, in a hostile environment can lead to corrosion of the ceramic material. For example, when the ceramic material is exposed to high temperatures, oxididation may occur. In order to prevent the porosity of the ceramic material leading to corrosion thereof, it is known to glaze the surface of the ceramic material to seal said porosity. One such glaze which is intended for use with reaction-bonded silicon nitride is described in GB 2 230 522 A. This known glaze comprises 35 to 45% by weight of manganese dioxide, 20 to 40% by weight of an alumina based composition having enhanced firing properties, and 30 to 40% by weight of silica together with a suitable binder.This known glaze is successful in many applications but it is desirable to provide an improved glaze which will allow an extension of the possible applications.

It is an object of the present invention to provide an improved glazing mixture which can be used to glaze ceramic materials such as reaction-bonded silicon nitride.

The invention provides a glazing mixture comprising a powder in a suitable binder, wherein the powder comprises: 15 to 40% by weight of silica, 10 to 35% by weight of alumina, and a balance comprising manganese dioxide and ferric oxide, the manganese dioxide forming 15 to 80% of said balance.

A grazing mixture according to the invention is found to provide, when fired, a vitreous glaze which is resistant to a wider range of hostile environments than the prior art glaze described above. This glazing mixture is specifically designed for use with reaction-bonded silicon nitride, having a closely-matched coefficient of thermal expansion, but may be useful with other ceramic materials.

In this specification and the claims, the use of chemical names is not intended to exclude the possibility of minor impurities so that, for example, alumina may contain small percentages of sodium monoxide, calcium oxide etc.

In a glazing mixture in accordance with the invention said balance may also comprise of up to 45% by weight of titanium oxide. For example, there may be between 40 and 45% by weight of titanium dioxide in said balance. Other metal oxides, such as cobalt oxide, may be added to said balance, instead of or in addition to titanium dioxide.

Preferably, the silica forms 30 to 40% by weight of the powder and the alumina forms 25 to 35% by weight of said powder.

Preferably the manganese dioxide forms 15 to 30% by weight of said balance.

The invention also provides a method of forming a vitreous glaze on a surface of an article made of ceramic material, the method comprising forming a glazing mixture, applying said glazing mixture to said surface, and firing at a temperature from 1200 degrees centigrade to 1300 degrees centigrade, the glazing mixture comprising a powder in a suitable binder, wherein the power comprises: 15 to 40% by weight of silica, 10 to 35% by weight of alumina, and a balance comprising manganese dioxide and ferric oxide, the manganese dioxide forming 15 to 80% by weight of said balance.

The ceramic material may, for example, be reactionbonded silicon nitride.

There now follows a detailed description of an example which is illustrative of the invention.

In the illustrative example, a glazing mixture was prepared comprising a powder in a suitable binder, specifically the binder comprised 5% by weight of ethyl cellulose powder dissolved in 95% by weight of 2-Ethoxyethanol. The powder comprised 25% by weight of alumina, 35% by weight of silica and a balance, ie 40% by weight of the total, comprising 25% by weight manganese dioxide and 75% by weight ferric oxide. These ingredients were prepared as a slurry obtained by ball milling the powder for 16 hours with Isopropylalcohol and sintered silicon nitride acting as a grinding medium. The slurry was then dried at a temperature between 100-150 degrees centigrade and the resulting solid material was passed through a 250 micrometre sieve.

The powder obtained was stirred into the binder to provide a required mixture to be applied to a ceramic material. The stirring was done using a magnetic stirrer on a hot plate. The relative proportions of the powder and of the binder were pre-determined to be suitable for application to reaction-bonded silicon nitride. The glazing mixture so formed was brushed on to a piece of reaction-bonded silicon nitride which was then fired at a temperature between 1200 and 1350 degrees centigrade in air. The temperature was raised at 75 degrees centigrade per hour until a temperature of 500 degrees centigrade was achieved. The temperature of 500 degrees centigrade was maintained for half an hour and then the temperature was raised at 600 degrees centigrade per hour until 1300 degrees centigrade was reached. The temperature of 1300 degrees centigrade was maintained for two hours and then cooling at 600 degrees centigrade per hour was applied until 1200 degrees centigrade was achieved and then maintained for two hours. The reaction-bonded silicon nitride was then allowed to cool in the furnace. The reaction-bonded silicon nitride was found to have vitreous glaze on the surface thereof which was found to enable the material to be exposed to temperatures greater than 1000 degrees centigrade for prolonged periods in the presence of corrosive gases without substantial damage to the material.

Claims

1 A glazing mixture comprising a powder in a suitable binder, wherein the powder comprises:

15 to 40% by weight of silica,

10 to 35% by weight of alumina, and a balance comprising manganese dioxide and ferric oxide, the manganese dioxide forming 15 to 80% by weight of said balance.

2 A glazing mixture according to claim 1, wherein said balance also comprises up to 45% by weight of titanium dioxide.

3 A glazing mixture according to either of claims 1 and 2, wherein the silica forms 30 to 40% by weight of the powder.

4 A glazing mixture according to any one of claims 1 to 3, wherein the alumina forms 25 to 35% by weight of the powder.

5 A glazing mixture according to any one of claims 1 to 4, wherein the manganese dioxide forms 15 to 30% by weight of said balance.

6 A glazing mixture substantially as hereinbefore described with reference to the illustrative example.

7 A method of forming a vitreous glaze on a surface of an article made of ceramic material, the method comprising forming a glazing mixture, applying said glazing mixture to said surface, and firing at a temperature of from 1200 degrees centigrade to 1350 degrees centigrade, the glazing mixture comprising a powder in a suitable binder, wherein the powder comprises:

15 to 40% by weight of silica,

8 A method according to claim 7, wherein the ceramic material is reaction-bonded silicon nitride.

9 A method according to either one of claims 7 and 8, wherein a glazing mixture according to any one of claims 1 to 6 is used.

10 A method of forming a vitreous glaze substantially as hereinbefore described with reference to the illustrative example.