This invention relates to a process for grinding ceramic powders.

With the recent increase in the use of ceramic materials such as zirconia in technologically advanced applications, there has been considerable interest in the production of fine, pure ceramic powders. The art of grinding materials to fine sizes is well known and any convenient method known to the art may be used. In order to maintain a suitable rheology in the grinding process, it is essential that a dispersant be used. Among the more effective and therefore the more widely used of the dispersants are organic compounds, one such class of compounds being amines such as triethanolamine and 2-am1no, 2-methyl propanol (AMP).

Processes utilising such amines have produced fine ceramic powders which have found ready acceptance in the marketplace. They are not, however, suitable for all uses, mainly because of the residual organic species adhering to the surface of the particles. Many manufacturers of ceramic articles wish the ceramic powder which they are to use to be completely free from any such organic residues. Hitherto, this has been achievable only either by the preferential adsorption of another more acceptable species on the particle surface thus displacing the organic species, or by burning off the organic residue by subjecting the powder to a suitably elevated temperature (the ceramic powder itself being unaffected by this treatment).

However, the burning process gives a powder which is more difficult to redispers^.. These additional processes also add to the cost of the end product. It has now been found that it is possible to grind ceramic powders to give a fine powder free of organic species. There is therefore provided, according to the present invention, a process for grinding a ceramic powder, comprising subjecting the powder to a grinding medium in the presence of a dispersant present in a quantity sufficient to allow efficient grinding, the dispersant being selected from at least one of ammonia and alkylamines having a boiling point of 100°C maximum.

The invention is surprising in that ammonia and low molecular weight alkylamines are not normally considered to be dispersing agents. By

"alkylamines" is meant of course compounds of the

1 2 3 1 3 formula NR R R where R , R and R are selected from hydrogen and C. - C. alkyl groups, at least one of R ¹ , R and R ³ being a C ₁ -C ₄ alkyl group. There

1s a particular preference for ammonia which is not

only very volatile (such that no residue remains on the ground particles) inexpensive and relatively non-toxic, but which also manifests a surprisingly high dispersion and wetting ability in these ceramics systems. Ammonia is thus the preferred material, but alkylamines such as methylamine and ethylamine may be used. It is possible and permissible to use a combination of one or more of these dispersants. The ceramic powders for use in this Invention comprise any of the ceramic powders used in the manufacture of high grade ceramics. These include zirconia (either pure or modified with, for example yttria, magnesia or silica), yttria, alumina, silicon nitride and silicon carbide, and blends of such ceramic powders.

The apparatus in which grinding of the ceramic powder is carried out may be any apparatus suitable for such purposes. It may be, for example a ball mill, an agitated media mill (such as an attritor), a sand grinder or a bead mill. An especially preferred apparatus is an attritor, a relatively low energy device whose mode of operation causes virtually no degradation of the walls of the containment vessel and therefore no contamination of the finished product.

The media used for grinding can be any media suitable for the grinding of ceramic powders, for example, balls of yttrla-modified zirconia, steatite or steel. In an especially preferred embodiment, the combination of fine grinding media of particle size 0.8-3.0 mm in an agitated media mill, the quantity of media being such that the average stand-off distance 1s from 50-90 urn, 1s an especially preferred one. Such a combination is described 1n copendlng Australian Patent Application No. 23770/88 and gives especially good results in the working of this invention.

The dispersant is added m & quantity sufficient to allow efficient grinding, that is, such that the contents of the grinding apparatus remain in a suitably fluid condition and grinding proceeds at a reasonable rate. This is readily ascertained by eye - if the contents are of a thick pasty consistency and do not flow readily, more dispersant is needed. This is added in the form of a solution in water (where the dispersant is a gas, such as ammonia or methylamine) or directly where the dispersant is a liquid at normal temperatures.

When grinding fs complete, the ground ceramic powder may be recovered by any suitable means known to the art. The dispersant will evaporate, leaving a fine, organic-free ceramic powder ready for use in any application.

The invention is further illustrated by the following examples.

Example 1

Preparation of a ceramic powder.

A yttria-modified zirconia powder of surface area 5.0 m 2 g-1 was milled in water in an attritor using 0.8 mm yttria-modified zirconia media (ex

Shinegawa Co., Japan). Ammonium hydroxide was added at a rate of 2% H ₃ by weight on zirconia, and this permitted grinding at a powder loading (weight percentage of powder in the powder plus water mixture) of 60%.

After four hours' grinding, a particle size measurement, as measured on a "Microtrac" (trade mark) particle size analyser (ex Leeds and Northrup), showed that 50% of particles were of a size of less than 0.25 urn, and the viscosity as measured on a "Bohlin" (trade mark) V0R rheometer using a cup and bob cell was 6.8 mPa.S over the shear rate range of from 30 to 300 sec .

Ex amp l e 2

A comparative example showing the effect of omitting the ammonia.

Example 1 was repeated but omitting the ammonium hydroxide. It was found that in order to maintain a suitably low viscosity which would allow a particle size reduction of 50% less than 0.25 urn after 5 hours' milling, the powder loading could not be higher than 30%. The viscosity of the final milled powder dispersion was 5 mPa.S.

This powder was dried and water was added while kneading the powder with a palette knife until a very stiff paste was obtained. The water absorption was found to be 26.4% by weight. To this paste was then added a 10% ammonia solution and kneading was continued. The stiff paste deflocculated to give a fluid dispersion with a viscosity of 7 mPa.S. The quantity of ammonia required was 0.12% NH, based on the zirconia solids.

Example 3

Preparation of a ceramic powder.

A co-precipitated Ti0 ₅ -Zr0 ₉ (equi olar)

2 -1 powder of surface area 23 m g was milled in water using the 0.8 mm yttria-modified zirconia media used in Examples 1 and 2. Ammonium hydroxide was added at a rate of 1% NH, by weight on powder and this permitted grinding at a powder loading (weight percentage of powder in the powder plus water mixture) of 65%. After 20 minutes' grinding, the particle size was measured as described in Example 1 and it was found that 50% of the particles were of a size of less than 0.75 urn. The dispersion had a low viscosity.