TECHNICAL FIELD

5 The invention relates to a method of manufacturing an essentially dense ceramic body, starting from a boron nitride-based ceramic powder, by hot pressing. The body shall be easily mechanically machined and removable from, for example, a steel body by blasting. The boron nitride- 10 based ceramic body is primarily used for forming of powder- metallurgically manufactured metallic bodies, for example as a core or mould in hot-isostatic pressing.

BACKGROUND ART

15

Ceramic bodies, with essentially full theoretical density, are suitable to use for forming in connection with the manufacture of products from metal powder by hot-isostatic pressing. Material such as aluminium oxide or zirconium

20. oxide cannot, however, be used as cores unless they are formed so as to be easily withdrawable from the formed product. The bodies should be easy to mechanically machine into moulds and cores and should be removable from the formed product by, for example, blasting. Possible

25 materials are then graphite and boron nitride, preferably in the form of hexagonal boron nitride. Graphite can only be used where carbonizing through diffusion and dissolution, at normal temperatures for hot-isostatic pressing, does not take place to any harmful extent and does not constitute a 0 problem, such as, for example, in the case of cobalt-base alloys. From this follows that boron nitride at present constitutes the preferred material. It is, of course, possible to use graphite if it is coated with a diffusion- inhibiting layer, for example of boron nitride.

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The manufacture of boron nitride-based ceramic bodies with essentially full density by hot pressing is a well- established technique. However, manufacturing bodies of

essentially theoretical density from a powder with low fill density by compaction and consolidation presents a problem. US Patent US 4 495 123 describes a method of manufacturing dense bodies of boron nitride, wherein a boron nitride powder, with a boron oxide content of less than 1 %, is filled into a capsule whereupon the powder is vibrated into a density higher than the fill density before the capsule is sealed, evacuated, compacted and consolidated by means of hot-isostatic pressing.

From German Patent DE-A-4 003 809 it is known, starting from a boron nitride power containing up to 4 per cent by weight boron oxide, to add up to 3 % A1N, up to 1 % Ca and up to 50 % of additives such as Zrθ ₂ , Si ₃ N4, SiC, B ₄ C,. Siθ ₂ , AI ₂ O ₃ , Y ₂ O ₃ , MgO or metals to optimize the technical properties.

The powder is preformed and pre-compressed by cold-isostatic pressing, possibly with the addition of 0.5-6 % of a binder, into a porous body with a density of at least 50 % of the theoretical density, in the following designated %TD. The preformed body is placed in a steel capsule and heated to between 500 and 1100°C, whereupon the binder is driven off. Thereafter, the capsule is evacuated and sealed gas-tightly ' to be finally compacted and consolidated by hot-isostatic pressing in the conventional manner at a temperature of between 1200 and 1500°C and a pressure of between 50 and 300 MPa.

In addition to pre-compaction to a density above the fill density, the manufacture of a boron nitride body according to the US patent requires extensive preparation regarding the purity of the boron nitride powder.

SUMMARY OF THE INVENTION

The present invention relates Lo a method of manufacturing an essentially dense and mechanically machinable ceramic body, starting from a boron nitride powder, without pre-

compaction or preforming, the powder being compacted and consolidated by means of hot pressing.

By hot pressing is meant in the following uniaxial pressing in a heated tool, uniaxial pressing of hot powder, hot- isostatic pressing as well as hot pressing using pseudo- isostatic methods.

Boron nitride powder in commercially available qualities usually contains boron oxide in contents of up to 2 per cent by weight. The boron oxide is a glass-forming material which acts as a sintering aid in connection with compaction and consolidation of boron nitride.

The invention is also applicable to the manufacture of boron nitride-based ceramic bodies to which additives such as Zrθ ₂ , Si ₃ N ₄ , SiC, B ₄ C, Siθ ₂ , AI ₂ O ₃ , Y ₂ O ₃ , MgO, mixtures of these ceramic materials, or metals are made to optimize the technical properties.

Prior to compaction and consolidation of the boron nitride- based ceramic powder mentioned, the content of sintering aid is corrected to a content of between 3 and 10 per cent by weight. According to the invention, the sintering aid substantially consists of a glass-forming material, i.e. a material which, per se or together with the boron oxide present in the boron nitride powder, while being heated to the pressing temperature, is transformed into a glass. During the pressing the glass forms a compact binder phase which essentially fills up the remaining spaces between the compressed grains in the ceramic powder and binds the grains into a dense and mechanically machinable ceramic body. The boron nitride-based ceramic powder with additives is compacted and consolidated by means of hot pressing into an essentially dense wuy, ρre_era_/_y into a jjo witr_ a density exceeding 90 %TD, at a temperature of between 500 and 1500°C and a pressure of between 50 and 300 MPa.

The content of sintering aid in the boron oxide-containing boron nitride-based ceramic powder is corrected, where necessary, to the intended content according to the invention in any of the following ways:

- addition of boron oxide powder or another powdered material which, per se or together with the boron oxide present in the boron nitride, while being heated forms a glass. Suitable materials, in addition to boron oxide and boron oxide-based glass, are other glass powders, ceramic clays or silicate minerals which together with the existing boron oxide form a glass with a suitable viscosity at the pressing temperature. In case of powdered additions, the mixing is advantageously performed by means of co-grinding, thus obtaining a suitable distribution of the sintering aid, evenly distributed over the surface of the boron nitride grains. In addition to this, co-grinding with boron oxide present in the powder provides a local liquid phase reaction, as a consequence of the temperature increase during the grinding, whereby the glass-forming sintering- promoting additive is at least partially dissolved in the boron oxide;

- addition of a powder suspension containing the sintering- promoting, glass-forming material, for example a boron oxide powder, a boron oxide-based glass powder, another glass powder or another glass-forming material such as amorphous silica. The powder is suspended in a liquid, such as water, ethanol, methanol or another alcohol which is driven off after being mixed in;

in addition, the oxide content, especially the boron oxide content in the boron nitride-containing cermic powder, may be increased by measures such as:

- admixture of boric acid, preferably in dissolved state, followed by drying;

- oxidation of boron nitride and/or another non-oxidic material in the ceramic powder while forming boron oxide and/or another oxide, which together with boron oxide forms a glass at the temperature prevailing during compaction and consolidation;

- addition of oxides or other oxygen-containing compounds, which during the process, grinding, heating, pressing, react with the boron nitride while forming boron oxide .

During the compacton and the consolidation, the glass- forming materials form a compact binder phase which essentially fills up the spaces between the grains and cements them together into a dense and mechanically machinable ceramic body which is essentially free from pores and microcracks.

In certain applications it is advantageous to granulate the boron nitride-based powder with additives before it is filled into the steel capsule. The powder is suitably granulated in connection with the content of sintering- promoting, glass-forming agent being corrected. By checking the addition and temperature during the co-grinding, the binder removal, the drying, the oxidation, etc., granules of the desired size as well as a desired distribution of the sintering aid on the surfaces of the powder grains are obtained.

The content of sintering aid is preferably corrected by correcting the boron oxide content in the boron nitride- based ceramic powder in any of the ways described above into a boron oxide content of between 4.5 and 7 per cent by weight.

During hot-ioostatic pressing of a boron nitride— sed ceramic powder according to the invention, the content of sintering aid is corrected to a content of between 3 and 10 per cent by weight before the powder is filled into a steel

capsule. During filling, the ceramic powder is packed into a fill density corresponding to 15 to 60 %TD. After filling of the steel capsule, it is evacuated and sealed essentially gas-tightly before the capsule and the ceramic powder contained in the capsule is compacted and sintered by hot- isostatic pressing at a temperature of between 500 and 1500°C and a pressure of 50 to 300 MPa. A ceramic body with a density of at least 90 %TD is then formed.

During compaction and consolidation of the boron nitride- based ceramic powder by means of hot-isostatic pressing, starting from a powder with a fill density of between 15 and 60 %TD, into a fully dense body, the powder is filled into a capsule which, according to one embodiment of the invention, is joined together and designed to be deformed symmetri¬ cally, hence taking up the heavy deformations of the capsule which arise during the compaction of a powdered material with low fill density into an essentially dense body. This symmetrical deformation is achieved in one embodiment by designing and joining together the capsule such that, at a critical value of the applied external overpressure, an instability arises in the capsule, the sheet is corrugated and the capsule deformed symmetrically and regularly, the great reduction in volume in connection with the compaction of the powder from a low fill density to an essentially dense body then being taken up by the capsule without leakage arising. Alternatively, the deformation is control¬ led by stiffening certain sheets such that other sheets are stretched during the compression.

In another embodiment, this symmetrical deformation of the capsule is achieved by joining together the sheets included in the steel capsule by means of joints with a high folded- up flange. By forming the joints with a high flange, they are able to absorb the βsvy deformation of the c sule which results from the great compression of the ceramic powder from a low fill density into an essentially dense body, without leakage arising.

In a preferred embodiment of the invention, a granulated boron nitride-based ceramic powder, in which the boron oxide content has been corrected to a content of between 4.5 and 7 per cent by weight, is filled into a steel capsule. The steel capsule is designed and joined together according to any of the methods described in the foregoing, to take up the great deformations during the compaction. During the filling, granulated ceramic powder is packed into a fill density corresponding to 20 to 60 %TD. After the steel capsule has been filled, it is evacuated and sealed essen¬ tially gas-tightly before the capsule and the ceramic powder enclosed in the capsule are compacted and sintered by hot- isostatic pressing at a temperature of between 1000 and 1250°C and a pressure of 100 to 140 MPa. This leads to the formation of a ceramic body with a density of at least 96 %TD.

Boron nitride-based ceramic bodies bonded and manufactured according to the invention are essentially dense, while at the same time being easily mechanically machinable. This renders them most suitable for use as shape-imparting cores or moulds during powdermetallurgical manufacture of com¬ ponents from metal powder. After forming, they are then easily mechanically removed, for example by blasting. To further improve the possibilities of machining of the cera¬ mic bodies, by making possible machining by electroerosive methods, so-called spark machining, 5 to 10 per cent by weight carbon is added to the ceramic powder in a special embodiment.