METHOD FOR THE MANUFACTURE OF AN OBJECT FROM POWDERED MATERIAL BY ISOSTATIC PRESSING

Abstract: The preformed body is provided with a casing (13) of glass which is made gas-impenetrable by heating before carrying out the isostatic pressing. Inside the glass casing there is arranged on the preformed body an intermediate layer (12) which counteracts the penetration of molten glass from the casing into the preformed body. The intermediate layer com­ prises a layer of one or more intermediate phases in the system Al₂O₃ - SiO₂, for example mullite 3 Al₂O₃·2 SiO₂, or a layer containing one or more such intermediate phases as main constituent. In a preferred embodiment a layer of boron nitride is arranged inside the intermediate layer. The pow­ dered material from which object is manufactured consists preferably of aluminium oxide or an aluminium oxide-based ceramic.

Description: The present invention relates to a method of manufacturing an object from powdered material by isostatic pressing according to the precharacterising part of Claim 1. When the preformed body is subjected to the isostatic pressing at sintering temperature, in order to obtain a desired dense, sintered product, the body is enclosed within a casing which is capable of preventing the pressure medium used, normally a gas, from penetrating into the powder body. As casing there is often used glass in the form or particles which may be applied on or around the preformed body in different ways. The preformed body with the applied casing is then heated, usually under vacuum, at such a temperature that the glass particles form a dense casing around the preformed body. When, later on during the isostatic pressing, the preformed body is subjected to pressure at an elevated temperature, problems may arise which are caused by the glass in the casing penetrating into pores in the preformed body and/or reacting with superficially located portions on the preformed body. To counteract such a process, an intermediate layer of a powdered material can be arranged on the preformed body inside the casing of glass. A well-known powdered material in such an intermediate layer is boron nitride. Boron nitride is suitable for this purpose because, on the one hand, it exhibits a small propensity to react with the solid ceramic or metallic material which is used for manufacturing objects by isostatic pressing and, on the other hand, a layer of boron nitride is easily removable from the finished object due to its relative softness and relatively low strength. The function of the boron nitride as a barrier against the penetration of the glass, however, has proved to be unsatisfactory in those cases where there are great demands on the quality of the pressed and sintered end product. The reason is that a layer of boron nitride has a certain porosity and therefore cannot entirely prevent the passage of a low-viscous glass and its penetration into pores in the preformed body and/or the reaction of this glass with the surface material of the powder body, so that the quality of the powder body is deteriorated by the presence of glass therein or by reaction products of glass and the material in the preformed body. It is also known to use, in intermediate layers with the described function, a finely-divided glass with a higher melting point than the glass in the casing or a finely-divided metallic material. However, such intermediate layers cannot sufficiently efficiently prevent the passage of a low-viscous glass and the disadvantages connected therewith. JP-A-58 88 179 discloses hot isostatic pressing of ceramics whereby the ceramics are coated with boron nitride and alumina and/or zirconium oxide and hot-pressed first at the sintering temperature for the alumina or zirconium oxide coating to cover the ceramics with an airtight layer and then at a higher temperature and pressure. The invention aims at finding a method of the above-mentioned kind which makes it possible, by selecting a suitable intermediate layer material, to completely eliminate or reduce to a minimum any damage inflicted to the formed object by molten glass from the surrounding casing. To achieve this aim the invention suggests a method for the manufacture of an object from powdered material by isostatic pressing according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. As examples of such intermediate phases in the system Al₂O₃-SiO₂ may be mentioned mullite 3 Al₂O₃·2 SiO₂, sillimanite Al₂O₃·SiO₂ and kyanite Al₂O₃·SiO₂ (a high pressure modification of sillimanite). The intermediate layer is preferably used when manufacturing objects at temperatures below the melting temperature or the decomposition temperature of the intermediate phase or phases used in the intermediate layer. A probable explanation of the good results obtained according to the present invention is that the above-mentioned intermediate phases only react very insignificantly with the surrounding glass. The mullite or other intermediate phase is in equilibrium with the surrounding glass and may, while maintaining its crystalline state, dissolve and take up some SiO₂ from the glass, which results in a slight sintering of the particles of the intermediate phase and hence in the intermediate layer functioning as a barrier against the penetration of glass into the preformed body located thereinside. In addition to the above-mentioned phases, the intermediate layer may contain a powdered additive such as aluminium oxide, zirconium oxide, titanium diboride, boron nitride, or a high-melting glass which does not react, or only insignificantly reacts, with the intermediate phase, so that this in all essentials retains its crystalline state. Particularly suitable additives are such powdered materials which are used for the manufacture of the object. As examples of high-melting glass materials that may be used may be mentioned quartz glass and a glass containing 96.7 per cent by weight SiO₂, 2.9 per cent by weight B₂O₃ and 0.4 per cent by weight Al₂O₃ (Vycor®). The percentage of the additive may amount to almost 50%, and preferably to at most 30% of the total weight of all the constituents in the intermediate layer. According to a particularly advantageous embodiment of the invention, there is arranged on the preformed body inside the intermediate layer a layer consisting at least substantially of boron nitride. In addition to its function of mechanically contributing to prevent glass from penetrating into the preformed body, the boron nitride constitutes an excellent release agent. This latter function has proved to be of particular importance when using the boron nitride inside an intermediate layer of mullite or other intermediate phase of the above-mentioned kind for the manufacture of objects of materials, among other things ceramics, having a high coefficient of thermal expansion. Upon cooling of the pressed object, and upon a shrinking thus occurring, the mullite or the corresponding intermediate phase functions as a coherent stiff shell around the object, thus protecting the object, before the glass has solidified, against viscous glass moving against the object. When the glass has solidified, because of the slight sintering previously described, the mullite is fixedly secured to the glass, and therefore, under the cooperation of the boron nitride, the object is efficiently released from the glass casing and the intermediate layer. In certain applications it may be advantageous to arrange between the above-mentioned layer, which at least for the main part consists of boron nitride, and the intermediate layer a layer consisting of a mixture of boron nitride and a high-melting glass such as the previously mentioned quartz glass or Vycor® glass. The volume of the high-melting glass suitably amounts to 20-80% and preferably to 30-70% of the total volume of the glass and the boron nitride. The boron nitride may consist of boron nitride of commercially available qualities. Such qualities contain a certain amount of oxygen in the form of boron oxide. The content of foreign substances in the boron nitride in layers, which at least substantially consist of boron nitride, preferably amounts to at most 10 per cent by volume. Instead of using boron nitride, it is possible in some cases to use other substances with layer structures such as graphite and molybdenum sulphide. The particle size of the material in the intermediate layer suitably amounts to 1-200 µm (micrometer) and preferably to less than 150 µm, of the boron nitride suitably to 1-100 µm and preferably to less than 45 µm, and of the high-melting glass suitably to 1-200 µm and preferably to less than 125 µm. The thickness of the intermediate layer suitably amounts to 0.1-3 mm and preferably to 0.3-0.6 mm, the thickness of the layer of boron nitride suitably to 0.1-2 mm and preferably to 0.2-0.4 mm, and the thickness of the layer of a mixture of boron nitride and high-melting glass to 0.1-3 mm and preferably to 0.3-0.6 mm. The layer of boron nitride and the layer of one or more intermediate phases of the system Al₂O₃-SiO₂ may be brought about by immersing the preformed body into or spraying it with a suspension of those particles from which the respective layer is to consist, in a solvent, for example acetone, ethanol or other alcohol, followed by a drying of the preformed body. The powdered material from which the object is manufactured preferably is a ceramic material or a metallic material. As examples of ceramic materials for which the present invention is particularly suited for use may be mentioned aluminium oxide and aluminium oxide-based ceramics, for example the systems Al₂O₃-SiC and Al₂O₃-ZrO₂ and zirconium oxide and zirconium-oxide-based ceramics. These ceramic materials have a greater coefficient of thermal expansion than usable glass materials, which causes them - for reasons previously mentioned - to become effectively released from the glass casing. Another extremely important property of the intermediate layer when using it for the manufacture of aluminium oxide and aluminium-oxide-based ceramics is that the intermediate phase in the intermediate layer is in chemical equilibrium with Al₂O₃ and does not take up any Al₂O₃ from the object. As examples of other ceramic materials for which the invention is applicable may be mentioned silicon carbide, boron carbide, titanium diboride ans silicon nitride. As examples of metallic materials may be mentioned, inter alia, steel, an iron-based alloy, for example 3% Cr-Mo steel containing 0.33%C, 0.30% Si, 0.40% Mn, 0.01% P, 0.01% S, 2.8% Cr, 0.6%Mo, the balance being Fe, or 12 % Cr-Mo-V-Nb steel containing 0.18% C, 0.25% Si, 0.60% Mn, 0.01% P, 0.01% S, 11,5% Cr, 0.5% Ni, 0.5% Mo, 0.30% V, 0.25% Nb, the balance being Fe, or an alloy containing 1.27% C, 0.3% Si, 0.3% Mn, 6.4% W, 5.0% Mo, 3.1% V, 4.2% Cr, the balance being Fe, or of a nickel-based alloy, for example an alloy containing 0.03% C, 15% Cr, 17% Co, 5% Mo, 3.5% Ti, 4.4% Al, 0.03% B, the balance being Ni, or an alloy containing 0.06% C, 12% Cr, 17% C, 3% Mo, 0.06% Zr, 4.7% Ti, 5.3% Al, 0.014% B, 1.0% V, the balance being Ni. The percentage here relates to percentage by weight. As examples of usable materials in glass in the casing around the preformed body may be mentioned a glass containing 80.3 per cent by weight SiO₂, 12.2 per cent by weight B₂O₃, 2.8 per cent by weight Al₂O₃, 4.0 per cent by weight Na₂O, 0.4 per cent by weight K₂O, and 0.3 per cent by weight CaO (Pyrex®), further an aluminium silicate containing 58 per cent by weight SiO₂, 9 per cent by weight B₂O₃, 20 per cent by weight Al₂O₃, 5 per cent by weight CaO and 8 per cent by weight MgO, as well as mixtures of particles of substances, for example SiO₂, B₂O₃ and alkaline and alkaline earth metal oxides which, upon heating, form glass, further a glass containing 96.7 per cent by weight SiO₂, 2.9 per cent by weight B₂O₃ and 0.4 per cent by weight Al₂O₃ (Vycor®) and mixtures of particles which, upon heating, form glass. The pressure and temperature for the isostatic pressing and the sintering of a ceramic or metallic material are, of course, dependent on the type of this material. Normally, however, the pressure should amount to at least 100 MPa and the temperature should amount to 1000-1900°C, preferably to at most 1800°C. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 schematically a preformed body provided with intermediate layers according to the present invention, Figure 2 the same body on a reduced scale in a glass casing, Figures 3 and 4 other preformed bodies provided with intermediate layers according to the present invention. Example 1 Aluminium oxide powder having a particle size of 0.1-2 µm (micrometer) is placed into a capsule of plastic, for example softened polyvinyl chloride or rubber, having approximately the same shape as the preformed body to be manufactured. The capsule is sealed and subjected with its contents to a compaction at 300 MPa for a period of 5 minutes at room temperature. After completed compaction the capsule is removed, and the body thus manufactured is machined into the desired shape. The preformed powder body thus obtained, is designated 10 in Figure 1 and has a cylindrical shape. On the preformed body there are applied first a layer 11 of boron nitride and on top of this layer a layer 12 of mullite. These layers are illustrated in greatly exaggerated thickness. For the application of the layer 11 a suspension is used containing 40 per cent by weight boron nitride with a particle size of less than 45 µm, 3 per cent by weight polymethyl methacrylate, and 57 per cent by weight acetone. The suspension is sprayed on the body so that all surfaces are covered with a layer of the suspension. When the acetone has evaporated, the layer has a thickness of 0.2 mm. Thereafter there is applied, similarly by spraying, a layer 12 of a suspension containing 45 per cent by weight mullite with a particle size of less than 150 µm, 5 per cent by weight butyl acrylate, and 50 per cent by weight isopropanole. After drying, the layer 12 has a thickness of 0.4 mm. The preformed body 10 with the applied boron nitride layer 11 and the intermediate layer 12 of mullite is provided, according to Figure 2, with a casing 13 of glass in the form of a mass of glass particles arranged in a graphite crucible 14, the inner wall of which is provided with a release layer 15 of boron nitride. The glass in this mass consists of 80.3 per cent by weight SiO₂, 12.2 per cent by weight B₂O₃, 8.2 per cent by weight Al₂O₃, 4.0 per cent by weight Na₂O, 0.4 per cent by weight K₂O, and 0.3 per cent by weight CaO. The crucible with its contents is placed in a furnace in which the binders in the layers 11 and 12, i.e. the acrylates, are driven off in vacuum while the temperature is successively raised to 400°C and is maintained at this temperature for a few hours. Thereafter, the crucible with its contents is placed in a high pressure furnace in which the necessary pressure can be generated by the supply of a gas, for example argon, and the necessary temperature can be obtained by conventional heating devices. First the crucible is heated to a temperature of 1000-1200°C at atmospheric pressure so that the glass casing forms a melt and becomes gas-impenetrable. Thereafter, the pressure is raised to 150 MPa and the temperature to 1350-1400°C and are maintained at these values for a period of 1 hour. The aluminium oxide will then sinter while forming a body with a density of 99.9% of the greatest theoretically possible density. When the body has been allowed to cool and the glass casing has been removed, the barrier layers 11 and 12 can be removed without difficulty by light sand blasting. Example 2 The same procedure as described in Example 1 is used to manufacture an object of aluminium oxide, with the difference that no layer of boron nitride is used but that the intermediate layer 12 of mullite is applied direct on the preformed body 10, as shown in Figure 3, and that the isostatic pressing of the body 10 with the layer 12 arranged in the glass casing 13 in the crucible 14 is carried out at a temperature of 1200-1300°C. Example 3 The same procedure as described in Example 1 is used to manufacture an object of zirconium dioxide with a particle size of less than 1 µm. Example 4 The same procedure as described in Example 1 is used to manufacture an object of an aluminium oxide-based ceramic consisting of 75 per cent by weight aluminium oxide and 25 per cent by weight silicon carbide as whiskers, the isostatic pressing, however, being carried out at a temperature of 1500-1650°C. Example 5 The same procedure as described in Example 1 is used to manufacture an object of titanium diboride with a particle size of less than 3 µm, the isostatic pressing, however, being carried out at a temperature of about 1500°C. Example 6 The same procedure as described in Example 1 is used to manufacture an object of silicon nitride (containing 2.5 per cent by weight yttrium oxide) with a particle size of less than 1 µm, the isostatic pressing, however, being carried out at a temperature of about 1700°C. Example 7 The same procedure as described in Example 1 is used to manufacture an object of a 12% Cr-Mo-V-Nb steel containing 0.18% C, 0.25% Si, 0.60% Mn, 0.091% P, 0.01% S, 11.5% Cr, 0.5% Ni, 0.5% Mo, 0.30% V, 0.25 Nb, the balance being Fe, and having a grain size of less than 800 µm. In this case the isostatic pressing is carried out at a temperature of 1200°C. Example 8 The same procedure as described in Example 1 is used to manufacture an object of boron carbide with the difference that outside the layer 11 there is applied, as illustrated in Figure 4, a layer 16 consisting of a mixture of Vycor® glass and boron nitride and that outside this layer 16 there is applied a layer 12 of mullite, the isostatic pressing, however, being carried out at a temperature of about 1750°C. For the application of the layer 16 there is used a suspension containing 20 per cent by weight Vycor® glass with a particle size of less than 125 µm, 18 per cent by weight boron nitride with a particle size of less than 45 µm, 3 per cent by weight butyl acrylate and 59 per cent by weight isopropanole. After drying, the layer has a thickness of 0.4 mm. For the application of the layer 12 there is used in this case a suspension containing 45 per cent by weight mullite with a particle size of less than 70 µm, 3 per cent by weight polymethyl methacrylate and 52 per cent by weight acetone. The thickness of this layer is 0.4 mm after drying. Example 9 The same procedure as described in Example 8 is used to manufacture an object of Al₂O₃ with 25 per cent by weight SiC whiskers, the isostatic pressing, however, being carried out at a temperature of 1500-1650°C. Instead of using mullite in the examples described, there may be used, as intermediate layer, mullite, sillimanite or kyanite, or mixtures of at least two of them in arbitrary proportions. Further, it is possible to add to these intermediate phases, or mixtures thereof in the cases illustrated in the examples, the previously mentioned additives, such as Al₂O₃, ZrO₂ or TiB₂ in a content of 10% of the total weight of all constituents in the intermediate layer.

Claims: Method of manufacturing an object from powdered material by isostatic pressing of a body (10) preformed from the powdered material with a gaseous pressure medium, the preformed body being provided with a casing (13) of glass or of a material forming glass upon heating, and with an intermediate layer (12) of a powdered material, being arranged on the preformed body inside the casing and counteracting the penetration of molten glass from the casing into the preformed body, the casing of glass being made gas-impenetrable by heating, before the isostatic pressing is carried out while sintering the preformed body, characterized in that the intermediate layer comprises a layer of one or more intermediate phases in the system Al₂O₃-SiO₂, or a layer containing one or more such intermediate phases as main constituent, and that the intermediate phase or phases consists/consist of one or more of the substances mullite 3 Al₂O₃·2 SiO₂, sillimanite Al₂O₃·SiO₂, or kyanite Al₂O₃·SiO₂. Method according to Claim 1, characterized in that the intermediate layer is used at temperatures below the melting temperature or decomposition temperature of the intermediate phase or phases. Method according to Claim 1 or 2, characterized in that on the preformed body inside the intermediate layer there is arranged a layer which at least for the main part consists of boron nitride. Method according to Claims 1 or 2, characterized in that on the preformed body inside the intermediate layer there is arranged a layer which at least for the main part consists of graphite or molybdenum sulphide. Method according to any of Claim 3 or 4, characterized in that the intermediate layer contains a powdered additive, such as aluminium oxide, zirconium oxide, titanium boride, boron nitride, or quartz glass, which does not react, or only insignificantly reacts, with the intermediate phase. Method according to any of Claims 3 to 5, characterized in that between the intermediate layer and the layer of boron nitride arranged inside the intermediate layer there is arranged a layer consisting of a mixture of boron nitride and a high-melting glass. Method according to any of the preceding Claims, characterized in that the powdered material from which the object is manufactured consists of aluminium oxide or an aluminium oxide-based ceramic. Method according to any of Claims 1 to 6, characterized in that the powdered material from which the object is manufactured consists of zirconium oxide or a zirconium oxide-based ceramic. Method according to any of Claims 1 to 6, characterized in that the powdered material from which the object is manufactured consists of titanium diboride or a titanium diboride-based ceramic.

Inventors: HERMANSSON, LEIF, TJERNLUND, ANNA-KARIN
Applicants: ASEA CERAMA AB
Application year: 1988
Main classification: B22F3
IPC codes: B22F3, C04B35, C22C33