This invention relates to leachable ceramic materials and to their use in investment casting. Investment casting

Investment casting is a process in which molten metals are poured into a refractory ceramic mould designed to create a duplicate of the desired part. Casting may be done around "cores". Ceramic materials used for such purposes include chemically bonded particles of materials such as silica, zircon, aluminium silicates (e.g. mullite), and alumina. Such materials are porous with low structural integrity and are simply required to resist the pressures and temperatures implicit in the casting process. After casting these materials are removed to leave the cast product remaining.

Examples of such "core" materials include those of US4073662, US4187266, US4837187, US5779809, GB1602027, and GB2126569. When making large items by investment casting it is frequently necessary to provide additional support to parts of the mould/core.

For such supports it is conventional to use dense ceramics, for example such as quartz and alumina rods. Such rods may be of a variety of sizes, but typically range in diameter from 0.2 to 40mm. The present disclosure is not however limited to such ranges of diameter.

Such supports need to be removed after the casting process and it is common to leach out the ceramic.

Comparison of quartz and alumina

Quartz and alumina both have low trace element presence which can be a requirement with casting some materials, but both represent compromises between mechanical strength and chemical resistance.

Quartz is the traditional material of choice, since it can be leached out easily using strong aqueous solutions of hydroxides such as NaOH. Alumina is increasingly being used as it has a higher mechanical strength than quartz. However, alumina is chemically more inert to hydroxides, and typically leach cycle times are increased from one day (for quartz) to three days (for alumina) and requires the more aggressive KOH as a strong aqueous solution. The scope of disclosure

This disclosure provides materials that are mechanically stronger than quartz, and yet chemically weaker than alumina, permitting shorter leach times in aqueous hydroxide solution than normal dense alumina without the strength limitation of quartz.

This requirement has been met by providing alumina materials comprising:- · a mechanically supportive continuous matrix phase comprising alumina;

• at least one second phase interpenetrating the matrix phase and providing a pathway for leachants to penetrate into the material.

The material preferably comprises at least lvol% of the at least one second phase, preferably more than 3vol%, and conveniently can comprise 5±3vol% or 5±2vol% or 5±lvol%. The amount of the at least one second phase may be up to 10vol%, 20vol% or more. The scope of the invention is as set out in the claims, and in any new and inventive features described herein with reference to the following non-limitative description.

General disclosure The present disclosure aims to use the mechanical strength of alumina to provide materials having a greater strength than quartz, but provides a second phase

interpenetrating the quartz that permits easier access to leachants than is provided by dense alumina.

Conventional aluminas used as supports in investment casting are dense ceramics comprising very low amounts of other components (typically being 95% or more pure alumina). Typical modulus of rupture for such a material would be of the order of 550 MPa (80kpsi). Dissolution by KOH leachant is by attack at the surface.

Conventional quartz used as supports in investment casting are glassy materials comprising essentially pure silica. Typical modulus of rupture for such a material would be of the order of 210 MPa (30kpsi). Dissolution by NaOH or KOH leachant is by attack at the surface, and quartz is attacked more vigorously than is alumina.

The present disclosure provides an alumina containing matrix (that may incorporate other materials) and at least one second phase that interpenetrates the matrix and that provides a pathway for leachants. The second phase may comprise porosity (that enables penetration of the leachants below the surface); and may comprise a leachable phase that is more readily leachable than the matrix, so that preferential leaching of the leachable phase permits leachant to penetrate below the surface.

In either case, the effect is to increase the area of the matrix phase that is exposed to the leachant above that of the outer surface of the support. This increased leachant contact permits quicker leaching of the material of the matrix.

In any event a modulus of rupture in excess of quarz is desired and (for example) this invention contemplates materials having modulus of rupture above 250 MPa, above 300 MPa, and above 350 MPa. Examples

Example 1 - porosity as second phase

A 99.8% pure alumina powder [Grade 998E powder from Morgan Advanced Materials (from their Latrobe facility); a sub-micron powder with a d50 less than one micron] was pressed or extruded to form rods and other shapes which were fired at 1350°C with a ramp time 2.5 hours to 1350°C, soak for 1.5 to 2 hours , ramp down time of 0.5 to 1.5 hours with a total cycle time of 5-6 hours to provide porous sintered alumina shapes, including cylindrical rods having a cross section ranging from ~0.25mm to ~40mm (0.010" to 1.6"), and having a porosity in the range 5-7%.

Example 2 - leachable material as second phase A 99.8% alumina powder [Grade 998E powder from Morgan Advanced Materials (from their Latrobe facility); a sub-micron powder with a d50 less than one micron] was mixed with sub-micron silica [Grade GP3i from Harbison Walker] in proportions to create a 97% alumina containing mixture. The mixture was pressed or extruded to form rods and other shapes which were fired at 1650°C with a ramp time of 14 hours to 1650°C, soak for 2 hours , ramp down time of 8 hours for a total cycle time of 24 hours to provide fully sintered alumino-silicate cylindrical shapes, including rods having a cross section ranging from ~0.25mm to ~40mm (0.010" to 1.6").

Example 3 - leachable material as second phase

A 99.8% alumina powder [Grade 998E powder from Morgan Advanced Materials (from their Latrobe facility); a sub-micron powder with a d50 less than one micron] was mixed with sub-micron silica [Grade GP3i from Harbison Walker] in proportions to create a 95% alumina containing mixture.

The mixture was pressed or extruded to form rods and other shapes which were fired at 1650°C with a ramp time of 14 hours to 1650°C, soak for 2 hours , ramp down time of 8 hours for a total cycle time of 24 hours to provide fully sintered alumino-silicate cylindrical shapes, including rods having a cross section ranging from ~0.25mm to ~40mm (0.010" to 1.6").

Properties of examples

Modulus of rupture of the rods of examples 1 to 3 were measured using a 3-point method [ASTM D790]. The samples measured were ~35mm (1 ¾") sections cut from 0.79mm (0.031") diameter circular cross-section rods of material.

All examples showed a modulus of rupture significantly above that of quartz measured under the same conditions, and showed greater susceptibility to leaching than dense alumina. It is to be noted that for best results the silica used in manufacture of alumino-silicates in accordance with this invention should be fine materials to avoid excessive weakening of the structure of the support material. Typically silicas with a d50<l^m are used and good results may be achieved with d50 in the range 0.5±0.2μιη or d50 in the range 0.5±0.1μιη. The present invention is not limited to these particular ranges however.

The above disclosure is illustrative and not limitative of the scope of the invention. Variants will readily occur to the person skilled in the art and be encompassed by the present invention. For example, the examples show only the use of porosity, or of a second material (leachable material) at grain boundaries of the matrix that is more readily attacked by leachants than the matrix. It will be apparent that both porosity and leachable material may be provided.