The present invention relates to composite structures of ceramic materials and high strength concrete known generally as super concrete.

Ceramic materials have a lot of advantageous properties such as: low weight, high strength, stiffness, hardness and resistance to abrasive and erosive wear, corrosion and high temperatures.

However, ceramic materials have some disadvantages such as high costs and special production and processing problems which limit the cost effectiveness, especially when in the case of complex shapes and large parts. The main disadvantageous property of ceramic materials is their brittleness which causes them to fail before any noticeable yield or plastic flow as with metals. As a consequence, ceramic materials are susceptible to surface damage.

The term "ceramic material" is intended to encompass any class of inorganic, non-metallic products, which are subjected to a temperature of 540°C, and above, during manufacture or use, including metallic oxides, borides, carbides or nitrides and mixtures or compounds of such materials. This definition is adopted from O'Bannon: Dictionary of Ceramic Science and Engineering. Plenum Press New York, 1984.

In recent years there have been developed the so-­called super concretes which are concretes having a very high strength but which can be processed at room temperature and can be combined with virtually any material, thus making the production of cost effective complex components possible. These super concretes have some very advantageous properties, such as being highly wear resistant, temperature resistant, corrosion resistant, electrically insulating and dimensionably and thermally stable.

However, these super concrete materials have some drawbacks, such as low tensile strength and low resistance to acidic environments.

The term "super concrete" is intended to mean a group of inorganic non-metallic materials made of densely packed, specially selected particles embedded in a very dense and strong matrix. The matrix is based on an hydraulic cement such as Portland cement or calcium aluminate cement or phosphate cement with the addition of special "densifiers" such as microsilica. Special particles to obtain certain effects such as low thermal expansion (cordierite and lithium-zirconium phosphate), thermal or electrical conduction (metal particles or fibres), and magnetic properties (ferrites) may be incorporated in the super concrete. The super concretes used according to the present invention have a compression strength of at least 70 MPa and preferably above 120 MPa according to ASTM standard C39-86.

It is an object of the present invention to provide combined or composite structures of ceramic materials and super concretes which are resistant and inert and which can be produced in a cost effective way.

Accordingly, the present invention provides a composite structure, material or article comprising, in combination, ceramic material selected from inorganic, non-metallic products which are subjected to a temperature of 540°C and above during manufacture or use and a super concrete comprising densely packed particles of inorganic materials embedded in a matrix based on hydraulic cement and having strength of at least 70 MPa, the material or article comprising an outer layer of ceramic material bonded to an internal structure of super concrete, or an outer structure of super concrete bonded to an inner layer of ceramic material.

Ceramic materials may be made by different shaping technics such as slip casting, extrusion, injection moulding, tape casting, roll forming and uniaxial as well as isostatic pressing, and by different heat treatment processes such as vacuum, inert gas, atmospheric and gas pressure sintering, uniaxial and isostatic hot pressing, reaction bonding, infiltration, and flame and plasma spraying. The ceramic materials may comprise different phases: crystalline, amorpheous, solid solutions and ceramic or metallic fibres embedded in a ceramic matrix. The ceramic materials include dense as well as porous materials.

Typical well known examples of ceramic materials according to the above definition are alumina containing 85 - 100 % of aluminium oxide, partially and fully stabilised zirconia, zirconia toughened alumina, silicon infiltrated silicon carbide, sintered silicon carbide, reaction bonded silicon nitride, sintered silicon nitride, tungsten carbide, titanium diboride, aluminatitanium carbide composite, titanium carbide and boron nitride. Further examples are traditional ceramic like porcelain, cordierite, mullite, and spinell.

Also functional ceramics such as ferrites, titanates and ceramic high temperature superconductors are meant to fall within the above definition of ceramic materials.

A typical super concrete mixture used in connection with the present invention may comprise about 400 parts by weight of calcium aluminate cement, 96 parts by weight of microsilica with a mean particles size of 0.15 microns and a surface area of about 20 m²/gram, 1147 parts by weight of bauxite (50 % 1-4mm and 50 % 0-1mm), added dispersants and accelerators to obtain a well dispersed mixture and favourable pouring and casting properties.

Materials sold under the trademarks CERAMITE and DENSIT are two classes of superconcrete which can be used in connection with the present invention. Their strength properties approach those of traditional ceramics at the high end and exceed those of high strength concrete at the low end. These materials can be processed at room temperature and can be combined with virtually any material, thus making the production of cost effective complex components possible.

The particles embedded in the matrix of the super concrete may be of virtually any inorganic material, the more typical being quartz, aluminium oxide, bauxite, silicon carbide and different silicates applied to give strength and high wear resistance.

The composite structures according to the invention, can be used for a large number of finished products and which are possible to produce more cost effectively than up til now. The products also have better overall properties than before. As examples of products which can be made using the advantages of the present invention the following products can be mentioned:

Tiles comprising a structure of super concrete with a layer of ceramic material applied to at least one side.

Pipes consisting of an inner tube of super concrete and an outer layer of ceramic material, and pipes consisting of an inner ceramic tube with an outer protection of super concrete.

Ceramic capacitors in the form of ceramic tubes having an outer electrically insulating and protecting layer of super concrete.

Valve balls comprising an inner part of super concrete with a surface layer of ceramic material.

Monoscrews comprising an inner part of super concrete having a surface layer of ceramic material.

The combined structures according to the present invention are preferably normally made by first forming and finishing the ceramic part of the structure by any conventional process as one or more pieces having the optimal wall thickness. The super concrete is then cast into or onto the the ceramic part or parts according to the conditions given by the over-all design. In this way very complex and large shapes can be produced which are not possible to make from ceramic materials alone in a cost effective way. Alternatively the ceramic layer can be applied to the cured super concrete structure by plasma or flame spraying.

A further advantage of the composite structures according to the present invention is that fixing devices such as steel bolts can be cast partly into the super concrete part of the structures. In this way the structures can be easily affixed to other structures.

This is normally not possible to obtain with ceramic materials alone as it is not possible to insert for example steel bolts during production of a ceramic material.

The invention may be carried into practice in various ways and will now be illustrated with reference to the following non-limiting examples.

EXAMPLE 1

A composite structure according to the present invention in the form of paddles for use in mechanical mixers working on highly abrasive and corrosive pasty mixtures was made in the following way: A 5mm thick shell of 99.8% alumina was made by slip casting. The outer surface of the ceramic shell corresponded to the shape of the paddle. A super concrete mixture comprising 400 parts by weight of calcium aluminate cement, 96 parts by weight of microsilica 1147 parts by weight of bauxite filler (50% 1-4mm and 50% 0-1mm), dispersants and accelerators was cast into the ceramic shell. Steel bolts for connection of the paddle to the mixer arm were inserted and kept in position in the super concrete. The concrete was allowed to cure for 24 hours. The paddle made according the present invention showed excellent wear and corrosion properties compared to conventional paddles made from stainless steel. It should be appreciated that paddles made from only ceramics would not be possible to use both due to very high costs and due to design problems.

EXAMPLE 2

This example relates to a combined structure of an outside super concrete body on an inner shell of ceramic material, which is used in connection with a sensor configuration for flow measurement devices.

A cold isostatically pressed straight tube of 8mm wall thickness made of zirconia toughened alumina (40% partially stabilised zircona, 60% alumina) was produced. Special electrodes were applied to the outside surface.

A layer of super concrete of the same composition as used in Example 1, was cast into the cavity between the outside surface of the ceramic tube and an outside steel pipe, thus protecting the electrodes and insulating them electrically and at the same time fixing the ceramic tube to the steel pipe.

Several significant advantages are provided by this structure. An inner surface of highly wear resistant material in a steel pipe is provided without any shrink fitting procedure that would also destroy the electrode system. The application of a super concrete body gives substantially higher temperature resistance and space filling capability than any known glue concepts, while still providing stability and stiffness., By applying the super concrete, the electrodes on the outside of the ceramic tubes are also electrically insulated from the steel pipe.

EXAMPLE 3

This example relates to a ball valve for use in abrasive and corrosive media, and especially for ball valves having a diameter of 40mm and above. The outer surface of such ball valves has to fulfil very strict requirements as to surface smoothness. Furthermore, such ball valves made completely from ceramic materials tend to be expensive and difficult to produce especially when considering the larger sizes. According to the present invention a ball valve was made of a shell of zirconia-aluminia composite by conventional methods. The shell was filled with a super concrete of the composition described in Example 1. The ball valve produced had excellent toughness, and very good wear resistance.

EXAMPLE 4

In this example, a monopump screw was produced consisting of an outer shell of a highly wear and corrosion resistant ceramic material 5mm in thickness made from reaction bonded silicon nitride and a core of super concrete of the composition described in Example 1 with a steel bolt joiner inserted in one end of the monoscrew. By slip casting the ceramic, any machining of the outer surface of the monoscrew is avoided, resulting in favourable production costs and better performance of the screw. The costs for producing a monopump screw made from ceramic material alone would be much higher due to raw material costs and higher production costs.