EP2086898A1

EP2086898A1 - Ceramic composition

Info

Publication number: EP2086898A1
Application number: EP07815393A
Authority: EP
Inventors: Gregory Willis; Andrew Ruijs
Original assignee: Millenium Research Pty Ltd
Current assignee: Military Ceramics Corp Pty Ltd
Priority date: 2006-10-19
Filing date: 2007-10-19
Publication date: 2009-08-12
Also published as: AU2007312952A1; IL198195A0; WO2008046152A1; AU2007312952B2

Abstract

The invention relates to a composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by continuous metal fibres configured to provide tensile constraint within the material. The invention also relates to processes of casting, fuse-casting and forming such a composite. The invention further relates to armour and building material formed from the composite material.

Description

CERAMIC COMPOSITION

The present invention relates generally to composite materials and more particularly to a ceramic composite which exhibits high toughness and impact resistance. The invention has particular application for use as an armour, as an impact-resistant/wear-resistant ceramic, and as an impact resistant building material, and the invention is herein described in that context.

Traditional ceramics are capable of significant structural load bearing in compression, but exhibit poor strength characteristics under impact loading or vibration. In the past, reinforced ceramics have been developed which are aimed at improving these structural characteristics. Fibre reinforcement has been found to be the most effective technique for toughening ceramics since it induces a number of toughening mechanisms including crack bridging, frictional debonding, fibre pullout, and crack deflection. These fibres are typically comprised of ceramic filaments or whiskers.

Due to their high strength and toughness, reinforced ceramics have been used extensively in the past in a broad range of applications. Nevertheless, there is a continuing need to improve the structural performance of the ceramics, particularly in certain applications such as armour, wear-resistant linings, and building claddings where strength and toughness are critical.

An aim of the present invention is to ameliorate these problems to provide a reinforced ceramic which has both advanced structural characteristics, particularly in relation to toughness and impact resistance and which may be produced at lower cost as compared to traditional reinforced ceramics.

Accordingly, in a first aspect, the present invention relates to a composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by continuous metal fibres configured to provide tensile constraint within the material. This tensile constraint is principally bi-axial, but may also have a lateral component.

The applicant has found that a composite material according to the present invention exhibits excellent characteristics in compression, tension, bending, toughness and impact resistance. The metal reinforcement used in this invention is in the form of continuous metal fibres (as distinct from discontinuous fibres such as chopped short fibres or whiskers). These continuous metal fibres may take the form of long rods or wires, woven wire mesh, or welded wire mesh. In each case, the wires preferably span almost the entire width of the ceramic body in at least two axes (x-axis, y-axis) to provide the biaxial constraint. It is preferable that the wires do not extend quite to the edges because of the advantages of having the wires sealed inside as a corrosion preventative. One advantage in the use of continuous fibres, in comparison with whiskers or chopped short fibres, is that the ratio of the peripheral oxidation zone to the unoxidised core is very much smaller for a thick wire than for a microscopic whisker or chopped fibre. A peripheral oxidation zone is an inevitable consequence of high temperature processing of a metal, and this oxidation zone is unable to perform the ductile structural role of the parent metal. Preferably the metal reinforcement includes nodes or ridges spaced along its outer surface to act as anchor-points to anchor the metal reinforcement into the ceramic body. The anchor-points provide mechanical interlocking in the ceramic body, thereby constraining the fibres from sliding or pulling through the ceramic body when loaded. In a particularly preferred form, the metal wire composition is chosen such that the thermal expansion co-efficient of the metal is higher than that of the ceramic body. As the ceramic composite is formed by heat treatment, on cooling of the material, the metal reinforcement thermally contracts more than the ceramic body, thereby placing the metal wires in tension and the ceramic body in compression.

In a preferred form, the metal reinforcement constitutes a relatively large part of the composite material. The actual amount of the reinforcement may vary depending on the application of the composite. However, preferably the metal content is in the range of 3 to 15 % volume of the composite material. In a particularly preferred form, the metal reinforcement is a common metal such as mild or stainless steel. An advantage of using common metals such as mild or stainless steel is that they can be incorporated in the high quantities as discussed above without making the material prohibitively expensive for applications such as armour, wear-resistant linings, and building claddings.

The use of a relatively large volume of continuous metal reinforcement which is anchored to the ceramic body and provides biaxial constraint, enables very high levels of tensile constraint to be achieved in the composite material. As the tensile constraint protects a ceramic material from tensile failure, the preferred forms of the present invention provide enhanced performance characteristics particularly in respect of its toughness and impact resistance.

The level of tensile constraint will vary depending on the mis-match of the coefficient of thermal expansion (α) in the ceramic body to the metal reinforcement. The use of a glass/ceramic-particle mix as the ceramic body is ideally suited to be reinforced by metal as its coefficient of thermal expansion can easily be established to suitably mis-match that of common metals such as mild or stainless steel. The optimum level of tensile constraint will vary depending on the application of the composite material. In an impact-resistant application, it is preferable that the tensile constraint is as high as possible to maximise the resistance of the ceramic to high energy impact and high explosive assault.

Preferably the thermal expansion coefficient of the ceramic body is in the range of 5 to 10 μm/mK. These ranges of values are ideally suited to be used with a mild steel which has a thermal expansion coefficient of 1 1.7 μm/mK.

Preferably, the metal reinforcement is in the form of a welded wire mesh. A mesh is ideal for providing the required biaxial tensile constraint and furthermore, the morphology of the mesh provides numerous anchor points (each welded junction acts as an anchor point). Woven mesh has a series of sharp bends in the wires as they pass over and under one another in the weave. This also represents an anchoring situation, albeit inferior to the welded mesh.

In a particularly preferred form, the metal reinforcement is arranged in a plurality of layers in the composite body. In this arrangement, each layer or combination of layers provides bi-axial tensile constraint, thereby enabling the tensile reinforcement to be distributed throughout the ceramic glass body. In a preferred form, the layers of reinforcement are interconnected by lateral (z-axis) reinforcement such as wire stirrups.

Placing a number of layers of continuous fibres on top of one another, and weaving them together with metal stirrups gives a 3-dimensional lattice of wires, thereby giving continuous fibre reinforcement in all three axes. This prevents the problem of delamination during fracture. Furthermore, there is an inherent manufacturing advantage in using pre-wired stacks of metal fibres as they can readily be incorporated in the ceramic body during processing.

In a preferred form, the ceramic body exhibits very low porosity. Preferably the material is less than 8% porous and more preferably less than 5%.

Porosity is a measure of the internal voids in a material. Typically there are two types of porosity; namely open porosity and closed porosity. Open porosity is where there are interconnected pores linked to the surface, whereas closed porosity is where there are internal unconnected pores.

Water absorbing can only occur through open porosity. Therefore, a material with no open porosity is not susceptible to corrosive moisture attack on its metal reinforcement. A material with closed porosity but no open porosity therefore has zero water absorption. Typically a ceramic with porosity below 5% generally only has closed pores.

The advantage of a low porosity in the ceramic body is that it can assist in preventing corrosion of the metal reinforcement with only minimum cover. A further significant advantage of having a low porosity structure is that it substantially improves performance of the composite material when used as an armour. The key means by which ceramics traditionally resist ballistic penetration include hardness, dilatancy, and compressive strength. Where the ceramic is harder than the projectile, the projectile cannot penetrate but rather is flattened or eroded on contact. Dilatancy in the impact zone is where the ceramic is pulverised into microscopic angular particles which cause localised expansion, thereby filling the hole the projectile is attempting to drill. Where the ballistic impacted ceramics can not be penetrated, the armour must fail by compressive fracture and the high inherent compressive strength of armour ceramics opposes failure in this mode. The use of a structure having very low porosity is important as the dilatancy mechanism will not work in a porous material, as the dilatant expansion will be lost through crushing of the pores. Furthermore, the compressive strength is highly porosity dependent and this is a prime reason for the poor ballistic performance of concrete which has a compressive strength of approximately 15 to 140 MPa whereas a non-porous silicon carbide has a compressive strength of 4,600 MPa.

In ballistic applications, any porosity whether open or closed degrades the dilatant ballistic response. Therefore, a ballistic ceramic should have negligible open porosity and negligible closed porosity. A material with a porosity of 5% does not exhibit any open pores, but it does have closed pores which can affect the dilatancy of the material. Accordingly, preferably when used as an armour, the composite material exhibits less than 5% porosity and more preferably less than 3% porosity. The same is pertinent for wear resistant ceramics, in which case pores act as sites of weakness.

In a further aspect, the present invention relates to a composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by metal, and wherein the composite material has a porosity of less than 8%.

In a particularly preferred form, the composite material according to this aspect of the invention has a porosity of less than 5%. Preferably when the material used as an armour the porosity is less than 3%. Preferably the metal reinforcement is in the form of mild steel. Further, preferably the metal reinforcement is arranged to provide bi-axial tensile constraint.

In a particularly preferred form, the ceramic material is produced by a hot-pressing process or by sintering or fuse-casting. The advantage of the methods of production used is that they can be used to produce a material with low porosity and are compatible with the use of a mild steel as the metal reinforcement.

The ceramic filler particles may be chosen from numerous mineral oxides, nitrides, borides, carbides or suicides or combinations thereof. The optimal choice is dependent on numerous factors such as the required structural characteristics and cost considerations, as well as compatibility with other aspects of the material.

In one application, the composite material is used as an armour or wear resistant material. When used in that application, the ceramic particles are ideally extremely hard, lightweight (low true density), and relatively low in cost. The three most suitable materials are silicon carbide, aluminium oxide, or boron carbide. However other materials which may be used include other hard relatively lightweight oxides, nitrides, or borides, such as AIN, ZrC, ZrB₂, TiC, TiB₂, Si₃N₄, and SiAION. Silicon carbide has high hardness (27 GPa), low density (3.1 g/cc) and low cost. However it exhibits poor chemical compatibility with molten glass. Aluminium oxide, has good chemical compatibility with molten glass and exhibits moderate hardness (16 GPa) and moderate density (4.0 g/cc) and is inexpensive. Boron carbide exhibits extreme hardness (30 GPa), very low density (2.5 g/cc) but is expensive (in the order of 10 times the cost of silicon carbide and aluminium oxide), and has poor chemical compatibility with molten glass. For uses as a building material, the filler particles may be chosen from numerous mineral particles, or refined ceramic particles such as oxides, nitrides, borides, carbides or suicides or combinations thereof. The optimal choice is dependent on numerous factors such as the required structural characteristics and cost considerations, as well as compatibility with other aspects of the material. More preferably these will be silicate or aluminosilicate mineral particles as these are cheap and stable.

Preferably the ceramic particle content of the ceramic body part of the composite is maximised, so as to maximise hardness, impact resistance, and wear resistance. Maximising the filler particle composition is preferably achieved by a process known as "gap grading". Gap grading involves blending two or more monosized grades of particles, a super-coarse, a coarse, a medium and a fine grade, in such ratios that the medium grade optimally fills the interstices of the coarse grade, and the fine grade optimally fills the interstices of the medium grade.

Preferable the super-coarse grade has a mean grade size in the range 1 to 6mm. Preferably the coarse grade has a narrow size distribution with a mean grade size which is chosen anywhere from 200 to 1000 microns in size, ideally 300-600 and preferably 400 microns. The amount of coarse grade in the blend is typically 50 to 70 % volume of the total volume of filler particles, and more preferably 60 % volume.

Preferably the medium grade has a narrow size distribution with a mean size of between 20 to 200 microns, ideally 50 microns. The amount of medium grade in the blend is typically 10 to 25 % volume of the total volume of the filler particles and ideally 20 % volume.

Preferably the fine grade has a narrow size distribution with a mean size of 1 to 20 microns, ideally 5 microns. The amount of fine grade in the blend is typically 10 to 25 % volume of the total volume of filler particles and more preferably 20 % volume.

The choice of material for the ceramic filler particles may be chosen depending on the requirements of the material. Boron carbide is suitable for a lightweight embodiment where cost is no object. The density of the boron carbide embodiment, including mesh in glass is approximately 3.0 g/cc. The cost is high owing to the extreme cost of boron carbide. This lightweight version is ideal for vehicular armour where weight is critical and cost is not. Silicon carbide is an ideal compromise between weight and cost. The density of the silicon carbide embodiment, including the mesh and glass, is approximately 3.5 g/cc. The cost is low owing to the low cost of silicon carbide powder. Owing to its moderate cost and moderate weight, this embodiment is ideal for armouring large ships and small sea-going vessels. It is also ideal for aircraft carrier runways and suitable for vehicular armour where cost is an object.

Aluminium oxide may be used where hardness and weight is not a critical issue. The density of the aluminium oxide embodiment including the mesh and glass is approximately 4.0 g/cc. The good chemical compatibility of aluminium oxide with glass means that this embodiment is easier to process as compared to boron carbide or silicon carbide. It is therefore ideal for very thick panels for use in bunkers where weight is not an issue and processing is an issue.

Generally, the composite material of the invention has 10 to 80% volume of glass matrix in the ceramic body.

When manufactured under a hot-pressing process, in a preferred form the ceramic body is within the range of 60 to 80 % volume ceramic filler particles (20 to 40 % volume glass) and more preferably 65 to 75% filler particles and ideally 70% filler particles to maximise the filler composition under optimal gap grading conditions. Similarly, when manufactured by a sintering process, the glass/ceramic-particle-filler composite is within the range of 60 to 80 % volume filler particles (20 to 40 % volume glass) and more preferably 65 to 75% filler particles and ideally 70% filler particles to maximise the filler composition under optimal gap grading conditions.

In another preferred form, there is 10 to 30%, more preferably 14 to 17%, volume glass matrix in the ceramic body. An optimal formula for a wear resistant material and armour contains about 15% volume glass matrix and about 85% volume ceramic particles.

When manufactured using fuse-casting processing, the glass content of the body is very high, typically 40 to 75 % volume of the total volume of the ceramic body, preferably 45 to 65 % volume, and ideally 50 % volume. An optimal formula for building material applications contains about 40% volume glass matrix and about 60% ceramic particles.

When the material is produced by a sintering process, gap-grading of the ceramic filler particles, and even of the glass powder particles, is critical in order to produce a green body (the unfired ceramic) with the maximum packing density. It is possible, with optimised gap-grading characteristics, to produce a green body with less than 15% porosity prior to sintering, and porosity approaching the 5 to 10% range after sintering without the use of melting or pressure. This requires very narrow particle size distributions and large size ratios coarse:medium and medium:fine should be over 10:1. When the material is formed from a fuse-casting process, the glass content is preferably relatively high, typically 60 to 90 % volume of the total volume of the ceramic body, so as to maximise fluidity for pouring into moulds. At least 10 % volume filler particles are needed, otherwise the crack deflection mechanism does not operate and the body is virtually as brittle as pure glass. The filler particles must also be relatively coarse for the same reason, preferably coarser than 200 mesh.

In accordance with earlier aspects of the invention, the composite material is reinforced by a metal. Preferably the metal is low cost, such as aluminium, stainless steel, mild steel, zinc coated or galvanised mild steel or copper. However, higher cost metals such as superalloy, nickel, chromium, tungsten, titanium, molybdenum, tantalum and niobium may be used if desired. However, it is to be appreciated that many of these higher cost metals, whilst providing adequate performance, make the cost prohibitive for widespread use. In one form, when a mild steel mesh is used, preferably it is coated typically by a zinc coating or galvanised, to prevent oxidation during processing.

The amount of metal reinforcement in the composite material will depend on the required structural characteristics. Preferably, however, the resultant metal content is between 3 and 15 % volume of the total volume of the composite material, the higher range being used in high strength applications such as armours, where preferably the metal is arranged in densely packed layers.

In the embodiment where a wire mesh is used, preferably the wire thickness is coarser than 0.1 mm and ideally between 1 and 6 mm. The thinner the wire, the more flexible the mesh. This can be problematic, however, during processing of the material. Also, the thinner the wire, the broader the oxidation zone in relation to the total cross-section after processing.

Preferably the grid size of the mesh is relatively small. For armour applications, a fine grid enables maximisation of fibre volume% in the material.

In a preferred form, the grid size is within the range of 5 to 25 mm.

The optimal glass composition used as the glass binding component in the composite may be formed from a combination of glass forming oxides such as silicate, borate, germanate, phosphate, lead-bisilicate; modifier oxides, and particularly fluxes. Optimisation of the glass component is made on the basis of three criteria: namely, minimal forming temperature; maximum protection of the metal reinforcement; and compatibility with the ceramic filler particles and the metal reinforcement.

The incorporation of a glass matrix within the composite material has various significant advantages. The use of a glass bonded matrix allows the composition to be formed at relatively low temperatures, depending on the composition of the glass. Furthermore, the glass can provide significant benefits to the performance of the composite. In that regard, glass as a component of a composite armour is extremely effective in stopping HEAT (high explosive, anti-tank rounds). By combining a glass matrix with hard ceramic particles, a material is produced which is effective against both armour piercing and HEAT rounds.

In making a composite using a hot-pressing process, forming temperatures are typically within the range of 500⁰C to 800⁰C, depending on the composition of the glass which is preferably added as a blend of pre-fritted and powdered inorganic glass particles. The following example illustrates the effect the composition of the glass has on the forming temperatures of the composition. Glass Type: Soda lime silica glass or borosilicate glass finely powdered frit (cullet). A material having a negligible porosity (that is, less than 10% pores) was hot-pressed at 1000⁰C and 20 MPa. This is a temperature-minimising example.

In making a composite using a sintering process, forming temperatures are only marginally higher than those for hot-pressing the same ceramic body, typically in the order of 1000⁰C, depending on the composition of the glass, which is preferably added as a blend of pre-fritted and powdered inorganic glass particles.

Glass Type: Soda lime silica glass or borosilicate glass finely powdered frit (cullet). A negligible porosity material was sintered at 1000⁰C.

For the fuse-casting processing technique, the temperature of the molten body is typically 800⁰C to 1400⁰C. In this processing, the glass forms a protective layer over the metal reinforcement immediately it is brought into contact with the metal, thereby affording oxidisation protection at these potentially damaging temperatures. Again, the composition of the glass is a fact of partially determining the temperature at which the material is formable, as exemplified by the following:

Glass Type: Soda lime silica glass or borosilicate glass finely powdered frit (cullet). A negligible porosity material was castable at 1400⁰C. In the production of the composite material according to the earlier aspects of the invention, various considerations need to be given in respect of hot-pressing processing techniques as well as in fuse-casting.

In forming the green body (which is the unfired ceramic) this may be done typically by slurry casting or from a dried granulate. Slurry casting involves the following steps:

1. batching the gap graded ceramic powders and glass powder (frit) and dry mixing, for example in a ball mill;

2. adding sufficient liquid to produce a thick slurry thin enough to be pourable, and wet mixing, for example in a ball mill. The liquid can be water containing chemical binders (eg. 10 g/L sodium carboxymethylcellulose) or ethanol containing chemical binders (eg. 90 g/L polyvinylbutyral). The liquid content is typically 20 to 50 % volume, ideally 35 to 40 % volume.

3. placing the metal reinforcement in a steel mould; and 4. pouring the slurry into the mould and oven drying the slurry at

100° to 12O⁰C.

The slurry casting method is preferred for hot-pressed armour embodiments.

When forming the body by a dry granulate method, a similar approach to the slurry method is followed, except that the slurry is formed into a granulate (1 to 5 mm sized fragments of oven dried ceramic body). The metal reinforcement is then placed in a steel mould and the granulate is poured into that mould.

The next stage of the process is the hot-pressing of the unfired ceramic. An important issue with hot-pressing is the type of mould that must be used. For temperatures below 65O⁰C, mild steel moulds can be used. From 65O⁰C to approximately 75O⁰C, mild steel moulds can be used only if the pressures are low, and even then their working life is short due to oxidisation. "Low pressures" means typically 10 MPa or less, ideally 5 MPa or less. Mild steel moulds are very cheap in terms of materials and machining costs. Above 65O⁰C at pressures greater than 10 MPa, and above 75O⁰C at any useful pressure, stainless steel moulds must be used owning their oxidisation resistance and temperature/strength characteristics (eg. Sandvik 253 MA Alloy or AISI 310 Alloy). With the use of stainless steel moulds, the mould cost becomes expensive and becomes a major component in the production costs. Preferably the moulds are coated with a non-stick agent. This can be done by a number of options, including clay, graphite, boron nitride or molybdenum compounds. Graphite is a preferred option due to its combination of low cost and effectiveness.

In relation to a fuse-casting process, this involves the use of either water-cooled metal moulds or sand moulds. These are filled with layers of mesh prior to pouring. Fuse-casting involves the following steps: i. batching the gap graded ceramic powders and glass powder (frit) and drying mixing, for example in a ball mill; ii. placing the dry mix in a crucible and heating to the melt temperature; iii. pouring the melt into the mesh-filled mould; and iv. placing a hot top on the mould to facilitate slow cooling. In preparation of the composite material, metal fasteners may be embedded in the material. These are typically metal bolts which are typically zinc coated steel. This may simply be achieved by placing the fasteners into the mesh filled mould prior to pouring (the slurry, granulate or melt as the case may be). For hot-pressed embodiments, the pressing plate must have matching holes so that the studs remain protruding after hot-pressing.

For sintered embodiments, the composite material is dried and removed from the mould prior to sintering to the required processing temperature. Therefore, moulds can be fabricated from any low-cost material, such as plaster, timber, plastic, etc. In accordance with the various aspects of the present invention, a composite structure may be formed which may exhibit either improved strength and/or be produced using low cost techniques. Armour embodiments of preferred structures of the composite are as follows: i. Armour Embodiments. Highly concentrated gap-graded hard filler particles in a glass matrix. The entire body permeated with densely-packed layers of steel mesh. The surface of the mesh has a thin oxidation layer, in the order of tens of microns. The hot-pressed embodiment has a minimal glass content sufficient to fill pores and interstices. The fuse-cast embodiment has a higher glass volume to filler particle volume ratio. It will be convenient to hereinafter describe embodiments of the present invention with reference to the accompanying drawings. It is to be understood that the particularity of these drawings and the related description does not supersede the preceding broad description of the invention.

In the drawings: Figure 1 is a schematic perspective view which is cut away of a ceramic composite;

Figure 2 is a perspective view of the metal reinforcement of the composite of Figure 1 ; and

Figure 3 is a sectional side view of a ceramic armour. Turning firstly to Figures 1 and 2, a ceramic composite 10 is disclosed which includes a ceramic body 1 1 reinforced by densely packed layers of metal reinforcement 12. The ceramic body is formed from ceramic particles which are bonded in a glass matrix. The composite material is formed at elevated temperatures using either a hot press, fusion cast, or sintering production technique.

As best illustrated in Figure 2, the metal reinforcement 12 comprises a plurality of layers of metal mesh 13. The mesh is formed from mild steel wires 14 which are welded together. In this way, each of the weld points 15 of the metal mesh provide nodes which act as anchor-points to assist in bedding in of the reinforcement 12 into the ceramic body 11.

Each of the layers of metal mesh provide biaxial tensile constraint which, using the reference coordinates of Figure 1 , act in the x-y plane. In the illustration of Figure 1 , the reinforcement 12 is shown for ease of identification. However, it is to be appreciated that in use, the reinforcement 12 is typically fully embedded within the ceramic body 1 1 so that the ceramic body provides adequate cover to prevent corrosion of the reinforcement 12.

As illustrated, the metal mesh 13 is arranged in layers which, in the embodiments of Figures 1 and 2, are interconnected by metal stirrups 16 which act as shear reinforcement. The metal stirrups which are typically looped wire, also aid in fixing of the reinforcement in production of the composite body 10. With the metal stirrups 16, the reinforcement 12 provides shear reinforcement and tensile constraint in the Z axis using the coordinates of Figure 1.

Figure 3 illustrates the ceramic composite 10 when used as an armour 20. In this embodiment, the metal reinforcement 12 is again produced by layers of metal mesh which are densely packed together. In this way, the content of metal reinforcement is relatively high, in the order of 12 to 15% of the total volume of the armour 20. Further, the armour panel 20 illustrated in Figure 3 is relatively thick as compared to its length and breadth. The following examples are given to illustrate embodiments of the present invention without limiting the invention's broad scope: Example 1

Embodiment for vehicular or vessel armour or wear-resistant tiles Filler particles: Silicon carbide. 60 parts of Coarse (400 micron), 20 parts of medium (50 micron), and 20 parts of fine (5 micron).

Glass: Soda lime silica glass or borosilicate glass finely powdered frit (cullet). 32 volume% in the ceramic body.

Mesh: 1.4 mm thick, 13 mm grid, zinc-coated steel welded mesh, 1 layer per 1.9 mm height. Forming Method: Slurry casting with 90g/L polyvinylbutyral in ethanol, 40% volume liquid. Firing Method: Hot pressing in a stainless steel mould at 1000⁰C and 20 MPa.

Example 2 Embodiment for Bunker Armour

(i) Filler particles: Aluminium oxide. 60 parts of Coarse (400 micron), 20 parts of medium (50 micron), and 20 parts of fine (5 micron). Glass: Soda lime silica glass or borosilicate glass finely powdered frit (cullet). 50 volume% in the ceramic body. Mesh: 5 mm thick, 25 mm grid, zinc-coated steel welded mesh, 1 layer per 8 mm height.

Forming Method: Fuse casting into mesh-filled sand moulds at >1400°C. (ii) Filler particles: Aluminium oxide. 60 parts of Coarse (400 micron), 20 parts of medium (50 micron), and 20 parts of fine (5 micron). Glass: Soda lime silica glass or borosilicate glass finely powdered frit

(cullet). 35 volume% in the ceramic body.

Mesh: 5mm thick, 25 mm grid, zinc-coated steel welded mesh, 1 layer per 8 mm height.

Forming Method: Sintering at 1000⁰C.

Example 3

Embodiment for Building Cladding

(i) Filler particles: Crushed flint. 60 parts of Coarse (400 micron), 20 parts of medium (50 micron), and 20 parts of fine (5 micron). Glass: Soda lime silica glass or borosilicate glass finely powdered frit

(cullet). 30 volume% in the ceramic body.

Mesh: 5 mm thick, 25 mm grid, zinc-coated steel welded mesh, 1 layer per 8 mm height.

Forming Method: Sintering at 1000⁰C.

Table 1. Tensile Constraint

Note 1. Tensile constraint is given in metric tonnes for a 1 square metre equi-axed (1 metre by 1 metre) panel of ceramic composite material.

Note 2. This tensile constraint acts in both the x-axis and y-axis directions in the horizontal plane.

Note 3. Tensile constraint in the x-axis is proportional to the dimension of the ceramic composite material in the y-axis, and vice versa. This is because tensile constraint in a given axis arises from the number of wires parallel to that axis

Note 4. Tensile constraint in the longitudinal (z-axis) direction is dependent on the number of stirrups used to wire together the mesh sheets.

Note 5. All of the above examples utilise a mild steel mesh with a wire thickness of 1.4mm and a mesh spacing (centre-centre) of 13 mm.

Note 6. "Volume% mesh" refers to the total volume of the ceramic composite comprised of mesh (the total volume% of ceramic is therefore 100 volume% mesh).

Note 7. It is clear from the above table that the tensile constraint increases with thickness. This is why the thick armour embodiments have such high constraint values.

Note 8. The tensile constraint in armour embodiments should be as high as possible, to maximise the resistance of the ceramic to high-energy impact and high-explosive assault. These extreme values of hundreds to thousands of tonnes per square metre result in a ballistic material that has a very high absorbency.

Table 2. Thermal expansion coefficient (α) data.

Note 1. α is given in its standard unit of μm/m.K (micrometres per metre per kelvin).

Note 2. Filler refers to the ceramic filler particles. Note 3. The thermal expansion mismatch is the number in column 3

(the ceramic body, i.e., the glass/ceramic-filler-particle mix) minus the number in column 4 (the wire).

Accordingly, the present invention provides a reinforced ceramic which exhibits improved strength characteristics in respect of toughness and impact resistance through the use of continuous fibre metal reinforcement. By selecting a higher thermal expansion coefficient for the wire as compared to the ceramic body, it is possible to introduce significant tensile constraint within the material to enhance the performance of the ceramic composite, particularly as an armour. Further, by use of common metals such as mild steel, high volumes of continuous metal fibre reinforcement can be included without making the material prohibitively expensive for large scale armour applications.

It is to be appreciated that various alterations or additions may be made to the examples previously described without departing from the spirit or ambit of the present invention. Future patent applications may be filed in Australia or overseas on the basis of or claiming priority from the present application. It is to be understood that the following provisional claims are provided by way of example only, and are not intended to limit the scope of what may be claimed in any such future application. Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions.

Claims

CLAIMS:

1. A composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by continuous metal fibres configured to provide tensile constraint within the material.

2. A composite material according to claim 1 where in the tensile constraint is bi-axial.

3. A composite material according to claim 1 or claim 2 wherein the continuous metal fibres take the form of long rods or wires, woven wire mesh, or welded wire mesh.

4. A composite material according to claim 3 wherein the metal fibres are welded wire mesh.

5. A composite material according to claim 3 or claim 4 wherein the wire thickness in the wire mesh is coarser than 0.1 mm.

6. A composite material according to claim 5 wherein the wire thickness is between 1 and 6 mm.

7. A composite material according to any one of claims 3 to 6 wherein the grid size of the wire mesh is in the range of 5 to 25 mm.

8. A composite material according to claim 2 wherein the metal fibres span almost an entire width of the ceramic body in at least two axes to provide biaxial constraint.

9. A composite material according to claim 8 wherein the axes are x and y axes.

10. A composite material according to any one of claim 3 to 9 wherein the wires or wire mesh do not extend to edges of the ceramic body.

1 1. A composite material according to any one of claims 1 to 10 wherein reinforcement further includes nodes or ridges spaced along an outer surface to act as anchor-points to anchor the metal fibres into the ceramic body.

12. A composite material according to any one of claims 1 to 11 wherein the thermal expansion co-efficient of the metal is higher than that of the ceramic body.

13. A composite material according to claim 12 wherein the thermal expansion coefficient of the ceramic body is in the range of 5 to 10 μm/mK.

14. A composite according to claim 13 wherein the metal is mild steel having a thermal expansion coefficient of 1 1.7 μm/mK.

15. A composite material according to any one of claim 1 to 14 wherein the metal fibres are 3 to 15% volume of the composite material.

16. A composite material according to any one of claims 1 to 13 wherein the metal is selected from aluminium, stainless steel, mild steel, zinc coated, galvanised mild steel, copper, superalloy, nickel, chromium, tungsten, titanium, molybdenum, tantalum or niobium.

17. A composite material according to claim 16 wherein the metal is selected from mild steel, galvanised mild steel or stainless steel.

18. A composite material according to claim 17 wherein the metal is galvanised mild steel.

19. A composite material according to any one of claims 1 to 18 wherein the metal fibres are arranged in a plurality of layers in the ceramic body.

20. A composite material according to claim 19 wherein the layers are interconnected by z-axis, lateral reinforcement.

21. A composite material according to claim 20 wherein the lateral reinforcement is wire stirrups.

22. A composite material according to any one of claims 1 to 21 wherein the material of the ceramic body is less than 8% porous.

23. A composite material according to claim 22 wherein the material of the ceramic body is less than 5% porous.

24. A composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by metal, and wherein the composite material has a porosity of less than 8%.

25. A composite material according to claim 24 wherein the porosity of the ceramic material is less than 3%.

26. A composite material according to claim 24 or claim 25 wherein the metal is mild steel.

27. A composite material according to any one of claims 24 to 26 wherein the metal is arranged to provide bi-axial tensile constraint.

28. A composite material according to any one of claim 1 to 27 wherein the ceramic particles are selected from oxides, nitrides, borides, carbides or suicides or combinations thereof.

29. A composite material according to claim 28 wherein the ceramic particles of the composite are selected from silicon carbide, aluminium oxide, aluminium nitride, boron carbide, ZrC, ZrB₂, TiC, TiB₂, Si₃N₄, or SiAION, or combinations thereof.

30. A composite material according to claim 29 wherein the density of the composite material when the ceramic particles are boron carbide is approximately 3.0g/cc.

31. A composite material according to claim 29 wherein the density of the composite material when the ceramic particles are silicon carbide is approximately 3.5g/cc.

32. A composite material according to claim 29 wherein the density of the composite material when the particles are aluminium oxide is approximately 4.0g/cc.

33. A composite material according to claim 28 wherein the ceramic particles are silicate or aluminosilicate mineral particles.

34. A composite material according to any one of claims 1 to 33 wherein the ceramic particles include a blend of two or more of a super- coarse, coarse grade, medium grade and fine grade of particles.

35. A composite material according to claim 34 wherein the super-coarse grade has a mean size in the range 1 mm to 6mm.

36. A composite material according to claim 35 wherein the coarse grade has a narrow size distribution with a mean size between 200 to 1000 microns.

37. A composite material according to claim 36 wherein the coarse grade has a mean size of 300-600 microns.

38. A composite material according to claim 37 wherein the coarse grade has a mean size of 400 microns.

39. A composite material according to any one of claims 34 to 38 wherein the amount of coarse grade in the blend is typically 50 to 70 % volume of the total volume of ceramic particles.

40. A composite material according to claim 39 wherein there is 60 % volume coarse grade of the total volume of ceramic particles.

41. A composite material according to any one of claims 34 to 40 wherein the medium grade has a narrow size distribution with a mean size of between 20 to 200 microns.

42. A composite material according to claim 41 wherein the medium grade has a mean size of 50 microns.

43. A composite material according to any one of claims 34 to 42 wherein the amount of medium grade in the blend is typically 10 to 25 % volume of the total volume of the ceramic particles.

44. A composite material according to claim 43 wherein there is 20 % volume of medium grade of the total volume of ceramic particles.

45. A composite material according to any one of claims 34 to 44 wherein the fine grade has a narrow size distribution with a mean size of 1 to 20 microns.

46. A composite material according to claim 45 wherein the fine grade has a mean size of 5 microns.

47. A composite material according to any one of claims 34 to 46 wherein the amount of fine grade in the blend is typically 10 to 25 % volume of the total volume of ceramic particles.

48. A composite material according to claim 47 wherein there is 20 % volume amount of fine grade in the blend.

49. A composite according to claim 1 wherein there is 10 to 80% volume glass matrix in the ceramic body.

50. A composite according to claim 49 wherein there is 10 to 30% volume glass matrix in the ceramic body.

51. A composite according to claim 50 wherein there is 14 to 17% volume glass matrix in the ceramic body.

52. A composite according to claim 51 wherein there is about 15% volume glass matrix and about 85% volume ceramic particles in the ceramic body.

53. A composite according to claim 1 wherein there is 60 to 80% volume ceramic particles and 20 to 40% volume glass matrix in the ceramic body.

54. A composite according to claim 53 wherein there is 65 to 75% volume ceramic particles.

55. A composite according to claim 54 wherein there is 70% ceramic particles.

56. A composite according to any one of claims 50 to 55 manufactured by a hot-pressing process.

57. A composite according to any one of claims 50 to 55 manufactured by a sintering process.

58. A composite according to claim 1 wherein there is 40 to 90 % volume glass matrix in the ceramic body.

59. A composite according to claim 58 wherein there is 40 to 75 % volume glass matrix in the ceramic body.

60. A composite according to claim 59 wherein there is 45 to 65 % volume glass matrix in the ceramic body.

61. A composite according to claim 60 wherein there is 50 % volume glass matrix in the ceramic body.

62. A composite according to any one of claims 58 to 61 wherein there is at least 10% volume of ceramic particles.

63. A composite according to claim 58 or claim 59 wherein there is about 40% volume glass matrix and about 60% volume ceramic particles.

64. A composite according to any one of claims 58 to 63 wherein the ceramic particles are coarser than 200 mesh.

65. A composite according to any one of claim 58 to 64 manufactured by a fuse-casting process.

66. A composite according to any one of claims 1 to 65 wherein the glass matrix is formed from a combination of glass forming oxides selected from silicate, borate, germanate, phosphate and lead-bisilicate.

67. A process of slurry casting a composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by continuous metal fibres, which process includes:

1. dry mixing gap-graded ceramic powders and glass powder;

2. adding sufficient liquid to produce a pourable, thick slurry, and wet mixing;

3. placing metal reinforcement in a steel mould; and 4. pouring the slurry into the mould and oven drying the slurry at

100° to 12O⁰C.

68. A process of forming a composite material by a dry granulate method, the composite material including a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by continuous metal fibres, which process includes:

1. dry mixing gap-graded ceramic powders and glass powder;

2. adding sufficient liquid to form a slurry, and wet mixing;

3. drying and calcining the slurry then crushing and sieving to produce a granulate; 4. placing metal reinforcement in a steel mould; and

5. pouring the granulate into the mould and then preparing the composite material by sintering or hot pressing.

69. A process of fuse-casting a composite material which includes a ceramic body incorporating ceramic particles bonded in a glass matrix and which is reinforced by continuous metal fibres, which process includes:

1. dry mixing gap-graded ceramic powders and glass powder to form a dry mix;

2. placing the dry mix in a crucible and heating to form a melt; 3. pouring the melt into a mould filled with metal fibres; and

4. placing a hot top on the mould to facilitate slow cooling.

70. Armour formed from a composite material of any one of claims 1 to 66.

71. Armour formed form a composite material of claim 1 in which the ceramic particles are boron carbide or silicon carbide.

72. Armour according to claim 71 in which the composite particle is silicon carbide.

73. Armour according to any one of claims 70 to 72 used on a large ship or small sea-going vessel.

74. An aircraft carrier runway formed from a composite material according to claim 1 in which the ceramic particles are silicon carbide.

75. A bunker panel formed from a composite material according to claim 1 in which the ceramic particles are aluminium oxide.

76. A building material formed from the composite material of any one of claims 1 to 66.

77. A building material formed from the composite material according to claim 1 in which the ceramic particles are selected from oxides, nitrides, borides, carbides or suicides or combinations thereof.

78. A building material according to claim 77 wherein the ceramic particles are silicate or aluminosilicate particles.

79. A building material according to any one of claims 76 to 78 used as building cladding.

80. An impact-resistant material formed from the composite material of any one of claims 1 to 66.

81. A wear resistant ceramic formed from a composite material of any one of claims 1 to 66.

82. A composite material substantially as herein described with reference to the Figures.

83. A composite material substantially as herein described, with reference to any one of Examples 1 , 2 or 3.

84. A vehicular or vessel armour, or a wear-resistant tile formed from a composite material substantially as herein described with reference to Example 1.

85. Armour for a bunker formed from a composite material substantially as herein described with reference to Example 2.

86. Building cladding formed from a composite material substantially as herein described with reference to Example 3.