GB2457688A

GB2457688A - A process for the manufacture of sialon-bonded silicon carbide composite materials

Info

Publication number: GB2457688A
Application number: GB0803159A
Authority: GB
Inventors: Anthony Norris Pick; Robert John Blacker Kyle
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-02-21
Filing date: 2008-02-21
Publication date: 2009-08-26
Also published as: GB0803159D0

Abstract

A process is disclosed for the manufacture of silicon carbide products with a complex nitrogen containing bonding system, including the forming and firing techniques for the shapes. The process comprises blending a mixture of silicon carbide, silicon, alumina and iron oxide and firing the mixture in a nitrogen rich atmosphere to form a bonding phase of b -sialon, a -silicon nitride and b -silicon nitride. The proportion of silicon nitride may be reduced by the addition of yttrium oxide (yttria) to the mixture prior to firing. The raw materials used are of high purity, which are then shaped either by a pressure casting technique or a slip casting technique. The raw materials are mixed to form a slip, which is then either pumped into porous pressure casting moulds or poured into a plaster mould to remove the water forming the desired shape. The resultant shapes are dried and then fired in a nitrogen atmosphere, either using a carefully controlled firing cycle and gas flow system or, where necessary, by using an inert gas to control the rate of reaction, to develop the bonding system by means of one or more separate firing cycles and subsequently may be fired for a second time in air to increase the oxidation resistance. The material formed may be used in wear resistant applications and as body armour.

Description

A PROCESS FOR MANUFACTURING A RANGE OF SILICON

CARBIDE PRODUCTS FOR KILN FURNITURE,

ENGINEERING AND PERSONAL BODY ARMOUR

APPLICATIONS

This invention relates to a process for the manufacture of a range silicon carbide products for use as kiln furniture, especially but not exclusively, in rapid firing applications, for engineering applications, for wear resistant applications and for personal body armour.

There is an increasing requirement for energy saving, not only to reduce production costs but also as a means of controlling the effect of global warming. Manufacturers of ceramic products, ranging from porcelain through technical ceramics to building materials, are now using methods to improve efficiency by automatically loading and unloading kiln cars and using faster firing cycles. The kiln furniture used for these applications must be low mass and thermally stable; have a high strength; have excellent thermal shock and oxidation resistance; and the ability to withstand high temperatures.

In addition, there is an increasing use of high strength, wear resistant ceramic products in a number of applications; including aircraft engines, brake discs for high performance cars, cyclones and hydro cyclones as well as for personal body armour. All of these applications require ceramic products, which have a high strength, excellent impact resistance and, in certain cases, excellent abrasion resistance and the ability to withstand sudden temperatures changes.

This invention not only relates to the composition and firing process, during which silicon metal powder and alumina are converted to a complex high strength nitride bonding system but also includes the method of manufacturing of the shapes required for the various applications.

The formation of a silicon nitride bonded silicon carbide from commercial refractory grade raw materials, which are formed into shape by hydraulic, hammer or vibration pressing; or extrusion is well known. These shapes are fired in a nitrogen atmosphere to convert the silicon metal powder to a-silicon nitride and n-silicon nitride, which forms the bonding system, in situ. The silicon metal powder used for this type of product, frequently only contains 96-98% silicon and the iron present, as an impurity, is known to act as a catalyst during the metal-nitrogen (gas/solid) reaction. It is well known that all silicon carbide refractory shapes are susceptible to oxidation, which eventually leads to their failure. This includes silicon nitride bonded silicon carbide refractones.

For certain shapes made by slip casting or vibration casting, clay is added to the composition to aid the casting process, which contains silica, alumina and a number of other oxides. During the firing process in a nitrogen atmosphere, these combine with the silicon metal to form a bond containing predominantly silicon oxynitride as well as some a-silicon nitride, -siIicon nitride and glassy phases. Silicon oxynitride and the glassy phases significantly improve the cold abrasion resistance but are detrimental to the thermal properties, particularly resistance to thermal shock and oxidation.

According to the present invention there is provided a process for the manufacture of high purity silicon carbide products with a complex nitride bonding system, consisting predominantly of -siaIon. High purity silicon metal powder (Si content = 99% mm), silicon carbide graded fractions (SiC content = 98% mm), a -alumina fine powder (A1203 content = 99% mm) and iron oxide powder (Fe203 content = 94% mm) or some other suitable catalyst are mixed into a slip, formed into shape by pressure casting or slip casting, dried and fired under a controlled nitrogen atmosphere. In the examples given, iron oxide is added as a catalyst to aid the nitriding process because neither iron nor any of its compounds are

I

present in sufficient quantity in any of the high purity raw materials. This forms a complex bonding system, which consists of 3-sialon with some a-silicon nitride and 3-silicon nitride, in varying ratios depending on raw materials used. The products formed are suitable for kiln furniture or other thermal or wear resistant applications. These compositions can be further modified by the addition of yttrium oxide or other rare earth oxide, alkaline earth oxide or mixed oxides of these groups. In the examples given, yttrium oxide is the preferred additive, which promotes the formation of 13-sialon and as a sintenng aid, particularly if fired to high temperatures in a nitrogen atmosphere. This greatly increases the strength making it potentially suitable for personal body armour.

The use of fine high purity raw materials and the formation of a fine high purity bonding system on firing under a controlled atmosphere produce a product having a very high density, very high strength, excellent thermal shock resistance and excellent abrasion resistance, ideal for kiln furniture in conventional and fast fire conditions, automatic handling for loading and unloading the kiln cars, and for other engineering applications.

The powdered raw materials should have a maximum particle size of 180 pm and are selected to give optimum packing density when combined together. This is important because the packing density helps to determine the final fired density of the product which must be as high as possible as this contributes towards the strength as measured by the modulus of rupture (MOR) by 3-point bending.

Having made a suitable slip, the shape required may be formed either by pressure casting or by slip casting, both techniques being well known to those skilled in the ceramic art.

The pressure casting forming technique used in this invention is modified to suit the characteristics of the slip, which is thixotropic and will solidify if subject to sudden increases in pressure. Pressure casting is widely used for the production of clay based products, such as tableware and sanitaryware. This technique has been modified to allow non-clay containing silicon carbide mixed slip, given above, to be formed into the required flat shapes and profiles by pressure casting.

Alternatively, large or complex shapes may be formed by slip casting into suitable gypsum plaster moulds, a technique also well known to those skilled in the ceramic art. It is very important that the moulds are made from a mixture of hard dense and porous plasters (eg Ultramix and Keramicast supplied by BPB Formula), which are mixed with water ideally in a vacuum mixer and cast into the required mould. For the best results, it is important that the plaster mould is homogeneous and has a consistent pore size distribution. Slip casting relies on the water being drawn from the slip by the plaster mould to form a solid shape and, by using a mixture of plasters, the rate of water removal can be controlled to prevent segregation in the walls of the cast piece. Generally, the plaster ratio has to be determined empirically and may vary with the complexity of the shape being formed.

The basic composition may comprise of an intimate mixture of silicon carbide powders, generally in the range 55-80%, preferably 60-75% and more substantially 65-75%, silicon metal powder, generally in the range 10-35%, preferably 12-30% and more substantially 15- 25%, a-alumina, generally in the range 6-20%, preferably 7-16% and more substantially 8- 14%. Iron oxide is added as a catalyst to a maximum level of 1%. Yttrium oxide or a mixed oxide containing yttrium may also be added to promote -sialon formation and increase the density.

The basic composition is mixed with water and a deflocculant, such as a hydroxide of an alkali metal, alkaline earth metal, and ammonium or alkali polymethacrylate to form the slip.

The deflocculant addition is generally in the range 0.1-0.8%.

For pressure casting and slip casting, the water addition is generally in the range 10-30%, preferably 10-25% and more substantially 12-18%.

Table 1. Examples of the compositions: Compositions 1 2 3 4 5 Silicon Carbide, 0-l5Opm 40.0 35.3 39.4 34.8 ________ Silicon Carbide, d = 2.0-1 0.Opm 32.0 35.5 31.5 34.4 63.7 Silicon Metal Powder, 0-53.im 20.0 16.0 19.7 15.8 24.5 a-Alumina,d=2.0-5.0pm 8.0 13.7 7.9 13.5 9.8 Yttrium Oxide d = 2.0-1 0.Opm ________ ________ 1.5 1.5 2.0 Iron Oxide, 0-45pm (addition) 0.5 0.5 0.5 0.5 0.5 Deflocculant (addition) 0.3-0.6 0.3-0.6 0.3-0.6 0.3-0.6 0.3-0.6 Water (addition) 12-14 12-14 13-15 13-15 16-20 Properties of the slip: pH = 7.0-8.5 Viscosity = 200 -600 cps Slip Density = 2.40-2.50 glcm3 The slips are preferably mixed in a non-metal lined ball mill or some other suitable slow mixer. All of the ingredients are weighed out and placed in the ball mill or mixer and mixed for a total of a minimum of between 6 hours and 48 hours, depending on the speed of the ball mill or mixer and the ambient temperature, to ensure that the slip is homogeneous and that the components are intimately mixed. After approximately 6 hours and 12 hours, pH, viscosity (as determined with a Gallenkamp Torsion Viscometer fitted with 11/ bob and 30 swg wire) and slip density are checked and adjustments made with further deflocculant or water additions, if necessary. It is important to keep pH within this range because, if the slip becomes acid (ie <7.0) or too alkaline (ie >8.5), there is a danger that silanes or similar volatile silicon compounds are formed, some of which spontaneously ignite in air. The slip performance is optimised by having a low viscosity with minimum water content and, hence, a high slip density, which means the solids content. In general, the higher the solids content of the slip, the less water to be removed during the casting process or subsequent drying.

After mixing, a final check is made of the slip properties before it is used for slip casting or pressure casting.

The slip as described above is pumped into the pressure casting moulds, which are made either of porous organic or inorganic materials such as plastic or metals, or a mixture of both, backed as necessary by solid organic or inorganic materials such as plastic or metals, or a mixture of both as required. The two halves of the mould are held together by hydraulic pressure, mechanical pressure, electro-mechanical pressure or a combination of these as appropriate. The slip is pumped into the moulds at between 1 bar and 500 bar (the pressure depends on the characteristics of the moulds, eg pore size and pore size distribution) so that the water is forced out of the piece through the porous moulds, leaving a semi-solid or solid shape which is then removed from the parted moulds as part of, or at the end of the machine cycle. This operation is normally performed at ambient temperature (but the slip can be preheated if location ambient temperature is below about 12°C) and the machine transfers the slip directly into the mould from either an internal or external container via appropriate pump and valve arrangements.

The green shape is then removed manually or by appropriate handling device or robotically and placed on a drying plate to dry.

Alternatively, if the pieces are to be formed by slip casting, the working faces of a suitable plaster mould is coated with a release agent, typically but not exclusively fine, flake graphite of particle size 0-75pm, which is applied dry, usually by sponge and the excess graphite removed by brush or air jet. The mould is assembled with a filling tube and header and the slip is poured in to fill the mould and header. This is left to stand until the optimum amount of water has been removed and the green shape is strong enough to be removed from the mould. This technique is well known in the art.

The green shape made either by pressure casting or by slip casting is dried in air and/or a drying oven at a maximum temperature of 150°C, until the moisture content is less than 0.5% and preferably less than 0.2%. If the moisture content is greater than 0.5%, the silicon metal powder may oxidise rapidly during firing causing the body to expand and become friable.

The dried green shape is then fired in a suitable kiln, which will provide a nitrogen rich atmosphere, which may be high purity nitrogen, ammonia or a mixture of nitrogen and hydrogen. It is important that no oxygen or water vapour is present in the atmosphere because the formation of undesirable compounds. The kiln can be gas or oil fired with an internal muffle to contain the nitrogen atmosphere but an induction furnace or an electrically heated kiln with a steel shell to contain the nitrogen atmosphere within the whole unit is preferred.

An electrically heated kiln or induction furnace is preferred for accuracy. During the firing process, nitrogen gas reacts with solid silicon metal and a-alumina to form a mixture of 3-sialon, a-silicon nitride and (3-silicon nitride. The kinetics and mechanism of the reaction is such that, in the early stages, a-silicon nitride and (3-silicon nitride are formed preferentially and these reactions are strongly exothermic, particularly at approximately 1180°C and 1280°C. If these exothermic reactions are not controlled, the result is a rapid increase in temperature within the shape, which may exceed that of the melting point of silicon (ie 14 10°C) and the reaction ceases because molten silicon will not react with gaseous nitrogen. For best results, the rate of reaction is controlled either by suitable modifications to the firing cycle and careful control of nitrogen flow rate to slow the rate of reaction between 1100°C and 1400°C or by the injection of an inert gas. The preferred method is to use an inert gas. These exothermic reactions are detected by strategically placed thermocouples within the load, which indicate that the temperature of the load is starting to exceed that of the programmed temperature, causing an inert gas to be injected into the reaction chamber.

The inert gas either replaces or dilutes the nitrogen, reducing and controlling the rate of reaction. Once the rate of reaction is reduced and the temperature, as indicated by the thermocouples placed within the load, falls below that of the programmed temperature, the inert gas is turned off and the full nitrogen atmosphere is restored. This process can be repeated as often as necessary.

For an electric kiln, the kiln is loaded with the green shapes and the temperature is raised at a steady rate of between 25°C/hour and 60°C/hour to 1380-1400°C and this temperature maintained for 16-20 hours. Alternatively, after approximately 6 hours at 1380-1400°C, the temperature may be increased to 1450-1500°C and maintained at that temperature for a further 14-16 hours, which not only ensures complete conversion of the a-alumina with a-silicon nitride and 3-silicon nitride to 13-sialon but also develops a strong bonding system with the silicon carbide grains. The link between the bonding phase (consisting predominantly of (3-sialon with small amounts of a-silicon nitride, and n-silicon nitride) and the silicon carbide is very complex and difficult to define, since it may be a mixture of one or more of the phases, depending on the composition.

For some applications, the shapes may be re-fired to 1200-1350°C in air to seal the surface and increase the oxidation resistance.

Table 2. The resultant products have the following properties: Property After Firing in Nitrogen After Firing in Air Bulk Density 2.78-2.82 g/cm3 2.78-2.82 glcm3 pparent Porosity 8-14% 0-3% Modulus of Rupture (at ambient) 160-220 MPa 160-220 MPa Modulus of Rupture (at 1400°C) 180-250 MPa 180-250 MPa Thermal Expansion (%) 0.48 _____________________ Oxidation Resistance at 1000°C (%) _____________________ 0.1 Silicon Carbide 60-70% 60-70% Bonding Phase 25-42% 25-42% The main application for these products is kiln furniture, low-thermal mass kiln furniture, automated processes and engineering applications.

After firing in nitrogen, as given above, compositions 3, 4 and 5 may be further treated, by reducing the gas flow to a minimum and raising the temperature at a rate of 60 -100°C/hr to between 1550°C and 1700°C and this temperature maintained for 2-6 hours. During this period the bond is fully converted to f3-sialon and sintenng occurs, which increases the density and strength and reduces the porosity.

Table 3 Properties of Silicon Carbide Products after High Temperature Firing Property After High Firing Temperature Bulk Density 2.85-2.90g/cm3 Apparent Porosity 3-6% Modulus of Rupture (at ambient) 250-400 MPa Silicon Carbide 50-70 -sialon 30-50 The main application for these products is personal body armour and high wear resistant applications.

Claims

CLAIMSWhat we claim is: 1. A process for the manufacture of a range of silicon carbide products from an admixture of high purity, finely divided silicon carbide fractions; high purity, finely divided silicon metal, high purity, finely divided alumina and iron oxide powder as catalyst, which is shaped and fired in a nitrogen rich atmosphere to form a complex bonding phase of -sialon, a-silicon nitride and 3-silicon nitride.
2. The product of claim 1 contains a bonding phase of between about 25 W/, and about 50 W/ of the final composition.
3. The 3-sialon content of the bonding phase can be increased to 100% by the addition of yttrium oxide or other rare earth or alkaline earth oxide or mixed oxides thereof.
4. The admixture of claim 1, with or without additions of yttrium oxide or other suitable additives of claim 3, can be mixed into a homogeneous slip with water and a suitable deflocculant by blending in a non-metallic ball mill or other slow mixer.
5. The admixture of claim 1, with or without additions of yttrium oxide or other suitable additives of claim 3, can be mixed into a slip and either slip cast into plaster moulds or pressure cast to form the required shapes.
6. The dried shapes of claim 5 are heat treated in a suitable kiln, fired by gas, oil or electricity or in an induction furnace, in a nitrogen rich atmosphere.
7. The dried shapes of claim 5 are fired using a carefully controlled firing cycle and controlled, variable nitrogen flow rates to prevent overheating caused by exothermic reactions between silicon and nitrogen.
8. The dried shapes of claim 5 are preferably fired in an electric kiln or induction furnace with an inert gas injected intermittently to control the exothermic reactions caused by the reaction of silicon and nitrogen.
9. When the dried shapes are fired to 1380 -1400°C for a complex bonding phase, which may contain one or more of -sialon, a-silicon nitride and 13-silicon nitride by reacting the silicon, alumina and other minor ingredients with the nitrogen.
10. When the dried shapes are fired to 1400-1500°C fora complex bonding phase containing 80-100% 13-sialon with possibly either or both of a-silicon nitride and 13-silicon nitride.
11. The shapes exhibit a density greater than 2.78g/cm3, a porosity of less than 14% and a 3-point bend strength greater than 16OMPa at ambient temperature, after firing in a nitrogen rich atmosphere.
12. The shapes may be re-fired in an oxidizing atmosphere, after which the re-fired article exhibits a porosity of less than 3%.
13. The fired shapes may be treated further in a nitrogen atmosphere and fired to 1400-1700°C to convert the bond to predominantly 13-sialon and to induce sintenng, increasing the density and strength.
14. The shapes of claim 13 exhibits a density greater than 2.80g/cm3, a porosity of less than 10% and a 3-point bend strength greater than 160MPa at ambient temperature. (Q Amendments to the claims have been filed as foHowsCLAIMSWhat we claim is: 1) A process for the manufacture of a range of silicon carbide products from an admixture of high purity, finely divided silicon carbide fractions; high purity finely divided Silicon metal powder; high purity finely divided alumina; high purity finely divided yttrium oxide or a mixed oxide containing yttrium or other rare earth or alkaline earth oxides or mixtures thereof and iron oxide powder used as a catalyst.This mixture is then shaped and fired in a nitrogen rich atmosphere to form a complex bonding phase of -sialon.2) The product of claim 1) contains a bonding phase of between 25w/a and about 50w/a of the final composition.3) The components of claim 1) can be mixed in a homogenous slip with water and a suitable deflocculant by blending in, for instance, a ball mill or other slow mixer.4) The admixture of claim 1) when mixed as a slip can be cast into plaster moulds or pressure cast to form the required shapes.5) The dried shapes from claim 4) can be fired in a suitable kiln or furnace, powered by gas, oil or electricity or in an induction furnace, in a nitrogen rich atmosphere.6) The dried shapes of daim 4) are fired preferably, but not exclusively in an electric kiln using a careful controlled firing cycle, with varying nitrogen flow rates and the intermittent injection of an inert gas in order to control and prevent any overheating provided by exothermic reactions between silicon metal powder and nitrogen which can cause the silicon metal powder to enter a molten, non-reactive state. The shapes are fired during this process to between 1380-1400°C to provide the 13-sialon bonding phase by reacting the components in claim 1) with nitrogen.7) The shapes from claim 6) exhibit a density greater than 2.78 g/cm3, an open porosity of less than 14% and a bending strength of greater than I6OMPa at ambient temperature.8) The shapes from claim 6) may be re-fired in an oxidising atmosphere after which the re-fired article exhibits a porosity of less than 4%.9) The shapes from claim 6) can continue to be fired or may be re-fired in the nitrogen atmosphere to between 1400-1700°C in an appropriate furnace or kiln as described in claims 5) & 6) in order to induce sintering, thereby increasing density to greater than 2.85g/cm3, decreasing the porosity below 6%, and with a bending strength *,. : greater than 25OMPa at ambient temperature *:: :110) The shapes in claim 9) may be re-fired in an oxidising atmosphere, after which the article exhibits a porosity of less than 4%..?l) A process for the manufacture of a range of silicon carbide products from an * admixture of high purity, finely divided silicon carbide fractions; high purity finely divided silicon metal powder; high purity finely divided alumina and iron oxide powder as a catalyst. This is shaped and then fired in a nitrogen rich atmosphere to form a *:* complex bonding phase of f3-sialon, a-silicon nitride and a-silicon nitride.t2) The product of claim 11) contains a bonding phase of around 25'/ and around 50tvI of the final composition.13) The components of claim 11) can be mixed into a homogenous slip with water and a suitable deflocculant by blending in a ball mill or other slow mixer and then cast into plaster moulds or pressure cast to form the required shapes Patent Application No: GB 0803159.3 14) The dried shapes of claim 13) are fired in a kiln powered by gas, oil or electricity or in an induction furnace in a nitrogen rich atmosphere as for claim 5).
15) The dried shapes of claim 13) are fired using a careful controlled firing cycle with varying nitrogen flow rates, preferably, but not exclusively in an electric kiln, and with intermittent injection of an inert gas in order to control & prevent any overheating provided by an exothermic reaction between silicon metal powder and nitrogen which can cause the silicon metal powder to enter a molten non-reactive state. During this process, the shapes are fired to between 1380-1400°C to provide a complex bonding phase which may contain one or more of -sialon, a-silicon nitride and 3-silicon nitride by reacting the components in daim 11) 16) When the dried shapes from claiml3) are fired to 1400-1500°C, using the same procedures as in claim 15) a complex bonding phase is formed containing 75-100% 13-sialon, with 0-20% of either or both a-silicon nitride and 3-silicon nitride.17) The shapes produced in claim 15) & 16) exhibit a density greater than 2.78g/cm3 with a porosity of less than 14% and a bending strength greater than I4OMPa at ambient temperature.18) The shapes from claim 15) & 16) may be re-fired in an oxidising atmosphere after which the article exhibits a porosity of less than 4%. a... a * *S * S *. . e 4 4 S. * . * a..S * *S * S *PS S * . a S