Compositions: ceramic – Ceramic compositions – Refractory
Patent
1997-04-28
1998-12-22
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 982, 501 983, 4233271, C04B 35599
Patent
active
058519432
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention comprises a process for the production of O'--SiAlON's and composite ceramic materials containing O'--SiAlON's.
BACKGROUND ART
The term SiAlON, or silicon aluminium oxynitride, encompasses a family of compounds or phases comprised of the elements: silicon, aluminium, oxygen and nitrogen. Each phase is described by a composition range for which that particular structure is stable. O'--SiAlON is stable over the composition range: Si.sub.2-x Al.sub.x O.sub.1+x N.sub.2-x where 0<x.ltoreq.0.4. O'--SiAlON has a similar structure to silicon oxynitride with equal amounts of aluminium and oxygen substituting for silicon and nitrogen respectively into the silicon oxynitride structure. The amounts of aluminium and oxygen which can be substituted into the structure increase with temperature. At 1600.degree. C., x can be as high as 0.2. At 1900.degree. C., x can be as high as 0.4.
SiAlON's are advanced ceramic materials which exhibit useful properties such as high strength and hardness, low density, wear resistance and corrosion resistance, and are able to retain these properties at high temperatures. SiAlON's are used in refractories and for a variety of engineering applications such as cutting tools, spray nozzles and pump seals. The exact properties of a given SiAlON depend on the chemical composition and fabrication variables, such as purity, grain size and shape, and the method of fabrication. O'--SiAlON has similar properties to silicon oxynitride which include excellent resistance to oxidation and thermal shock. Silicon oxynitride is commonly used as a refractory material.
SiAlON's and O'--SiAlON's are known compounds and common methods for producing them include:
(i) Reaction Sintering
Mixtures of two or more of the following: Si.sub.3 N.sub.4, SiO.sub.2, Si.sub.2 N.sub.2 O, Al.sub.2 O.sub.3 and AlN, are sintered at .gtoreq.1600.degree. C. under a nitrogen atmosphere, usually in the presence of a rare earth sintering aid such as Y.sub.2 O.sub.3 or CeO.sub.2. This process involves expensive raw materials and high temperatures, but allows good control over the composition and purity of the product.
(ii) Carbothermal Reduction
Aluminosilicate materials are blended with carbon and fired at .gtoreq.1350.degree. C. under a flowing nitrogen atmosphere. This process is described as carbothermal reduction because the carbon acts by reducing the aluminosilicate, allowing nitridation to occur. This process involves cheap raw materials and lower firing temperatures than for reaction sintering but impurities in the aluminosilicate can degrade the properties of the product. The process is difficult to control because it involves stopping a reaction at a specific point prior to completion.
(iii) Combustion Synthesis
A mixture containing silicon metal powder is ignited under a nitrogen atmosphere. The energy evolved by this strongly exothermic nitridation of silicon propagates a reaction front through the reaction mixture. This method is very rapid and energy efficient but is difficult to control.
Methods (ii) and (iii) both yield SiAlON powders which must then be formed and sintered to obtain a ceramic body. Method (i) is the most commonly used method for preparing O'--SiAlON. As is apparent from the above known methods, in order to get good control over the composition and purity of the product expensive raw materials and/or extreme reaction conditions are required.
European patent application EP 0153000 to Kennecott Corporation discloses a bonded material consisting of granular silicon carbide and a bonding phase comprising Si.sub.3 N.sub.4 which has been modified by the presence of oxygen and aluminium. Also disclosed is a method for the manufacture of the bonded material which includes the use of bentonite clay as a temporary binder. The amount of this bentonite clay in the raw starting material is disclosed as being about 0.5% by weight. The starting materials are disclosed as being silicon carbide, aluminium powder, silicon powder plus optional processing aids and temporary binders.
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Barris Glen Clifton
Hodren Gregory Michael
Group Karl
Industrial Research Limited
Mandragouras Amy E
Monks Lawrence E
Pryotek Products Limited
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