Compositions: ceramic – Ceramic compositions – Refractory
Patent
1998-05-27
2000-05-23
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 982, 501 983, C04B 35599
Patent
active
060665831
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a process for the production of SiAION's and in particular to the production of .beta.'SiAlON, .alpha.'SiAlON, and composites with these.
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. .beta.'-phase SiAlON (.beta.'SiAlON) is stable over the composition range: Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z=0 to 4.2. This includes silicon nitride (.beta.Si.sub.3 N.sub.4) as the z=0 end member. .beta.'SiAlON has the same structure as silicon nitride (.beta.Si.sub.3 N.sub.4), and can be regarded as a solid solution formed by substituting equal amounts of aluminium and oxygen for silicon and nitrogen respectively into the silicon nitride structure. The amounts of aluminium and oxygen which can be substituted into this structure increase with temperature. At 1750.degree. C., z can range from 0 to 4.2. In general terms .beta.'SiAlON compositions can be referred as low z compositions and high z compositions with low z being <3 and high z being .gtoreq.3. The z value basically refers to the aluminium content of the composition.
.alpha.'-phase SiAlON (.alpha.'SiAlON) has a structure derived from .alpha.Si.sub.3 N.sub.4 which is stabilised by metal cation (M) such as Y, Li, Ca. The general formula is and n(Al--O) bonds and v represents the valency of the metal cation M. The limits of the .alpha.SiAlON composition are restricted and vary with the size and nature of the stabilising cation. For example, the limits of solubility of yttrium have been found to vary m/v from 0.33 to 0.67 for one composition range. A limited range of metal cations stabilise the .alpha.SiAlON structure. These are Li, Ca, Mg, Y and a number of the rare earth metals but not La or Ce. SiAlONs 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. .alpha.'SiAlON, when fully dense, is a very hard material but brittle. .beta.'SiAlON is less hard but tough. A composite of the two is a good compromise and yields excellent mechanical strength and wear resistance. SiAlONs 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. .beta.'SiAlON has similar properties to silicon nitride which include excellent resistance to attack by molten metal. Silicon nitride is commonly used as a refractory material.
Documents indicating the state of the art include:
U.S. Pat. No. 3,960,581 to Ivan B Cutler discloses a process for making SiAlONs from readily available raw materials, such as clay, together with carbon. There is no teaching or recognition however of the use of silicon metal in the process. Use of silicon metal allows synthesis of low z SiAlON compositions. In addition, the use of carbon is preferable, but not essential, in the process of the present invention. It is also low z .beta.'SiAlON compositions that allow the formation of .alpha.'SiAlON compositions by the process of the present invention as will be further described herein.
DD 263749 to Akad Wissenschaft DDR, inventor Schikore H, which describes the production of SiAlON-based materials from a charge containing by weight (A) 75-95% clay, 5-25% carbon, and 0-50% aluminium compounds; or (B) 50-80% clay, 20-50% silicon carbide, and 0-50% aluminium compounds. No disclosure of the use of silicon metal is made and carbon or silicon carbide must be used.
U.S. Pat. No. 4,360,506, inventor Paris R A, which discloses the formation of .beta.'SiAlON's from a paste comprising silico-aluminous material (clay), carbon, and fine particles of a ligneous m
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Choi et al, "Synthesis of .beta.'-Sialon with Various Compositions from Kaolin", Journal of the Korean Ceramic Society, vol. 23, No. 5, pp. 17-24 (1986). (With English translation (pp) 1-8). No month.
Group Karl
Industrial Research Limited
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