Compositions: ceramic – Ceramic compositions – Refractory
Patent
1992-11-25
1998-12-01
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Refractory
501 88, 501 96, 501 98, 501103, 501106, 501118, 264 65, 264 66, C04B 35185, C04B 3565
Patent
active
058438599
DESCRIPTION:
BRIEF SUMMARY
THIS APPLICATION IS A 371 OF PCT/EP91/00986, FILED MAY 28, 1991.
Single and multiphase moulded bodies with a ceramic matrix are being increasingly used as temperature-resistant and wear-resistant components in mechanical engineering and in machine manufacture. Such bodies are either fabricated by means of classical ceramic (i.e. powder-metallurgical) processes by pressing and sintering or they are manufactured by reaction of metallic precursors with a gaseous or liquid phase (see. e.g. "Reaction-Formed Ceramics", Am. Ceram. Soc. Bull., 67 (1988) 356). These so-called reaction-formed ceramics have been almost exclusively limited up to now to Si.sub.3 N.sub.4 (RBSN) and SiC (RBSC). A further approach to the production of multiphase ceramics in particular is to infiltrate a porous ceramic body with a metallic or ceramic phase (see e.g. Melt Infiltration Approach to Ceramic Composites", J. Am. Ceram. Soc. 71 (1988) C-96). In another new method a ceramic phase which grows through a porous ceramic body which is formed by reaction of a gas with a molten metal and can enter the porous ceramic precursor body on one or several sides (see. e.g. "Formation of Lanxide Ceramic Composite Materials", J. Mater. Res. 1 (1986) 81 DIMOX process).
All these classes of ceramics and their production processes have characteristic disadvantages. The decisive disadvantage of ceramics or ceramic composites formed by classical processes is the high linear shrinkage which occurs between the green body and the final product; it is usually 15 to 25%. This typical shrinkage is problematic when trying to maintain the shape and dimension of a moulded part and it leads to the formation of cracks and other defects which reduce the quality. The shrinkage of a ceramic matrix has a disadvantageous effect on the structural coherence particularly when strengthening elements such as fibres, platelets and whiskers or other components which do not have a concomitant shrinkage are included. The high tensions which occur as a result of the differences in shrinkage almost always lead to damaging cracks. A further characteristic disadvantage, in particular of the classically fabricated oxide ceramics, is the formation of a glass-like intergranular phase which, although accelerating the sinter and densification process, strongly impairs the mechanical high temperature properties.
Although reaction-bound ceramics, above all Si.sub.3 N.sub.4 (RBSN) and SiC and also ceramics fabricated by means of the DIMOX process (among others Al.sub.2 O.sub.3 and AlN), either have no shrinkage or only a very slight shrinkage, the production process is seriously impeded by the reaction periods which often take weeks. A further disadvantage of the RBSN-like ceramics is the fact that it is only possible to obtain bodies with seldom more than 85% of the theoretical density which can be attributed to the increasing reduction in nitrogen diffusion as the porosity decreases.
Experience shows that such a low density results in poor mechanical properties (see also "Review: Reaction-Bonded Silicon Nitride: its Formation and Properties", J. Marter, Sci., 14 (1979) 1017). The production of ceramic composites based on Al.sub.2 O.sub.3 and AlN by a melting reaction and associated growth through a porous ceramic precursor body is described inter alia in the recently published European patent applications 0155831, 0169067, 0193292 and 0234704. Also in this case the reaction proceeds very slowly. In addition it is very time-consuming to control the dimension of the bodies.
Due to their finely-dispersed porosity, their good corrosion resistance and their resistance to temperature shock, mullite ceramics are of great importance in applications at high temperatures and under corrosive conditions. However, these ceramics have crucial disadvantages because they usually have to be sintered at high temperatures (>1650.degree. C.) in order to achieve adequate density and strength as a result of which the SiO.sub.2 components contain intergralular glass phases and usually have sinter shrinkag
REFERENCES:
patent: 4921818 (1990-05-01), Lesher et al.
patent: 4923832 (1990-05-01), Newkirk et al.
patent: 4988645 (1991-01-01), Holt et al.
patent: 5158916 (1992-10-01), Claussen
Reed, "Introduction to the Principles of Ceramic Processing", 1988, John Wiley & Sons, pp. 261 & 266, No Month.
Bell Mark L.
Marcheschi Michael
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