Compositions: ceramic – Ceramic compositions – Refractory
Patent
1990-09-27
1992-10-27
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 96, 501 98, 501128, 501 87, 501 89, 501105, 264 60, C04B 3510, C04B 3565
Patent
active
051589168
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
Single and multiple phase formed bodies with ceramic matrix are increasingly used as temperature- and wear-resistant components in machine and apparatus construction. Such bodies are produced either via classical ceramic (i.e. powder metallurgical) processes by pressing and sintering or they are produced 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-bound ceramics have hitherto been limited almost exclusively to Si.sub.3 N.sub.4 (RBSN) and SiC (RBSC). A further possibility especially to produce multiphase ceramics is the infiltration of 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 a further novel process, a porous ceramic body is grown through by a ceramic phase which results by reaction of a gas with a metallic melt which possesses a one or multi-sided entry to the porous ceramic preliminary body (see e.g. "Formation of Lanxide Ceramic Composite Materials", J. Mater. Res. 1 (1986) 81).
All these ceramic classes and their production processes have characteristic disadvantages which the following described invention possesses scarcely or only to a small extent. The decisive disadvantage of classically produced ceramics or ceramic composite materials lies in the high linear shrinkage which occurs between the green body and the end product; usually, it amounts to 15 to 25%. This typical shrinkage makes the shape and measurement stability of a formed part problematical, it leads to crack formation and other quality-reducing faults. The shrinkage of a ceramic matrix acts disadvantageously especially also in the case of the incorporation of reinforcing elements, such as fibres, platelets and whiskers or other, non-co-shrinking components, on the structure cohesion. The high stresses arising as a result of the shrinkage differences lead almost always to damaging cracks. A further characterising disadvantage, especially of the classically produced oxide ceramics, lies in the formation of a glass-like intergranular phase which admittedly accelerates the sinter and consolidation process but strongly impairs the mechanical high temperature properties.
Reaction-bound ceramics, above all Si.sub.3 N.sub.4 (RBSN) and SiC and also ceramics produced via the lanxide process (inter alia Al.sub.2 O.sub.3 and AlN), admittedly show no or only small shrinkage but the course of the production is sensitively impaired by the often week-long reaction times. A further disadvantage of the RBSN-like ceramics lies in the fact that only bodies with seldom more than 85% of the theoretical density can be achieved, which is to be attributed to the nitrogen diffusion reducing ever more with decreasing porosity. According to experience, such a low density results in poor mechanical properties (see "Review: Reaction-Bonded Silicon Nitride: its Formation and Properties", J. Mater. Sci., 14 (1979) 1017). The production of ceramic composite material based on Al.sub.2 O.sub.3 and AlN by melt reaction and therewith involved growing through of a porous ceramic preliminary body is, inter alia, described in the recently published European Patent Applications 0155831, 0169067, 0193282 and 0234704. The reaction hereby also proceeds very slowly. Furthermore, the dimension control of the bodies is involved with great expense.
Therefore, the task forming the basis of the invention is to provide a ceramic formed body which does not display or only in substantially small extent the above-mentioned disadvantages of the known formed bodies.
This task is solved by a powder-metallurgically produced ceramic formed body consisting of at least one dispersed inorganic component which is embedded in a phase of aluminium oxide and/or aluminium nitride which is characterised in that it the green state, from a green body which, besides the dispersed inorganic components, contains at least 10 vol. % of aluminium powder, which ca
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