Production of an aluminide containing ceramic moulding

Details Production of an aluminide containing ceramic moulding Production of an aluminide containing ceramic moulding

1997-11-19

2000-02-15

Jones, Deborah

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

4283191, 428325, 428501, 4285395, 75228, 501 87, 501 89, 501 972, 501127, 501105, 419 10, C04B 3566, C22C 105

Patent

active

060250658

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention relates to a process for making ceramic bodies and the ceramic bodies thus formed.


BACKGROUND AND PRIOR ART

Hard metal (Co bonded WC), developed in the thirties, which had been successful until today in numerous variants due to its good wear resistance, has always been the trigger for experiments to ductilize also non-carbide ceramics with metallic phases. Especially in the sixties during which the term "Cermets" was coined (c.f. e.g. "Structure and Properties of Cermets") Z. Metallkde., 59 (1968) 170) numerous experiments concentrated on oxide ceramics of which a reduction in brittleness would have drastically improved the prospects for increased technical application. The desired advantageous combination of good ductility and fracture toughness of metals with the excellent high temperature properties, the wear resistance and the hardness of ceramics, however, was unsuccessful in most cases. On the contrary, cermets even combined the negative properties of both classes of materials. One cause lies in the typically very bad wetting of oxide ceramics by liquid metals which causes, during liquid phase sintering, the metal phase to sweat out of the body. In order to prevent this, such composite bodies must be hot pressed or hot forged as for instance carried out in the system Al.sub.2 O.sub.3 --Al (UK-Patent 2,070,068A; U.S. Pat. No. 5,077,246). Another cause for the bad mechanical behavior can be sought in the characteristic microstructure of cermets which typically results from a powder metallurgical mixture of the two phases. The ceramic component is mainly embedded in the metallic matrix which often amounts to less than 20 vol % (c.f. e.g. "Processing of Al.sub.2 O.sub.3 /Ni Composites", J. Eur. Ceram. Soc., 10 (1992) 95). In metal bonded carbides or boride cermets the ceramic phase can additionally form a skeleton, however, the metallic properties usually dominate in most cermets ("Cermets", Reinhold Publ. Co., New York, 1960). Today only carbide combinations, especially TiC-Ni, are designated as cermets.
New strengthening concepts for ceramic materials are based on the introduction of a second phase into the ceramic matrix such that the positive properties of the ceramic are essentially retained. Examples are transformation toughenable ZrO.sub.2 particles ("Strengthening Strategies for ZrO.sub.2 -toughened Ceramics at High Temperatures", J. Mat. Sci. Eng., 71 (1985) 23) or SiC whiskers ("TZP Reinforced with SiC Whiskers", J. AM. Ceram. Soc., 69 (1986) 288) in an Al.sub.2 O.sub.3 matrix. At first, the inclusion of metals was considered unreasonable because, according to conventional composite theories, metals with low yield stress and low modulus of elasticity could not improve brittle ceramics especially with respect to their strength. Admittedly, it has been shown recently that this is not always the case ("Effect of Microstructure on Thermal Shock Resistance of Metal-Reinforced Ceramics", J. Am. Ceram. Soc. 77 (1994) 701 and "Metalle verbessern mechanische Eigenschaften von Keramiken", Spektrum der Wissenschaft, Januar (1993) 107). However, the strengthening effect is only achieved if the microstructure of the classical cermets is reversed, i.e. the ceramic material forms a rigid matrix which is premeated by a monocrystalline metallic phase. The designation "metcers" would in this case constitute a conceptual characterization of the reversal in microstructural components. Quite apart from the modified microstructure of these composites, it is also the significantly smaller quantity of the metal phase which is responsible for the improvement compared to conventional cermets. The metal embedded in the ceramic matrix has considerably better mechanical properties than it does in the "free" state, a phenomenon which seems to apply even for otherwise brittle intermetallic phases ("Metcers--A Strong Variant of Cermets", Cfi/ber. DKG 71 (1994) 301).
So far, a number of different methods have been used to produce these novel metal-ceramic composites, for example the dir

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patent: 5077246 (1991-12-01), Weaver et al.
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patent: 5326519 (1994-07-01), Claussen
patent: 5607630 (1997-03-01), Claussen
patent: 5679297 (1997-10-01), Hwang

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