Metal-ceramic formed body and process for producing it

Details Metal-ceramic formed body and process for producing it Metal-ceramic formed body and process for producing it

1998-11-11

2003-06-03

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S095200, C501S095300, C501S153000, C260S66500B, C260S66500B, C260S66500B, C419S013000, C419S014000, C419S019000, C428S539500

Reexamination Certificate

active

06573210

ABSTRACT:

The invention relates to a ceramic metal composite with a matrix of Al
2
O
3
, its production and use.
Ceramic materials represent a promising class of materials due to their excellent high temperature stability, their wear resistance and hardness as well as due to their excellent oxidation and corrosion properties. An essential disadvantage, however, is their brittleness which especially limits their application as engineering material. For many years, intensive research activities have therefore been undertaken to improve the fracture toughness of ceramic materials.
One such trial to solve this problem represents, for instance, the inclusion of transformable ZrO
2
particles (“Strengthening Strategies for ZrO
2
-Toughened Ceramics at High Temperatures”, J.Mater.Sci.Eng., 71 (1985) 23) or SiC-Whiskers (“TZP Reinforced with SiC Whiskers”, J.Am.Ceram.Soc., 69, (1986) 288) into an Al
2
O
3
matrix.
In the sixties, the development of cermets, two-phase composites consisting of a ceramic matrix with a ductile metal inclusion phase (“Aufbau und Eigenschaften von Cermets”, [Microstructure and Properties of Cermets], was considered especially promising for the increase of damage tolerance of ceramic materials. The hope of combining the positive properties of both material classes into cermets remained, however, unfulfilled in almost all cases. In most cases, pressureless sintering is not possible due to the bad wetting behavior of liquid metals to oxide ceramics, because the liquid metal phase sweats out of the green body during the temperature treatment. Even an improvement of the wetting behavior by alloying elements or coatings (“Fabrication of Cermets from Alumina and Ni-Based Alloys”, Ceram. Trans., Ceramic Science III, (Ed. G. L. Messing), Vol. 12, (1990) 911 and “Effect of Ti on Sinterability of 70% Al
2
O
3
-30% Cr Cermets”, Ceram.Bull. 57 (1978) 1056) was unsuccessful. It must therefore be assumed that, accept for the wetting, also the presence of oxide layers on the added metal particles considerably influences the sintering properties of cermets (“Effect of Sintering Atmosphere on the Properties of Cermets”, Powder Metallurgy International, 23, [4] (1991) 224). Therefore, expensive production technologies are required, e.g. hot pressing or hot forging (UK-Patent 2,070,068A; U.S. Pat. No. 5,077,246) or post treatment by hot isostatic pressing. The characteristic microstructure of cermets, however, where the ceramic phase is mainly embedded in the metallic phase which often makes up less than 20 vol % of the total volume (“Processing of Al
2
O
3
/Ni-Composites”, J.Eur.Ceram.Soc., 10 (1992) 95) usually leads to bad mechanical properties. For this reason, only metal bonded non-oxide cermets, especially carbides, e.g. TiC—Ni (“The Sory of Cermets”, Powder Metallurgy International, 21 (1989) 37) were successful in technical application.
In the development of novel metal ceramic composites, it is therefore desired to produce a microstructure consisting of a ceramic matrix in which an interpenetrating metal phase is embedded. These composites enable a marked improvement of mechanical properties (“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”, Sprektrum der Wissenschaft, Januar (1993) 107), because the metal embedded in the ceramic matrix exhibits much better mechanical properties than in its “free” state (“Metcers-A Strong Variant of Cermets”, Cfi/Ber. DKG 71 (1994) 301).
Methods to produce such metal ceramic composites are, for example, the directed oxidation of molten metals (DMO) (“Formation of Lanxide TM Ceramic Composite Materials”, J.Mater. Res., 1 (1986) 81 and “Directed Oxidation of Molten Metals” in: Encyclopedia of Mat. and Eng. (Ed. R. W. Cahn), Supplementary Vol. 2, Pergamon, Oxford (1990) 1111), pressure casting (“Application of the Infiltration Technique to the Manufacture of Cermets”, Ber.Dt.Keram.Ges., 48 (1971) 262), infiltration of porous ceramic preforms with liquid metal (“Method for Processing Metal-Reinforced Ceramic Composites” J.Am.Ceram.Soc., 73 [2] (1990) 388), which, if need be, can also take place under pressure in order to also infiltrate non-wetting metals (“Microstructure and Properties of Metal Infiltrated RBSN Composites” J.Eur.Ceram.Soc. 9 (1991) 61) as well as reactive metal infiltration of SiO
2
-containing preforms (“Al2O3/Al Co-Continuous Ceramic Composite (C
4
) Materials Produced by Solid/Liquid Displacement Reactions: Processing Kinetics and Microstructures”, Ceram.Eng.Sci.Porc. 15 (1994) 104).
All these methods exhibit, except for the high processing effort, characteristic disadvantages. For example, the two reaction forming methods, the directed oxidation of molten metals and C
4
are only suitable for the production of composites that consist of an Al
2
O
3
matrix and an Si or Mg containing Al alloy. Furthermore, in both cases, the reaction rates are very slow (2 cm/day) and therefore the processing times are extremely long. For technical reasons (no suitable pressure vessel material), only Al
2
O
3
/Al composites can be produced. Similar problems are also associated with infiltration because the processing temperatures required for the infiltration of refractory metals (100-200° C. above the melting points of the metal) cannot be realized for technical reasons.
Even powder metallurgical manufacturing of metal ceramic composites has, in many cases, been shown to be problematic and expensive. For example, Al
2
O
3
-containing metal ceramic composites can be prepared by thermite reactions (SHS: Self-Propagating High-Temperature Synthesis) according to the following reaction scheme.
Me
x
O
y
+Al→Al
2
O
3
+Me
x,y=1,2, . . . , n
In this reaction, a metal oxide is reduced by Al to form the respective metal, while Al oxidizes to Al
2
O
3
(“Combustion Synthesis of Ceramic and Metal-Matrix Composites”, J.Mat.Synth.Proc. 1 (1994) 71). In most cases, this reaction proceeds very exothermally, it is therefore very difficult to control. For this reason, SHS composites are usually porous and inhomogeneous. The bad mechanical properties exclude their use as engineering components.
A further approach to solving the problem of powder metallurgical manufacturing of metal reinforced ceramics is presented in the German application DE-P 44 47 130.0. According to this invention, composites consisting of Al
2
O
3
and intermetallic aluminide compounds are prepared from metal oxides and aluminum according to the reaction
Me
x
O
y
+Al→Me
x
Al
y
+Al
2
O
3
x,y=1,2, . . . , n
It is, however, a certain disadvantage, similar to SHS materials, that the reaction is strongly exothermic and therefore requires an expensive and time-consuming process control. Aluminide usually exhibit good high temperature stability and good corrosion and oxidation resistance, however, a disadvantage is their extreme brittleness at room temperature (“Intermetallics”, G. Sauthoff, VCH Verlag, Weinheim, 1995, ISBN 3-527-29320-5)). These aluminide-reinforced ceramics can therefore not be expected to show a considerable increase in fracture toughness by the incorporation of the intermetallic phase.
The object of the present invention is thus to provide composites that comprise an Al
2
O
3
matrix that is permeated by an interpenetrating network of a ductile metal phase which melts at temperature higher than Al and, that does not exhibit the previously discussed disadvantages.
According to the invention, this task is achieved by a ceramic metal composite with an Al
2
O
3
matrix and embedded metal wherein the Al
2
O
3
matrix of the sintered composite is permeated by an interpenetrating network of a ductile metal phase melting at higher temperatures than aluminum that accounts for 15 to 80 vol % of the total volume, the Al
2
O
3
matrix forming an interconnecting network with a volume content of 20 to 85 vol % and wherein the material contains 0.1 to 20 atom-% aluminum with respect to the

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