Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
Patent
1994-09-30
1997-03-04
Fiorilla, Christopher A.
Plastic and nonmetallic article shaping or treating: processes
Including step of generating heat by friction
264 65, 264 66, 419 10, 419 13, 419 14, 419 19, 501108, 501127, 501 89, 501 96, 501 97, 501 98, C22C 3200, C04B 3558, C04B 3503
Patent
active
056076304
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a process for the production of fine-grained ceramic molded bodies containing Al.sub.2 O.sub.3 using aluminum metal in a powder form.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a tool for reforming cylindrical disks-shaped ceramic molded bodies according to the invention.
The production of Al.sub.2 O.sub.3 ceramics using metallic aluminum is known. Thus such a process is described in U.S. Pat. No. 5,024,795 which is based on directed melt oxidation. Processes are described in DE-A-38 12 266, 40 17 262, 40 39 530, 40 39 531 and 41 26 738 which are based on the principle of oxidizing mixtures of aluminum metal and Al.sub.2 O.sub.3 at temperatures above the melting point of aluminum and mainly at temperatures above 900.degree. C. Ceramic moulded bodies manufactured according to these processes are characterized by a particularly low shrinkage, high green strength and grain boundaries free of glass phases. However, a disadvantage of these processes is that it is not possible to embed oxidizable metallic shaped parts in the ceramics, in particular aluminum wires, without these being destroyed during the heat treatment. This process is also not suitable for a superplastic forming of the bodies obtained or for processing by the flame spraying process. Moreover the necessity of always having to work with an addition of Al.sub.2 O.sub.3 in the known processes is a disadvantage with regard to the manufacture of particularly crack-resistant and very finely pored bodies. Thus according to the teaching of the aforementioned DE-A documents at least 50 vol. % Al.sub.2 O.sub.3 is required for passivation, as an aid to sintering and for the complete conversion of the aluminum metal.
The object of the invention is therefore to create an improved process for the production of such Al.sub.2 O.sub.3 ceramic moulded bodies which eliminates or reduces the aforementioned disadvantages and can also be carried out without addition of Al.sub.2 O.sub.3.
This object is achieved according to the invention by a process for the production of a fine-grained ceramic molded body containing at least 10 vol. % Al.sub.2 O.sub.3 which is characterized in that O.sub.3 or/and, if desired, further suitable inorganic substances for ceramic formation in a non-aqueous liquid in the presence of oxygen until 10 to 80% of the metallic aluminum powder has reacted to form one or several aluminum oxidation precursors from the group amorphous Al.sub.2 O.sub.3, .gamma.-Al.sub.2 O.sub.3 and aluminum-organic compounds and the powder exhibits a maximum oxidation rate between 510.degree. and 610.degree. C. according to differential thermoanalytical measurement temperature which is below the melting temperature of aluminum until at least 50% of the metallic aluminum powder remaining after the milling has reacted to Al.sub.2 O.sub.3 and subsequently
The invention is based on the surprising finding of a connection between the maximum oxidation rate of aluminum powder at temperature below the melting point of aluminum (660.degree. C.) and the extremely fine granularity of the ceramic molded body obtained as an oxidation product that is achieved after oxidation by heat treatment in an oxygen-containing atmosphere. As a result of this extremely fine granularity of the ceramic molded body obtained according to the invention 1. superplastic forming of the body is possible at relatively low temperatures and pressures and 2. metallic shaped bodies such as wires and sheet bars can be embedded in the green body in step (b) which retain their shape and their metallic character in the subsequent oxidation treatment and sintering in steps (c) and (d). Steps (b), (c) and (d) of this process can also be carried out by flame spraying or plasma spraying in which the ceramic molded bodies are obtained in the form of spray layers which have a finer particle size than that which was previously obtainable.
An essential feature of the invention is the intensive wet milling of the aluminum powder carried out in step (a) of
REFERENCES:
patent: 3676309 (1972-07-01), Dolomont
patent: 4587225 (1986-05-01), Tsukuma et al.
patent: 4938928 (1990-07-01), Koda et al.
Journal of the American Ceramic Society, Bd. 76, Nr. 4, Apr. 1993, Westervill, Ohio, US, pp. 970-980 XP360957, S-WU et al. `Mechanisms And Kinetics Of Reaction-Bonded Aluminum Oxide Ceramics`.
Journal of the European Ceramic Society, Bd. 5, Nr. 1, 1989, pp. 29-35 XP70883, Claussen et al., `Low-Shrinkage Reaction-Bonded Alumina`.
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