Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Including plural heating steps
Patent
1997-10-09
1999-01-05
Fiorilla, Christopher A.
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Including plural heating steps
264674, C04B 3332
Patent
active
058558420
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for the production of dense polycrystalline silicon carbide shaped articles and the superior shaped articles produced by the process. In one aspect the invention provides a new firing cycle for the production of dense polycrystalline silicon carbide shaped articles. In accordance with this firing cycle, powder compacts containing silicon carbide and alumina or a precursor thereof are first heated to an intermediate temperature, as will be explained in more detail below. After an extended dwell at the intermediate temperature, the temperature is raised in a second stage to a higher temperature to complete the densification process.
In the second stage, magnesia or a precursor is thereof may optionally be present as a sintering assist, and the atmosphere is essentially carbon monoxide. It is preferred that the original compact contains magnesia, or a precursor thereof, but the magnesia may be introduced during the firing cycle. Depending upon the particular conditions, it may also be advantageous to have a controlled amount of carbon present, as will be explained in more detail below. In a preferred embodiment of the invention, the first stage is carried out in an inert atmosphere such as argon, and the atmosphere is changed after the intermediate temperature dwell, for example by purging the furnace chamber and introducing carbon monoxide for the remainder of the firing cycle.
BACKGROUND OF THE INVENTION
Shaped articles comprising high density polycrystalline silicon carbide are well known. They are characterised by excellent physical properties such as high resistance to thermal shock, abrasion and oxidation together with high levels of strength and thermal conductivity. It is this combination of properties which makes silicon carbide materials leading candidates for engineering applications. However, the production of satisfactory high density materials has been fraught with difficulties.
Early workers (eg. Alliegro, Coffin and Tinkepaugh J. Amer. Ceram. Soc., 39 high density bodies with the aid of sintering aids such as aluminium and iron, and aluminium plus one of the metals zirconium, boron or iron. It was further disclosed that for the hot pressing of silicon carbide, magnesium additions and magnesium and aluminium additions were ineffective, and impaired the densification process as compared to a control sample of silicon carbide, hot pressed under identical conditions disclosed the hot pressing of silicon carbide using alumina as the densification aid. The limitations of hot pressing for the attainment of dense bodies are well known.
The selection of suitable densification aids for the sintering of silicon such as boron, aluminium, iron, nickel and cobalt could be effective densification aids. Using these principles, alumina, beryllia, yttria, hafnia and rare earth oxides are considered to be potential densification aids as they do not decompose silicon carbide during sintering. Metal oxides including zirconia, calcia, magnesia are not considered suitable as they tend to decompose silicon carbide to metallic silicon. In addition the use of carbon with metal oxide additions was reported to be beneficial for oxides such as alumina, beryllia, ytrria, rare earth oxides, calcia, zirconia, and hafnia. It is stated that the carbon is added to react with the said oxides to form the corresponding metal carbide and silicon metal. The formation of the metal carbides was seen as desirable. In the process according to the present invention, the formation of such metal carbides was not observed. Furthermore, in contrast to the work of Negita, in the current work it has been found that the reaction of carbon with the metal oxide densification aids is undesirable and impairs the densification of the bodies. This indicates that the role of carbon in the present work is different to that proposed by Negita and others. In addition, given the unstable nature of metal carbide phases, for some even in air at room temperature, the formation of such phases is seen as undesirable and a
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patent: 4829027 (1989-05-01), Cutler et al.
Phase Transformation and Thermal Conductivity of Hot Pressed Silicon Carbide Containing Alumina and Carbon-Tosikazu Sakai, Journal of the American Ceramic Society, vol. 71, No. 1, issued Jan. 1988, pp. C-7 to C-9.
International Search Report, International Application No. PCT/AU92/00661.
Oh Chull Hee
Trigg Mark Brian
Commonwealth Scientific and Industrial Research Organisation
Fiorilla Christopher A.
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