Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1995-06-20
1997-09-09
Group, Karl
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
501 93, 501 963, C04B 35577
Patent
active
056656610
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method for producing silicon carbide ceramics.
BACKGROUND ART
Glass-free silicon carbide ceramics are known for maintaining their strength at high temperatures. They are made by sintering silicon carbide powder containing boron and carbon additives at temperatures in excess of 2000.degree. C., which, in contrast to the majority of other ceramics, results in a dense ceramic that contains no amorphous or glassy phases between the grains. Because SiC is a covalently bonded material, it is difficult to sinter by solid state diffusion processes. The addition of boron and carbon has been found to enhance the surface diffusion of the grains, which allows the material to sinter, although at very high temperatures.
Recently, attempts have been made to liquid-phase sinter SiC at lower temperatures by using oxide additives such as Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3. The goal has been to add these oxides in a ratio that produces YAG (yttrium aluminum garnet) upon cooldown from sintering in order to obtain a totally crystalline product. A material that has been commercialized using this method is Carborundum HEXALOY-SX. However, the oxide sintering aids react with the surface of the silicon carbide to yield a silicate glass. This glass is difficult to crystallize, and a thin amorphous layer remains between the grains. Consequently, the major problem with this approach is that the excellent high temperature properties of SiC are compromised due to the presence of a glassy film that results from the addition of the oxides.
SUMMARY OF THE INVENTION
The process of this invention for crystallizing the grain boundaries of a silicate glass-containing silicon carbide composition comprises heating a composition comprising (1) from about 20% to about 99% silicon carbide, (2) from 0.5% to about 20% of a silicate glass, and (3) from 0.001% to about 80% of a high metal content silicide of a transition metal of IUPAC groups 3, 4, 5, 6, 7, 8, 9, 10 or 11, all percentages being by weight based on the total weight of the composition, to a temperature of 1300.degree. to 2100.degree. C. under vacuum until oxygen is removed from the silicate glass as SiO gas and the glass that remains within the silicon carbide ceramic crystallizes.
The starting material for the process of this invention can be (1) a powder mixture of silicon carbide, a silicate glass-forming sintering aid and a transition metal or transition metal compound, (2) a powder mixture of silicon carbide, a silicate glass-forming sintering aid and a high metal content silicide, or (3) a sintered ceramic comprising silicon carbide, a silicate glass and a high metal content transition metal silicide.
DETAILED DESCRIPTION OF THE INVENTION AND REFERRED EMBODIMENTS
It has been discovered that the grain boundaries of a silicate glass-containing silicon carbide composition can be crystallized by heating the composition to a temperature of 1300.degree.-2100.degree.C. under a vacuum in the presence of a high metal content transition metal silicide until oxygen is removed from the silicate glass as SiO gas, and the glass that remains within the silicon carbide ceramic crystallizes.
The process of this invention for eliminating the glassy grain boundary phases that result from the use of oxide additives in the liquid phase sintering of silicon carbide ceramics takes advantage of the catalytic nature of certain high metal content transition metal silicides, under certain conditions, in removing oxygen from these glassy grain boundaries, with resultant crystallization of the glass. The conditions required are processing under a vacuum and at a temperature high enough to form SiO gas as the oxidation product of the metal silicide. The catalytically active silicides suitable for the practice of this invention are high metal content silicides that oxidize in such a fashion that oxygen-silicon bonds are formed in preference to oxygen-metal bonds. High metal content silicides are defined as transition metal silicides that have a metal to sil
REFERENCES:
patent: 5204294 (1993-04-01), Matsumoto
patent: 5281564 (1994-01-01), Matsumoto
A. I. Burykina, L. V. Strashinskaya, and T. M. Evtushok, "Investigation of the Interaction of Silicon Carbide with Refractory Metals and Oxides." Fiziko-Khimicheskaya Mekhanika Materialov, vol. 4, No. 3, pp. 301-305, 1968.
Group Karl
Lanxide Technology Company, LP
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