Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1991-06-20
1993-08-17
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
501 88, 264 65, C04B 3556
Patent
active
052368750
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to the production of dense articles of SiC.
BACKGROUND OF THE INVENTION
Shaped articles comprising polycrystalline SiC are known. They are characterized 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 SiC materials leading candidates for engineering applications. However, this combination of properties only concurs in high density materials.
During high temperature heat treatments of prerequisite powder compacts, a reduction in the surface energy of the system can occur. The reduction in surface energy is through the diffusion of atoms by either grain boundary diffusion and subsequent densification, or by grain growth through surface diffusion mechanisms with virtually no macroscopic densification. At the high temperatures required for the sintering of SiC powder compacts, surface diffusion typically prevails over grain boundary diffusion. This results in coarsening of the SiC grains in a powder compact with little macroscopic densification taking place.
The oldest process for production of dense articles of SiC is that of reaction sintering, in which silicon liquid or vapour is infiltrated into a compacted body of SiC powder and C. The Si reacts with the C to form SiC in situ which bonds the powder particles. However, this process typically leaves from 8 to 12 volume percent of free Si, which sets a maximum operating temperature of about 1300.degree. to 1400.degree. C. for the resultant densified article.
In more recent times, attention has been directed to the use of certain additives which promote grain boundary diffusion over surface diffusion for pressureless sintering of SiC. However, apart from B or certain compounds thereof, found to be effective in increasing grain boundary diffusion, there does not appear to have been any successful proposal, at least in terms of commercial utility. Moreover, even with use of B or a B compound, problems still exist.
In use of B or a B compound, C usually is added as disclosed in U.S. Pat. Nos. 4,004,934, 4,041,117 and 4,108,929 all to Prochazka and U.S. Pat. No. 4,124,667 to Coppola et al. It is indicated that the C reduces the surface SiO.sub.2 layer on the SiC powder to SiC and CO. In U.S. Pat. No. 4,041,117, Prochazka suggests that the SiO.sub.2 can halt densification of SiC compacts so that little or no shrinkage can occur. Prochazka also suggests that the addition of C can limit exaggerated grain growth during densification. However, he further indicates that grain growth can only be inhibited by strict control of temperature and pressure within narrow limits. Also, the final product usually contains C particles in the microstructure which can lead to degradation of mechanical properties of the product.
The literature on effective sintering aids for SiC powder, other than B or B compounds, is credited as having commenced with Alliegro et al, J. Amer. Ceram. Soc. 39 [11] 386-89 (1056). This reference discloses that 1% Al addition to .alpha.- or .beta.-SiC powder enables densification by hot-pressing to about 98% of the theoretical density. The .beta.-SiC powder was synthesised from a Si/C mixture, in which case, the Al usually was added to that mixture as oxide that was reduced during the synthesis. With use of .alpha.-SiC powder, the Al evidently was added as the metal powder. Alliegro et al report that Fe, Li, Ca and Cr also aided densification, but that Mg, Ta, Co, Ba, Mo, W, Sr and Cu were not beneficial whether used alone or with Al.
Artemova et al, in Neoroanicheskie Materialy, Vol. 10, No. 12, pp 2228-9, Dec. 1974, report on the preparation of a densified product by shock compression employing an explosive charge. Powdered SiC/Al.sub.2 O.sub.3 mixtures ranging from 10/90 to 90/10 mole percent, in 10 mole percent increments, were used and attained in excess of 98% of the theoretical density for the mixtures. This mode of densification, having some si
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Dietrich Rainer
Drennan John
Hay David G.
Oh Chull H.
Trigg Mark B.
Bell Mark L.
Foseco Pty. Ltd.
Gallo Chris
Western Mining Corporation Ltd.
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