Compositions: ceramic – Ceramic compositions – Refractory
Patent
1997-01-31
2000-05-23
Marcheschi, Michael
Compositions: ceramic
Ceramic compositions
Refractory
501128, 501132, 501153, 264653, 264603, 264645, C04B 3510
Patent
active
06066584&
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the field of ceramics and concerns sintered Al.sub.2 O.sub.3 materials and processes for their production. The sintered materials can be used as medical implants, as wear products, as cutting tools or as abrasives.
The intensive efforts of recent years aimed at producing monolithic sintered Al.sub.2 O.sub.3 products having grain sizes of less than 2 .mu.m and improved mechanical properties from corundum powder followed essentially two routes: full density.
Although the very fine starting materials have the desired high sintering activity, owing to their poor densification behaviour they at the same time bring with them considerable problems in shaping. The low-cost uniaxial dry pressing process, which has hitherto been predominantly employed, leads to insufficient green density or to density inhomogeneities in the shaped body and, on sintering, defects which reduce the hardness and strength. For these reasons, other shaping processes such as cold isostatic pressing, extrusion, pressure or centrifugal casting or gel casting are employed.
An Al.sub.2 O.sub.3 grain size in the sintered body of 0.8 .mu.m and a hardness of HV20=1920 have been achieved by extrusion of ceramic compositions comprising submicron powders and sintering at 1400.degree. C. (G. Riedel, et al., Silicates Industriels (1989) 1/2,29-35). The powder used here had a mean particle size of 0.45 .mu.m and a d.sub.84 value, which describes the width of the coarse particle fraction of the particle size distribution, of 1.0 .mu.m.
In the case of very fine Al.sub.2 O.sub.3 powders having d.sub.50 <0.4 .mu.m and a d.sub.84 value, which describes the width of the coarse particle fraction of the particle size distribution, of <0.7 .mu.m, slip casting has also been successfully used recently (T.-Sh. Yeh et al., J. Am. Ceram. Soc. (1988), pp. 841-844), also in combination with the application of pressure (pressure filtration: F. F. Lange et al., Bull. Am. Ceram. Soc. (1987), pp. 1498-1504; Vacuum Pressure Filtration: H. Mizuta et al., J. Am. Ceram. Soc. (1992), pp. 469-473). As shown, for example, by Mizuta et al., these complicated processes enable the best mechanical properties up to now for pure sintered corundum to be achieved, but according to this prior art no Vickers low-load hardnesses of .gtoreq.2000 or flexural strengths of .gtoreq.800 MPa were measured even when hot isostatic pressing (HIPping) was employed.
The usefulness of pressureless casting processes such as gel casting or enzyme-controlled coagulation for producing pure Al.sub.2 O.sub.3 ceramics has hitherto been restricted to corundum powders having mean particle sizes of more than 0.4 .mu.m (A. C. Young, et al., J. Am. Ceram. Soc. (1991)3, 612-618), so that the mean grain sizes of the dense sintered microstructures produced were always greater than 1.5 .mu.m. In the example cited, the strengths achieved remained below 300 MPa.
The opportunities for improving the mechanical properties and the grain fineness of sintered Al.sub.2 O.sub.3 products produced from corundum powder by addition of substances which promote sintering are very limited. The temperature required for sintering submicron Al.sub.2 O.sub.3 powders to full density is, in particular, reduced to 1200.degree. and less by addition of more than 1% of dopants which form liquid phases during sintering, but the strengths remain at the level usual for traditional sintered corundum, viz. about 400 MPa (L. A. Xue et al., J. Am. Ceram. Soc., (1991), pp. 2011-2013), and the widespread formation of grain boundary phases brings with it unfavourable high-temperature properties.
Although the relationship between defect structure and strength of brittle solids has been well known for a long time, most studies are restricted to the purely qualitative determination of relevant defect types; even a characterization of only relative defect size distributions (H. E. Exner et al., Mater. Sci. Eng. 16 (1974), pp. 231-238) is rare. As regards the technology dependence of the actually important absolute defect freque
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Blank Paul
Krell Andreas
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Marcheschi Michael
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