Anisotropies and mechanical properties of hot-pressed Al 2 O 3 and SiC-whisker/Al 2 O 3 composites
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The anisotropies and mechanical properties of hot-pressed AI2O3 and SiC-whisker/A12O3 composites were investigated. The c-axis of A12O3 crystals in monolithic AI2O3 was oriented in the direction of hot-pressing. The same tendency was observed in A12O3 matrices of composites, but orientation of A12O3 crystals was smaller than that of monolithic A12O3. SiC-whiskers obstructed plastic deformation and mass transfer of A12O3 matrices. The twist mode crack deflection was remarkable on fractured surfaces of polycrystalline A12O3 in which the main crack propagated perpendicular to hot-pressing direction. This was caused by the orientation of A12O3 crystals. This anisotropy resulted in a small difference in fracture toughness; however, other mechanical properties didn't change. The fracture mode of SiC-whisker/Al 2 O 3 composites was obviously changed, depending on the direction of crack extension as compared with the changes observed in monolithic A12O3. However, there was no difference in mechanical properties among the composites. Macroscopic fracture mode seemed to have less effect than microscopic fracture mechanism, such as whisker bridging and pull-out.
I. INTRODUCTION To toughen ceramics, SiC-whisker-reinforced ceramic composites have been investigated.1 Because structural ceramics for high temperature use are hard to be sintered in general and the addition of whiskers obstructs densifying of ceramic matrices, conventional hot-pressing has been used for sintering of whiskerreinforced ceramic composites. It is reported that SiC-whiskers in hot-pressed composites are distributed in two-dimensional random perpendicular to the hotpressing direction.2 Therefore, it can be predicted that there are anisotropies of texture and mechanical properties in hot-pressed composites. The anisotropies of properties in whisker-reinforced ceramic composites are reported in several papers.2"10 In these papers, two problems are pointed out: (i) In the case of Si 3 N 4 matrix composites, it seems to be difficult to discuss the effect of whisker addition and their orientation, because Si3N4 grains themselves grow into needle-like ones such as whiskers, (ii) The difference of fracture pattern caused by anisotropy has not been investigated in detail. Fiber orientation is one of the most basic and important factors of material design for fiber- or whisker-reinforced composites.11 In this paper, relationships between anisotropies and mechanical properties in hot-pressed A12O3 and SiC-whisker/Al 2 O 3 are investigated, and then the effect of whisker orienJ. Mater. Res., Vol. 9, No. 1, Jan 1994
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tation on the texture and fracture of the composite is discussed. II. EXPERIMENTAL A. Fabrication of materials Two kinds of SiC-whiskers were used in this study. One was manufactured by Tateho Chemicals Co. Ltd. as SCW#1-S (average diameter: 0.35 /z.m; average length: 17.1 yttm from manufacturer's catalog) and the other was made by Tokai Carbon Co. Ltd. as TWS400 (average diameter: 1.1 /urn; averag
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