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Microstructural Development to Toughen Sic

Published online by Cambridge University Press:  10 February 2011

W. J. Moberlychan ,
R. M. Cannon ,
L. H. Chan ,
J. J. Cao ,
C. J. Gilbert ,
R. O. Ritchie  and
L. C. De Jonghe
W. J. Moberlychan
Affiliation:
Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720
R. M. Cannon
Affiliation:
Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720
L. H. Chan
Affiliation:
Komag, Inc., 591 Yosemite Dr., Milpitas, CA 95035
J. J. Cao
Affiliation:
Materials Science & Engineering Dept., UC Berkeley, Berkeley, CA 94720
C. J. Gilbert
Affiliation:
Materials Science & Engineering Dept., UC Berkeley, Berkeley, CA 94720
R. O. Ritchie
Affiliation:
Materials Science & Engineering Dept., UC Berkeley, Berkeley, CA 94720
L. C. De Jonghe
Affiliation:
Materials Science & Engineering Dept., UC Berkeley, Berkeley, CA 94720
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Abstract

SiC offers a promise for high strength applications at high temperature; however, poor fracture resistance has inhibited its utility. Recent developments to control microstructure during hot pressing have improved fracture toughness >3 fold, while also improving strength 50% above that of a commercial SiC, Hexoloy. This ABC-SiC (designated for the Al, B, and C additives) utilizes liquid phase sintering to obtain full densification at 1650°C, and to induce the β-3C to α-4H phase transformation below 1900°C. Interlocking, plate-like, α grains, coupled with a thin (˜1 nm) amorphous layer, provide for tortuous intergranular fracture and high toughness.

This study focuses on the developing microstructure; how the α-4H polytype grows as a stacking modification of the β-3C grains, and how amorphous grain boundaries and crystalline triple point phases develop and interact with the crack geometry. HR-TEM and Image-Filtered EELS characterize the amorphous grain boundaries. Field Emission - SEM, EDS and Auger Electron Spectroscopy characterize the fracture morphology and the chemistry of grain boundaries and triple points. Electron Diffraction and HR-TEM depict an epitaxial relationship between triple point phases (Al8B4C7 and A14O4C) and matrix α-SiC grains, the development of which affects the mechanical toughening. The transformation to toughen SiC is compared to the well-studied transformation processing in Si3N4. A distinct advantage is the interlocked nature of the plate-like grains which causes strong elastic bridging behind the crack tip.

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References

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