Microstructural Development to Toughen SiC
- PDF / 3,586,615 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 86 Downloads / 207 Views
MICROSTRUCTURAL DEVELOPMENT to TOUGHEN SiC W. J. MOBERLYCHAN*, R. M. CANNON*, L. H. CHAN**, J. J. CAO***, C. J.GILBERT***, R. 0. RITCHIE***, and L. C. DE JONGHE*** *Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720. **Komag, Inc., 591 Yosemite Dr., Milpitas, CA 95035. ***Materials Science & Engineering Dept., UC Berkeley, Berkeley, CA 94720. 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 13-3C to ct-4H phase transformation below 1900'C. Interlocking, plate-like, cXgrains, coupled with a thin (-I nm) amorphous layer, provide for tortuous intergranular fracture and high toughness. This study focuses on the developing microstructure; how the ct-4H polytype grows as a stacking modification of the 3-3C 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 (Al 8B4 C7 and A140 4 C) and matrix o•-SiC grains, the development of which affects the mechanical toughening. The transformation to toughen SiC is compared to the well-studied transformation processing in Si 3 N 4 . A distinct advantage is the interlocked nature of the plate-like grains which causes strong elastic bridging behind the crack tip. INTRODUCTION SiC can have the Zinc-blende (ZnS) cubic crystal structure common to many compound semiconductors or it can exhibit one of many hexagonal polytype structures based on the Wurzite crystal structure. Numerous studies have observed a transformation from 3-to-cL at temperatures near 2000'C, with the seminal work of Mitchell, Heuer, et al [1-4] presenting the most thorough evaluation of the P3-3C - to - ct-6H structure. Theoretical energy calculations have determined the 6H polytype (ABCACBABCACB... stacking of Si planes with C atoms in 1/2 the tetrahedral interstices) to be the equilibrium phase above 2000'C; however, other commonly observed polytypic structures are the a-4H (ABACABAC...), the ot-15R (ABACABCBABCACBC...), and the P3-3C (ABCABCABC...) phases. Various impurities in SiC can affect not only the particular polytype phase that is present at a given temperature but also the order in which the phase transformation(s) occur [5-7]. The processing of dense polycrystalline SiC requires the use of sintering additives, often present in concentrations upwards of 10 vol%; and these additi
Data Loading...
