1634 MOBERLYCHAN et aL: ROLES OF AMORPHOUS GRAIN BOUNDARIES phase pockets, >1 um in size. Cracks propagating Gilbert, M. Gopal, R. Ritchie and G. Thomas for their along grain boundaries were further deflected by ical assistance and discussions. M. Sixta is also small(5-10 nm) crystalline triple junctions [36]. authors were grateful for use of the facilities of the over-sintering at temperatures National Center for Electron Micr >1900C led to small voids at triple junctions, for the assistance of staff members D. Ah-Tye, C. Nelson which may correlate to reductions in strength and and J. Turner toughness [8]. Nevertheless, nanometric-scale voids at triple junctions would be expected to have less detrimental effects on strength than other toughen- REFERENCES ing mechanisms, such as pre-existing cracks and /or 10-20% of a weaker phase. 1. Suzuki. K. Pressureless-sintered silicon carbide with The deflection round elongated in Silicon Carbide grains [Fig. 1(b) and the observation of crack brid ics 2, ed S Somy Inomata. elsevier ging [Fig. 2(a) have provided a similar toughening 2 Science. amsterdam 1,pp.163-18 M. A. and Krstic. V.D.J. Mater. Sci. 1994 mechanism as reported for the Y AG-SiC ceramics 29,934-938 However. a lower volume fraction of additives in 3. Mulla, M. A. and Krstic, V.D., Acta metall. mater ABC-SiC corresponded to a more confined and 1994,42,303-308 han in YAG-Sic 4. Kim. D. H. and Kim, C. H. J. Am. Ceram. So 1990,73,1431-143 ceramics [1-8]. The ABC-SiC exhibited less crack 5. Lee, S. K. and Kim. C.H. J. Am. Ceram. Soc branching than the YAG-Sic ceramics, especially 994,77,1655-165 far from the main crack surface. Also the interlock- 6. Padture, N. P.,J.Am. Ceram. Soc., 1994, 77, ing nature of the plate-like ABC-Sic grains hin- 7. Padture. N P and Lawn. B. R.J. Am. Ceran. Soc dered a simple pullout mechanism as have been 994.77,2518-2522 proposed for the fiber-like grains of Si3N4 [10, 11]. 8. Cao, J. J, Moberly Chan, W.J., De Jonghe, L.C he presence of fewer short cracks, a lower volume An. Ceram fraction of secondary phases, and an interlocking Soc.1996,79,46l-469 rain morphology provided coexisting high strength 10. Li. C.W. Lee. D- 1. and Lui. s- C.D. d.n. Ceran and high toughness in this ABC-SiC [8, 24] Both the recently developed ABC-SiC and monosilicate matrix composites. American Ceramic ommercial SiC(Hexoloy-SA) had an a hexagonal crystal structure and a grain size ranging from C. H. Acta metall. mater, 1990. 38, 403-409 10 um. In addition, both materials exhibited an 14 J. D, Shetty, D. K, Griffin, C. w. and amorphous phase at the grain boundaries, resulting ge, S.Y., J.Am from the sintering additives used. However, the 15. Clarke. D.R.J. 4m. Ceram Soc., 1987. 70, 15-22 ABC-SiC started with submicron B powder and 16. Kleebe, H K. Cannon.R. M. and sed sintering additives (Al, B and c) which enabled liquid phase sintering at temperature 17. Mitchell. T. D J and Ritchie. R.,J. Am. Ceram. Soc. 1995. 78 below which are typically reported for SiC. The B- to-z phase transformation could be controlled at 18. Cao, J.J., Moberly Chan, W.J., De Jonghe, L.C these lower temperatures to produce an interlock- Dalgleish. B. and Niu. M. Y. in ing, plate-like microstructure consisting of a-4H Matrix Composites 11, Ceramic Trans. Publications, Westerville OH, 1995, pp grains rather than the equiaxed a-6H grains in 19. MoberlyChan, W.J, Cao, J. J, Niu, M. Y and De Hexoloy-SA. The elongated grains(with an aspect Jonghe. L. C, Toughened B-Sic ratio upwards of 10)in the ABC-SiC cracks to deflect through the weaker ame Composites, ed. K. K. Chawla, P. K. Liaw and S phase at the grain boundaries, resulting f grains behind the propagating crack tip 20. Lin, B-W.. Imai. M. Yano. T and Iseki, T.,J. Am Ceran.Soc,1986,69,C67-C68 microstructure enhanced the toughness of ABC-Sic 21. MoberlyChan,W.J,Cao,J.J,Niu,M.Y,De y a factor of three over that of the commercial Jonghe, L C and Schwartzman, A F, SiC cor Hexoloy-SA as well as providing >50% improve- sites with alumina-coated a-SiC platelets in B-SiC ment in strength ontrolling toughness through microstructure. Microbeam Analysis Proceedings, ed. J. Friel VCH, New York, 1994, pp 49- Acknowledgements-This work was Director, Office of Energy Research. 。 of Basic 23. Chia, K. Y. and Lau, S. K, Cera. Engng Sci. Proe.,1991,12.1845-1861 Laboratory. The authors wish to C. ct No. 24. Gilbert, C J, Cao, J, J, Moberly Chan, W J,De DE-AC03-76SF00098 with the Lawrence Jonghe. L. C. and Ritchie, R. O. Acta metall. mater,1996,44,3199-3214.phase pockets, >1 mm in size. Cracks propagating along grain boundaries were further de¯ected by small (5±10 nm) crystalline triple junctions [36]. However, ``over-sintering'' at temperatures >19008C led to small voids at triple junctions, which may correlate to reductions in strength and toughness [8]. Nevertheless, nanometric-scale voids at triple junctions would be expected to have less detrimental e€ects on strength than other toughen￾ing mechanisms, such as pre-existing cracks and/or 10±20% of a weaker phase. The de¯ection of the crack around elongated grains [Fig. 1(b)] and the observation of crack brid￾ging [Fig. 2(a)] have provided a similar toughening mechanism as reported for the YAG±SiC ceramics. However, a lower volume fraction of additives in ABC±SiC corresponded to a more con®ned and more dramatic toughening than in YAG±SiC ceramics [1±8]. The ABC±SiC exhibited less crack branching than the YAG±SiC ceramics, especially far from the main crack surface. Also the interlock￾ing nature of the plate-like ABC±SiC grains hin￾dered a simple pullout mechanism as have been proposed for the ®ber-like grains of Si3N4 [10, 11]. The presence of fewer short cracks, a lower volume fraction of secondary phases, and an interlocking grain morphology provided coexisting high strength and high toughness in this ABC±SiC [8, 24]. 4. SUMMARY Both the recently developed ABC±SiC and the commercial SiC (Hexoloy±SA) had an a hexagonal crystal structure and a grain size ranging from 3± 10 mm. In addition, both materials exhibited an amorphous phase at the grain boundaries, resulting from the sintering additives used. However, the ABC±SiC started with submicron b powder and used sintering additives (Al, B and C) which enabled liquid phase sintering at temperatures below which are typically reported for SiC. The b￾to-a phase transformation could be controlled at these lower temperatures to produce an interlock￾ing, plate-like microstructure consisting of a-4H grains rather than the equiaxed a-6H grains in Hexoloy±SA. The elongated grains (with an aspect ratio upwards of 10) in the ABC±SiC enabled cracks to de¯ect through the weaker amorphous phase at the grain boundaries, resulting in bridging of grains behind the propagating crack tip. This microstructure enhanced the toughness of ABC±SiC by a factor of three over that of the commercial Hexoloy±SA as well as providing >50% improve￾ment in strength. AcknowledgementsÐThis work was supported by the Director, Oce of Energy Research, Oce of Basic Energy Sciences, Materials Sciences Division, of the United States, Department of Energy under Contract No. DE-AC03-76SF00098 with the Lawrence Berkeley Laboratory. The authors wish to thank R. Cannon, C. Gilbert, M. Gopal, R. Ritchie and G. Thomas for their technical assistance and discussions. M. Sixta is also acknowledged for a critical reading of this paper. 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