Acta mater. VoL 46, No 5, pp 1625-1635. 1998 8y丿 Pergamon Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PI!:Sl359-6454(97)00343-1 1359-6454/98s19.00+0.00 THE ROLES OF AMORPHOUS GRAIN BOUNDARIES AND THE B-a TRANSFORMATION IN TOUGHENING SiC W.J. MOBERLYCHANT J.J. CAO and L C DE JONGH r for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720 and Department laterals Science and Mineral Engineering. University of California at Berkeley. Berkeley, CA 94720.USA Received 22 July 1996, accepted 12 September 1997) Abstract--Controlled development of the ceramic microstructure has produced silicon carbide (SiC)with ughness three times that of a commercial SiC. Hexoloy-SA, coupled with 50% improvement in Stength. AL, B and C were used as sintering additives, hence the designation ABC-SiC. These additives fa- itated full densification at temperature as low as 1700C, the formation of an amorphous phase at the grain boundaries to enhance inter lar fracture, and the promotion of an elongated microstructure to enhance crack deflection and crack bridging. Comparisons of microstructures and fracture properties have een made between the present ABC-SiC, Hexoloy-SA and other reported Sic ceramics sinter YAG or Al2O]. The Alo chemistry of the amorphous phase in the ABc-Sic accounted for the nular fracture vs the transgranular fracture in Hexoloy-SA. An interlocking, plate-like grain integra eveloped during the B to a transformation without limiting densification. The combined microstructural developments improved both strength and toughness. 1998 Acta Metallurgica Inc. 1 INTRODUCTION phous phas I nm [15, 16. This would al quantity of additives is phase-sintering: sufficient to involved"in situ toughening"via the formation of oat grain boundaries, yet limiting the final volume plate-like grains during the transformation from the fraction of secondary phases -cubic to the az-hexagonal crystal structure This study has characterized microstructural (Suzuki [] Mulla and Krstic [2,3]. Lee and differences and commonalities between a commer- Kim [4, 5]. Padture and Lawn [6, 7] and Cao et cial SiC(Hexoloy-SA, Carborundum, Inc, Niagara [8D. An elongated grain structure, coupled with Falls, NY, U.S.A )and a recently developed Sic intergranular fracture, provided a tortuous crack (subsequently referred to as ABC-Sic [8 indicating path and a toughening mechanism similar to that toughened with plate-like grains formed during the obtained for silicon nitride [9-1l]. The transform- B to a transformation. Microstructural comparisons ation and elongated microstructure has been have also been made to other Sic ceramics (referred induced in SiC by additives which promoted liquid to as AlO3-SiC when Al2O, is the major sintering phase sintering at temperatures 200-400C lower additive[1-3], and YAG-Sic when the predomi- than the typical SiC sintering temperature of nant secondary phase is amorphous and/or crystal- -2100'C [1-8. In silicon nitride [9-lI] the use of line yttria-alumina-garnet [4-7D toughened by appropriate concentrations of sintering additives similar B to a transformations. Table I lists the and controlling the processing temperature have compositions and processing parameters reported also resulted in the formation of an amorphous for these high toughness phase at the enabled intergranular fracture and improved tough ness. Where a secondary phase coating provides a 2. MATERIALS PROCESSING AND weak interface and promotes crack bridging, in both monolithic and composite material system has been noted that the interfacial phase need Previously reported toughened Sic ceramics have ypically incorporated a significant volume fraction only slightly thicker than the interface roughness of of second phase(s), such as 5-20%A12O3 the strengthening fiber or platelet [11-14]. Grain 10-20% YAG [5, 7]. The ABC-SiC developed here boundary fracture could be induced by an amor- utilized less sintering additives: 3%AL. <1%B and x2% C. Although secondary phases also tCurrent address: Komag, Inc, San Jose, CA 95131, resulted from these additives, predominantly the ternary phases AlgBC7 and Al4 CO4 [17-19THE ROLES OF AMORPHOUS GRAIN BOUNDARIES AND THE b±a TRANSFORMATION IN TOUGHENING SiC W. J. MOBERLYCHAN{, J. J. CAO2 and L. C. DE JONGHE1 1 Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720 and 2 Department of Materials Science and Mineral Engineering, University of California at Berkeley, Berkeley, CA 94720, U.S.A. (Received 22 July 1996; accepted 12 September 1997) AbstractÐControlled development of the ceramic microstructure has produced silicon carbide (SiC) with a toughness three times that of a commercial SiC, Hexoloy±SA, coupled with >50% improvement in strength. Al, B and C were used as sintering additives, hence the designation ABC±SiC. These additives fa￾cilitated full densi®cation at temperature as low as 17008C, the formation of an amorphous phase at the grain boundaries to enhance intergranular fracture, and the promotion of an elongated microstructure to enhance crack de¯ection and crack bridging. Comparisons of microstructures and fracture properties have been made between the present ABC±SiC, Hexoloy±SA and other reported SiC ceramics sintered with YAG or Al2O3. The Al0O chemistry of the amorphous phase in the ABC±SiC accounted for the intergra￾nular fracture vs the transgranular fracture in Hexoloy±SA. An interlocking, plate-like grain structure developed during the b to a transformation without limiting densi®cation. The combined microstructural developments improved both strength and toughness. # 1998 Acta Metallurgica Inc. 1. INTRODUCTION Recent development in the processing of silicon car￾bide (SiC) for improved fracture resistance have involved ``in situ toughening'' via the formation of plate-like grains during the transformation from the b-cubic to the a-hexagonal crystal structure (Suzuki [1], Mulla and Krstic [2, 3], Lee and Kim [4, 5], Padture and Lawn [6, 7] and Cao et al. [8]). An elongated grain structure, coupled with intergranular fracture, provided a tortuous crack path and a toughening mechanism similar to that obtained for silicon nitride [9±11]. The transform￾ation and elongated microstructure has been induced in SiC by additives which promoted liquid phase sintering at temperatures 200±4008C lower than the typical SiC sintering temperature of 021008C [1±8]. In silicon nitride [9±11], the use of appropriate concentrations of sintering additives and controlling the processing temperature have also resulted in the formation of an amorphous phase at the grain boundaries, which has thereby enabled intergranular fracture and improved tough￾ness. Where a secondary phase coating provides a weak interface and promotes crack bridging, in both monolithic and composite material systems, it has been noted that the interfacial phase need be only slightly thicker than the interface roughness of the strengthening ®ber or platelet [11±14]. Grain boundary fracture could be induced by an amor￾phous phase as thin as 1 nm [15, 16]. This would in￾dicate that a minimal quantity of additives is desirable for liquid-phase-sintering: sucient to coat grain boundaries, yet limiting the ®nal volume fraction of secondary phases. This study has characterized microstructural di€erences and commonalities between a commer￾cial SiC (Hexoloy±SA, Carborundum, Inc., Niagara Falls, NY, U.S.A.) and a recently developed SiC (subsequently referred to as ABC±SiC [8] indicating the sintering additives used). This ABC±SiC was toughened with plate-like grains formed during the b to a transformation. Microstructural comparisons have also been made to other SiC ceramics (referred to as Al2O3±SiC when Al2O3 is the major sintering additive [1±3], and YAG±SiC when the predomi￾nant secondary phase is amorphous and/or crystal￾line yttria±alumina±garnet [4±7]) toughened by similar b to a transformations. Table 1 lists the compositions and processing parameters reported for these high toughness SiC ceramics. 2. MATERIALS PROCESSING AND CHARACTERIZATION Previously reported toughened SiC ceramics have typically incorporated a signi®cant volume fraction of second phase(s), such as 5±20% Al2O3 [1, 2] or 10±20% YAG [5, 7]. The ABC±SiC developed here utilized less sintering additives: 03% Al, <1% B and 02% C. Although secondary phases also resulted from these additives, predominantly the ternary phases Al8B4C7 and Al4CO4 [17±19], the Acta mater. Vol. 46, No. 5, pp. 1625±1635, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S1359-6454(97)00343-1 1359-6454/98 $19.00 + 0.00 {Current address: Komag, Inc., San Jose, CA 95131, U.S.A. 1625