《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_SELF-TOUGHENED-SIC

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 [17-19

THE 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

MOBERLYCHAN et aL: ROLES OF AMORPHOUS GRAIN BOUNDARIES Table 1. Processing parameters of toughened Sic additives crystal structure ABC-SiC8.17-19.36 3%Al,99%)at 1900 C. In ad- elsewhere [8, 24] dition, the size of the initial Al particles has been Vickers microhardness indentations were made correlated with the resulting size of regions of the on the polished surfaces of the ABC-Sic ceramic secondary phases [19, 21]. When added aluminum and the commercial Hexoloy-sA, and the lengths powders were >3 um in size, residual secondary and configurations of cracks emanating from the phases were common, with only a limited amount corners of the indents were examined using a the additives actually incorporated as the amor- SEMt. Although complete fracture of a bar with a phous grain boundary interlayer. controlled surface flaw has been shown to provide a Disks (2.5" in diameter) of the ABC-SiC were nore quantitative assessment hot pressed at 50 MPa, and at various temperature toughness [8, 24], observation of surface cracks ema- ranging from 1650 to 1950 C. Densification >98% nating from microhardness indentations provided a was achieved at all temperatures by modifying the good qualitative assessment of the toughness [25- concentration of the Al sintering additive. both pre. 27]. The crack in the Hexoloy-SA followed a rela- sintering anneals and post-sintering anneals have tively straight path [Fig. I(a).In contrast,the been investigated to determine how best to control cracks in the ABC-Sic [Fig. I(b)] exhibited deflec- are [8]. Beams, 3 mmx- 30 1 tions. Similar comparisons of crack paths have been ng, were sliced from the hot pressed disks reported between Hexoloy-SA and the Sic"in situ four-point bend tests to evaluate mechanical toughened the incorporation of 20% YA strength and fracture toughness. The tensile surfaces interpret higher toughness [7] were polished to a I um diamond finish SEM fractography on surfaces broken in four The Hexoloy-SA in this study was commercially point bend tests exhibits distinctive morphologies for the two sic materials. The surface of the abc- obtained from Carborundum. and the additives u process it have not been extensively di Sic exhibited intergranular fracture between elongated grains [Fig. 2(a)], with bridging regions cussed in the literature [22, 23]. The most prominent behind the crack tip Fig. I(b)]. The fractography of econdary phase observed in Hexoloy-SA in this the commercial Hexoloy-SA exhibited strictly study was graphite, which was detected both as par- transgranular fracture, with an overall smoothness ticulates within SiC grains and at large triple junc- similar to brittle glasses [Fig. 2(b)]. Dark regions tions. Also the porosity(>2-5%)in commercial observed by SEM of the Hexoloy-SA [Fig. I(a)and Hexoloy-SA appeared substantially greater than 2(b) were indicative of voids and occasionally sec- hat measured in the ABC-SiC. During polishing ondary phases for scanning electron microscope (SEM) obser- Bright field TEM* imaging defined major micro- vation and ion milling for transmission electron structural differences between ABC-SiC hot pressed microscope(TEM)sample preparation, the graphite at 1900 C and Hexoloy-SA(Figs 3 and 4, respect ively). Hot pressed ABC-SiC exhibited elongated TA Topcon ISI-DS130C was operated at 3-20 kv grains, with an aspect ratio >10 for the larger fa Philips EM400 was operated at 100 kv. grains, which were consistent with the seM obser

transformation and observed toughening were cor￾related to the presence of a thin amorphous phase along grain boundaries in ABC±SiC. The volume fraction of sintering additives used was a trade-o€ of processing parameters and properties. High aluminum content enhanced densi®cation, lowered sintering temperature and increased the amount of triple junction phases. A smaller Al concentration lowered the fraction of detrimental secondary phases and tended to improve high temperature strength. To provide densi®cation of beta SiC when hot pressed for 1 h at 16508C 5 wt%, Al was necessary [17, 20]; yet only 1% Al was sucient to provide densi®cation (>99%) at 19008C. In ad￾dition, the size of the initial Al particles has been correlated with the resulting size of regions of the secondary phases [19, 21]. When added aluminum powders were >3 mm in size, residual secondary phases were common, with only a limited amount of the additives actually incorporated as the amor￾phous grain boundary interlayer. Disks (2.50 in diameter) of the ABC±SiC were hot pressed at 50 MPa, and at various temperatures ranging from 1650 to 19508C. Densi®cation >98% was achieved at all temperatures by modifying the concentration of the Al sintering additive. Both pre￾sintering anneals and post-sintering anneals have been investigated to determine how best to control the microstructure [8]. Beams, 03 mm2 030 mm long, were sliced from the hot pressed disks for four-point bend tests to evaluate mechanical strength and fracture toughness. The tensile surfaces were polished to a 2±5%) in commercial Hexoloy±SA appeared substantially greater than that measured in the ABC±SiC. During polishing for scanning electron microscope (SEM) obser￾vation and ion milling for transmission electron microscope (TEM) sample preparation, the graphite is preferential removed, thereby confusing the volume fraction of porosity and graphite. The measured fracture toughness of the ABC± SiC, based on the controlled surface ¯aw method, was 7.1 MPaZm vs 2.2 for the Hexoloy±SA [8]. Moreover, measurements of bend strengths yielded a value of 0650 MPa for the ABC±SiC vs 0400 MPa for the commercial Hexoloy±SA. Thus the strength, and especially the fracture toughness, of ABC±SiC compared favorably with other SiC ceramics [1±7]. Further details of processing, characterization of microstructure and mechanical properties of these SiC ceramics have been detailed elsewhere [8, 24]. Vickers microhardness indentations were made on the polished surfaces of the ABC±SiC ceramic and the commercial Hexoloy±SA, and the lengths and con®gurations of cracks emanating from the corners of the indents were examined using a SEM{. Although complete fracture of a bar with a controlled surface ¯aw has been shown to provide a more quantitative assessment of the KIc toughness [8, 24], observation of surface cracks ema￾nating from microhardness indentations provided a good qualitative assessment of the toughness [25± 27]. The crack in the Hexoloy±SA followed a rela￾tively straight path [Fig. 1(a)]. In contrast, the cracks in the ABC±SiC [Fig. 1(b)] exhibited de¯ec￾tions. Similar comparisons of crack paths have been reported between Hexoloy±SA and the SiC ``in situ toughened'' via the incorporation of 20% YAG to interpret higher toughness [7]. SEM fractography on surfaces broken in four￾point bend tests exhibits distinctive morphologies for the two SiC materials. The surface of the ABC± SiC exhibited intergranular fracture between elongated grains [Fig. 2(a)], with bridging regions behind the crack tip [Fig. 1(b)]. The fractography of the commercial Hexoloy±SA exhibited strictly transgranular fracture, with an overall smoothness similar to brittle glasses [Fig. 2(b)]. Dark regions observed by SEM of the Hexoloy±SA [Fig. 1(a) and 2(b)] were indicative of voids and occasionally sec￾ondary phases. Bright ®eld TEM{ imaging de®ned major micro￾structural di€erences between ABC±SiC hot pressed at 19008C and Hexoloy±SA (Figs 3 and 4, respect￾ively). Hot pressed ABC±SiC exhibited elongated grains, with an aspect ratio >10 for the larger grains, which were consistent with the SEM obser￾Table 1. Processing parameters of toughened SiC Name Ref. Processing temperature Sintering additives Final crystal structure Secondary phases Grain length (m m) ABC±SiC [8, 17±19, 36] 1650±1950 3% Al, <1% B, 02% C a-4H Al8B4C7, Al4CO4, Al2O3, B4C 5±10 YAG±SiC [4, 5] 1850±2000 5±20% YAG a-4H, (6H if seeded) YAG, Al2O3 10±25 YAG±SiC [6, 7] 1850±2000 5±20% YAG a-4H, (6H if seeded) YAG, Al2O3 10±25 Al2O3±SiC [1, 2] 1950±2050 2±25% Al2O3 a-4H Al2O3 5±15 Al2O3±SiC [3] 1950±2050 2±25% Al2O3 a-4H Al2O3 5±15 Hexoloy [22, 23] ? ? a-6H Graphite, ? 3±8 {A Topcon ISI-DS130C was operated at 3±20 kV. {A Philips EM400 was operated at 100 kV. 1626 MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES

MObERLYCHAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1627 Hexoloy 5 um HP@1900°c/1hr x、5pm Fig. 1. SEM micrographs of crack path that emanated from the corner of a Vickers microhardness indentation made on the polished surface of a commercial SiC, Hexoloy-SA(a) and ABC-Sic (b) Dark features in the Hexoloy-SA were indicative of porosity in this material. The crack path deflected around elongated grains of ABC-SiC and did not deflect toward secondary phases(characterized as Alg B C7, Al CO4, Al2O3 and BC [17-19, 21]). vations of intergranular fracture around plate-like sistent with reported mechanisms for the B-to-z grains [Fig. 2(a). Much of the contrast observed transformation [28, 29]. The transformation of B-3C within the ABC-Sic grains was from stacking to a-6H has been the typical reported faults within the a-4H microstructure. This growth reaction [28, 29). however, a-4H was the major of hexagonal Sic with grains elongated along basal transformational product observed in the present ell as residual stacking defects, was con- ABC-SiC. Details of this transformation are the

vations of intergranular fracture around plate-like grains [Fig. 2(a)]. Much of the contrast observed within the ABC±SiC grains was from stacking faults within the a-4H microstructure. This growth of hexagonal SiC with grains elongated along basal planes, as well as residual stacking defects, was con￾sistent with reported mechanisms for the b-to-a transformation [28, 29]. The transformation of b-3C to a-6H has been the typical reported reaction [28, 29], however, a-4H was the major transformational product observed in the present ABC±SiC. (Details of this transformation are the Fig. 1. SEM micrographs of crack path that emanated from the corner of a Vickers microhardness indentation made on the polished surface of a commercial SiC, Hexoloy±SA (a) and ABC±SiC (b). Dark features in the Hexoloy±SA were indicative of porosity in this material. The crack path de¯ected around elongated grains of ABC±SiC and did not de¯ect toward secondary phases (characterized as Al8B4C7, Al4CO4, Al2O3 and B4C [17±19, 21]). MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1627

1628 MOBERLYCHAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES subject of a separate paper. The elongated micro- structure of the ABC-Sic developed during inter locking growth of plate-like grains. These interlocking grains, similar to a-alumina [30], would be more ideal manner than a microstructure of elongated, fibrous grains, such as that reported for the transformation in toughened Si3N4 [9-11]. The Hexoloy-SA, although having a grain size similar to ABC-SiC, had a more equiaxed micro- HP@1900°c Fig. 4). Pores were commonly observed in Hexoloy-SA, as well as secondary phase regions of 5 um graphite(see black arrow). The diffraction contrast within grains in Fig 4 was determined to be due to variations in thickness and bend contours. Stacking faults and microtwins, commonly observed by tEM and hr-tem in ABC-Sic and other sic ma- terials, were not present in Hexoloy-SA Bright field imaging was also utilized to observe crack paths in thin TEM specimens of the ABC- Hexoloy a-6H SiC, The crack imaged in Fig s was propagated by aration. This particular ABC-SiC ceramic(Fig. 5) SEM fract of aBc-sic ressed at had been hot pressed at 1950C for I h, which 900° c for I h(a) Hexoloy-SA(b), from con- resulted in a larger grain size but reduced aspect trolled flaw bending tests. The tortuous surface mor- ratio as compared to material hot pressed at ind bridging of ated, plate-like a-4H grains. The sur- 1900 C(Fig. 3). Grain boundaries, which were not face morphology of the Hexoloy-SA indicated transgranu. easily resolved by optical metallography nor SEM ar fracture of the -6H grains [see Fig. 1(b) were easily distinguished in TEM a-4H HP 19009c/1hr um Fig 3. Bright field TEM image of the microstructure of ABC-Sic hot pressed at 1900C for I h Elongated late- like grains, with an interlocking microstructure developed during p to a phase transformation. Streaks within grains were determined to be stacking faults and microtwins in the a-4H structure. Black arrow denotes secondary phases at triple junction[36], which are also present in larger pockets [17, 21

subject of a separate paper.) The elongated micro￾structure of the ABC±SiC developed during inter￾locking growth of plate-like grains. These interlocking grains, similar to a-alumina [30], would be expected to cause good creep resistance, in a more ideal manner than a microstructure of elongated, ®brous grains, such as that reported for the transformation in toughened Si3N4 [9±11]. The Hexoloy±SA, although having a grain size similar to ABC±SiC, had a more equiaxed micro￾structure, with numerous triple junctions exhibiting close-to-ideal 1208 angles (see white arrows in Fig. 4). Pores were commonly observed in Hexoloy±SA, as well as secondary phase regions of graphite (see black arrow). The di€raction contrast within grains in Fig. 4 was determined to be due to variations in thickness and bend contours. Stacking faults and microtwins, commonly observed by TEM and HR-TEM in ABC±SiC and other SiC ma￾terials, were not present in Hexoloy±SA. Bright ®eld imaging was also utilized to observe crack paths in thin TEM specimens of the ABC± SiC. The crack imaged in Fig. 5 was propagated by bending a doubly-dimpled TEM sample after prep￾aration. This particular ABC±SiC ceramic (Fig. 5) had been hot pressed at 19508C for 1 h, which resulted in a larger grain size but reduced aspect ratio as compared to material hot pressed at 19008C (Fig. 3). Grain boundaries, which were not easily resolved by optical metallography nor SEM [see Fig. 1(b)], were easily distinguished in TEM Fig. 2. SEM fractographs of ABC±SiC hot pressed at 19008C for 1 h (a) and of Hexoloy±SA (b), from con￾trolled ¯aw bending tests. The tortuous surface mor￾phology in ABC±SiC resulted from intergranular fracture and bridging of elongated, plate-like a-4H grains. The sur￾face morphology of the Hexoloy±SA indicated transgranu￾lar fracture of the a-6H grains. Fig. 3. Bright ®eld TEM image of the microstructure of ABC±SiC hot pressed at 19008C for 1 h. Elongated, plate-like grains, with an interlocking microstructure developed during b to a phase transformation. Streaks within grains were determined to be stacking faults and microtwins in the a-4H structure. Black arrow denotes secondary phases at triple junction [36], which are also present in larger pockets [17, 21]. 1628 MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES

mObERLYChAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 2 OL-6H Hexoloy Fig 4. Bright field TEM image of the microstructure of Hexoloy-SA. Voids and regions of graphi black arrows) were commonly observed in this material. The white arrows indicated the predominan f - 120 triple junctions between equiaxed x-6H grains images acquired using diffraction contrast. The grain in Fig. 6(b) was oriented close to a (8 10 2 crack imaged in Fig. 5 propagated along grain 3)2-6H zone axis for imaging oundaries, producing a tortuous crack path similar In general, no specific crystallographic relation- to that observed in the SEM image of Fig. I(b). ship existed between the two zone axes orientations The crack path did not seek out voids nor weaker on either side of a grain boundary. However, the secondary phases. Diffraction contrast detailed hexagonal basal plane of the lower grain (of the merous stacking faults(and microtwins) within the ABC-SiC) also represented the surface of a plate a-4H grains of this SiC, even though it had been like grain, and this grain boundary facet had to hot pressed at1950°C fracture intergranularly to allow for the bridging which provided the improved toughness. The grains High resolution TEMf was used to determine the depicted in Fig. 6(a) and 6(b)were not oriented presence of amorphous phases at the grain bound- exactly along their respective zone axes, thereby aries of both the ABC-Sic and the Hexoloy-Sa sacrificing good high resolution imaging conditions [Fig. 6(a)and 6(b), respectively]. For both images, The probability was low that two randomly the lower grain was oriented to a (2110) zone axis, oriented grains had parallel, low-index axes,while with the [000l] direction normal to the grain also having a parallel grain boundary. Since the im- boundary layer. As has been noted, ABC-SiC pro- portant grain boundaries(for toughness due to cessed at the higher temperatures had been trans- bridging) involved a basal plane as the long facet formed to the a-4H structure with numerous for one grain, this grain boundary face was first stacking faults, whereas Hexoloy-SA exhibited the rotated to be imaged parallel to the TEM electron 2-6H structure. The ABC-SiC imaged in Fig. 6(a) beam. Subsequent tilting along the grain boundary had only been hot pressed at 1780.C for I h. and was conducted until a compromise image within 5o therefore retained substantial B phase, both as sep- of two, zone axes in the adjacent grains, was arate B grains and as dual-phase grains comprised obtained. As long as the basal plane in the lower grain was discretely presented without tilt in the of a-4H and B-3C. The upper grain in Fig. 6(a)was lattice image, the thickness of the amorphous grain tilted close to a(110)B zone axis for high resolution boundary layer could be measured. The amorphous imaging. On the other hand, all grain boundaries in grain boundary layer observed in the ABC-SiC was Hexoloy-SA separated two a-6H grains. The upper always <2 nm and usually <I nm thick.Most grain boundary layers observed in the Hexoloy-SA tA JEOL ARM1000 was operated at 800 kV, and a were also <2 nm thick; however, some amorphous Topcon ISI-002B was operated at 200 kV. egions were up to 5 nm thick [Fig. 6(b)]. The

images acquired using di€raction contrast. The crack imaged in Fig. 5 propagated along grain boundaries, producing a tortuous crack path similar to that observed in the SEM image of Fig. 1(b). The crack path did not seek out voids nor weaker secondary phases. Di€raction contrast detailed nu￾merous stacking faults (and microtwins) within the a-4H grains of this SiC, even though it had been hot pressed at 19508C. High resolution TEM{ was used to determine the presence of amorphous phases at the grain bound￾aries of both the ABC±SiC and the Hexoloy±SA [Fig. 6(a) and 6(b), respectively]. For both images, the lower grain was oriented to a h2110i zone axis, with the [0001] direction normal to the grain boundary layer. As has been noted, ABC±SiC pro￾cessed at the higher temperatures had been trans￾formed to the a-4H structure with numerous stacking faults, whereas Hexoloy±SA exhibited the a-6H structure. The ABC±SiC imaged in Fig. 6(a) had only been hot pressed at 17808C for 1 h, and therefore retained substantial b phase, both as sep￾arate b grains and as dual-phase grains comprised of a-4H and b-3C. The upper grain in Fig. 6(a) was tilted close to a h110ib zone axis for high resolution imaging. On the other hand, all grain boundaries in Hexoloy±SA separated two a-6H grains. The upper grain in Fig. 6(b) was oriented close to a h8 10 2 3ia-6H zone axis for imaging. In general, no speci®c crystallographic relation￾ship existed between the two zone axes orientations on either side of a grain boundary. However, the hexagonal basal plane of the lower grain (of the ABC±SiC) also represented the surface of a plate￾like grain, and this grain boundary facet had to fracture intergranularly to allow for the bridging which provided the improved toughness. The grains depicted in Fig. 6(a) and 6(b) were not oriented exactly along their respective zone axes, thereby sacri®cing good high resolution imaging conditions. The probability was low that two randomly oriented grains had parallel, low-index axes, while also having a parallel grain boundary. Since the im￾portant grain boundaries (for toughness due to bridging) involved a basal plane as the long facet for one grain, this grain boundary face was ®rst rotated to be imaged parallel to the TEM electron beam. Subsequent tilting along the grain boundary was conducted until a compromise image within 58 of two zone axes in the adjacent grains was obtained. As long as the basal plane in the lower grain was discretely presented without tilt in the lattice image, the thickness of the amorphous grain boundary layer could be measured. The amorphous grain boundary layer observed in the ABC-SiC was always <2 nm and usually <1 nm thick. Most grain boundary layers observed in the Hexoloy±SA were also <2 nm thick; however, some amorphous regions were up to 5 nm thick [Fig. 6(b)]. The Fig. 4. Bright ®eld TEM image of the microstructure of Hexoloy±SA. Voids and regions of graphite (black arrows) were commonly observed in this material. The white arrows indicated the predominance of 01208 triple junctions between equiaxed a-6H grains. {A JEOL ARM1000 was operated at 800 kV, and a Topcon ISI-002B was operated at 200 kV. MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1629

1630 mObERLYChAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1950°c Fig. 5. Bright field TEM image of a crack path along grain boundaries in ABC-SiC hot pressed at 1950C for I h. This crack was propagated by bending a doubly-dimpled TEM specimen prior to final hicker amorphous layers in Hexoloy-SA also phase could have resulted from different sintering appeared to vary in thickness along a single grain additives in other reports boundary; however, this observation could have Techniques other than HR-TEM EDS(Energy been partially inflated by imaging conditions chan- Dispersive X-ray Spectroscopy), PEELS(Parallel ging due to the grain boundaries not being atomic- Electron Energy Loss Spectroscopy ) energy-filtered ally flat and or to subtle bending of the TEM thin TEM [36] were utilized to corroborate the presence of this grain boundary layer. Scanning auger elec- The nature of TEM sample preparation of poly- tron spectroscopy(AES) of the fracture sur crystalline materials and the requirements of HR. of ABC-Sic provided the most definitive chemical TEM imaging precluded detection of an amorphous analysis(Fig. 7). AEs detected a thin alumina-con- phase at every grain boundary. Even the grain taining pha Ise on all exposed grain boundaries boundary interface incorporating a basal plane had which was removed with <I nm ion etching occasional steps, often I nm in height. On the [Fig. 7(a)and 7(b)]. The aluminum detected on the atomic level, the grain boundaries which did not in- as fractured surface exhibited a shift in energy typi cal for an oxidized form of al such merous steps. This typically resulted in the thin The intensity of the o signal was diminished as amorphous boundary layer being neither parallel to compared to ideal sapphire [34], due to the thin the electron beam nor discrete in its projected po- nature of the amorphous layer and possibly the tential throughout the thickness of the TEM speci- amorphous phase being oxygen deficient. The Si men. Thus a(possible) thin amorphous layer often present in the spectrum of the as fractured surface could not be observed, most probably as a result of did not exhibit an oxidized signature, thereby indi nonideal imaging orientations. Such conditions are cating most of this Si signal was from the Sic grain more pronounced for smaller grain sizes and shorter below a thin surface layer. a trace sulfur signal was detected as fractured [Fig. 7(a) and after removal grain boundary facets; and this may account for of <I nm ion etching (Fig. 7(b)]. but was removed which reported no amorphous grain boundary by subsequent I nm) ion etching. Because of its layer[1, 21, 23, 31-33]. Also different observed grain came primarily from the SiC grain beneath the boundary phases or the lack of a grain boundary grain boundary layer. However, some C was believed soluble in the amorphous boundary layer, 个 A Perkin Elmer小660 was operated at3kV as many of the crystallized triple junctions con

thicker amorphous layers in Hexoloy±SA also appeared to vary in thickness along a single grain boundary; however, this observation could have been partially in¯ated by imaging conditions chan￾ging due to the grain boundaries not being atomic￾ally ¯at and/or to subtle bending of the TEM thin foils. The nature of TEM sample preparation of poly￾crystalline materials and the requirements of HR￾TEM imaging precluded detection of an amorphous phase at every grain boundary. Even the grain boundary interface incorporating a basal plane had occasional steps, often 1 nm in height. On the atomic level, the grain boundaries which did not in￾corporate a basal plane were twisted and had nu￾merous steps. This typically resulted in the thin amorphous boundary layer being neither parallel to the electron beam nor discrete in its projected po￾tential throughout the thickness of the TEM speci￾men. Thus a (possible) thin amorphous layer often could not be observed, most probably as a result of nonideal imaging orientations. Such conditions are more pronounced for smaller grain sizes and shorter grain boundary facets; and this may account for many of the HR-TEM analyses in the literature which reported no amorphous grain boundary layer [1, 21, 23, 31±33]. Also di€erent observed grain boundary phases or the lack of a grain boundary phase could have resulted from di€erent sintering additives in other reports. Techniques other than HR-TEM EDS (Energy Dispersive X-ray Spectroscopy), PEELS (Parallel Electron Energy Loss Spectroscopy), energy-®ltered TEM [36] were utilized to corroborate the presence of this grain boundary layer. Scanning auger elec￾tron spectroscopy (AES){ of the fracture surfaces of ABC±SiC provided the most de®nitive chemical analysis (Fig. 7). AES detected a thin alumina-con￾taining phase on all exposed grain boundaries, which was removed with 1 nm) ion etching. Because of its higher energy and greater escape depth, the C signal came primarily from the SiC grain beneath the grain boundary layer. However, some C was believed soluble in the amorphous boundary layer, as many of the crystallized triple junctions con￾Fig. 5. Bright ®eld TEM image of a crack path along grain boundaries in ABC±SiC hot pressed at 19508C for 1 h. This crack was propagated by bending a doubly-dimpled TEM specimen prior to ®nal ion beam thinning. {A Perkin Elmer F-660 was operated at 3 kV. 1630 MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES

MOBERLYCHAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1631 HP@1780°c/1hr x;计 的:2话a Hexoloy 0-6H 1. 5nm Fig. 6. HR-TEM es of amorphous grain boundaries. (a) Between an -4H 22110) orientation) and a B grain(<3 from a( 110)orientation) in ABC-SiC he sed at I780°C for I h. The amorphous phase in ABC-SiC was typically < I nm thick. (b) Between two a-6H grains in xoloy-SA(upper and lower grains within 5 of a (8 10 2 3)and a(2110)orientation, respectively) Although some thick amorphous grain boundaries existed in Hexoloy-SA, the amorphous phase was typically <2 nm thick tained ternary Al-o-C phases [36]. The Auger in materials often not experimentally signal at 180 eV was at the detection limit (2-5%) detected [20, 32, 35]. Weak beam dark field ima- in Fig. 7(a)and 7(b), thus the existence of B in the ging, which has been utilized in some material sys- grain boundary layer was inconclusive. (The ma- tems to depict amorphous films at all grain jority of the B in this ABC-SiC is incorporated in boundaries, was unsuccessful at imaging the grain the ternary Alg B C7 phase [17, 21, 8, 36]; however, boundaries in ABC-SiC. The difficulties with dark Bs influential role in sintering and grain boundaries field imaging were due to a combination of the pre

tained ternary Al0O0C phases [36]. The Auger signal at 180 eV was at the detection limit (2±5%) in Fig. 7(a) and 7(b), thus the existence of B in the grain boundary layer was inconclusive. (The ma￾jority of the B in this ABC±SiC is incorporated in the ternary Al8B4C7 phase [17, 21, 8, 36]; however, B's in¯uential role in sintering and grain boundaries in materials is often not experimentally detected [20, 32, 35].) Weak beam dark ®eld ima￾ging, which has been utilized in some material sys￾tems to depict amorphous ®lms at all grain boundaries, was unsuccessful at imaging the grain boundaries in ABC±SiC. The diculties with dark ®eld imaging were due to a combination of the pre￾Fig. 6. HR-TEM images of amorphous grain boundaries. (a) Between an a-4H grain (<58 from a h2110i orientation) and a b grain (<38 from a h110i orientation) in ABC±SiC hot pressed at 17808C for 1 h. The amorphous phase in ABC±SiC was typically <1 nm thick. (b) Between two a-6H grains in Hexoloy±SA (upper and lower grains within 58 of a h8 10 2 3i and a h2110i orientation, respectively). Although some thick amorphous grain boundaries existed in Hexoloy±SA, the amorphous phase was typically <2 nm thick. MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1631

MOBERLYCHAN et aL: ROLES OF AMORPHOUS GRAIN BOUNDARIES ABC-Sic and the commercial Hexoloy-SA As· Fractured Surface However, the thin nature of these interfacial layers and the low atomic number elements in these layers prevented a definitive chemical analysis by tra ditional analytical TEM. (Energy filtered imaging, a recently emerging TEM technique, has established an enrichment of al and o at or near the grain oundaries of ABC-Sic [36)Coupled with AES analysis of the intergranular fracture surfaces, the amorphous layer in the ABC-Sic was determined After <I nm lon Etching to be primarily an Al(and O)-containing amor- phous phase. Since the Hexoloy-SA material did not fracture along grain boundaries, and the sinter ing additives used to process this commercial ce ig. 7. AES spectra acquired of an intergranular fracture amic have not been published, the chemistry of the see Fig. 2(a). The Al- amorphous layer in the Hexoloy-SA has not yet ntaining amorphou Ar ion etching(b) and was removed with -I nm been ascertained. EDS in high voltage TEM yielded Sic grain was detected all grains. a minimal C signal as compared to Si, thus any possible C enrichment has not been established Although the exact effect of amorphous phase sence of us stacking defects, nonplanar chemistry on intergranular fracture could not be grain boundaries, fine grain size and thickness of assessed, the observed difference in fracture mode amorphous phase often being < I nm. indicated chemistry of the amorphous phase was more critical than its thickness 3. DISCUSSION necessary to induce intergranular fracture appeared The higher toughness reported for the five [1-8] to be more subtle than just the SiC ceramics listed in the Introduction (Table 1), as amorphous, oxide phase. Auger electron spec- compared to the commercial ceramic Hexoloy-SA, troscopy of the ABC-Sic boundaries not only has been attributed to a commonality in their exhibited alumina-bonding but also a trace of sul- microstructures. All toughened Sic exhibited fur. This sulfur impurity appeared to be preferen- elongated grain structures [1-8], whereas the tially segregated between the Sic and the Al- Hexoloy-SA displayed an equiaxed microstructure. containing layer [Fig. 7(a)and 7(b)]. Other analyses The ABC-SiC also was comprised of a different of amorphous grain boundary phases also have crystal structure, a-4H, as compared to a-6H for observed segregated secondary impurities. In SiC, Hexoloy-SA. The starting B-Sic powders for most sintered with Al2O3 additives, Suzuki [1] discussed of the five toughened Sic ceramics [1-8 trans- the presence of Ca on intergranular fracture sur- formed, at least partially, during processing to pro- faces. Other work [37, 38 has also indicated the duce the a-4H alpha phase. One toughened Sic presence of Ca segregated between the amorphous used seeded a-6H grains to enhance the B-a phase and the Si3N4 grain, as well as its influence transformation [7], and X-ray diffraction(XRD) of on altering the thickness of the amorphous phase the type of a phase was not reported for some of Computer modeling has established the infuence of the SiC ceramics [2, 3, 7]. The formation of a-4H S impurities to weaken grain boundaries [39]. The instead of a-6H has been reported as related to the influence of sulfur impurities in SiC, as well as con- resence of Al and /or Al2O3 as a sintering trol of their location within the grain boundary, is additive [1, 35, although different processing tem- being pursued with further experimental eratures may also influence the final crystal struc- observations [38, 40 ture. The thermal processing conditions for The grain sizes of the five tough SiC ceramics Hexoloy-SA have not been detailed in the litera- listed in Table 1 varied from 2 to 25 um in ture, and it has not been published whether all length [1-8]. The grain size of the Hexoloy-SA and starting powders were B or a. However, the predo- the present ABC-Sic were similar, however the minance of 120c triple junctions and fault-free aspect ratio upwards of 10 in the ABc-Sic ceramic suggested long times and or high tempera- differed considerably from the equiaxed grains of ere utilized to produce the equiaxed, a 6H the Hexoloy-SA. The aspect ratio, in conjunction ucture. In contrast, the high aspect ratio of with intergranular fracture provided high toughnes the elongated grains of ABC-Sic was intentionally in ABC-SiC. Analysis of monolithic SiC, sintered enhanced by a processing temperature lower than with the same Al, B and c additives at a tempera- that utilized for other sic ceramics ture low enough to prevent the B to transform- HR-TeM determined the existence of ation, also detected an alumina-containing the grain boundaries of bo amorphous grain boundary [19, 21]. These weak

sence of numerous stacking defects, nonplanar grain boundaries, ®ne grain size and thickness of amorphous phase often being <1 nm. 3. DISCUSSION The higher toughness reported for the ®ve [1±8] SiC ceramics listed in the Introduction (Table 1), as compared to the commercial ceramic Hexoloy±SA, has been attributed to a commonality in their microstructures. All toughened SiC exhibited elongated grain structures [1±8], whereas the Hexoloy±SA displayed an equiaxed microstructure. The ABC±SiC also was comprised of a di€erent crystal structure, a-4H, as compared to a-6H for Hexoloy±SA. The starting b-SiC powders for most of the ®ve toughened SiC ceramics [1±8] trans￾formed, at least partially, during processing to pro￾duce the a-4H alpha phase. One toughened SiC used seeded a-6H grains to enhance the b±a transformation [7], and X-ray di€raction (XRD) of the type of a phase was not reported for some of the SiC ceramics [2, 3, 7]. The formation of a-4H instead of a-6H has been reported as related to the presence of Al and/or Al2O3 as a sintering additive [1, 35], although di€erent processing tem￾peratures may also in¯uence the ®nal crystal struc￾ture. The thermal processing conditions for Hexoloy±SA have not been detailed in the litera￾ture, and it has not been published whether all starting powders were b or a. However, the predo￾minance of 1208C triple junctions and fault-free grains suggested long times and/or high tempera￾tures were utilized to produce the equiaxed, a-6H microstructure. In contrast, the high aspect ratio of the elongated grains of ABC±SiC was intentionally enhanced by a processing temperature lower than that utilized for other SiC ceramics. HR-TEM determined the existence of an amor￾phous phase at the grain boundaries of both the ABC±SiC and the commercial Hexoloy±SA. However, the thin nature of these interfacial layers and the low atomic number elements in these layers prevented a de®nitive chemical analysis by tra￾ditional analytical TEM. (Energy ®ltered imaging, a recently emerging TEM technique, has established an enrichment of Al and O at or near the grain boundaries of ABC±SiC [36].) Coupled with AES analysis of the intergranular fracture surfaces, the amorphous layer in the ABC±SiC was determined to be primarily an Al (and O)-containing amor￾phous phase. Since the Hexoloy±SA material did not fracture along grain boundaries, and the sinter￾ing additives used to process this commercial cer￾amic have not been published, the chemistry of the amorphous layer in the Hexoloy±SA has not yet been ascertained. EDS in high voltage TEM yielded a minimal C signal as compared to Si, thus any possible C enrichment has not been established. Although the exact e€ect of amorphous phase chemistry on intergranular fracture could not be assessed, the observed di€erence in fracture mode indicated chemistry of the amorphous phase was more critical than its thickness. The chemistry of an amorphous grain boundary necessary to induce intergranular fracture appeared to be more subtle than just the presence of an amorphous, oxide phase. Auger electron spec￾troscopy of the ABC±SiC boundaries not only exhibited alumina-bonding but also a trace of sul￾fur. This sulfur impurity appeared to be preferen￾tially segregated between the SiC and the Al￾containing layer [Fig. 7(a) and 7(b)]. Other analyses of amorphous grain boundary phases also have observed segregated secondary impurities. In SiC, sintered with Al2O3 additives, Suzuki [1] discussed the presence of Ca on intergranular fracture sur￾faces. Other work [37, 38] has also indicated the presence of Ca segregated between the amorphous phase and the Si3N4 grain, as well as its in¯uence on altering the thickness of the amorphous phase. Computer modeling has established the in¯uence of S impurities to weaken grain boundaries [39]. The in¯uence of sulfur impurities in SiC, as well as con￾trol of their location within the grain boundary, is being pursued with further experimental observations [38, 40]. The grain sizes of the ®ve tough SiC ceramics listed in Table 1 varied from 2 to 25 mm in length [1±8]. The grain size of the Hexoloy±SA and the present ABC±SiC were similar, however the aspect ratio upwards of 10 in the ABC±SiC ceramic di€ered considerably from the equiaxed grains of the Hexoloy±SA. The aspect ratio, in conjunction with intergranular fracture provided high toughness in ABC±SiC. Analysis of monolithic SiC, sintered with the same Al, B and C additives at a tempera￾ture low enough to prevent the b to a transform￾ation, also detected an alumina-containing amorphous grain boundary [19, 21]. These weak Fig. 7. AES spectra acquired of an intergranular fracture surface of plate-like a-4H grains [see Fig. 2(a)]. The Al￾containing amorphous phase (a) was removed with 01 nm Ar ion etching (b), and only the SiC grain was detected after 2 nm etching in all grains. 1632 MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES

mObERLYChAN et al: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1633 interfaces promoted intergranular fracture; however, With the large amount of YAG(up to 20% by the more equiaxed shape of the submicron beta volume), these YAG-Sic ceramics could be con- grains did not allow for enhanced toughening due sidered as composites, with the YaG being a to grain bridging. Thus, high toughness in this cer-*weak" phase and an easy path for crack propa- amic required an amorphous grain boundary layer gation. In many SEM images the YAG formed a with a chemistry to weaken the grain boundary and nearly continuous network, and therefore would a grain shape with a high aspect ratio. Lack of any have provided a simple means for all crack propa- f these three ingredients produced an inherently gation to have occurred within the YAG. Thus brittle material when a continuous and relatively thick network of Analyses of the two reported Al2O3-SiC [1-3 YAG existed, the propagation of the crack through have indicated both similarities and disparities as the YAG suggested much of the strength of the ompared to the present ABC-SiC. Since the Al in composite was dependent on this weaker phase his ABC-sic partially reacted with oxides on the Although the fracture toughness was improved, the surface of Sic powders, the alumina-containing room temperature strength of YAG-Sic was amorphous phase at the grain boundaries of ABC- reduced as compared to Hexoloy-SA [7. In con- Sic could have acted similarly to the Al-containing trast, the lower concentration of sintering additives purities on the intergranular fracture surfaces of in the ABC-Sic enabled a much higher fracture Al2O3-SiC [1]. Although Suzuki [1] detected Al on strength to be realized in conjunction with higl tergranular fracture surfaces with Auger, he did toughness. not observe an amorphous phase by HR-TEM in Since the primary utility of these toughened sic did not include Auger analysis of fracture surfaces cations s is for high-temperature structural appli- nor HR-TEM of grain boundaries. The disparity in additives)on high temperature strength must be Suzukis aEs and HR-teM data could be attribu- considered. No high temperature mechanical testing ted to the fact that the Suzukis hr-tem images of these Yag-sic ceramics nor of ABC-Sic have did not depict one of the well-faceted 10001) grain yet been published. The decrease in strength of boundaries. Other research of grain boundaries in YAG at high temperatures [41], should correspond Sic typically did not resolve cross fringes to make to a decrease in strength of Sic containing 10-20% analysis of the boundary unambiguous [1, 23, 31- YAG, especially for an amorphous secondary 33]. ABC-SiC sintered at lowered temperatures phase. YAG-SiC has been proposed to offer good resolved no amorphous phase using HR- ductility at high temperature [7, but with a degra TEM [17, 21]. Yet, the analysis of amorphous layers dation in strength. The thin amorphous phase 10% YAG) exhibited expansion(CTE) between the Sic and YAG. Such a similar rough crack path around the elongated a CTE mismatches would be a further reason for lim- Sic grains [4-7. The crack paths suggested more iting the volume fraction of a second phase. a tor- the presence of deflection rather than grain pullout. tuous crack path which occurred because it was However, a TEM analysis of the grain boundaries connecting-up numerous, pre-existing short cracks has not been reported for either YAG-Sic would provide a rough fracture surface, but would ceramic 14-7. Nor has it been determined whether also correlate to a lowering of the overall fractur the YAG secondary phase is fully crystalline or strength. amorphous. Since the volume fraction of YAG was In the ABC-Sic no pre-existing cracks were substantial, most Sic grains were coated with a observed after processing. In addition, the tortuous relatively thick layer of YAG, and SEM images crack paths, which were propagated during mechan- showed that the crack path remained in the YAG. ical testing, did not connect weaker secondary

interfaces promoted intergranular fracture; however, the more equiaxed shape of the submicron beta grains did not allow for enhanced toughening due to grain bridging. Thus, high toughness in this cer￾amic required an amorphous grain boundary layer with a chemistry to weaken the grain boundary and a grain shape with a high aspect ratio. Lack of any of these three ingredients produced an inherently brittle material. Analyses of the two reported Al2O3±SiC [1±3] have indicated both similarities and disparities as compared to the present ABC±SiC. Since the Al in this ABC±SiC partially reacted with oxides on the surface of SiC powders, the alumina-containing amorphous phase at the grain boundaries of ABC± SiC could have acted similarly to the Al-containing impurities on the intergranular fracture surfaces of Al2O3±SiC [1]. Although Suzuki [1] detected Al on intergranular fracture surfaces with Auger, he did not observe an amorphous phase by HR-TEM in the same material. Other reported Al2O3±SiC [2, 3] did not include Auger analysis of fracture surfaces nor HR-TEM of grain boundaries. The disparity in Suzuki's AES and HR-TEM data could be attribu￾ted to the fact that the Suzuki's HR±TEM images did not depict one of the well-faceted {0001} grain boundaries. Other research of grain boundaries in SiC typically did not resolve cross fringes to make analysis of the boundary unambiguous [1, 23, 31± 33]. ABC±SiC sintered at lowered temperatures resolved no amorphous phase using HR￾TEM [17, 21]. Yet, the analysis of amorphous layers 10% YAG) exhibited a similar rough crack path around the elongated a SiC grains [4±7]. The crack paths suggested more the presence of de¯ection rather than grain pullout. However, a TEM analysis of the grain boundaries has not been reported for either YAG±SiC ceramic [4±7]. Nor has it been determined whether the YAG secondary phase is fully crystalline or amorphous. Since the volume fraction of YAG was substantial, most SiC grains were coated with a relatively thick layer of YAG, and SEM images showed that the crack path remained in the YAG. With the large amount of YAG (up to 20% by volume), these YAG±SiC ceramics could be con￾sidered as composites, with the YAG being a ``weak'' phase and an easy path for crack propa￾gation. In many SEM images the YAG formed a nearly continuous network, and therefore would have provided a simple means for all crack propa￾gation to have occurred within the YAG. Thus, when a continuous and relatively thick network of YAG existed, the propagation of the crack through the YAG suggested much of the strength of the composite was dependent on this weaker phase. Although the fracture toughness was improved, the room temperature strength of YAG±SiC was reduced as compared to Hexoloy±SA [7]. In con￾trast, the lower concentration of sintering additives in the ABC±SiC enabled a much higher fracture strength to be realized in conjunction with high toughness. Since the primary utility of these toughened SiC ceramics is for high-temperature structural appli￾cations, the e€ects of microstructure (and residual additives) on high temperature strength must be considered. No high temperature mechanical testing of these YAG±SiC ceramics nor of ABC±SiC have yet been published. The decrease in strength of YAG at high temperatures [41], should correspond to a decrease in strength of SiC containing 10±20% YAG, especially for an amorphous secondary phase. YAG±SiC has been proposed to o€er good ductility at high temperature [7], but with a degra￾dation in strength. The thin amorphous phase in the ABC±SiC also has been expected to enhance creep, as has been reported for Si3N4 [42]. However, the residual triple junctions were typically crystalline ternary phases [36]. Since SiC has a higher melting temperature than does Si3N4, an improved creep resistance could be expected as compared to Si3N4 at similar temperatures. Further work has been initiated to determine which micro￾structures would provide the best compromise of high temperature strength and room temperature toughness [43]. The improved fracture toughness of YAG±SiC was reported to be due to a corresponding increase in ``short crack'' formation [7]. This was speculated [7] to occur (in part) as a result of stress arising from a mismatch in coecient of thermal expansion (CTE) between the SiC and YAG. Such CTE mismatches would be a further reason for lim￾iting the volume fraction of a second phase. A tor￾tuous crack path which occurred because it was connecting-up numerous, pre-existing short cracks would provide a rough fracture surface, but would also correlate to a lowering of the overall fracture strength. In the ABC±SiC no pre-existing cracks were observed after processing. In addition, the tortuous crack paths, which were propagated during mechan￾ical testing, did not connect weaker secondary MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES 1633

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. The authors were grateful for use of the facilities of the National Center for Electron Microscopy, and especially for the assistance of sta€ members D. Ah-Tye, C. Nelson and J. Turner. REFERENCES 1. Suzuki, K., Pressureless-sintered silicon carbide with addition of aluminum oxide, in Silicon Carbide Ceramics V2, ed. S. Somiya and Y. Inomata. Elsevier Applied Science, Amsterdam, 1991, pp. 163±182. 2. Mulla, M. A. and Krstic, V. D., J. Mater. Sci., 1994, 29, 934±938. 3. Mulla, M. A. and Krstic, V. D., Acta metall. mater., 1994, 42, 303±308. 4. Kim, D. H. and Kim, C. H., J. Am. Ceram. Soc., 1990, 73, 1431±1434. 5. Lee, S. K. and Kim, C. H., J. Am. Ceram. Soc., 1994, 77, 1655±1658. 6. Padture, N. P., J. Am. Ceram. Soc., 1994, 77, 519± 523. 7. Padture, N. P. and Lawn, B. R., J. Am. Ceram. Soc., 1994, 77, 2518±2522. 8. Cao, J. J., MoberlyChan, W. J., De Jonghe, L. C., Gilbert, C. J. and Ritchie, R. O., J. Am. Ceram. 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Ceramic± Matrix Composites II, Ceramic Trans. Vol. 46. ACS Publications, Westerville OH, 1995, pp. 277±288. 19. MoberlyChan, W. J., Cao, J. J., Niu, M. Y. and De Jonghe, L. C., Toughened b-SiC composites with alumina-coated a-SiC platelets, in High Performance Composites, ed. K. K. Chawla, P. K. Liaw and S. G. Fishman. TMS Publications, Warrendale, PA, 1994. 20. Lin, B-W., Imai, M., Yano, T. and Iseki, T., J. Am. Ceram. Soc., 1986, 69, C67±C68. 21. MoberlyChan, W. J., Cao, J. J., Niu, M. Y., De Jonghe, L. C. and Schwartzman, A. F., SiC compo￾sites with alumina-coated a-SiC platelets in b-SiC matrix: controlling toughness through microstructure, in Microbeam Analysis Proceedings, ed. J. Friel. VCH, New York, 1994, pp. 49±50. 22. Dutta, S., J. Mater. Sci., 1984, 19, 1307±1313. 23. Chia, K. Y. and Lau, S. K., Ceram. Engng Sci. Proc., 1991, 12, 1845±1861. 24. Gilbert, C. J., Cao, J. J., MoberlyChan, W. J., De Jonghe, L. C. and Ritchie, R. O., Acta metall. mater., 1996, 44, 3199±3214. 1634 MOBERLYCHAN et al.: ROLES OF AMORPHOUS GRAIN BOUNDARIES