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 facilitated 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 intergranular 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 carbide (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 transformation 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 toughness. 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 amorphous phase as thin as 1 nm [15, 16]. This would indicate that a minimal quantity of additives is desirable for liquid-phase-sintering: sucient to coat grain boundaries, yet limiting the ®nal volume fraction of secondary phases. This study has characterized microstructural dierences and commonalities between a commercial 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 predominant secondary phase is amorphous and/or crystalline 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,<1%B.~2%C AlgB.C,, Al,, Al_O3. B C 5-10 1950-2050 2-25%Al2O3 Hexoloy Graphite, transformation and observed toughening were cor- preferential removed, thereby confusing the elated to the presence of a thin amorphous phase volume fraction of porosity and graphite along grain boundaries in ABC-SiC. The volume The measured fracture toughness of the ABC- fraction of sintering additives used was a trade-off SiC. based on the controlled surface flaw method f processing parameters and properties. High was Paym vs 2.2 for the Hexoloy-SA aluminum content enhanced densification, lowered Moreover, measurements of bend strengths yielded sintering temperature and increased the amount of a value of -650 MPa for the ABC-SiC VS-400 triple junction phases. A smaller Al concentration MPa for the commercial Hexoloy-SA. Thus the lowered the fraction of detrimental secondary strength, and especially the fracture toughness, of phases and tended to improve high temperature ABC-SiC compared favorably with other strength. To provide densification of beta Sic when ceramics [1-7]. Further details of processing, hot pressed for I h at 1650.C 5 wt%, Al was characterization of microstructure and mechanical necessary [17, 20 ]; yet only 1% Al was sufficient to properties of these Sic ceramics have been detailed provide densification (>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 correlated 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 sucient to provide densi®cation (>99%) at 19008C. In addition, 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 amorphous 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 presintering 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 <1 mm diamond ®nish. The Hexoloy±SA in this study was commercially obtained from Carborundum, and the additives utilized to process it have not been extensively discussed in the literature [22, 23]. The most prominent secondary phase observed in Hexoloy±SA in this study was graphite, which was detected both as particulates within SiC grains and at large triple junctions. Also the porosity (>2±5%) in commercial Hexoloy±SA appeared substantially greater than that measured in the ABC±SiC. During polishing for scanning electron microscope (SEM) observation 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 emanating from microhardness indentations provided a good qualitative assessment of the toughness [25± 27]. The crack in the Hexoloy±SA followed a relatively straight path [Fig. 1(a)]. In contrast, the cracks in the ABC±SiC [Fig. 1(b)] exhibited de¯ections. 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 fourpoint 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 secondary phases. Bright ®eld TEM{ imaging de®ned major microstructural dierences between ABC±SiC hot pressed at 19008C and Hexoloy±SA (Figs 3 and 4, respectively). Hot pressed ABC±SiC exhibited elongated grains, with an aspect ratio >10 for the larger grains, which were consistent with the SEM obserTable 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 consistent 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 microstructure of the ABC±SiC developed during interlocking 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 microstructure, 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 diraction 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 materials, 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 preparation. 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 controlled ¯aw bending tests. The tortuous surface morphology in ABC±SiC resulted from intergranular fracture and bridging of elongated, plate-like a-4H grains. The surface morphology of the Hexoloy±SA indicated transgranular 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 diraction 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. Diraction contrast detailed numerous 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 boundaries 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 processed at the higher temperatures had been transformed 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 separate 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 relationship 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 platelike 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 important 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