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 obsertransformation 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 <1 mm diamond ®nish. The Hexoloy±SA in this study was commercially obtained from Carborundum, and the additives uti￾lized to process it have not been extensively dis￾cussed in the literature [22, 23]. The most prominent secondary phase observed in Hexoloy±SA in this study was graphite, which was detected both as par￾ticulates within SiC grains and at large triple junc￾tions. 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) 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