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 weaksence 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