S Gustafsson et al. /Joumal of the European Ceramic Sociery 29(2009)539-550 than the expected 60%0. The glassy pockets at multi-grain junc- rich in silicon. Impurities were not detected in any of these tions were rich in silicon. The mol fraction SiO2 in the glass was analyses 93.0+3.2%. Some of the analysed glassy pockets showed an alumina content close to the eutectic Al2O3-SiO2 composition 4.2. The as-sintered mullite/SiC nanocomposite of approximately 5 mol%Al2O3, reported in several studies. 26,2 EDX also showed that the amorphous grain boundary films were The microstructure of the as-sintered nanocomposite material shown in Figs 6a and 7. Most grain sections were equiaxed (a) and an average matrix grain size of 0.7 um was determined from SEMimages(Figs 6 and 7a). The1 granular cavities also in this microstructure, and these cavities were usually faceted and less than 100 nm in diameter(Fig. 7b) The majority(around 80%)of the Sic particles were located at grain boundaries and multi-grain junctions, see Fig. 7. The siz of these particles was in the range 30-90 nm, and a number of them formed agglomerates as shown in Fig. 7. Particle agglom- erates present in multi-grain junctions were generally associated with cavities, see Fig 7c. The intragranular SiC particles did not form clusters, and were smaller, typically 10-50 nm, see Fig. 7a. Strain contours were only occasionally observed in the mullite matrix immediately surrounding a SiC particle 200nm Some dislocations were pinned by, or emerging from, the rity of the observed dislocations were. as in the mullite microstructure associated with intragranular cavities, see Fig. 7b. Although the overall dislocation density was low, dislocation pile-ups at triple grain junctions or at grain boundaries were observed in a few areas Thin glassy films were observed in the matrix grain bound- aries along with smaller volumes of glass separating both intragranular and intergranular SiC particles from the surround ing mullite grains, see Figs. 8 and 9. The thickness of the amorphous grain boundary films varied between 0.6 and 0.9nm Very small (10-15 nm) amorphous pockets were observed at some multi-grain junctions, see Fig 8. The Al?O3 mol f 100nm to 55.9+1.5%,1.e also in this case lower than for 3: 2 mullite The intergranular glass was enriched in silicon, although the 200nm 50 nm Fig. 7. The microstructure of the as-sintered nanocomposite (TEM).(a)SiC par- ticles present in both inter- and intragranular positions(black and white arrows, Fig. 8. Thin glassy grain boundary films(white arrows) merging into a glas respectively).(b) Dislocations pinned by intragranular cavities(C)and SiC par- containing triple grain junction(black arrow) in the as-sintered nanocomposite ticles(P).(c)Clusters of SiC particles(arrowed), associated with intergranular material. The glass appears with bright contrast in the diffuse TEM dark fieldS. Gustafsson et al. / Journal of the European Ceramic Society 29 (2009) 539–550 545 than the expected 60%. The glassy pockets at multi-grain junc￾tions were rich in silicon. The mol fraction SiO2 in the glass was 93.0 ± 3.2%. Some of the analysed glassy pockets showed an alumina content close to the eutectic Al2O3–SiO2 composition of approximately 5 mol% Al2O3, reported in several studies.26,27 EDX also showed that the amorphous grain boundary films were Fig. 7. The microstructure of the as-sintered nanocomposite (TEM). (a) SiC par￾ticles present in both inter- and intragranular positions (black and white arrows, respectively). (b) Dislocations pinned by intragranular cavities (C) and SiC par￾ticles (P). (c) Clusters of SiC particles (arrowed), associated with intergranular porosity. rich in silicon. Impurities were not detected in any of these analyses. 4.2. The as-sintered mullite/SiC nanocomposite The microstructure of the as-sintered nanocomposite material is shown in Figs. 6a and 7. Most grain sections were equiaxed, and an average matrix grain size of 0.7 m was determined from SEM images (Figs. 6 and 7a). The mullite grains contained intra￾granular cavities also in this microstructure, and these cavities were usually faceted and less than 100 nm in diameter (Fig. 7b). The majority (around 80%) of the SiC particles were located at grain boundaries and multi-grain junctions, see Fig. 7. The size of these particles was in the range 30–90 nm, and a number of them formed agglomerates as shown in Fig. 7. Particle agglom￾erates present in multi-grain junctions were generally associated with cavities, see Fig. 7c. The intragranular SiC particles did not form clusters, and were smaller, typically 10–50 nm, see Fig. 7a. Strain contours were only occasionally observed in the mullite matrix immediately surrounding a SiC particle. Some dislocations were pinned by, or emerging from, the intra- and intergranular SiC particles, but the majority of the observed dislocations were, as in the mullite microstructure, associated with intragranular cavities, see Fig. 7b. Although the overall dislocation density was low, dislocation pile-ups at triple grain junctions or at grain boundaries were observed in a few areas. Thin glassy films were observed in the matrix grain bound￾aries along with smaller volumes of glass separating both intragranular and intergranular SiC particles from the surround￾ing mullite grains, see Figs. 8 and 9. The thickness of the amorphous grain boundary films varied between 0.6 and 0.9 nm. Very small (∼10–15 nm) amorphous pockets were observed at some multi-grain junctions, see Fig. 8. The Al2O3 mol fraction of the mullite matrix was determined to 55.9 ± 1.5%, i.e. also in this case lower than for 3:2 mullite. The intergranular glass was enriched in silicon, although the Fig. 8. Thin glassy grain boundary films (white arrows) merging into a glass containing triple grain junction (black arrow) in the as-sintered nanocomposite material. The glass appears with bright contrast in the diffuse TEM dark field image.