S Gustafsson et al. / Journal of the European Ceramic Society 29(2009)539-550 P (ah 500nm 200nm 500nm Fig. 3. The microstructure of the as-sintered polycrystalline mullite (TEM). a) Intergranular porosity(P), and a dislocation pile-up at a grain boundary (arrowed ).(b) Dislocation network(arrowed) associated with cavities on a larger 25 nm homogenised by milling for I h in a planetary mill using Si3N4 balls whereafter 3 wt% of a polyethylene glycol binder was added to the slip. In order to retain a homogeneous distribution of(c) the Sic nanoparticles, the slip was freeze granulated by sprayin into liquid nitrogen. The ice was removed by sublimation using a freeze dryer and the resulting granules were hot pressed into plates at 1600C for I h in an argon atmosphere at a maximum pressure of 40 MPa. Hot pressing has been widely used for pro- ducing dense nanocomposite materials since the nanoparticles may suppress full densification 24.6 The nanocomposite mate- rial in the present study reached nearly full density, 99.8%,as measured by a helium pycnometer. 3. 2. Microstructural characterization and instrumentation 20nm The as-sintered and creep tested materials included in the microstructural characterization are shown in Table 1. Polished Fig. 4. Amorphous pockets at triple grain junctions and glassy grain bound- and thermally etched (45 min at 1300C in argon)specimens ary films in the as-sintered polycrystalline mullite (TEM).(a) Glass containing were imaged in a SEM(Leo ULTRA 55)equipped with a field triple grain junctions(arrowed).(b)Diffuse dark field image of a thin glassy emission gun(FEG)in order to assess grain size and overall grain boundary film(arrowed) merging into an amorphous pocket. The glass homogeneity. The average grain size was determined by the tion Fresnel fringes(arrowed) extending along the grain boundaries reveal the mean linear intercept method, and the average intercept length presence of thin intergranular films merging into a pocket at the triple grain was multiplied by a factor of 1.5542 S. Gustafsson et al. / Journal of the European Ceramic Society 29 (2009) 539–550 Fig. 3. The microstructure of the as-sintered polycrystalline mullite (TEM). (a) Intergranular porosity (P), and a dislocation pile-up at a grain boundary (arrowed). (b) Dislocation network (arrowed) associated with cavities on a larger elongated grain section. homogenised by milling for 1 h in a planetary mill using Si3N4 balls whereafter 3 wt% of a polyethylene glycol binder was added to the slip. In order to retain a homogeneous distribution of the SiC nanoparticles, the slip was freeze granulated by spraying into liquid nitrogen. The ice was removed by sublimation using a freeze dryer and the resulting granules were hot pressed into plates at 1600 ◦C for 1 h in an argon atmosphere at a maximum pressure of 40 MPa. Hot pressing has been widely used for pro￾ducing dense nanocomposite materials since the nanoparticles may suppress full densification.2–4,6 The nanocomposite mate￾rial in the present study reached nearly full density, 99.8%, as measured by a helium pycnometer. 3.2. Microstructural characterization and instrumentation The as-sintered and creep tested materials included in the microstructural characterization are shown in Table 1. Polished and thermally etched (45 min at 1300 ◦C in argon) specimens were imaged in a SEM (Leo ULTRA 55) equipped with a field emission gun (FEG) in order to assess grain size and overall homogeneity. The average grain size was determined by the mean linear intercept method, and the average intercept length was multiplied by a factor of 1.5. Fig. 4. Amorphous pockets at triple grain junctions and glassy grain bound￾ary films in the as-sintered polycrystalline mullite (TEM). (a) Glass containing triple grain junctions (arrowed). (b) Diffuse dark field image of a thin glassy grain boundary film (arrowed) merging into an amorphous pocket. The glass appears with bright contrast. (c) Defocus Fresnel image of a triple grain junc￾tion. Fresnel fringes (arrowed) extending along the grain boundaries reveal the presence of thin intergranular films merging into a pocket at the triple grain junction