C. Kaya et al /Journal of the European Ceramic Society 23(2003)935-94 Z i 50w m 六5 Fig 9. SEM images of 3rd stage co-extruded alumina/zirconia com- ponent, showing the fine microstructure of zirconia phase and dense Fig. 8. SEM images of co-extruded boehmite/zirconia multiphase microstructure of a-alumina(A: a-alumina, Z: zirconia). Sample sin- tered at I400°for2h interface if the rheological behaviour of two pastes is controlled to similar and the difference in sintering shrinkage is <3%.(a)cross average grain size of alumina and zirconia within the sectional and(b) longitudinal view(sample was sintered at 1400Cfor co-extruded and sintered bi-phase component was determined to be 1.6 and 0.45 um, respectively. The conia microstructure contains only equiaxed grains the any phase or at the interface is avoided, as shown in whilst the majority of grains in the alumina matrix are Fig. 8(a)and(b). It should also be noted from Fig8 near equiaxed with the presence of some elongated ones. that there is a shape distortion in alumina and zirconia as shown in Fig 9. These fine scale microstructures are due to difference in water removal behaviour of the expected to provide good mechanical and thermo- pastes during extrusion as they have different particle mechanical properties. Figs. 7-9 show the micro- shapes and packing densities in the paste form. How- structural features in assessing the effectiveness of the ever, this is considered not to be a critical issue provid- co-extrusion technique in order to produce di-phasic ing the each filament within the co-extruded structure is aligned microstructures and it can be concluded from conunuous these micrographs that the rheological matching of the Fig 9 demonstrates a 3rd stage co-extruded alumina/ two different pastes coupled with differential sintering zirconia microstructure with the absence of any inter or shrinkage's play the most critical role for a successful intragranular porosity as well as cracks after sintering at co-extrusion Shape distortion of each filament during 1400C for 2 h. One of the main objectives of the pre- co-extrusion is also noted as shown in Figs. 8 and 9 ent work is to control the fine-scale sintered micro- This distortion in shape can be eliminated using lower structure of the each phase present in the final sintered extrusion reduction ratios or extrusion rates component using ultrafine starting powders (<100 nm) The microstructures of zirconia coated monofilament Fig 9 shows the very fine and dense microstructures of using dip-coating are shown in Fig. 10. Homogeneous zirconia (in TZP form by the addition of 3 mol%Y203) coating around an alumina rod in green state is evident and a-alumina after sintering at 1400oC for 2 h. The from the micrograph shown Fig 10(a) and its thicknessthe anyphase or at the interface is avoided, as shown in Fig. 8(a) and (b). It should also be noted from Fig. 8 that there is a shape distortion in alumina and zirconia due to difference in water removal behaviour of the pastes during extrusion as theyhave different particle shapes and packing densities in the paste form. How￾ever, this is considered not to be a critical issue provid￾ing the each filament within the co-extruded structure is continuous. Fig. 9 demonstrates a 3rd stage co-extruded alumina/ zirconia microstructure with the absence of anyinter or intragranular porosityas well as cracks after sintering at 1400 C for 2 h. One of the main objectives of the pre￾sent work is to control the fine-scale sintered micro￾structure of the each phase present in the final sintered component using ultrafine starting powders (<100 nm). Fig. 9 shows the veryfine and dense microstructures of zirconia (in TZP form bythe addition of 3 mol% Y2O3) and a-alumina after sintering at 1400 C for 2 h. The average grain size of alumina and zirconia within the co-extruded and sintered bi-phase component was determined to be 1.6 and 0.45 mm, respectively. The zir￾conia microstructure contains onlyequiaxed grains whilst the majorityof grains in the alumina matrix are near equiaxed with the presence of some elongated ones, as shown in Fig. 9. These fine scale microstructures are expected to provide good mechanical and thermo￾mechanical properties. Figs. 7–9 show the micro￾structural features in assessing the effectiveness of the co-extrusion technique in order to produce di-phasic aligned microstructures and it can be concluded from these micrographs that the rheological matching of the two different pastes coupled with differential sintering shrinkage’s playthe most critical role for a successful co-extrusion. Shape distortion of each filament during co-extrusion is also noted as shown in Figs. 8 and 9. This distortion in shape can be eliminated using lower extrusion reduction ratios or extrusion rates. The microstructures of zirconia coated monofilament using dip-coating are shown in Fig. 10. Homogeneous coating around an alumina rod in green state is evident from the micrograph shown Fig. 10(a) and its thickness Fig. 8. SEM images of co-extruded boehmite/zirconia multiphase component, showing the absence of cracks within the phases or at the interface if the rheological behaviour of two pastes is controlled to be similar and the difference in sintering shrinkage is <3%. (a) cross sectional and (b) longitudinal view (sample was sintered at 1400 C for 2 h). Fig. 9. SEM images of 3rd stage co-extruded alumina/zirconia com￾ponent, showing the fine microstructure of zirconia phase and dense microstructure of a-alumina (A: a-alumina, Z: zirconia). Sample sin￾tered at 1400 C for 2 h. 940 C. Kaya et al. / Journal of the European Ceramic Society 23 (2003) 935–942