C. Kaya et al. /Journal of the European Ceramic Society 23(2003)935-942 8 Shear Rate,(s1)×104 力sg (a) Extrudate Velocity, (m/s)x104 000834 1500 as a function of extrudate velocity for a constant L/D: 16 for boehmite and zirconia sol-derived pastes. Fig. 7. SEM images of co-extruded boehmite/zirconia multiphase component, showing the presence of cracks within the zirconia phase (light phase)if the rheological behaviour of two pastes are not similar and 2, both pastes used in this work are water based and and the difference in sintering shrinkage is about 7%.(a) cross sectional contains very small amount of additives as binder or and(b) longitudinal view(sample was sintered at 1400C for 2 h ) solvent. therefore, it can be concluded that the rheolo- gical behaviour of each paste is dependant on the part the interface when zirconia shows "shear thickening cle characteristics such as size, shape and loading as well behaviour during extrusion and the difference in linear as the chosen processing technique of the pastes. The sintering shrinkage between these two phases is about developed technique for the preparation of sol-derived 7% as shown in Fig. 7(a). The presence of extensive pastes provides homogeneously mixed pastes suitable cracks(caused by the difference in sintering shrinkage) for co-extrusion under the same extrusion parameters. perpendicular to the extrusion direction within the zir- Otherwise, it is not possible to prepare a paste from conia filaments is also evident from the longiditunal nano-size particles using conventional paste preparation section of co-extruded sintered filament shown in echniques Fig. 7(b). If the right rheology, drying cycle and sinter- The other critical parameter is the drying behaviour ing shrinkage are optimised using the necessary addi of the pastes. Fig. 7(a) shows a cross-sectional SEM tions (see Figs. 1 and 2), crack free co-extruded micrograph, indicating the effect of the rheology filaments are produced as shown in Fig 8. Fig 8(a)and matching of the two pastes on the microstructure of co-(b)show the cross-sectional and longidutional micro extruded component. For successful co-extrusion, both structure of a 2nd stage co-extruded alumina/ zirconia bi rheology, the drying and the sintering shrinkage of the phase filament after sintering at 1400C for 2 h In order pastes should be controlled in order to control the to optimise the linear sintering shrinkage of zirconia, 2 internal stresses and cracking. Fig. 7(a) shows a lst wt. coarse zirconia powders(300 nm)were added to the stage co-extruded alumina/zirconia microstructure after main composition as described in Fig. 2, so that the dif- sintering at 1400C for 2 h Significant crack formation ference in sintering shrinkage between these two phases is is visible within the zirconia phase (light phase) also controlled to be less than 3%. Crack formation withinand 2, both pastes used in this work are water based and contains verysmall amount of additives as binder or solvent, therefore, it can be concluded that the rheolo￾gical behaviour of each paste is dependant on the parti￾cle characteristics such as size, shape and loading as well as the chosen processing technique. of the pastes. The developed technique for the preparation of sol-derived pastes provides homogeneouslymixed pastes suitable for co-extrusion under the same extrusion parameters. Otherwise, it is not possible to prepare a paste from nano-size particles using conventional paste preparation techniques. The other critical parameter is the drying behaviour of the pastes. Fig. 7(a) shows a cross-sectional SEM micrograph, indicating the effect of the rheology matching of the two pastes on the microstructure of co￾extruded component. For successful co-extrusion, both rheology, the drying and the sintering shrinkage of the pastes should be controlled in order to control the internal stresses and cracking. Fig. 7(a) shows a 1st stage co-extruded alumina/zirconia microstructure after sintering at 1400 C for 2 h. Significant crack formation is visible within the zirconia phase (light phase) also at the interface when zirconia shows ‘‘shear thickening’’ behaviour during extrusion and the difference in linear sintering shrinkage between these two phases is about 7% as shown in Fig. 7(a). The presence of extensive cracks (caused bythe difference in sintering shrinkage) perpendicular to the extrusion direction within the zir￾conia filaments is also evident from the longiditunal section of co-extruded sintered filament shown in Fig. 7(b). If the right rheology, drying cycle and sinter￾ing shrinkage are optimised using the necessaryaddi￾tions (see Figs. 1 and 2), crack free co-extruded filaments are produced as shown in Fig. 8. Fig. 8(a) and (b) show the cross-sectional and longidutional micro￾structure of a 2nd stage co-extruded alumina/zirconia bi￾phase filament after sintering at 1400 C for 2 h. In order to optimise the linear sintering shrinkage of zirconia, 2 wt.% coarse zirconia powders (300 nm) were added to the main composition as described in Fig. 2, so that the dif￾ference in sintering shrinkage between these two phases is controlled to be less than 3%. Crack formation within Fig. 6. (a) Shear stress–shear rate and (b) measured pressure changes as a function of extrudate velocityfor a constant L/D: 16 for boehmite and zirconia sol-derived pastes. Fig. 7. SEM images of co-extruded boehmite/zirconia multiphase component, showing the presence of cracks within the zirconia phase (light phase) if the rheological behaviour of two pastes are not similar and the difference in sintering shrinkage is about 7%. (a) cross sectional and (b) longitudinal view (sample was sintered at 1400 C for 2 h.). C. Kaya et al. / Journal of the European Ceramic Society 23 (2003) 935–942 939