C. Kaya et al /Journal of the European Ceramic Society 23(2003)935-94 pastes of alumina and zirconia are co-extruded in par- The seeded sol was first stirred magnetically for 10 h allel, and layed-up in closed-packed linear array to form and then ultrasonic agitation was employed at 15 kHz a heterogeneous macro-plug for subsequent extrusion for 3 h for further dispersion of any particle agglomer provided that the flow properties of the chemically dif- ates which might be present. 6 The final sol composition ferent pastes are similar. i.e., boehmite +2 wt. seeding powder+I wt. gly cerol+I wt. celacol was ball-mixed for 2 days using high purity zirconia balls. Before and after ball-milling 2. Experimental wor the zirconia balls were weighted to ensure that there was no contamination resulting from the milling media. The 2.1. Paste preparation from seeded boehmite (y- mixed seeded sol was then vacuum filtered in order to A1OOH)sol obtain a gel structure. The resulting soft white gel was further compacted using pressure filtration apparatus to Boehmite (r-AlOOH)sol(Remal A20, Remet corp, squeeze out the excess water, and obtain an extrudable USA) was used as the alumina source. The sol ha Paste I average particle size and solids-loading of 40 nm and 20 wt%, respectively. The as received sol is stable at a pl 2.2. Zirconia sol-paste preparation value of 4. The received boehmite sol was seeded with 2 wt% of the total mass using ultrafine a-Al2O3(30 nm, Zirconia sol was prepared using ultrafine and high emi gh purity polishing powder) purity zirconia powders(average grain size is 30 nm, VP powders. The flow chart for the seeding process, fol- zirconia, Degussa Ltd, Germany) with the addition of 3 lowed by paste preparation is given in Fig. 1. To seed mol% yttria. Kinetically stable and well dispersed zir the boehmite sol, the seeding powders was first dis- conia sol having 20 wt solids-loading was prepared persed in distilled water and then added into boehmite by the addition of small amount of zirconia to the sol. Glycerol (1 wt %)and celacol (I wt %)were also water, while the suspension was magnetically stirred added in order to minimise the surface roughness of the The best pH value in order to obtain the maximum sta extruded and increase the green strength, respectively. bility was found to be 8.5 and this ph value was ma Celacol (1 wt %)was first dissolved in water at 85C tained using ammonia. I wt. glycerol and celacol were and then added to the seeded sol in order keep it plasti- added to prepared zirconia sol. Cyclohexanone cally deformable during the multiple extrusion stage. ( C6H1oO)(I wt %)and I wt% boehmite sol were also added in order to minimise the shear-thickening effect The flow chart for zirconia gel and paste preparation is iven in Fig. 2. Each paste was then laboratory scale extrusion apparatus with an extrusion reduction ratio of 4: 1. The ram velocity chosen was very low(0.5 or I mm/min )to minimise edge-tearing defects Dispersed in water, ultrasonic that might form at higher velocities. Flow behaviour of each paste was determined using a computer program linked to the Instron test machine. Rheological beha Boehmite(AIOOH viour of each material. i.e. boehmite and zirconia was controlled to be similar for multiple extrusion Ball-milling for 2 The rheological behaviour of boehmite and zirconia Kinetically stable sol-derived pastes was characterised by a capillary rheometer 18-20 Three dies with different L/D(length/ Vacuum filtering diameter)ratios were used. The ram speeds ranged from 24 to 0.5 mm/min. and the load at each-pre-set velocity vas recorded. The paste rheology was characterise 40-45 wt so solids using the Benbow-Bridgwater relationship Pressure filtration P=2Ln(Do/D(oo +aI)+4(L/D(To +B,)(1) Extrusion where P is the pressure, Do and D are the diameters of the barrel and the die, respectively, L is the length of the Fig. 1. Flow chart for the preparation of boehmite sol-derived past die land and v is the extrudate velocity. The pastepastes of alumina and zirconia are co-extruded in par￾allel, and layed-up in closed-packed linear array to form a heterogeneous macro-plug for subsequent extrusion provided that the flow properties of the chemicallydif￾ferent pastes are similar. 2. Experimental work 2.1. Paste preparation from seeded boehmite (
- AlOOH) sol Boehmite (g-AlOOH) sol (Remal A20, Remet corp., USA) was used as the alumina source. The sol has average particle size and solids-loading of 40 nm and 20 wt.%, respectively. The as received sol is stable at a pH value of 4. The received boehmite sol was seeded with 2 wt.% of the total mass using ultrafine a-Al2O3 (30 nm, BDH Chemical, UK., high puritypolishing powder) powders. The flow chart for the seeding process, fol￾lowed bypaste preparation is given in Fig. 1. To seed the boehmite sol, the seeding powders was first dis￾persed in distilled water and then added into boehmite sol. Glycerol (1 wt.%) and celacol (1 wt.%) were also added in order to minimise the surface roughness of the extruded and increase the green strength, respectively. Celacol (1 wt.%) was first dissolved in water at 85 C and then added to the seeded sol in order keep it plasti￾callydeformable during the multiple extrusion stage. The seeded sol was first stirred magneticallyfor 10 h and then ultrasonic agitation was employed at 15 kHz for 3 h for further dispersion of anyparticle agglomer￾ates which might be present.16 The final sol composition i.e., boehmite+2 wt.% seeding powder+1 wt.% gly￾cerol+1 wt.% celacol was ball-mixed for 2 days using high purityzirconia balls. Before and after ball-milling, the zirconia balls were weighted to ensure that there was no contamination resulting from the milling media. The mixed seeded sol was then vacuum filtered in order to obtain a gel structure. The resulting soft white gel was further compacted using pressure filtration apparatus to squeeze out the excess water, and obtain an extrudable paste.17 2.2. Zirconia sol-paste preparation Zirconia sol was prepared using ultrafine and high purityzirconia powders (average grain size is 30 nm, VP zirconia, Degussa Ltd., Germany) with the addition of 3 mol% yttria. Kinetically stable and well dispersed zir￾conia sol having 20 wt.% solids-loading was prepared bythe addition of small amount of zirconia to the water, while the suspension was magneticallystirred. The best pH value in order to obtain the maximum sta￾bilitywas found to be 8.5 and this pH value was main￾tained using ammonia. 1 wt.% glycerol and celacol were added to prepared zirconia sol. Cyclohexanone (C6H10O) (1 wt.%) and 1 wt.% boehmite sol were also added in order to minimise the shear-thickening effect. The flow chart for zirconia gel and paste preparation is given in Fig. 2. Each paste was then extruded using a laboratoryscale extrusion apparatus with an extrusion reduction ratio of 4:1. The ram velocitychosen was very low (0.5 or 1 mm/min.) to minimise edge-tearing defects that might form at higher velocities. Flow behaviour of each paste was determined using a computer program linked to the Instron test machine. Rheological beha￾viour of each material, i.e., boehmite and zirconia was controlled to be similar for multiple extrusion. 2.3. Paste rheology The rheological behaviour of boehmite and zirconia sol-derived pastes was characterised bya capillary rheometer.1820 Three dies with different L/D (length/ diameter) ratios were used. The ram speeds ranged from 124 to 0.5 mm/min. and the load at each-pre-set velocity was recorded. The paste rheologywas characterised using the Benbow–Bridgwater relationship:21 P ¼ 2Lnð Þ Do=D o þ 1Vm ð Þþ 4ð Þ L=D o þ 1Vm ð Þð1Þ where P is the pressure, Do and D are the diameters of the barrel and the die, respectively, L is the length of the Fig. 1. Flow chart for the preparation of boehmite sol-derived paste. die land and V is the extrudate velocity. The paste 936 C. Kaya et al. / Journal of the European Ceramic Society 23 (2003) 935–942