C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 central layer with graded composition (Al2O3-Y-TZP) EPD was performed with the Al_ Y-TZP sol under a and a hard outer surface layer of pure alumina were constant voltage of 10 V using a deposition time of 2.5 produced from nano-size sols using EPD in an attempt min and with magnetic stirring during the EPD. Under to generate a continuous property variation across thethe applied electric field, the boehmite and zirconia final component and to improve the microstructural particles(possessing a net negative surface charge at pH features in terms of grain size, hardness and fracture 9.2) migrated towards the positive electrode, i.e., the toughnes deposition electrode (see Fig. 1). The particles were deposited until a matrix thickness of 1.4 mm was achieved. The central deposition electrode was con 2. Experimental work nected to a balance linked to a computer. The apparatus Two different colloidal sols, i. e. pure alumina and alumina plus Y-TZP, to be used in the EPd experi- Electrode什+) ments were prepared. In order to obtain the Al2O3-Y Electrode(-) TZP structure after sintering, equal amounts of very fine (the average particle size is 30 nm, VP zirconia, Degussa td, Germany), medium size(150 nm, BDH Chemicals, UK) and coarse(400 nm, Dai-chi Ltd, Japan) zirconia powders were first dispersed in distilled water with the addition of 3 mol% Y2O3. To prepare a kinetically First step of stable zirconia sol. the distilled water was stirred vigo AlOOH-Y-TZP sol ously while the nano-size zirconia powders were added at a rate of 0.5 g min=. The low rate of addition pre- rented the formation of large heteroflocculated clusters The resultant sol and 0.5 wt. binder(Celacol, in order to increase the green strength) were then added to a boehmite sol (r-AlOOH) containing spherical particles with an average particle size of 100 nm(Alcan Chemi- cals, UK)and then ball-mixed all together for I day(the EPD formed boehmite sol was seeded with 2 wt. T1O2 particles in order to lower the a-alumina transformation tempera- ture). The solids-loading of the final sol was adjusted to be 20 wt. of the dispersion liquid with simultaneous ultrasonic agitation to enhance powder dispersion Homogeneous, well-dispersed and agglomerate-free stable suspensions were obtained at a pH value of 9.2. A commercial a-Al2O3 powder (Tai-micron, Japan) (+) was used as the pure alumina source. As received alu mina powders contain spherical particles with an aver age particle size of 150 nm. These powders were dispersed in distilled water at a pH value of 4 with the addition of 0.5 wt. binder(Celacol)and the solids- ding of the prepared suspension was adjusted to be Second step of 20 wt. of the dispersion medium. The final sol was Pure AlO3 sol ball-mixed for I day An in-situ electrophoretic deposition cell was used in order to manufacture functionally graded compo- nents. 7 A stainless steel rod 0.3 mm in diameter was used as deposition electrode (+) whilst a tubular stain Pure AhO3 surface less steel electrode 40 mm in diameter was used as the negative electrode (cathode). The central electrode was EPD formed coated with a very thin C layer in order to allow easy FGM of removel of the EPD formed green deposit before sin- tubula Graded Al,O3-Y-TZP tering. The distance between the two electrodes was hape chosen to be 17 mm The epd cell electrodes were con Fig. I. Processing of Al2Or-Y-TZP/Al2O3 FGM of tubular shape nected to a 0-60 v d.c. power supply. The first stage of using double-step EPDcentral layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina were produced from nano-size sols using EPD in an attempt to generate a continuous property variation across the final component and to improve the microstructural features in terms of grain size, hardness and fracture toughness. 2. Experimental work Two different colloidal sols, i.e. pure alumina and alumina plus Y–TZP, to be used in the EPD experi￾ments were prepared. In order to obtain the Al2O3–Y￾TZP structure after sintering, equal amounts of very fine (the average particle size is 30 nm, VP zirconia, Degussa Ltd, Germany), medium size (150 nm, BDH Chemicals, UK) and coarse (400 nm, Dai-chi Ltd, Japan) zirconia powders were first dispersed in distilled water with the addition of 3 mol% Y2O3. To prepare a kinetically stable zirconia sol, the distilled water was stirred vigor￾ously while the nano-size zirconia powders were added at a rate of 0.5 g min
1 . The low rate of addition pre￾vented the formation of large heteroflocculated clusters. The resultant sol and 0.5 wt.% binder (Celacol, in order to increase the green strength) were then added to a boehmite sol (g-AlOOH) containing spherical particles with an average particle size of 100 nm (Alcan Chemi￾cals, UK) and then ball-mixed all together for 1 day (the boehmite sol was seeded with 2 wt.% TiO2 particles in order to lower the a-alumina transformation tempera￾ture). The solids-loading of the final sol was adjusted to be 20 wt.% of the dispersion liquid with simultaneous ultrasonic agitation to enhance powder dispersion. Homogeneous, well-dispersed and agglomerate-free stable suspensions were obtained at a pH value of 9.2. A commercial a-Al2O3 powder (Tai-micron, Japan) was used as the pure alumina source. As received alu￾mina powders contain spherical particles with an aver￾age particle size of 150 nm. These powders were dispersed in distilled water at a pH value of 4 with the addition of 0.5 wt.% binder (Celacol) and the solids￾loading of the prepared suspension was adjusted to be 20 wt.% of the dispersion medium. The final sol was ball-mixed for 1 day. An in-situ electrophoretic deposition cell was used in order to manufacture functionally graded compo￾nents.17 A stainless steel rod 0.3 mm in diameter was used as deposition electrode (+) whilst a tubular stain￾less steel electrode 40 mm in diameter was used as the negative electrode (cathode). The central electrode was coated with a very thin C layer in order to allow easy removel of the EPD formed green deposit before sin￾tering. The distance between the two electrodes was chosen to be 17 mm. The EPD cell electrodes were con￾nected to a 0–60 V d.c. power supply. The first stage of EPD was performed with the Al2O3–Y-TZP sol under a constant voltage of 10 V using a deposition time of 2.5 min and with magnetic stirring during the EPD. Under the applied electric field, the boehmite and zirconia particles (possessing a net negative surface charge at pH 9.2) migrated towards the positive electrode, i.e., the deposition electrode (see Fig. 1). The particles were deposited until a matrix thickness of 1.4 mm was achieved. The central deposition electrode was con￾nected to a balance linked to a computer. The apparatus Fig. 1. Processing of Al2O3–Y-TZP/Al2O3 FGM of tubular shape using double-step EPD. 1656 C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660