C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 is able to record the weight gain per millisecond during the deposition process, i.e., in real time. After the first AlO, Surface stage EPD, the deposition electrode surrounded with he boehmite plus zirconia deposit layer in green state was put in the pure alumina suspension and the central deposition electrode was connected to the negative terminal of the power supply as alumina particles have positive surface charge at the working pH value of 4 The second stage EPD was performed using the same Graded AL Oj-Y-T7P voltage of 10 V d. c. for a deposition time of 1.5 min in order to obtain a thin alumina layer around the first formed thicker boehmite plus zirconia layer. The final green body specimens in tubular shape containing an inner layer of boehmite plus zirconia surrounded by a thin layer of pure alumina were dried under humidity Fig. 2. SEM micrograph of the EPD-formed FGM of tubular shape controlled atmosphere for I day and left in ambient air showing the XRD and SEM analysed points within the graded layer for another day before being pressureless sintered at and the thickness of the pure alumina surface layer to be 100 um 1400°Cfor2h. The surface properties of the colloidal sol particles in terms of their electrophoretic mobility and net surface using X-ray diffraction and also an image analyser for harge were determined using a surface charge analyser comparison (DELSA 440 surface charge analyser). Microstructural The graded internal layer of Al2O3-Y-TZP is formed observations were carried out by using a field emission by the controlled and engineered EPD using nano sol un scanning electron microscope(FEG SEM, Hitachi particles having different particle sizes. The electro- FX-4000, Japan) on sintered, fractured and thermally phoretic mobility of each particle within the mixed sol etched(1300C for 20 min) surfaces. Fracture tough- will be different as this depands on the particle sizes, and ness and hardness were determined using the Vickers hence the masses. 14, 17 Particles with smaller diameter indentation technique. 2I Grain size measurements were will have higher mobility than those of larger size.The conducted using the linear intercept technique. 2 and SEM pictures shown in Fig 3 indicate the difference in the interfacial behaviour of the composites between the composition from the centre to the alumina layer prov graded layer and the pure alumina surface layer was ing the validity of the 'particle mobility-particle size characterised using the crack path propagation tech- relationships concept. The SEM micrograph shown in Fig. 3a represents the microstructure at the point A (dark phase is alumina and the light phase is TZP). The volume fraction of the TZP phase was found to be 71% 3. Results and discussion at this point (r=150 um). The composition of the gra- ded layer shows a gradual change in composition as The sintered (1400oC for 2 h) microstructure of the shown in Fig. 3b and c. The volume fraction of TZP tubular Al2O3-Y-TZP/Al2O3 functionally graded com- phase decreases from 35%(point B, r=350 um) to 13% posite incorporating a tough central layer with graded (point C, r=550 um). sEM pictures taken along the composition(AlOxY-TZP)and a hard outer surface radius of the sintered FGM show an increase in alumina layer of pure alumina is shown in Fig. 2. From the SEM volume fraction from the centre to the pure alumina picture it is clear that the thickness of the outer alumina layer and they also show a significant change in micro layer is about 100 um indicating the ability of electro- structure as shown in Fig. 3. Table I shows the rela- phoretic deposition to produce a homogeneous and tionships between the composition of the graded layer and controlled surface layer. It is also seen from Fig. 2 that the grain size of the alumina and tzp grains. The finest the pure alumina surface layer is continuous and alumina grain size(0.65 um)was found at the point A homogeneous in thickness and also that there is no where the volume fraction of TZP is 71% whilst this value crack formation within the graded layer or alumina increases to 1.85 um at the point C when the TZP volume surface layer. The FGM produced has a central hole fraction decreases to 13%. However, the TzP grain size 0.25 mm in diameter, a layer of Al2OxY-TZP 1 4 mm along the radius of the FGM seems to be independent of in diameter and a surface alumina layer 100 um in he composition. The SEM microstructure of the pure thickness. In order to analyse the composition within alumina surface layer is shown in Fig. 3d indicating the the graded section of the composite, detail SEM pictures presence of a dense microstructure with an average were taken from the points shown in Fig. 2 and the alumina grain size of 2.6 um. It can be concluded from amount of alumina and Y-TZP phases were calculated the results presented in Fig. 3 that the Tzp grainsis able to record the weight gain per millisecond during the deposition process, i.e., in real time. After the first stage EPD, the deposition electrode surrounded with the boehmite plus zirconia deposit layer in green state was put in the pure alumina suspension and the central deposition electrode was connected to the negative terminal of the power supply as alumina particles have positive surface charge at the working pH value of 4. The second stage EPD was performed using the same voltage of 10 V d.c. for a deposition time of 1.5 min in order to obtain a thin alumina layer around the first formed thicker boehmite plus zirconia layer. The final green body specimens in tubular shape containing an inner layer of boehmite plus zirconia surrounded by a thin layer of pure alumina were dried under humidity controlled atmosphere for 1 day and left in ambient air for another day before being pressureless sintered at 1400 C for 2 h. The surface properties of the colloidal sol particles in terms of their electrophoretic mobility and net surface charge were determined using a surface charge analyser (DELSA 440 surface charge analyser). Microstructural observations were carried out by using a field emission gun scanning electron microscope (FEG SEM, Hitachi FX-4000, Japan) on sintered, fractured and thermally etched (1300 C for 20 min) surfaces. Fracture tough￾ness and hardness were determined using the Vickers indentation technique.21 Grain size measurements were conducted using the linear intercept technique.22 and the interfacial behaviour of the composites between the graded layer and the pure alumina surface layer was characterised using the crack path propagation tech￾nique.23 3. Results and discussion The sintered (1400 C for 2 h) microstructure of the tubular Al2O3–Y-TZP/Al2O3 functionally graded com￾posite incorporating a tough central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina is shown in Fig. 2. From the SEM picture it is clear that the thickness of the outer alumina layer is about 100 mm indicating the ability of electro￾phoretic deposition to produce a homogeneous and controlled surface layer. It is also seen from Fig. 2 that the pure alumina surface layer is continuous and homogeneous in thickness and also that there is no crack formation within the graded layer or alumina surface layer. The FGM produced has a central hole 0.25 mm in diameter, a layer of Al2O3–Y-TZP 1.4 mm in diameter and a surface alumina layer 100 mm in thickness. In order to analyse the composition within the graded section of the composite, detail SEM pictures were taken from the points shown in Fig. 2 and the amount of alumina and Y–TZP phases were calculated using X-ray diffraction and also an image analyser for comparison. The graded internal layer of Al2O3–Y-TZP is formed by the controlled and engineered EPD using nano sol particles having different particle sizes. The electro￾phoretic mobility of each particle within the mixed sol will be different as this depands on the particle sizes, and hence the masses.14,17 Particles with smaller diameter will have higher mobility than those of larger size. The SEM pictures shown in Fig. 3 indicate the difference in composition from the centre to the alumina layer prov￾ing the validity of the ‘particle mobility-particle size relationships’ concept. The SEM micrograph shown in Fig. 3a represents the microstructure at the point A (dark phase is alumina and the light phase is TZP). The volume fraction of the TZP phase was found to be 71% at this point (r=150 mm). The composition of the gra￾ded layer shows a gradual change in composition as shown in Fig. 3b and c. The volume fraction of TZP phase decreases from 35% (point B, r=350 mm) to 13% (point C, r=550 mm). SEM pictures taken along the radius of the sintered FGM show an increase in alumina volume fraction from the centre to the pure alumina layer and they also show a significant change in micro￾structure as shown in Fig. 3. Table 1 shows the rela￾tionships between the composition of the graded layer and the grain size of the alumina and TZP grains. The finest alumina grain size (0.65 mm) was found at the point A where the volume fraction of TZP is 71% whilst this value increases to 1.85 mm at the point C when the TZP volume fraction decreases to 13%. However, the TZP grain size along the radius of the FGM seems to be independent of the composition. The SEM microstructure of the pure alumina surface layer is shown in Fig. 3d indicating the presence of a dense microstructure with an average alumina grain size of 2.6 mm. It can be concluded from the results presented in Fig. 3 that the TZP grains Fig. 2. SEM micrograph of the EPD-formed FGM of tubular shape showing the XRD and SEM analysed points within the graded layer and the thickness of the pure alumina surface layer to be 100 mm. C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660 1657