H. Hadraba et al /Ceramics International 30(2004)853-863 。 25bz=0z9z6u25°> 100 120140 TIME [min ig. 10. Dependence of alumina volume concentration in alumina/zirconia suspension in alumina/zirconia functinally gradient material FGM HP/SY-5 on deposition time was calculated using the relation ticles on the electrode. To express the change in suspen- (h/2.78)-1 sion composition caused by particle deposition the relations 0.0338 (2) and values were used that had been found for the deposi- tion of one-component deposits [11]. The good agreement which is inverse to the relation for one-component deposit between the theoretical suspension composition and the de- Al203 [11]. Applying this relation also in the case of the posit composition established(see Fig. 10) implies the va- functionally gradient composite Al2O3/ZrO2 is justified lidity of all the assumptions made: electrophoretic mobility view of the fact that identical electrophoretic mobility of of both components in the suspension was identical [10], the Al2O3 and ZrO2 particles in isopropanol suspensions stabi- relations derived in the preceding work [11] for deposition lized with MCAA was established [10] and the validity of kinetics are valid and they also hold for the two-component this equality has also been confirmed in the case of a mix system ture of Al2O3 and ZrO2 particles( Section 3.2) As reported in the preceding work [ll the length of The theoretical time dependence of the suspension com- indentation cracks propagating from the corners of inden- position was calculated on the basis of an iterative pro- tation by Vickers indentor to the material surface provides cedure, when for every interruption of deposition the new for conclusions as to the size of fracture toughness of the volume concentrations Al2O3(C1) and ZrO2(ch+)) in the material. Using this method, Nicholson[17] demonstrated solution were calculated from the initial volume concentra- that the fracture toughness of functionally gradient material tions Al2O3(cA)and ZrO2(c)according to the relations is directly proportional to its composition. Fig. 11 gives CVI-ciVo 0.0338△r the fracture toughness of functionally gradient material (3) FGM HP/Y-5 as a function of its composition. As can be seen from Fig. 11, the fracture toughness in transversal c2,=W0-2=2v(1-e-03842)+c4 direction is proportional to the composition of functionally (4) gradient composite and it is higher than the fracture tough ness of pure phases. The fracture toughness in longitudina ere Vo is the total suspension volume, Vi is the volume of direction deflects from the linear dependence. As stated noved and added ZrOz suspension when the deposition is above, due to the different green densities throughout the interrupted (5 ml)and At is the time between individual de- deposit, the deposit was deformed during drying and sin- position interruptions (5 min). The initial Al2 O3 suspensio tering. This deformation was caused by internal stresses concentration was =3.61 voL %, 6=0 and the con- acting in parallel to the electrode, i.e. in the transversal centration of added ZrO2 suspension was cs= 2.40 vol %. direction. The part of deposit containing predominantly Relations(3)and(4)express both the change in suspen- ZrO2 was exposed to tensile stresses in transversal direc sion composition, which is given by the removal and ad- tion. These stresses opened up the crack in longitudinal dition of a small amount of suspension, and the change in direction and the fracture toughness established was in this this composition, which is given by the deposition of par- direction lower than in transversal direction. Conversely, inH. Hadraba et al. / Ceramics International 30 (2004) 853–863 861 Fig. 10. Dependence of alumina volume concentration in alumina/zirconia suspension in alumina/zirconia functinally gradient material FGM HP/3Y-5 on deposition time. deposited was calculated using the relation: t = ln (h/2.78) − 1 0.0338 (2) which is inverse to the relation for one-component deposit Al2O3 [11]. Applying this relation also in the case of the functionally gradient composite Al2O3/ZrO2 is justified in view of the fact that identical electrophoretic mobility of Al2O3 and ZrO2 particles in isopropanol suspensions stabi￾lized with MCAA was established [10] and the validity of this equality has also been confirmed in the case of a mix￾ture of Al2O3 and ZrO2 particles (Section 3.2). The theoretical time dependence of the suspension com￾position was calculated on the basis of an iterative pro￾cedure, when for every interruption of deposition the new volume concentrations Al2O3(cA i+1) and ZrO2(cZ i+1) in the solution were calculated from the initial volume concentra￾tions Al2O3(cA i ) and ZrO2(cZ i ) according to the relations: cA i+1 = cA i V0 − cA i V1 − cA i V0(1 − e−0.0338 t) V0 (3) cZ i+1 = cZ i V0 − cZ i V1 − cZ i V0(1 − e−0.0338 t) + cZ s V1 V0 (4) where V0 is the total suspension volume, V1 is the volume of removed and added ZrO2 suspension when the deposition is interrupted (5 ml) and t is the time between individual de￾position interruptions (5 min). The initial Al2O3 suspension concentration was cA 0 = 3.61 vol.%, cZ 0 = 0 and the con￾centration of added ZrO2 suspension was cZ s = 2.40 vol.%. Relations (3) and (4) express both the change in suspen￾sion composition, which is given by the removal and ad￾dition of a small amount of suspension, and the change in this composition, which is given by the deposition of par￾ticles on the electrode. To express the change in suspen￾sion composition caused by particle deposition the relations and values were used that had been found for the deposi￾tion of one-component deposits [11]. The good agreement between the theoretical suspension composition and the de￾posit composition established (see Fig. 10) implies the va￾lidity of all the assumptions made: electrophoretic mobility of both components in the suspension was identical [10], the relations derived in the preceding work [11] for deposition kinetics are valid and they also hold for the two-component system. As reported in the preceding work [11] the length of indentation cracks propagating from the corners of inden￾tation by Vickers indentor to the material surface provides for conclusions as to the size of fracture toughness of the material. Using this method, Nicholson [17] demonstrated that the fracture toughness of functionally gradient material is directly proportional to its composition. Fig. 11 gives the fracture toughness of functionally gradient material FGM HP/3Y-5 as a function of its composition. As can be seen from Fig. 11, the fracture toughness in transversal direction is proportional to the composition of functionally gradient composite and it is higher than the fracture tough￾ness of pure phases. The fracture toughness in longitudinal direction deflects from the linear dependence. As stated above, due to the different green densities throughout the deposit, the deposit was deformed during drying and sin￾tering. This deformation was caused by internal stresses acting in parallel to the electrode, i.e. in the transversal direction. The part of deposit containing predominantly ZrO2 was exposed to tensile stresses in transversal direc￾tion. These stresses opened up the crack in longitudinal direction and the fracture toughness established was in this direction lower than in transversal direction. Conversely, in