C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 :始 Fig. 4. SEM micrographs of the fracture surfaces of (a) Al2O]Y-TZP graded layer, (b) pure alumina surface layer, both showing dominantly inter- granular fracture behaviour and (c) intergranular propagation of an indenter-induced crack along the graded and surface alumina grain boundary graded layer and pure alumina boundary. Fig. 4c of 7. 1 MPa m/2 and Vicker's hardness of 10.4 GPa hows that the bonding between these two phases is whilst the lowest volume fraction(13%)results in a weak enough to deflect a crack which should con- fracture toughness of 3. 8 MPa m/2 and hardness of 15.7 tribute to the damage-tolerant behaviour of the overall GPa. Increase in volume fraction of Tzp phase within FGM composites. the graded layer decreases hardness and increases the fracture toughness and also controls the alumina grain growth during sintering. Pure alumina surface layer 4. Conclusions (100 um in thickness) with a hardness value of 19.4 GPa is introduced for tribological applications. The FGM Electrophoretic deposition(EPD) was used to pro- composite shows mainly intergranular fracture beh luce tubular Al2Ox-Y-TZP/AlO3 functionally graded viour and the interfacial region between the graded composites incorporating a tough central layer with layer and the alumina surface layer is weak as an graded composition(Al2O3-Y-TZP)and a hard outer indenter-induced crack propagates intergranularly surface layer of pure alumina from nano-size sols in an along the boundary between these two phases. The attempt to generate a continuous property variation results presented in this work suggest that the overall across the final component and to control the micro- FGM produced could be an ideal micro-component for structure at a nanometer scale. Hardness, fracture room and high temperature applications where toughness and alumina grain size within the graded"damage-tolerantbehaviour and high wear resistance layer were found to be controlled by the volume frac- are required. Experimental work is being conducted at tion of Tzp grains and the highest volume fraction present in order to determine the thermomechanical (71%)of Tzp phase provides a fracture toughness value properties of the Fgmgraded layer and pure alumina boundary. Fig. 4c shows that the bonding between these two phases is weak enough to deflect a crack which should con￾tribute to the damage-tolerant behaviour of the overall FGM composites. 4. Conclusions Electrophoretic deposition (EPD) was used to pro￾duce tubular Al2O3–Y-TZP/Al2O3 functionally graded composites incorporating a tough central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina from nano-size sols in an attempt to generate a continuous property variation across the final component and to control the micro￾structure at a nanometer scale. Hardness, fracture toughness and alumina grain size within the graded layer were found to be controlled by the volume frac￾tion of TZP grains and the highest volume fraction (71%) of TZP phase provides a fracture toughness value of 7.1 MPa m1/2 and Vicker’s hardness of 10.4 GPa whilst the lowest volume fraction (13%) results in a fracture toughness of 3.8 MPa m1/2 and hardness of 15.7 GPa. Increase in volume fraction of TZP phase within the graded layer decreases hardness and increases the fracture toughness and also controls the alumina grain growth during sintering. Pure alumina surface layer (100 mm in thickness) with a hardness value of 19.4 GPa is introduced for tribological applications. The FGM composite shows mainly intergranular fracture beha￾viour and the interfacial region between the graded layer and the alumina surface layer is weak as an indenter-induced crack propagates intergranularly along the boundary between these two phases. The results presented in this work suggest that the overall FGM produced could be an ideal micro-component for room and high temperature applications where ‘‘damage-tolerant’’ behaviour and high wear resistance are required. Experimental work is being conducted at present in order to determine the thermomechanical properties of the FGM. Fig. 4. SEM micrographs of the fracture surfaces of (a) Al2O3–Y-TZP graded layer, (b) pure alumina surface layer, both showing dominantly inter￾granular fracture behaviour and (c) intergranular propagation of an indenter-induced crack along the graded and surface alumina grain boundary. C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660 1659