C. Kaya/Journal of the European Ceramic Society 23(2003)1655-1660 A- 7I voL% TZP B 35 voL% TZP (o) C- 13 voL, %TZI Pure AL, O, Layer Fig 3. SEM micrographs taken from the points within the Al2OrY-T'ZP graded layer shown in Fig. 2: (a)71 vol. TZP(point A), (b)35 vol% TZP (point B),(c)13 vol. TZP (point C)and (d) the microstructure of pure alumina surface layer(dark phase represents alumina and the light phase represents TzP grains Table I Propertices of electrophoretically formed Al,0-Y-TZPJALO, FGM of tubular shape rea in graded layer Grain size(um) Volume fraction of Hardness Fracture toughness (see Fig. 2) TZP (%) (GPa) MPa m2) TZP 000 3.8 Pure Al2O3 layer 3.1 within the graded layer play an important role in sup The SEM micrographs of fracture surfaces of the pressing alumina grain growth, the highest volume graded layer and pure alumina surface layer are shown fraction providing sub-micron alumina grains within in Fig. 4a and b, respectively. Inter-granular fracture the graded layer is the dominant failure mode for both layers, however, The relationships between the composition of the a few alumina grains within the pure alumina surface FGM and hardness/fracture toughness are also given in layer fail trans-granularly, as shown in Fig. 4b. The Table 1. As the volume fraction of TzP phase within the presence of fine, medium size and also coarser TZP functionally graded Al2O3-Y-TZP layer decreases, the grains within the graded layer is visible from the hardness value increases whilst the fracture toughness image shown in Fig. 4a. The nature of the interfacial decreases. The highest toughness value of 7. 1 MPa m/2 region between the graded Al,O3-Y-TZP and the pure was determined at the point A and the highest hardness Al2O3 surface layer in terms of the crack deflection value of 15.7 GPa at the point C. It is also seen from the and propagation behaviour was characterised by using Table I that the pure alumina surface layer has a hard- a crack path propagation test, as shown in Fig. 4c ness value of 19.4 GPa and a moderate fracture tough- An indenter-induced crack propagates intergranularly ness value of 3. 1 MPa m, resulting from the dense and from the pure alumina surface layer and when it fine grained alumina microstructure. interacts with the graded layer, it propagates along thewithin the graded layer play an important role in sup￾pressing alumina grain growth, the highest volume fraction providing sub-micron alumina grains within the graded layer. The relationships between the composition of the FGM and hardness/fracture toughness are also given in Table 1. As the volume fraction of TZP phase within the functionally graded Al2O3–Y-TZP layer decreases, the hardness value increases whilst the fracture toughness decreases. The highest toughness value of 7.1 MPa m1/2 was determined at the point A and the highest hardness value of 15.7 GPa at the point C. It is also seen from the Table 1 that the pure alumina surface layer has a hard￾ness value of 19.4 GPa and a moderate fracture tough￾ness value of 3.1 MPa m1/2, resulting from the dense and fine grained alumina microstructure. The SEM micrographs of fracture surfaces of the graded layer and pure alumina surface layer are shown in Fig. 4a and b, respectively. Inter-granular fracture is the dominant failure mode for both layers, however, a few alumina grains within the pure alumina surface layer fail trans-granularly, as shown in Fig. 4b. The presence of fine, medium size and also coarser TZP grains within the graded layer is visible from the image shown in Fig. 4a. The nature of the interfacial region between the graded Al2O3–Y-TZP and the pure Al2O3 surface layer in terms of the crack deflection and propagation behaviour was characterised by using a crack path propagation test, as shown in Fig. 4c. An indenter-induced crack propagates intergranularly from the pure alumina surface layer and when it interacts with the graded layer, it propagates along the Fig. 3. SEM micrographs taken from the points within the Al2O3–Y-TZP graded layer shown in Fig. 2: (a) 71 vol.% TZP (point A), (b) 35 vol.% TZP (point B), (c) 13 vol.% TZP (point C) and (d) the microstructure of pure alumina surface layer (dark phase represents alumina and the light phase represents TZP grains. Table 1 Properties of electrophoretically formed Al2O3–Y-TZP/Al2O3 FGM of tubular shape Area in graded layer (see Fig. 2) Grain size (mm) Volume fraction of TZP (%) Hardness (GPa) Fracture toughness (MPa m1/2) Al2O3 TZP A 0.65 0.70 71 10.4 7.1 B 1.35 0.75 35 12.9 6.3 C 1.85 0.85 13 15.7 3.8 Pure Al2O3 layer 2.6 – 0 19.4 3.1 1658 C. Kaya / Journal of the European Ceramic Society 23 (2003) 1655–1660