T-S. Kim et al. Scripta Materialia 52(2005)725-729 tainer of 30 mm in diameter, and then extruded through a die of 3.5 mm in diameter each filament was com- ■ Relative density Bending strength formly as designated. Each pha ase became thin ner by the repeated extrusion. The grain size became fine as the number of extrusion passes increased, in which the thicknesses of both the Al,O(m-zrO2) and Zro yers were about 375 50 um, 5 um and 0.7 um for the second-, third-, fourth- and the fifth-pass filament It is anticipated that microstructural change hap- pened during the extrusion using a relationship between Pass of extrusion the mean diameter(Dn)of the filaments with the pass of extrusion; the relationship can be described as follows, Fig. 2. Relative density and bending strength of the Al2Or-(m-ZrO2) ZrO2 sintered composite with the number of extrusion passes. Dn=D-1/R/ strength is attributed to the microstructural refinement where D,_I is initial diameter of the container, and R is (shown in Fig. 1)as well as the densification an extrusion ratio. The equation indicates that the diam Fig. 3 shows fracture surfaces of fibrous Al,O3- eter of both the extruded bar and the fiber varies only (m-ZrO2)Zro2 composites examined by Bse (a)and depending on the reduction ratio(R). The resultant val- secondary electron(b and c) modes on SEM. The sec- ues(D) for the first- to fifth-pass filaments are 3.5 ond-pass filament fractured well and maintains well 418 um, 45 um, 6 um and 0. 7 um, respectively. It is seen the as extruded microstructure, in which the Al2O3- that the data agrees well, indicating that the equation is (m-ZrO2) particles are darker than Zro2. It is seen that a convenient way for the materials designer to anticipate fracture of the second filament progresses in a transgra he microstructural variations for a ceramic composite nular way with a partial intergranular way due to a formation of different fracture patterns between Al2O The rather heterogeneous microstructure in the fifth- (m-ZrO2) and ZrO, phases. This is identified from pass filament is attributed to factors such as a relatively he magnified photos of ZrO2(Fig. 3b)and Al2O3- large size mismatch between the as received powders (m-ZrO2)(Fig. 3c), where fracture with fine dimples (AlO3 300 nm and Zro2 80 nm in diameter), discret observed from the Al2O3-(m-ZrO2) parts. The com ancy in the plastic flow rate between Al2O,m-ZrO2)/ bined fracture modes in the fibrous composites could polymer and ZrO2/polymer layers during extrusion possibly modify the intrinsic low fracture toughness of and the large flexibility of the polymer compared to ceramic materials as reported elsewhere [5-9]. Not only the ceramic [10 that, the homogeneous and fibrous microstructure The relationship between the microstructure and the formed in the extruded composites could improve the materials properties is as shown in Fig. 2, in which rel- other mechanical properties, such as fracture toughness ative density and bending strength of fibrous Al2O3- Fig. 4 shows a fracture pattern taken from the third (m-zrO2)/ZrO2 composites are plotted against the num passed composite. The fractured specimen presents a ber of extrusion passes. Both values are simultaneously microstructure identical to the one as extruded(Fig increased as the number of extrusion passes increased, 4a), and builds up a sound interface between Al_- where the density varies from 95.8% for the second-pass (m-ZrO2) and ZrO2 phases(Fig 4b ). It is seen that the filaments to 98.5% for the fifth-pass filaments, while the transgranular fracture with the dimples formed in the respective bending strength changes from 160to hird-pass filament, but the portion of intergranular 564 MPa. The increase in density might be due to the re- mode determined from the dimples is less than that peated extrusion, whereas the increase in bending for the second one. ZrO2(Fig. 4c)presents a still more Fig. 3. BSE-SEM fractography of the second passed composite examined at(a) low magnification and high magnification of (b) ZrO2 and (c)Al2O3-tainer of 30 mm in diameter, and then extruded through a die of 3.5 mm in diameter. Each filament was com￾bined uniformly as designated. Each phase became thin￾ner by the repeated extrusion. The grain size became fine as the number of extrusion passes increased, in which the thicknesses of both the Al2O3–(m-ZrO2) and ZrO2 layers were about 375 lm, 50 lm, 5 lm and 0.7 lm for the second-, third-, fourth- and the fifth-pass filaments, respectively. It is anticipated that microstructural change hap￾pened during the extrusion using a relationship between the mean diameter (Dn) of the filaments with the pass of extrusion; the relationship can be described as follows, Dn ¼ Dn1=R1=2 ð1Þ where Dn1 is initial diameter of the container, and R is an extrusion ratio. The equation indicates that the diam￾eter of both the extruded bar and the fiber varies only depending on the reduction ratio (R). The resultant val￾ues (Dn) for the first- to fifth-pass filaments are 3.5 mm, 418 lm, 45 lm, 6 lm and 0.7 lm, respectively. It is seen that the data agrees well, indicating that the equation is a convenient way for the materials designer to anticipate the microstructural variations for a ceramic composite bar. The rather heterogeneous microstructure in the fifth￾pass filament is attributed to factors such as a relatively large size mismatch between the as received powders (Al2O3 300 nm and ZrO2 80 nm in diameter), discrep￾ancy in the plastic flow rate between Al2O3–(m-ZrO2)/ polymer and ZrO2/polymer layers during extrusion and the large flexibility of the polymer compared to the ceramic [10]. The relationship between the microstructure and the materials properties is as shown in Fig. 2, in which rel￾ative density and bending strength of fibrous Al2O3– (m-ZrO2)/ZrO2 composites are plotted against the num￾ber of extrusion passes. Both values are simultaneously increased as the number of extrusion passes increased, where the density varies from 95.8% for the second-pass filaments to 98.5% for the fifth-pass filaments, while the respective bending strength changes from 160 to 564 MPa. The increase in density might be due to the re￾peated extrusion, whereas the increase in bending strength is attributed to the microstructural refinement (shown in Fig. 1) as well as the densification. Fig. 3 shows fracture surfaces of fibrous Al2O3– (m-ZrO2)/ZrO2 composites examined by BSE (a) and secondary electron (b and c) modes on SEM. The sec￾ond-pass filament fractured well and maintains well the as extruded microstructure, in which the Al2O3– (m-ZrO2) particles are darker than ZrO2. It is seen that fracture of the second filament progresses in a transgra￾nular way with a partial intergranular way due to a formation of different fracture patterns between Al2O3– (m-ZrO2) and ZrO2 phases. This is identified from the magnified photos of ZrO2 (Fig. 3b) and Al2O3– (m-ZrO2) (Fig. 3c), where fracture with fine dimples is observed from the Al2O3–(m-ZrO2) parts. The com￾bined fracture modes in the fibrous composites could possibly modify the intrinsic low fracture toughness of ceramic materials as reported elsewhere [5–9]. Not only that, the homogeneous and fibrous microstructure formed in the extruded composites could improve the other mechanical properties, such as fracture toughness. Fig. 4 shows a fracture pattern taken from the third￾passed composite. The fractured specimen presents a microstructure identical to the one as extruded (Fig. 4a), and builds up a sound interface between Al2O3– (m-ZrO2) and ZrO2 phases (Fig. 4b). It is seen that the transgranular fracture with the dimples formed in the third-pass filament, but the portion of intergranular mode determined from the dimples is less than that for the second one. ZrO2 (Fig. 4c) presents a still more Fig. 2. Relative density and bending strength of the Al2O3–(m-ZrO2)/ ZrO2 sintered composite with the number of extrusion passes. Fig. 3. BSE-SEM fractography of the second passed composite examined at (a) low magnification and high magnification of (b) ZrO2 and (c)Al2O3– (m-ZrO2). T.-S. Kim et al. / Scripta Materialia 52 (2005) 725–729 727