B -T Lee et al. / Joumal of the European Ceramic Society 28(2008)229-233 the values of hardness and fracture toughness increased grad ually due to the increased densification of the composite The hardness values of the 4th passed network type fibrous (AlO3-m-zrO2)/t-ZrO2 composites sintered at 1300 and 4 0o tigate the trend of the fracture toughness, Kic was measured by the indentation method using Evans equation. The equation is, 点 Bending strength KrC=0.16h1/2(c)-32 where h is the vickers hardness a the half of indentation dia Temperature(C) onal and c is the half of crack length from indentation center. Fig 3. Relative density and bending strength of 4th passed network type fibro In the sample sintered at 1300C the values of fracture Al2O3-(m-zrO2)t-ZrO2 composites. toughness in both the longitudinal and transverse sections were comparatively low, about 4.8 and 4.7 MPa m", respectively where the span length for the testing apparatus was 10 mm. However, their values increased with the sintering temperature and at 1500C the maximum fracture toughness in the longitu Bending strength= 16/-D3 dinal and transverse sections were about 8.6 and 7.4 MPam /2 0.1013 respectively. However, in the transverse section, the fracture toughness values were slightly lower than those in the longitudi- where I is the load (kg), m the span length(mm) and D is the nal section due to the orientation of the fibrous microstructure diameter of the bend bar(mm) In the transverse section the crack tip traveled along axis which In the sample sintered at 1300C, the values of the relative was almost symmetrical in morphology. The crack tip had to tensity and bending strength were comparatively low with about cross the inner core/sell arrangement(Al2O3-m-ZrO2)/t-zrO2 91% and 567 MPa, respectively, due to low densification. How- and the outer thick boundary of t-ZrO2. The crack propagation er, as the sintering temperature increased, the values increased energy was dissipated by the microcracks of the two-phase core due to the enhanced densification. The maximum relative density and by the t-m phase transformation of the t-ZrO2 phases of and bending strength values were obtained at 1500C and their the inner and outer network. However, in case of the indentation values were about 98.5 and 1006 MPa, respectively. These val- ues are higher than those of individual monolithic ceramics. 17 in the longitudinal section the crack tip had to travel across the Compared to the other network type(Al203-m-ZrO2)1-ZrO2 the(Al2O3-m-ZrO2) fiber. The unidirectional orientation of composites l the value of the bending strength also increased. the phases imparts slightly higher fracture toughness. In case of The fibrous, hierarchical microstructure imparted this superior the crack propagation along the axis of the fibrous alignment the mechanical property cylindrical confinement by the inner and outer network of t-ZrO2 Fig. 4 shows the hardness and fracture toughness of 4th hinders the crack tip to propagate along the weaker(Al2O3-m- passed network type fibrous(Al2O3-m-ZrO2)t-ZrO2 com- ZrO)core phase and the crack length was comparable in the posites depending on the sintering temperature. The fracture two anisotropic axes in the longitudinal section tions of the composites. As the sintering temperature increased, network type(Al203-m-ZrO2 /t-Z-0O2 composites sintered at 1500C. In Fig. 5(a), the fracture surface shows the outer t-ZrO2 10 network and inner(Al2 O3-m-ZrO)/t-ZrO2 network. However .9g in the inner(Al2O3-m-ZrO2)/t-ZrO2 zone, a rougher surface is observed compared to the outer t-zrO2 network zone, which wi propagation path. The higher roughness means higher deflection of the crack propagation path which in turn can improve the frac- ● Hardness(Hv) ture toughness of the composites In Fig. 5(b and c), the enlarged images of the fracture surface of the outer and inner network are shown. The outer t-ZrO network zone had a mixed fracture P Fracture Toughness(L) [2 mode with both inter-and trans-granular fractures, whereas, in the inner(AlO3-m-zrO2/t-ZrO2 zone the fracture mode also showed a mixed fracture mode with the Al2O3 phase predomi Temperature(C) nantly in the trans-granular mode of fracture. In previous reports R1m时地 sed netwok tepe fibrous system with a soft interface in which strone del1232 B.-T. Lee et al. / Journal of the European Ceramic Society 28 (2008) 229–233 Fig. 3. Relative density and bending strength of 4th passed network type fibrous Al2O3–(m-ZrO2)/t-ZrO2 composites. where the span length for the testing apparatus was 10 mm. Bending strength = 16l m π D3 1 0.1013 where l is the load (kg), m the span length (mm) and D is the diameter of the bend bar (mm). In the sample sintered at 1300 ◦C, the values of the relative density and bending strength were comparatively low with about 91% and 567 MPa, respectively, due to low densification. How￾ever, as the sintering temperature increased, the values increased due to the enhanced densification. The maximum relative density and bending strength values were obtained at 1500 ◦C and their values were about 98.5 and 1006 MPa, respectively. These val￾ues are higher than those of individual monolithic ceramics.17 Compared to the other network type (Al2O3–m-ZrO2)/t-ZrO2 composites,11 the value of the bending strength also increased. The fibrous, hierarchical microstructure imparted this superior mechanical property. Fig. 4 shows the hardness and fracture toughness of 4th passed network type fibrous (Al2O3–m-ZrO2)/t-ZrO2 com￾posites depending on the sintering temperature. The fracture toughness was measured in both longitudinal and transverse sec￾tions of the composites. As the sintering temperature increased, Fig. 4. Hardness and fracture toughness of 4th passed network type fibrous Al2O3–(m-ZrO2)/(t-ZrO2) composites depending on sintering temperature. the values of hardness and fracture toughness increased grad￾ually due to the increased densification of the composite. The hardness values of the 4th passed network type fibrous (Al2O3–m-ZrO2)/t-ZrO2 composites sintered at 1300 and 1500 ◦C were about 1149 and 1452 Hv, respectively. To inves￾tigate the trend of the fracture toughness, KIC was measured by the indentation method using Evan’s equation. The equation is, KIC = 0.16Ha1/2 c a −3/2 where H is the Vickers hardness, a the half of indentation diag￾onal and c is the half of crack length from indentation center. In the sample sintered at 1300 ◦C the values of fracture toughness in both the longitudinal and transverse sections were comparatively low, about 4.8 and 4.7 MPa m1/2, respectively. However, their values increased with the sintering temperature and at 1500 ◦C the maximum fracture toughness in the longitu￾dinal and transverse sections were about 8.6 and 7.4 MPa m1/2, respectively. However, in the transverse section, the fracture toughness values were slightly lower than those in the longitudi￾nal section due to the orientation of the fibrous microstructure. In the transverse section the crack tip traveled along axis which was almost symmetrical in morphology. The crack tip had to cross the inner core/sell arrangement (Al2O3–m-ZrO2)/t-ZrO2 and the outer thick boundary of t-ZrO2. The crack propagation energy was dissipated by the microcracks of the two-phase core and by the t–m phase transformation of the t-ZrO2 phases of the inner and outer network. However, in case of the indentation in the longitudinal section the crack tip had to travel across the unidirectionally aligned inner and outer t-ZrO2 cylinders and the (Al2O3–m-ZrO2) fibers. The unidirectional orientation of the phases imparts slightly higher fracture toughness. In case of the crack propagation along the axis of the fibrous alignment the cylindrical confinement by the inner and outer network of t-ZrO2 hinders the crack tip to propagate along the weaker (Al2O3–m￾ZrO2) core phase and the crack length was comparable in the two anisotropic axes in the longitudinal section. Fig. 5 shows the SEM fracture surfaces of the 4th passed network type (Al2O3–m-ZrO2)/t-ZrO2 composites sintered at 1500 ◦C. In Fig. 5(a), the fracture surface shows the outer t-ZrO2 network and inner (Al2O3–m-ZrO2)/t-ZrO2 network. However, in the inner (Al2O3–m-ZrO2)/t-ZrO2 zone, a rougher surface is observed compared to the outer t-ZrO2 network zone, which was due to the effect of the fibrous microstructure. This phenomenon indicates that the microstructure had an effect on the fracture propagation path. The higher roughness means higher deflection of the crack propagation path which in turn can improve the frac￾ture toughness of the composites. In Fig. 5(b and c), the enlarged images of the fracture surface of the outer and inner network are shown. The outer t-ZrO2 network zone had a mixed fracture mode with both inter- and trans-granular fractures, whereas, in the inner (Al2O3–m-ZrO2)/t-ZrO2 zone the fracture mode also showed a mixed fracture mode with the Al2O3 phase predomi￾nantly in the trans-granular mode of fracture. In previous reports the fibrous monolithic ceramic was fabricated by the Si3N4/BN system with a soft interface in which strong delamination dur-