Y. Xu et al. Ceramics International 27(2001)565-570 Fig 8. Impact fracture surface of Hi-Nicalon Sic/SiC composites. where p is the fracture load, I is the span, and a is the super-alloy (ak=80-160 kJ m). The impact fracture notch depth, b and w are thickness and width of the surface was brush-like in Fig 8a. It was very interesting imple respectively. to observe that the present composite materials could The fracture toughness calculated from Eq(2)was withstand the hitting impact of a steel nail(Fig. 8b) 41.5 MPa m /2. which was 10 times that of monolithic These results revealed that the 3d textile hi-nicalon ceramic materials(3-5 MPa m /)and two times that of SiC/SiC composites exhibited the excellent resistance 3D C/SiC composites(20. 3 MPa m/2)(22]. The fracture against dynamic impact toughness differences between 3D C/SiC and 3D H icalon SiC/SiC composites could be illustrated from the microstructure difference of these two kinds of 4. Conclusions materials. In Fig. 6, it was observed that the surface of Hi-Nicalon sic fiber was much smoother than that of High performance three dimensional textile Hi-Nica T300 carbon fiber. Consequently, it was very easy for lon SiC fiber reinforced silicon carbide composites were Hi-Nicalon SiC fiber to be pulled out from the Sic fabricated by chemical vapor infiltration. The density of matrix, leading to higher fracture toughness the composites was 2.5g cm-3 after the three-dimen- Here, the work of fracture was introduced to repre- sional carbon preform was infiltrated for 30 h. The sent the toughness of the 3D Hi-Nicalon SiC/Sic textile values of flexural strength were 860 MPa at room tem- composite materials. The work of fracture was obtained perature and 1010 MPa at 1300 C in vacuum. Above the from the characteristic area under the load-displacement infiltration temperature, the failure behavior of the com- curve divided by the cross-section of the specimen. In posites became brittle because a compressive stress was order to determine the work of fracture effectively, we generated cross the interfacial layer caused by the mis defined the characteristic area(Ac)which started from match of thermal expansion coefficients between fiber the initial point to the 10% drop of the curve(Fig. 7). and matrix. The obtained value of the shear strength This gives an average work of fracture as high as 28. 1 kJ was 67.5 MPa. The fracture toughness and work of m-, which is the nearly three times that of 3D C/Sic fracture were as high as 41. 5 MPa m/2 and 28. 1 kJ m-2 composites and six times that of laminated SiC ceramic respectively. The value of dynamic fracture toughness matrix composites (4625 J m -), respectively [22, 23] was 36. kJ m-- 3.4. Impact loading Acknowledgements Instrumented Charpy impact tests on un-notched amples were conducted to determine the energy The authors wish to thank the National natural sci- absorbing capability and dynamic fracture behavior of entific Foundation of China, Chinese Aeronautics Foun- the composite materials. The dynamic fracture toughn dation and National Defense Foundation of China for (ak)was calculated by using the following equation: the financial support ak=△w/bh References where w is the absorbing energy of materials during impact processing, b and h are the thickness and width [1 T M. Besmann R.A. Lowden, of specimen, respectively tration, in:R. Naslain(Ed. ) perature Ceramic Matrix Composites, Woodhead Public The value of ak is 36 kJ m- for 3D Hi-Nicalon SiC/ (23S. Jacques, A.Guette,F.La bordeaux, 1993, pp. 215 Naslain. S. GowJard Sic composite materials, and is lower than that of Preparation and characterization of Sic/Sic composites withwhere p is the fracture load, l is the span, and a is the notch depth, b and w are thickness and width of the simp1e respectively. The fracture toughness calculated from Eq. (2) was 41.5 MPa m1/2, which was 10 times that of monolithic ceramic materials (3–5 MPa m1/2) and two times that of 3D C/SiC composites (20.3 MPa m1/2)[22]. The fracture toughness differences between 3D C/SiC and 3D Hi￾Nicalon SiC/SiC composites could be i1lustrated from the microstructure difference of these two kinds of materials. In Fig. 6, it was observed that the surface of Hi-Nicalon SiC fiber was much smoother than that of T300 carbon fiber. Consequently, it was very easy for Hi-Nicalon SiC fiber to be pulled out from the SiC matrix, leading to higher fracture toughness. Here, the work of fracture was introduced to repre￾sent the toughness of the 3D Hi-Nicalon SiC/SiC textile composite materials. The work of fracture was obtained from the characteristic area under the load-displacement curve divided by the cross-section of the specimen. In order to determine the work of fracture effectively, we defined the characteristic area (Ac) which started from the initial point to the 10% drop of the curve (Fig. 7). This gives an average work of fracture as high as 28.1 kJ m
2 , which is the nearly three times that of 3D C/SiC composites and six times that of laminated SiC ceramic matrix composites (4625 J m
2 ), respectively [22,23]. 3.4. Impact loading Instrumented Charpy impact tests on un-notched samples were conducted to determine the energy absorbing capability and dynamic fracture behavior of the composite materials. The dynamic fracture toughness (k) was calculated by using the following equation: k ¼ w=bh ð3Þ where w is the absorbing energy of materials during impact processing, b and h are the thickness and width of specimen, respectively. The value of k is 36 kJ m
2 for 3D Hi-Nicalon SiC/ SiC composite materials, and is lower than that of super-alloy (k=80–160 kJ m
2 ). The impact fracture surface was brush-1ike in Fig. 8a. It was very interesting to observe that the present composite materials could withstand the hitting impact of a steel nail (Fig. 8b). These results revealed that the 3D textile Hi-Nicalon SiC/SiC composites exhibited the excellent resistance against dynamic impact. 4. Conclusions High performance three dimensional textile Hi-Nica￾lon SiC fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. The density of the composites was 2.5g cm
3 after the three-dimen￾sional carbon preform was infiltrated for 30 h. The values of flexural strength were 860 MPa at room tem￾perature and 1010 MPa at 1300C in vacuum. Above the infiltration temperature, the failure behavior of the com￾posites became brittle because a compressive stress was generated cross the interfacial 1ayer caused by the mis￾match of thermal expansion coefficients between fiber and matrix. The obtained value of the shear strength was 67.5 MPa. The fracture toughness and work of fracture were as high as 41.5 MPa m1/2 and 28.1 kJ m
2 respectively. The value of dynamic fracture toughness was 36.0 kJ m
2 . Acknowledgements The authors wish to thank the National Natural Sci￾entific Foundation of China, Chinese Aeronautics Foun￾dation, and National Defense Foundation of China for the financial support. References [1] T.M. Besmann, R.A. Lowden, Overview of chemical vapor infil￾tration, in: R. Naslain (Ed.), High Temperature Ceramic Matrix Composites, Woodhead Publications, Bordeaux, 1993, pp. 215. [2] S. Jacques, A. Guette, F. Langlais, R. Naslain, S. GouJard, Preparation and characterization of SiC/SiC composites with Fig. 8. Impact fracture surface of Hi-Nicalon SiC/SiC composites. Y. Xu et al. / Ceramics International 27 (2001) 565–570 569