wwceramics. org/ACT Properties of Sic Matrix Composite 313 shown in Fig. 4b, which was consistent with its brittle E=E2(1-2) fracture behavior(Fig. 3). The low flexural strengt (117 MPa)and fracture toughness(3.3 MPam")are at- where subscripts f and m refer to the fber and the matrix, tributed to the brittle fracture behavior of sample B. The E" is the plane strain modulus for the phase x, Eand v are fect of heat treatment of PyC interphase on the fracture espective behavior of composites was studied. The fracture surface Phases, and longitudinal coordinate r is the ratio of frac- morphology of sample C showed that carbon fibers were ture energy of interphase to the adjacent fiber apparently pulled out, as shown in Fig. 4c, which implies Whether the interphase can be debonded or not that the heat-treated PyC interpahse can work on carbon depends on the characteristics(modulus and fracture fibers. As a result, the Flexural strength of sample C was energy) of the matrix, fiber, and interphase. The re- increased from 117 to 504 MPa compared with that of quired data for determining whether the interphase sample B, and the fracture toughness was increased from debonding or not in the present work are listed in Ta- 3.3 to 18.6MPam/2 ble I. The likely results were calculated and marked in In the present work, the difference in the failure Fig. 5. For SiC/PyC/SiC composite, asic/SiCm is behaviors and the fracture modes of the composites are -0.13, and the corresponding TPyc/sic is in the range attributed to the different properties of fibers and different of 0. 1-0.3, and hence a line segment Li is marked in interfacial bonding between fiber and matrix, in which Fig. 5. For CAPyC/SiC composite, ac /Sicm is-0. 23, PyC interphase plays a key role in the mechanical behav- and the corresponding I Pyc/c is in the range of 0.23- ior of a ceramic matrix composite. For two dissimilar 0.70, and hence a line segment L2 is marked in Fig. 5 elastic materials, the crack propagation behavior at th It can be found that most part of segment Li is in interphase has been studied by He and Hutchinson, the debonding region(below the cross-hatched curve) and a debonding diagram has been obtained, as shown in and Fig 6a shows the debonding of Py C interphase on Fig 5, which contains two regions showing the interphase SiC fiber in the sample A. It means that the PyC inter- debonding and fiber failure. Dundurs elastic mismatch phase can debond when the composites were during rameter o can be described as follows: loading. As a comparison, most part of segment L2 is on ES-Em the top of the cross-hatched line and the debonding of ig. 6b, which means the cracks will penetrate the n (1) PyC interphase was not found in sample B, as shown in However, carbon fibers were pulled out in the sample C (Fig. 4c), which indicates the heat-treated PyC inter- phase can be debonded when the composites were dur- ing loading, as shown in Fig. 6c. The reason for the result was that the heat treatment can improve the crys- L2 talline degree clear that crystallization degree of 0.5 heat-treated PyC(Fig. 7a) was higher than that of th untreated one(Fig. 7b),and the higher crystalline de gree of the heat-treated Py C interphase was beneficial Matrix for the pullout of carbon fibers. Meanwhile, it has been found that the fracture energy of PyC interphase can be reduced to by heat treatment owing to the improvement of the crystallization degree. The heat-treated PyC in- 0.5-0.23-0.130.0 templ er c ystalline degree had the lower Elastic mismatch a fracture energy(0-2J/m) than the untreated PyC Fig. 5. Comparison of the measured fra for ceramic interpahse(2-6]/m) /c. of C/ narx co mposite(CMC) Pyc /SiC composite was lowered in the range of 0- He and Hutchinson. 18 19 L, Ly and L, represent the calculated 0. 23, which is labeled as segment Ls in the debonding range ofI/ for SiG /RyC/SiC, C/l)yCSiC, and C/lyC"/SiC region of Fig. 5. As a result, carbon fibers in sample C mposites, respectively can be pulled out during loadishown in Fig. 4b, which was consistent with its brittle fracture behavior (Fig. 3). The low flexural strength (117MPa) and fracture toughness (3.3MPa m1/2) are at￾tributed to the brittle fracture behavior of sample B. The effect of heat treatment of PyC interphase on the fracture behavior of composites was studied. The fracture surface morphology of sample C showed that carbon fibers were apparently pulled out, as shown in Fig. 4c, which implies that the heat-treated PyC interpahse can work on carbon fibers. As a result, the flexural strength of sample C was increased from 117 to 504MPa compared with that of sample B, and the fracture toughness was increased from 3.3 to 18.6MPa m1/2. In the present work, the difference in the failure behaviors and the fracture modes of the composites are attributed to the different properties of fibers and different interfacial bonding between fiber and matrix, in which PyC interphase plays a key role in the mechanical behav￾ior of a ceramic matrix composite.16 For two dissimilar elastic materials, the crack propagation behavior at the interphase has been studied by He and Hutchinson,18,19 and a debonding diagram has been obtained, as shown in Fig. 5, which contains two regions showing the interphase debonding and fiber failure. Dundur’s elastic mismatch parameter a can be described as follows: a ¼ E f E m E f þ E m ð1Þ E x ¼ Ex ð1 v2 x Þ ð2Þ where subscripts f and m refer to the fiber and the matrix, E x is the plane strain modulus for the phase x, E and n are the elastic modulus and Poisson’s ratio for the respective phases, and longitudinal coordinate Gi Gf is the ratio of frac￾ture energy of interphase to the adjacent fiber. Whether the interphase can be debonded or not depends on the characteristics (modulus and fracture energy) of the matrix, fiber, and interphase. The re￾quired data for determining whether the interphase debonding or not in the present work are listed in Ta￾ble I. The likely results were calculated and marked in Fig. 5. For SiCf/PyC/SiC composite, aSiCf =SiCm is 0.13, and the corresponding GPyC=SiCf is in the range of 0.1–0.3, and hence a line segment L1 is marked in Fig. 5. For Cf/PyC/SiC composite, aCf =SiCm is –0.23, and the corresponding GPyC=Cf is in the range of 0.23– 0.70, and hence a line segment L2 is marked in Fig. 5. It can be found that most part of segment L1 is in the debonding region (below the cross-hatched curve), and Fig. 6a shows the debonding of PyC interphase on SiC fiber in the sample A. It means that the PyC inter￾phase can debond when the composites were during loading. As a comparison, most part of segment L2 is on the top of the cross-hatched line and the debonding of PyC interphase was not found in sample B, as shown in Fig. 6b, which means the cracks will penetrate the car￾bon fiber when the composites were during loading. However, carbon fibers were pulled out in the sample C (Fig. 4c), which indicates the heat-treated PyC inter￾phase can be debonded when the composites were dur￾ing loading, as shown in Fig. 6c. The reason for the result was that the heat treatment can improve the crys￾talline degree. It is clear that crystallization degree of heat-treated PyC (Fig. 7a) was higher than that of the untreated one (Fig. 7b), and the higher crystalline de￾gree of the heat-treated PyC interphase was beneficial for the pullout of carbon fibers. Meanwhile, it has been found that the fracture energy of PyC interphase can be reduced to by heat treatment owing to the improvement of the crystallization degree. The heat-treated PyC in￾terphase with higher crystalline degree had the lower fracture energy (0–2 J/m2 ) 16 than the untreated PyC interpahse (2–6 J/m2 ),12 and hence the GPyCHT =Cf of C/ PyCHT/SiC composite was lowered in the range of 0– 0.23, which is labeled as segment L3 in the debonding region of Fig. 5. As a result, carbon fibers in sample C can be pulled out during loading. Fig. 5. Comparison of the measured fracture energies for ceramic matrix composite (CMC) system with the debonding criterion of He and Hutchinson.18,19 L1, L2, and L3 represent the calculated range of Gi /Gf for SiCf /PyC/SiC, Cf /PyC/SiC, and Cf /PyC HT/SiC composites, respectively. www.ceramics.org/ACT Properties of SiC Matrix Composite 313