S.M. Dong et al./ Ceramics International 28(2002 )899-905 TSA/SiC. Although microcracks around some fibers Push-out·Push have been formed, the average IDS still reaches 161.1 MPa. In composites TSA/C/SiC, relatively weak inter- faces were formed. The data for interfacial frictional stress reveal the same trend as that of the IDs. TSA/SiC composite also shows a higher IFS value. However, it should be mentioned that debonding between fibers and matrix could still be realized under certain load during push-out test in TSA SiC although the matrix and fibers were directly bonded during the PIMP 00 The differences of interfacial debonding strength and interfacial frictional stress might be ascribed to the interaction between fibers and matrix [12-15. Gen- erally, the radial stress on the fibers can be described by where o is radial thermal residual stress given by ar=k(am-a)△T (3) or is fiber surface roughness induced radial misfit stress given by Fig. 5. Typical single fiber push-out and push back load/ displacem curves(a), and SEM micrographs of the protruding fibers after singl (4) fiber push-out test(b) for composite TSA/ SiC. wEmer means the load for fiber debonding and sliding as well Em(l-ur)+ er(I Um) as the contributions from fiber elastic/plastic deforma- tion and Poisson expansion effects [ll]. It is used in this oP is the radial stress induced by Poisson's effect. ar study for estimating the interfacial debonding strength, And af are the thermal expansion coefficients of the IDS via the formula fiber and matrix in the radial direction. AT is the tem- IDS= Po/IDt () perature difference between composite fabrication tem- perature (1373 K)and room temperature (298 K),at which the push-out and push-back test were conducted where D and t are the fiber diameter and the specimen Er, Em, Uf and umare Youngs moduli and Poisson,s thickness of the slice for push-out and push-back test. ratios of the fiber and matrix. 8 Is the fiber surface The same formula was also used for determining the roughness. In this experiment, fiber surface roughness interfacial frictional stress(IFS), where Po was sub- profiles for Tyranno SA fiber was examined by an opti- stituted by Pb, which is mainly used to initiate the slid al interferometric microscope, MicromapTM, with of resolution of less than 1 nm in z-direction and 8 was From the calculated results listed in Table 2. it can be determined to be 3. 74. o Is a factor for the effect of fiber bserved that a relatively strong interface is present in volume fraction in the composites. Here, oP and o are simply assumed to be zero and l, respectively. Since the Table 2 Tyranno SA is well crystallized and near stoichiometric Physical and mechanical properties of the composites [17], their thermal expansion coefficients can be assumed to be similar as those of the sintered sic monolithic Composites ISA/SIC TSA/C/SIC ceramics(about 4.0x10-6/K)[18]. For the polymer Bulk density (g/cm) 2.51士0.0 2.30±0.04 derived matrix, thermal expansion coefficient can be Flexural strength(MPa) 402.4*35.9 258.7-+26.0 simply assumed to be 3.0-3.5x10-6/K(similar to that Interfacial debonding strength(MPa) 161.1+31.6 61.5+17.8 of a Si-C[O] amorphous phase)[18, 19]. Based on those Interfacial frictional stress(MPa) 56.9±18.2 Modulus of elasticity(GPa) 1020±5.3 49.7±4.8 assumptions, the radial stress on the fibers in the com- posites can be roughly estimated using Eqs. (2H5)means the load for fiber debonding and sliding as well as the contributions from fiber elastic/plastic deforma￾tion and Poisson expansion effects [11]. It is used in this study for estimating the interfacial debonding strength, IDS via the formula: IDS ¼ Po=Dt ð1Þ where D and t are the fiber diameter and the specimen thickness of the slice for push-out and push-back test. The same formula was also used for determining the interfacial frictional stress (IFS),where Po was sub￾stituted by Pb,which is mainly used to initiate the slid￾ing of the debonded fiber. From the calculated results listed in Table 2,it can be observed that a relatively strong interface is present in TSA/SiC. Although microcracks around some fibers have been formed,the average IDS still reaches 161.1 MPa. In composites TSA/C/SiC,relatively weak inter￾faces were formed. The data for interfacial frictional stress reveal the same trend as that of the IDS. TSA/SiC composite also shows a higher IFS value. However,it should be mentioned that,debonding between fibers and matrix could still be realized under certain load during push-out test in TSA/SiC although the matrix and fibers were directly bonded during the PIMP process. The differences of interfacial debonding strength and interfacial frictional stress might be ascribed to the interaction between fibers and matrix [12–15]. Gen￾erally,the radial stress on the fibers can be described by [16], N r ¼ T r þ R r þ P r ð2Þ where T r is radial thermal residual stress given by T r ¼  r m  r f
T ð3Þ R r is fiber surface roughness induced radial misfit stress given by R r ¼   Rf  ð4Þ  ¼ !EmEf Emð Þþ 1  f Ef ð Þ 1 þ m ð5Þ P r is the radial stress induced by Poisson’s effect. r m And r f are the thermal expansion coefficients of the fiber and matrix in the radial direction. T is the tem￾perature difference between composite fabrication tem￾perature (1373 K) and room temperature (298 K),at which the push-out and push-back test were conducted. Ef, Em, r f and r mare Young’s moduli and Poisson’s ratios of the fiber and matrix.  Is the fiber surface roughness. In this experiment,fiber surface roughness profiles for Tyranno SA fiber was examined by an opti￾cal interferometric microscope,MicromapTM,with a resolution of less than 1 nm in Z-direction,and  was determined to be 3.74. ! Is a factor for the effect of fiber volume fraction in the composites. Here, P r and ! are simply assumed to be zero and 1,respectively. Since the Tyranno SA is well crystallized and near stoichiometric [17],their thermal expansion coefficients can be assumed to be similar as those of the sintered SiC monolithic ceramics (about 4.0106 /K) [18]. For the polymer derived matrix,thermal expansion coefficient can be simply assumed to be 3.0–3.5106 /K (similar to that of a Si–C[–O] amorphous phase) [18,19]. Based on those assumptions,the radial stress on the fibers in the com￾posites can be roughly estimated using Eqs. (2)–(5). Table 2 Physical and mechanical properties of the composites Composites TSA/SiC TSA/C/SiC Bulk density (g/cm3 ) 2.510.02 2.300.04 Flexural strength (MPa) 402.435.9 258.726.0 Interfacial debonding strength (MPa) 161.131.6 61.517.8 Interfacial frictional stress (MPa) 56.918.2 32.810.1 Modulus of elasticity (GPa) 102.05.3 49.74.8 Fig. 5. Typical single fiber push-out and push back load/displacement curves (a),and SEM micrographs of the protruding fibers after single fiber push-out test (b) for composite TSA/SiC. S.M. Dong et al. / Ceramics International 28 (2002) 899–905 903