International Journal of Applied Ceramic Technolog-Sauder, Brusson and Lamon Vol. 7, No. 3, 2010 Fig. 5. Micrographs offracture surfaces of SA3/SiC minicomposites: a)M2 with composite behavior, (b)M2 with brittle-like behavior, and (c) M4. large. In the Hi-Nicalon S/SiC minicomposites, Rmax is A≈b-cPC(-Bc) much smaller than epyc The radial stress induced by the fiber surface rough ness amplitude in the interface crack during tensile where h is the peak amplitude and Epyc is interphase deformation during sliding, Epyc<19 owever loading was estimated using the following equation may depend on the PyC structure. But this shoule Emer not affect the analysis drastically, provided epyc does (1+vm)+ Em(1-vrR)(4) not exceed 1% Equations (4)and (5)indicate that thicker coatings decrease A and oR. The clamping stress where A is the amplitude of lateral displacements in- OR+0 when the coating thickness exceeds the peak duced by surface roughness( Fig 8)and R is the fiber amplitude Thus, it appears that differences Setting A to 2RRMS yields OR=930 MPa for SA3/ haviors are related to contribution of fiber surface SiC and or= 130 MPa for Hi-NicalonS. These are roughness in the interface crack rather than to fiber/ rough estimates of the average values of oR in the ab matrix interface sence of a thin interphase. Then, setting A to /max yields uppe Residual stresses OR=850 MPa for Hi-Nicalon S/SiC. These values are useful for comparison purposes. They clearly indicate Residual stresses build up during cooling down that significant effects of fiber surface roughness are ex- fro processing temperature(a 1000.C)dep pected with the SA3 fibers, when compared with Hi- on the thermal mismatch Nicalon. The influence of Py C fiber coating can be Figure 9 shows that fibers and the CVI matrix have anticipated using Eq. (5) comparable coefficients of thermal expansion, suggestinglarge. In the Hi-NicalonS/SiC minicomposites, Rmax is much smaller than ePyC. The radial stress induced by the fiber surface rough￾ness amplitude in the interface crack during tensile loading was estimated using the following equation10: sR ¼ EmEf Efð1 þ nmÞ þ Emð1  nfÞ A Rf ð4Þ where A is the amplitude of lateral displacements in￾duced by surface roughness (Fig. 8) and Rf is the fiber radius. Setting A to 2RRMS yields sR 5 930 MPa for SA3/ SiC and sR 5 130 MPa for Hi-NicalonS. These are rough estimates of the average values of sR in the ab￾sence of a thin interphase. Then, setting A to hmax yields upper bounds: sR 5 3080 MPa for SA3/SiC and sR 5 850 MPa for Hi-NicalonS/SiC. These values are useful for comparison purposes. They clearly indicate that significant effects of fiber surface roughness are ex￾pected with the SA3 fibers, when compared with Hi￾NicalonS. The influence of PyC fiber coating can be anticipated using Eq. (5): A h  ePyCð1  ePyCÞ ð5Þ where h is the peak amplitude and ePyC is interphase deformation during sliding, ePyCo1%. However, it may depend on the PyC structure. But this should not affect the analysis drastically, provided ePyC does not exceed 1%. Equations (4) and (5) indicate that thicker coatings decrease A and sR. The clamping stress sR-0 when the coating thickness exceeds the peak amplitude. Thus, it appears that differences in minicomposite behaviors are related to contribution of fiber surface roughness in the interface crack rather than to fiber/ matrix interface opening strength. Residual Stresses Residual stresses build up during cooling down from the processing temperature ( 10001C) depend￾ing on the thermal expansion mismatch. Figure 9 shows that fibers and the CVI matrix have comparable coefficients of thermal expansion,26 suggesting Fig. 5. Micrographs of fracture surfaces of SA3/SiC minicomposites: (a) M2 with composite behavior, (b) M2 with brittle-like behavior, and (c) M4. 298 International Journal of Applied Ceramic Technology—Sauder, Brusson and Lamon Vol. 7, No. 3, 2010