wwceramics. org/ACT Infuence of Interface Characteristics on the Mechanical Properties and that it tends to be quite brittle when the fiber coat ing becomes thinner Table iv also reports the interface shear stress mea sured on the damage-tolerant specimens. It is much posites, with the same PyC fiber coating thickness. It could not be determined on the quite brittle test spec- imens, due to the lack of hysteresis loops. Figure 5 shows that the fracture surface of the damage-tolerant SA3/SiC minicomposites contained quite long pullout fibers. A few short pullout fibers can be observed in the fracture surface of the quite brittle minicomposites. This confirms that matrix cracks formed before ultimate fail- and that fiber/matrix debonding occurred. Th 0 200M pening width of hysteresis loops can also be noticed, as well as tow-controlled ultimate failure Fiber Surface Roughness at a deformation 0.6% For the other specimens from batch M2 and for Figure 6 shows images of the surface of pullout all the rom batch ma. the behar fibers during tensile tests. They demonstrate that devi was essentially linear and brittle like. But it is ation of the matrix crack occurred at the fiber surface in worth pointing out that the strain to failure was both Hi-NicalonS/SiC and A3/SiC. minicomposites slightly larger than 0.1% and that acoustic emis- Thus, it can be concluded that bonding of SA3 fibers signals initiated before failure, which indi- was not significantly stronger than that of Hi-NicalonS cates that the first matrix crack did not cause fibers, and it can be reasonably conjectured that the in- minIco terface opening strength was comparable in both sys tems. It was not sufficiently high to cause deviation Table IV shows that for those specimens from within the interphase, as it has been observed when th batch M2 that exhibited the conventional features of surface of SiC fibers has been treated to strengthen the omposite behavior, the tensile characteristics are similar Then, the surface of SA3 fibers appears to be rather to those of Hi-NicalonS/SiC minicomposites. This rough, with peaks and valleys confirms that SA3 fibers were not degraded during pro- cessing because SA3 fibers strength Micrographs of cross-sections as well as AFM im- 1400..2324 For those specimens from batch M4, the ages of fiber surface highlight this topography(Figs. 6 failure characteristics are very close to those at the onset and 7). Table IV summarizes the fiber surface roughness highly dependent on thickness of the Py C fiber coating, tude observed in the analyzed portion of the fiber sur- (r Table IV. Characteristics of Fiber Surface Rough- face)clearly indicate that the surface roughness ness Determined Using AFM and the Magnitude of amplitude of SA3 fibers is much more significant than Clamping Stresses oR (Eq(4)) that of Hi-NicalonS fibers. Note that Rmax may be as high as 60 nm for the SA3 fibers, which is much larger RRMs(nm) Rmax(nm) OR(MPa) than the thickness of the PyC coating(epyc) in the M4 It should also be noticed that the diam- 2.33 eter is three times as small as that of Hi-Nicalot Fiber SA3 558 930 whereas the roughness amplitude is about four times asopening width of hysteresis loops can also be noticed, as well as tow-controlled ultimate failure at a deformation B0.6%. For the other specimens from batch M2 and for all the specimens from batch M4, the behavior was essentially linear and brittle like. But it is worth pointing out that the strain to failure was slightly larger than 0.1% and that acoustic emis￾sion signals initiated before failure, which indi￾cates that the first matrix crack did not cause minicomposite failure. Table IV shows that for those specimens from batch M2 that exhibited the conventional features of composite behavior, the tensile characteristics are similar to those of Hi-NicalonS/SiC minicomposites. This confirms that SA3 fibers were not degraded during pro￾cessing because SA3 fibers strength is stable up to 14001C.23,24 For those specimens from batch M4, the failure characteristics are very close to those at the onset of matrix cracking. It seems obvious that the behavior is highly dependent on thickness of the PyC fiber coating, and that it tends to be quite brittle when the fiber coat￾ing becomes thinner. Table IV also reports the interface shear stress mea￾sured on the damage-tolerant specimens. It is much larger than t obtained for Hi-NicalonS/SiC minicom￾posites, with the same PyC fiber coating thickness. It could not be determined on the quite brittle test spec￾imens, due to the lack of hysteresis loops. Figure 5 shows that the fracture surface of the damage-tolerant SA3/SiC minicomposites contained quite long pullout fibers. A few short pullout fibers can be observed in the fracture surface of the quite brittle minicomposites. This confirms that matrix cracks formed before ultimate fail￾ure, and that fiber/matrix debonding occurred. The damage-tolerant composite behavior exists, but it is not reflected by the stress–strain curve because it is very limited. Fiber Surface Roughness Figure 6 shows images of the surface of pullout fibers during tensile tests. They demonstrate that devi￾ation of the matrix crack occurred at the fiber surface in both Hi-NicalonS/SiC and SA3/SiC minicomposites. Thus, it can be concluded that bonding of SA3 fibers was not significantly stronger than that of Hi-NicalonS fibers, and it can be reasonably conjectured that the in￾terface opening strength was comparable in both sys￾tems. It was not sufficiently high to cause deviation within the interphase, as it has been observed when the surface of SiC fibers has been treated to strengthen the fiber/coating bond.25 Then, the surface of SA3 fibers appears to be rather rough, with peaks and valleys. Micrographs of cross-sections as well as AFM im￾ages of fiber surface highlight this topography (Figs. 6 and 7). Table IV summarizes the fiber surface roughness amplitudes determined using the AFM. Both RRMS (root mean square) and Rmax (maximum peak ampli￾tude observed in the analyzed portion of the fiber sur￾face) clearly indicate that the surface roughness amplitude of SA3 fibers is much more significant than that of Hi-NicalonS fibers. Note that Rmax may be as high as 60 nm for the SA3 fibers, which is much larger than the thickness of the PyC coating (ePyC) in the M4 test specimens. It should also be noticed that the diam￾eter is three times as small as that of Hi-NicalonS, whereas the roughness amplitude is about four times as Fig. 4. Scanning electron micrograph of the fracture surface of a Hi-NicalonS/SiC minicomposite test specimen. Table IV. Characteristics of Fiber Surface Rough￾ness Determined Using AFM and the Magnitude of Clamping Stresses rR (Eq. (4)) RRMS (nm) Rmax (nm) rR (MPa) Fiber HiS 2.33 15.2 130 Fiber SA3 8.04 55.8 930 www.ceramics.org/ACT Influence of Interface Characteristics on the Mechanical Properties 297