790 Journal of the American Ceramic Society-Bertrand et al Vol 84. No 4 increase fiber/matrix interactions. These stresses are induced by the curved fibers that try to stretch under a tensile load The I data indicate that the fiber/matrix bond is strengthened in the minicomposites reinforced with treated fibers. Thus, the Is 1=E measured in the HNT/(C/SiC)Io minicomposites is significantl smaller than that in their counterpart with as-received fibers (HN/(C/SiC)o). Although the Is measured in the internal matrix b2 _(1+v)EE+(1-2)E falls within the same range for most minicomposites, that obtained for the minicomposites reinforced with treated fibers is the shortest where o is the applied stress in the unloading-reloading sequence (Table I) that corresponds to 84, o, the initial stress level at unloading, E Matrix cracking essentially involved transverse cracks the Youngs modulus of the minicomposite, R the fiber radius, v over, a few longitudinal cracks also were detected(Fig. 5) the Poisson's ratio (v=vm= v, Em the Youngs modulus of the cracks were not identified on the specimens inspected before matrix, Er that of fiber, and ve the fiber volume fraction. T was testing. They were created during the tensile tests derived from the SA-o data measured during the last unloading- Unlike the minicomposites with as-received fibers in which reloading sequence before ultimate failure of the minicomposites ignificant fiber/coating debonds were observed before testing, The number of matrix cracks in gauge length, N, was determined those minicomposites with treated fibers did not exhibit such from SEM inspection of the minicomposites after failure preexisting interface features. (2) The spacing distance of the matrix cracks at saturation. Figure 6 illustrates the significant differences observed in the Estimates of T are given by the following equations deflection of matrix cracks, depending on the fiber/coating bond In those minicomposites reinforced with as-received fibers, deflec- sRe tion of the matrix cracks occurs in the fiber/interphase interface and also in the matrix/interphase interface(Fig. 6). It is worth 24(1+ pointing out that the crack-opening displacement is rather large (0. 2 um). In those minicomposites reinforced with treated fibers, rrl matrix cracks are deflected within the coating in the PyC sublayers (Fig. 6). Crack branching can also be noticed on Fig. 6. Finally, the crack reaches the PyC sublayer bonded to the fiber surface. Unlike where o, is the applied stress at matrix-cracking saturation. in minicomposites with as-received fibers, the crack-openin 3) The force-deformation curves. A curve fitting procedur displacement is now very small (-10 nm been detailed and validated in previous works. s This model has based on a model of the tensile behavior was used e T adj (4 Interfacial Shear Stress (T) ment involved predictions of the force-deformation curves(Fig. results of tensile tests and crack examination, estimates of the T ters given in Table t properties and the flaw-strength parame- To assess the trends that have been identified based 7)from the cons were extracted from various data provided by the tensile tests The T estimates given in Table Iv evidence an unquestionable (1) The width of the hysteresis loops(8A)measured durin strengthening of the fiber/coating bond associated with the treated equation(established elsewhere for microcomposites 2. O/lowing fibers. The different methods agree satisfactorily in indicating this trend, despite certain discrepancies that can be noticed. The scatter in T measurements previously noticed does not affect this trend biN(I-a1Ve-Ro Therefore the results can be summarized as follows .t data are 2:E ( closer to 200 MPa for minicomposites reinforced with treated fibers, and they are close to 100 MPa for those reinforced with as-received fibers These trends compare satisfactorily with those established on Nicalon/SiC composites. This agreement can be attributed to the presence of a silicon/carbon/oxygen layer at the surface of as- received Nicalon and Hi-Nicalon fibers, and a free carbon layer at the surface of treated Nicalon and Hi-Nicalon fibers. which has been shown to dictate the fiber/coating bond (5) Lifetime in Static Fatigue at 700C n did not fail after 140 and 200 h. Further comparison of the data showed that the multilayered coating improved lifetime: from 2 to 20 h for those minicomposites reinforced with as-received fibers The influence might appear to be less clear for those minicompos ites reinforced with treated fibers because of scatter in the data 1.5pm for the HNT/(C/SiC)o minicomposites However, some HNT/ (C/SiC)o minicomposites were not broken after 200 and 140 h, was 114 h. This trend agreed with previous results reported elsewhere. 7, 8 The results suggest an effect of the fiber/coating bond. Strong nIcon matrix cracks. SEM images(Fig. 9)of the interfacial regions after the static fatigue tests show that PyC layers have disappeared Fig. 5. SEM images of longitudinal cracks detected in Hi-Nicalon/SiC However, some interesting differences can be noticed, depending minicomposite after tensile tests. on the batch, that can be related to the location of the debond crackincrease fiber/matrix interactions. These stresses are induced by the curved fibers that try to stretch under a tensile load. The ls data indicate that the fiber/matrix bond is strengthened in the minicomposites reinforced with treated fibers. Thus, the ls measured in the HNT/(C/SiC)10 minicomposites is significantly smaller than that in their counterpart with as-received fibers (HN/(C/SiC)10). Although the ls measured in the internal matrix falls within the same range for most minicomposites, that obtained for the minicomposites reinforced with treated fibers is the shortest (Table III). Matrix cracking essentially involved transverse cracks. More￾over, a few longitudinal cracks also were detected (Fig. 5). Such cracks were not identified on the specimens inspected before testing. They were created during the tensile tests. Unlike the minicomposites with as-received fibers in which significant fiber/coating debonds were observed before testing, those minicomposites with treated fibers did not exhibit such preexisting interface features. Figure 6 illustrates the significant differences observed in the deflection of matrix cracks, depending on the fiber/coating bond. In those minicomposites reinforced with as-received fibers, deflec￾tion of the matrix cracks occurs in the fiber/interphase interface and also in the matrix/interphase interface (Fig. 6). It is worth pointing out that the crack-opening displacement is rather large (0.2 mm). In those minicomposites reinforced with treated fibers, matrix cracks are deflected within the coating in the PyC sublayers (Fig. 6). Crack branching can also be noticed on Fig. 6. Finally, the crack reaches the PyC sublayer bonded to the fiber surface. Unlike in minicomposites with as-received fibers, the crack-opening displacement is now very small (;10 nm). (4) Interfacial Shear Stress (t) To assess the trends that have been identified based on the results of tensile tests and crack examination, estimates of the t were extracted from various data provided by the tensile tests: (1) The width of the hysteresis loops (dD) measured during unloading–reloading cycles, which is related to t by the following equation (established elsewhere for microcomposites15): t 5 b2N~1 2 a1Vf! 2 Rf 2V f 2 Em S sp 2 dDDS s sp DS1 2 s sp D (1) with a1 5 Ef Ec (2) b2 5 ~1 1 n! Em@Ef 1 ~1 2 2n!Ec# Ef@~1 1 n!Ef 1 ~1 2 n!Ec# where s is the applied stress in the unloading–reloading sequence that corresponds to dD, sp the initial stress level at unloading, Ec the Young’s modulus of the minicomposite, Rf the fiber radius, n the Poisson’s ratio (n5nm 5 nf ), Em the Young’s modulus of the matrix, Ef that of fiber, and Vf the fiber volume fraction. t was derived from the dD–s data measured during the last unloading– reloading sequence before ultimate failure of the minicomposites. The number of matrix cracks in gauge length, N, was determined from SEM inspection of the minicomposites after failure. (2) The spacing distance of the matrix cracks at saturation. Estimates of t are given by the following equations:16,17 t 5 ssRf 2VflsS1 1 EfVf EmVm D (3) t 5 ssRfVm 2Vfls (4) where ss is the applied stress at matrix-cracking saturation. (3) The force–deformation curves. A curve fitting procedure based on a model of the tensile behavior was used. This model has been detailed and validated in previous works.5,12 The t adjust￾ment involved predictions of the force–deformation curves (Fig. 7) from the constituent properties and the flaw–strength parame￾ters given in Table II. The t estimates given in Table IV evidence an unquestionable strengthening of the fiber/coating bond associated with the treated fibers. The different methods agree satisfactorily in indicating this trend, despite certain discrepancies that can be noticed. The scatter in t measurements previously noticed12 does not affect this trend. Therefore, the results can be summarized as follows: t data are closer to 200 MPa for minicomposites reinforced with treated fibers, and they are close to 100 MPa for those reinforced with as-received fibers. These trends compare satisfactorily with those established on Nicalon/SiC composites.3 This agreement can be attributed to the presence of a silicon/carbon/oxygen layer at the surface of as￾received Nicalon and Hi-Nicalon fibers, and a free carbon layer at the surface of treated Nicalon and Hi-Nicalon fibers, which has been shown to dictate the fiber/coating bond. (5) Lifetime in Static Fatigue at 700°C The lifetime data are shown in graphical form in Fig. 8. The lifetimes obtained for those minicomposites reinforced with treated fibers were unambiguously the longest. Some specimens did not fail after 140 and 200 h. Further comparison of the data showed that the multilayered coating improved lifetime: from 2 to ;20 h for those minicomposites reinforced with as-received fibers. The influence might appear to be less clear for those minicompos￾ites reinforced with treated fibers because of scatter in the data for the HNT/(C/SiC)10 minicomposites. However, some HNT/ (C/SiC)10 minicomposites were not broken after 200 and 140 h, whereas the maximum lifetime of those HNT/C minicomposites was 114 h. This trend agreed with previous results reported elsewhere.7,8 The results suggest an effect of the fiber/coating bond. Strong bonds have been shown to reduce debonding and crack-opening displacement that limit the amount of oxygen migrating within the matrix cracks. SEM images (Fig. 9) of the interfacial regions after the static fatigue tests show that PyC layers have disappeared. However, some interesting differences can be noticed, depending on the batch, that can be related to the location of the debond crack Fig. 5. SEM images of longitudinal cracks detected in Hi-Nicalon/SiC minicomposite after tensile tests. 790 Journal of the American Ceramic Society—Bertrand et al. Vol. 84, No. 4