September 1999 Hi-Nicalon/SiC Minicomposites with(Pyrocarbon/SiC)n Nanoscale Multilayered Interphases 2469 41 gardless of the method and batch. The T values obtained from the crack-spacing distance at saturation are 17-49 MP whereas those determined from the hyst are 32-70 MPa. Therefore, because of the scatter usually observed Interphase oer with T measurements, the T values do not indicate any signifi- cant effect of multilayering the interphase. The T values are comparable to those measured on the SiC/SiC minicomposites with a single PyC layer(batch(100/0)1). They also are com- parable to those measured on SiC/SiC minicomposites tha have been reinforced with as-received Nicalon fiber 3 this agreement may be attributed to the presence of the SiC-O layer of at the surface of the Hi-Nicalon and Nicalon fibers which has been demonstrated to dictate the devia matrix cracks Fiber Table I indicates that the T values derived from the hysteresis oop widths are generally larger than those obtained from the matrix-crack ng distance. As discussed in a following sec- tion, this discrepancy may be attributed to an effect of the initial twisting of the tows which enhances F/M interactions pared via P-CVI(n= h of a nanoscale multilayered interphase pre- Fig. 4. SEM micrograp V. Discussion interphase. Recall the previously mentioned feature, that the n It has been emphasized that the tensile mechanical behavior confirms debonding in the interface between the fiber and the the minicomposites was not affected by the presence of region between the fiber and the interphase consists mainly of multilayered interphases that involved stiff sublayers of Sic Similar features have been identified, regardless of the inter- been identified in the composites that have been reinforced phases, including proportional limit, saturation stress, strain to ith as-received Nicalon fibers. In these composites, the F/M nteractions are weak. and deflection of the matrix cracks oc- Slightly different results have been obtained for Hi-N curs at the fiber/interphase interfaces. SiC microcomposites with(PyC/SiC)n nanoscale mult interphases. 4 That author found that the stress-strain 3) Interfacial Shear Stress of the microcomposites was dependent on the respective thick- The interfacial shear stress (T)was estimated from the fol- nesses of the sublayers. The best properties were observed for lowing data he following sublayer conditions: e(Byc)=3 nm, e(sic)= 30 d during nm, and n= 10. The corresponding(3/30)10 minicomposites nloading-reloading cycles. T is given by the following equa- exhibited an interfacial shear stress that seemed to be slightly ion, which has been established elsewhere for microcompos- larger than that obtained with the other minicomposites. How- because of the scatter generally observed with T data b2NI-aVeR e The estimates of the interfacial shear stress in the minicom- () posites must be regarded as comparable. They do not indicate an effect of multilayering the interphase. This is a satisfactory result, because the properties of the minicomposite s nave been degraded by the presence of an interphase that contains ayers of a stiff material such as Sic a1=EJEc Comparable shear stresses have been estimated and crack deviation at the fiber surface has been observed (1+v)Em[Er+(1-2v)E the batch. Furthermore. similar interfacial shear stresses and E[(1+v)Er+(1-v)E crack deviations have been obtained with sic/sic minicom- posites that have been reinforced with as-received Nicalon fi- where &A is the hysteresis loop width, o the corresponding ber(NL 202)3 These features characterize a weak interfacial applied stress during the unloading-reloading sequence, a, the bond, as opposed to the rather strong bonding observed in the initial stress level at unloading, Ec the Youngs modulus of the SiC/SiC composites that are reinforced with treated Nicalon minicomposite, Rr the fiber radius, and v the Poissons ratio fiber. A layer of SiO/anisotropic carbon has been identified at measured s v). T was derived from the 8A-o data that were the surface of the fibers in the minicomposites of the present during the last unloading-reloading sequence, before study and in those SiC/SiC minic ites and 2D woven the ultimate failure of the minicomposites. N, the number of composites that have been reinforced with as-received Nicalon atrix cracks, was determined from SEM inspection of the NL 202)fiber; these minicomposites and composites exhibit a minicomposites after failure (2) The spacing distance of the matrix cracks at saturation, the composites that have been reinforced with treated Nicalon fiber and exhibit strong interfacial bonds. In these latter com- UsRe posites, the crack deviation occurs within the interphase. It has T been demonstrated that the weakest link is located within the 2 I interface in the former composites. 4, n the fiber/interphase interphases in the latter composites and Therefore, the debonding may be considered to be dictated by the surface of the fiber in the minicomposites of the present (4) study. Because of the presence of a layer of Sio /anisotropi carbon, the fiber/interphase interface is the weakest link where os is the applied stress at matrix cracking saturation. interfacial regio Table I shows that comparable T values were obtained, re- Debonding at the matrix/interphase interface also hasconfirms debonding in the interface between the fiber and the interphase. Recall the previously mentioned feature, that the region between the fiber and the interphase consists mainly of a SiO2/anisotropic carbon sublayer. Such sublayers also have been identified in the composites that have been reinforced with as-received Nicalon fibers. In these composites, the F/M interactions are weak, and deflection of the matrix cracks oc￾curs at the fiber/interphase interfaces.1,4 (3) Interfacial Shear Stress The interfacial shear stress (t) was estimated from the fol￾lowing data: (1) First, the width of hysteresis loops measured during unloading–reloading cycles. t is given by the following equa￾tion, which has been established elsewhere for microcompos￾ites:25 t = b2N~1 − al Vf! 2 Rf 2Vf 2 Em S sp 2 dDD F s spS1 − s sp DG (1) with a1 = Ef/Ec (2a) b2 = ~1 + n!Em@Ef + ~1 − 2n!Ec# Ef@~1 + n!Ef + ~1 − n!Ec# (2b) where dD is the hysteresis loop width, s the corresponding applied stress during the unloading–reloading sequence, sp the initial stress level at unloading, Ec the Young’s modulus of the minicomposite, Rf the fiber radius, and n the Poisson’s ratio (n 4 nm 4 nf ). t was derived from the dD–s data that were measured during the last unloading–reloading sequence, before the ultimate failure of the minicomposites. N, the number of matrix cracks, was determined from SEM inspection of the minicomposites after failure. (2) The spacing distance of the matrix cracks at saturation, using the following equations:26,27 t = sSRf 2 VflS S1 + Ef Vf EmVm D (3) t = sSRf Vm 2 VflS (4) where sS is the applied stress at matrix cracking saturation. Table I shows that comparable t values were obtained, re￾gardless of the method and batch. The t values obtained from the crack-spacing distance at saturation are 17–49 MPa, whereas those determined from the hysteresis loop widths are 32–70 MPa. Therefore, because of the scatter usually observed with t measurements, the t values do not indicate any signifi￾cant effect of multilayering the interphase. The t values are comparable to those measured on the SiC/SiC minicomposites with a single PyC layer (batch (100/0)1). They also are com￾parable to those measured on SiC/SiC minicomposites that have been reinforced with as-received Nicalon fiber.3 This agreement may be attributed to the presence of the Si–C–O layer of at the surface of the Hi-Nicalon and Nicalon fibers, which has been demonstrated to dictate the deviation of the matrix cracks.28 Table I indicates that the t values derived from the hysteresis loop widths are generally larger than those obtained from the matrix-crack-spacing distance. As discussed in a following sec￾tion, this discrepancy may be attributed to an effect of the initial twisting of the tows, which enhances F/M interactions within the minicomposites. IV. Discussion It has been emphasized that the tensile mechanical behavior of the minicomposites was not affected by the presence of multilayered interphases that involved stiff sublayers of SiC. Similar features have been identified, regardless of the inter￾phases, including proportional limit, saturation stress, strain to failure, etc. Slightly different results have been obtained for Hi-Nicalon/ SiC microcomposites with (PyC/SiC)n nanoscale multilayered interphases.14 That author found that the stress–strain behavior of the microcomposites was dependent on the respective thick￾nesses of the sublayers. The best properties were observed for the following sublayer conditions: e(PyC) 4 3 nm, e(SiC) 4 30 nm, and n 4 10. The corresponding (3/30)10 minicomposites exhibited an interfacial shear stress that seemed to be slightly larger than that obtained with the other minicomposites. How￾ever, the difference cannot be considered to be significant, because of the scatter generally observed with t data. The estimates of the interfacial shear stress in the minicom￾posites must be regarded as comparable. They do not indicate an effect of multilayering the interphase. This is a satisfactory result, because the properties of the minicomposites have not been degraded by the presence of an interphase that contains layers of a stiff material such as SiC. Comparable shear stresses have been estimated and crack deviation at the fiber surface has been observed, regardless of the batch. Furthermore, similar interfacial shear stresses and crack deviations have been obtained with SiC/SiC minicom￾posites that have been reinforced with as-received Nicalon fi￾ber (NL 202).3 These features characterize a weak interfacial bond, as opposed to the rather strong bonding observed in the SiC/SiC composites that are reinforced with treated Nicalon fiber.4 A layer of SiO2/anisotropic carbon has been identified at the surface of the fibers in the minicomposites of the present study and in those SiC/SiC minicomposites and 2D woven composites that have been reinforced with as-received Nicalon (NL 202) fiber; these minicomposites and composites exhibit a weak interfacial bond. By contrast, this layer is not present in the composites that have been reinforced with treated Nicalon fiber and exhibit strong interfacial bonds. In these latter com￾posites, the crack deviation occurs within the interphase. It has been demonstrated that the weakest link is located within the interphases in the latter composites and in the fiber/interphase interface in the former composites.1,4,28 Therefore, the debonding may be considered to be dictated by the surface of the fiber in the minicomposites of the present study. Because of the presence of a layer of SiO2/anisotropic carbon, the fiber/interphase interface is the weakest link in the interfacial region. Debonding at the matrix/interphase interface also has been Fig. 4. SEM micrograph of a nanoscale multilayered interphase pre￾pared via P-CVI (n 4 30). September 1999 Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases 2469