pril 2001 Influence of Interfaces on Mechanical Behavior and Lifetime of Hi-Nicalon/(PyC/SiC) Sic 791 M (Fiber surface Sic 1 um (Fiber surface) Fiber Fibe Fig icrographs showing deflection of matrix microcracks in interfacial region of minicomposites: (a) SEM micrograph of a HN/(C/SIC)o minicomposite,(b)TEM micrograph of a HNT/(C/SiC)o minicomposite in the interfacial region. In the HN/(C/SiC)o minicomposites, on (a)250pT「·T·冂 the Pyc sublayer that lay on the fibers has been eliminated whereas, in the HNT/(C/SiC)o minicomposites, all the PyC layers exp in which matrix cracks are found to be deflected are affected V. Discussion 2150 It has been confirmed again that multilayered interphases are not detrimental to the mechanical behavior of sic/sic minicom- 100 posites, although these contain stiff sublayers of SiC and very thin (nanometer-scale)PyC sublayers The presence of rather strong fiber/coating interfaces in those minicomposites reinforced with treated Hi-Nicalon fibers has been Batcha revealed by a set of data, including features of the force deformation curves. location of the debond crack in the interfacial 002040.60.8 region, composition of the fiber surface, and estimates of T. Deformation (%) The force-deformation curves pertinent to those minicompos- ites reinforced with treated fibers exhibited the features previousl observed on Nicalon/SiC minicomposites and two-dimensional woven composites reinforced with treated fibers(NLM 202) including(i) a wide curved domain, (ii)a large stress/strain at saturation of matrix cracking that coincides with ultimate failures, (b)250 (iii) small residual deformations at zero load, and (iv) a gentle modulus decrease during tensile tests. The matrix cracks were deflected within each Pyc sublayer of the interphase in those minicomposites reinforced with treated fibers and at the fiber surface in those minicomposites reinforced 150 experiment vith as-received fibers. This debond patten was similar to that observed on Nicalon/SiC composites. 10 Deviation within the interphase resulted from the presence of a stronger bond between 100 the interphase and the fib In those minicomposites reinforced with as-received fibers, weakening of the fiber/interphase region has been evidenced usin SEM. The preexisting debond cracks were attributed to the lateral contraction of as-received Hi-Nicalon fibers during minicomposite Batch C rocessing. Yun et al. have shown that the lateral contraction of as-received Hi-Nicalon fibers during processing of composites can 0 xceed 1% at 1200C. The contraction was associated with fiber Deformation(%) shrinkage and with the transformation of the Sic amorphous phase nto a B-sic crystallized phase. Fig. 7. Comparison of predicted and experimental force deformation curves for SiC/SiC minicomposites(a) HN/(C/SiC)o and(b)HNT/(C/SiC)o The interfacial bond was characterized by the interfacial shear tress (T). As previously mentioned, all the methods indicated tha the fiber/coating bond was stronger in those minicomposites fibers and T > 200 MPa for those reinforced with treated fibers reinforced with treated fibers. However, the range of t data was In study, T 100 MPa for those Hi-Nicalon/SiC arrower than that observed on Nicalon/Sic two-dimensional InIc tes reinforced with as-received fibers. and T s 200 woven composites. In those Nicalon/SiC composites, the respe MPa e reinforced with treated fibers. A contribution of tive t values were different by more than I order of magnitude: twisting could be expected. Twisting generated radial compressive T 10 MPa for those composites reinforced with as-received stresses that enhanced fiber/matrix interactions and can tend toin the interfacial region. In the HN/(C/SiC)10 minicomposites, only the PyC sublayer that lay on the fibers has been eliminated; whereas, in the HNT/(C/SiC)10 minicomposites, all the PyC layers in which matrix cracks are found to be deflected are affected. IV. Discussion It has been confirmed again that multilayered interphases are not detrimental to the mechanical behavior of SiC/SiC minicom￾posites, although these contain stiff sublayers of SiC and very thin (nanometer-scale) PyC sublayers. The presence of rather strong fiber/coating interfaces in those minicomposites reinforced with treated Hi-Nicalon fibers has been revealed by a set of data, including features of the force– deformation curves, location of the debond crack in the interfacial region, composition of the fiber surface, and estimates of t. The force–deformation curves pertinent to those minicompos￾ites reinforced with treated fibers exhibited the features previously observed on Nicalon/SiC minicomposites and two-dimensional woven composites reinforced with treated fibers (NLM 202) including (i) a wide curved domain, (ii) a large stress/strain at saturation of matrix cracking that coincides with ultimate failures, (iii) small residual deformations at zero load, and (iv) a gentle modulus decrease during tensile tests. The matrix cracks were deflected within each PyC sublayer of the interphase in those minicomposites reinforced with treated fibers and at the fiber surface in those minicomposites reinforced with as-received fibers. This debond pattern was similar to that observed on Nicalon/SiC composites.3,10 Deviation within the interphase resulted from the presence of a stronger bond between the interphase and the fiber.6 In those minicomposites reinforced with as-received fibers, weakening of the fiber/interphase region has been evidenced using SEM. The preexisting debond cracks were attributed to the lateral contraction of as-received Hi-Nicalon fibers during minicomposite processing. Yun et al.20 have shown that the lateral contraction of as-received Hi-Nicalon fibers during processing of composites can exceed 1% at 1200°C. The contraction was associated with fiber shrinkage and with the transformation of the SiC amorphous phase into a b-SiC crystallized phase.21 The interfacial bond was characterized by the interfacial shear stress (t). As previously mentioned, all the methods indicated that the fiber/coating bond was stronger in those minicomposites reinforced with treated fibers. However, the range of t data was narrower than that observed on Nicalon/SiC two-dimensional woven composites. In those Nicalon/SiC composites, the respec￾tive t values were different by more than 1 order of magnitude: t ' 10 MPa for those composites reinforced with as-received fibers and t . 200 MPa for those reinforced with treated fibers.3 In the present study, t ' 100 MPa for those Hi-Nicalon/SiC minicomposites reinforced with as-received fibers, and t ' 200 MPa for those reinforced with treated fibers. A contribution of twisting could be expected. Twisting generated radial compressive stresses that enhanced fiber/matrix interactions and can tend to Fig. 7. Comparison of predicted and experimental force deformation curves for SiC/SiC minicomposites (a) HN/(C/SiC)10 and (b) HNT/(C/SiC)10. Fig. 6. Micrographs showing deflection of matrix microcracks in interfacial region of minicomposites: (a) SEM micrograph of a HN/(C/SiC)10 minicomposite, (b) TEM micrograph of a HNT/(C/SiC)10 minicomposite. April 2001 Influence of Interfaces on Mechanical Behavior and Lifetime of Hi-Nicalon/(PyC/SiC)n/SiC 791