September 1998 Multilayered Interphases in SiC/SiC CHI Composites with"Weak"and" Strong" Interfaces 5000 苏 untreated 1000fe 5 n(C,SIC)layer Fig 9. Debond and maximum stresses in SiC/SiC composites, as a function of the number of(C-SiC) layers(n)in the inter o untreated fiber plateau+treated fibers n(C,SIC) layer Fig. 10. Interfacial shear stress in SiC/SiC composites, as a function of the number of( C-Sic) layers(n)in the interphase face. Thus, o is expressed as the sum of the true thermally bonding has not been yet modeled, only the following values induced residual stress and a compressive stress component could be determined: (i) the debonding stress(oa)(also called that accounts for a roughness contribution. The amplitude(A) the critical debonding stress for unstable debonding ),(ii)the of the roughness was derived from o using the following maximum stress, (iii) the displacement at the maximum load, and (iv) the frictional shear stress(Plateau)(Table IV) E(1 +vm)+Em(l-ve It was very difficult to push the fibers out of the matrix, AaArr(3) and satisfactory fiber push-out was possible only for embed- ded lengths of <190 um. At these lengths, the applied loads ete to unity for an infinite remained moderate so that the fibers were not damaged On the basis of the above estimates of radial (maximum load is -120 g for a Nicalon fiber) and the dia- stresses, a roughness amplitude A of -50 nm was mond indentor did not penetrate the matrix before the load Table Ill). The above-mentioned results apply to the multilay- decrease(diamond penetration is evidenced by a steep stiffness ered interphases. The residual stresses operating on the fiber are determined by the amount of carbon in the interphase. They The stresses required to debond(2500 MPa)or to push out are dependent on the total thickness of carbon layers (4000 MPa) the fibers are uncommonly high for such thin (2) Composites Reinforced with Treated Fibers samples(-150 um)(Fig. 9). Although fiber displacements are similar to those measured on composites with weakly bonded (A) Push-Out Tests: Because the push-out behavior ob- fibers, the embedded lengths are only half as long. The fric- served for the composites with strong fiber/coating interface tional shear stresses calculated from the plateau are -100 MPaface. Thus, sc is expressed as the sum of the true thermally induced residual stress and a compressive stress component that accounts for a roughness contribution. The amplitude (A) of the roughness was derived from sc using the following expression:18 A = H− scF Ef~1 + nm! + Em~1 − nf! qEmEf G − DaDTJr (3) where q is a parameter equal to unity for an infinite matrix. On the basis of the above estimates of radial residual stresses, a roughness amplitude A of ∼50 nm was obtained (Table III). The above-mentioned results apply to the multilay￾ered interphases. The residual stresses operating on the fiber are determined by the amount of carbon in the interphase. They are dependent on the total thickness of carbon layers. (2) Composites Reinforced with Treated Fibers (A) Push-Out Tests: Because the push-out behavior ob￾served for the composites with strong fiber/coating interface bonding has not been yet modeled, only the following values could be determined: (i) the debonding stress (sd) (also called the critical debonding stress for unstable debonding14), (ii) the maximum stress, (iii) the displacement at the maximum load, and (iv) the frictional shear stress (tplateau) (Table IV). It was very difficult to push the fibers out of the matrix, and satisfactory fiber push-out was possible only for embed￾ded lengths of <190 mm. At these lengths, the applied loads remained moderate so that the fibers were not damaged (maximum load is ∼120 g for a Nicalon fiber) and the dia￾mond indentor did not penetrate the matrix before the load decrease (diamond penetration is evidenced by a steep stiffness increase). The stresses required to debond (>2500 MPa) or to push out (>4000 MPa) the fibers are uncommonly high for such thin samples (∼150 mm) (Fig. 9). Although fiber displacements are similar to those measured on composites with weakly bonded fibers, the embedded lengths are only half as long. The fric￾tional shear stresses calculated from the plateau are ∼100 MPa Fig. 9. Debond and maximum stresses in SiC/SiC composites, as a function of the number of (C–SiC) layers (n) in the interphase. Fig. 10. Interfacial shear stress in SiC/SiC composites, as a function of the number of (C–SiC) layers (n) in the interphase. September 1998 Multilayered Interphases in SiC/SiC CVI Composites with ‘‘Weak’’ and ‘‘Strong’’ Interfaces 2321