September 1999 Hi-Nicalon/SiC Minicomposites with(Pyrocarbon/SiC)n Nanoscale Multilayered Interphases Table Il. Properties of the Minicomposite Constituents oisson's ratio in Coefficient of thermal expansion, (GPa) plane axial conditions TE(X 10/C) Constitu o°(MPa) Ii-Nicalon sic fiber 280 464.71 4.6-4.7 PyC interphase 120.2 280 0.12 P-CVI SIC 400 400170170 0.20 5.7 scripts"T"and"L "denote transverse and longitudinal conditions, respectively. ' Data for the PyC interphase are taken from Bobet and Lamon, whereas data for the SiC 3) Tensile Tests on Minicomposites Uniaxial tension tests were performed at room temperature at a constant strain rate(50 um/min). The load was measured -SiC matrix using a 500 N load cell. The minicomposite elongation was measured using two parallel linear-variable differential trans- former(LVDT) extensometers that were mounted on the grips The minicomposites ends were glued within metallic tubes that were then gripped into the testing machine. The gauge length was 20 mm. The system compliance(Cs) was determined or Fiber- Interphase dry fiber tows with decreasing gauge lengths(Cs =0.3 um/N) Unloading-reloading cycles were conducted on a few speci mens of each batch to evaluate the residual strains and the F/m bonding. After ultimate failure, the test specimens were exam- d using SEM, to measure the matrix-crack-spacing distance II. Results ( Material Characterization Figure 2 shows a general view of the failure surface of a minicomposite,some fiber pullout and residual porosity are visible. Figure 3 shows a multilayered(PyC/SiC)n, interphase at different scales. The microstructure of the SiC-based sublayers quasi-amorphous and nanocrystallized (Lu =15 nm). In such a microstructure. the number and size of microstructural Matrix defects are reduced, in comparison to those observed in the SiC-based sublayers made via I-CVI. 3 The PyC exhibits a rough, laminar microstructure(Ae= 18.5). Because of the strong microstructural anisotrophy that is indicated by the value of the extinction angle, the PyC sublayers are expected to cause deviation of the matrix microcracks, as observed by Droillard and co-workers. 4, 12 Both the PyC and SiC sublayers are generally parallel to the fibers axis(Figs. 3 and 4)and Fig. 2. SEM micrographs of the fracture surface of SiC(Py C/SiC),/ continuous(Fig 3). Their thicknesses e(sic) and e(pyc) are con- SiC minicomposites, showing the constituents, fiber pullout, and stant(Fig 3) debonding between the fiber and the first PyC interfacial sublayer AES depth profiles performed on Hi-Nicalon fiber show the presence of an oxygen-enriched layer(15 nm thick)at the fiber surface. This Si-C-O phase consists of SiO2 and free carbon It is thought to be related to the heating of the tows, under The scatter of the stress-strain curves falls within the range vaccum, before the deposition of the interphase and the matrix hat is usually observed with CMCs. Features of the force- In the minicomposites, the first interfacial Py C sublayer that is elongation curves(Table D)do not indicate a significant differ deposited is bonded to this Si-C-O layer ence between the batches. Figure 5 shows that the presence of a multilayered interphase does not affect the tensile behavior, (2) Tensile Behavior of Minicomposites in comparison with the reference minicomposites with a single (A) Force-Deformation Curves: The force-deformation Py C layer as the interphase curves of the minicomposites(Fig. 5)are markedly nonlinear Figure 6 shows typical hysteresis loops that and indicate nonbrittle mechanical behavior. All the curves when unloading-reloading the minicomposites. The elastic display the following typical features modulus that is pertinent to the cracked minicomposite is giver (1) An initially linear region, reflecting the elastic defor- by the slope of the linear portion of the reloading curve(mini mation of the minicomposites. The proportional limit is ob- mum tangent modulus). The tangent to this linear portion gen- served to be -0.1% deformation erally intercepts the origin. The nt strain at zero lo (2) A nonlinear domain of deformations, resulting from includes contributions from misfit relief e* and sliding Es. The multiple matrix cracking wide hysteresis loops, and the large permanent elongations at (3) After saturation of matrix ng. a second linear zero load(10-20 um, as shown in Table I), region that is attributed to the elas gation of the fibers ence of rather weak F/M interactions. 12 suggest the I 4) A second, slightly nonlinea prior to maximum Figure 7 shows the typical elastic moduli that have beer load. which is attributed to individual fiber break measured during the tensile tests. For most minicomposites, the (5) Finally, the ultimate failure modulus decreases to a minimum value that coincides with the(3) Tensile Tests on Minicomposites Uniaxial tension tests were performed at room temperature at a constant strain rate (50 mm/min). The load was measured using a 500 N load cell. The minicomposite elongation was measured using two parallel linear–variable differential trans￾former (LVDT) extensometers that were mounted on the grips. The minicomposites ends were glued within metallic tubes that were then gripped into the testing machine. The gauge length was 20 mm. The system compliance (CS) was determined on dry fiber tows with decreasing gauge lengths (CS 4 0.3 mm/N). Unloading–reloading cycles were conducted on a few speci￾mens of each batch, to evaluate the residual strains and the F/M bonding. After ultimate failure, the test specimens were exam￾ined using SEM, to measure the matrix-crack-spacing distance. III. Results (1) Material Characterization Figure 2 shows a general view of the failure surface of a minicomposite; some fiber pullout and residual porosity are visible. Figure 3 shows a multilayered (PyC/SiC)n interphase at different scales. The microstructure of the SiC-based sublayers is quasi-amorphous and nanocrystallized (L111 4 15 nm).14 In such a microstructure, the number and size of microstructural defects are reduced, in comparison to those observed in the SiC-based sublayers made via I-CVI.13 The PyC exhibits a rough, laminar microstructure (Ae 4 18.5°). Because of the strong microstructural anisotrophy that is indicated by the value of the extinction angle, the PyC sublayers are expected to cause deviation of the matrix microcracks, as observed by Droillard and co-workers.4,12 Both the PyC and SiC sublayers are generally parallel to the fibers axis (Figs. 3 and 4) and continuous (Fig. 3). Their thicknesses e(SiC) and e(PyC) are con￾stant (Fig. 3). AES depth profiles performed on Hi-Nicalon fiber show the presence of an oxygen-enriched layer (15 nm thick) at the fiber surface. This Si–C–O phase consists of SiO2 and free carbon. It is thought to be related to the heating of the tows, under vaccum, before the deposition of the interphase and the matrix. In the minicomposites, the first interfacial PyC sublayer that is deposited is bonded to this Si–C–O layer. (2) Tensile Behavior of Minicomposites (A) Force–Deformation Curves: The force–deformation curves of the minicomposites (Fig. 5) are markedly nonlinear and indicate nonbrittle mechanical behavior. All the curves display the following typical features: (1) An initially linear region, reflecting the elastic defor￾mation of the minicomposites. The proportional limit is ob￾served to be ∼0.1% deformation. (2) A nonlinear domain of deformations, resulting from multiple matrix cracking. (3) After saturation of matrix cracking, a second linear region that is attributed to the elastic elongation of the fibers. (4) A second, slightly nonlinear domain prior to maximum load, which is attributed to individual fiber breaks. (5) Finally, the ultimate failure. The scatter of the stress–strain curves falls within the range that is usually observed with CMCs. Features of the force– elongation curves (Table I) do not indicate a significant differ￾ence between the batches. Figure 5 shows that the presence of a multilayered interphase does not affect the tensile behavior, in comparison with the reference minicomposites with a single PyC layer as the interphase. Figure 6 shows typical hysteresis loops that are obtained when unloading–reloading the minicomposites. The elastic modulus that is pertinent to the cracked minicomposite is given by the slope of the linear portion of the reloading curve (mini￾mum tangent modulus). The tangent to this linear portion gen￾erally intercepts the origin. The permanent strain at zero load includes contributions from misfit relief e* and sliding e0 S. The wide hysteresis loops, and the large permanent elongations at zero load (10–20 mm, as shown in Table I), suggest the pres￾ence of rather weak F/M interactions.12 Figure 7 shows the typical elastic moduli that have been measured during the tensile tests. For most minicomposites, the modulus decreases to a minimum value that coincides with the Table II. Properties of the Minicomposite Constituents† Constituent‡ Young’s modulus (GPa) Shear modulus (GPa) Poisson’s ratio in plane axial conditions Coefficient of thermal expansion, CTE (× 10−6/°C) Statistical parameters EL ET GL GT n12 n13 aT aT m s0 § (MPa) Hi-Nicalon SiC fiber 280 280 125 125 0.12 0.12 4.6–4.7¶ 4.6–4.7 4.2 6 PyC interphase 80 12 36 5 0.12 0.20 2 20–28 SiC interphase 280 280 125 125 0.12 0.12 3.9 3.9 P-CVI SiC 400 400 170 170 0.20 0.20 4.6 4.6 5.5 5.7 † Subscripts “T” and “L” denote transverse and longitudinal conditions, respectively. ‡ Data for the PyC interphase are taken from Bobet and Lamon,19 whereas data for the SiC interphase and P-CVI SiC are taken from Heurtevent14 (however, the statistical parameters for P-CVI SiC have been taken from Fredefon et al.22). § For a reference volume (V0) of 1 m3 . ¶ For a temperature range of 523–1073 K. From Cabot et al.21 Fig. 2. SEM micrographs of the fracture surface of SiC/(PyC/SiC)n / SiC minicomposites, showing the constituents, fiber pullout, and debonding between the fiber and the first PyC interfacial sublayer. September 1999 Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases 2467