December 2000 Table Il. Deposition Time, Interlayer Thicknesses, and Average Peak Load for Multilayer C/SiC Minicomposite Samples Tensile testing Peak load Sample Nicalon fiber type (μm) (um) MC-22 0.25 0.25 0.2 110±6 MC-3 Ceramic grade 0.5 90±37 Ceramic grade l5/15 0.008 0.03 85±41 Ceramic grade 83±15 MC-80 0.05 MC-85 Hi-Nicalon 15/30 0.4 122±15 MC-97 Hi-Nicalol 0.5 0.083 0.1 150±14 000 MC-114 Hi-Nicalon 120 C only 0.25 187±22 iNdividual layers were not discernible 250 025um carbon only 0 0. 4 um 03m 1.5 2.5 Displacement(mm) (a) Nicalon minicomposites. The curves have been shifted for clarity crosshead displacement at the peak load than those of minicom- posites made with ceramic-grade Nicalon fibers but do not exhibit their gradual strength reduction after peak load. The load versus crosshead displacement curves are nonlinear after the matrix cracking load is exceeded, and the small discontinuities along the curves typically indicate the occurrence of matrix cracks. The peak loads and crosshead displacements were greatest for the specimens that had only a single carbon-layer interface(MC-114) (2) Composite Plates The results of infiltration and the properties of the composite plates are listed in Table Ill, The density of each sample was I um calculated from geometrical measurements and the mass of the sample with the preform containing 40 vol% fiber. The variation in filtration time was, in part, due to difficulties in performing (b) FCVI on such thin components. Experience with preforms less than 10 mm in thickness suggests that there is a tendency fo Fig. 3. SEM images of polished and etched cross sections of specimens premature sealing of the entrance surface by deposited Si of the Hi-Nicalon multilayer interface minicomposites with total carbon Interrupting the infiltration after several hours and refixturing the thicknesses of (a)0.3 um(MC-80)and(b)0.5 Hm(MC- sample to allow for a small gap between the bottom(cooled side) of the graphite holder and the sample solved this problem. The result, however, is poorer reproducibility with regard to infiltratio time that caused greater MTS depletion before the deposition of SiC on An example of the alternating carbon and Sic interface laye the fiber tow seen in the TEM image of Fig. 5. There appears to be some small Representative tensile load-displacement curves for Hi-Nicalon variability in the layers, which are 0. 1 to 0.2 um in thickness, and fiber-reinforced minicomposites are plotted in Fig. 4. The maxi- the SiC layers become increasingly rougher away from the fiber, mum load of these materials was found to be -50% higher than apparently because of the fairly large grains of the deposited Si nose of the ceramic-grade Nicalon minicomposites, probably Each layer is conformal with respect adjacent layers. Imagesthat caused greater MTS depletion before the deposition of SiC on the fiber tow. Representative tensile load–displacement curves for Hi-Nicalon fiber-reinforced minicomposites are plotted in Fig. 4. The maxi￾mum load of these materials was found to be ;50% higher than those of the ceramic-grade Nicalon minicomposites, probably because of the better strength retention of Hi-Nicalon fibers during high-temperature matrix deposition.25,26 The curves show greater crosshead displacement at the peak load than those of minicom￾posites made with ceramic-grade Nicalon fibers but do not exhibit their gradual strength reduction after peak load. The load versus crosshead displacement curves are nonlinear after the matrix￾cracking load is exceeded, and the small discontinuities along the curves typically indicate the occurrence of matrix cracks. The peak loads and crosshead displacements were greatest for the specimens that had only a single carbon-layer interface (MC-114). (2) Composite Plates The results of infiltration and the properties of the composite plates are listed in Table III. The density of each sample was calculated from geometrical measurements and the mass of the sample with the preform containing 40 vol% fiber. The variation in infiltration time was, in part, due to difficulties in performing FCVI on such thin components. Experience with preforms less than 10 mm in thickness suggests that there is a tendency for premature sealing of the entrance surface by deposited SiC. Interrupting the infiltration after several hours and refixturing the sample to allow for a small gap between the bottom (cooled side) of the graphite holder and the sample solved this problem. The result, however, is poorer reproducibility with regard to infiltration time. An example of the alternating carbon and SiC interface layers is seen in the TEM image of Fig. 5. There appears to be some small variability in the layers, which are 0.1 to 0.2 mm in thickness, and the SiC layers become increasingly rougher away from the fiber, apparently because of the fairly large grains of the deposited SiC. Each layer is conformal with respect to adjacent layers. Images obtained from other areas of the specimen are similar, indicating relatively good through-thickness uniformity. Table II. Deposition Time, Interlayer Thicknesses, and Average Peak Load for Multilayer C/SiC Minicomposite Samples Sample Nicalon fiber type C/SiC deposition times (min/layer) Total interlayer thickness (mm) Total carbon layer thickness (mm) Average carbon layer thickness (mm) Average SiC layer thickness (mm) Tensile testing Peak load (N) Number of samples MC-22 Ceramic grade 120 C only 0.25 0.25 0.25 110 6 6 8 MC-31 Ceramic grade 15/10 0.5 ††† 90 6 37 2 MC-36 Ceramic grade 15/15 0.2 0.05 0.008 0.03 85 6 41 6 MC-32 Ceramic grade 15/20 1.3 0.3 0.05 0.2 83 6 15 6 MC-80 Hi-Nicalon 30/15 0.8 0.3 0.05 0.1 129 6 9 10 MC-85 Hi-Nicalon 15/30 1.4 0.4 0.067 0.2 122 6 15 10 MC-97 Hi-Nicalon 60/2 1.0 0.5 0.083 0.1 150 6 14 10 MC-114 Hi-Nicalon 120 C only 0.25 0.25 0.25 187 6 22 15 † Individual layers were not discernible. Fig. 3. SEM images of polished and etched cross sections of specimens of the Hi-Nicalon multilayer interface minicomposites with total carbon thicknesses of (a) 0.3 mm (MC-80) and (b) 0.5 mm (MC-97). Fig. 4. Representative tensile load–displacement curves for the Hi￾Nicalon minicomposites. The curves have been shifted for clarity. December 2000 Ceramic Composites with Multilayer Interface Coatings 3017