f the American Ceramic Sociery-l VoL. 83. No. 12 Table Ill. Specifications and Properties for the Fabricated Composite Tensile Samples Tensile specimen average Tensi Sample no Cv-1172 1225 2.41±0.036 0.833±0.015 69±27 4±1 CvI-1174 2.49±0.040 0.835±0.016 254±19 66+9 CVI-1175 Hi-Nicalon 28.5 2.61±0.018 0.872±0.007 260±24 300 200 50 ramic Grade Hi-Nicalon/Carbon Hi-Nicalon/Multilayer on/ Multilayer Interface Interface Carbon Nicalon Fibe 7. Comparison of average ultimate strength measurements of bricated and oxidized tensile bars obtained from the composite pla Fig. 5. TEM image of a specimen of the composite ceramicgrade Nicalon with a multilayer interface(CVI ealing for 100 h and then were fast-fractured at room temperature. The the alternating carbon and SiC layers. specimens had ground, exposed surfaces with no seal coat. The epresentative tensile curves for the oxidized specimens(Fig. 6) In monotonic tensile testing. the cessed specimens eveal little strain tolerance. As can be seen from the ultimate trength values(Table Ill and Fig. 7), less than one-third of the surfaces with uncorrelated behavior (i.e, fabric layers failed at as-fabricated strength was retained after exposure different locations ). Representative tensile curves are seen in Fig Specimens fabricated with Hi-Nicalon fibers were stronger than IV. Discussion those fabricated with ceramic-grade Nicalon, a condition that is haracteristic of the better strength retention of Hi-Nicalon, as was In the specimens of Naslain and co-workers, ",and in those also seen in the minicomposites(Table Ill and Fig. 7). In addition, examined here, cracks are ultimately directed between the fiber the higher overall density of the Hi-Nicalon samples likely and the first carbon layer when the fibers are not treated to increas contributes to the reduced standard deviation in the data(Table adhesion between the carbon laver and the fibers. Even in these materials. however. some crack deflection can be observed within In a test to determine resistance to oxidation, approximately half the multilayers, as seen in Fig 8, which shows a fiber and interface of the specimens of each of the samples were held at 950C in air 300 NIcalon/ Crack deflection within ade nicalon/ 100 Oxid bed Oxidized 0.0020.40.6081.0121.4 Fig. 8. SEM image of a polished cross section of a ceramic-grade Fig. 6. Representative stress-strain curves for as-fabricated and oxidized Nicalon multilayer interface minicomposite(MC-32)revealing some crack specimens obtained from composite plates. The curves have been shifted deflection within the interface and extension of cracks along the interface for clarity between the first carbon layer and the fiberIn monotonic tensile testing, the as-processed specimens showed nonlinear stress–strain behavior and very fibrous fracture surfaces with uncorrelated behavior (i.e., fabric layers failed at different locations). Representative tensile curves are seen in Fig. 6. Specimens fabricated with Hi-Nicalon fibers were stronger than those fabricated with ceramic-grade Nicalon, a condition that is characteristic of the better strength retention of Hi-Nicalon, as was also seen in the minicomposites (Table III and Fig. 7). In addition, the higher overall density of the Hi-Nicalon samples likely contributes to the reduced standard deviation in the data (Table III). In a test to determine resistance to oxidation, approximately half of the specimens of each of the samples were held at 950°C in air for 100 h and then were fast-fractured at room temperature. The specimens had ground, exposed surfaces with no seal coat. The representative tensile curves for the oxidized specimens (Fig. 6) reveal little strain tolerance. As can be seen from the ultimate strength values (Table III and Fig. 7), less than one-third of the as-fabricated strength was retained after exposure. IV. Discussion In the specimens of Naslain and co-workers,4,7 and in those examined here, cracks are ultimately directed between the fiber and the first carbon layer when the fibers are not treated to increase adhesion between the carbon layer and the fibers. Even in these materials, however, some crack deflection can be observed within the multilayers, as seen in Fig. 8, which shows a fiber and interface Table III. Specifications and Properties for the Fabricated Composite Tensile Samples Sample no. Interface Nicalon grade Infiltration time (h) Tensile specimen density g/cm3 Tensile specimen fractional density Unoxidized average strength (MPa) Oxidized average strength (MPa) CVI-1172 C/SiC Ceramic 12.25 2.41 6 0.036 0.833 6 0.015 169 6 27 54 6 18 CVI-1174 Carbon Hi-Nicalon 17 2.49 6 0.040 0.835 6 0.016 254 6 19 66 6 9 CVI-1175 C/SiC Hi-Nicalon 28.5 2.61 6 0.018 0.872 6 0.007 260 6 24 80 6 5 Fig. 5. TEM image of a specimen of the composite plate prepared from ceramic-grade Nicalon with a multilayer interface (CVI-1172) revealing the alternating carbon and SiC layers. Fig. 6. Representative stress–strain curves for as-fabricated and oxidized specimens obtained from composite plates. The curves have been shifted for clarity. Fig. 7. Comparison of average ultimate strength measurements of the as-fabricated and oxidized tensile bars obtained from the composite plates. Fig. 8. SEM image of a polished cross section of a ceramic-grade Nicalon multilayer interface minicomposite (MC-32) revealing some crack deflection within the interface and extension of cracks along the interface between the first carbon layer and the fiber. 3018 Journal of the American Ceramic Society—Besmann et al. Vol. 83, No. 12