can Ceramic SocienBe Vol. 83. No. 12 oth load and crosshead displacement were recorded during the test at a constant rate of 3 Hz. Specimens were cut from each of the plates and were prepared following ASTM specification C 1275-95 for shoulder-loaded tensile specimens(the overall specimen length was 101 mm with 100-grit section of 32 mm x 6.5 mm X 3. 18 mm). 2Surfaces were in multiple overlapping passes with a 12.7-mm-diameter, diamond wheel mounted on a Harig Slicer/Grinder (Model 718, Bridgeport Corp, Birmingham, AL). All specimens were tested in tension at ambient conditions(20.C, 18% relative humidity) under a constant loading rate of 50 N/s in an electro- mechanical testing machine(Instron Model 1380, Canton, MA) equipped with a low-contact force-capacitance extensometer with 100-nm resolution. The shoulder-loaded specimens were gripped by a pair of in-house-developed grips connected to the load train through a pair of self-aligning swivels. Load was transferred from the grips to the specimens through the shoulders. Both load and strain were recorded during the test at a constant rate of 5 Hz. Oxidative exposures under zero load were conducted with the specimens resting on an alumina plate in a hot-wall, resistively Fig. 1. SEM heated box furnace at 950C in ambient air (exterior 20oC, 18% Nicalon specimen(MC-32)revealing the interface layer relative humidity ) The 950C exposure temperature was chosen to be consistent with similar exposures of materials in other ceramic composite programs. No measurements were made of possible temperature gradients within the furnace 250 (5) Microstructural Characterization 200 All the minicomposite samples were examined with a scanning electron microscope (SEM)(Hitachi S-800, NSA Hitachi Scien- tific Instruments, Mountain View, CA). SEM images of polished 21500.25 um ross sections were used for obtaining interface layer thicknesses, which were determined by visual measurement of the length between tangents applied to the boundaries in the images. Reso- lution of the measurements was estimated to be 5 nm Layers 0.3 um Some of the later samples that had thinner coatings were also etched with a NaoH-K, Fe(CN) solution to emphasize the bound- aries between layers. Fracture surfaces of samples were also examined to look for indications of fiber pullout and crack deflection. Transmission electron microscopy (TEM) was used to examine the structure of the interfacial coatings in greater detail 00511.522.53 (Hitachi HF2000, NSA Hitachi Scientific Instruments, Mountain Displacement(mm) Fig. 2. Representative tensile load-displacement curves for the ceramic- grade Nicalon minicomposites. The curves have been shifted for clarity Il. Results (I Minicomposites The interface-layer deposition times varied from 10 to 60 min to fiber strengths within a Hi-Nicalon process batch has been dentify the periods that would produce the desired interface-layer eported to be -22%, and therefore the differences fall within thickness. The initial efforts utilized ceramic-grade Nicalon(sam the error limit ple Nos. MC-22, 31, 32, and 36): an example of the deposited The crosshead displacement at peak load recorded during interlayer structure can be see in Fig. 1. The dark region between evaluation of the minicomposite specimens with multilayer inter- the fiber and the first interface layer is void space where the fiber facial coatings was consistently less than 200 um; those recorded has separated from the interface layer. Either the layer thicknesses for specimens with a single carbon-layer interface were generally for the sample with minimal deposition times, 15 min carbon and greater than 200 um. The shape of the load versus displacement 10 min SiC (MC-31), were too small to discern or there was curves is instructive with regard to the failure behavior of these insufficient material deposited to form coherent layers materials. The minicomposite that did not show any distinct layers Figure 2 contains representative tensile load-displacement MC-31)also did not exhibit strength retention after peak load curves for minicomposite specimens containing ceramic-grade Strength retention was observed only when both the interfacial Nicalon fibers. The actual tensile strengths of the minicomposite shear stress and the fiber-bond strength were low, resulting in large specimens were not determined because it was impractical to matrix crack spacing and long fiber-pullout lengths measure the cross-sectional area of the fracture plane. Neverthe The results from the evaluation of minicomposites with less, the peak loads listed in Table Il are useful measures of ceramic-grade Nicalon fibers guided the development of multilay strength. Furthermore, all specimens contain the same number of ered coatings on Hi-Nicalon fibers. For example, longer deposition fibers(500), and the strength of the minicomposite is dictated times were explored, particularly for carbon (Table ID). Figure 3 primarily by the distribution of fiber strengths and the character- contains micrographs of polished and etched cross sections of the of the fiber-matrix interface. Therefore, comparisons be- interfacial regions of two selected samples. The Hi-Nicalon sample peak loads indicate the effect of fibers and interfacial MC-85, with the longest SiC deposition time and ther gs on the composite tensile strength. The average peak load SiC layer, exhibited rougher layer surfaces than thos other for minicomposites with multilayered interfacial coating (Table l) samples because of its larger SiC grain size. The marked departure appears to be 18%25% lower than that for minicomposites with time versus thickness of sample MC-97 compared with a single layer of carbon(MC-22). However, the standard error fo samples was caused by a change in furnace configurationBoth load and crosshead displacement were recorded during the test at a constant rate of 3 Hz. Specimens were cut from each of the plates and were prepared following ASTM specification C 1275–95 for shoulder-loaded tensile specimens (the overall specimen length was 101 mm with a gauge section of 32 mm 3 6.5 mm 3 3.18 mm).23 Surfaces were ground in multiple overlapping passes with a 12.7-mm-diameter, 100-grit diamond wheel mounted on a Harig Slicer/Grinder (Model 718, Bridgeport Corp., Birmingham, AL). All specimens were tested in tension at ambient conditions (20°C, 18% relative humidity) under a constant loading rate of 50 N/s in an electro￾mechanical testing machine (Instron Model 1380, Canton, MA) equipped with a low-contact force-capacitance extensometer with 100-nm resolution. The shoulder-loaded specimens were gripped by a pair of in-house-developed grips connected to the load train through a pair of self-aligning swivels. Load was transferred from the grips to the specimens through the shoulders. Both load and strain were recorded during the test at a constant rate of 5 Hz. Oxidative exposures under zero load were conducted with the specimens resting on an alumina plate in a hot-wall, resistively heated box furnace at 950°C in ambient air (exterior 20°C, 18% relative humidity). The 950°C exposure temperature was chosen to be consistent with similar exposures of materials in other ceramic composite programs. No measurements were made of possible temperature gradients within the furnace. (5) Microstructural Characterization All the minicomposite samples were examined with a scanning electron microscope (SEM) (Hitachi S-800, NSA Hitachi Scien￾tific Instruments, Mountain View, CA). SEM images of polished cross sections were used for obtaining interface layer thicknesses, which were determined by visual measurement of the length between tangents applied to the boundaries in the images. Reso￾lution of the measurements was estimated to be ;5 nm. Some of the later samples that had thinner coatings were also etched with a NaOH–K3Fe(CN)6 solution to emphasize the bound￾aries between layers. Fracture surfaces of samples were also examined to look for indications of fiber pullout and crack deflection. Transmission electron microscopy (TEM) was used to examine the structure of the interfacial coatings in greater detail (Hitachi HF2000, NSA Hitachi Scientific Instruments, Mountain View, CA). III. Results (1) Minicomposites The interface-layer deposition times varied from 10 to 60 min to identify the periods that would produce the desired interface-layer thickness. The initial efforts utilized ceramic-grade Nicalon (sam￾ple Nos. MC-22, 31, 32, and 36); an example of the deposited interlayer structure can be see in Fig. 1. The dark region between the fiber and the first interface layer is void space where the fiber has separated from the interface layer. Either the layer thicknesses for the sample with minimal deposition times, 15 min carbon and 10 min SiC (MC-31), were too small to discern or there was insufficient material deposited to form coherent layers. Figure 2 contains representative tensile load–displacement curves for minicomposite specimens containing ceramic-grade Nicalon fibers. The actual tensile strengths of the minicomposite specimens were not determined because it was impractical to measure the cross-sectional area of the fracture plane. Neverthe￾less, the peak loads listed in Table II are useful measures of strength. Furthermore, all specimens contain the same number of fibers (500), and the strength of the minicomposite is dictated primarily by the distribution of fiber strengths and the character￾istics of the fiber–matrix interface. Therefore, comparisons be￾tween peak loads indicate the effect of fibers and interfacial coatings on the composite tensile strength. The average peak load for minicomposites with multilayered interfacial coating (Table II) appears to be 18%–25% lower than that for minicomposites with a single layer of carbon (MC-22). However, the standard error for fiber strengths within a Hi-Nicalon process batch has been reported24 to be ;22%, and therefore the differences fall within the error limit. The crosshead displacement at peak load recorded during evaluation of the minicomposite specimens with multilayer inter￾facial coatings was consistently less than 200 mm; those recorded for specimens with a single carbon-layer interface were generally greater than 200 mm. The shape of the load versus displacement curves is instructive with regard to the failure behavior of these materials. The minicomposite that did not show any distinct layers (MC-31) also did not exhibit strength retention after peak load. Strength retention was observed only when both the interfacial shear stress and the fiber-bond strength were low, resulting in large matrix crack spacing and long fiber-pullout lengths. The results from the evaluation of minicomposites with ceramic-grade Nicalon fibers guided the development of multilay￾ered coatings on Hi-Nicalon fibers. For example, longer deposition times were explored, particularly for carbon (Table II). Figure 3 contains micrographs of polished and etched cross sections of the interfacial regions of two selected samples. The Hi-Nicalon sample MC-85, with the longest SiC deposition time and therefore thickest SiC layer, exhibited rougher layer surfaces than those of the other samples because of its larger SiC grain size. The marked departure of coating time versus thickness of sample MC-97 compared with the other samples was caused by a change in furnace configuration Fig. 1. SEM image of a polished cross section of a ceramic-grade Nicalon specimen (MC-32) revealing the interface layers. Fig. 2. Representative tensile load–displacement curves for the ceramic￾grade Nicalon minicomposites. The curves have been shifted for clarity. 3016 Journal of the American Ceramic Society—Besmann et al. Vol. 83, No. 12