January 1999 Radial variations in Modulus and Hardness in SCS-6 Silicon Carbide Fibers Carbon Core SiC Outer sheath 480 6g0 on Core SiC Outer Sheath H 600 30Q Fig. 8. Modulus and hardness as a function of indentation depth in Fig. 9. Modulus and hardness as a function of indentation depth in the outer siC sheath. V. Discussion the graphitic carbon layer which lies at the edge of the carbon The layer consists of small of textured turbostratic The modulus and hardness data in Figs 4 and 6 clearly show arbon. Note that the magnitude of the measured modulus and that the mechanical properties of scs-6 Sic fibers vary dra hardness are less than the values recorded for the carbon core matically with distance from their centers. Youngs modulus The texturing in the coating is believed to produce these lower increases from 28 GPa at the inner core to approximately 360 values of E and H compared to the carbon filament. It is most GPa in the outer sheath, and hardness increases from roughly 4 likely due to the(0002) planes buckling and sliding over each to 34 GPa over the same range. These 13x and 9x increases are other very significant and suggest that the fibers will behave as com It is interesting to note that due to the extreme anisotropy of ite materials. This result matches DiCarlo's predictions graphite the Reuss (isostress)modulus for polycrystalline and it also helps to explain Zywicz et al's XTM report that aphite is many times smaller than the Voigt(isostrain )modu- SCS fibers fail first in their cores prior to their SiC sheaths lus(e.g, Ref. 19). Since the Reuss value is closest to the t is also worth noting that the graphitic layer between the core measured moduli of both the carbon core and the graphitic core and the Sic has a lower indentation modulus and hardness than coating, the indentation process must test the carbon under any other part of the fiber, and hence, it must be regarded as a conditions of isostress. In real applications of the fibers, the potential site of failure. The mechanical properties of each of loading is unlikely to resemble conditions of isostrain, and ons will now be considered in relation to their chem- hence, indentation testing may not be the most effective way of istry and microstructure examining the modulus of the carbon core and the graphiti The inner cores of the fibers consist of carbon filaments that layer uniform grain sizes and are expected to be isotropic and com- s the next region within the SCS fibers is the inner Sic measure 16.5 um in radii. These carbon filaments have small pliant. Note that the data points that fall within the inner core caath. This region consists of B-SiC crystals imbedded in a ig. 4 give consistent values of 25-30 GPa for Youngs nificantly with radius&, 10 The section closest to the carbon core modulus and 4.0-4.4 GPa for hardness. These results imply is strongly carbon-rich while the section bordering the outer that the carbon core is very uniform in its elastic and plastic sheath has a nearly equal mix of C and Si atoms as shown in properties. The magnitude of the measured modulus is lowe Fig. 2. The data of Fig. 4 show that the modulus and hardness an, but similar to, at least three other reports in the literature. increase as the indentations move away from the core. Most of DiCarlo reported an approximate value of 41 GPa for E of the the increase is confined to the first few micrometers of the core, Eldridge et al. cited an estimated value of 35 GPa for E, inner SiC sheath where the chemistry and structure of the Sic while most recently Sathish et al. 15 reported an experiment change very sharply. As stated earlier, close to the carbon core ralue of 40 GPa. The difference between our value and those the indentation data fall into two types, the first with a high of other researchers probably results from the unique geometry modulus(=300 GPa)and hardness(=33 GPa) and the second of the indentation test. As stated previously, the Ye ith an intermediate modulus(=175 GPa) and hardness(=22 otained is very he Reuss, isostress, GPa). Sathish et al. have reported a region in the SiC, close of 25.5 GPa to the carbon core. where the modulus has an intermediate Before moving on to the inner sheath, it is usefi onsider value of 141-215 GPa. Further from the core. but still withinV. Discussion The modulus and hardness data in Figs. 4 and 6 clearly show that the mechanical properties of SCS-6 SiC fibers vary dra￾matically with distance from their centers. Young’s modulus increases from 28 GPa at the inner core to approximately 360 GPa in the outer sheath, and hardness increases from roughly 4 to 34 GPa over the same range. These 13× and 9× increases are very significant and suggest that the fibers will behave as com￾posite materials. This result matches DiCarlo’s predictions,3,4 and it also helps to explain Zywicz et al.’s XTM report that SCS fibers fail first in their cores prior to their SiC sheaths.11 It is also worth noting that the graphitic layer between the core and the SiC has a lower indentation modulus and hardness than any other part of the fiber, and hence, it must be regarded as a potential site of failure. The mechanical properties of each of the regions will now be considered in relation to their chem￾istry and microstructure. The inner cores of the fibers consist of carbon filaments that measure 16.5 mm in radii. These carbon filaments have small, uniform grain sizes and are expected to be isotropic and com￾pliant. Note that the data points that fall within the inner core in Fig. 4 give consistent values of 25–30 GPa for Young’s modulus and 4.0–4.4 GPa for hardness. These results imply that the carbon core is very uniform in its elastic and plastic properties. The magnitude of the measured modulus is lower than, but similar to, at least three other reports in the literature. DiCarlo reported an approximate value of 41 GPa for E of the core, Eldridge et al. cited an estimated value of 35 GPa for E, while most recently Sathish et al.15 reported an experimental value of 40 GPa. The difference between our value and those of other researchers probably results from the unique geometry of the indentation test. As stated previously, the Young’s modulus that we obtained is very close to the Reuss, isostress, average of 25.5 GPa. Before moving on to the inner sheath, it is useful to consider the graphitic carbon layer which lies at the edge of the carbon core. The layer consists of small grains of textured, turbostratic carbon. Note that the magnitude of the measured modulus and hardness are less than the values recorded for the carbon core. The texturing in the coating is believed to produce these lower values of E and H compared to the carbon filament. It is most likely due to the (0002) planes buckling and sliding over each other. It is interesting to note that due to the extreme anisotropy of graphite the Reuss (isostress) modulus for polycrystalline graphite is many times smaller than the Voigt (isostrain) modu￾lus (e.g., Ref. 19). Since the Reuss value is closest to the measured moduli of both the carbon core and the graphitic core coating, the indentation process must test the carbon under conditions of isostress. In real applications of the fibers, the loading is unlikely to resemble conditions of isostrain, and hence, indentation testing may not be the most effective way of examining the modulus of the carbon core and the graphitic layer. The next region within the SCS fibers is the inner SiC sheath. This region consists of b-SiC crystals imbedded in a carbon matrix and its chemistry and microstructure vary sig￾nificantly with radius.8,10 The section closest to the carbon core is strongly carbon-rich while the section bordering the outer sheath has a nearly equal mix of C and Si atoms as shown in Fig. 2. The data of Fig. 4 show that the modulus and hardness increase as the indentations move away from the core. Most of the increase is confined to the first few micrometers of the inner SiC sheath where the chemistry and structure of the SiC change very sharply. As stated earlier, close to the carbon core the indentation data fall into two types, the first with a high modulus (≈300 GPa) and hardness (≈33 GPa) and the second with an intermediate modulus (≈175 GPa) and hardness (≈22 GPa). Sathish et al.15 have reported a region in the SiC, close to the carbon core, where the modulus has an intermediate value of 141–215 GPa. Further from the core, but still within Fig. 8. Modulus and hardness as a function of indentation depth in the carbon core. Fig. 9. Modulus and hardness as a function of indentation depth in the outer SiC sheath. January 1999 Radial Variations in Modulus and Hardness in SCS-6 Silicon Carbide Fibers 115