116 Journal of the American Ceramic Society-Mann et al. Vol. 82. No he inner SiC sheath, Sathish et al. report modulus values Youngs modulus of 28 GPa and a hardness of 4.2 GPa. The m a range of 220-310 GPa to a range of modulus and hardness appeared to be uniform in this core,as 315-405 GPa. Thus, our values for Youngs modulus across expected. The graphitic coating on the core appeared to be the SiC sheath(shown by Fig 4)are in good agreement with weaker, though, with a modulus of 21 GPa and a hardness of recent results of sathish et al. Is 1.7 GPa. The inner sheath, which contained a varying chem- an earlier paper Sathish et al. 4 used Auger spectroscopy stry, showed a sharp increase in stiffness and hardness from from Ning and Pirouz(Fig. 2) to predict variations of he inner core. The modulus and hardness increased radially bulk and shear moduli across the inner and outer sheaths of within this region to 360 and 34 GPa, respectively. Most of this SCS SiC fibers. They predicted that both moduli increase rap increase was confined to the first few micrometers of the inner idly across the inner sheath as reported here. However, the SiC sheath, and thereafter there were no signs of local minima so predicted a large, local minima in the inner SiC sheath or decreases, just a gradual increase to the maximum values This is something which they did not see in their subsequent observed. The outer sheath, which contained a relatively uni- xamination of the fibers 15 The results of these indentation form chemistry, was consistently stiff and hard. The average periments suppo arp nse in modulus and hardness were measured to be 360 and 34 GP like Sathish et al.'s later results they show no sign of a local which are similar to previous measurements or estimates. The minima. To help verify this finding, large areas of a fiber were average modulus for the full fiber was 333 GPa. The measured canned using an atomic force microscope(AFM) and an ac values of modulus and hardness clearly showed that the me technique that yields a measure of material stiffness 20, 21 No chanical properties of ScS-6 fibers changed by an order abrupt changes or local minima in stiffness were observed in magnitude across their diameters. Such dramatic variations he region of the inner sheath. Thus, all of the results reported should be considered in modeling the behavior of these here suggest that the variation in mechanical rtes across materials the inner sheath is continuous everywhere, except close to the carbon core. with no identifiable local minima Acknowledgments: We gratefully acknowledge Textron Speciality The last region that was investigated within the SCS fibers Materials for supplying the fibers, Professor W. D. Nix, Jerry English and Bob he outer sheath. This section consists of strongly textured SiC Kershaw for help during the initial testing grains and is relatively uniform in chemistry. The ratio of C to egion, as shown in Fig References 2. Young's modulus shows some scatter but is basically con- TExtron Specialty Materials, Textron CO., Lowell, MA. tant at an average value of 360 GPa. The distinct plateau S R Nutt and F E wawner. Silicon Carbide Filaments: Microstructure dulus in Fig. 4 is attributed to the uniform chemistry and J.Mer.Sc.,20,1953-60(1985) microstructure of the region. Hardness also shows a plateau J. A. DiCarlo and w. williams, ""Dynamic Modulus and Damping of Boron, Silicon Carbide, and Alumina Fibers, " Ceram. Eng. Sct. Proc., 1, 671-92 with an average value of 34 GPa. The magnitude of youngs modulus, 360 GPa, is a little lower than expected for SiC. For of CVD Silicon Carbide Fi- single crystals of B-SiC the modulus varies from 379 GPa Ceramics. Edited by R.A. the [100] crystallographic direction to 547 GPa in the [1ll Bradley. ASM International direction. 2 A Voigt(isostrain)average for the modulus, based a SCS-6 Fiber/Ti-6Al on single-crystal elastic constants, is 459 GPa, while the Reuss (isostress)average is 414 GPa. Thus, all of the expected values bJ. Chung: M.Sc. Thesis. Case Western Reserve University, Cleveland, OH, X J. Ning, P. Pirouz, K. P. D. Lagerlof, and J. DiCarlo, " The Structure of may be due to nonideal stoichiometry throughout the outer Car-76(1990). aly Vapor Deposited SiC Monofilaments, "JMater Res, 5, Young,s modulus averaged over the full cross-sectional area X J Ning and P. Pirouz, "The Microstructure of ScS-6 SiC Fibers, "J. of the fiber was calculated to be 333 GPa. which is close to the Mater.Res,6,2234-48(1991) T. Bhatt and D. R Hull, ""Microstructural and Strength Stability of CVD average moduli measured by Sathish et al 5 of 335-365 GP SiC Fibers in Argon Environment, Ceram. Eng. Sci Proc 12, 1832-44 but lower than the other reported values of 400 GPa and 390 (1g91) GPa. The disparity between the current measurements of M. L. Sattler, J. H, Kinney, E. Zywicz, R. Alani, and M, C. Nichols, "The modulus and earlier reports may stem from the difficulty of Microstructure of SCS-6 and SCS-8 SiC Reinforcing Fibers, ""Ceram. Eng. Sci Proc,l3,227-37( accurately determining contact areas and therefore propertie IE. Zywicz, J. H. Kinney, M. L. Sattler, T. M. Breunig, and M. C. Nichol the absolute magnitudes of modulus and hardness in Fig. 4 crast, 169, 247-53(1993)structures and Their Influence on Failure,"JMi- during indentations with significant elastic recovery. However, show a clear trend which includes dramatic variations in me 12X. J. Ning, P Pirouz, and S. C. Farmer, " Microchemical Analysis of the SCS-6 Silicon Carbide Fiber, " J. Am. Ceram Soc., 76, 2033-41(1993). chanical properties across the SCS SiC fibers Within the core SJ. I. Eldridge, J. P. Wining, T S. Davison, and M.-J. Pindera, the modulus and hardness are consistently low, but then drop in Strength of SCs-6 Silicon Carbide Fibers,"J.Am. Ceram Soc., 76, 3151-54 the graphitic core coating before rising by an order of magni-(943). tude within the first few micrometers of the sic sheath. this 4S, Sathish, J H. Cantrell, and W. T. Yost, "Radial Variations of Elastic sharp increase is attributable to the dramatic change in chem- Sathish, J H. Cantrell, and w. T. Yost, ""Scanning Acoustic Microscopy istry and structure in the SiC sheath close to the core. Across of SCS-6 Silicon Carbide Fiber, 'J. Am. Ceram Soc., 79, 209-212(1996) the bulk of the SiC sheath, where the chemistry and structure ary slowly, the modulus and hardness are smooth and con- w. C. Oliver and G M. Pha, "An Improved Technique for Determining tinuous and largely mimic the trend in fiber chemistry tion Experiments, J. Mater. Res, 7, 1564(1992 O. L. Blakslee, D. G. Proctor, E J. Seldin, G. B. S andT. w Elastic Constants of Compression-Annealed Pyrolytic Graphite, J. App. VI Conclusions Phs,4l,3373-82(1970 J. R. Dryden and G. R. Purdy, " The Effect of Graphite on the Mechanical The mechanical properties of SCS-6 Sic fibers were mea- Proper msaovald. H.J. Butt, S.A.C. Gould, C.B. Prater, and B. Drake, J.A. niques. Polished cross sections of fibers Berkovich diamond tip to depths between 50 and 450 nm. The Surface Elasticities with the Atomic Force Microscope, Nanotechnology, 2, It data from each indentation were analyze all. G. W. Marshall. Jr. and T P to determine youngs modulus and the hardness of the material urements of the Hardness and Elas. all of the major regions of these fibers: the carbon core, the an Dentin, J. Biomech. Eng, 118, graphitic core coating, the inner sheath, and the outer sheath The carbon core of the fibers was extremely compliant with a and Technology, Lit. Sthe inner SiC sheath, Sathish et al. report modulus values which increase from a range of 220–310 GPa to a range of 315–405 GPa. Thus, our values for Young’s modulus across the SiC sheath (shown by Fig. 4) are in good agreement with the recent results of Sathish et al.15 In an earlier paper Sathish et al.14 used Auger spectroscopy data from Ning and Pirouz8 (Fig. 2) to predict variations of bulk and shear moduli across the inner and outer sheaths of SCS SiC fibers. They predicted that both moduli increase rap￾idly across the inner sheath as reported here. However, they also predicted a large, local minima in the inner SiC sheath. This is something which they did not see in their subsequent examination of the fibers.15 The results of these indentation experiments support the presence of a sharp rise in moduli, but like Sathish et al.’s later results they show no sign of a local minima. To help verify this finding, large areas of a fiber were scanned using an atomic force microscope (AFM) and an AC technique that yields a measure of material stiffness.20,21 No abrupt changes or local minima in stiffness were observed in the region of the inner sheath. Thus, all of the results reported here suggest that the variation in mechanical properties across the inner sheath is continuous everywhere, except close to the carbon core, with no identifiable local minima. The last region that was investigated within the SCS fibers is the outer sheath. This section consists of strongly textured SiC grains and is relatively uniform in chemistry. The ratio of C to Si atoms is close to 1.0 throughout the region, as shown in Fig. 2. Young’s modulus shows some scatter but is basically con￾stant at an average value of 360 GPa. The distinct plateau in modulus in Fig. 4 is attributed to the uniform chemistry and microstructure of the region. Hardness also shows a plateau with an average value of 34 GPa. The magnitude of Young’s modulus, 360 GPa, is a little lower than expected for SiC. For single crystals of b-SiC the modulus varies from 379 GPa in the [100] crystallographic direction to 547 GPa in the [111] direction.22 A Voigt (isostrain) average for the modulus, based on single-crystal elastic constants, is 459 GPa, while the Reuss (isostress) average is 414 GPa. Thus, all of the expected values are slightly higher than the measured modulus. This difference may be due to nonideal stoichiometry throughout the outer sheath. Young’s modulus averaged over the full cross-sectional area of the fiber was calculated to be 333 GPa, which is close to the average moduli measured by Sathish et al.15 of 335–365 GPa, but lower than the other reported values of 400 GPa1 and 390 GPa.3 The disparity between the current measurements of modulus and earlier reports may stem from the difficulty of accurately determining contact areas, and therefore properties, during indentations with significant elastic recovery. However, the absolute magnitudes of modulus and hardness in Fig. 4 show a clear trend which includes dramatic variations in me￾chanical properties across the SCS SiC fibers. Within the core the modulus and hardness are consistently low, but then drop in the graphitic core coating before rising by an order of magni￾tude within the first few micrometers of the SiC sheath. This sharp increase is attributable to the dramatic change in chem￾istry and structure in the SiC sheath close to the core. Across the bulk of the SiC sheath, where the chemistry and structure vary slowly, the modulus and hardness are smooth and con￾tinuous and largely mimic the trend in fiber chemistry. VI. Conclusions The mechanical properties of SCS-6 SiC fibers were mea￾sured as a function of fiber radius using nanoindentation tech￾niques. Polished cross sections of fibers were indented with a Berkovich diamond tip to depths between 50 and 450 nm. The force–displacement data from each indentation were analyzed to determine Young’s modulus and the hardness of the material in all of the major regions of these fibers: the carbon core, the graphitic core coating, the inner sheath, and the outer sheath. The carbon core of the fibers was extremely compliant with a Young’s modulus of 28 GPa and a hardness of 4.2 GPa. The modulus and hardness appeared to be uniform in this core, as expected. The graphitic coating on the core appeared to be weaker, though, with a modulus of 21 GPa and a hardness of 1.7 GPa. The inner sheath, which contained a varying chem￾istry, showed a sharp increase in stiffness and hardness from the inner core. The modulus and hardness increased radially within this region to 360 and 34 GPa, respectively. Most of this increase was confined to the first few micrometers of the inner SiC sheath, and thereafter there were no signs of local minima or decreases, just a gradual increase to the maximum values observed. The outer sheath, which contained a relatively uni￾form chemistry, was consistently stiff and hard. The average modulus and hardness were measured to be 360 and 34 GPa, which are similar to previous measurements or estimates. The average modulus for the full fiber was 333 GPa. The measured values of modulus and hardness clearly showed that the me￾chanical properties of SCS-6 fibers changed by an order of magnitude across their diameters. Such dramatic variations should be considered in modeling the behavior of these materials. Acknowledgments: We gratefully acknowledge Textron Speciality Materials for supplying the fibers, Professor W. D. Nix, Jerry English and Bob Kershaw for help during the initial testing. References 1 Textron Specialty Materials, Textron CO., Lowell, MA. 2 S. R. Nutt and F. E. Wawner, ‘‘Silicon Carbide Filaments: Microstructure,’’ J. Mater. Sci., 20, 1953–60 (1985). 3 J. A. DiCarlo and W. Williams, ‘‘Dynamic Modulus and Damping of Boron, Silicon Carbide, and Alumina Fibers,’’ Ceram. Eng. Sci. Proc., 1, 671–92 (1980). 4 J. A. DiCarlo, ‘‘High Temperature Properties of CVD Silicon Carbide Fi￾bers’’; pp. 1–8 in Whisker and Fiber Toughened Ceramics. Edited by R. A. Bradley. ASM International, Metal Park, 1988. 5 M. Lancin, J. Thibault-Desseauc, and J. S. Bour, ‘‘Structure of the Interface in a SCS-6 Fiber/Ti-6A1-4V Composite,’’ J. Microsc. Spectrosc. Electron., 13, 503 (1988). 6 J. Chung; M.Sc. Thesis. Case Western Reserve University, Cleveland, OH, 1990. 7 X. J. Ning, P. Pirouz, K. P. D. Lagerlof, and J. DiCarlo, ‘‘The Structure of Carbon in Chemically Vapor Deposited SiC Monofilaments,’’ J. Mater. Res., 5, 2865–76 (1990). 8 X. J. Ning and P. Pirouz, ‘‘The Microstructure of SCS-6 SiC Fibers,’’ J. Mater. Res., 6, 2234–48 (1991). 9 R. T. Bhatt and D. R. Hull, ‘‘Microstructural and Strength Stability of CVD SiC Fibers in Argon Environment,’’ Ceram. Eng. Sci. Proc., 12, 1832–44 (1991). 10M. L. Sattler, J. H. Kinney, E. Zywicz, R. Alani, and M. C. Nichols, ‘‘The Microstructure of SCS-6 and SCS-8 SiC Reinforcing Fibers,’’ Ceram. Eng. Sci. Proc., 13, 227–37 (1992). 11E. Zywicz, J. H. Kinney, M. L. Sattler, T. M. Breunig, and M. C. Nichols, ‘‘Heterogeneous Fibre Microstructures and Their Influence on Failure,’’ J. Mi￾crosc., 169, 247–53 (1993). 12X. J. Ning, P. Pirouz, and S. C. Farmer, ‘‘Microchemical Analysis of the SCS-6 Silicon Carbide Fiber,’’ J. Am. Ceram. Soc., 76, 2033–41 (1993). 13J. I. Eldridge, J. P. Wiening, T. S. Davison, and M.-J. Pindera, ‘‘Transverse Strength of SCS-6 Silicon Carbide Fibers,’’ J. Am. Ceram. Soc., 76, 3151–54 (1993). 14S. Sathish, J. H. Cantrell, and W. T. Yost, ‘‘Radial Variations of Elastic Properties of SCS-6 Silicon Carbide Fiber,’’ J. Mater. Res., 9, 2298–303 (1994). 15S. Sathish, J. H. Cantrell, and W. T. Yost, ‘‘Scanning Acoustic Microscopy of SCS-6 Silicon Carbide Fiber,’’ J. Am. Ceram. Soc., 79, 209–212 (1996). 16Nano Instruments, Inc., Knoxville, TN 37914. 17W. C. Oliver and G. M. Pharr, ‘‘An Improved Technique for Determining Hardness and Elastic Modulus Using Load and Displacement Sensing Indenta￾tion Experiments,’’ J. Mater. Res., 7, 1564 (1992). 18O. L. Blakslee, D. G. Proctor, E. J. Seldin, G. B. Spence, and T. Weng, ‘‘Elastic Constants of Compression-Annealed Pyrolytic Graphite,’’ J. Appl. Phys., 41, 3373–82 (1970). 19J. R. Dryden and G. R. Purdy, ‘‘The Effect of Graphite on the Mechanical Properties of Cast Irons,’’ Acta Metall., 31, 1999–2006 (1989). 20P. Maivald, H. J. Butt, S. A. C. Gould, C. B. Prater, and B. Drake, J. A. Gurley, V. B. Elings, and P. K. Hansma, ‘‘Using Force Modulation to Image Surface Elasticities with the Atomic Force Microscope,’’ Nanotechnology, 2, 606–17 (1991). 21J. H. Kinney, M. Balooch, S. J. Marshall, G. W. Marshall, Jr., and T. P. Weihs, ‘‘Atomic Force Microscope Measurements of the Hardness and Elas￾ticity of Peritubular and Intertubular Human Dentin,’’ J. Biomech. Eng., 118, 133 (1996). 22Landolt-Borstein, Numerical Data and Functional Relationships in Science and Technology, Lit. S. 36. Springer-Verlag, New York, 1969. h 116 Journal of the American Ceramic Society—Mann et al. Vol. 82, No. 1