Radial variations in modulus and hardness in SCS-6 silicon carbide fibers

The mechanical properties of SCS-6 SiC fibers were measured as a function o f fiber radius using nanoindentation techniques. Hardness and Young's modul us were characterized for the material in all of the major regions of these fibers: the carbon core, the graphitic core coating, the inner SiC sheath, and the outer SiC sheath. The carbon core of the fibers was determined to be uniform in properties but extremely compliant, Young's modulus of 28 GPa and a hardness of 4.2 GPa were measured. The graphitic core coating was fo und to exhibit considerable anelasticity and to have both a low modulus (21 GPa) and a low hardness (1.7 GPa), The inner sheath of the fiber, which co ntained a varying chemistry, showed a sharp increase in stiffness and hardn ess from the inner core. Modulus and hardness increased by an order of magn itude over just 1 or 2 mu m when transversing radially away from the core i nto the SiC. This change in properties was pronounced and clearly defined. The outer sheath, which contained a uniform chemistry and microstructure, n as consistently stiff and hard when transversing radially. The average modu lus and hardness for the full fiber was 333 GPa, The values reported for Yo ung's modulus and hardness clearly showed that the mechanical properties of SCS SiC fibers exhibit dramatic changes across their diameters.