Microstructure and role of outermost coating for tensile strength of SiC fiber

Abstract

The detailed microstructure of the SiC fiber surface and the outermost coating of SiC(SCS-6) fiber are observed using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The tensile strengths of the SiC fibers: uncoated fiber (SCS-0), coated fiber (SCS-6) and extracted fiber from fatigue-loaded SiC(SCS-6) fiber-reinforced Ti-15-3 composite are determined. Fractographic analysis is done on these fibers and the mirror radius is compared with the tensile strength. Thickness of the outermost coating is ≈3.6 μm and it consists of three different layers (i.e. sublayers I, II and III). Basically, these sublayers consist of a carbon matrix in which β-SiC crystallites are dispersed. The fracture toughness of the SiC fiber is ≈3.3 MPa m 1/2. The outermost coating increases the fiber strength twofold because it reduces stress concentration at the surface of the SiC fiber. The tensile strength of the extracted fiber (SCS-6) from fatigue-loaded specimens shows a reduced strength which is attributed to the debonding of the outermost coating while the composite is loaded.