Investigation of bond strength and failure mode between SiC-coated mesophase ribbon fiber and an epoxy matrix Original Research Article

The use of mesophase pitch-based carbon fibers in composite materials has been limited by their poor oxidation resistance at high temperatures. Ribbon-shaped mesophase fibers with excellent axial thermal conductivity were spun at Clemson University and coated with SiC at the University of Erlangen-Nürnberg using chemical vapor deposition (CVD) in an effort to protect them from high-temperature oxidation. Using the microbond technique with a newly developed microfixture to apply an axisymmetric load to each specimen, the failure modes and interfacial shear stress (IFSS) were investigated for untreated ribbon fibers and ribbon fibers with 0.75-μm and 1.2-μm thick SiC coatings. For microbond testing, an Epon® 828-based epoxy was used as a model thermoset matrix material. The three distinct failure modes found for SiC-coated fibers included microdrop debonding, fiber breakage, and coating failure. The uncoated fibers exhibited the debonding failure mode for approximately 90% of the samples. However, the fibers with a 0.75-μm thick coating exhibited all three failure modes with uniform frequency, while the fibers with a 1.2-μm thick coating exhibited coating failure for 70% of the samples. The SiC coating was found to increase the IFSS by approximately 40%. However, the coating thickness had no effect on the increase of the IFSS.