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

The use of mesophase pitch-based carbon fibers in composite materials has b een limited by their poor oxidation resistance at high temperatures. Ribbon -shaped mesophase fibers with excellent axial thermal conductivity were spu n at Clemson University and coated with SIC at the University of Erlangen-N urnberg 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 un treated ribbon fibers and ribbon fibers with 0.75-mu m and 1.2-mu m thick S iC coatings. For microbond resting, an Epon(R) 828-based epoxy was used as a model thermoset matrix material. The three distinct failure modes found f or SiC-coated fibers included microdrop debonding, fiber breakage, and coat ing failure. The uncoated fibers exhibited the debonding failure mode for a pproximately 90% of the samples. However, the fibers with a 0.75-mu m thick coating exhibited all three failure modes with uniform frequency, while th e fibers with a 1.2-mu m thick coating exhibited coating failure for 70% of the samples. The SIC coating was found to increase the IFSS by approximate ly 40%. However, the coating thickness had no effect on the increase of the IFSS. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.