Microstructural and mechanical characterization of carbon coatings on SiC fibers

Abstract

A series of carbon coatings was deposited on a 1040 SiC monofilament using chemical vapor deposition, and failure of the fiber-matrix interfacial region under transverse tension was studied. Deposition substrate temperatures were approximately 920, 1000, and 1080 °C, and all other deposition parameters were held constant. The microstructures of these carbon-coated fibers were examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). TEM observations were made using bright-field imaging, dark-field imaging, selected-area diffraction, and high-resolution lattice imaging. Tensile testing of single-fiber composite samples was performed transverse to the fiber axis to determine the stress required to cause debonding of the fiber from the titanium alloy matrix. Adhesion experiments were used to examine differences in bond strength of the SiC–C interfaces of the three coatings. A systematic increase in the grain size of the SiC substrate fiber within 3 μm of the SiC–C interface with increasing deposition temperature was observed. The crystallographic texturing of the basic structural units of carbon within the coatings was also found to increase with increasing deposition temperature. The SiC–C interface strength increased with increasing deposition temperature and correlates with the microstructural changes in both the SiC and carbon at the interface. The overall composite transverse strength was not affected by the change in deposition temperature, although the fracture location was affected. The carbon coating with the lowest SiC–C interface strength failed at this interface, and the coatings with more highly textured carbon failed within the coating, where the proportion of weak van der Waals bonds parallel to the tensile direction was correspondingly higher.