Microstructural and mechanical characterization of carbon coatings on SiC fibers

A series of carbon coatings was deposited on a 1040 SiC monofilament using chemical vapor deposition, and failure of the fiber-matrix interfacial regi on under transverse tension was studied. Deposition substrate temperatures were approximately 920. 1000, and 1080 degreesC, and all other deposition p arameters were held constant. The microstructures of these carbon-coated fi bers were examined using optical microscopy, scanning electron microscopy. and transmission electron microscopy (TEM). TEM observations were made usin g bright-field imaging, dark-field imaging, selected-area diffraction. and hi,,h-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. A dhesion experiments were used to examine differences in bond strength of th e SiC-C interfaces of the three coatings. A systematic increase in the grai n size of the SiC substrate fiber within 3 mum of the SiC-C interface with increasing deposition temperature was observed. The crystallographic textur ing of the basic structural units of carbon within the coatings was also fo und to increase with increasing deposition temperature. The SiC-C interface strength increased with increasing deposition temperature and correlates w ith 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 int erface. 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.