Microstructural investigations in C-f-SiC composites in as-sintered state and after creep experiments

The high interest in ceramic matrix composites during the last decade has l ed to a considerable number of studies devoted to their thermomechanical pr operties and damage processes. Despite their sensitivity to oxygen partial pressure, carbon fibres appear to possess higher stability and better mecha nical properties if they are treated under protective atmospheres than othe r ceramic fibres (especially classical silicon carbide fibres), The aim of this investigation is to characterize at the nanoscale the main microstruct ural parameters of C-f-SiC composites provided by the SEP (Division of SNEC MA. Bordeaux, France). This material was fabricated from a 2.5D preform mad e of high strength polyacrylonitrile (PAN)-based carbon fibres densified ac cording to the chemical vapour infiltration process. A pyrocarbon (PyC) int erphase was deposited on the fibre prior to the beta-SiC matrix infiltratio n. A careful high resolution electron microscopy (HREM) microstructural inv estigation focused on the fibre microstructure as well as on the different interfaces in the material: pyrocarbon/fibre and matrix/pyrocarbon interfac es. All these observations have been realized in longitudinal and transvers e sections of the specimen, These observations are found in good agreement with Guigon's model for high strength ex-PAN carbon fibres. The PyC interph ase texture was strongly anisotropic at the fibre/interphase and interphase /matrix interfaces over a mean thickness of 8-15 nm, Tensile creep tests we re performed under partial pressure of argon between 1273 and 1673 K for st ress levels ranging from 110 to 220 MPa, Scanning electron microscopy and h igh resolution electron microscopy were used to study the microstructural m odifications inside the fibres and at the different interfaces. A discussio n of the possible creep mechanisms based on the microstructural investigati on and the creep results is presented.