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

The high interest in ceramic matrix composites during the last decade has led to a considerable number of studies devoted to their thermomechanical properties and damage processes. Despite their sensitivity to oxygen partial pressure, carbon fibres appear to possess higher stability and better mechanical properties if they are treated under protective atmospheres than other ceramic fibres (especially classical silicon carbide fibres). The aim of this investigation is to characterize at the nanoscale the main microstructural parameters of Cf-SiC composites provided by the SEP (Division of SNECMA, Bordeaux, France). This material was fabricated from a 2.5D preform made of high strength polyacrylonitrile (PAN)-based carbon fibres densified according to the chemical vapour infiltration process. A pyrocarbon (PyC) interphase was deposited on the fibre prior to the beta-SiC matrix infiltration. A careful high resolution electron microscopy (HREM) microstructural investigation focused on the fibre microstructure as well as on the different interfaces in the material: pyrocarbon/fibre and matrix/pyrocarbon interfaces. All these observations have been realized in longitudinal and transverse sections of the specimen. These observations are found in good agreement with Guigon's model for high strength ex-PAN carbon fibres. The PyC interphase texture was strongly anisotropic at the fibre/interphase and interphase/matrix interfaces over a mean thickness of 8-15 nm. Tensile creep tests were performed under partial pressure of argon between 1273 and 1673 K for stress levels ranging from 110 to 220 MPa. Scanning electron microscopy and high resolution electron microscopy were used to study the microstructural modifications inside the fibres and at the different interfaces. A discussion of the possible creep mechanisms based on the microstructural investigation and the creep results is presented.