Multiscale investigation of the creep behaviour of a 2.5D C-f-SiC composition

This paper deals with some results on the creep behaviour of a 2.5D C-f-SiC composite. This material fabricated by CVI was tested in tension under an argon partial pressure for temperatures ranging from '1273 to 1673 K and st resses between 110 and 220 MPa. Results regarding creep curves (strain-time ) and strain rate-time curves tend to confirm the existence of a secondary stage. Damage-stress and damage-time curves are also presented. The limits of the Dorn's formalism are evidenced as well as the occurrence of a damage process leading to a so-called damage-creep mechanism. In order to explain this macroscopic creep behaviour of the composite, investigations at the m esoscopic, microscopic and nanoscopic scales were necessary. Five modes of matrix microcracking are observed together with different pull-out features regarding the extracted fibre surface. The damage accumulation via matrix microcracking appears to be a time dependent mechanism. Two modes of interf acial sliding are evidenced: at 1473 K and 220 MPa, the pyrocarbon (PyC) in terphase is fractured leading to debonding between carbon layers, while at 1673 K, there is a loss of anisotropy of the PyC layer close to the matrix and, thus, an interfacial sliding appearing as a viscous flow. To elucidate the role of the carbon fibres, a nanoscale study via HREM has been conduct ed. An increase of the mean diameters of the basic structural units (BSUs) and of the areas of local molecular orientation (LMOs) within the fibres ha s been observed when increasing temperature under 220 MPa. In fact, these c hanges do not contribute to the macroscopic strain. Therefore, this restruc turation effect has been called "nanocreep" of the carbon fibre as it appea rs to have a negligible contribution to the macroscopic creep behaviour of the 2.5D Cf-SiC composite, (C) 1999 Kluwer Academic Publishers.