G Boitier et al. / Composites: Part A 33 (2002)1467-1470 leaves then the interfacial sliding can be assimilated to a dry friction between two rough solids. Close to the fibers and the matrix the structure of the PyC is turbostratic type, with atomic carbon planes parallel to the fiber direction That has also clearly been evidenced by SEM observations of the surface of the fibers: that surface is very rough(see the small insert in Fig. 2a). Although at 1473 K the fiber/matrix interface is radially in compression on. under stress some lenticular pores appear(Fig. 2a). These lenticular pores due to local decohesion between carbon planes are, in fact, the nuclei for the microcrack development. When the debond- ing is not total, carbon ribbons bridge the two parts of the microcracks. At 1673K, the PyC interphase is a little degraded over about 100 nm from the matrix; there are a disappearing of the previous anisotropic texture, and a certain amorphization; then the interfacial sliding can be assimilated to a viscous flow (Fig. 2b). That was also confirmed by SEM observations: the fiber surfaces are smooth [8-10)(see the small insert in Fig. 2b) If observations are performed at a higher scale one notes that the nanotexture of the carbon fibers increases with the test temperature. Consequently it leads to an increase of the local molecular orientation of oriented volumes of carbon planes parallel to the fibers. Although this phenomenon has been called carbon fiber nanocreep'[16], this contribution of the fibers and matrix to the macroscopic deformation is negligible, but it can be considered as the nuclei of the dama In the case of Sicr-SiBC same type of featu observed, but a little more complex due to the existence of the different matrix layers based on the Si-B-C system. In addition to the damage observed in Cr-SiC composites, Fig. 1. HRTEM micrographs of the matrix/pyrocarbon interface in a there are also matrix microcrack deviations via some matrix as-received Cr-SiC (a) and of the matrix/pyrocarbon/fiber interfaces in multilayers where carbon exists [11]: this carbon is located -received SiCr-SiBC (b). at the interfaces of specific matrix layers and presents a thin 5 nm Fig. 2. HRTEM and SEM micrographs of the pyrocarbon interphase in Cr-SiC creep tested specimen under 220 MPa and in Ar:(a) with the presence of some lenticular pores at 1473 K:(b)and the presence of carbon rollings at 1673 K Inserts correspond to SEM images of the surface features of the SiCr fibers at theseleaves: then the interfacial sliding can be assimilated to a dry friction between two rough solids. Close to the fibers and the matrix the structure of the PyC is turbostratic type, with atomic carbon planes parallel to the fiber direction. That has also clearly been evidenced by SEM observations of the surface of the fibers: that surface is very rough (see the small insert in Fig. 2a). Although at 1473 K the fiber/matrix interface is radially in compression, under stress some lenticular pores appear (Fig. 2a). These lenticular pores due to local decohesions between carbon planes are, in fact, the nuclei for the microcrack development. When the debond￾ing is not total, carbon ribbons bridge the two parts of the microcracks. At 1673 K, the PyC interphase is a little degraded over about 100 nm from the matrix; there are a disappearing of the previous anisotropic texture, and a certain amorphization; then the interfacial sliding can be assimilated to a viscous flow (Fig. 2b). That was also confirmed by SEM observations: the fiber surfaces are smooth [8–10] (see the small insert in Fig. 2b). If observations are performed at a higher scale one notes that the nanotexture of the carbon fibers increases with the test temperature. Consequently it leads to an increase of the local molecular orientation of oriented volumes of carbon planes parallel to the fibers. Although this phenomenon has been called ‘carbon fiber nanocreep’ [16], this contribution of the fibers and matrix to the macroscopic deformation is negligible, but it can be considered as the nuclei of the damage process. In the case of SiCf–SiBC same type of features are observed, but a little more complex due to the existence of the different matrix layers based on the Si–B–C system. In addition to the damage observed in Cf–SiC composites, there are also matrix microcrack deviations via some matrix multilayers where carbon exists [11]: this carbon is located at the interfaces of specific matrix layers and presents a thin Fig. 2. HRTEM and SEM micrographs of the pyrocarbon interphase in Cf–SiC creep tested specimen under 220 MPa and in Ar: (a) with the presence of some lenticular pores at 1473 K; (b) and the presence of carbon rollings at 1673 K. Inserts correspond to SEM images of the surface features of the SiCf fibers at these two temperatures. Fig. 1. HRTEM micrographs of the matrix/pyrocarbon interface in a as-received Cf–SiC (a) and of the matrix/pyrocarbon/fiber interfaces in a as-received SiCf–SiBC (b). 1468 G. Boitier et al. / Composites: Part A 33 (2002) 1467–1470