J.Woltersdorf and E.Pippel

The paper presents the results of the development of a brazing technique for monolithic SiC and SiCf/SiC composites. This brazing technique is based on the use of Si-16Ti (at. %) and Si-18Cr (at %) eutectic alloys. The brazing temperature of the used alloys allows to avoid the degradation of the fibre/matrix-interfaces in the composite materials at least for advanced stoichiometric SiC fibre composites. All the joints showed excellent adhesion and no discontinuities and defects at the interface, while the brazing layer revealed a fine eutectic structure. In particular, in the composite joints the brazing layer appeared well adherent both to the matrix, the fibres and the fibre-matrix interphase, and the brazing alloy infiltration looked sufficiently controlled. The brazing alloys were characterized by X ray diffraction and scanning electron microscopy (SEM). All the brazed joints were analysed by SEM and the Si-16Ti joints were also characterized by high resolution transmission electron microscopy investigations of the microstructure and of the nanochemistry (HREM, EELS, esp. ELNES). These analyses revealed atomically sharp interfaces without interdiffusion or phase formation at the interface leading to the conclusion that direct chemical bonds are responsible for the adhesion. Shear tests performed at room temperature and 600°C on lap joint specimens gave remarkable results: the samples manufactured with monolithic SiC exhibit a high shear stress level and mainly cracked in the SiC bulk (150 MPa at RT for Si-18Cr joints), while the composite samples exhibited a RT shear strength up to 70 MPa for Si-16Ti and up to 80 MPa for Si-18 Cr with failure occurring mainly in the base material