In order to use SiC/SiCf composites for nuclear fusion applications, brazing with metallic fillers cannot be used, for avoiding the production of radioactivity by nuclear transmutation of the metallic elements, and in any case the metal alloys currently available for such a type of brazing cannot withstand the service temperatures required.

A silicon oxycarbide (SiOC) glass, derived from the pyrolysis of polysiloxane (silicone resin) preceramic polymer, has been used by us in the past as joining medium to join 2D-SiC/SiC fiber composites at temperatures of 1000-1200 C. The obtained joints were homogeneous, dense, had a thickness of about 2 to 4 micro-m and appeared to be free from residual stresses. The joint quality in terms of mechanical strentgh, as determined by fracture shear stress test (ASTM D905-89), showed nevertheless a large results variability, due to the high surface roughness intrinsic of such composites, because of the ceramic fabric layers used in their manifacturing.

In order to overcome such difficulties, a new approach is being investigated, using different types of fillers in the joint layer produced with the preceramic polymer. Experiments have been carried out using unidirectional fibers of carbon, unidirectional fibers of SiC (Nicalon), 2D fabrics of carbon fibers, 2D fabrics of SiC fibers and SiC nanopowders (about 40 nm grain size, laser produced). The use of fibers in the joining layers, although not increasing the mechanical shear strength of the joints, change completely the fracture mechanics of them. In this case, in fact, the stress-strain curves, as determined by shear tests, display features typical of a composite material, while the fracture of joints produced from preceramic polymer alone was always brittle, typically like the monolithic ceramics fracture. The SiC fibers in any case behave better than the carbon fibers.

The use of SiC nanopowed was investigated dispersing them in the preceramic polymer in the ratio of 10, 20, 30 and 40 wt%. The better results have been obtained using the 20 % ratio and pyrolysing the polymer at 1200 C: in this case a maximum mechanical shear strength of 12.4 MPa was obtained. At higher pyrolysis temperatures the joint strength is still better, increasing to a maximum of 17 Mpa at 1400 C. The control of the surface roughness and morphology of the composite material continue to be a key factor for the further improvement of the mechanical strength of the joints.

This work has been performed in the frame of EC Fusion Technology Long-Term Programme - Materials 1995/98 Task SDS 3.1.1