Fibre-matrix interfaces in titanium matrix composites made with sigma monofilament

A review of the development of coatings for Sigma monofilament is given. The coating must protect the underlying silicon carbide before, during and after consolidation. This requires the coating outer surface to be under negative or zero residual stress at room temperature. The coating should also be well bonded to the SiC. It is shown that stoichiometric TiB2 is under a tensile stress of around 3 GPa under these conditions and hence is unsuitable. The boron-rich outer surface of SM1240 is essentially unstressed and the carbon surface of SM1140+ is under ≈300 MPa axial compressive stress. Failure of monofilament in the composite initiates at the W–SiC interface, rather than at the metal–fibre interface characteristic of SCS-6. In order to ensure this behaviour, the coating in the composite must be thick enough to ensure that the stress concentration field arising from irregularities at the TiC–C boundary do not initiate fracture of the SiC. This requires a minimum thickness of around 1 μm of carbon.