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

A review of the development of coatings for Sigma monofilament is given. Th e coating must protect the underlying silicon carbide before, during and af ter consolidation. This requires the coating outer surface to be under nega tive or zero residual stress at room temperature. The coating should also b e well bonded to the SIG. It is shown that stoichiometric TiB2 is under a t ensile stress of around 3 GPa under these conditions and hence is unsuitabl e. The boron-rich outer surface of SM1240 is essentially unstressed and the carbon surface of SM1140 + is under approximate to 300 MPa axial compressi ve stress. Failure of monofilament in the composite initiates at the W-SiC interface, rather than at the metal-fibre interface characteristic of SCS-B . In order to ensure this behaviour, the coating in the composite must be t hick enough to ensure that the stress concentration field arising from irre gularities at the TiC-C boundary do not initiate fracture of the SiC. This requires a minimum thickness of around 1 mu m of carbon. Crown Copyright (C ) 1999 Published by Elsevier Science S.A.