Diamond fibres were produced in a hot filament reactor by chemical vapour deposition of a 40 mu m thick layer of diamond on to titanium-coated iron and iron-chromium wires. In the 40 h deposition time carbon diffused through the titanium and steel and produced carbide microstructures with hardness values of similar to 3234 and 5390 N mm(-2) for the Fe and Fe-Cr cores, respectively. During cooling thermal stress fracture of the diamond was avoided by stress relief cracks in the disordered graphite layer at the diamond/Ti-rich layer interface.