Ti-TiN hardmetals prepared by in situ formation of TiN during reactive ball milling of Ti in ammonia

Vapour deposition of titanium nitride on WC/Co or hard ferrous-based cuttin g tips generally results in significant increases in cutting tool life. How ever, a major limitation of such nitrided tips is that they cannot be resha rpened for re-use. Although monolithic TiN may be too brittle for cutting t ool applications, with appropriate microstructural design, Ti-TiN composite s should have the required combinations of toughness, ductility, hardness, wear resistance and thermal conductivity to replace coated tips for a range of machining applications. We report the synthesis of monolithic Ti-TiN co mposites from nanostructural precursor powders. Reactive ball milling of Ti in nitrogen or ammonia under controlled conditions eventually results in t he formation of nanostructural TiN. Furthermore, by ending the reaction aft er an appropriate period a homogeneous and uniform mixture of Ti and TiN ph ases can easily be produced. Due to the highly reactive, nanostructural nat ure of the powder product this synthesis route has the potential to elimina te wetting problems generally associated with the current technology of con ventional liquid-phase sintering. Moreover, by controlling nitriding gas pr essure changes during milling good control of both the Ti to TiN ratio and final crystallite size distributions can be achieved. It was found that pre cursor Ti-TiN nanostructural powders synthesised in this way can be success fully compacted and liquid phase sintered without sintering aids. Such comp acts show high densities and nanoindentation hardnesses in the range of 18- 23 cpa. Structural characterization was performed using X-ray analysis, tra nsmission and scanning electron microscopy as well as optical microscopy. T he mechanical. properties were characterised using micro- and macroindentat ion techniques. (C) 2000 Elsevier Science S.A. All rights reserved.