Ahmed Y. Mosbah

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

Abstract Vapour deposition of titanium nitride on WC/Co or hard ferrous-based cutting tips generally results in significant increases in cutting tool life. However, a major limitation of such nitrided tips is that they cannot be resharpened for re-use. Although monolithic TiN may be too brittle for cutting tool applications, with appropriate microstructural design, Ti–TiN composites 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 composites from nanostructural precursor powders. Reactive ball milling of Ti in nitrogen or ammonia under controlled conditions eventually results in the formation of nanostructural TiN. Furthermore, by ending the reaction after an appropriate period a homogeneous and uniform mixture of Ti and TiN phases can easily be produced. Due to the highly reactive, nanostructural nature of the powder product this synthesis route has the potential to eliminate wetting problems generally associated with the current technology of conventional liquid-phase sintering. Moreover, by controlling nitriding gas pressure changes during milling good control of both the Ti to TiN ratio and final crystallite size distributions can be achieved. It was found that precursor Ti–TiN nanostructural powders synthesised in this way can be successfully compacted and liquid phase sintered without sintering aids. Such compacts show high densities and nanoindentation hardnesses in the range of 18–23 GPa. Structural characterization was performed using X-ray analysis, transmission and scanning electron microscopy as well as optical microscopy. The mechanical properties were characterised using micro- and macroindentation techniques.

Nanotube and Nanorod Synthesis under High Frequency Electric Discharge Assisted Mechanical Milling

Electric discharge assisted mechanical milling using a 50 Hz power supply has been used to produce a range of fine and nanostructural products, including nanocrystalline aglomerates and individual nano-particles and nano-fragments. Processing variables include; starting powder sample size; electric arc parameters such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results of an experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a new pulsed power supply working at frequencies in the kHz range. The aims of this preliminary investigation were to determine processing parameters required for the synthesis of potentially useful high surface area particles, nanostructural powders and nanoparticles, and to compare products with those synthesised by Hz frequency discharge milling. Microstructural, morphological, and phase changes induced by kHz discharge milling were characterised by x-ray diffractommetry and transmission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nano-particles, agglomeration of powder particles, particle melting and/or sintering, and chemical reaction induced by mechanoprocessing in the presence of a particular type of plasma. Discharge milling of graphite under Ar/4%H2 resulted in a range of products including; graphite nanostructures, carbon nanotubes and other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical milling of graphite resulted in higher yields of carbon nanotubes. hematite resulted in partial reduction to magnetite and FeO,and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co-WC resulted in products including; micron and submicron fracture products, nanostructural regions of Co and WC, and carbon rich nanorods and nanotubes.