L Journal of Alloys and Compounds 309 (2000) 201–207 www.elsevier.com / locate / jallcom Ti–TiN hardmetals prepared by in situ formation of TiN during reactive ball milling of Ti in ammonia * D. Wexler, A. Calka , Ahmed Y. Mosbah Department of Materials Engineering, Wollongong University, Northfields Ave, Wollongong, NSW 2522, Australia Received 29 March 2000; accepted 15 May 2000 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.  2000 Elsevier Science S.A. All rights reserved. Keywords: Nitride materials; Mechanical alloying; Mechanical properties 1. Introduction nitrides include direct combustion synthesis from Ti-pow- der in liquid nitrogen [1] and reactive ball milling of Ti in Metal and metalloid nitrides are of considerable interest nitrogen [2,3] followed by sintering. In this paper, a simple due to their desirable properties including high to extreme synthesis method for producing bulk metal and metalloid hardness, high temperature stability, high thermal con- nitrides using mechanosynthesis followed by hot pressing ductivity, high corrosion resistance and attractive colour. is investigated. It has been found that the direct reaction of Bulk materials are usually synthesised on a commercial metals with nitrogen can be achieved at room temperature scale by direct reaction of the metal with nitrogen at high during ball milling of elemental powders if a sufficient temperatures and pressures. This process is generally amount of molecular nitrogen (N ) is present in the mill 2 carried out under near isothermal conditions. However, the chamber [2]. This initial investigation was carried out in a final product is often inhomogeneous and may contain ball mill specifically designed for enhancing solid state unreacted material. There are also a large number of reactions between different species [4,5]. In follow-up conventional nitriding processes employed to produce work using this technique, nitration of elements such as Si, nitrides mainly in the form of coatings. These include, gas Fe and Ti was found to proceed more rapidly when done in and salt bath nitriding, ion nitriding, plasma nitriding and a NH rather than in nitrogen gas [6,7]. The reaction with 3 range of other deposition methods. NH was found to result in the formation of nitrides, 3 Novel methods for the synthesis of bulk titanium hydrides and hydrogen gas, with corresponding changes in pressure within the sealed mill chamber. Generally, re- action with NH in the sealed mill chamber results in an *Corresponding author. 3 E-mail address: andrzej calca@uow.edu.au (A. Calka). initial rapid loss in pressure, due to some combination of ] 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388(00)01030-6