New Technique for Composite Formation

Abstract

In general, composites are formed from a powder mixture of binder and reinforcement materials. The softer binder material surrounds and binds together the much stronger reinforcement material. The reinforcement material can be in the form of fibers or particles. The cutting tools are an example of composite materials where softer cobalt (Co) is used as a binder and the stronger tungsten carbide (WC) hard material as a reinforcement. Liquid phase sintering is applied for the processing of cutting tools and high densification is usually achieved after short periods of sintering at high temperatures. Life of cutting tools is usually extended by applying thin coating layers of titanium nitride (TiN) on WC/Co composite or hard ferrous-based cutting tip. However, such coated tools can not be re-sharpened and therefore, they can only be used once. That is a major limitation. Monolithic TiN has high enough hardness for cutting tool applications but it is too brittle for such applications. It may be made tougher through the addition of titanium (Ti) metal as a binder and the design of appropriate microstructures of Ti-TiN composites. Such composites may be designable to provide the right combination of properties such as toughness, hardness; and thermal conductivity required for replacing the coated tips in many machining applications. This paper covers a new method for designing such composites. The monolithic Ti-TiN composites synthesized from nano-structural precursor powders have shown that designing a certain composite maybe possible through mechno-synthesis using precursor elemental materials. Formation of nanostructural titanium nitride was achieved by ball milling of titanium powder in nitrogen or ammonia under controlled conditions. More importantly, by controlling the milling time, homogeneous and uniform distribution of TiN particles in Ti matrix was produced. The potential of this synthesis process in producing other composites with the required combination of properties and overcoming the wetting problems that exist in the conventional liquid-phase sintering as a result of the high reactivity of the powder product is illustrated. Moreover, control of the matrix to hard phase ratio and the final size distributions of the crystallites were possible using this synthesis method by controlling the pressure changes of the nitriding gas during the milling process. Full compaction and liquid phase sintering of the nanostructural Ti-TiN powders synthesized by this route were achieved without using sintering aids. High hardness values (up to 23 GPa) were achieved. X-ray analysis, optical microscopy and scanning electron microscopy techniques were employed in the structural characterization of the processed compacts. Micro- and macro-indentation techniques were used in evaluating the mechanical properties of such compacts.