Diffusion of grain-growth inhibitors in hardmetals

The research project "Diffusion of grain-growth inhibitors in hardmetals" refers to materials which are of vital importance for components in modern production processes. In principle, hardmetal is a sintered composite material made from hard tungsten carbide (WC) and ductile cobalt (Co). It is used in many areas of mechanical engineering, electronics, aircraft and vehicle manufacturing, transport in general, paper industry, petrochemical industry, and in mining and road construction. Recent technical innovations like smartphones, tablet PCs, intelligent automobile electronics and the general trend towards miniaturisation of electronic components demand tool properties which can only be covered by very fine-grained hardmetals which offer a combination of high bending strength, high hardness, and long tool life (e.g. for high-performance precision drilling of printed circuit boards (PCBs) with microdrills). For maintaining the fine microstructure of these high-performance materials, so- called grain-growth inhibitors (GGIs) are applied. GGIs are metals, added as carbides, which retard the growth of the WC particles during sintering. The mechanism of their effect is partly known - it seems to be based on the modification of the interface energy between W and Co, by adsorption of GGIs on the WC surface, respectively their dissolution in the Co-based binder. Because of this, the distribution of the GGIs, which should be as homogeneous as possible, is crucial for the production of fine-grained hardmetals. The aim of the research project is to study the behaviour of the inhibitor metals in the course of the early sintering stages (during heating up) and the influences of this behaviour (diffusion) on the fully sintered hardmetal. Research includes the adjustment of hardmetals with different additions of GGIs, different grain sizes of WC and GGI particles, accurately controlled heat treatment (sintering), and analysis of the GGI distribution in the microstructure by employing state-of-the-art analytical methods like WDS-EPMA (wavelength-dispersive electron probe microanalysis), GDOES (glow- discharge optical emission spectroscopy), FIB/FESEM (ion milling in the electron microscope and analysis with energy dispersive systems). Apart from methodical studies on the analysis of such samples (first preparation experiments and analyses have been successfully performed), fundamental understanding and detailed quantitative results on diffusion and distribution behaviour of conventional and alternative GGIs are expected. This facilitates a closer specification of material selection and processing, ultimately leading to improved hardmetals. A further consequence is an improved efficiency with regard to raw materials and energy consumption in hardmetal manufacturing and application.

The aim of this research project was to gain fundamental knowledge on the production of modern, near-nano grained hardmetals by an optimised application of grain-growth inhibitors such as chromium, vanadium or molybdenum. Hardmetals are compound materials consisting of a hard carbide phase, usually tungsten carbide (WC) and a tough metallic phase such as cobalt. They are produced from powder mixtures by so-called liquid phase sintering where the metallic binder phase melts at temperatures around 1400C and subsequently fills the pores between the WC particles. Hardmetals exhibit a remarkable combination of hardness and toughness. This combination makes them mandatory for a variety of industrial key applications such as metal cutting, wood cutting, wear protection or mining. Each of these applications requires specific properties, which can be adjusted by varying the initial parameters such as the binder content or the grain size of the WC particles. For modern applications such as drilling of printed circuit boards industry requires very fine WC grain sizes following the general trend to nano-materials. Such nano-sized WC powders are already commercially available. However, upon the sintering process the WC grains start to grow, limiting the feasibility of producing nano-grades. To overcome this issue so-called grain-growth inhibitors (GGIs) such as chromium, vanadium or molybdenum are added. Since they are added in form of powder particles they have to distribute in the material before the growth of WC particles initiates. For standard grades grain-growth mainly happens upon liquid phase sintering and GGIs can distribute fast via the liquid binder phase. In near-nano grades WC growth initiates already upon early sintering stages at temperatures around 1000C and, hence, GGIs have to resolve and distribute via the solid binder phase. Less is known about that process and the inhibiting mechanism at low temperatures are not yet fully understood. The present project gained fundamental knowledge on the dissolution and distribution kinetics of the most effective grain-growth inhibitors vanadium, chromium and molybdenum. A variety of parameters influencing the distribution such as amongst others the temperature, sintering atmosphere, cobalt content, initial cobalt grain size or pressing forces were characterised. It was found that chromium spreads significantly faster as compared to vanadium or molybdenum and vanadium, the most effective inhibitor upon liquid phase sintering, is less effective at low temperatures. The distribution of GGIs caused inhomogeneity of the hardmetal microstructure which can be controlled by a proper application of various parameters. From the results in this project a detailed description of the behaviour of GGIS at initial sintering stages was created. This description can be used by industry in order to produce near-nano hardmetals with improved properties by an optimised application of grain-growth inhibitors.