Tooling up with tungsten
Maxim Vreeswijk of Imperial College London, UK, discusses his research on characterising the mechanical properties of interfaces in tooling material tungsten carbide-cobalt composites.
Tungsten carbide (WC)-cobalt (Co) is our industrial workhorse, frequently used for material removal tools and wear parts. It is applied to high-wear components of industry, examples of which are tools used in metal cutting, rock drilling and wire drawing. The history of WC-Co systems began in the early 1920s, when the system was developed by Osram studiengesellschaft – then a research laboratory in Berlin, Germany – as a lower cost alternative to the diamond wire drawing dies used by Osram GmbH in the production of the tungsten filaments used in light bulbs. As the composite had sufficient hardness to compete with diamond, the product was brought to the market under the name WIDIA, from the German, wie diamant or like diamond.
The first commercial WIDIA products demonstrated at the 1927 Leipzig spring fayre were not the wire drawing tools the material had been originally developed for, it had instead been utilised in metal cutting tools. Here, it was found that the cutting speeds could be increased by a factor of two or three, by replacing the creep-resistant tool steels. The use of WC-Co in cutting and milling tools has since become one of its main applications.
The WC-Co system is a member of the cemented carbides family of materials – composite materials consisting of a ductile metallic alloy phase (the binder) and a hard phase, generally carbide or nitride-based. The latter phase shows high hardness and inappreciable plasticity when deformed at room temperature. These hard phases differentiate themselves from other ceramic materials by having a much higher thermal and electrical conductivity. The most widely used cemented carbide material contains tungsten carbide as the hard phase and uses cobalt as binder phase. The WC-Co grades applied in industrial products generally contain between 6–16 wt % cobalt – the high carbide content allows the cobalt phase to be interpenetrated by a WC skeleton, a microstructure which is the basis for its excellent mechanical properties.
Liquid state processing
The key to success for WC-Co is its ability to be consolidated pressurelessly at low temperatures, via a liquid phase sintering process. Processing starts off with the constituent powders being milled to form a homogeneous mixture of the WC and Co. Solvents and pressing aids are added, the former to enhance mixing and avoid Co oxidation, and the latter to lubricate the pressing die and improve part cohesion during pressing. The mixture is then spray dried, where the wet mix is sprayed into a heated chamber at 200°C, the ethanol is evaporated, and deformable granules are formed. These granules are pressed into the desired pre-sintering shape through uniaxial pressing or cold isostatic pressing. The formed shape is then sintered in a two-step cycle. The initial pre-sintering treatment allows evaporation of the pressing aids from the part while it is still porous. Next, the part is heated above the liquid formation temperature and held at the sintering plateau.
Initially, the softened or liquated cobalt binder phase flows in-between the gaps of the WC crystals and rearranges the solid to maximise the area of the WC/Co interfaces. The energy of these interfaces is very low, hence the redistribution is energetically favoured. Then the liquated binder will take small WC grains into solution, which will reprecipitate on larger grains. This process is known as dissolution-precipitation. It forms a skeleton of solid WC, where neighbouring WC grains are connected by narrow WC necks. Finally, further thickening of these necks will occur during the coalescence process where material is transported away from the necks to the bulk of the WC grains. Coalescence results in the particle centres coming closer together, thus causing further densification. However, it also results in deleterious grain growth, therefore, time in this stage should be limited. Once the system has fully densified it is allowed to cool, forming the finished part.
Metal milling, cutting and drilling are demanding applications, resulting in a limited lifetime of cutting tools. What makes these applications particularly demanding is that the material is under high stresses due to localised loads applied to the small cutting tip. The tip scrapes over a surface causing substantial wear. Additionally, metal chips are generated, which reside between the machined part and the cutting tool, causing localised crater wear. Finally, friction between the surface and the tip generates heat, causing large temperature rises at the extremity of the cutting tip, which may exceed 1,000°C.
By considering the mechanical properties of WC-Co and the modes of failure of cutting tools, it becomes clear that the material is most suited for these extreme applications. The primary failure mode of cutting tools is excessive wear, which decreases as the material hardness is enhanced. The hardness needs to be excellent at high temperatures, therefore the hot-hardness becomes the figure of merit for this failure mode. The secondary failure mode is chipping of the tip, which is effectively fracture of the tool.
The effect of chipping can be reduced by enhancing the toughness of the material and having some ductility to plastically accommodate the stresses, thus avoiding brittle fracture.
When the properties of the WC-Co system are compared with conventional metals and ceramics, we find that the composite has better balance of hardness and toughness, thus combining the key properties for the aforementioned applications. Thus, WC-Co combines the hardness to resist wear and the toughness and ductility to accommodate plastic deformation rather than failing through brittle fracture. A further benefit of the WC-Co system in milling applications is the high thermal conductivity of WC compared with other ceramic compounds.
Due to the friction between the cutting tool and the metal there is an effective heat source on the tool tip. The accumulation of heat at this point could occur if the material was to have insufficient thermal conductivity to transport the heat away from the source. The result of heat accumulation would be higher creep rates as a result of the high temperatures experienced at the tip. Hence, the ability of WC to level thermal gradients further enhances the materials’ performance in milling applications.
The extreme temperatures reached during operation can cause creep mechanisms to play a role in the wear of cutting tools. During creep deformation, a variety of deformation mechanisms can be active. There are three key deformation mechanisms that are active during such high-temperature deformation. The dominant one depends strongly on the applied stresses and strain rates. At low strain rates and stresses, deformation can occur through diffusional flow of the WC phase. The diffusional mechanism is accommodated by a WC grain shape change. In the case of a tensile applied stress, the diffusion process results in significant grain growth of WC grains parallel to the loading axis and shrinking in the load plane.
At intermediate strain rates, grain boundary sliding (GBS) becomes a possible deformation mechanism. For sliding to occur the interfacial areas must be deformable, which requires the presence of cobalt in-between WC grains. GBS of WC/WC interfaces is preceded by decohesion of these interfaces, allowing the grain boundary to be infiltrated by a thin cobalt lamella (~1.2nm). This layer is highly deformable and hence allows for grain boundary sliding. Lastly, at maximal strain rates and stresses in the high-temperature domain, the plastic deformation of WC becomes an available deformation mechanism, which can occur through dislocation slip and climb.
Towards interfacial design
Recent studies have shown that compositional modulations of the cobalt binder phase, for example by adding nickel and chromium to the binder, greatly enhance the creep resistance of the composite. It has been shown that these additions preferentially segregate to grain boundaries, and could subsequently affect a variety of creep mechanisms which operate along grain boundaries such as grain shape change, grain boundary sliding or grain boundary decohesion. At Imperial’s research at the Centre for Advanced Structural Ceramics, we aim to develop methods to directly assess the effects of these additions on the strength of the interfaces. In these experiments, we isolate individual microstructural units and test their properties. By subsequently combining the measured properties of the units a model for the bulk behaviour can be constructed.
The micro dual cantilever beam test is an example of such a method, which is performed in situ inside a scanning electron microscope. In the test, two cantilevers are milled, using a focused ion beam, such that they sandwich an interface. At the top of the cantilevers, a notch is made to allow them to be pulled apart. Next, a state of pure bending is applied to the cantilevers by driving a wedge-shaped indenter into the notch, breaking apart the interface. An example
of such a test from previous work in our group was where a single crystal, in this case made from silicon carbide, is being cleaved along a particular crystallographic plane.
Cleaving the crystal allows its fracture energy to be measured, which is achieved by continuously measuring the crack length and the beam displacement inside the microscope and relating these to the fracture energy using elastic beam theory. The ultimate goal of this method is to extend our analysis of cleavage planes to include interfaces as well. We aim to be able to compare the properties of a variety of WC/WC interfaces, to characterise the effect of interface crystallography, and with that understanding be able to assess the effects of alloying additions on the interfacial behaviour. This could provide essential information needed for the design of new material compositions with enhanced creep resistance, as a result of their improved interfacial strength.
*Maxim Vreeswijk is studying for an MEng in aerospace materials at Imperial College London, UK.