Probing hardmetal

Materials World magazine
,
3 Sep 2010

Scientists in France and Sweden have made progress in understanding why some properties of hardmetal improve when doped with vanadium. The findings could help manufacturers control the hardness and durability of hardmetal.

Doping with vanadium restricts the growth of tungsten carbide grain – the component that most contributes to the hardness – by one tenth. It is this restriction in growth that further hardens the material. Professor Göran Wahnström of Chalmers University of Technology in Sweden managed a project that investigated how the doping had this effect.

He explains, ‘The main result from theoretical calculations is the prediction that an ultra-thin film of cubic vanadium carbide is present at liquid phase sintering conditions’, and that this is responsible for grain growth inhibition. ‘The theoretical calculations were based on the quantum mechanical density functional theory. We also added some results from thermodynamic modelling using Thermo-Calc [software].’

Using high-resolution electron microscopy to characterise the surfaces, scientists at the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), in Grenoble, France, confirmed that a film two-atom layers thick of a cubical structure is constructed on the tungsten carbide grains. Researchers in Sweden applied atom-probe tomography to scrutinise the interfaces.

‘Large grains [of tungsten carbide] are thermodynamically unstable with the composition of the film, but the thin film is stabilised by strong bindings on the interface between the film and the cementing phase [of cobalt],’ adds Wahnström.

Dr Bryan Roebuck, a fellow at the National Physical Laboratory (NPL) in Teddington, UK, who specialises in the characterisation and performance of materials, notes, ‘The underlying reason for the material’s hardness has been known for a long time – 20 or 30 years – as are the optimal amounts of tungsten carbide, cobalt and so on. But the mechanism of how the hardness works has not been fully understood.’

Roebuck’s comments suggest that the work is useful and could have a significant effect, but not in all related industrial activities. ‘This is world leading research but it doesn’t cover all applications because some applications need a very big grain.’ Different types of hardmetal contain a range of grain sizes. The hardmetal used in the Chalmers/CEA tests contained smaller grains, he says, which means it can be improved in terms of hardness but not toughness. Its improvements, he suggests, will mainly affect circuit board drills, woodworking and composite material tools.

‘Advances in understanding the mechanisms by which small amounts of other elements affect properties and performance of the final tools will provide a competitive advantage and should yield ideas for new and improved tool compositions,’ Roebuck adds.

According to Wahnström, work is continuing on the theoretical study of other carbide phases, the effect of stochiometry, and the effect of the thin films on the mechanical properties.