"The size of bore can vary, from a diameter of 10 meters for large tunnel bores down to three hundredths of a millimeter, thinner than a human hair, for applications in the electronics industry.This places great demands on the manufacturing process to attain precise properties," said Göran Wahnström, professor of physics at the University.

Hardmetal is made of fine powders of WC and Co heated up to create a solid material consisting of a hard skeleton of wolfram carbide grains surrounded by the tougher cobalt-rich cement phase. The size of these grains is key to the hardness of the hard metal, and the challenge is to be able to control the growth of these grains during the sintering process. By combining experimental and theoretical methods, the researchers now understand how they can control the structure of the material in detail, down to the level of the atom, during the production process.

By doping the material (adding another substance in tiny portions) scientists have known that the growth of the grain can be dramatically limited. A tiny addition of vanadium can limit the growth of the grains to one tenth, from a particle size of one thousandth of a mm down to one ten-thousandth mm. In the doped materials, a research group in Grenoble found, using high-resolution electron microscopy, that an extremely thin layer, only two atom layers thick, of a cubical structure can be built on the wolfram carbide grains. At Chalmers, researchers used atom-probe tomography, a technology unique in Sweden, to analyze the interfaces atom by atom.

"These films can affect the growth, but the question is whether they are there during the actual sintering process when the WC particles are growing, when the experimental microscopy technology cannot be used. The theoretical prediction is that these films can also exist at the high sintering temperatures. Large grains with the composition of the film are then thermodynamically unstable, but the thin film is stabilized by strong bindings on the interface between the film and the cementing phase," said Wahnström.

"Our work has focused on characterizing and understanding the interfaces in the material, on the one hand between the wolfram carbide grains, so-called granular interfaces and, on the other hand, between the wolfram carbide grain and the cementing phase, what are called phase interfaces,” he added. “The theoretical part made use of quantum mechanical density-functional theory to describe and understand how the electrons in the material bind together the material."

The work was carried out as a twin doctoral project with funding from the Swedish Research Council and the industry (Sandvik and Seco Tools) and in collaboration with a research team in Grenoble.