Drilling bits in the mining industry and cutting tools for metalworking in the manufacturing industry are often made of hard metal -- a material nearly as hard as diamond. Researchers have long tried to control the manufacturing process for the material to be able to steer in detail the hardness and other key properties to make it more durable. By combining theory and experiments, researchers at Chalmers University of Technology in Sweden have now taken a crucial step toward being able to micromanage the performance of the material, down to the level of the atom.
"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. The results of Sven Johansson's and Jonathan Weidow's research are of great interest to industry, and what they have managed to do is unique," says Göran Wahnström, professor of physics.
Hard metal is a mixture of a hard carbide phase, wolfram carbide (WC), and a tougher metal phase, cobalt (Co). It is produced by sintering, whereby fine powders of WC and Co are heated up so the cobalt melts and the material is pulled together by capillary force. The result is a solid material consisting of a hard skeleton of wolfram carbide grains surrounded by the tougher cobalt-rich cement phase.
The size of the wolfram carbide grains is key to the hardness of the hard metal. The great 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. 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.
"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. 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," says Göran Wahnström.
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. But they did not know why.
In the doped materials, the 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, Jonathan Weidow 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," says Göran Wahnström.
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