Striking Gold at the Mechanical Metallurgy Laboratory

Andreas Mortensen, Director of EPFL’s Institute of Materials (IMX) and his colleagues Ludger Weber, Senad Hasanovic and Reza Tavangar have created gold that is virtually unscratchable and more resilient than tempered steel, to meet the needs of the Swiss watchmaking industry. He has also just been awarded a €2.5 million European Research Council (ERC) Advanced Grant to study defects in composite materials and alloys at a very small scale.

By combining a gold alloy with boron carbide, an extremely hard ceramic that’s used in bulletproof vests and as an abrasive, Andreas Mortensen and his team from the Mechanical Metallurgy Laboratory in EPFL’s Institute of Materials have succeeded in making the world’s hardest 18-karat gold (75% gold). With a Vickers hardness of 1000, it is harder than most tempered steels and thus almost impossible to scratch in daily life, except with diamond and other hard ceramics (quantitative tests are being organized with a specialized laboratory). This discovery is the result of a three-year collaboration between the laboratory and the Swiss watchmaking company Hublot.  

Infiltration under pressure
The process for developing this material is relatively complicated. Powdered boron carbide is heated to almost 2000°C, where it forms a rigid, porous structure by a process called sintering, which creates many small solid bridges between neighbouring carbide particles. A liquid alloy of gold is infiltrated under very high pressure into the pores of this structure, and then solidified, yielding a pore-free composite material. The final material is thus made up of two phases, metal and ceramic, that are intimately interconnected in space, like two three-dimensional interwoven labyrinths. Because the molten gold used is a previously-made alloy based on 24-karat gold and aluminum (3%) for strength, the final gold is thus a little less than 3% aluminum, 75% gold and a bit more than 22% boron carbide by weight.

Overcoming the 18-karat limit
Like nearly all metals, gold is very soft. Managing to harden it to this degree while still maintaining 18-karat purity was a real challenge for the EPFL scientists. They overcame the obstacle by using a ceramic-metal composite approach. Composite materials are created by artificially combining several materials that conserve their individual characteristics even after they are assembled. In this they are different from alloys, in which atoms mix together to form a new, homogeneous, material. The first watches created using this new gold will be presented in 2012 at the BaselWorld Watch and Jewellery show.

A €2.5 million ERC grant
Unscratchable gold is just the tip of the iceberg for research in Andreas Mortensen’s laboratory, however. The scientist has just received a five-year, €2,496,000 Advanced ERC grant, which he will use to unveil the nanoscale mysteries (a nanometer is a millionth of a millimeter) of hard phases present within composite materials and metallic alloys. He plans to use cutting-edge testing equipment to observe and understand the nanomechanics of particles in composite materials and alloys, aiming then to suppress defects that weaken them. The research should eventually allow researchers to create composite materials and alloys that are more resistant and efficient than those currently in existence.

Defects that weaken materials
In order to grasp the nature and implications of this project, it is important to understand that when composite materials and alloys are created, particles of a very resistant material are often added or precipitated within the ductile metallic base material in order to obtain a final product with improved properties (more resistant, harder, etc.). Within the final product, "soft" and "hard" phases coexist. Mortensen is primarily interested in the microstructure of such "hard" particles whose role is to harden the material, because they often carry defects that negatively affect the final result. He will thus conduct nano-indentation tests (which test the particles’ resistance to deformation under pressure exerted by a very small diamond tip). These tests will be accompanied by focused ion beam milling to prepare microsamples that can then be probed mechanically to reveal the local strength and defect distribution within such hard particles.

Ceramic particles, carbide precipitates and silicon

Three kinds of particles will be tested and analyzed: ceramic particles, which are used for making metal matrix composites; silicon particles in aluminum, which play an important role in the resistance and ductility of most cast aluminum alloys; and carbides that are added to steel in order to increase its strength and hardness.

Nano- and micro-mechanical testing will be accompanied by microstructural characterization of the particles by electron microscopy, which will allow the scientists to identify defects that weaken them, and, by association, the resistance of the alloys and composites they are part of. A study of this kind has never been undertaken at the nanoscale. It is now only possible thanks to modern methods of nanometer-scale mechanical characterization. "If it is successful, this project will make it possible to determine much of what defines the mechanical properties of many metallic materials, and thereby perhaps to improve them" concludes Mortensen.

Laure-Anne Pessina