(533e) A Novel Route to Access Metal-Metal Nanocomposites: Bismuth-Cobalt as a Low-Friction Material

Bulk nanocomposite materials are one of the most rapidly moving fields in materials science since their properties can be designed through non-traditional combinations of their constituents. Metal/metal nanocomposites result from non-miscible metals and give access to fascinating alternatives to traditional metal alloys with superior mechanical, magnetic or tribological properties. These beneficial properties are the result of combining metals of completely different physical properties at the nanometer scale. Generally, this has been difficult when using traditional metallurgical procedures and motivated the development of mechanical alloying, severe plastic deformation or rapid solidification. These top-down approaches have yielded fascinating improvements in metallurgy but they require vast amounts of energy and long processing times.

Grass et al. recently demonstrated the homogeneous mixing of over 20 volume percent of a hard ceramic (cerium oxide, a refractory ceramic) into a soft metal (bismuth) at the 50 nm scale ? this extension of classical oxide reinforcement to one order of magnitude higher loadings was the result of mixing the constituents bottom-up, as nanoparticles [1]. The only way to avoid de-mixing was to manufacture the constituents at the same time and irreversibly agglomerate them right during powder collection. We further demonstrated a tripled Vickers hardness both in nanocrystalline cobalt [2] and superalloy (Ni/Mo) [3] prepared using a similar bottom-up method from the corresponding metal or alloy nanoparticles.

Here, we demonstrate the preparation of bulk metal/metal nanocomposites from non-miscible metals by using reducing flame spray synthesis [4, 5] for the simultaneous fabrication of the individual metal constituents. As a model system, we chose bismuth and cobalt as they represent two extremely different metals: Bismuth is one of the softest elements with a melting point of around 270°C. Cobalt is a key part in hard metals and refractory alloys and melts at 1495°C. Combining the hardness of cobalt and the softness of bismuth afforded a metal/metal nanocomposite with excellent lubrication properties. A potential application of such composites is in the replacement of lead or carbon-based bearings in engines where reduced wear at high lubrication are crucial to achieve high mechanical efficiency. This is of particular importance in combustion engines where over 30 % of the fuel losses are attributed to friction.

References

[1] R. N. Grass, T. F. Albrecht, F. Krumeich, W. J. Stark, J. Mater. Chem. 2007, 17, 1485.

[2] R. N. Grass, M. Dietiker, C. Solenthaler, R. Spolenak, W. J. Stark, Nanotechnology 2007, 18.

[3] E. K. Athanassiou, R. N. Grass, N. Osterwalder, W. J. Stark, Chem. Mater. 2007, 19, 4847.

[4] E. K. Athanassiou, R. N. Grass, W. J. Stark, Nanotechnology 2006, 17, 1668.

[5] R. N. Grass, W. J. Stark, J. Mater. Chem. 2006, 16, 1825.