Fabrication and thermo-mechanical behavior of nanoporous materials

Nanoporous materials are enormously interesting for future applications due to many excellent properties including: high surface-to-volume ratio, high strength-to-weight ratio, electrical and thermal conductivity, or radiation tolerance. These excellent properties can be used for combining structural purpose and a certain functional use in the same material at the same time. To use these foams more efficiently in the future, it is necessary to acquire information about the foam manufacturing, their thermo-mechanical properties, and the plastic deformation mechanisms.

Therefore, the objective of this work was to manufacture nanoporous copper (NPC), to determine the thermo-mechanical properties, and to elucidate the deformation behavior at elevated temperatures. The experimental approach for manufacturing the foam structures used high-pressure torsion, subsequent heat treatments, and selective dissolution. Scanning electron microscopy was used for identifying the shape and size of the foam structures and their thermal stability. In-situ nanoindentation was conducted to determine mechanical properties and deformation mechanisms at elevated temperatures.

SEM image showings the obtained foam structure after selective dissolution Hardness over temperature for the NPC up to 300°C

The high-temperature nanoindentation on nanoporous copper, shows a room temperature hardness of 220 MPa. During high temperature experiments, unexpected oxidation of the copper occurred even at GPa. A model was developed, taking into account the mechanical properties of the copper oxides, which allows to explain the measured mechanical properties in dependence of the proceeding oxidation. The strain rate sensitivity of the copper foam strongly correlates with the strain rate sensitivity of ultra fine grained bulk copper. Although oxidation occurred near the surface, the rate-controlling process was still the deformation of the softer copper. An increase in the strain rate sensitivity with increasing temperature was observed, comparably to that of ultra fine grained copper, which can be linked to thermally activated processes at grain boundaries. Important insights into the effects of oxidation on the deformation behavior were obtained by assessing the activation volume. Oxidation of the copper foam, thereby hindering dislocations to exit to the surface, 50·b3, typical for ultra fine grained materials. These basic mechanistic insights shall contribute to a better understanding of the deformation processes of nanoporous materials at a microscopic level.

In a related study on nanoporous Au we could show that the porous material shows pornounced incluences of testing temperature on the localized deformation behavior as well as the thermally activate deformation characteristics.

Deformation under nanoindents in nanoporous Au at different telperatures

More details can be found in recent open access articles on nanoporous Cu and nanoporous Au.

Acknowledgement: This work was supported by the Austrian Science Fund FWF (I-1020), the  Marshall Plan Foundation, the Montanuniversität Leoben and the country of Lower Austria.