Formation of metal-metal nanocomposites is a particularly attractive proposition, addressing the need to produce hard, conductive, but still plastic coatings which reduce wear. The Cu-Ag system can be a suitable model for investigation the structure development and the structure-property relations in metal-metal nanocomposites.
The aim of the present research was to study structural and mechanical properties of Cu-Ag alloy thin films in a broad range of compositions. For these investigations micro-combinatorial samples were prepared by simultaneous thermal evaporation of Cu and Ag. The maximum deposition rate of both constituents was 1 nm/s, the substrates were kept at ambient temperature and the background pressure was (4-6) 10-6 mbar during growth. The composition along the gradient on the sample was measured by EDS.
For determination of the structure of the Cu-Ag system 40 nm layers were deposited. Structural characterization was carried out by conventional TEM. We found that the Ag-rich part (of 60% Ag) of the sample develops very small grains (below 10nm) forming a metal-metal nanocomposite structure and simultaneously a very strong <111> texture appears in both components. With increasing Cu concentration the grain size of the layer is increasing and the texture disappears gradually.
To explore how composition modifies the mechanical properties 600 nm layer was deposited in micro-combinatorial arrangement onto oxidized Si at ambient temperature. Mechanical properties were evaluated by a NanoTest 600 nano-mechanical tester equipped with a Berkovich indenter tip. The mechanical properties of Cu-Ag nanocomposites show the optimum of their properties (H, E, H/E) in different ranges of composition.
The hardness values are well above the hardness of constituent components and a maximum occurs around 80 at% of Cu due to changing from Hall-Petch effect to grain boundary sliding. The Young modulus reaches its maximum at the copper-rich region of the sample according to the rule of mixtures, while H/E is hardly dependent on the composition.
This work was supported by the Hungarian National Science Foundation; OTKA T-043437 and 048699 projects.
|