Analysis and optimization of the Cu-C interface in copper matrix composites reinforced with carbon nanotubes and nanofibres

Resumen

Lately, the metal matrix composites reinforced by nanoreinforcements have been broadly studied due to the beneficial effect that can provide in thermomechanical properties. Particularly carbon nanotubes (CNTs) and nanofibres (CNFs) have been investigated as reinforcement because of their superior thermal conductivity and tensile strength. These properties make these reinforcements potential candidates for metal matrix nanocomposites. Nevertheless, the main challenges in these composites are to obtain a good dispersion of the reinforcement within the metal matrix and achieve a strong interface between matrix and reinforcement. The purpose of this research project was to analyse copper based composites reinforced by carbon nanotubes and nanofibres (CNTs and CNFs). Copper-carbon interface was studied particularly. Several techniques have been developed to enhance the mechanical and thermal properties of this interface. The mechanical adhesion energy of amorphous carbon substrate with copper thin film samples was measured quantitatively. The aim was to measure the adhesion energy and a subsequent analysis of the interface with chromium and titanium interlayer, and without interlayer. The technique of Nanoindentation Induced Delamination (Top Nanoindentation) was used for this purpose. It was concluded that this technique is a valid method in order to determine the adhesion energy in a bidimensional sample which consists of a copper thin film over a carbon substrate. The experimental results were also compared with a cohesive elements based Finite Element Model. The trend of the results was the same in both cases. Finally, it was found that the presence of titanium and chromium interlayers sensibly enhance the adhesion energy. Electroless Plating was developed. This is an electrochemical deposition technique. It was used in order to deposit copper over carbon nanofibres (CNFs). Different working conditions of pH and temperature were analysed, and CNFs with different structures were coated by copper under optimal working conditions. It was proved that heat treatment in non graphitized CNFs enhances the CNFs coating. Best copper deposition was observed in longitudinally aligned CNFs. Also, a coating mechanism was studied. A good dispersion of the CNFs within the copper matrix can be achieved by this technique. Cu/CNT (both with Ti and Cr coating and without coating) samples were consolidated by means of Hot Press and High Pressure Torsion. A comparison was done between both consolidation techniques. The structure and mechanical and thermal properties of the composites were studied. Additionally, the Cu/CNF interface in a Cr coated CNF reinforced copper composite, consolidated by Hot Press, was analysed. Chromium carbides were obtained in the Cu/CNF interface. These carbides did not change the nanofibre structure. Concerning Cu/CNT composites, it was discovered that the grain size is considerably finer in next to CNTs grains, especially in Cr coated CNTs reinforced copper composites. In relation to mechanical properties, it was observed that titanium acts as hardness enhancer, despite the coarser grain size in these composites.