Generation of ultrafine grained Ni-CNT composites by severe plastic deformation

Nearly all metallic materials are crystalline and consist of close-packed crystallites, which can also be described as grains. With conventional manufacturing methods like casting, forging or rolling at enhanced temperatures or by sintering powders, materials with a grain size of a few to a few hundred micrometers can be produced. With decreasing grain size, the mechanical strength of metallic materials increase. This effect is very significant for materials with grain sizes below one micrometer, often termed ultrafine grained materials. Hence, it would be advantageous for many technical applications if it would be possible to manufacture materials with an ultrafine grain size. Severe plastic deformation (SPD) methods are very effective and novel procedures to generate ultrafine grained materials. Briefly summarized, large strains are applied to a metallic material under high hydrostatic pressure during SPD which induces significant grain refinement without changing the overall dimensions of the material. Unfortunately, the SPD processed materials are thermally unstable due to the high amount of stored energy in their large grain boundary area, and coarsening of the microstructures (even at room temperature) alters material properties considerably. As a consequence, the thermal stability of ultrafine grained materials is a fundamental issue for future potential applications. The aim of this project is the development of novel SPD-processed composite materials, in which the major issue of structural instability is overcome. Carbon nanotubes (CNTs) dispersed throughout a Ni matrix can stabilize the microstructure against grain growth. Additionally, due to their outstanding intrinsic properties, CNTs are expected to improve the mechanical and tribological properties as well. Comprehensive microstructural characterization in combination with an examination of the CNT influence on the mechanical and tribological properties as well as on thermal stability of SPD processed CNT-reinforced metal matrix composites will be performed and used to describe the optimal material performance. Therefore, a tool for the optimization of CNT-based composites with well-defined properties will be provided. The proposed project can further serve as a starting point regarding further research in this area with other matrix materials and other types of reinforcement phases. Additionally, these results will be useful for the evaluation of these composites from a scientific perspective as suitable candidates for mechanical applications.

Nanocrystalline (< 100 nm) and ultrafine grained (< 1 m) materials have been a subject of extensive research over the past couple of decades due to their unique mechanical and functional properties. Although several routes to obtain such materials are currently known, one of the most efficient processes is severe plastic deformation. Severe plastic deformation cannot only be used to refine the grain size of a metal, but also the range of available nanostructures processed by severe plastic deformation has been continuously extended. One novel approach uses severe plastic deformation for the fabrication of metal matrix composites (MMCs). This project investigated the application of severe plastic deformation as novel synthetization process of carbon based reinforced MMCs with nickel and silver matrices. As carbon reinforcement phase, carbon nanotubes and nanodiamonds, were used. The project covered different aspects, ranging from the evolution of the microstructure of the metal matrix, the redistribution and homogenization of the reinforcement material, the structural state of the carbon reinforcement phase, the thermal stability of the composites to their mechanical and tribological properties. It could be shown that it is possible to tailor the microstructure of the metal matrix by severe plastic deformation using the carbon reinforcements. The matrix grain size and the content of the reinforcement phase had a strong influence on the hardness and strength of the MMCs. By adding 5 wt% (or 15 vol%) nanodiamonds in silver MMCs, for example, the microhardness could be enhanced by up to 70 % and the tensile strength by up to 60 % compared to severe plastically deformed pure silver. The achieved values were furthermore 5 times higher than in microcrystalline pure silver. The reinforcement content had also a strong influence on the ductility and on the anisotropy of the mechanical properties. Additionally, an excellent thermal stability using nanodiamonds as reinforcement phase could be achieved. Even after annealing at very high temperatures (>70% of the melting temperature of the metal), the matrix grain size remained very stable with sizes of about 100 nm. We also showed that the application of severe plastic deformation cannot be considered without reservation, since significant damage was introduced in the carbon based reinforcements after a certain amount of deformation. This further affected the tribological properties of the MMCs. The results of the project can be used as a basis for creating stable bulk nanocrystalline carbon reinforced MMCs with different metallic matrices in the future, which will increase potential applications. The success of the project can be easily seen by the vast number of presentations at international conferences (more than 19 including 4 invited talks) and 11 publications in the renowned international journals, which have been published up to date.