Top 1: First, metal nitrides were studied for electrochemical energy storage technologies. However, its electrochemical stability was insufficient. Therefore, we moved on to carbon nanoparticle/metal oxide systems. Using hydrothermal synthesis, we showed a 10-fold increase in energy storage capacity when using manganese oxide/carbon hybrids. Using atomic layer deposition, we achieved battery-like energy storage capacities when using titania/vanadia metal oxides. Using nanoscale engineered carbon fiber/carbon nanoparticle electrodes, we enhanced the energy storage capacity by a factor of 5-6. This was possible by capitalizing on the high electrical conductivity of the carbon fibers and the large surface area provided by carbon nanoparticles. A key factor to enable the high performance was the use of redox-active surface groups at the fluid/solid interface.
Top 2 and 3: production of composites reinforced with carbon materials. The best distribution of carbon nano particles (CNP) within the metallic matrix was achieved for the nanodiamond (ND)-containing composites, followed by onion like carbon (OLC) and by carbon nanotube (CNT). This feature is related primarily to the hybridization of the C atoms in the nanoparticles and in the case of OLCs and CNTs (both sp2) the difference is related to their shape, since CNTs tends to enhance their interlinking, resulting in larger agglomerates. A new developed processing route which takes advantage of the optimal dispersability of sp3 carbon (nanodiamonds), allows to control the sp3/sp2 ratio by a thermal treatment, and thus to tailor the global physical properties. Very high densification was achieved by hot pressing and by spark plasma sintering, without compromising the structure of the CNTs.
Top 2: with tribological tests and modeling of contact mechanics, we were able to identify the self-lubricating mechanisms brought by the CNPs and its influence on friction and wear. The embedded nanoparticles produce a continuous feeding of solid lubricant which reduces the overall coefficient of friction. This effect has been even enhanced by combining composites with a surface structuring and a coating with CNPs.
Top 3: test facilities were developed for studying contact resistance of composites. The thermal diffusivity and electrical conductivity of the composite could be slightly increased against the pure metal, having the reduction of contact resistance a much more significant effect, opening good perspectives for this material to be used as electrical contact. Low voltage sparking experiments showed that the composites have a reduced arc duration and energy input, which translates into a larger duty-life.
Top 4: cemented carbides with different binders were produced by hot pressing consolidation and liquid phase sintering, presenting a functionally graded microstructure regarding chemical composition, particle size of WC and hardness profiles. We developed a method to analyze in-situ the thermal stress behavior during an individual thermal cycle. Despite of the functionalization of the surface, the alternating stress behavior cannot be avoided indicating that the overall composition of the cemented carbide strongly affects the stress behavior of the system. The addition of Cr and (Ta,Nb)C to the WC-Co substrate enhances the corrosion resistance of the binder, reducing fatigue induced cracks. Differences between the coatings (ZrCN and TiCN) in thermomechanical experiments could be explained with the help of Finite Elements Simulations and is based on the different mechanical behavior. Finally, prototypes of different carbide variants and coatings could be tested in plane milling of motor blocks made of cast iron, showing that Zr(CN) coatings improved the resistance to crack formation and propagation.
