Advanced design of highly functionalised CNT reinforced composites:

Carbon nanotubes (CNT) are interesting materials to be used in electrical applications, such as electrical contact components permitting to obtain better properties than the currently used Ag-MeO. The difficulties of producing MMC reinforced with CNT are on the one side, to purify the pristine CNT, and on the other side to avoid the bundling, which can produce very inhomogeneous samples. The bundling is produced because pristine CNT have low solubility due to weak interactions with solvent molecules as opposed to strong ?-? interaction between individual nanotubes. Therefore, the required increase in solubility could be achieved by attachment of functional groups that efficiently interact with solvents and, due to steric effects, decrease the interaction between neighbouring nanotubes. In the present project, different approaches of nanotube functionalisation are proposed: i) covalent bonding of functional groups, ii) non-covalent interaction, and iii) electrochemical deposition of metals. The first approach will be to induce the reaction of the aromatic ring with aryl cations, the formation of acyl derivatives, and the hydrolysis of organometallic compounds. The second will be the adsorption of soluble polyanilines, polynuclear aromatic molecules or polyelectrolites. The polyanilines will be produced by combinatorially selected chemical reactions on the polyaniline backbone. The aromatic compounds will be produced by diazonium coupling of aromatic amines and activated aromatic rings (phenols, amines, polynuclear molecules). The latter will be produce by direct reduction of metals on the surface of the CNT. The compounds will be tested by high throughput screening in terms of their ability to solubilise multiwall carbon nanotubes (MWNT). The functionalised CNT are then used as reinforcements for nickel-based composites. It is well known that nickel is an acceptable electrical and thermal conductor and has shown a better arcing resistance than other good conductors (Ag and Cu). A number of techniques have been developed in order to obtain Ni-CNT composites. We propose a consolidation technique based on the press-sintering procedure. After the debundling and functionalisation of the CNT, metallic powders will be mixed with them and will then be compressed and sintered. Once the composite is obtained, a complete microstructural (including 3D-architecture), thermal, electrical and mechanical characterisation will be carried out.

Advanced design of Al-MMC and effects of interfaces

MMC properties have direct relation with the microstructure, grain shape and size, impurities and mainly, the bonding between the constituent elements. Aluminum based alloys have been extensively studied and used due to their low density and high atmospheric corrosion resistance. Moreover, they can offer mechanical resistance similar to ferrous alloys, which can be used for different applications. Metal matrix composites can be produced through several techniques being Rheological Rolidification in Semisolid State (RSSS) ideal for obtaining adequate grain size and shape. In using the RSSS-technique, microstructure can be strongly modified and ceramic particles (or fibres) can be distributed in the metal matrix in accordance with the processing parameters. Thus, hardness and mechanical resistance which are difficult to obtain by conventional techniques can be achieved through RSSS. Another production process consists in mixing the metal powder with particles, compacting and sintering. The necessary blending of, for instance, short fibres with powder prior to consolidation hot-pressing makes the final MMC susceptible to defects owing to imperfect metal/fibres interfaces. The following systems will be studied:
• Al – TiB2: these samples will be produced by RSSS. Alloy elements and impurities inherent to the method can improve or degrade mechanical properties. Boron combines with other metals to form borides, such as Al2B and TiB2. Titanium boride forms stable nucleation sites for interaction with active grain-refining phases such as TiAl3 in molten aluminium, but metallic borides reduce tool life in machining operations, and in coarse particle form they have detrimental effects on mechanical properties and ductility. For environmental and economic reasons, recycling of Al is extensively used for obtaining Al-alloys (recycling process needs 25% of the electrical energy needed for primary production). However, one of the most important problems in recycling Al is the classification and separation of the scrap. Isolation of some elements from scrap, which plays a crucial role in the properties of the alloy, is not easy. The purpose of the project is to study the effects of different elements (impurities and alloy elements) on the Al alloy.
• Al - Al2O3; Al(Cu) - Al2O3; Al(Cu) – SiCO: These samples have already been produced in one of the groups by powder metallurgy. The effect of contact between grains of Al and Cu in the densification of Alumix 13 (Al-4%Cu), Alumix/short alumina or silicon carbide fibres, via the effect of the liquid phase produced by reaction of Al and Cu at temperatures higher than 548ºC has been addressed. The spread of such liquid (within the grain boundaries and into pockets of alumina fibres) is very rapid and the distribution of the liquid tends to be non-uniform. For composites made of pure Al matrix and reinforced with short amorphous SiCO Nicalon fibres the matrix bonds very well to the fibres, enabling optimum creep behaviour for such composites. The incidence of the interface state (modified by selected heat treatments of the MMC) on certain properties, like the thermal expansion behaviour after cycling within 20 and 500ºC, will be assessed during the investigation. Additionally, MMC samples of Al-Al2O3 and Al-Cu-Al2O3 will be prepared by semisolid solidification and the differences in the microstructures, phases and properties with respect to the product by means of powder metallurgy will be evaluated.
Modern characterisation techniques like Focused Ion Beam (FIB) / Scanning Electron Microscopy (SEM) Dual Beam system and Transmission Electron Microscopy (TEM) on FIB-preparated foils, will be used to characterise the matrix/particle interfaces in order to clarify the formation of reaction products. 3D analysis by means of Synchrotron Tomography (ST) and FIB-tomography will be used to study the distribution of particles and to correlate them with mechanical properties: yield stress, traction resistance, elastic modulus, fatigue resistance.

Advanced production and characterisation of nano-structured bulk metal-ceramic composites

Metal-ceramic composites for wear applications involve materials composed of hard resistant phases embedded in a tough metal binder phase. Most well known materials for wear applications are cermets, hard metals and Fe-based MMC. The properties and functionalities of these materials are closely related to the grain size of the hard particles and their interaction with the metal binder phase. Wear resistance increases with decreasing hard particle grain size. Processing and manufacturing of nano-metal-ceramic composites deals with the challenge of controlling the grain growth during manufacturing. In this context, we use a hot press sintering process. This is a processing technique with a high potential to process nano-bulk materials with good interface bonding, where a reasonable control of grain growth during sintering is achieved. However, the main mechanisms involved in hot pressing, and the interplay between the manufacturing parameters (heating rate, pressure, etc.) and densifying materials (initial grain size, amount of hard phases, binder-metal phase type) must be clarified. Another point is the comparison with the microstructure formation and properties of MMC produced by conventional techniques (sintering, rheocasting) and hot pressing consolidation. In the framework of this project we aim to clarify both topics using the following strategy: • Production of nano-bulk materials of the type “hard phase particle–metal binder” by hot pressing compaction.
• Production of MMC of the type “hard phase particle–metal binder” by conventional production techniques.
• Determination of the sintering mechanism and interplay between hot press parameters and material characteristics with “model alloys” (i.e. Cu, Al or Ag, and particle reinforced Cu and Ag alloys).
• Thermodynamic and kinetic modelling of diffusion processes between particle and metal.
• Fully microstructure characterisation and mechanical properties measurements by TEM, FIB-tomography, ST, synchrotron X-ray/neutron diffraction, nano-hardness, nano-scratch tests.

Advanced production and characterisation of nano-structured coatings

The wear resistance of MMC can be further improved by coating the surface with thin films. Typical coatings are multilayers combining nitrides (TiN), carbonitrides (i.e Ti(C,N), Zr(C,N)), oxides (Al2O3) or combinations like (Ti,Al)N. The typical manufacturing processes are Chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD) or plasma-assisted deposition processes. By carefully adjusting the deposition parameters, the architecture and composition of coatings can be controlled. A recent development shows the possibility of producing nano-Ti(C,N) coatings with “composite characteristics”. The surface treatment prior to the coating process plays a very important role to enhance adhesion of the coatings to MMC. This method aims at grading the surface in order to reduce the pile-up of stresses between the MMC and the coating, which occur during wear applications. Under cutting conditions these coated MMC will undergo cyclic thermomechanical loads as well as wear processes. Thus, the stability of the microstructure and the residual stress state as well as the mechanisms causing damage initiation should be investigated during thermomechanical fatigue and after different wear conditions using non-destructive synchrotron X-ray diffraction and 3D-micro-tomography techniques as well as high-resolution microscopy. Macro residual stresses caused by temperature gradients during conventional sintering and Spark Plasma Consolidation (SPC), as well as phase-specific micro residual stresses due to the thermoelastic mismatch between the phase constituents will be analysed with enhanced spatial resolution (?100 (m3) using synchrotron X-ray diffraction and separated from each other. These investigations should reveal the stability of the residual stress states under friction and dynamic loads. The residual stress gradients and their stability in the nano-coatings will be assessed by synchrotron XRD after different wear/cutting conditions. The following strategy is foreseen: • Production of nano-coatings by CVD and PVD on nano-MMC substrates, and nano-MMC substrates with prior surface treatment. • Determination of the influence of deposition parameters and surface treatment on coating architecture. • Investigations of internal stresses by X-ray techniques and high energy beams (synchrotron and neutron diffraction) between the MMC and the coating, also as a function of temperature. • Full characterisation of microstructure and mechanical properties by TEM, FIB tomography, nano-hardness, nano-scratch tests.

MMCs reinforced with diamond particles

MMC reinforced with diamond particles are in development for heat sink applications in power electronic devices. Diamonds with the best-known thermal conductivity (~1000Wm-1K-1), are promising as reinforcements to improve thermal conductivity and also reduce the thermal expansion of a MMC, which is important to avoid delamination between the heat sink and the ceramic substrate. Residual stresses caused by the thermal expansion mismatch are transported from the macroscopic interface within the electronic package into the inner structure of the MMC. High amounts of matrix stresses can be expected causing debonding and voids within the MMC during thermal cycles. The evolution of the debonding and voids as well as of the residual stresses during thermal cycling can be investigated in situ by simultaneous ST and synchrotron diffraction. This will help to understand the influence of the 3D architecture of the reinforcements and their effects on the long-term stability of the MMC and its thermal fatigue resistance. The debonding mechanisms between the metal matrix (Al or Cu) and the diamonds will be visualized using tomography. The goal is to investigate the elastic deformation by stress evaluation and the plastic deformation by void kinetics in the matrix during thermal cycling in relation to differences in architectures of particle reinforced MMC. The samples have already been produced and in this project we will concentrate on the stress measurements.

Modern characterisation of MMC

In the above mentioned systems it is of great importance to investigate the interfaces between the particles and matrix. This can be achieved by TEM. However, it has to be possible to define the place for the production of the TEM foil with a precision of some nanometers. This target preparation can be done by FIB, available at one of the groups of the network. The use Electron Backscatter Diffraction (EBSD) allows the determination of phases and the orientation of the crystals, both of the matrix and of the reinforcing particle. The challenge is to prepare the surface of the sample with enough quality (free of lattice deformation), since EBSD is very sensitive to surface quality. The preparation of heterogeneous samples can be very difficult. Different methods have to be investigated for the different systems, like for example: mechanical polishing, electrolytic polishing, ion polishing, and focussed ion beam polishing. Owing to the differences in physical and mechanical properties between the metal matrix and the reinforcements, internal stresses are also an intrinsic feature of MMC. Macro residual stresses caused by temperature gradients during the fabrication process as well as phase-specific micro residual stresses due to the thermoelastic mismatch between the phase constituents can be analysed with enhanced spatial resolution ((100 (m3) using synchrotron X-ray diffraction and. For the residual stress analysis in WC- and Fe-base MMCs neutron diffraction might be preferred due to its higher penetration depth even in case of high-Z materials, such as WC. The residual stress gradients in nano-coatings can be assessed by synchrotron XRD under constant penetration depth, i.e. by a coordinated variation of the diffractometer angles (, ( and (. These investigations should reveal the stability of the residual stress states under friction and dynamic loads.

3D characterisation of MMC

Different tomographic methods with different spatial resolution scales are available: • Microtomography with X-rays micro focus tube (resolution ~ 8µm). • Synchrotron tomography (resolution ESRF ID19 >300nm, with mirrors ID22>50 nm). • FIB-tomography as serial slices technique (resolution ~5 nm) with its different contrast possibilities such as SE, EDX, EBSD. • TEM-tomography and Atom Probe Tomography (APT) (resolution <1nm). In all of these Methods, a set of voxels are produced that can be processed and evaluated quantitatively with adequate 3D-image analysis software. In this way, all information can be used to create or validate microstructure models and simulations. In the last years, the X-Ray tomography has been widely used in the field of materials science. High resolution synchrotron tomography has been applied to Ti-alloys achieving 0,3 µm resolution in phase contrast images of alpha and beta phases. In recent years, FIB-nano tomography has been developed. This offers new possibilities not only due to the dimension scale but also due to its additional contrast modes. These methods will be tested regarding their applicability in the different composite materials. Their complementarities regarding the different scales and hierarchies of microstructures will be studied, i.e. the spatial resolution capability on one hand and the size of the volume to be analysed on the other hand.

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