Papers by Author: Harald Bolt

Abstract: Thin TiCx films with a range from pure Ti to stoichiometric TiC (0 ≤ x ≤ 1) have been deposited on carbon-based materials by dual magnetron sputter deposition. The wetting behaviour of a Cu Ti alloy on TiCx coatings has been characterized using the sessile drop method. For these experiments a contact angle measurement device was constructed and successfully tested. Both, stoichiometric TiC and Ti coatings improve wetting dramatically. In between there was no significant wetting improving effect compared to the uncoated case. Subsequently, TiCx coated C/Cu braze joints have been tested on their ability to withstand mechanical loads and analysed with respect to their fracture behaviour.

Abstract: Thin graded W/Cu coatings plus subsequent heat treatment at 800°C are used to improve the interfacial adhesion between W and Cu. Specimen with ~500 nm thick graded W/Cu coatings were characterized and analyzed after thermal treatment at 550°C, 650°C and 800°C. At 800°C a significant change in the nanostructure is observed. The better adhesion is caused by W/W grain boundary diffusion processes and Oswald ripening of the nanometre-sized grains leading to their interconnection between each other and the W substrate. The phase and texture analysis of graded W/Cu indicates grain growth and diffusion processes of pure W and Cu. The stress analysis shows that the changes in the nanostructure of the W/Cu coatings correlates to the stress relieve of W at temperature starting from 650°C. After cooling of the coating to RT the residue intrinsic tensile stress is caused by thermal mismatches of the substrate and the 100% W layer of the coating.

Abstract: Copper matrix composites reinforced with silicon carbide fibres (SiCf/Cu) are considered as heat sink materials for the divertor of DEMO as they combine high thermal conductivity and good mechanical strength at high temperature. A new method was developed to synthesise a metal matrix composite (MMC) consisting of about 3-6 unidirectional reinforced layers (UD-layers). The UD-layers were prepared by two subsequent electroplating processes which allow to adjust various fibre volume fractions. These single UD layers were stacked with different relative fibre orientations (0°/0° and 0°/90°) and consolidated by vacuum hot pressing to form the MMC specimen. The thermal conductivity perpendicular to fibre direction was obtained by laser flash apparatus (LFA) measurements. It is about 310 Wm-1K-1 for electroplated copper (Cu) and above 200 Wm-1K-1 for MMC specimens with a fibre volume fraction of 8-13%. Due to the manufacturing process, boundaries within the matrix were found resulting in a reduction of the values. In addition, DSC (differential scanning calorimetry) measurements were performed which gave similar results.

Abstract: The mechanical properties of a SiC-fiber/copper matrix composite, reinforced with SCS-0 SiC-fibers ( 140µm, Specialty Materials), can significantly be increased by applying a Ti-Ta-C multilayer between fiber and matrix. This interlayer is deposited with a magnetron sputter device directly on the single fibers. By changing the deposition parameters of this sputter process the Ti-Ta-C interlayer can be optimized regarding fiber strength and fiber/matrix adhesion. Experiments with different deposition pressures, bias voltages and layer thickness’ were performed to increase the bond strength and the ultimate tensile strength when compared to the Ti-Ta-C reference sample.