In cold spraying, the required heat for bonding is provided by plastic deformation of the impacting ductile particles. Therefore, cold spraying is a well-established method for metal on metal coatings. However, few authors have investigated the impact phenomena and layer formation process for impacting brittle ceramic particles on ductile metal surfaces. For this study, titanium dioxide (TiO 2 ) on metal surfaces was chosen as a model system, and layer formation on aluminum, copper, titanium and steel substrates was investigated by SEM, TEM, XRD and Raman spectroscopy. The results show that the deposition efficiency depends on spray temperature, powder properties, and in particular on substrate ductility, even for an impact of ceramic particles during a second pass over already coated areas. High-resolution TEM images revealed no crystal growth or phase transitions at the ceramic/metal interfaces. Nevertheless, a clear dependence of the photocatalytic activity on spray parameters and substrate material could be observed. Cold-sprayed TiO 2 -coatings have potential applications in biomedical implants or as photo-catalytic functional systems

In cold spraying, for successful bonding powder particles have to exceed a critical velocity upon impact, which is dependent on properties of the particular material. So far a detailed physical explanation of the underlying mechanisms of bonding is still lacking. In the present study, computational materials science and high resolution microscopical methods are used to investigate the microstructural development at the particle-substrate interface. The modelling can show that the critical velocity is related to a transition in the flow-behaviour on the particle or substrate surfaces. The presence of microstructural features predicted by modelling in detail could be confirmed by SEM and TEM analyses of internal interfaces of cold sprayed coatings. By describing the mechanisms of bonding, the calculations could also demonstrate the influence of material properties or microstructural defects on the conditions for successful impacts. With respect to particular powder properties, the results should promote the development of optimum process parameters in cold spraying.

This paper investigates the microstructure and properties of TiC-based nanocrystalline coatings produced by HVOF and VPS processes. It assesses the extent to which nanocrystalline cermet powders can be tailored for specific applications using high-energy milling and the commercial viability of HVOF sprayed coatings made from such powders. Paper includes a German-language abstract.