In this study, an experimental approach to investigate basic dependencies on impact and bonding of agglomerated ceramic particles in cold spraying is presented. Single impact morphologies of ceramic particles obtained from wipe tests are correlated with data obtained from powder compression experiments with a modified nanoindenter. Different feedstock powders of agglomerated TiO 2 -nanoparticles were used and also partially heat treated. The powder shapes and sizes prior and after the compression tests were analyzed by confocal microscopy. The single particle impacts were characterized by SEM. Besides the expected influence of substrate material, substrate temperature, and spray conditions, the deformation and bonding of ceramic particles to metal substrates critically depend on the powder properties. To which degree particles fracture or contribute to layer formation upon the high-energy impact is highly correlated to their individual deformation behaviour in quasi-static compression tests.

Cold spraying was applied to deposit Ti2AlC on different substrate materials. The study of single impacts by scanning electron microscopy indicates that bonding of the first layer is mainly attributed to the deformation and shear instabilities of the substrates. Nevertheless, the irregularly shaped particles appear to flatten by the impact. This deformation seems to be attributed to local, internal shear, but also to internal fracture. By applying up to five passes under more sophisticated spray parameters, Ti2AlC - coatings with thicknesses of about 110 to 155 µm can be achieved. XRD analysis of the coating proves that the crystallographic structure of the feedstock can be retained during cold spraying. The coating microstructures show rather low porosity, but several cracks between spray layers. Successful build-up of more than one monolayer can probably be attributed to internal deformation and occurring shear instabilities within the highly anisotropic Ti2AlC - phase.

Functional titanium dioxide coatings have a high potential for photocatalytic applications. However, the high process temperatures of conventional thermal spray methods lead to undesirable phase transformations. Cold gas spraying seems to be beneficial, since for ceramic coatings no phase transformations have been observed so far. In this study, bonding of cold-sprayed ceramic particles on metal substrates is elucidated by investigating single impacts obtained from wipe tests. Different feedstock powders were used, consisting of agglomerated titanium dioxide nanoparticles. The single particle impacts were investigated by scanning electron microscopy and confocal microscopy. The results demonstrate that bonding of particles is influenced by the substrate material, substrate temperature, powder properties and spray conditions. In addition, powder characteristics seem to be of particular importance. The results allow for conclusions regarding favorable conditions for cold spraying of ceramics.

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

The metastable anatase structure of titanium dioxide (TiO 2 ) is photocatalytically active and thus able to purify air and water. This makes anatase TiO 2 a desirable material for instance in environmental technology. The present work provides a comparative study on the photocatalytic activity of TiO 2 coatings, prepared by different spray methods: conventional atmospheric plasma spraying, HVOF spraying, suspension spraying and cold spraying. The spray experiments were performed with anatase powder as feedstock and the spray parameters were optimized in order to obtain coatings with high photocatalytic performance. With respect to spray technique and spray conditions, different amounts of the photocatalytic active anatase phase could be retained. The photocatalytic activity of the coatings was evaluated by degradation of dichloroacetic acid. The results show that the photocatalytic activity is highly dependent on the amount of preserved anatase structure obtained under the different spray techniques.

Titanium dioxide in anatase phase structure has high antibacterial activity. For this study, titanium dioxide coatings on stainless steel were produced by cold spraying. The bactericidal effect of the coatings was tested with Pseudomonas aeruginosa bacteria at a high concentration of more than 107 CFU (colony-forming units) per milliliter. The bacteria were applied on the surface and exposed to UV light with a peak intensity of 360 nm. A kill rate of 99,99% was already achieved after 5 minutes, while the raw stainless steel reference did not show any significant reduction even after 60 min. The results show that cold-sprayed titanium dioxide coatings can serve as self-disinfecting surfaces.