What would be the most appropriate parameters, namely, gas temperature and gas pressure, for cold spraying of a given feedstock material? This question is the focus of the present contribution. Initially, it is shown that main coating characteristics can be described as a unique function of a dimensionless parameter, defined as the ratio of particle velocity to critical velocity. Subsequently, these velocities and the respective ratio are worked out and expressed explicitly in terms of key process and material parameters, such as gas temperature and particle size. In this way, final properties of cold-sprayed deposits are linked directly to primary cold-spray parameters. Moreover, it is shown that the window of deposition, as well as the relationship between final properties of the deposit and the spraying conditions, can be incorporated conveniently into simple 2-D diagrams, showing contours of the velocity ratio, or the desired coating property, on the plane of the primary process parameters. Based on these diagrams, the process parameters related to a given coating property can be identified and selected, without a need to refer to intermediate variables such as particle velocity. The paper includes examples of the application of these maps for cold spraying of copper.

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.