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.

Cold spraying has been developed as a high-quality coating process. However, the deposition materials were limited as metallic materials. In this paper, titanium dioxide (TiO 2 ) ceramic particles are deposited on several substrate materials and formed thick coatings, making this approach suitable for a wide range of photocatalytic applications. In order to understand the adhesion mechanism of solid ceramic particles, the structures of feedstock particles are carefully observed with a scanning electron microscope (SEM) and a high-resolution transmission electron microscope (TEM). We find a porous structure which is agglomerated with nano-scaled primary particles. It is assumed that the break down phenomenon occurs due to the cold spray process and influences the adhesion of ceramic particles. The primary particles are bonded within a single crystal. This particular structure is the main factor to make adhesion between the particle and the substrate. SEM and TEM analysis clearly reveals adhesion mechanisms related to the impact of spraying ceramic particles toward the substrate.

Hydroxyapatite Composite (HAC) Coatings made by Low Pressure Gas Dynamic Spray (LPGDS) on the grit blasted Ti alloy substrate were studied. The powder feedstock for the HAC coating by LPGDS was prepared by mixing HA powder with Phosphate Glass particles in polyvinyl alcohol, drying and stirring. As-sprayed HAC films on the substrate were heat-treated by two steps (400-500°C and 600-900°C) and their physical characteristics were investigated by scanning electronic microscopy (SEM), X-ray diffractometer (XRD). During sintering-heat treatment, HA particles were consolidated and melted to form a dense and homogeneous coating layer which exhibits the grain structure. Application of HA-glass feedstock is found to be effective for Cold Spray HA based coatings.

The synthetic hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ) is a very useful biomaterial for numerous applications in medicine, especially in view of using as fine powder for suspension plasma spraying. The powder was synthesized using aqueous solution of ammonium phosphate (H 2 (PO 4 )NH 4 ) and calcium nitrate (Ca(NO 3 ).4H 2 O) in carefully controlled experiments. The synthesized fine powder was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The powder was formulated into water and alcohol based suspension and used to carry out the initial tests of plasma spraying onto titanium substrate. The phase analysis of sprayed coating was made with the used of XRD.