Effect of thermal spray processes on anatase–rutile phase transformation in nano-structured TiO₂ photo-catalyst coatings

TiO₂ photo-catalytic coatings were deposited using three different thermal spray techniques, namely, flame, plasma and high velocity oxy-fuel (HVOF). TiO₂ nanopowders were used as feedstock. X-ray diffraction analysis and scanning electron microscopy were used to study the deposited TiO₂ coating. It was concluded that a high anatase content of 80 vol.% can be achieved for the coating deposited by HVOF if kerosene is used as fuel. On the contrary, the anatase contents in the coating deposited by the flame and plasma spray processes were reduced to 29 and 5 vol.% respectively due to their structural transformation of anatase to rutile. Microstructure shows that the as sprayed HVOF TiO₂ coating had nanostructure characteristics with smaller grain size than feedstock powder. It was found that the structural transformation of anatase to rutile is significantly influenced by the thermal spray processes. Liquid fuelled HVOF technique can prevent this transformation.