Available online at www.sciencedirect.com Journal of the European Ceramic Society 30 (2010) 3131–3136 Plasma sprayed TiO 2 : The influence of power of an electric supply on particle parameters in the flight and character of sprayed coating Pavel Ctibor ∗ , Milan Hrabovsk´ y Institute of Plasma Physics, v.v.i., Academy of Science of the Czech Republic, Za Slovankou 3, 182 00, Praha 8, Czech Republic Received 25 November 2009; received in revised form 20 May 2010; accepted 29 May 2010 Available online 8 July 2010 Abstract The influence of input power on sprayed powder and final coating was quantified for a water-stabilized plasma spray torch (WSP ® ) and ceramic coating formed from titanium dioxide (TiO 2 ). All other spray setup parameters were secured during the experiment with electric supply power as the only variable factor. In-flight particles were characterized by a Doppler particle velocimetry, the microstructure of the coatings was observed by microscopic techniques with computer image analysis, and phase composition was studied by X-ray diffraction. Various mechanical properties were measured – microhardness, surface roughness, and wear resistance in a slurry. Also other particular physical characteristics of the coatings – reflectivity and bandgap energy – were observed in their dependence on the supply power because they are associated with applications of the coatings. The higher the power the higher the coating quality will be: both its microstructure and mechanical performance. This substantial difference has the same trend for both power supplies utilized for the testing. © 2010 Elsevier Ltd. All rights reserved. Keywords: Plasma spraying; DC plasma torch; Power supply; TiO 2 ; Wear testing; Bandgap 1. Introduction In plasma spraying, electric power is one of the most important parameters controlling the product character. 1–3 By changing the quantity of heat available for plasma jet formation it is possible to change the quality of plasma sprayed coatings. 1 Particles of the feedstock powder attain a different degree of overheating and different kinetic energy for their movement towards the substrate when electric power is changed. This could be quantified by in-flight monitoring of the velocity, temperature and apparent diameter of the particles. Resulting coatings should exhibit different microstructure and properties, namely, mechan- ical ones. 1–4 Regarding the water-stabilized plasma spray torch the influence of power supply on produced coatings has never been systematically studied before. Plasma sprayed coatings starting from TiO 2 feedstock pow- der usually consist of non-stoichiometric suboxides. 1,2,5 TiO 2 starts to lose oxygen at a temperature above 1600 ◦ C in atmo- spheres. The partial pressure of oxygen required to reduce TiO 2 ∗ Corresponding author. Tel.: +42 266053717; fax: +42 286586389. E-mail addresses: ctibor@ipp.cas.cz, ctibor@ensil.unilim.fr (P. Ctibor). to Ti 2 O 3 , Ti 3 O 5 or Ti 4 O 7 is of the order of 10 -5 Pa at around 2000 ◦ C. During plasma spraying in the air, oxygen partial pres- sure does not go below 1Pa. However, oxygen loss occurs resulting in the formation of non-stoichiometric titanium oxide. Oxygen deficient phases of TiO 2 (especially Ti 3 O 5 , Ti 6 O 11 ) are sometimes observed in the coatings by XRD, namely, when the feedstock is an agglomerated titania nanopowder. 1 The loss of oxygen leads to the formation of titanium suboxides TiO x . In the range 1.75 ≤ x ≤ 1.85 (i.e., between Ti 4 O 7 and Ti 7 O 13 ) a homol- ogous series (Magneli phases) of ordered structures with planar stacking faults exists. 5 The formation of non-stoichiometric and oxygen deficient lattice in rutile TiO 2 is due to the formation of structural defects. This material is the subject of longstanding interest by the authors because of the variability of its promising properties – mechanical, namely, sliding and wear, as well as electrical and photocatalytic properties. Oxygen deficiency brings such a high variation in dielectric permittivity and the quality factor of tita- nia, which facilitates studying it by a microwave microscopy. 6 Also Raman spectroscopy has been performed on a laser-treated surface and in reference to Ti 4 O 7 /Ti 5 O 9 Magneli samples. 7 Low friction regimes were predicted near TiO 1.93 to TiO 1.98 and also near TiO 1.70 . 8 The tendency of the rutile stoichiometry (ergo, 0955-2219/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2010.05.029