Microstructural characterization of plasma sprayed TiO 2 functional coating with gradient of crystal grain size R. Tomaszek a , L. Pawlowski a, ⁎ , L. Gengembre b , J. Laureyns c , Z. Znamirowski d , J. Zdanowski d a Service of Thermal Spraying at Ecole Nationale Supérieure de Chimie de Lille, BP 90108, F-59652 Villeneuve d'Ascq, France b Laboratory of Catalyse of Lille at University of Science and Technology of Lille, F-59655 Villeneuve d'Ascq, France c Laboratory of Infrared and Raman Spectrochemistry at University of Science and Technology of Lille, F-59655 Villeneuve d'Ascq, France d Faculty of Microsystems Electronics and Photonics at Wroclaw University of Technology, Pl-50-372 Wroclaw, Poland Received 31 May 2005; accepted in revised form 25 October 2005 Available online 5 December 2005 Abstract The paper aims at development of a coating with a gradient in grain size. A first layer was deposited directly onto stainless steel substrate by plasma spraying of a coarse TiO 2 powder. This layer had thickness in the range from 30 to 50 μm. A second layer was deposited on the first one using plasma spraying of different aqueous suspensions of two TiO 2 pigments. The layer had thickness smaller than 10 μm. Morphologies of TiO 2 powder and pigments were characterized using a scanning electron microscope (SEM) and their sizes were tested with a laser sizer. Phase analysis of powder and pigments, made by X-ray diffraction (XRD), enabled only rutile phase to be discovered. A mixture of rutile and anatase was found in the suspension sprayed coatings. The presence of anatase in sprayed coatings was an interesting and original result, which was also confirmed by Raman spectroscopy. The formation of this phase was tentatively explained by a high cooling rate of small splats. The suspension plasma spraying was optimized in order to obtain homogeneous and dense deposits. The morphology of sprayed coatings was characterized using SEM. Crystal grain sizes were estimated using XRD. The sizes were in the range of a few tenths to one hundred nanometers in the initial powders. They were clearly smaller in the coatings sprayed using coarse powder but remained quite similar in suspension sprayed layers. Finally, X-ray photoelectron spectroscopy (XPS) was used to analyze quantitatively the chemical composition of powders and coatings. © 2005 Elsevier B.V. All rights reserved. Keywords: Suspension plasma spraying; Atmospheric plasma spraying; TiO 2 coatings; XRD phase analysis; Raman spectroscopy; Thermally sprayed coatings microstructure; XPS chemical analysis 1. Introduction TiO 2 is an oxide having a lot of applications such as, e.g.: • Coloring agent in mixture for painting; • Support for catalysis; • High permittivity dielectrics in capacitors; • Oxygen sensor; • Coating for wavelength dependent optical transmission. The material is often used as a thin film deposited by PVD or as a thick coating obtained by thermal spraying [1,2]. Among the emerging applications of the latter, two seem to be studied intensively at present: • Photocatalytic coatings [2–6]; • Electronic emission coatings [7,8]. The photocalytic activity seems to be promoted mainly by an appropriate phase composition of coatings. The coating should possibly include anatase phase. The tested coatings were pre- pared by vacuum or atmospheric plasma spraying (APS) of powders and by atmospheric plasma spraying of suspension [3–6]. The coatings for electronic emission were prepared using TiO 2 pure and alloyed with Al 2 O 3 by APS. Some of them were submitted to post spray laser treatment. The principal microstructural feature determining electronic field emission was the size of conducting grains [9]. Small size of the grains Surface & Coatings Technology 201 (2006) 45 – 56 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +33 320 33 61 65; fax: +33 320 33 61 65. E-mail address: lech.pawlowski@ensc-lille.fr (L. Pawlowski). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.10.033