Thermal stability of reactive sputtered tungsten oxide coatings N.M.G. Parreira, T. Polcar, A. Cavaleiro ⁎ ICEMS-Grupo de Materiais e Engenharia de Superfícies, Faculdade de Ciências e Tecnologia da Universidade de Coimbra-Pólo II-Pinhal de Marrocos, 3030-788 Coimbra, Portugal Received 10 November 2006; accepted in revised form 5 January 2007 Available online 17 January 2007 Abstract The thermal stability of different W–O coatings, W 100 ,W 90 O 10 ,W 54 O 46 ,W 30 O 70 and W 25 O 75 , were studied by in-situ X-ray diffraction at elevated temperatures up to 900 °C. The coatings were deposited by DC reactive magnetron sputtering from a pure tungsten target in an Ar + O 2 atmosphere onto Fecralloy alloy. The evolution of the structure of the coatings was studied in both protective and oxidant atmospheres. Three groups of films were identified: (1) W 25 O 75 which showed structural evolution following the W–O phase diagram; (2) amorphous O-deficient WO 3 whose structure followed the W–O phase diagram either as WO 3 or as its chemical composition depending on annealing in oxidant or protective atmospheres, respectively; and (3) low O-content crystalline films that oxidized from 500 °C. © 2007 Elsevier B.V. All rights reserved. Keywords: Tungsten oxide; Reactive sputtering; Structural characterization; Thermal stability 1. Introduction Tungsten oxide is a very important semiconductor material. It has been found to have great application as an electrochromic device [1], as a semiconductor gas sensor [2] or for use in catalytic activity [3] as confirmed by the great number of publications in the field. However, the majority of these studies dealt with tungsten trioxide, while sub-stoichiometric tungsten oxides have not been yet intensively studied. Moreover, tungsten is one possible material for the inner wall of a fusion reactor [4]. In the W–O phase diagram [5] the most important monophase regions are α-W, WO 2 and WO 3 corresponding to three oxidation states of tungsten, W, W 4+ and W 6+ [6]. There are also many phases with chemical compositions between WO 2 and WO 3 , such as W 18 O 49 ,W 24 O 68 ,W n O 3n-2 and W n O 3n-1 [5]. WO 3 itself is known as a polymorphous material that can exist as a function of the temperature, as triclinic, monoclinic, orthorhombic and tetragonal forms [7]; moreover, a hexagonal form can be synthesized under special conditions [8]. With low oxygen content, a metastable W phase, known as β-W but also attributed to β-W 3 O, can also be observed [9]. Among many deposition processes of tungsten oxides, reactive sputtering is one of the most versatile. Metastable structures can be easily deposited with this method, such as supersaturated solid solutions of oxygen in the α-W phase, nanocrystalline or amorphous phases, as reported in a previous author's work [10]. In that study, W–O coatings were deposited and structurally characterized at room temperature. The structure of the coatings could be divided into four distinct zones: i) crystalline films with oxygen in solid solution in the b.c.c. α-W or f.c.c. β-W (or β-W 3 O) phases [for oxygen content lower than 30 at.%]; ii) in the range of oxygen from 30 up to 67 at.%, the coatings were amorphous; iii) a transition region with a quasi-amorphous structure was defined in the range [67 at.% b O b 75 at.%]; and iv) WO 3 nanocrystalline structure was observed in the case of the coatings with compositions close to that of stoichiometric WO 3 . All the coatings with oxygen content higher than 30 at. % exhibited short-range order and, thus, it would be important to investigate their crystallization temperature. On the other hand, all coatings deficient in oxygen should, in a normal atmosphere, incorporate oxygen to reach stoichiometric composition. Therefore, knowledge of the structural evolution Surface & Coatings Technology 201 (2007) 7076 – 7082 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +351 239 790 745; fax: +351 239 790 701. E-mail address: albano.cavaleiro@dem.uc.pt (A. Cavaleiro). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.01.019