Synthesis, structural and mechanical characterization of sputtered tungsten oxide coatings

Abstract

Tungsten oxide coatings were deposited without substrate bias by DC reactive magnetron sputtering of a tungsten target using oxygen as reactive gas. By tuning the partial pressure of oxygen ( p O 2 /p Ar ) between 0 and 4, the oxygen content of the films was changed from 0 to 75 at.%. The structure of the films (investigated by X-ray diffraction) depends on their oxygen content. For low oxygen contents, the a-W and h-W 3 O phases were observed (< 30 at.%), and with the increase of oxygen content (30 at.% < O < 67 at.%) the structure became amorphous. A transition region was obtained for oxygen content between 67 at.% and 75 at.%, and when O > 75 at.%, a nanocrystalline (WO 3 ) structure was reached.The hardness and Young's modulus were evaluated by depth sensing indentation. The decrease in hardness followed the four different ranges of chemical compositions accordingly, from å 23 GPa for pure W down to å 7 GPa for WO 3 films. A similar behaviour was observed for the Young's modulus, which ranged from 450 GPa to 150 GPa. The cohesion/adhesion of the films were investigated using a scratch-test apparatus. These coatings displayed a low adhesion (critical load, L c < 15 N) to the steel substrate because the depositions were carried out intentionally without an adhesion interfacial layer. D

Thermal stability of reactive sputtered tungsten oxide coatings

“…3. At room temperature, as shown in previous work [10], these samples exhibit a quasi-amorphous structure with a coherent domain of diffraction close to 1-1.2 nm. However, they have a different short-range organization: the sample W 30 O 70 has a similar structure to the W 25 O 75 sample, with two broad peaks centred in positions close to 30 and 40°whereas the W 54 O 46 coating only exhibits one broad peak centred at 2θ = 45.2°, which could be attributed to the position of the (110) α-W line [10].…”

“…Bearing in mind the achievements of previous research [10], from all the deposited coatings ( Table 1) only some of them, gathered in three groups, were selected for the structural characterization at elevated temperature, according to their composition and structure at room temperature: i) the W 25 O 75 coating, since no further oxygen incorporation is possible and only phase transformations were expected; ii) two amorphous coatings at room temperature, W 54 O 46 and W 30 O 70 , which were analysed in protective atmosphere to analyse crystallization, and in air to investigate the incorporation of the oxygen in the coating and consequent formation of new phases; iii) the crystalline W 100 and W 90 O 10 coatings, which were analysed only in oxidant atmosphere since these coatings already exhibited crystalline phases at room temperature, α-Wand a mixture of α-Wand β-W (β-W 3 O), respectively. The structural evolution of the W 25 O 75 coating with increasing temperature is shown in Fig.…”

“…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.…”

Modelling of Magnetron Sputtering of Tungsten Oxide with Reactive Gas Pulsing

“…In our case, the layered structure is rather pronounced due to the relatively high deposition rate of more than 2 nm Á s À1 , which prevents oxidation of the already deposited film by diffusion. [19] Since the pulsing parameters influence the stoichiometry, deposition rate and thickness of each single layer and define the interfaces, they can be tuned according to desired properties of the deposited coating. A prolonged transition time is beneficial to obtain higher deposition rates.…”

Tungsten oxide with different oxygen contents: Sliding properties

“…There are few research works dealing with tungsten oxide with different oxygen contents focused mainly on electrical and optical properties [2]. Only recently, the structural and the mechanical properties of the W 100Àx O x , with x from 0 to 75 at%, have been studied in an author's work [3]. Moreover, the tribological properties of tungsten oxides remain almost unknown.…”