Thermomechanical properties of amorphous metallic tungsten-oxygen and tungsten-oxide coatings

Abstract

Metallic amorphous tungsten-oxygen and amorphous tungsten-oxide films, deposited by Pulsed Laser Deposition, are characterized. The correlation is investigated between morphology, composition, and structure, measured by various techniques, and the mechanical properties, characterized by Brillouin Spectroscopy and the substrate curvature method. The stiffness of the films is correlated to the oxygen content and the mass density. The elastic moduli decrease as the mass density decreases and the oxygen-tungsten ratio increases. A plateau region is observed around the transition between the metal-like (conductive and opaque) films and the oxide ones (non conductive and transparent). The compressive residual stresses, moderate stiffness and high local ductility of compact amorphous tungsten-oxide films are interesting for applications involving thermal or mechanical loads. The coefficient of thermal expansion is quite high (8.9 · 10 −6 K −1 ), being strictly correlated to the amorphous structure and stoichiometry of the films. Upon thermal treatments the coatings show a quite low relaxation temperature of 450 K. Starting from 670 K, they crystallize into the γ monoclinic phase of WO 3 , the stiffness increasing by about 70%. The measured thermomechanical properties provide a guidance for the design of devices which include a tungsten based layer, in order to assure their mechanical integrity. mitigating or favoring crack formation. Similarly, in high temperature applications, a significant mismatch between the coefficients of thermal expansion of the coating and of the substrate can induce high interface stresses, with possible coating delamination and device failure. More specifically, in an electrochromic system the W oxide film is part of a complex multilayer system: it is deposited on a transparent conductor, like ITO, and faces the electrolyte, solid or liquid, containing the ions responsible of the electrochromic effect, and can be subject to various and very different stress states [15]. Moreover, in some applications (e.g. solar-cells, thermophotovoltaic) tungsten oxide coatings operate at temperatures above room temperature [16]; this could induce phase transition or recrystallization, with a consequent variation of the as-deposited properties. Although the thermomechanical properties of tungsten based coatings can be crucial for the design of devices which exploit them, relatively fewer studies have investigated the relationship between their nanostructure, composition and mechanical properties [17,18,19]. The goal of this work is to achieve a more comprehensive understanding of the effects of structure, morphology and chemical composition on the thermomechanical properties of different systems of amorphous W-O and WO x coatings, providing useful results for the design of devices. We investigate amorphous films characterized by different oxygen/tungsten ratios and morphologies. To produce them we selected the Pulsed Laser Deposition (PLD) technique, which allows a significant versatility in tailoring the str...

Study of the Thermal Annealing on Structural and Morphological Properties of High-Porosity A-WO3 Films Synthesized by HFCVD

“…The Raman spectra for both the as-synthesized samples and the samples annealed at 100 °C do not show any peaks, indicating that the porous films are amorphous, which is in accordance with the XRD results. These characteristic lines in Raman spectra have been reported earlier in [53], where authors observed Raman peaks only when samples were annealed above 400 °C.…”

“…The slight shift towards lower wavenumbers of peaks at 272 and 719 cm −1 could be due to the growth occurring in a non-equilibrium process, so the synthesized films are obtained with high stress (compressive or tensile) mainly because of the gradient temperature between filament and substrate. This shift can also be attributed to the increment in the particle size [53,61,62].…”

Post-mortem analysis of tungsten plasma facing components in tokamaks: Raman microscopy measurements on compact, porous oxide and nitride films and nanoparticles

“…According to [70], tungsten oxides with O content higher than ≈73 % behaves as a dielectrics whereas for concentrations lower than 73 % the sample behaves as a metal. According to [71], tungsten nitrides are good absorbers but with k values which can be ≈ 60% lower than the corresponding value for tungsten.…”

Ultrasonic and Spectroscopic Techniques for the Measurement of the Elastic Properties of Nanoscale Materials

“…The combination of the techniques gave access to a wide range of frequencies, allowing detailed determination of the elastic properties of the film [113]. Brillouin spectroscopy turned out to be a useful characterization tool also for other types of films of interest in materials science, like boron films [114] and amorphous and nanocrystalline tungsten films [115].…”