Amorphous, ultra-nano- and nano-crystalline tungsten-based coatings grown by Pulsed Laser Deposition: mechanical characterization by Surface Brillouin Spectroscopy

Abstract

Pulsed Laser Deposition allows to obtain W and W-Ta alloy coatings with different nanostructures, monitored by X-ray diffraction. The correlation between such structures and the elastic properties is investigated for amorphous-like, ultra-nano-and nano-crystalline coatings obtained by tuning the gas pressure during deposition, annealing temperature and Ta concentration. The full elastic characterization is achieved by surface Brillouin spectroscopy, interpreted by isotropic and anisotropic film models. Amorphous like coatings are obtained with He pressures of tens of Pa. In comparison with bulk W, they have lower stiffness, by about 60%, closely correlated to the mass density (lower by about 40%). In the nanocrystalline regime the stiffness is more correlated to the average grain size, approaching the bulk values for increasing crystallite size. Vacuum annealing of amorphous like coatings leads to the nucleation of ultra-nano crystalline seeds, embedded in an amorphous matrix with intermediate values for mass density and stiffness. Here, the stiffness results from an interplay between the crystal size and the density. Alloying with Ta leads to properties which are consistent with the lever rule in the nanocrystalline regime, and deviate from it when the higher Ta concentration, interfering with crystal growth, induces an ultra-nano crystalline structure.

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

“…However, tree-like nanostructure in porous samples can induce a substantial anisotropy of the mechanical properties of the films, such as the in-plane properties can differ from the outof-plane ones. Possible anisotropy effects on the elastic moduli of nanostructured tungsten films measured by Brillouin spectroscopy have already been considered [23]. The elastic moduli reported here for porous samples, computed under the isotropic assumption, can be seen as lower bounds for the real anisotropic moduli Brillouin spectra recorded for a-W(O) and a-WO 3−x samples are shown in figure 6a and 6b respectively.…”

“…The minimization is performed with C 11 and C 44 as the only free parameters, for a fixed value of ρ, obtaining the most probable estimates for C 11 and C 44 . The number of the experimentally observed modes determines, in turn, the accuracy in the determination of the elastic properties of the films [23].…”

“…, ν being the Poisson's ratio of the film. In the case of W based materials, the Poisson's ratio has been found in the range between 0.28 and 0.45 [23]. With ν in this range, the values of f (ν) remain between two close bounds: 0.933 ± 0.015, such that considering v T ≈ 0.933v R introduces at most a 1.6% error in the approximation of n. For this reason, we compute v R from the R peak frequency (see fig.…”

“…The thermomechanical characterization is performed exploiting Brillouin spectroscopy (BS) and the substrate curvature method (SC). The coupling of these two techniques has been shown to be a powerful tool for the characterization of nanostructured films, providing a broad, non destructive characterization of the samples [23,24,25]. In particular, when transparent oxide coatings are investigated, BS can be able to derive, through the detection of surface and bulk acoustic waves, all the elastic moduli of the films (i.e.…”

Nanosecond laser pulses for mimicking thermal effects on nanostructured tungsten-based materials

“…The elastic properties (i.e. Young's modulus E, Poisson's ratio ν and shear modulus G) and the coefficient of thermal expansion α of the coatings were determined by the combined use of Brillouin spectroscopy and the substrate curvature method in previous works [40,41]. They are summarized in table 1.…”