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

Tungsten (W) and W-alloy materials are valuable for a wide range of applications, as they possess a peculiar combination of properties like high melting point, good heat conductivity, high resistance to thermal loads, high density and corrosion resistance. The W binary alloys are especially interesting for applications involving high power, high voltage, thermal management and radiation shielding. Among them, the W-Ni alloys are particularly interesting for micro- and nano-mechanical systems, and the W-Ta ones for the aerospace industry.

Pure W films also have low sputtering yield and low tritium retention; these properties make them attractive for nuclear fusion reactors. Initially, this application was investigated by relatively thick W layers, deposited on structural materials by plasma spray, which can cover wide areas with limited deposition time and costs. However, plasma spray does not allow a fine control of the film structure, resulting in undesired properties, up to the failure of the coatings. Other physical and chemical vapour deposition techniques are available to deposit W coatings, with a better control of stoichiometry, structure and morphology.

Marco Beghi and colleagues exploited Pulsed Laser Deposition (PLD): a flexible technique, already employed to deposit various mono- or multi-elemental films such as metals, compounds and carbon based materials, also replicating complicated target stoichiometries. In particular, the deposition parameters provide different possibilities of tailoring the film structure and morphology. The study is now published in peer-reviewed journal, Materials and Design.

Growing films with thicknesses ranging from a few nanometers to a micrometer, it has been proven that it is possible to obtain, by pulsed laser deposition, W coatings with different morphologies, from compact to porous, and with different structures, from nanocrystalline to amorphous-like.

Pulsed laser deposition W coatings have also been exploited, in the frame of plasma-wall interaction investigations, as a proxy of redeposited W. The authors deposited W and W-Ta films on silicon (Si) substrate, and exploited three different variants of the pulsed laser deposition process to produce different nanostructures. The deposition parameters which have been varied are the He pressure in the deposition chamber, a post-deposition thermal annealing of a-W, in vacuum, and alloying with Ta, in percentages up to 24%.

The obtained nanostructures can be grouped in three classes: amorphous-like (a-W), ultra-nanocrystalline (u-nano-W) and nanocrystalline (nano-W). While a-W and u-nano-W have lower densities, the mass density of nano-W approaches the bulk value. The elastic moduli have been determined by Brillouin spectroscopy, in the isotropic film approximation, and taking into account anisotropy for the nano-W, which tends to have a columnar structure. The stiffness is mainly correlated to the crystallite size; those of the W-Ta alloys are consistent with the lever rule for sufficiently large grains, and deviate from it for smaller grains.

This study confirms the versatility of the pulsed laser deposition technique in obtaining films with different nanostructures. These nanostructures can be exploited to tailor the film properties, from values approaching the bulk W ones to appreciably lower values of mass density and stiffness (down to 40 % of the bulk value).