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

TL;DR

This study investigates how nanostructure variations in pulsed laser deposited tungsten-based coatings affect their elastic properties, using surface Brillouin spectroscopy to correlate structure, density, and stiffness.

Contribution

It provides a comprehensive elastic characterization of amorphous, ultra-nano-, and nano-crystalline tungsten coatings, revealing how nanostructure influences mechanical properties.

Findings

Amorphous-like coatings have about 60% lower stiffness than bulk tungsten.

Stiffness in nanocrystalline coatings correlates with grain size, approaching bulk values as grains grow.

Annealing induces ultra-nano crystalline seeds, affecting density and stiffness.

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.