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

摘要

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