Coefficient of thermal expansion of nanostructured tungsten based coatings assessed by thermally induced substrate curvature method

TL;DR

This study measures the thermal expansion and residual stress of nanostructured tungsten coatings, revealing how nanostructure and porosity influence their thermal properties using a laser-based substrate curvature method.

Contribution

It introduces an optimized experimental setup for assessing CTE and residual stress in nanostructured W coatings and elucidates the relationship between nanostructure, stress, and thermal expansion.

Findings

All coatings exhibit compressive residual stress decreasing with nanostructure transition.

CTE of coatings exceeds that of bulk tungsten, increasing with grain size reduction.

Porosity and material softness influence the thermal expansion behavior.

Abstract

The in plane coefficient of thermal expansion (CTE) and the residual stress of nanostructured W based coatings are extensively investigated. The CTE and the residual stresses are derived by means of an optimized ad-hoc developed experimental setup based on the detection of the substrate curvature by a laser system. The nanostructured coatings are deposited by Pulsed Laser Deposition. Thanks to its versatility, nanocrystalline W metallic coatings, ultra-nano-crystalline pure W and W-Tantalum coatings and amorphous-like W coatings are obtained. The correlation between the nanostructure, the residual stress and the CTE of the coatings are thus elucidated. We find that all the samples show a compressive state of stress that decreases as the structure goes from columnar nanocrystalline to amorphous-like. The CTE of all the coatings is higher than the one of the corresponding bulk W form. In particular, as the grain size shrinks, the CTE increases from 5.1 10$^{-6}$ K$^{-1}$ for nanocrystalline W to 6.6 10$^{-6}$ K$^{-1}$ in the ultra-nano-crystalline region. When dealing with amorphous W, the further increase of the CTE is attributed to a higher porosity degree of the samples. The CTE trend is also investigated as function of materials stiffness. In this case, as W coatings become softer, the easier they thermally expand.