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

摘要

The in plane coefficient of thermal expansion (CTE) and the residual stressof nanostructured W based coatings are extensively investigated. The CTE andthe residual stresses are derived by means of an optimized ad-hoc developedexperimental setup based on the detection of the substrate curvature by a lasersystem. 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 Wcoatings are obtained. The correlation between the nanostructure, the residualstress and the CTE of the coatings are thus elucidated. We find that all thesamples show a compressive state of stress that decreases as the structure goesfrom columnar nanocrystalline to amorphous-like. The CTE of all the coatings ishigher than the one of the corresponding bulk W form. In particular, as thegrain size shrinks, the CTE increases from 5.1 10$^{-6}$ K$^{-1}$ fornanocrystalline W to 6.6 10$^{-6}$ K$^{-1}$ in the ultra-nano-crystallineregion. When dealing with amorphous W, the further increase of the CTE isattributed to a higher porosity degree of the samples. The CTE trend is alsoinvestigated as function of materials stiffness. In this case, as W coatingsbecome softer, the easier they thermally expand.