Structural, morphological and mechanical characterization of Mo sputtered coatings

In this work, the intrinsic properties of molybdenum films deposited by DC magnetron sputtering (DCMS) on titanium substrates have been investigated, as a function of working gas (Ar, Kr, Xe) and pressure. Morphology, microstructure, hardness, elastic modulus, residual stress have been studied through high resolution methodologies. Microstructural analyses consisted of Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Focused Ion Beam (FIB) cross section analysis. Moreover, nano-mechanical properties of the films (hardness and modulus) were analyzed by nano-indentation testing while adhesion was evaluated by scratch tests. A recently proposed approach was used to investigate the average residual stress of the produced coatings. It involves incremental FIB milling of annular trenches at material surface, combined with high resolution SEM imaging [1]. SEM-FIB and XRD results showed a significant modification of films' microstructure due to selected deposition parameters: it goes from dense-columnar (high crystallite size) at low working pressure and/or high gas mass to nano-porous (small grains) at high pressure and/or low gas mass. This means that the momentum flux carried to the substrate considerably change. In addition residual stress field and mechanical property results are consistent with the coatings microstructure variations and can be successfully used to confirm them. Finally, it has been found that microstructure and/or mechanical properties of the coatings help to foresee the film/substrate system adhesion. This is fundamental to predict its functional behavior. Therefore, an effective route to tailor the structure and properties of molybdenum coatings was investigated. Modifying synthesis conditions, Mo films can present dense microstructures and good mechanical properties (high Young's modulus and load bearing capacity) demonstrating their potential for applications in harsh environments like thermonuclear fusion plants. Otherwise changing sputtering parameters, they can show a nano-porous structure, suggesting novel applications as for example in the lubrication and catalysis fields.