Surface Electronic Structure and Mechanical Characteristics of Copper–Cobalt Oxide Thin Film Coatings: Soft X-ray Synchrotron Radiation Spectroscopic Analyses and Modeling

Novel copper–cobalt oxide thin films with different copper/cobalt molar ratios, namely, [Cu]/[Co] = 0.5, 1, and 2, have been successfully coated on aluminum substrates via a simple and cost-effective sol–gel dip-coating method. Coatings were characterized using high resolution synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy, in combination with nanomechanical testing and field emission scanning electron microscopy (FESEM). The surfaces of both [Cu]/[Co] = 0.5 and 1 samples consisted primarily of fine granular nanoparticles, whereas the [Cu]/[Co] = 2 has a smoother surface. The analyses reveal that the increase of copper concentration in the synthesis process tends to promote the formation of octahedral Cu 2+ which minimizes the development of octahedral Cu +, and these octahedral Cu 2+ ions substitute the Co 2+ site in cobalt structure host. The local coordinations of Co, Cu and O are not substantially influenced by the change in the copper to cobalt concentration ratios except for the [Cu]/[Co] = 2 coating where the local coordination appears to slightly change due to the loss of octahedral Cu + . The present film coatings are expected to exhibit good wear resistance especially for the [Cu]/[Co] = 1.0 sample due to its high hardness/elastic modulus ( H / E ) ratio. Finite element modeling (FEM) indicated that, under spherical loading conditions, the high stress and the plastic deformation were predominantly concentrated within the coating layer, without spreading into the substrate.