Structural, optical, and mechanical properties of cobalt copper oxide coatings synthesized from low concentrations of sol–gel process

Thin films of CoxCuyOz have been coated on aluminum substrates via sol–gel route using low concentration of copper and cobalt precursors at annealing temperatures in range of 500–650 °C. The coatings were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-Vis-NIR spectrophotometry, and nanoindentation. The XRD analysis in 2θ-range of 30°–42° revealed that the coatings exhibited low crystallinity of CoCu2O3, CoCuO2, and CuCoO2. The surface bonding structure analyzed using XPS indicated that the coating contained: Cu (tetrahedral Cu+ and octahedral Cu2+), Co (octahedral Co3+, tetrahedral Co2+, and mixed Co2+ and Co3+), and O (lattice, surface, and sub-surface oxygens). The optical properties characterized using UV-Vis-NIR showed that the reflectance spectra of coatings formed a spectrally solar selective absorber profile associated with the interference peaks and the absorption edges around wavelengths of below 1.2 μm. The maximum absorptance (α = 75.8%) was shown by coating synthesized at 500 °C. The mechanical properties of coatings showed that the increase of annealing temperature increased the coating's hardness (H) and the elastic modulus (E) due to the enhancement of the [CoCuO2/CuCoO2]:[CoCu2O3] oxide phases ratio, as the result, an excellent stability of the wear resistance (H/E) of around ∼0.035 was recorded.