Tailoring the physicochemical and mechanical properties of optical copper-cobalt oxide thin films through annealing treatment

Sol-gel dip-coated optical coatings, copper-cobalt oxides on aluminum substrates, were thermally treated at different annealing temperatures in the range 500-650 °C. The resulting films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-Vis-NIR spectrophotometry and nanoindentation techniques. The crystallinity of CoCu2O3 enhanced significantly, with increasing annealing temperature from 500 to 650 °C, while the electronic structure and bonding states of the copper-cobalt oxides matrix remained unchanged. UV-Vis-NIR analysis showed that the solar absorptance (α) of the coatings changed with increase of annealing temperature and an optimum α (84.4%) was achieved at 550 °C, which also coincides to the maximum tensile residual stress of the coating. Nanoindentation tests exhibited an increasing trend in both the hardness (H) and elastic modulus (E) of the coatings with increase in annealing temperature, although a slight decrease in the H/E ratio was also observed. The experimental studies were complemented by finite element modeling (FEM). The results showed that, under mechanical loading, the stress deformation and plastic deformation were concentrated within the coating layers. As such, the likelihood of delamination of the coating layer upon unloading would be reduced.