Nano-indentation investigations of the mechanical properties of thin TiO2, WO3 and their composites layers, deposited by spray pyrolysis

The aim of the present work is to determine the indentation hardness (HIT) and indentation modulus (EIT) of pure TiO2 and WO3 thin films, as well as thin films composed of different TiO2 and WO3 proportions and deposited by spray pyrolysis on a stainless-steel (OC 404) substrate.Since the HIT and EIT of the films are properties expected to depend on the phase-chemical composition, morphology, structure and their changes when increasing the WO3 content in the TiO2-WO3 composite film, the correlation between the mechanical and structural properties is also addressed.The obtained results show that HIT and EIT strongly depend on the concentration of the co-deposited WO3.The determined values of HIT and EIT noticeably decrease (in comparison with HIT and EIT of the pure (100 %) TiO2 layer) when very low concentrations of WO3 (up to 2.5 % of W) are co-deposited.At higher concentrations of the co-deposited WO3 (more than 2.5 % of W), the HIT and EIT values increase almost linearly with an increase of the WO3 in the precursor.The observed non-proportional behavior of HIT and EIT is associated with specific changes of the structure and a development of defects in the deposited TiO2-WO3 composite phase, as well as with the increase in the amount of the formed separate WO3 phase (with increasing of WO3 (H2W3O12) in the working solution) surrounded by solitary TiO2 particles.