Investigation of various properties of HfO₂-TiO₂ thin film composites deposited by multi-magnetron sputtering system

Highlights: • HfO_2, mixed HfO₂-TiO₂ with various amount of Ti content and TiO₂ thin films were deposited using magnetron co-sputtering. • The change of the Ti content in mixed oxide had significant impact on the properties of deposited coatings. • All deposited thin films were well transparent in visible light range. • With the increase of Ti content in thin films hardness increased from 4.9 GPa to 13.7 GPa. - Abstract: In this work the properties of hafnium dioxide (HfO₂), titanium dioxide (TiO₂) and mixed HfO₂-TiO₂ thin films with various amount of titanium addition, deposited by magnetron sputtering were described. Structural, surface, optical and mechanical properties of deposited coatings were analyzed. Based on X-ray diffraction and Raman scattering measuremets it was observed that there was a significant influence of titanium concentration in mixed TiO₂-HfO₂ thin films on their microstructure. Increase of Ti content in prepared mixed oxides coatings caused, e.g. a decrease of average crystallite size and amorphisation of the coatings. As-deposited hafnia and titania thin films exhibited nanocrystalline structure of monoclinic phase and mixed anatase-rutile phase for HfO₂ and TiO₂ thin films, respectively. Atomic force microscopy investigations showed that the surface of deposited thin films was densely packed, crack-free and composed of visible grains. Surface roughness and the value of water contact angle decreased with the increase of Ti content in mixed oxides. Results of optical studies showed that all deposited thin films were well transparent in a visible light range. The effect of the change of material composition on the cut-off wavelength, refractive index and packing density was also investigated. Performed measurements of mechanical properties revealed that hardness and Young's elastic modulus of thin films were dependent on material composition. Hardness of thin films increased with an increase of Ti content in thin films, from 4.90 GPa to 13.7 GPa for HfO₂ and TiO₂, respectively. The results of the scratch resistance showed that thin films with proper material composition can be used as protective coatings in optical devices.