Nanoindentation of Chromium Oxide Possessing Superior Hardness among Atomic-Layer-Deposited Oxides

Chromium (III) oxide is a technologically interesting material with attractive chemical, catalytic, magnetic and mechanical properties. It can be produced by different chemical and physical methods, for instance, by metal-organic chemical vapor deposition, thermal decomposition of chromium nitrate Cr(NO₃)₃ or ammonium dichromate (NH₄)₂Cr₂O₇, magnetron sputtering and atomic layer deposition. The latter method was used in the current work to deposit Cr₂O₃ thin films with thicknesses from 28 to 400 nm at deposition temperatures from 330 to 465 °C. The phase composition, crystallite size, hardness and modulus of elasticity were measured. The deposited Cr₂O₃ thin films had different structures from X-ray amorphous to crystalline α-Cr₂O₃ (eskolaite) structures. The averaged hardness of the films on SiO₂ glass substrate varied from 12 to 22 GPa and the moduli were in the range of 76-180 GPa, as determined by nanoindentation. Lower values included some influence from a softer deposition substrate. The results indicate that Cr₂O₃ could be a promising material as a mechanically protective thin film applicable, for instance, in micro-electromechanical devices.