Nanostructure, mechanical properties and oxidation resistance of understoichiometric ZrB2-x films deposited by high power impulse magnetron sputtering

Diborides of transition metals from group IVB (TMB2, TM = Ti, Zr) are desirable materials in demanding industrial conditions due to their excellent mechanical properties. Direct current magnetron sputtering (DCMS) leads to the growth of overstoichiometric (TMBx, x > 2) film with nanocomposite structure consisting of crystalline hexagonal TMB2 nanocolumns surrounded by a thin, amorphous boron-rich rich tissue phase. At elevated temperatures, the presence of the tissue phase has a negative effect on the films' mechanical properties and oxidation resistance. An innovative approach using effective ionization of sputtered species during high-power pulsed magnetron sputtering (HiPIMS) growth of ZrB2 films is presented. While layers grown using the conventional DCMS method are overstoichiometric (B/Zr = 2.2), the films grown by HiPIMS are understoichiometric, with a B/Zr ratio ranging from 1.6 to 1.9. In understoichiometric ZrB1.9 and ZrB1.6 films, detailed structural analysis using transmission electron microscopy revealed a nanocrystalline structure comprised of densely packed 10-20 nm wide nanograins. In addition, the understoichiometric films exhibit high hardness values above 42 GPa and improved high-temperature oxidation resistance compared to the ZrB2.2 film deposited by DCMS.