Improved oxidation properties from a reduced B content in sputter-deposited TiBx thin films

Transition-metal diboride thin films, which have high melting points, excellent hardness, and good chemical and thermal conductivity, severely suffer from rapid oxidation in air. Here, we explore the influence of varying B content and resulting nanostructure change on the oxidation properties of TiBx thin films, with x = 1.43, 2.20, and 2.70. Results show that all as-deposited layers have columnar structure. The column boundaries of as-deposited TiB2.20 and TiB2.70 films grown by direct current magnetron sputtering (DCMS) are B-rich, while the as-deposited TiB1.43 films grown by high-power impulse magnetron sputtering (HiPIMS) show no apparent grain boundary phases and contain Ti-rich planar defects. The oxidation rate of TiB1.43 air-annealed at 400 °C up to 48 h is significantly lower than that of TiB2.20 and TiB2.70 films. The oxidation rate of TiB1.43, TiB2.20, and TiB2.70 films was measured at 2.9 ± 1.5, 7.1 ± 1.0, and 20.0 ± 5.0 nm/h, respectively, with no spallation of even as thick oxide scales as 0.5 μm in any of the films. The improved oxidation resistance can be explained by the absence of B-rich tissue phase at the column boundaries of understoichiometric TiBx films, a phase that interlaces the nanocolumnar TiB2 structures in the corresponding overstoichiometric films. An easy oxidation pathway is thus eliminated. •Improved oxidation resistance in TiBx thin films with reduced boron content•Microstructural differences in over- and understoichiometric TiBx dictates oxidation kinetics.•Understoichiometric TiBx thin films exhibit superior oxidation resistance and mechanical properties compared to overstoichiometric TiBx.