Strategy for simultaneously increasing both hardness and toughness in ZrB₂-rich Zr_1−xTa_xB_y thin films

Refractory transition-metal diborides exhibit inherent hardness. However, this is not always sufficient to prevent failure in applications involving high mechanical and thermal stress, since hardness is typically accompanied by brittleness leading to crack formation and propagation. Toughness, the combination of hardness and ductility, is required to avoid brittle fracture. Here, the authors demonstrate a strategy for simultaneously enhancing both hardness and ductility of ZrB₂-rich thin films grown in pure Ar on Al₂O₃(0001) and Si(001) substrates at 475 °C. ZrB_2.4 layers are deposited by dc magnetron sputtering (DCMS) from a ZrB₂ target, while Zr_1−xTa_xB_y alloy films are grown, thus varying the B/metal ratio as a function of x, by adding pulsed high-power impulse magnetron sputtering (HiPIMS) from a Ta target to deposit Zr_1−xTa_xB_y alloy films using hybrid Ta-HiPIMS/ZrB₂-DCMS sputtering with a substrate bias synchronized to the metal-rich portion of each HiPIMS pulse. The average power $PTa$ (and pulse frequency) applied to the HiPIMS Ta target is varied from 0 to 1800 W (0 to 300 Hz) in increments of 600 W (100 Hz). The resulting boron-to-metal ratio, y = B/(Zr+Ta), in as-deposited Zr_1−xTa_xB_y films decreases from 2.4 to 1.5 as $PTa$ is increased from 0 to 1800 W, while x increases from 0 to 0.3. A combination of x-ray diffraction (XRD), glancing-angle XRD, transmission electron microscopy (TEM), analytical Z-contrast scanning TEM, electron energy-loss spectroscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and atom-probe tomography reveals that all films have the hexagonal AlB₂ crystal structure with a columnar nanostructure, in which the column boundaries of layers with 0 ≤ x < 0.2 are B-rich, whereas those with x ≥ 0.2 are Ta-rich. The nanostructural transition, combined with changes in average column widths, results in an ∼20% increase in hardness, from 35 to 42 GPa, with a simultaneous increase of ∼30% in nanoindentation toughness, from 4.0 to 5.2 MPa√m.