W-ion irradiation promotes dense TiBx film growth during magnetron sputtering without substrate heating

Abstract

Lowering energy consumption during thin film growth by magnetron sputtering techniques is essential for future industry as a step towards reaching the United Nations (UN) sustainable development goals. Large potential for energy savings has been shown by employing high-mass metal ion irradiation from targets operated in high-power impulse magnetron sputtering (HiPIMS) mode. With this approach, demonstrated for transition metal (TM) nitrides, thermally-induced adatom mobility is replaced with that supplied by overlapping collisions cascades of low-energy recoils, resulting in a strong reduction of the necessary external heating. Here, the novel method is tested for TiB_x, which is a model system for TM-based diborides, another class of promising materials to be used as protective coatings for cutting tools. We show that such films grown with no external substrate heating develop porosity over a wide range of B/Ti ratios. However, (Ti_1-yW_y)B_x films grown by the hybrid W₂B₅-HiPIMS/Ti-TiB₂-DCMS co-sputtering with substrate bias synchronized to W⁺/W²⁺-rich ion fluxes, are dense (without porosity), irrespective of x. Nanoindentation hardness increases from ∼20 GPa for TiB_x to ∼40 GPa for (Ti_1-yW_y)B_x. The slightly understoichiometric (Ti_0.86W_0.14)B_1.91 film exhibits super-hardness and is nearly stress-free. These results prove that selected heavy ion irradiation is beneficial for low-temperature growth of hard diboride films, which demonstrates the versatility of this approach and calls for evaluation in other material systems.