Yttrium-induced structural evolution and oxidation resistance in TiB2+∆ coatings deposited by conventional magnetron sputtering and HiPIMS

Abstract

Titanium diboride (TiB₂) is a promising candidate for high-temperature applications due to its chemical inertness, phase stability, and excellent mechanical properties. However, its typical nanocomposite microstructure with a B-tissue phase promotes low-temperature oxidation. In this study, we employ a dual approach to suppress B-tissue formation and enhance oxidation resistance: yttrium alloying, due to its strong oxygen affinity, and the use of high-power impulse magnetron sputtering (HiPIMS) to reduce boron content in the growing film. Two Ti_1−xY_xB_2±∆ coatings with ∼9 at.% Y were deposited: overstoichiometric X-ray amorphous Ti_0.68Y_0.32B_2.8 via conventional direct current magnetron sputtering (DCMS) and understoichiometric crystalline Ti_0.76Y_0.24B_1.4 via HiPIMS. Thermally induced structural evolution and mechanical performance were analyzed using X-ray diffraction, scanning transmission electron microscopy, and nanoindentation. The X-ray amorphous coating crystallized above 900 °C into TiB₂ and YB₆ phases, while the HiPIMS coating retained its nanocolumnar, stacking fault-rich α-Ti_1−xY_xB_2-∆ structure up to 1100 °C. The Ti_0.68Y_0.32B_2.8 coating exhibited moderate hardness (∼ 28 GPa), whereas the Ti_0.76Y_0.24B_1.4 coating reached superhardness (> 40 GPa) with higher Young's modulus (∼ 420 GPa). Both coatings showed improved oxidation resistance compared to TiB₂, with delayed crystalline oxide formation above 700 °C, while slower oxidation kinetics was observed for the understoichiometric coating. These results demonstrate the effectiveness of alloying and highly ionized deposition techniques for tuning the structure and high-temperature performance of TiB₂-based coatings.