ICMCTF 2026 Session MA4-2-TuM: Boron-containing Coatings II

Both experimental and theoretical studies have reported TaBₓ to be promising for high-temperature and wear-resistant applications due to its exceptional hardness and oxidation resistance. However, achieving dense, nanocrystalline TaB_x with controlled stoichiometry remains a challenging task. To date, TaBₓ films have been deposited only by conventional magnetron sputtering and high target utilization sputtering. This study demonstrates the pressure-controlled structure transformation of TaBₓ films deposited by HiPIMS.

TaBₓ coatings were deposited from a TaB₂ target in an Ar atmosphere at 340 °C using HiPIMS. Three groups of films were grown under three pressures (0.3 – 0.9 Pa) and two substrate bias conditions. The target current density was maintained at 1 A/cm² by adjusting the pulse frequency. The microstructure was analyzed using XRD and TEM. TaBₓ films exhibited an amorphous structure at low pressure, a nanocomposite nature at moderate pressure, and a dense nanocrystalline structure at high pressure. This work presents the first systematic investigation into how energy flux controlled by pressure influences phase evolution in TaBₓ deposited by HiPIMS. Cross-sectional SEM confirmed uniform thicknesses and sufficient adhesion. The chemical composition, as determined by WDS, showed that the B/Ta ratios rose from 1.05 to 1.4. This might be attributed to the reduced resputtering of B by thermalized reflected Ar neutrals at higher pressure. The nanoindentation method yielded a nanohardness of 26.2 GPa and Young’s modulus of 344.6 GPa for amorphous films, improving to 42.4 GPa and 469.8 GPa for nanocrystalline ones. Fracture toughness (K_IC) was also determined by cantilever fracture testing in SEM.

These results establish working pressure in HiPIMS as a powerful parameter for controlling the crystallinity, stoichiometry, and mechanical properties of refractory borides. This study clarifies how changes in microstructure affect the densification and hardening of boron-rich films. The demonstrated ability to obtain dense, hard, and tough TaBₓ films by HIPIMS highlights its potential for preparing next-generation materials for extreme environments. This work was funded by the Slovak Research and Development (No. APVV-24-0038) and the European Union under grant agreement No. 101158464 (COLOSSE).

Chemical ordering in layered materials, such as MAX and MAB phases, enables precise tailoring of functional properties through structural motifs like in-plane (i-MAB, i-MAX) and out-of-plane (o-MAB, o-MAX) configurations. This study introduces super-ordered MAB phases (s-MAB), a novel structural material combining simultaneous in-plane and out-of-plane chemical ordering, previously unreported in synthesized MAB phases. Using first-principles density functional theory, we systematically investigated the thermodynamic stability of hypothetical quinary s-MAB phases with 314 (M1₄M2₂M3₃Al₃B₁₂) and 416 (M1₄M2₂M3₆Al₃B₁₈) compositions (M1, M3 = transition metals; M2 = Sc, Y) against a comprehensive set of competing phases, including ternary and quaternary MABs, disordered configurations, and binary/ternary borides at 2000 K. Our calculations predict 27 thermodynamically stable s-MAB compositions, predominantly featuring M1 = Cr, Mo, W, Mn, Fe and M3 = V, Nb, Ta, with M2 = Sc or Y promoting the combined in-plane and out-of-plane order. Notably, 416 structures exhibit conditions favoring partial or complete disorder, highlighting the nuanced stability landscape. These s-MAB phases offer a theoretical roadmap for synthesizing novel boron-containing compounds with precisely controlled atomic arrangements, potentially yielding unique mechanical, thermal, and electronic properties. Additionally, stable s-MABs can serve as precursors for 2D boridene (MBene) synthesis, expanding compositional flexibility for property tuning compared to existing boridenes. This work establishes a framework for designing advanced boride-based materials, with implications for high-temperature coatings, structural components, and electronic devices. Future experimental efforts should prioritize optimizing synthesis conditions to validate predicted chemical ordering and explore application-driven property enhancements.