Hard and electrically conductive multicomponent diboride-based films with high thermal stability

We report high-quality, hard (31–41 GPa), crack-resistant (hardness-to-effective Young's modulus ratio of 0.13–0.16) and electrically conductive (2.8–4.2 × 10⁵ S m^?1) HfB₂-based ceramic materials with high thermal stability. The materials were prepared in the form of well adhesive films using a simple deposition process: pulsed magnetron sputtering (B₄C target overlapped by metal stripes) onto floating substrates. We go through a wide range of compositions resulting from incorporating five other metals (Ti, Y, Zr, Ho or Ta) which partially replace Hf, and focus on the effect of the number and characteristics of elements in the metal sublattice. Regardless of the number of incorporated elements, no segregation to more than one AlB₂-type crystalline phase was observed. Growing number of metal elements leads to decreasing crystal size as indicated by the width of diffraction peaks, strongly decreasing compressive stress to less than 2 GPa and slightly decreasing hardness and electrical conductivity. The experimental data are explained by Monte-Carlo calculations of the energy delivered into the growing films. The thermal stability of septenary diboride-based films Hf₈Zr₄Ti₄Ta₄Y₅B₆₀C₉ and Hf₁₀Zr₄Ti₄Ta₄Ho₅B₅₈C₉ was superior to that of quaternary films Hf₂₂Y₅B₅₈C₉ and Hf₂₂Ho₅B₅₈C₉ when annealed to 1300 °C. Both the single solid solution crystalline phase (X-ray diffraction) and the dense and pinhole-free structure (scanning electron microscopy) were observed for the septenary films not only before but also after annealing. The results are important for the design and industry-friendly preparation of thin-film materials combining multiple functional properties for various technological applications.