Three-fold superstructured superlattice HfN/HfAlN thin films for improved damage tolerance

• Single crystal 3-fold superstructured superlattices of HfN 1.33 / Hf 0.76 Al 0.24 N 1.15 . • Strain, mosaicity and superlattice quality determined by reciprocal space maps. • Structural analysis by aberration corrected high-resolution HAADF STEM. • Micropillar compression reveal crack fragmentation on {1 1 0} <1 1 0> slip systems. • Uniformly high strength of ∼7 GPa, hardness of ∼36 GPa and improved toughness. Achieving both high hardness and toughness in protective coatings is a formidable challenge. Here, we harness an original superlattice architecture that synergistically combines Koehler hardening with coherent interfaces to reduce the crack driving forces and enhance toughness, enabling coatings with improved damage tolerance. We engineer epitaxial HfN 1.33 /Hf 0.76 Al 0.24 N 1.15 superlattices on MgO(0 0 1) substrates using low-energy, high-flux ion-assisted reactive magnetron sputtering. These superlattices, with bilayer periods ranging from 6 to 20 nm exhibit a unique three-fold superstructure, where each layer forms a three-dimensional checkerboard pattern. HfN 1.33 forms a checkerboard period of 7.5 Å from self-organization of Hf-vacancies and N-interstitials, whereas Hf 0.76 Al 0.24 N 1.15 forms a period of 12.5 Å due to spinodal decomposition. Mechanical testing reveals high hardness (∼36 GPa), matching that of Hf 0.76 Al 0.24 N 1.15 and exceeding the softer HfN 1.33 (∼27 GPa), reflecting interface-driven Koehler strengthening. Micropillar compression tests show distributed cracking along {1 1 0} 〈1 1 0〉 slip systems and faster mechanical recovery than Hf 0.76 Al 0.24 N 1.15 for improved toughness, corroborated by cube-corner indentation fracture analysis, while not reaching the superb toughness and plasticity of HfN 1.33 . These results demonstrate that epitaxial HfN 1.33 /Hf 0.76 Al 0.24 N 1.15 superlattices combine high hardness and strength with good toughness for an improved film damage tolerance.