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

Abstract

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 HfN1.33/Hf0.76Al0.24N1.15 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. HfN1.33 forms a checkerboard period of 7.5 Å from self-organization of Hf-vacancies and N-interstitials, whereas Hf0.76Al0.24N1.15 forms a period of 12.5 Å due to spinodal decomposition. Mechanical testing reveals high hardness (∼36 GPa), matching that of Hf0.76Al0.24N1.15 and exceeding the softer HfN1.33 (∼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 Hf0.76Al0.24N1.15 for improved toughness, corroborated by cube-corner indentation fracture analysis, while not reaching the superb toughness and plasticity of HfN1.33. These results demonstrate that epitaxial HfN1.33/Hf0.76Al0.24N1.15 superlattices combine high hardness and strength with good toughness for an improved film damage tolerance.