Grain boundary rotation-mediated strengthening in nano-columnar cr-based hard coatings

Abstract

To investigate the hardening mechanisms of CrN-based hard coatings, transmission electron microscopy (TEM) was employed to correlate nanoindentation-induced deformation with microstructural evolution. Among the investigated coatings, CrAlN exhibits the highest hardness and Young's modulus (41.2 GPa/402.8 GPa), outperforming CrSiN (30.5 GPa/336.2 GPa) and CrN (26.7 GPa/305.7 GPa). The superior performance of the CrAlN is attributed to a coordinated deformation mechanism involving ∼9° deflection of columnar grains under compression loading, which facilitates stress redistribution while maintaining structural continuity. Additionally, stress-induced phase transformations at acute-angle grain boundaries—from FCC CrAlN to HCP Cr₂N and FCC AlN—further enhance strain accommodation, contributing to an improved strength-toughness balance. In contrast, CrSiN coating develops internal crack formation accompanied by Cr₂N phases. While these transformations increased hardness, they also introduced embrittlement, compromising ductility. The CrN coating, lacking both solid-solution strengthening and grain refinement, displayed extensive trans-granular fracture and grain disintegration, indicative of limited capacity for stress delocalization. These findings provide mechanistic insights into the structure–property relationships of CrN-based coatings and offer design guidelines for the development of advanced wear-resistant coatings with tailored mechanical responses.