Toughening nitride hard coatings by deflecting cracks along grain boundaries

Grain boundaries (GBs) in hard coatings are often considered as the weakest link, acting as preferred pathways for crack propagation and thereby limiting the coating's fracture toughness. In this study, we investigate whether continuous crack deflection along GBs can mitigate this limitation and enhance the fracture resistance of hard coatings. Three model systems were examined: CrN, AlN and their multilayered structure coatings, all characterized by columnar GB structures. Fracture toughness was quantitatively assessed using an in situ SEM micro-cantilever fracture testing. The key approach of this study is the use of two different loading geometries, with notches aligned either parallel or perpendicular to the coating's growth direction, allowing us to compare the influence of the crack propagation direction and deflection. Across all three systems, the perpendicular notch configuration—aligned across the columnar microstructure—resulted in approximately 8 % higher fracture toughness. This enhancement is attributed to continuous crack deflection along GBs during deformation. Additionally, the extent of crack deflection was found to depend on the local GB arrangement, with transgranular fracture observed when no well-aligned GBs were present along the crack path. These findings provide quantitative insights into the toughening mechanisms enabled by GB-mediated crack deflection and offer design strategies for mechanically robust hard coatings.