Substrate geometry-controlled microstructure-stress-property relationships in nanocrystalline thin films

The functional properties of nanocrystalline thin films are governed by cross-sectional gradients of microstructure and residual stress. However, despite decades of intensive research, thin-film science has focused predominantly on planar substrates, leaving the influence of substrate geometry on films’ process-microstructure-stress-property relationships largely underexplored. Here, we demonstrate that thin-film growth on sharp cutting edges produces microstructural states and micromechanical responses that differ fundamentally from those formed on planar substrates. A nanocrystalline Al _0.6Ti _0.4N film was deposited on WC–Co inserts by DC magnetron sputtering and characterized using cross-sectional X-ray nanodiffraction combined with analytical scanning and transmission electron microscopy. Hardness, Young’s modulus, and fracture behavior were evaluated by nanoindentation and in situ microcantilever bending. Measurements reveal pronounced grain refinement at the cutting edge, accompanied by compressive residual stresses that are up to 500 MPa higher than those observed in the planar regions. Electron microscopy further identifies a previously unreported bimodal microstructure localized to the edge region, arising from superimposed self-shadowing effects. This manifests as a periodic grain size modulation with sharply demarcated coarse- and fine-grained regions and associated residual stress oscillations. Microcantilever bending tests performed directly at the cutting edge show that the fracture response is governed by the dominant grain morphology, producing distinctive fracture surface characteristics and load–deflection behavior. These results establish cutting edges as microscale platforms where substrate geometry fundamentally governs thin-film microstructural evolution and the resulting mechanical properties. They reveal substrate curvature as a controllable design parameter for engineering spatially resolved microstructure-stress states, providing a framework for tailoring nanocrystalline films in performance-critical, non-planar regions.