Dimension-dependent mechanical features of Au-nanocrystalline nanofilms

For metal nanofilms composed of nanocrystals, the multiple deformation mechanisms will coexist and bring unique and complex elastic-plastic and fracture mechanical properties. By successfully fabricating large quantities of uniform doubly-clamped suspended gold (Au) nanobeams with different thicknesses and nanograin sizes, we obtain full-spectrum mechanical features with statistical significance by combining atomic force microscopy (AFM) nanoindentation experiments, nonlinear theoretical model, and numerical simulations. The yield and breaking strengths of the Au nanobeams have a huge increase by nearly an order of magnitude compared with bulk Au and exhibit strong nonlinear effects, and the corresponding strong-yield ratio is up to 4, demonstrating extremely high strength reserve and vibration resistance. The strong-yield ratio gradually decreases with decreasing thickness, identifying a conversion of the failure type from ductile to brittle. Interestingly, the Hall–Petch relationship has been identified to be still valid at the nanoscale, and K in the equation reaches 4.8 Gpa·nm^1/2, nearly twice of bulk nanocrystalline Au, which is ascribed to the coupling effect of nanocrystals and nanoscale thickness.