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·nm1/2,nearly twice of bulk nanocrystalline Au,which is ascribed to the coupling effect of nanocrystals and nanoscale thickness.