A Computational Study of Plastic Deformation of Nanoscale Freestanding Thin Films Using a Hyperelastic-Viscoplastic Crystal Plasticity Constitutive Model: Application to the Characterization of NEMS Materials And Devices

Nanoscale materials show significantly different mechanical behavior and deformation mechanism from their macro- scale counterparts. Hence, a separate theoretical framework should be developed to reliably investigate these types of scale-dependent materials’ mechanical properties, especially their behavior at the plastic deformation regime. However, the available frameworks are far from complete. In this research, we develop a hyper-elastic based crystal plasticity constitutive model, which takes microstructure characteristics into account, to investigate the plastic deformation of freestanding gold (Au) thin films, which have a wide variety of application in MEMS/NEMS devices. According to the computationally fast hyper-elastic- based crystal plasticity theory, the yield stress of single-crystalline Au thin films is as high as 375 MPa, and the thin film shows a high degree of strain-rate and strain-path (deformation mode) dependency. In uniaxial tensile deformation mode, the UTS value of the single-crystal Au thin film reaches to 514 MPa in 0.001 strain rate, and by increasing the strain rate to 0.002, the UTS increases to 580 MPa. Moreover, while the material has 514 UTS in uniaxial tensile mode, the UTS increases to 586 MPa in equal-biaxial deformation mode. The result of this study may be of great interest in NEMS materials and device characterization.

Keywords:

Metallic thin films , Freestanding thin layers , Single crystalline Au , Plastic deformation , Crystal plasticity , Strain rate