Exceptional fracture resistance of ultrathin metallic glass films due to an intrinsic size effect

Oleksandr Glushko,Christoph Gammer,Jürgen Eckert,Christian Mitterer,Megan J Cordill,Marlene Mühlbacher

doi:10.1038/s41598-019-44384-z

Abstract

Metallic glasses typically fail in a brittle manner through shear band propagation but can exhibit significant ductility when the sample size is reduced below a few hundreds of nanometers. To date the size effect was mainly demonstrated for free-standing samples and the role of extrinsic setup parameters on the observed behavior is still under debate. Therefore, in the present work we investigated the mechanical properties of polymer-supported sputtered amorphous Pd82Si18 thin films with various thicknesses. We show that the films exhibit brittle fracture for thicknesses far below 100 nm. A pronounced size effect resulting in extended crack-free deformation up to 6% strain was observed only in films as thin as 7 nm – a thickness which is lower than the typical shear band thickness. This size effect results in exceptional cyclic reliability of ultrathin metallic glass films which can sustain cyclic strains of 3% up to at least 30,000 cycles without any indication of fatigue damage or electrical conductivity degradation. Since the enhancement of mechanical properties is observed at ambient conditions using inexpensive substrates and an industrially scalable sputter deposition technique, a new research avenue for utilization of ultrathin metallic glasses in microelectronics, flexible electronics or nanoelectromechanical devices is opened up.

Highlights

IntroductionAdditional film thickness measurements were carried out by X-ray reflectivity (XRR) in a Rigaku SmartLab 5-axis diffractometer operated at 40 kV and 30 mA with Cu–Kα radiation parallelized in a parabolic multilayer mirror
To sum up the main implications of the presented work, first and foremost, a strong size effect in the deformation behaviour of metallic glasses is demonstrated in thin films with macroscopic lateral dimensions at ambient conditions
It is important to note that the observed size effect is attributed to the fundamental deformation mechanisms of amorphous metallic alloys and not to a particular chemical composition

Summary

Introduction

Additional film thickness measurements were carried out by X-ray reflectivity (XRR) in a Rigaku SmartLab 5-axis diffractometer operated at 40 kV and 30 mA with Cu–Kα radiation parallelized in a parabolic multilayer mirror. The device was equipped with a 0.1 mm vertical slit, a double-bounce Ge(220) monochromator, and a 5.0 mm length limiting slit in the incident beam as well as 0.2 and 0.4 mm vertical slits and a 5.0° Soller slit in the scattered beam. The incidence angle Θ was increased from 0 to 3° in 0.005° steps. The collected XRR data were evaluated with the Python-based refinement program GenX [for details see M. 40, 1174 (2007)], utilizing the Parratt recursion formula for reflectivity simulation and the genetic differential evolution algorithm for data refinement. The comparison of film thicknesses measured by a profilometer and obtained from XRR measurements measured thicknesses is given in Supplementary Fig. S

Methods

Results

Conclusion