Time-dependent deformation behavior of freestanding and SiN x -supported gold thin films investigated by bulge tests
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tlev Cassel Hochschule Kaiserslautern, Standort Zweibrücken, D-66482 Zweibrücken, Germany
Mathias Göken Materials Science & Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute I, D-91058 Erlangen, Germany (Received 12 March 2015; accepted 10 June 2015)
A novel strain-rate jump method was developed for the plane-strain bulge test and used to investigate the time-dependent deformation behavior of gold thin films in the thickness range 100–400 nm. The experimental method is based on an abrupt variation of the pressurization rate. The evaluated strain-rate sensitivity was found to be five times higher for films in freestanding condition (m 5 0.094) than for films tested on a SiNx substrate (m 5 0.020). Bulge creep tests confirmed this increased time-dependence. The observation of the surface of the freestanding films after the creep tests provided evidence of apparent grain boundary sliding taking place next to intragranular plastic deformation. The out-of-plane deformation was presumably favored by the columnar microstructure of the samples, with grains extending between both free surfaces. In the case of SiNx-supported films, grain boundary sliding was prevented by the good adhesion of gold to the SiNx substrate.
I. INTRODUCTION
Metallic thin films, unlike their coarse-grained bulk counterparts, often exhibit a time-dependent deformation behavior. This is especially the case with submicron films, the strength of which usually depends on the straining velocity. The knowledge of this strain-rate sensitivity is, therefore, essential for successfully designing microchips and microelectro-mechanical systems. Measuring the strain-rate sensitivity can also contribute to improve our scientific understanding of the deformation mechanisms of thin films. The strain-rate sensitivity m is defined in the Mukherjee– Bird–Dorn equation as the power-law exponent, which relates the stress level experienced by a sample to the strain-rate imposed during deformation. It is, therefore, mathematically calculated as: m¼
@ ln r @ ln e_
:
ð1Þ
Measuring the strain-rate sensitivity of macroscopic samples is a relatively common practice, especially in the Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.184 J. Mater. Res., Vol. 30, No. 14, Jul 28, 2015
field of severe plastic deformation and for nanostructured materials.1–4 There is, on the other hand, only scarce data available on thin films—especially freestanding ones5–11—mostly due to the lack of suitable experimental techniques. The approach chosen for the present work was to build upon one of the most robust methods available for thin films, the bulge test. Actually, this is not the first attempt to do so. As early as 1975 in USSR, Solonovich12 applied an increment of stress to a circular copper membrane during its transient creep relaxation. He observed an instantaneous change of the creep rate, whence he could calculate an activation volume. More recently, Alaca et al.6 tested rectangul
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