Experimental Investigation of Strain Rate Dependence of Nanocrystalline Pt Films

Abstract

A new microscale uniaxial tension experimental method was developed to investigate the strain rate dependent mechanical behavior of freestanding metallic thin films for MEMS. The method allows for highly repeatable mechanical testing of thin films for over eight orders of magnitude of strain rate. Its repeatability stems from the direct and full-field displacement measurements obtained from optical images with at least 25 nm displacement resolution. The method is demonstrated with micron-scale, 400-nm thick, freestanding nanocrystalline Pt specimens, with 25 nm grain size. The experiments were conducted in situ under an optical microscope, equipped with a digital high-speed camera, in the nominal strain rate range 10 −6 -10 1 s −1 . Full field displacements were computed by digital image correlation using a random speckle pattern generated onto the freestanding specimens. The elastic modulus of Pt, E=182±8 GPa, derived from uniaxial stress vs. strain curves, was independent of strain rate, while its Poisson's ratio was v=0.41± 0.01. Although the nanocrystalline Pt films had the elastic properties of bulk Pt, their inelastic property values were much higher than bulk and were rate-sensitive over the range of loading rates. For example, the elastic limit increased by more than 110% with increasing strain rate, and was 2-5 times higher than bulk Pt reaching 1.37 GPa at 10 1 s −1 .

Strain rate sensitivity of nanocrystalline Au films at room temperature

“…The rate-insensitive modulus reported in this work indicates that, if present, the rate of such anelastic deformation processes may be negligible at the time scales of our measurements and for the particular Au films. Similar rate insensitivity was reported by Jonnalagadda et al [21], which implies that the specific material fabrication and post-processing treatments may determine the anelastic behavior of metallic films.…”

“…This argument, however, does not provide a general mechanism for nanocrystalline materials. Experiments conducted by Jonnalagadda et al [21] on 400 nm thick nanocrystalline Pt films with an average grain size of 25 nm resulted in an elastic modulus that was independent of the strain rate and nearly the same as that of bulk Pt.…”

“…3 and according to the method described in Ref. [21]. The particles adhered to the free-standing thin film specimens due to van der Waals forces, providing high-fidelity full-field displacements via digital image correlation (DIC) [21] so that both axial and transverse displacements could be calculated.…”

“…[21]. The particles adhered to the free-standing thin film specimens due to van der Waals forces, providing high-fidelity full-field displacements via digital image correlation (DIC) [21] so that both axial and transverse displacements could be calculated. This method provided $25 nm resolution in film extensions [21], which is comparable to the resolution by experiments conducted inside scanning or transmission electron microscopes, with the additional capability to probe strain rates varying by almost eight orders of magnitude, which is not possible in experiments conducted inside analytical chambers.…”

Influence of Grain Structure and Doping on the Deformation and Fracture of Polycrystalline Silicon for MEMS and NEMS

“…The experimental apparatus for the strength and fracture experiments was described in previous reports [32]. In this experimental technique, the specimen ends are gripped to loadcell and PZT actuator mounted on three dimensional micro-positioners that facilitates loading in uniaxial tension.…”