Deformation-mechanism dependent stretchability of nanocrystalline gold films on flexible substrates
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tchability of polyimide-supported nanocrystalline Au films with a thickness ranging from 930 to 20 nm was evaluated by uniaxial tensile testing. The results show that the fracture strain gradually decreased with decreasing the film thickness. Such degraded stretchability depends on plastic deformation mechanisms associated with the length scales. As the film thickness is larger than 90 nm, local thinning in the grown grains contributed to the high stretchability. Full dislocation behaviors including dislocation pileup in the 930 nm-thick film, the activation of Frank–Read dislocation source in the 170 nm-thick film and the grain boundary dislocation source in the 90 nm-thick film were dominated plastic deformation. As the film thickness is less than 40 nm, low stretchability of thin films resulted from intergranular fracture, and partial dislocation behaviors became prevailed. Evident grain growth happened in the films studied except for the 20 nm-thick film, which is expected to be involved in the stretchability of the nanocrystalline metal films on flexible substrates.
I. INTRODUCTION
Due to low weight, ruggedness, low costs, large area, and ease of integration, flexible electronics have become a major research trend in recent years.1–3 Mechanical stability is a precondition for flexible electronics to function well. The thin metal film as an electrical interconnect between electronic units is one of the weakest components in the system against mechanical deformation.1,3 Thus, it is necessary to understand the fundamental deformation mechanisms of metal films bonded to flexible substrates so as to build high-performance flexible electronics. The mechanical behavior of metal films bonded to the flexible substrates is substantially different from that of their bulk counterparts. For freestanding films, the fracture strain is quite low (about 1–2%) and they rupture by forming a single necking.4–6 By contrast, a flexible substrate can suppress strain localization and retard necking in the film, and it finally causes multiple neckings before rupture in the film.7–10 The rupture strains (stretchability) of thin metal films (including Cu, Ag and Au films) on flexible substrates can be up to 10%.5,11–15 For metal films well bonded to the substrates, the failure strain can be dramatically enhanced beyond 50%.7–9,16 The film thickness studied is mostly above the nanometer scale. Recently, the ongoing miniaturization in thin film technology has led to a drastic reduction in the Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.349
dimensions of the component elements and an urgent need to understand the basic deformation behaviors of small-scale materials. Due to confinement effects, metal films on flexible substrates exhibit prominent sizedependent deformation mechanisms. It has been found that thinner films with a thickness below 200 nm failed by intergranular fracture at elongations by only a few percent, while thicker films were ruptured by ductile transgranular fracture and
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