Failure by simultaneous grain growth, strain localization, and interface debonding in metal films on polymer substrates
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In a previous paper, we have demonstrated that a microcrystalline copper film well bonded to a polymer substrate can be stretched beyond 50% without cracking. The film eventually fails through the coevolution of necking and debonding from the substrate. Here we report much lower strains to failure (approximately 10%) for polymer-supported nanocrystalline metal films, the microstructure of which is revealed to be unstable under mechanical loading. We find that strain localization and deformation-associated grain growth facilitate each other, resulting in an unstable deformation process. Film/substrate delamination can be found wherever strain localization occurs. Therefore, we propose that three concomitant mechanisms are responsible for the failure of a plastically deformable but microstructurally unstable thin metal film: strain localization at large grains, deformation-induced grain growth, and film debonding from the substrate.
I. INTRODUCTION
Flexible electronics are being developed for diverse applications, such as paper-like displays that can be folded or rolled,1 electronic skins for robots and humans,2 drapeable and conformable electronic textiles,3 and flexible solar cells providing portable and renewable sources of energy.4 In some designs, small islands of stiff functional materials and thin metal interconnects are deposited on a polymer substrate. When the structure is stretched, the stiff islands experience small strains, but the metal interconnects must deform along with the substrate. Such considerations have motivated us to study the behavior of polymer-supported metal films undergoing large deformation. A polymer-supported metal film behaves differently from a freestanding metal film. When stretched, a freestanding film of a ductile metal ruptures by forming a neck within a narrow region. Although strain within the neck is large, strain elsewhere in the film is small. Recall that the film typically has an extraordinarily large length-to-thickness ratio. Consequently, the net elongation of the freestanding film upon rupture is small, typically less than a few percent.5–9 For a ductile metal film well bonded to a polymer substrate, finite element simulations have shown that the polymer substrate can retard necking in the metal film, so that the film can elongate infinitely, limited only by rupture of the polymer substrate.10,11 Experimentally, however, most polymer-supported thin metal films rupture at a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0048 J. Mater. Res., Vol. 24, No. 2, Feb 2009
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