Bilayers are constructed from two materials with distinct properties and one physical dimension an order of magnitude smaller than the others. Due to the property mismatch, dimensional changes in each material will be different when the bilayer is exposed to different environments, resulting in out-of-plane deformations to minimize potential energy. The resulting curvature change can be tuned by altering the material elastic properties and relative thicknesses. In addition, instabilities or defects may initiate during the fabrication process, impacting the eventual curvature of the bilayer.
Existing analyses predict that surface instabilities, such as wrinkles and folds, form in high-property-mismatch bilayers and become more prominent with increased compressive strains locked at the interface. Under compressive stress, cracks are assumed to form only when the interfacial strength is weak
The aim of this work is to describe synthesis parameters and explain observed phenomena on bilayers formed when thin metal-alloy films are sputter deposited on compliant substrates. The deposition parameters create residual compressive strains and strong adhesion in the bilayers, resulting in instabilities that deviate from theory. The experiments revealed cracks on surfaces with strong interfacial adhesion, and nested wrinkles, with smaller wavelengths than predicted.
A numerical model, adopted to explain the observations, revealed that nested wrinkles, with wavelengths comparable to experimental wavelengths, form under certain mismatch-strain profiles. The model suggests that cracks can initiate from the peak of wrinkles when the critical fracture strength of the coating is exceeded.
The work impacts design of bilayers with extreme property mismatches since the fracture toughness of the film may be low enough to initiate cracks and possibly compromise formation of any desired instabilities.