| Film-substrate bilayer structures are ubiquitous in nature and artificial systems such as crustal structures,plant fruits,animal skins,surface coatings and various film devices.Usually there exists a mismatch of mechanical properties between the film and substrate,therefore a high compressive or tensile stress can be introduced into the film owing to a variety of factors such as force,heating,light and solution.Under tensile stress,the film will fracture into various crack patterns.When the compressive stress is beyond the critical stress,the surface is destabilized to form various surface patterns,such as wrinkling and buckle.The surface pattern is strongly dependent on substrate property.The film is susceptible to form buckle patterns on rigid substrates and homogenous wrinkle patterns tend to be formed on soft substrates.The appearance of wrinkle patterns was usually accompanied by failure of the film properties.In recent years,however,many studies also showed that the wrinkle patterns can be useful in a wide range of applications including flexible electronics,optical devices,fluidic channels,sensors and energy harvester.In order to make full use of wrinkle patterns in various fields,it is necessary to precisely control the morphology and size of wrinkling.Nowadays,regulation of the wrinkle patterns via tuning the mechanical stress has been investigated deeply.However,a systematically research on the regulation of the wrinkle patterns via tuning substrate property remains insufficient.In this thesis,the author studied the effects of substrate property,film thickness and mechanical stress on the surface patterns and explored the pressure sensing effect of wrinkled surface.The main results are listed as following:Firstly,Sylgard 527 gel and PDMS solution were uniformly mixed in different proportions to prepare soft substrates with different elastic moduli.Molybdenum films were deposited on these soft substrates and the quantitative relationship between substrate property and wrinkle patterns was studied.The experiment shows that two classes of distinct wrinkles spontaneously form on the sample surfaces,which are strongly dependent on the substrate modulus.The first class of wrinkling emerges on very soft surfaces,originating from the surface instability of soft substrate during sputtering and its wavelength is independent on the film thickness.On the contrary,the second class of wrinkling emerges on relatively stiff surfaces,originating from the thermal compression of film-substrate system after deposition and its wavelength increases linearly with the increase of film thickness,in consistent with the prediction of continuous elasticity theory.Secondly,elasticity-gradient substrates were prepared by curing liquid PDMS substrates in a gradient temperature field.Then nickel films were deposited on the elasticity gradient substrates and the effects of substrate property on surface patterns were further studied.When the sputtering time is short,networked folds are dominant for soft surfaces while the uniform wrinkle patterns are dominant for stiff surfaces.As the sputtering time increases further,the folds disappear completely and two distinct wrinkling patterns can be observed on the gradient samples.The coexistence of these two classes of wrinkles can be observed in the transition region from soft surface to stiff surface.Thirdly,silver films with different thicknesses were deposited on prestretched PDMS substrates and the morphological evolution of surface patterns under mechanical loading were investigated.The experiment shows that the wrinkle wavelength decreases with the increase of strain under the same film thickness and it increases linearly with the increase of film thickness under the same film strain,in consistent with the theoretical predictions of continuous elasticity theory.The pressure sensors were fabricated by adopting the wrinkle patterns of prestretched samples,and the performances of the sensors were tested and analyzed in detail. |
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