Fabrication And Strain Effect Of Periodic Thickness-gradient Films On Flexible Substrates

Thin film-substrate bilayer(or multilayer)structure systems are very common in nature,such as atmosphere,soil rocks,lake ices,epidermal tissues of animals and plants,etc.In artificial systems,thin film materials are widely used in various high-tech engineering fields,such as integrated circuits,information storage,solar cells,electronic components,sensors and so on.Due to the mismatch of the mechanical properties of the film and the substrate and their complex interfacial effects,various instability modes such as folding,delaminating and cracking are frequently observed on the film surfaces driven by stresses.Recently,the researchers in the world have performed a lot of researches on the instability structures in homogeneous film systems.However,the real films and substrates frequently possess non-uniform structures,so the heterogeneous film-substrate system has become a hot topic among the recent researches.In this thesis,a periodic thickness-gradient metal film system is successfully constructed on flexible polydimethylsiloxane(PDMS)substrates by magnetron sputtering.The regulations of the film thickness,the strain and the periodic structure of the mask on the surface instability modes are systematically studied.The main work is as follows.Firstly,periodic thickness-gradient metal silver films on flexible PDMS substrates are prepared by using magnetron sputtering technique and by masking weaved copper grid,and their morphological evolution laws during uniaxial compression/tension process have been investigated.It is found that under the uniaxial mechanical loading,the silver film surface forms stripe wrinkles and multiple cracks,which are aligned perpendicular to each other.The gradient of film thickness well modulates the in-plane stress distribution of the film.As a result,the wrinkles first form in the region with smaller film thickness,and then extend into the region with larger thickness,while the cracks are always restricted to the region with smaller thickness.The morphological characteristics,evolutional behaviors and physical mechanisms of such wrinkles and cracks are analyzed deeply based on the stress theory and finite element calculation.This study could promote better understanding of the strain effects of heterogeneous film systems and find applications in the field of flexible electronics by structural design of thickness-gradient films.Secondly,silver films with different thicknesses are prepared on PDMS substrates by controlling the deposition time.It is found that the film surface forms wrinkle mode or buckle-delamination mode under uniaxial compressive strain.As the deposition time increases,the wrinkle mode will transit into buckle-delamination mode gradually and the out-of-plane displacement increases significantly.The experiment shows that the particularity of periodic thickness-gradient well regulates the distribution of surface buckle-delamination mode.The strain distribution of silver film surface is closely related to the period of mask,and the buckle-delaminations always appear in the areas of smaller film thickness.This provides the possibility to control periodic buckle-delaminations of film surface by copper grid masking.Based on the experimental observation and theoretical analysis,the conditions for forming buckle-delaminations of silver films under compressive strain are discussed.The relationship between the height and width of buckle-delaminations and the compressive strain is in good agreement with the theoretical model.Thirdly,periodic thickness-gradient iron film system is prepared on the PDMS substrate using transmission electron microscope copper mesh with flat surface as the mask template.The effects of the shape,size,height of the mask and deposition time on the wrinkle morphologies are studied.The experimental results show that the periodic thickness-gradient iron film will spontaneously form three main wrinkle modes under the combined action of residual compressive stress and thickness gradient.In the areas of smaller film thickness,wavy wrinkles along the gradient direction and transverse wrinkle domains will be formed.As the film thickness increases,the wavy wrinkles evolve into stripe wrinkles along the gradient direction gradually.In the hollow areas of the mask,the film has the maximum thickness and the surface remains flat.Based on the competition between the compressive stress and the thickness gradient,the formation mechanism of such complex wrinkle mode has been analyzed.It is found that the wrinkle patterns with different shapes can be controlled by changing the shape and size of the mask template.The friction property of the gradient film has been investigated.The existence of periodic wrinkles makes the surface friction exhibiting corresponding periodic oscillation behavior,which provides useful guidance for preparation of friction-related micro devices.