Mechanical Research And Preparation Of Multilevel Topography Based On Film Instability

The surface multi-level morphology widely exists in nature and daily life.For example,there are a lot of "micro-convex" on the surface of the lotus leaf,forming a micro-nano structure,which makes it superhydrophobic.Hairy protrusions and bifurcated ends enhance the adhesion of the toes so that they can climb on extremely smooth and vertical surfaces.These phenomena have caused people to explore the surface micro-topography.With the development of nanotechnology and microelectronics industry,surface microstructures are widely used in the field of engineering technology,and surface thin film instability has attracted people’s attention as an effective method for constructing surface microstructures.In this paper,we study the formation and evolution of surface morphology through theoretical,finite element simulation and experiments for hard film-soft substrate systems with sinusoidal structures.The research on this method is helpful to solve related engineering structural problems and expand the application of membrane-based systems in the field of modern science and technology.It has broad application prospects in the fields of micro-nano manufacturing,flexible electronic devices and three-dimensional self-assembly.Firstly,for the hard film-soft substrate system with a sinusoidal structure surface,the instability behavior of the film on the surface is theoretically analyzed.The theoretical model of the surface instability of the two-dimensional sinusoidal membrane-based system is established,assuming that the film and the substrate are the ideal interface,and there is no slip,debonding and other phenomena.Considering the critical strain and the wavelength of instability of the sinusoidal structure film under uniaxial tensile load,the correction is made based on the solution of the planar film,and the unstable shape of the surface film under different amplitudes is predicted by numerical calculation.Secondly,the finite element method is used to simulate the dynamic buckling process of the surface instability of the sine structure membrane-based system.More emphasis is focused on the geometric parameters of the model(film thickness,model pretension,sinusoidal structure amplitude)and material properties(film-substrate modulus ratio).Effect on surface instability.The results show that all the factors including film-to-substrate modulus ratio,film thickness,stretching amount,and amplitude / wavelength of the sinusoidal structure play a part in affecting the instability of the film.Among them,the film-to-substrate modulus ratio increases,the overall instability wavelength increases,and the wavelength difference between the sinusoidal peak and trough regions does not change;when the film thickness increases,the overall instability wavelength increases linearly,and the wavelength difference hardly changes;With the increase of the stretching amount,the overall instability wavelength of the film decreases linearly,and the wavelength difference is almost unchanged;the amplitude / wavelength of the sinusoidal structure increases,the overall instability wavelength changes less,and the wavelength difference increases linearly.At the same time,using the finite element simulation method to study the instability processof planar thin films,based on the existing theory of critical instability wavelength,an analytical expression of the final instability wavelength of planar thin films is obtained.For sinusoidal film-based systems,the instability mode when the upper film is a non-uniform film is also considered,and a method for obtaining complex surface morphology by changing the geometric parameters and material properties of the film is proposed.Then,the effects of the amplitude of the sinusoidal structure,the thickness of the surface film,and the Young’s modulus on the instability morphology are studied experimentally.A 3D printing technology is used to obtain a mold with a sinusoidal structure,and a PDMS sample with a sinusoidal structure is obtained by pouring.Then,the self-made pre-stretching device is used to complete the stretching-irradiation-releasing process.The surface of the sample forms a "hard layer" due to irradiation.After release,instability ripples are generated.The thickness of the "hard layer" and Young’s modulus are controlled by the irradiation time.The field emission scanning electron microscope is used to observe the instability ripples on the sample surface.The experimental results show that the longer the irradiation time,the longer the surface instability ripples.The surface instability can be effectively controlled by controlling the irradiation time in different areas of the surface,which is qualitatively consistent with the finite element simulation results.Finally,the surface of the sinusoidal sample with instability ripple is coated with nano-scale silica particles to build a stable micro-nano composite structure.The surface hydrophobicity and friction resistance are tested.The results show that the surface with micron-level unstable ripples has better hydrophobic performance and abrasion resistance than the surface with only silica coating.