NANOINDENTATION OF THIN-FILM-SUBSTRATE SYSTEM DETERMINATION OF FILM HARDNESS AND YOUNG'S MODULUS

In the present paper,the hardness and Young's modulus of film-substrate systems are determined by means of nanoindentation experiments and modified models.Aluminum film and two kinds of substrates,i.e.glass and silicon,are studied.Nanoindentation XP Ⅱ and continuous stiffness mode are used during the experiments.In order to avoid the influence of the Oliver and Pharr method used in the experiments,the experiment data are analyzed with the constant Young's modulus assumption and the equal hardness assumption.The volume fraction model(CZ model)proposed by Fabes et al.(1992)is used and modified to analyze the measured hardness.The method proposed by Doerner and Nix(DN formula)(1986)is modified to analyze the measured Young's modulus.Two kinds of modified empirical formula are used to predict the present experiment results and those in the literature,which include the results of two kinds of systems,i.e.,a soft film on a hard substrate and a hard film on a soft substrate.In the modified CZ model,the indentation influence angle,(?), is considered as a relevant physical parameter,which embodies the effects of the indenter tip radius, pile-up or sink-in phenomena and deformation of film and substrate.

Experimental Research of Dynamic Response of Laser-Induced Film-Substrate System

Film-substrate’s interfacial bonding strength is closely related to film quality. An excellent interfacial bonding strength is the premise for the well use of film. The laser detecting technique of discrete scratches based on laser shockwave effect is a new method, which can measure interfacial bonding strength. With this technique, film-substrate system is of transient load of different laser energy, the relation between the dynamic response characteristics of such film-substrate system and film-substrate’s interfacial bonding strength is a core problem to be solved urgently. On this basis, this paper conducted research on the dynamic response characteristics of film-substrate system during laser loading process using detecting technique of PVDF patch sensor. Results show that under the irradiation of different laser energy, it can detect dynamic responses of theory models of different film-substrate system using PVDF patch sensor, wherein shockwave dynamic response and dynamic strain response are included. Laser energy and interfacial bonding strength are of a regular influence to the dynamic response of film-substrate system theory model.

Efficient model for the elastic load of film-substrate system involving imperfect interface effects

In this paper,an efficient calculation method based on discrete Fourier transformation is developed for evaluating elastic load induced elastic deformation fields of film-substrate system.Making use of 2 D discrete Fourier transformation,the elastic fields induced by Hertz load is harvested in frequency domain,and the displacement and stress fields across the interface are enforced to satisfy the elasticity conditions for each Fourier modes.Given arbitrary distributed stress field at free surface plane of the three types of film-substrate systems,unique resultant elastic field within the can be harvested.Hertz load of half space,elastic film on elastic substrate,elastic film on rigid substrate system and elastic film-substrate system with three types of imperfect interface models are investigated:(1)the spring-like imperfect interface model which can be described as:u^fk|zf=-h-u^sk|z^s=0=KTσKZ and u^fz|zf=-h-u^sz|z^s=0=KNσZZ;(2)the dislocation-like interface model,where interface displacement and stress components relation can be described as:u^fi|zf=0=k^uiju^si|z^s=0 andσ^fiz|z^f=0=σ^siz|zf=0=σ^siz|z^s=0;(3)the force-like interface model,where interface displacement and stress components relation can be described as:u^fi|z^f=0=u^si|z^s=0 andσ^fiz|z^f=0=k^tijσ^siz|z^s=0 respectively.Finally,several simulation examples are performed for verification of the reliability and efficiency of the proposed semi-analytical methods.

Bending characteristic of a cantilevered magnetostrictive film-substrate system

The bending problem of a film-substrate cantilever with arbitrary film-to-substrate thickness ratio is solved exactly by employing the force equilibrium equation, and then the optimization and application of the bending characteristic of the magne-tostrictive cantilever is discussed. Furthermore, the influence of geometrical and physical parameters of the two cantilever components on the maximum free-end deflection of the cantilever is addressed. The results indicate that as the substrate thickness is kept constant, the greater film-to-substrate stiffness ratio will induce a larger deflection, while for the case of fixed total cantilever thickness, the optimal cantilever deflection is independent of the physical parameters of the materials such as Young’s modulus and Poisson’s ratio.

Asymptotic solutions for buckling delamination induced crack propagation in the thin film-compliant substrate system

In a thin film-substrate system in-plane compressive stress is commonly generated in the film due to thermal mismatch in operation or fabrication process. If the stress exceeds a critical value, part of the film may buckle out of plane along the defective interface. After buckling delamination, the interface crack at the ends may propagate. In the whole process, the compliance of the substrate compared with the film plays an important role. In this work, we study a circular film subject to compressive stress on an infinitely thick substrate. We study the effects of compliance of the substrate by modeling the system as a plate on an elastic foundation. The critical buckling condition is formulated. The asymptotic solutions of post-buckling deformation and the corresponding energy release rate of the interface crack are obtained with perturbation methods. The results show that the more compliant the substrate is, the easier for the film to buckle and easier for the interface crack to propagate after buckling.

In situ method for stress measurements in film-substrate electrodes during electrochemical processes:key role of softening and stiffening

Electrode stress is one of the main driving forces of electrochemical degradation,which is directly related to battery cycle life,thus attracting great interest.Herein,we propose an in situ method to measure bilayer stresses in film-substrate electrodes during electrochemical processes.This method consists of two parts:stress models featuring Li-dependent material modulus and in situ deformation measurements,through which electrode bilayer stresses evolution accompanied by Li-dependent material modulus can be quantitatively characterized.As application of the method,typical silicon-composite and carbon-composite film-substrate electrodes are selected for in situ mechanical measurements and experimental analysis is performed.Results show that silicon material and carbon material exhibit significant,continuous softening and stiffening,respectively.In two film-substrate electrodes,electrode material films experience compressive stress and current collector substrates undergo a tensile-to-compressive conversion across the thickness.Besides,moduli and stresses in both electrodes vary nonlinearly with capacity,presenting non-overlapping paths between lithiation and delithiation.Based on experimental data,we further demonstrate the key role of Li-dependent modulus on electrode stresses,finding that silicon material softening decreases and carbon material stiffening increases electrode stresses.The deficiencies of current stress measurement method based on Stoney equation and the applicability of our method are discussed.

Regulating wrinkling patterns by periodic surface stiffness in film-substrate structures

Surface wrinkling of materials holds promise for important applications in diverse fields such as multifunctional surfaces and biomedical engineering. For these applications, it is of interest to attain various surface wrinkles with tunable wavelengths and amplitudes. Through a combination of experiments and numerical simulations, we here propose a method to regulate the wrinkling patterns in a film-substrate system by introducing periodic surface stiffness, which is generated through sequential specified ultraviolet-ozone(UVO) treatments. Both experiments and numerical simulations demonstrate that the proposed technique can produce various patterns with wide, tunable geometrical features and anisotropy. The effects of surface stiffness distribution, the exposure durations of UVO-treatments, and the loading biaxiality are examined on the generated surface patterns.

Design and optimization of cantilevered magnetostrictive film-substrate microactuator

The exact solution for the bending problem of a free-end point loaded film- substrate cantilever with arbitrary film-to-substrate thickness ratio is obtained by using the basic mechanical equilibrium equation. And then the problem of design and optimization for microactuator buildup of film-substrate cantilever is discussed by taking into account the effect of geometrical and physical parameters of the cantilever components. Furthermore, the optimal condition for actuator application is presented and some theoretical problems are clarified. The results show that, in general, the greater the film-to-substrate thickness ratio, the higher the ability of taking load, namely the larger the exerted force of the cantilever when the thickness of substrate is kept constant. When the total thickness of the cantilever is kept constant, however, the free-end exerted force will experience a maximum and this maximum value of the exerted force will decrease with the increasing film-to-sub- strate stiffness ratio. Meanwhile, the optimal thickness ratio corresponding to this maximum exerted force also decreases with the increasing stiffness ratio. Whether for the cases of fixed substrate or fixed total thickness, the influence of Poisson’s ratio of two cantilever components on the exerted force is remarkable, and should not be neglected.

An estimation method on failure stress of micro thickness Cu film-substrate structure

The failure of thin film-substrate structure occurs mainly at the thin film or the interface. However, the characterizing and estimating methods of failure stress in thin film are neither uniform nor effective because there are some complex effects of such as size, interface and stress state on the failure behavior of thin film-substrate structure. Based on the scanning electron microscope (SEM) in-situ in- vestigation on the failure models of the Cu thin film-substrate structure and the nano scratched testing results, the failure stresses in different thicknesses of the Cu film-substrate were characterized, which were compared and confirmed by other methods, such as Stoney formula and other empiric equations. These results indicate that the novel estimating method of failure stress in thin film based on the critical wavelength of surface unstable analysis is better than other methods. The main reason is that the novel estimating method of failure stress in meso thickness film fully considered the effect factors of free surface unstable behavior and elastic anisotropy of thin film. Therefore, the novel estimating method of failure stress assists people to understand the critical interfacial strength and to set up the failure criterion of thin film-substrate structure.

Analysis of film strain and stress in a film-substrate cantilever system

The bending problem of a magnetic film-nonmagnetic substrate cantilever system is studied by using the principle of energy minimization. Emphasis is placed on the analysis of geometrical and physical parameter dependence of the neutral plane,internal film stress and strain of the cantilever system,and then the influence of such a parameter on the bending characteristic is presented. The results indicate,owing to the anisotropic expanding feature of the magnetostriction,that the neutral plane is generally anisotropic,and moves downwards rapidly with the increasing thickness ratio. Meanwhile,the bounding rigidity of substrate on the film will de-crease with the increasing thickness ratio,and thus release the film stress,i.e.,it decreases,but the film strain increases. The effect of Poisson’s ratio of the materi-als on the film strain,the stress and the neutral plane in the direction transverse to the magnetization is prominent. For the strain and the stress in the magnetization,however,the role of Poisson’s ratio is inconspicuous. This property is due to the initiative elongating (or contracting) feature of the magnetic film along its mag-netization.

Emergent antisymmetric wrinkling patterns in films on ridged substrates

We report the formation of antisymmetric wrinkling patterns in films on ridged substrates induced by the buckling instability of the substrates via finite element simulations and experiments.Our simulated results reveal that the uniaxial compression along the ridge can trigger both the wrinkling instability of the film and the lateral buckling instability of the ridge.The latter could change the wrinkles from a symmetric pattern to an antisymmetric pattern in a range of film-substrate modulus ratio and aspect ratio of the ridge profile,as validated by the experimental observations.A three-dimensional phase diagram with four buckling patterns,i.e.,sole ridge buckling pattern,antisymmetric wrinkling pattern with different wavelengths from ridge buckling,symmetric wrinkling pattern without ridge buckling,and antisymmetric wrinkling pattern with the same wavelength as ridge buckling,is built with respect to the uniaxial compression,modulus ratio,and aspect ratio.The results not only elucidate how and when the interplay between the wrinkling instability and the ridge instability results in the formation of the antisymmetric wrinkling pattern but also offer a way to generate controllable complex wrinkling patterns.

Preparation and characteristics of a recycled cement-based superhydrophobic coating with dirt pickup resistance

We introduce a low-cost and effective technique that can transform waste cement-based dust into a superhydrophobic coating with dirt pickup resistance. An organic-inorganic hybrid superhydrophobic coating is prepared by the sol-gel method using methyltriethoxysilane as a precursor and waste cement-based dust as a film-forming material. Orthogonal experiments and a comprehensive scoring method were used to optimize the composition and production technologies. Our results show that this superhydrophobic organic-inorganic hybrid coating has an average static contact angle of 151.65° and low water adhesion. Related tests reveal that the dirt pickup resistance, washing resistance and film-substrate cohesion of this coating are also outstanding. The multi-scale physical and chemical mechanisms behind the properties of the coating are investigated. This recycled cement-based coating can be used as the external cover of engineering structures to protect them from corrosion.

Multi-scale simulation of three-dimensional thin-film lubrication

For three-dimensional(3D)mono-layer molecular thin-film lubrication,the elasticity of the substrate affects the tribological behaviors of a thin fluid film confined by two solid substrates.To account for the elastic effects,this study establishes a multi-scale method that combines an atomistic description of the near region with a coarse-grained description of the far region of the solid substrate to simulate the thin-film lubrication.It is demonstrated that for a given temperature range and film-substrate coupling strength,the multi-scale method is in excellent agreement with the fully atomistic simulation.This study reveals that the elastic response of the substrate can be effectively rendered in the hybrid scheme.In the application of the multi-scale method to investigate the tribological properties of the multi-layer molecular thin-film lubrication,it is determined that the systematic static friction coefficient monotonously decreases as the molecular layer thickness in the fluid film increases.In comparison to the mono-layer molecular thin-film lubrication,the multi-layer molecular thin-film lubrication plays a role in reducing the friction and wear of the system.