Micro And Nano Scales Adhesion Study Based On The Contact Geometry

With the rapid development of micro-scale/nano-scale devices and instruments, theadhesion in these scales becomes more and more important. The small-scale mechanicalsystems have high surface-area-to-volume ratios. Therefore, these systems are moreinfluenced by surface effects rather than inertia effects. The adhesion force is the chief factorof the failures of micro electro mechanical systems (MEMS) in the manufacture and in use.Meanwhile, in order to further the practicability and miniaturization of MEMS, it is urgent todevelop a sophisticated understanding and control of adhesion between solid-solid surfaces.In this thesis, a single-asperity adhesion model is established based on contact geometry.Meanwhile, the adhesion behaviors between two surfaces, the adhesion of special surfacestructure and the effect of reducing adhesion of diamond-like carbon (DLC) films are studiedby using an atomic force microscopy (AFM).(1) The precise contact geometry is crucial to the quantification of adhesion force. Thesingle-asperity adhesion model is established based on contact geometry. On the basis ofsmoothness assumption of the asperity and surface, the principal curvatures of a point on thesurface are obtained by using the surface theory of differential geometry. By using the firstapproximation of the surface, the contact between a parabolic-shaped asperity andarbitrary-shaped surface patch is transformed into the contact between an elastic ellipticparaboloid and a rigid flat surface. The gaps between the asperity and surface of two kinds ofcontacts are the same when applying normal load, and the elastic properties are not changedafter transformation. By using the theories of continuous medium contact mechanics andHamaker approach, the contact area and adhesion force of each point on the surface areobtained by using numerical methods. At last, two examples are given to show theeffectiveness of the model.(2) In order to study the adhesion between two solid surfaces and prevent wear, a flat tipwith a diameter~1.7μm is used. The adhesion forces of some surface have been determinedby recording the force-displacement curves with the AFM. The adhesion measurements werecarried out under ambient conditions, in a nitrogen-filled glove box, under distilled water, and under potassium chloride (KCl) solution. The outcome shows that the real contact areawithout the applied load is only a small proportion of the apparent contact area. The adhesionforce between solid surfaces cannot be predicted by the theory of thermodynamic surface freeenergy. The measurement stability and repeatability of adhesion by the AFM depend on thesurface characterization, measurement methods and the environment. Under differentenvironments, there are different interactions and factors affecting the adhesion force, and thedominant interactions and factors may be different too. The various interactions and factorsare mutually coupled to determine the final adhesion force.(3) In order to study the influence of temperature on the adhesion, the contact geometryof smooth surface and rough surface is selected. The adhesion measurements were carried outunder ambient conditions and in a nitrogen-filled glove box. The substrate temperaturechanges from30°C to200°C. The results show that when the temperature is less than200°C,the influence of temperature on the normal spring constant can be ignored. In this temperaturerange, the adhesion distribution for each temperature exhibits a Gaussian-like distribution. Inthe glove box, the mean adhesion force decreases with increasing temperature. However, inhumid environment, with increasing temperature, the mean adhesion force first increases andreaches the maximum at~100°C, then begin to decline. At about150°C, the mean adhesionforce decreases dramatically, and remains relatively stable in the high temperature state.(4) Under ambient conditions and in a nitrogen-filled glove box, the adhesion of aninsulating periodic structure is studied by using a sharp tip, a flat tip and a ball tip. Theoutcomes show that the adhesion behaviors are largely dependent on the contact geometry,surface topography and the environment. And these factors are mutually coupled with eachother to determine the final adhesion. When numerous measurements are carried out on thesame location, the adhesion of multi-asperity contact exhibits stratification phenomena. In theglove box (without static charge removal) and under ambient conditions, the adhesion force indifferent layers increases with the increasing measurement number, and jumps betweendifferent layers. The increase of adhesion is attributed to the increase of electrostatic chargesand the increase scale of the capillary meniscus. The electrostatic charges will increase only after contact and separation and have an additive effect. When the charges are saturated, theadhesion behavior becomes stable. In the glove box (with static charge removal), the adhesionforce in different layers changes slightly. By using the ball tip, we found that the difference ofinteraction of adhesion will lead to the different adhesion fluctuation situations on the samespot. That is: when there are some changes, the fluctuation is small; by removing the changes,the fluctuation is the largest; under ambient conditions, the fluctuation is the smallest.(5) Under ambient conditions and in a nitrogen-filled glove box, the effect of reducingadhesion of metal-doped diamond-like carbon (DLC) films is studied based on differentcontact geometries. The outcome shows that the DLC film can effectively reduce adhesionand wear for different contact geometries under different environments. The reduction ofadhesion is attributed to the decrease of surface energy and the increase of the contact angle towater. The reduction ratio is closely related to contact geometry, the roughness of DLC film,material characteristics paired to DLC film and the environment. And these factors aremutually coupled to determine the final reduction ratio.