Influence Of Surface Effects On Nano-contact Problems

With the incessant development of nanoscience and nanotechnology,Nonmaterial’s are widely used in industrial production. It is important to correctlyunderstand the mechanical behaviors of nanomaterials to achieve successful designand fabrication of nanoscale devices. Since the surface-to-volume ratio of a nanoscaledomain is relatively high compared to that of a macro-scale domain, the nano-scalematerial and structure components show some completely different performance fromthe macro-scale material and structure devices. However, since there is no intrinsiclength scale involved in the constitutive laws of the classical elastic theory, it is unableto predict the size-dependent behavior of nanosized materials and structures devicesinduced by surface effects. Comparison with atom simulation indicates surfaceelasticity is one effective method to analyze size-dependenct mechanical behaviors ofnanodevices. In this thesis, based on surface elasticity, the effects of surface onnano-contact problems are studied. the main researches are listed as follows:(1) The effects of the uncompleted surface stress (only residual surface tension)on contact problems under normal pressure at nano-scale are investigated. Thecomplex variable function method is adopted to derive the fundamental solutions ofthe nano-contact problem. The deformations induced, respectively, by a uniformdistributed pressure and a concentrated force are analyzed in detail. The results revealthat, at nanoscale, the displacement gradient on the deformed surface transitscontinuously across the uniform distributed loading boundary as a result of surfaceeffects, which are distinctly different from those in classical elasticity. In addition, fornano–indentation, the indent depth depends strongly on the surface stress.(2) The effects of completed surface stress which is not only about to residualsurface tension but also to consider the surface energy on the contact problem withshear load at nanometers are studied in the frame of surface elasticity theory. Fourierintegral transform method is adopted to derive the fundamental solution of thenano-contact problems under shear load. The deformations induced by a uniformly distributed shear load and a concentrated shear force are discussed in detail,respectively. The numercial results indicate that, stress and displacements are found todepend on surface elastic constants. There are some interesting characteristics innano-contact mechanics, which are distinctly different from those in macro-contactproblem. At nanoscale, both the contact stresses and the displacements on thedeformed surface transit continuously across the uniform distributed shear loadboundary as a result of surface stress. In addition, the indent depth and the contactstress depend strongly on the surface stress for nanoindentation. Selected numericalresults are presented to study the surface effects.