Researches On The Adhesive Contact And Rolling Friction At Micro And Nano Scales

Adhesion between contact bodies at the micro/nano-scale plays a critical role in the reliability and service life of micro/nano-mechanical systems. To avoid the adhesion failure and increase the system performance, on one hand, the generalized adhesive contact model considering size effect and surface effect should be established to provide theoretical foundations for the accurate prediction of adhesive contact and friction characteristics. On the other hand, effects of surface coating and surface texture on the adhesive contact behaviors should be investigated to provide criteria for the adhesion-reduction design of micro/nano-systems. However, the present researches on the above problems are still in a relatively backward stage. In this thesis, five research topics including the adhesive contact model, adhesion map, surface coating, surface texture and microscale rolling friction are investigated. The main contents are as follows.The equation of effective adhesive contact pressure including parameters of size ratio of the microsphere, surface energy of the coating and that of the substrate is derived based on the Hamaker summation method and the Lennard-Jones intermolecular potential law. After combining with the elastic responses of the coating/substrate system, the model of adhesive contact between a microsphere and a coating surface is proposed. Then, the model of adhesive contact between a microsphere and a textured surface is established.For the adhesive contact of smooth surfaces, a new adhesion map which considers the size effect and provides approaches at jump instabilities is constructed. Fitting equations for describing the variations of critical approach, interaction force, contact radius and pull-off force with the Tabor parameter and the size ratio are obtained.For the adhesive contact of hard coating/substrate systems, the effects of coating thickness and coating-to-substrate elastic modulus ratio on the pull-off force and the jump instabilities are analyzed emphatically. Results show that the pull-off force decreases at first and then increases with the increase of the coating thickness. When the coating-to-substrate elastic modulus ratio increases, the pull-off force decreases at smaller coating thicknesses, decreases at first and then increases at middle coating thicknesses, while increases monotonously at larger coating thicknesses. The jump instabilities of the system are mainly influenced by the coating thickness.For the adhesive contact of textured surfaces, the influences of texture shape, contact position on the surface, texture density, maximum height and radius on the pull-off force are inspected. Results demonstrate that effects of sphere-shaped textures on reducing adhesion are more obvious than cylinder-shaped or cube-shaped textures when the coverage area ratio, maximum height and interval of textures are fixed. For surfaces with sphere-shaped textures, there exists an optimal range of texture density in which the mean pull-off force is small and its variation is insignificant. With the increase of the texture maximum height or the decrease of the texture radius, the pull-off force decreases, while when the maximum height-diameter ratio of textures is fixed, a minimum pull-off force can be achieved.A numerical model of the microscale rolling friction considering adhesion hysteresis is proposed. Effects of the size ratio, relative amount of adhesion hysteresis, Tabor parameter and the external load on the maximum rolling friction torque are inspected. Results indicate that due to adhesion hysteresis at microscale, the maximum rolling friction torque presents a sublinear relationship with the external load, and its dimensionless value at zero load is not zero which not only increases with decreasing size ratio, showing clear size effect, but also increases with increasing relative amount of adhesion hysteresis and Tabor parameter.Based on the silicon microball bearing, an experimental platform for measuring the microscale rolling friction is built, and in-situ measurements of the angular displacement and normal force of the rotor are realized. The variation of the rolling friction torque with load is obtained through spin-down deceleration tests. It is found that the microscale rolling friction torque for a single contact pair between the microball and the raceway exhibits a sublinear relationship with load, which is the same as the numerical results in variation trend.