Study Of Friction Characteristics On Micro-textured Silicon Surfaces

Friction between micro components is the key factor that affects the performance and reliability of MEMS (microelectromechanical system) because of scale effect and surface effect. One of the research directions of tribological design in MEMS is exploring and controlling the friction behaviors of micro components by micro surface texturing. In this paper, the micro surface texturing was made on silicon wafer surface, and experiments and theoretical analysis of its friction behaviors were carried out from nano to micro scale.Because there are not satisfied commercial tribology test instruments, a new micro-tribotester was made to measure the pull-off and friction forces in situations of ball-flat and flat-flat contacts, with 0.6-5mN load in varied relative humidity conditions. The resolution of the tester is 8μN. A series of friction experiments of micro textured surfaces were carried out by the micro-tribotester.This paper classified the effects of micro surface topography of components on frcition behavior into two categories, which are effect of interior and exterior contact area. Dry frcition experiments of varied textured surfaces in pin-flat, ball-flat and flat-flat contacts were carried out. The results show that the topography parameters which affect friction force are roughness, radius of curvature of asperity, correlation length and direction of texture. There is the possibility of optimization of surface texturing.The topography of exterior contact area mainly affects the stability of friction. The effects are studied by a new elastic ratchet model presented by this paper. The model indicates that the friction force is affected not only by slope but also radius of curvature of surface topography. In the model, the friction is dominated by slope at nano scale, and by radius of curvature at micro scale. It was validated by experiments. Stick-slip model was developed based on the above elastic ratchet model. The model indicates that the upper head is sliding with high frequency vibrancy in"stick"process, which well explaines the "creep-slip" phenomena. The friction instability of textured surfaces was studied by AFM, UMT and micro-tribotester. The results show that the slip distance is affected by topography parameters.Friction model of flat-flat contact in relative humidity conditions was developed. The model indicates there is a critical relative humidity, upon which the friction force will greatly increase. Adhesion and friction experiments of textured surfaces were carried out. The results show that the pull-off and friction forces increases with dimple area ratio.