Molecular Dynamics Simulation Of Adhesive Contact Mechanics Between Nanoscale Rough Surfaces

In the macro world, the force and movement are transferred through the contact of local boundary among two or more objects, which is known as contact problems. It is one of the important branches of the Tribology and the fundament to analyze wear and lubrication problems of friction. Contact problems widely exist in a wide variety of areas; Micro/Nano contact problems have become problems that seriously restrict the development of MEMS technology. Contact interface is usually much smoother in MEMS. When it's a stationary contact or the gap of two surfaces is in nano-scale, it is the surface adhesion force making two surfaces contact together that lead to high friction and high wear. It not only affects the performance of micro-machine but also causes the operation to be invalid, thus this is one of the factors of waste products in micro-machine manufacturing, resulting in low success rate and increasing cost of MEMS .To explore the essence of nano-contacts and establishing the proper theoretical system especially for the rough surface contact theory close to the real surface, we can make reasonable explanations for some phenomena such as adhesion, friction and wear in design and manufacturing process of MEMS device, and it is also useful in guiding the design and manufacture of MEMS devices, improving products performance and percent of pass, manufacturing new products. In this paper, with molecular dynamics method, setting diamond as an example, we study the adhesive contacts between rigid sphere probe with smooth surface and flexible substrate using Tersoff Potential, to demonstrate whether the macro-contact model is applicable for predicting the nano-scale contact behavior. It shows that for small load, it is the adhesion that affects contact behavior and the relation of load and contact area agrees with the COS theory, however, for large loads, the bulk properties of materials contribute to the contact behavior which is consistent with the Hertz theory.At the same time, we characterize the fractal rough surface effectively with the W-M function, adopting Tersoff Potential to research the contact process of rough surface of rigid sphere tip with different fractal dimension and elastic sample, preliminary discussing the effect and impact of surface morphology on adhesion properties, and analyzing the relation of load, contact displacement and contact area with different fractal dimension. Simulation results indicate that rough topography has influences on load-displacement curve, which is similar with the role of adhesion at smooth contact, fractal topography only has impacts in a small load range, however, with large loads, the bulk properties of materials contribute to the load-displacement curve. The relation of real contact area with load is linear, which also agrees with G-W model qualitatively.