Dry friction between rough surfaces of silicon and functionalized gear microelectromechanical systems

Microelectromechanical systems (MEMS or micro-electro-mechanical systems) is a branch of nanotechnology studying microscopic devices, especially those with moving parts. Microgear MEMS is probably the most common type of MEMS that involves transmission of rotational motion between a gear pair. Understanding of physical and chemical mechanisms related to friction between components of microgear MEMS is important for their design and accurate prediction of their tribological properties. In this thesis, various problems related to modelling of tribological performance of silicon microgear MEMS are under investigation. The microgear MEMS teeth have been simulated in the vacuum environment and, therefore, the energy dissipation mechanisms may be reduced the dissociation of chemical and van der Waals interactions as well as the elastic interlocking between counterparts subscale asperities. The models developed have been used to simulate various tribological phenomena, including adhesion, friction, wear and the elastic interlocking of the tooth surfaces. The MEMS tooth roughness is described using statistical approach in accordance with the experimental data obtained by AFM (Atomic Force Microscopy). It has been shown that the microgear MEMS tooth surface roughness does not have any microscale roughness and hence, there is no plastic deformation of the atomic and adhesive subscale asperity due to the Polonsky-Keer effect. The tooth asperity of microgear MEMS is modelled as a nanoblock obtained by superposition of two hierarchical subscales that are specified by the character of interactions at the subscale: an atomic subscale, where chemical interactions are likely to occur, adhesive subscale, where molecular adhesion (van der Waals interaction) is significant and bulk elastic material. The tooth surface is covered by the nanoasperity blocks. According to the roughness studies, a real silicon rough surface has been described at different scales: nanoscale that include atomic subscale of active chemical interactions and molecular subscale of active van der Waals interactions, and bulk elastic scale. The Borodich-Savencu (B-S) one level model that have been developed before for tribology of nominally flat surfaces has been developed. In order to mirror the specific features of interactions between MEMS teeth, the B-S model assumed the gap between the surfaces is constant, while the gap in a gear MEMS pair is changing during the mesh cycle. This was taken into account by the iterative solutions of two-dimensional frictional Hertz-type problems using the Cardiff numerical solver. The new model allows us to model tribology of curved teeth using nanoblocks consisting of atomic and molecular subscales located at varying levels. The apparent friction force and coefficient of friction μ have been calculated by estimations of the total energy per unit length dissipated through the above-mentioned physical and chemical mechanisms. It has been shown that there is a high possibility of stiction (cohesion or the so-called cold welding) between pure silicon MEMS teeth. To improve the system performance and to find ways for controlling III friction effects and reducing stiction possibility, it is suggested to functionalise the MEMS microgear tooth surfaces by self-assembled monomolecular (SAM) layers. A damage model has been developed to study the damage accumulation and wear of these carbon-based functionalised monomolecular layers. The model is based on the Goryacheva-Torskaya model for damage accumulation in fatigue elements. The maximum damage occurs under action of the maximum load, hence the dry friction contact of a single tooth contact is considered. To use the damage model, the surface stresses are calculated. Numerical simulations for silicon-based MEMS micro-tooth surfaces functionalised by monomolecular layer carbon-based coatings show that initially the surfaces do not stick to each other. However, the stiction occur after some number of cycles because the functionalised monomolecular is gradually worn away due to damage accumulation in the layer