Abstract
Micro-Electro-Mechanical-Systems (MEMS) are miniaturized devices that operate at small-scale. Actuators-based MEMS have not been commercially realized yet, owing to the manifestation of high surface forces such as adhesion and friction between their moving elements. In recent years, inspiration from ‘Lotus Effect' has opened a new direction in the field of micro/nano-tribology to manipulate/control surface forces at small-scale. The underlying principle discovered from the ‘super-hydrophobic nature' of the leaves of water-repellent plants has led to the design and development of various ‘biomimetic tribological surfaces' that exhibit remarkable reduction in surface forces under tribological contact. In this chapter are presented the tribological issues in MEMS devices, examples of conventional solutions for the tribological issues and unique ‘bioinspired solutions' that have great capability to mitigate surface forces at micro/nano-scales.
TopIntroduction
Micro-Electro-Mechanical Systems (MEMS) are miniaturized devices that perform intelligent functions. They can be classified as: (i) sensors based, which have sensing elements and (ii) actuators based, which have elements that undergo mechanical motion. Micro-motors/engines, micro-gears and micro-shutters are examples of actuators based devices. Figure 1 shows a micro-gear made from silicon (http://www.sandia.gov/mstc/_assets/images/mems/gallery/gears/1.jpg). In these devices, tribological issues such as adhesion, friction and wear strongly manifest, which undermine the mechanical motion of MEMS elements (Kim, Asay, & Dugger, 2007). Therefore, it is imperative to solve these tribological issues in order to realize smooth operation and increased operating lifetimes of actuators based MEMS. Conventionally, thin films/coatings have been researched for their application to MEMS as solutions to mitigate the tribological issues. Examples include self-assembled monolayers (SAMs), diamond-like carbon (DLC) coatings, polymers and perfluoropolyether films (PFPE) (Bhushan, 2001). In recent years, bioinspired approaches have attracted attention as better alternative solutions for MEMS. This chapter highlights bioinspired approaches that are promising tribological solutions for actuators based MEMS devices.
TopBackground
MEMS are built at micro/nano-scale. At these scales, the ratio of surface area to volume is high. Hence, body forces such as inertia and gravity become insignificant. In contrast, surface forces such as capillary, van der Waals, electrostatic, and chemical bonding dominate. These surface forces cause adhesion at the interface of contacting MEMS elements. Amongst these forces, the capillary force that arises due to the condensation of water from the environment is the strongest. Further, adhesion strongly influences friction at micro/nano-scale (Maboudian & Howe, 1997). The magnitude of these surface forces is comparable with those that drive the motion of MEMS elements, thereby rendering the elements completely inoperable. MEMS are traditionally made from silicon due to availability of the process knowledge developed for the material in semiconductor industries. However, silicon does not have good tribological properties (Bhushan, 2001). Silicon due to its inherent hydrophilic nature experiences high surface forces, and because of its brittle nature it undergoes severe wear. Thus, the improvement of tribological performance of silicon is the key to realize the smooth operation of actuators based MEMS. Williams and Le have presented an excellent review on the tribological issues in MEMS devices (Williams & Le, 2006).
Tribological properties of materials at micro/nano-scale are evaluated using atomic force microscopes (AFM) and micro-tribo testers. With these instruments, contact conditions similar to those in MEMS can be easily simulated (loads ~nN to μN-mN; area ~few hundreds of nm2).
Key Terms in this Chapter
Lotus Effect: It is the phenomenon of super hydrophobicity and self-cleaning displayed by the leaves of lotus plant. The unique characteristic of lotus leaves to avoid getting wet is due to the presence of micro-protuberances and waxy nano-crystals on the lotus leaves surfaces.
Tribology: It is the science and engineering of interacting surfaces in relative motion. It encompasses the study and application of the principles of friction, wear and lubrication.
Real Contact Area: It is the real area of contact between two surfaces that are in contact under tribological conditions. The real contact area constitutes the areas where asperities of the contacting surfaces meet. The real area of contact is lower by 3 to 4 orders of magnitude when compared to the apparent area of contact (geometrical area).
MEMS (Micro-Electro-Mechanical Systems): They are micro/nano-scale devices that perform sensing and/or actuating functions. They are made using the techniques of microfabrication. Silicon is the most popular MEMS material.
SAMs (Self-Assembled Monolayers): They are molecular assemblies of organic molecules, which are formed on surfaces by adsorption or chemical grafting. The thickness of SAMs is typically in the range of few nanometers to tens of nanometers.
Friction: It is the resistance to motion. It is the resistance that one surface encounters when moving over another. Friction causes loss in energy.
Adhesion: It is the tendency of surfaces to cling/stick to one another. Materials with higher surface energy exhibit higher surface adhesion and vice versa.
Biomimetics: It is the use and implementation of concepts and principles found in nature/natural materials for man-made technology.
Wear: It is the unintentional material removal/loss under relative mechanical motion between contacting surfaces.
DLC (Diamond-Like Carbon): It is a class of amorphous carbon material that has certain properties similar to that of diamond (e.g. hardness). Usually it is applied to surfaces in the form of thin films/coatings by physical/chemical vapor deposition techniques. DLCs are semi-hydrophobic and are good lubricant materials.