Background

It has been shown that the mechanisms of friction and wear on the nanometer scale can be absolutely different from their micrometer or larger scale counterparts. It was found that friction stress is constant and of the order of the theoretical shear strength, when a sliding contact size is small. However, at a critical contact size, there is a transition beyond which the frictional stress decreases with increasing contact size, until it reaches a second transition where the friction stress gradually becomes independent of the contact size. Hence, the mechanisms of slip are size-dependent, or in other words, there exists a scale effect. This phenomenon is not observed in larger scale contacts.[5,6]

Making use of such understanding of tribology on the nanometer scale, recent developments in nanotech-nology have revealed wondrous potential in modifying surfaces on the nanometer scale. By observing lizard's feet, researchers have proven that clusters of nanometer scale contacts can provide strong adhesion just like the spatula at the ends of the hairs on a lizard's feet that allow them to walk on walls and ceilings.[7] On the other hand, based on nanostructures of the surfaces of lotus leaves, engineered surfaces with the right surface energy and nanoscopic bumps can ensure that dust particles do not stick to the surface easily, hence resulting in self-cleansing capabilities.[8]

Hard coatings such as diamond-like carbon layers are widely used as protective coatings on metal substrates, such as steel. Recently, the passivating of silicon surfaces within micro-electromechanical systems using carbon films has been proposed to prevent stic-tion and to reduce friction. Coatings have industrial applications in many engineering components. Both soft and hard coatings can inhibit adhesion between the substrate and the counterface resulting in decreased wear. They may also make surface defects less harmful by moving crack initiation sites from the surface down to subsurface regions, thus improving the performance of the substrate.

Various techniques such as chemical vapor deposition (CVD) and pulse laser deposition are being used to deposit carbon on to silicon.[9] Depending on the technique and conditions, either amorphous carbon films or diamond-like carbon films can be produced, of which the latter exhibit high density, extreme hardness, high thermal conductivity, chemical inertness, and infrared transparency. However, the films often suffer from adhesion problems—partially or totally delaminated at the interface or in the substrate due to high compressive stresses.[10-21] For example, Freller et al.[11] showed that when the compressive stress in a film exceeds the ultimate tensile strength of the substrate, failure occurs in the substrate. Zehnder et al.[12] reported that a film deposited at 20°C showed no evidence of failure in the bulk, but delaminated at the interface, all over the sample. On the other hand, a film grown at 500°C would adhere very well to the silicon substrate, but would delaminate when its thickness exceeds 600 nm. Som et al.[13] studied the delamination

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment