Surface engineering issues in nanomaterials

As feature sizes in modern materials get smaller, the surface/interface to volume ratio drastically increases. For nano-structured solids (those with one or more dimensions in the 1-100 nm range), it can be argued that the few atomic layers that constitute the interfacial region will dominate most properties. Modification and tailoring of the surface or interface may be the most effective approach of controlling these materials, and incorporating them in engineering applications. This requires an in-depth understanding of structural, physical and chemical properties of these interface regions. The general concept of "surface engineering" is not new, but when the bulk structure to be coated is itself < 100 nanometers in dimension, new challenges emerge. The coating thickness has to be substantially smaller than the bulk dimensions, yet be durable and effective. In this paper, some aspects of effective nano-meter scale coatings have been discussed. Since these are deposited by non-line of sight (plasma) techniques, they are capable of modifying nano-fibers, near net shape cellular foams, and other high porosity materials. Two types of coatings will be focused upon: (i) those that make the surface inert and (ii) those designed to enhance surface reactivity and bonding. The former has been achieved by forming 1-2 nm layer of -CF2- (and/or CF3) groups on the surface, and the latter by creating a nano-layer of SiO2-type compound. Nucleation and growth study of these coatings at the initial stages of formation are performed on model single-crystal surfaces of silicon, sapphire and graphite. It is seen that these coatings completely cover the surface of all these materials within a 3-4 nm coating thickness, and are fully functional by then. They are therefore applicable to nano-structural solids. The effectiveness of these coatings in engineering applications is investigated by applying them on carbon nanofibers and micro-cellular foam structures. Coated and uncoated carbon structures are infiltrated with epoxy matrix to form composites and their mechanical behaviors are compared. Carbon nanofibers are also dispersed in an organic medium, and the influence of coatings on their dispersion behavior is investigated by electron microscopy. In addition, the influence of these nano-films on graphite-metal interfaces is studied with the idea of understanding their applicability to metal matrix composites. The oxide coating is seen to provide significant improvement in epoxy-carbon and metal-carbon bond formation, thereby improving composite behavior. The fluorocarbon coating is seen to work as a deterrent to bond formation, and therefore may be used as a passivatiing layer on nanomaterials.