All carbon hybrid multi-functional composite thin films

Abstract

The use of nanoscale materials has become common practice in a variety of industries such as electronics, medicine, cosmetics, construction and automobile to name a few. Whether used as dispersed nanoparticles or in the form of coatings and thin films, these nanomaterials open up a world of possibilities for new emerging technologies. Carbon nanomaterials in particular have great potential with a dynamic range of interesting properties in different dimensional structures. Under most application conditions the nanomaterials need to be protected from mechanical and/or chemical disruption by a protective matrix. In this thesis the feasibility and viability of fabricating thin composite films by embedding carbon nanomaterials in a protective matrix is explored. Tetrahedral amorphous carbon (ta-C) containing high sp3 bonded carbon fraction has been selected as the protective matrix due to its high hardness, chemical inertness, optical transparency and ability to be deposited as a thin film. Carbon nanomaterials in the form of single wall carbon nanotubes (SWCNTs) and detonation nanodiamonds (DNDs) were tested in this thesis, due to their increasing usage in applications related to conductive films, thin film transistors, sensing elements and photonic components. SWCNT networks were coated by ta-C using a pulsed filtered cathodic arc process (p-FCVA) and tested to ascertain the electrical and mechanical properties. Experimental results supported by simulations show that ta-C coating with high sp3 carbon fraction of 75% could be deposited onto the SWCNT networks. The ta-C coating encapsulates the SWCNT bundles providing superior mechanical and chemical protection with 60% reduction in wear and limited immunity to oxygen plasma in comparison to uncoated network. The increase of electrical resistance by around 4500% for the SWCNT network post deposition of ta-C coating, has been investigated. The predominant cause of this resistance increase is attributed to the intrinsic compressive stress of the ta-C coating which moves the SWCNT bundle junctions apart, thus increasing the contact resistance leading to drastic increase in the network resistance. It has been shown that use of an intermediate coating between the SWCNT network and ta-C coating limits the resistance increase to manageable proportions. A 9 nm thick evaporated carbon coating as intermediate layer, limited the network resistance increase to 90% and improved the mechanical wear by 50% in comparison to ta-C coated network without intermediate coating. Furthermore, a novel method to deposit DND embedded ta-C composite films in a single step using p-FCVA process has been demonstrated here. The DND-ta-C composite film, with DND concentration of the order of 0.1 vol% has 15% increase of hardness and 40% lower wear in comparison to ta-C film. Even under high load and multiple wear test conditions the DND-ta-C composite film has mechanical performance superior to ta-C films making them an ideal candidate for tribological applications.