Synthesis and characterization of hydrogenated amorphous carbon-based tribological coatings

The development of low friction surfaces is needed to improve performance and energy efficiency for macroscopic and microscopic mechanical systems. Minimizing unwanted friction and wear can lead to dramatic economic and environmental benefits. Such research is an important approach to addressing the world's increasing energy concerns.;Hydrogenated amorphous carbon (CHx) thin films are ideal for some tribological applications because of their low wear rates and low coefficients of friction. The primary goal of this research is to develop and characterize modified CHx coatings so that they can be used in a variety of applications in humid environments and under higher contact loads.;Doping CHx films with a small amount of sulfur (CHx+S) enables them to achieve ultralow coefficients of friction in ambient humidity. Temperature-programmed desorption and quartz crystal microbalance were used to determine that sulfur reduces water adsorption onto the film surface. Sulfur-doped films showed a decrease in the activation energy for desorption of water, or weaker film-water bonding. This decrease causes a shorter residence time of water on the surface and less equilibrium water adsorption. At a given relative humidity, sulfur-doped films adsorbed less water than undoped films. Even at 90% relative humidity, sulfur-doped films adsorbed less than 1 monolayer of water. Sulfur acts to passivate dangling bonds at the film surface susceptible to oxidation and reduces the number of surface dipoles available to attract water. This enhanced hydrophobicity increases the contact angle of adsorbed water islands, which lowers the likelihood of coalescence into a water meniscus on the film surface. The decreased quantity and discontinuity of adsorbed water molecules are responsible for CHx+S being able to achieve lower friction in humid environments.;Adding titanium diboride (TiB2) to the CHx coatings yielded films with improved mechanical properties. TiB2 and CH x were synthesized in nanolayer and nanocomposite structures using a dual target magnetron sputtering system. Film crystallinity and hardness were evaluated as a function of coating composition and deposition parameters. The TiB2 component was nanocrystalline and tended to grow in an (001) preferred orientation. Nanolayer films required TiB2 thicknesses of around 0.8nm or greater to retain crystallinity. The hardness of pure TiB 2 coatings averaged around 30GPa. Hardness and modulus of nanostructured coatings increased with the addition of TiB2, generally according to rule of mixtures values. Nanolayer and nanocomposite coatings were also synthesized with a sulfur-doped CHx component. The addition of sulfur had little effect on the crystallinity and hardness of these coatings since it is such a small percentage of the film.;Friction properties of nanolayer films were investigated in humid air. Tribology of a nanolayer coating of 2nm TiB2 : 2nm CHx showed that it had low friction, mu=0.052, which was between the values for pure TiB2 and CHx. Sulfur-doped nanolayer coatings tested against uncoated counterfaces showed lower friction and wear than their undoped counterparts. Thus, the presence of sulfur in the amorphous component of the nanolayer structure was still able to have an appreciable effect on water adsorption. Considering the mechanical and tribological properties of these nanostructured coatings, a film could be designed to have a compromise of improved hardness and low friction.