TiB2/VN/Ag composite system for smart chameleon coatings development

To overcome drawbacks of conventional protective coatings, adaptive nanostructured composite films can be considered. Although challenging, practical realization of such composite systems is extremely rewarding for tribological pairs subjected to multiple changes of the environment and/or applied solicitation. Smart tribological coatings can resist friction and wear by adjusting dynamically surface microstructure and chemistry in response to changing operating conditions, thus providing an appropriate response to external stimuli [1-3]. By optimizing the interaction between the constituent phases and the design of the entire coating, an independent control of generally antagonistic tribological properties can be obtained. Such coatings require a through careful control of composition, morphology and architecture, and can be obtained using a multi-(nano)layer (superlattice) architecture where the intermixing of the different layers during wear produces a lubricous tribofilm. The present work aims to develop anti-wear chameleon coatings, with low environmental impact, based on nitrides and borides, containing a solid lubricating phases. Based on the synergistic effect of the various components, these coatings are capable to maintain low friction and wear values in different environments and temperature ranges. In particular, TiB2/VN/Ag composite system is currently under construction and investigation, where TiB2 represents the hard phase and VN/Ag the lubricant. At various temperatures, the lubrication function is provided by different lubrication phases, which evolve chemically and physically, are activated separately and operate in synergy. The chosen deposition technique is the High Power Impulse Magnetron Sputtering (HiPIMS) [4], a PVD technology able to produce ultra-dense hard films with increased toughness, which represent a suitable matrix for incorporating nano/micrometric lubricating phases. In order to achieve a deposition proof for the complete architecture of TiB2/VN/Ag system, the optimization of each constituent is needed. Following the preliminary study on VN, herein, the optimization study about titanium diboride (TiB2) coatings on Ti-6Al4V alloy is presented [5]. It is a hexagonal ceramic material having high hardness, excellent corrosion resistance, and good thermal oxidation and wear resistance. Several research works show that TiB2, in addition to having high hardness and thermal stability, can be very efficient in non-ferrous alloy cutting tools, when a strong buildup edge forms. TiB2 films have been physically, chemically, tribologically and mechanically investigated, by XRD, SEM/X-EDS, nanoindentation, scratch and wear tests.