Hydrogen is a clean and sustainable form of carrier of energy that can be used in mobile and stationary applications. At present hydrogen is produced mostly from fossil sources. Solar photoelectrochemical processes are being developed for hydrogen production. Storing hydrogen can be done in three main ways: in compressed form, liquid form and by chemical bonding. Near term spaceport operations are one of the prominent applications for usage of large quantities of liquid hydrogen as a cryogenic propellant. Efficient storage and transfer of liquid hydrogen is essential for reducing the launch costs. A Two Stage Reverse Turbo Brayton Cycle (RTBC) CryoCooler is being developed at University of Central Florida. The cryocooler will be used for storage and transport of hydrogen in spaceport and space vehicle application. One part in development of the cryocooler is to reduce the friction and wear between mating parts thus increasing its efficiency. Tribological coatings having extremely high hardness, ultra-low coefficient of friction, and high durability at temperatures lower than 60 K are being developed to reduce friction and wear between the mating parts of the cryocooler thus improving its efficiency. Nitrides of high-melting-point metals (e.g. TiN, ZrN) and diamond-like-carbon (DLC) are potential candidates for cryogenic applications as these coatings have shown good friction behavior and wear resistance at cryogenic temperatures. These coatings are known to have coefficient of friction less than 0.1 at room temperature. However, cryogenic environment leads to increase in the coefficient of friction. It is expected that a composite consisting of a base layer of a hard coating covered with layer having an ultra-low coefficient of friction would provide better performance. Extremely hard and extremely low friction coatings of titanium nitride, molybdenum disulphide, TiN/MoS2 bilayer coatings, DLC and DLC/MoS2 bilayer coatings have been chosen for this application. TiN film was deposited by reactive DC magnetron sputtering system from a titanium target and MoS2 film was deposited by RF magnetron sputtering using a MoS2 target. Microwave assisted chemical vapor deposition (MWCVD) technique was used for preparation of DLC coatings. These composite coatings contain a solid lubricating phase and a hard ceramic matrix phase as distinctly segregated phases. These are envisioned as having the desired combination of lubricity and structural integrity. Extremely hard coatings of TiN and DLC were chosen to provide good wear resistance and MoS2 was chosen as the lubricating phase as it provides excellent solid lubricating properties due to its lamellar crystal structure. This thesis presents preparation; characterization (SEM and XRD), microhardness and tribological measurements carried out on TiN and TiN/MoS2 coatings on aluminum and glass substrate at room temperature. It also presents initial development in preparation of DLC coatings.

Bulk forming and cutting are widely used to shape metals in industrial production. Bulk forming is characterized by large strains, extensive plastic deformation and large surface expansions. Cutting is characterized by high speeds, high pressures and high temperatures. The prevailing conditions during these processes lead to transfer of work material to tools. In bulk forming this is a significant problem. The transferred work material is hardened and becomes harder than the work material, causing galling. This leads to high friction and high forming forces, bad surface finish of the formed products and significant difficulties to produce complicated geometries. In cutting, transfer of work material can be desired for protection of the tool surface. However, the transfer film has to be of the correct type to provide a stable and predictive behaviour during operation. In this thesis the influence from tool material and surface treatment on work material transfer has been studied for both applications, with the use of simplified laboratory test methods followed by extensive surface studies. Both the tendency to, appearance of and chemical composition of work material transfer is evaluated. The results are compared with real industrial examples, to ensure that the correct mechanisms are mimicked. In forming, the problems arise when poor lubrication prevails, due to high forming forces or large surface expansions. The transfer of work material can then be avoided with the use of a galling resistant coating, offering low adhesion. However, the coating has to be as smooth as possible, to avoid activation of the work material and subsequent transfer. In cutting, the desired transfer film can be obtained by choosing the correct cutting parameters. The geometry and material of the fabricated component is often predetermined, setting the general cutting conditions, but the cutting speed influences the formation of the transfer film. Too low speed or too high speed leads to an unstable cutting process and poor surface finish of the piece. The speed intervals for each mechanism are partly determined by the tool material and thus by the tool coating.