Development of TiN/MoS₂ Composite Tribological Coatings for Cryocoolers

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. The efficiency of the compressor and motor of the cryocooler depends mainly on the friction and wear between the mating parts. Tribological coatings having extremely high hardness, ultra-low coefficient of friction, and high durability at temperatures lower than 60 K are being developed with an aim to reduce the friction and wear, thereby resulting in improved efficiency of the cryocooler. Extremely hard and extremely low friction coatings of DLC, TiN coatings and DLC/MoS₂ and TiN/MoS₂ bilayer coatings have been chosen as part of this ongoing research aimed for this application. This paper presents preparation; characterization (SEM and XRD), microhardness and tribological measurements carried out on TiN and TiN/MoS₂ coatings on aluminum and glass substrate.