Anti-wear and friction-reduction mechanism of Sn and Fe nanoparticles as additives of multialkylated cyclopentanes under vacuum condition

► A space liquid lubricant and two nanoparticle additives were studied under vacuum condition. ► In vacuum, the steel balls exhibited transient seizure-like high friction lubricated with MACs. ► Sn and Fe nanoparticles could effectively avoid the transient high friction. ► Shear modulus and crystal structure of nanoparticles were the main factors. ► The main tribological mechanism is the formation of a protective metallic film.