Electrodeposited MoxSyOz/Ni Tribological Coatings

Deposition of molybdenum disulfide (MoS₂) coatings using physical vapor deposition (PVD) and mechanical burnishing has been widely assessed for solid lubricants in space applications but still suffers from line-of-sight constraints on complex geometries. Here, we highlight one of the first demonstrations of electrodeposited Mo_xS_yO_z and Mo_xS_yO_z/Ni thin-film coatings from aqueous solutions of ammonium tetrathiomolybdate for solid lubricant applications and their remarkable ability to provide low coefficients of friction and high wear resistance. Characterization of the coating morphology shows amorphous microstructures with a high oxygen content and cracking upon drying. Even so, electrodeposited Mo_xS_yO_z can achieve low steady-state coefficients of friction (μ ∼ 0.05-0.06) and wear rates (2.6 × 10^-7 mm³/(N m)) approaching those of physical vapor deposited coatings (2.3 × 10^-7 mm³/(N m)). Additionally, we show that adding dopants such as nickel increased the wear rate (7.5 × 10^-7 mm³/(N m)) and initial coefficient of friction (μ_i = 0.23) due to compositional modifications such as dramatic sub-stoichiometry (S/Mo ∼ 1) and expression of a NiO_x surface layer, although doping did reduce the degree of cracking upon drying.