Electrodeposited MoxSyOz/Ni Tribological Coatings

Abstract

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.

Figure 1

SEM and average roughness (R_a) measured by SWLI (R_a inset) of (a) electrodeposited Mo_xS_yO_z, (b) electrodeposited Mo_xS_yO_z doped with Ni, and (c) PVD MoS_x.

Figure 2

TEM and corresponding EDS profiles through the thickness of (a) electrodeposited Mo_xS_yO_z, (b) electrodeposited Mo_xS_yO_z/Ni, and (c) PVD MoS_x. Electrodeposited coatings appear amorphous, while the PVD MoS_x coating is nanocrystalline. EDS shows Ni diffusion into the coating for the electrodeposited MoS₂ coating and a ∼10 nm thick NiO_x surface layer for the electrodeposited Mo_xS_yO_z/Ni coating. The intensity of Ni, Fe, and Ti are scaled by 0.2, indicated with an asterisk (*).

Figure 3

X-ray diffractograms of the electrodeposited Mo_xS_yO_z coatings showing no discernible peaks relating to Mo_xS_yO_z and of the PVD MoS_x coating with a strong (002) peak. Peaks corresponding to iron are from the substrate.

Figure 4

Rutherford backscattering spectra with corresponding simulated elemental fits from SIMNRA of (a) electrodeposited Mo_xS_yO_z, (b) electrodeposited Mo_xS_yO_z doped with Ni, and (c) PVD MoS_x. (d) Coating elemental compositions calculated from RBS show high concentrations of oxygen for the electrodeposited coatings and a trace of nickel in the electrodeposited pure Mo_xS_yO_z coating.

Figure 5

High-resolution XPS spectra of Mo 3d with corresponding fits of (a) electrodeposited Mo_xS_yO_z, (b) electrodeposited Mo_xS_yO_z doped with nickel, and (c) PVD MoS_x. The concentration of surface oxide to sulfide is shown in panel (d), where oxide is grouped as all components that cannot be assigned to the Mo(IV) S peak.

Figure 6

(a) Average coefficient of friction as a function of sliding cycles with standard deviation (shaded regions) of the three tested coatings. (b) Wear rate at different numbers of sliding cycles showing convergence to similar wear rates after 10,000 cycles of sliding. Note that the numbers above each point are in units of 10^–6 mm³/(N m).