Paper AS+BI+EM+NL+NS+SS-ThA9
Complimentary XPS and AES Analysis of MoS₃ Solid Lubricant Coatings

Thursday, October 31, 2013, 4:40 pm, Room 204

Molybdenum disulfide (MoS₂) nanoparticles are an ideal additive in solid coating for lubricating mechanisms in vacuum environments, with widespread application in the spacecraft industry. The formation of these nanoparticles can be complex, and the use of MoS₃ nanoparticles, which are produced using a simple wet chemical synthesis is being explored as an alternate approach.¹ The use of MoS₃ as a tribological material has not been explored beyond its use as an oil additive.² There is new interest in investigating its potential for use in solid lubricant coatings.

To aid in the evaluation of the tribological performance of a MoS₃-formulated coating compared to MoS₂ based coatings, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are utilized as complimentary techniques for the surface characterization of the contact wear regions created on the coating surface.

The unique scanning micro-focused monochromatic x-ray source was used to provide x-ray excited secondary electron images that help reveal topographical and surface chemical information which aid in resolving and pinpointing the analysis area of interest within the contact region of the wear track that is 50 µm to 100 µm wide. The micro-XPS results provided quantitative chemical characterization that complement high spatial resolution imaging AES analysis of the sub 100 nm molybdenum sulfide particles.

Tribometer testing showed the MoS₃-formulated coating perform similar to the MoS₂-based coatings, with similar coefficients of friction and endurances in dry nitrogen. MoS₃ nanoparticles produced using simple wet chemical synthesis, and the tribology of resin-bonded MoS₃ nanoparticle coating is comparable to similarly prepared bonded coatings containing MoS₂. The surface analysis results show a lubricating effectiveness that is consistent with the production of a thin film of MoS₂ in the contact region, with an increase in the presence of sulfide relative to polysulfide in the wear track and surface segregation of lubricating species.

We will present results of micro-area XPS and AES surface analyses on worn coatings to reveal changes in composition and chemical state of the coating surface, which might explain the observed friction results of the mechanical testing.

References

1. P. Afanasiev, “Synthetic approaches to the molybdenum sulfide materials,” Comptes Rendus Chimie, 11(1–2) (2008) 159–182.
2. O.P. Parenago, V.N. Bakunin, G.N. Kuz’mina, A.Yu. Suslov, and L.M. Vedeneeva, “Molybdenum Sulfide Nanoparticles as New-Type Additives to Hydrocarbon Lubricants,” Doklady Chemistry, 383(1-3) (2002) 86-88.