MoS2 coatings are used in a wide variety of applications where low friction is needed and performance is required. For parts manufactured in the aerospace sector (such as satellites) this means coatings must withstand a history of testing and storage in humid environments before launch and outside of targeted operating conditions in low earth orbit. This study utilizes MoS2 coatings deposited via rf-magnetron sputtered (amorphous) and N2 sprayed (highly ordered basal texture). Effects of oxidation resistance for amorphous and highly ordered MoS2 were investigated with a high-sensitivity, low energy ion scattering (HS-LEIS) spectrometer. Recent studies have shown that N2 sprayed MoS2 coatings possess a preferential surface parallel basal plane texture as deposited due to the kinetic energy imparted during spraying, effectively shearing MoS2 particles onto the surface. As such, the highly ordered structure of the sprayed coatings both at the surface and throughout the bulk of the film possess large continuous crystallites of MoS2. It is hypothesized that the large fraction of crystallites on the surface as compared to their sputtered counterparts help to act as safeguards against intrusion of environmental contaminants. Coatings were exposed to molecular oxygen at 250°C and atomic oxygen at 20°C for 30 minutes each and subsequently depth profiled in the HS-LEIS. Results show that N2 sprayed coatings were successful in limiting the depth of oxidation for both types of exposure. The main contributor, however, to increased initial friction post exposure was the type of coating (amorphous vs highly oriented). Tribological experiments in dry nitrogen and humid air showed the initial friction response to be unaffected for sprayed samples while greatly affected for sputtered. Spiral orbit tribological testing was utilized in dry and humid nitrogen environments to further assess the effect of prolonged sliding on purely amorphous MoS2 with and without formation of a transfer film. It is hypothesized that water does not poison friction behavior of established films of highly oriented MoS2, but it does poison the ability to form long range order and sintering of crystallites during run-in.

Understanding friction in MoS2: Structure, oxidation and run-in

253rd National Meeting (2017)

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