Paper TR+BI+SE+TF-ThA12
Understanding Friction in MoS2, Part 2: Water, Oxidation and Run- in

Thursday, November 10, 2016, 6:00 pm, Room 101A

Effects of water vapor and oxidation resistance for amorphous (sputtered) and highly ordered (N₂ sprayed) MoS₂ were investigated with a high-sensitivity, low energy ion scattering (HS-LEIS) spectrometer, molecular dynamics simulations and accompanying tribological testing in each environment of interest. 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 are hypothesized to act as a diffusion barrier to 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 and humid nitrogen 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.