Tribological Properties of MoS₂ Lubricants with Different Morphologies in an Ionic Liquid

Tribological properties of MoS₂ lubricants with different morphologies were investigated in an ionic liquid of 1-hexyl-3-methylimidazolium hexafluorophosphate. Both MoS₂ hollow spheres and nano-platelets improved the friction reduction and wear resistance of ionic liquid under relatively low loads and sliding velocities. However,MoS₂ micro-platelets had less effect on the friction reduction and even slightly weakened the anti-wear capability of ionic liquid. When the load and sliding velocity was increased to a great extent,the hollow spheres still showed good properties in friction reduction and wear resistance,whereas the nano-platelets accelerated the lubrication failure of ionic liquid. MoS₂ micro-platelets slightly improved the friction reduction and wear resistance of ionic liquid under relatively high load and sliding velocity. Fatigue wear occurred on steel balls lubricated by the pure ionic liquid. When the nano-platelets or the hollow spheres were added into the ionic liquid,the fatigue wear was absent from the steel balls. The MoS₂ lubricants were transferred to the friction surface lubricated with hollow spheres and nano-platelets. Small amount of MoS₂ still remained on the friction surface,whereas so much MoS₂ was transformed into MoO₃ after tribochemical reactions. A lubricating film,containing MoO₃,MoS₂,FeF₂,FePO₃,FeSO₄,and N,S-containing organic compounds,was formed on the friction surface by the tribochemical reactions among MoS₂,friction materials and base oil. However,MoS₂ micro-platelets didn't enagaged in the formation of lubricating film,and thus,did not show good tribological properties.