Tribological properties of MoS2 nano-balls as filler in polyoxymethylene-based composite layer of three-layer self-lubrication bearing materials

POM/MoS2 nano-balls composite was prepared by adding MoS2 nano-balls synthesized from Na2MoO4 and CH3CSNH2 into polyoxymethylene (POM). The comparative POM-based composite blended with micro-MoS2 particles was also prepared. The obtained POM/MoS2 composites were used as the polymerical layer in the three-layer self-lubrication materials. The results of tribological tests showed that the POM with MoS2 nano-balls presented better tribological properties than that with micro-MoS2. When the content of MoS2 nano-balls was not more than 1.0 wt%, the POM/MoS2 nano-balls samples presented lower friction coefficients and smaller wear volumes. However, higher contents of MoS2 nano-balls than 1.0 wt% were very disadvantageous to the tribological performances. DSC results showed the excessive MoS2 nano-balls affected the POM crystallinity, and accordingly, the self-lubricating capabilities of these samples were influenced as well. SEM micrographs for wear scars confirmed that the worn manner of the POM sample was changed when the content of MoS2 nano-balls was increased. XPS analysis showed that MoS2 nano-balls was transferred to the mated friction surface, on which Mo(IV) was oxidized into Mo(VI) via tribochemical reaction. TEM micrographs of worn debris proposed a wear manner concerning the exfoliation of nano-sheets from MoS2 nano-balls. The reason for the stable self-lubrication properties of POM/MoS2 nano-balls composite was ascribed to the forming-destroying of debris clusters in a long-time sliding process.