High lubricity of re-doped fullerene-like MoS ₂ nanoparticles

The electrostatic effects in tribological systems have been studied in the past, especially with regards to data storage media. Nanoparticles (NP) of WS2 and MoS2 with fullerene-like structure (IF) have been studied in the past and showed very good tribological behavior. Being semiconductors, their electrical properties can be controlled by, e.g., substituting the lattice Mo (W) atoms with Re (n-type conductivity) and Nb (p-type conductivity) atoms. In this study doping of IF-MoS2, and to a lesser degree IF-WS ₂, NP with small amounts (<1 at.%) of rhenium atoms has been studied. For this purpose two new synthetic approaches have been pursued. The doped nanoparticles were characterized by various techniques. In particular, the doping density was determined by ICP-MS technique. The resistivity of the nanoparticles was shown to decrease significantly with increasing doping level. In contrast to the undoped nanoparticles, the doped NP were shown to exhibit reduced agglomeration and produce stable suspensions in PAO-4 and PAO-6 oils. Extensive tribological measurements with these PAO oils formulated with 1 wt % of the doped NP showed friction coefficients as low as 0.01 in mixed lubrication conditions and negligible wear. Microscopy analysis of the tribological surfaces reveal very smooth but discontinuous and dense film of the doped NP on the tribological surfaces. It is proposed that the doped NP are negatively charged at their surface eliciting mutual repulsion, which has a remarkable influence on their rheological properties and their tribological behavior.