Inorganic fullerene-like nanoparticles as new lubricant additives: A drug delivery mechanism

When dispersed in a synthetic polyalphaolefin (PAO) base oil, inorganic fullerene-like (IF-MS₂) nanoparticles of metal dichalcogenides (IF-MoS₂, IF-WS₂, IF-NbS₂) lead to a significant reduction of both friction and wear under boundary lubrication. The effect of the contact pressure on the tribological properties of IF nanoparticles is particularly interesting. Results show that the higher is the pressure, the lower is the friction coefficient. The effect of the concentration shows that, even used at a low concentration (0.1%wt), IF-MS₂ is able to decrease friction (0.05) compared to base oil only (0.08). A steady state friction coefficient of 0.04 was reached with IF-WS₂ at 1%wt in PAO. Friction-induced transformation of the IF-MS₂ nanoparticles into H-MS₂ single sheets was evidenced by High Resolution Transmission Electron Microscopy (HRTEM). Some of these superimposed sheets are found in incommensurate positions, thus possibly explaining the very low friction coefficient of 0.04 obtained with IF-WS₂. In-situ Raman spectroscopy was performed during a friction test to follow this structural modification. The lubrication mechanism of IF-MS₂ is very similar to a "drug delivery system". A very low concentration of additives is sufficient and the activation is obtained by the opening of the nested structure, like in certain micellar structures. Furthermore, no chemical reaction is required to obtain interesting properties. Thus, fullerene-like nanoparticles are active at the very beginning of the test and even at ambient and low temperature.