Effect of dispersant on nano-PTFE: Striking the balance between stability and tribo-performance

Nanoparticles (NPs) of PTFE (polytetrafluoroethylene) in suspension form were reported as very efficient extreme pressure (EP) additive in 150N API Group II oil in our earlier work. The less stability of a nanosuspension due to agglomeration of NPs beyond 3wt%, however, was a major issue, which needed to be addressed for its possible industrial exploitation in lubricants. Using another additive, eg polyisobutylene succinimide (PIBSI), as a dispersant appeared to be a plausible solution. Its inclusion (1wt%) increased the stability of the oil by a factor of 4. It was necessary to investigate, however, whether it influenced other properties synergistically or antagonistically. The present paper highlights the positive and negative aspects of inclusion of PIBSI on the tribo-performance (EP and anti-wear) of the nano-PTFE-based oils. A series of nano-PTFE oils (1%-6%) stabilised with PIBSI (1%) were developed and evaluated for EP and anti-wear performance. The PIBSI-PTFE combination clearly showed antagonistic behaviour due to competition for film formation on the surfaces. However, with an increase in concentration of NPs, the film formation mechanism was dominated by them. Scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were used for the examination of the topography and surface chemistry of the protective films formed on the steel specimens.