Dispersion stability of nano additives in lubricating oils – an overview of mechanisms, theories and methodologies

ABSTRACT With the advancement in the field of nanotechnology, various researchers have reported an improvement in the friction and wear behaviour of different lubricating oils by the addition of different nano additives. However, the stability of these nano additives remains a challenge. In this paper, the dispersion stability of various nano additives in lubricating oils has been studied in detail. The paper aims to summarize various dispersion stability theories and several methods reported in the literature to improve dispersion stability. Apart from this, a special focus has been laid to highlight the various surfactants used to improve the dispersion stability, particularly the tribological properties. The literature suggests that although a number of studies have been carried out to study the effect of surfactant addition on dispersion stability, however, future developments could be focused to improve the dispersion stability of the existing mono and hybrid nano fluids and their underlying mechanisms. Abbreviations: SWCNH, single-walled carbon nano-horns; PAO, poly alpha olefin; MoS2, molybdenum di sulphide; H, Hamaker constant, MWCNTs, multi-walled carbon nanotubes; GNF, graphene nanoflakes; DLVO, Derjaguin and Landau Verway and Overbeek; DLS, dynamic light scattering; HLB, hydrophobic lyophilic balance; FERTEM, freeze etching replication transmission electron microscope; SDS, sodium dodecyl sulphate; PTFE, polytetrafluoroethylene; FTIR, Fourier-transform infrared spectroscopy; TBAAc, tetra butyl ammonium acetate; THF, tetrahydrofurane; UV–Vis, ultraviolet visible light; SEM, scanning electron microscope; TEM, transmission electron microscope GRAPHICAL ABSTRACT