Effect of oil quality on electrostatic charge generation and transport

This paper presents an experimental investigation into the effect of lubricating oil quality on tribocharging. The work forms part of a larger investigation into the causes of electrostatic charging events detected prior to severe scuffing during base oil lubricated wear experiments. A variable speed spinning disc charger was used for measurements of tribocharging, whilst controlling the oil viscosity, temperature, ageing and disc surface roughness. The results are discussed with reference to interactions between the double layer and fluid boundary layers and shear stress which could lead to charge separation at the disc-oil interface. Viscosity and surface roughness have been shown to affect the tribocharging of a base oil in a direct relationship, while temperature effects indicate a maxima due to transportation-recombination competition. Ageing experiments indicate an increase in conductivity, which will eventually lead to an increase in charging ability. Charge sign inversion is discussed in relation to specific adsorption of charged species (additives) on the disc surface, the presence of oppositely charged impurities and the differential of adsorption and/or diffusion rates for the negatively and positively charged species within the oils.