Biodiesel lubricity: an atomic-scale frictional characterisation approach

Abstract

Biodiesel lubricity is generally characterised through the size of wear scar produced using high frequency reciprocatinrig (HFRR).More recently, the adequacy of Stribeck curve in characterising biodiesel lubricity is also being investigated. However, for a rough surface contact, asperity interact ion sat nano-scale are predominantly the root cause for excessive friction in the boundary and mixed lubrication regimes that could eventually lead to material wear. Mitigation of this frictional loss depends heavily on the form action of boundary adsorbed tnbo-film, which is a consequence of molecular chemical reaction with the surface. It is therefore critical to also characterise the lubricity of biodiesel in respect to it s frictional behaviour at asperity level. Using lateral Force Microscopy (lFM), assuming that the tip-an -surface represents an asperity pair contact, the study examines the frictional characteristics for various biodiesels derived from canota. coconut, olive, palm and soybean. At different tip sliding velocities, it can be seen that the coefficient of friction measured for coconut methyl ester is consistently the high est, with palm methyl ester giving the lowest value. However, the trend is no longer consistent when the friction force is investigated separately for load and velocity dependence, with soybean methyl ester producing higher friction forces for most of the test conditions except at higher normal loadings. These ob servations reflect the deficiency in exist ing lubricity characterisation approaches, which clearly require better fundamental understanding of tribology at different length scales.