Study of Tribological Synergistic Effect of N-Containing Heterocyclic Borate Ester with Tricresyl Phosphate as Rapeseed Oil Additive

Abstract

Abstract A single borate ester additive sometimes fails to meet the requirements of modern working conditions, it needs two or more additives for the use. Two N-containing heterocyclic borate esters, 2-(1H-benzotriazol-1-yl) ethyl bis(2-aminoethyl) borate (BEB) and bis-(2-amionethyl) (2-benzothiazol-2-ylthio)ethyl borate (MEB) have been synthesized. Their tribological properties were studied by a four-ball tester after mixing with tricresyl phosphate (TCP) in rapeseed oil (RSO). The results show that these complex additives have strong synergistic effects on load carrying and anti-wear properties. At a mass ratio of BEB (MEB) to TCP of 0.5:0.5, the load carrying capability (PB value) of RSO can be improved by 160.6 % and 204.2 %, respectively. At the same mass ratio, the load carrying capability of the complex additive from the TMx series (MEB with TCP) was better than that of the TBx series (BEB with TCP). The anti-wear effect of TBx was better than that of TMx at high load. The friction reduction of 196 N showed little difference because of the small difference in physical adsorption capacity between TCP and BEB (MEB). The analytical results of Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) on the worn steel ball surfaces showed that the complex additives were adsorbed on the lubricating surface and formed a stable protective film during the sliding process due to absorption and tribochemical reactions between the borate ester, TCP and metal surface which helped to improve the tribological properties of RSO.