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Purpose -The fast growing concern for maintaining integrity of the environment has built up development of environmentally-adapted lubricants. Because of their toxicity, most of the traditional lubricating additives cannot be used in this kind of lubricant. The purpose of this paper is to find a borate ester derivative which can be used as a multifunctional additive in rapeseed oils (RSO). Design/methodology/approach -A borated S-2-hydroxypropyl N, N-dibutyl dithiocarbamate (BDTC) was synthesized and characterized. Its tribological properties in rapeseed oil as multifunctional additive were evaluated, using a four-ball tribometer and compared with one kind of commercial MoDTC. In addition, its thermal stability was identified using TGA and antioxidative ability tested by PDSC. The action mechanism of BDTC was studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and atmospheric pressure chemical ionization mass spectrometer (APCI/MS) analysis. Findings -Results show that BDTC exhibits high thermal stability, possesses comprehensive tribological performance in rapeseed oil, and has evident effect in controlling the oxidation of RSO. Analysis of worn surface indicates that BDTC was decomposed and reacted with metal during the friction process. The lubricating film mainly contains inorganic boron compound (B 2 O 3 ), organic nitrogen derivatives and FeSO 4 . Originality/value -This paper provides a borate ester derivative which possesses excellent tribological properties and can be used as a multifunctional ashless additive in environmentally-adapted lubricants. Furthermore, an innovative method, APCI/MS, was used to analyze the tribofragmentation behavior of BDTC.

Three ashless and nonphosphorus benzothiazole-derived borate esters were synthesized by a halide-free green synthesis method. The results show that the hydrolytic stability and tribological characteristics have been significantly improved. Scanning electron microscopy was used to analyze the morphologies of the rubbing surfaces. X-ray absorption near edge structure spectroscopy was also adopted to investigate the thermal films and tribofilms of the synthesized additives. The analysis results indicate that the thermal films are mainly composed of FeSO4 and B2O3, while the tribofilms consist of the components of FeS, FeSO4, and B2O3.

Nowadays, looking for environment-friendly lubricant additives to replace phosphorus-based derivatives in automotive lubricating system is one of the main objectives. Organic borate ester is one kind of potential substitutes for phosphorus-based derivatives. In this paper, the interactions of organic borate esters and dioctylphosphite ester on tribological performances were studied. Results showed that they had a synergistic effect on antiwear performance and friction-reducing property as lubricant additives in mineral oil. The chemical nature of worn surface was explored by X-ray absorption near-edge structure spectroscopy. Borate ester could facilitate the decomposition of phosphorus-based additive and the formation of phosphate. What is more, boron-containing species could react with phosphate to form boron–phosphorus-based thin film on the surface. Boron mainly forms B2O3 on the surface. In the near surface of tribofilm, nitrogen-containing borate ester reduced the polyphosphate chain length due to the formation of ammonium cation. The polyphosphate chain length of the tribofilm generated from dioctylphosphite ester mixed with nitrogen-containing alkyl borate ester is shorter compared to that of the tribofilm generated from dioctylphosphite ester mixed with nitrogen-containing heterocyclic borate ester.

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