Preparation Of Borate Ester Lubricant Additives And Study On Their Tribological Properties

Organic borate esters are recognized as "green" additives and have received extensive attention due to their low toxicity, pleasant odor, and excellent AW and EP performance.However, borate esters are sensitive to hydrolysis, which results in precipitation or insolubility in oil. The hydrolysis process is common because the electron-deficient boron element can be easily combined to water molecule in the air. To solve this problem, some of studies are focus on the introduction of nitrogen to the additive molecules to compensate the electron-deficient nature of boron element, and some of studies are focus on the increase of additive molecule's steric hindrance; both of the methods are important.Base on this idea, four borate esters coded as YXB, YX2, YX3 and SNPA were prepared, a Mo-containing additive YXM was synthesized, and ZDDP was used for comparison. Infrared spectroscopy (IR) and elemental analysis (EA) were used to characterize the synthesized additives. Their oil solubility and hydrolytic stability were tested.Their tribological performances were evaluated using a four-ball machine. The results indicate that the additives possess excellent extreme pressure, anti-wear and friction-reducing properties.1. Excellent extreme pressure properties. When 3.0 wt% YXB was added into the base oil, the PB value increased by 175%; when 3.0 wt% YXM was added into the base oil, the PB value increased by 194%; when 3.0 wt% SNPA was added into the base oil, the PB value increased by 203%.2. Excellent anti-wear properties. When 2.5 wt% YXM was added into the base oil,WSD reached the minimum, 0.44 mm, reduced 42.1% compared to the base oil; when 2.5 wt% SNPA was added into the base oil, WSD reached the minimum, 0.49 mm, reduced 29.6% compared to the base oil;3. Excellent friction-reducing properties. When 1.0 wt% YXB was added into the base oil, the average friction coefficient was 0.0766, reduced 24.4% compared to the base oil;when 1.0 wt% YXM was added into the base oil, the average friction coefficient was 0.0796, reduced 21.4% compared to the base oil PAO; when 2.5 wt% SNPA was added into the base oil, the average friction coefficient was 0.0787, reduced 22.3% compared to the base oil PAO.4. According to the relative results of SNPA and ZDDP,it can be found that SNPA shows a better extreme pressure,antiwear and friction-reducing performences.SEM and XPS were used to analyze the composition and structure of boundary films on the worn surfaces. According to the SEM images, the worn surface lubricated by base oil showed severe scuffing signs and the furrows formed were deep and wide, while the scuffing on the worn surfaces lubricated by PAO containing additives were abated to some extent. XPS results showed that the additives reacted with the steel surface and tribofilms consisting of N-containing organic debris,Fe2O3,BN,B2O3,MoO3,or FeSO4 were generated. This tribofilm can effectively protect the steel surfaces from direct contact.