Tribological properties of nano-calcium borate as lithium grease additive

Nano-calcium borate (NCB) with an average particle size of about 70 nm was synthesised via ethanol supercritical fluid drying technique, and the morphology and microstructures of as-prepared particles were characterised by means of scanning electron microscope (SEM, JEOL LTD., Tokyo, Japan) and X-ray powder diffraction. The friction and wear behaviour of the NCB as additive in lithium grease were evaluated with an Optimol-SRV IV (Optimol Instruments Prüftechnik GmbH, Munich, Germany) oscillating friction and wear tester (SRV tester). The morphology and surface composition of the worn surfaces of lower discs after SRV test were analysed by SEM and X-ray photoelectron spectroscopy (XPS, Physical Electronics, Inc., USA). The result demonstrated that the anti-wear and load-carrying capacities of the lithium grease were significantly improved, and the friction coefficient of the lithium grease decreased with the addition of NCB additive. The analytical results of XPS indicate that the good tribological performance of NCB is attributable to the formation of a boundary lubrication film composed of deposited NCB and the tribochemical reaction products such as B₂O₃, CaO and iron oxides on the rubbing surface.