Preparation of Water-Soluble Lanthanum Fluoride Nanoparticles and Evaluation of their Tribological Properties

Abstract

Water-soluble LaF3 nanoparticles surface-capped by two kinds of dialkyl polyoxyethylene glycol thiophosphate ester (denoted as DTP-10 and DTP-20) were synthesized via a surface-modification method. The morphology and microstructure of resultant surface-modified LaF3 nanoparticles (denoted as LaDTP-10 and LaDTP-20) were characterized by means of X-ray powder diffraction, transmission electron microscopy, and Fourier transform infrared spectrometry, and their thermal stability was examined by thermogravimetric analysis. Moreover, the tribological properties of as-synthesized LaF3 nanoparticles as additives in distilled water were evaluated with a four-ball friction and wear tester, and the morphology of wear scar and the chemical states of some typical elements thereon were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. It has been found that as-prepared LaDTP-10 and LaDTP-20 nanoparticles have a size of 19.6 and 8.5 nm, respectively, and they have good dispensability in distilled water. Moreover, as-synthesized LaDTP-10 and LaDTP-20 nanoparticles as lubricant additives in distilled water exhibit good friction reducing, antiwear, and extreme pressure properties as well as high load-carrying capacity even at a concentration of 1 % (mass fraction). This is because LaF3 nanoparticles can be deposited on sliding steel surfaces to afford a surface protective layer, and they may also tribochemically react with rubbing steel surfaces to generate a boundary lubricating film mainly composed of phosphate, sulfide, sulfate, La2O3, and LaF3. Therefore, it is feasible for LaDTP-10 and LaDTP-20 nanoparticles to be used as water-soluble lubricant additives under harsh conditions.