Tribological investigation of CaF2 nanocrystals as grease additives |
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Authors: | Libo Wang Bo Wang Xiaobo Wang Weimin Liu |
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Affiliation: | aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China;bGraduate School, Chinese Academy of Sciences, Beijing 100039, China |
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Abstract: | Calcium fluoride (CaF2) nanocrystals with average grain size of 60 nm were synthesized via a precipitation method. The morphology and structure of nanocrystals were characterized by means of transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). TEM and XRD showed that CaF2 nanocrystals are cubic particles in submicron scale. The tribological properties of the prepared CaF2 nanocrystals as an additive in lithium grease were evaluated with a four-ball tester. The results indicated that these nanocrystals exhibit excellent antiwear, friction reduction and extreme pressure (EP) properties. It was also found that the EP and antiwear capabilities of the grease are not proportional to the content of CaF2 nanocrystals but there existed a certain value. The rubbed surface after friction test was investigated with X-ray photoelectron spectroscopy and scanning electron microscopy to understand the action mechanism. The results show that a boundary film mainly composed of CaF2, CaO, iron oxide and some organic compounds was formed on the rubbed surface after friction test and the thickness of boundary film was about 12 nm. The disproportion of stoichiometric ratio of Ca and F in boundary lubrication film indicates that tribochemical reaction of CaF2 nanocrystals occurred on the worn steel surface at severe tribological conditions. |
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Keywords: | CaF2 Nanocrystals Lubrication additive Antiwear Lithium grease |
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