外文翻譯---caf2納米晶體作為潤(rùn)滑脂添加劑的摩擦學(xué)調(diào)查（英文）

1、Tribology International 40 (2007) 1179–1185Tribological investigation of CaF2 nanocrystals as grease additivesLibo Wanga,b, Bo Wanga, Xiaobo Wanga, Weimin Liua,?aState Key Laboratory of Solid Lubrication, Lanzhou Institu

2、te of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China bGraduate School, Chinese Academy of Sciences, Beijing 100039, ChinaReceived 17 May 2006; received in revised form 22 September 2006; accepted 20

3、 December 2006 Available online 12 February 2007AbstractCalcium fluoride (CaF2) nanocrystals with average grain size of 60 nm were synthesized via a precipitation method. The morphology and structure of nanocrystals were

4、 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 p

5、repared 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) pr

6、operties. 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 inves

7、tigated 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

8、 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 re

9、action of CaF2 nanocrystals occurred on the worn steel surface at severe tribological conditions. r 2007 Elsevier Ltd. All rights reserved.Keywords: CaF2; Nanocrystals; Lubrication additive; Antiwear; Lithium grease1. In

10、troductionBecause of the special physical and chemical properties, nanoscale materials have received considerable attention in various research fields [1,2]. In tribology, some nanomater- ials have been proved to have gr

11、eat potential as lubricating materials or for development of advanced lubrication technology. So far, a number of nanocrystals have been synthesized and used as additives of lubricating oil [3–6]. They can greatly improv

12、e the antiwear capability, reduce friction coefficient, exhibit extreme pressure properties and even retard thermoinduced oxidation of lubrication oil/ grease. Most of reported nanoadditives either contain heavy metals l

13、ike Zn, Cu, Pb et al. or involve sulphur atoms, which is a potential threat to environment. Green nanoparticle additives are strongly required in this regard.Alkali metal salt is an alternative choice. Alkali fluorides (

14、CaF2, LiF2, BaF2) generally have low shear strength and stable thermophysical and thermochemical properties at elevated temperatures. Therefore, they could only be used as high-temperature lubricant. For example, CaF2 is

15、 brittle at low temperature and has no lubricating properties. But when the temperature rises, it undergoes a transition from a brittle state to a plastic state and then can function as a lubricant in making self-lubrica

16、ting composites [7–9]. When being used in the form of thin lubricant films or component of self-lubricated metal matrix composite, CaF2 exhibits effective self-lubrication properties at tem- peratures as low as 400 1C [1

17、0–13], which is largely a result of reduced size. Due to small-scale effect, nanosized materials usually have dramatically lowered melting points compared with their bulk counterparts. Therefore, it seems attractive to h

18、ave a study on CaF2 nanocrystals being used as effective additives for grease or oil at lowered temperature. So present work reported on the synthesis of CaF2 nanocrystals obtained through a simple precipita-ARTICLE IN P

19、RESSwww.elsevier.com/locate/triboint0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2006.12.003?Corresponding author. Tel.: +86 931 4968166; fax: +86 931 8277088. E-mail ad

20、dress: wmliu@lzb.ac.cn (W. Liu).quite uniform and the average grain size is about 60 nm, which is in good agreement with the calculated result from the XRD patterns. Most of the CaF2 particles are in cubic shape, and wel

21、l dispersed on the copper grid. Fig. 3 shows TEM image of the CaF2 dispersed in lithium hydroxystea- rate (soap) fibre after removing the oil. From this picture, we can see that the CaF2 nanocrystals could be well disper

22、sed in hydroxystearate (soap) fiber when added in grease. There is no apparent aggregation even after removal of oil.3.2. Tribological properties of CaF2 nanocrystalsFig. 4 discloses variation of the friction coefficient

23、 and wear scar diameter with the concentration of CaF2additives. It can be seen that the antiwear and friction- reduction properties of lithium grease is improved by the addition of CaF2 nanocrystals. Upon addition of ev

24、en 0.5% of CaF2, both friction coefficient and wear scar decreased dramatically. However, it shows a transit concentration at around 1%, after which friction coeffi- cient and wear scar slightly increased. Therefore, the

25、 optimum concentration is about 1 wt%. At this point, the wear scar diameter and the friction coefficient could be reduced 29% and 19%, respectively. The maximum non-seizure loads (PB value) of lithium grease and lithium

26、 grease containing 1wt% CaF2 additives are shown in Table 1. It is seen that the additive is able to increase the PB values of the lithium grease considerably, roughly by 48%. It means the extreme-pressure property of th

27、e lithium grease can be remarkably improved by CaF2 nanocrystals.Figs. 5 and 6 give the friction coefficient and the wear scar diameter as a function of applied loads for lithium grease alone and lithium grease containin

28、g 1wt% of CaF2 nanocrystals. It can be seen that the friction coefficient of the lithium grease contains 1wt% CaF2 nanocrystals is much lower and more stable than that of pure lithium grease at all applied loads. We also

29、 noticed that during the tribology test there usually exists fricative noise for the lithium grease only, especially under high applied loads and the fricative noise disappeared at all test loads when CaF2 nanocrystals w

30、ere added. So CaF2 nanocrystals are very effective in improving the friction-reduction properties of lithium grease. When pure lithium grease is used, the wear scar diameter is much larger, especially under a load of 200

31、N. Contrary to the above, the wear scar diameter is much smaller when the grease containing 1wt% of CaF2 nanocrystals was adopted. In other words, lithium grease containing CaF2 nanocrystals also has good antiwear proper

32、ty.3.3. SEM analysis of the worn surfacesThe SEM micrographs of the worn surfaces and the elemental distributions of Ca, F, Fe and O on the wornARTICLE IN PRESSFig. 2. TEM and EDX images of CaF2 nanocrystals.Fig. 3. CaF2

33、 dispersed in lithium hydroxystearate (soap) fibre structure after removing the oil.0.0 0.5 1.0 1.5 2.0 2.5 3.00.080.090.100.110.12FrictionCoefficientWSDwt%Friction Coefficient0.300.360.420.480.540.60WSD / mmFig. 4. Fric