经热处理的蛇纹石粉体对金属磨损特性的影响陈文刚，高玉周，张会臣，许晓磊(大连海事大学机电与材料工程学院，辽宁大连 116026)摘要：用X射线衍射仪分析了在550℃和900℃热处理的两种蛇纹石粉体的相结构变化，并研究了不同的分散剂对蛇纹石微细粉体在润滑油中的分散性。
在MM–200型摩擦磨损试验机上进行摩擦磨损试验，使用的润滑油为基础油和分别含有2种经热处理的蛇纹石粉体的润滑油。
借助扫描电子显微镜和能谱仪分析了在上述3种润滑油润滑下#45钢环的表面形貌和成分。
结果表明：在900℃热处理后蛇纹石发生相变；用钛酸酯偶联剂进行表面修饰的蛇纹石粉体在润滑油中具有良好的分散性；润滑油中，2种经热处理的蛇纹石都可以在金属摩擦磨损表面形成保护膜。
在摩擦过程中，两接触表面上的微凸部分对粉体的研磨作用使粉体粒子表面带有大量的悬挂键，同时接触的金属摩擦副也形成活化表面，从而使粉体颗粒吸附在金属表面。
在载荷的作用下，蛇纹石粉体颗粒在磨损表面铺展形成自修复膜。
关键词：蛇纹石；热处理；添加剂；磨损特性；自修复中图分类号：TH117.3 文献标识码：A 文章编号：0454–5648(2008)01–30–05INFLUENCE OF HEAT-TREATED SERPENTINE POWDER ON WEAR PROPERTIES OFMETAL SURFACESCHEN Wengang，GAO Yuzhou，ZHANG Huichen，XU Xiaolei(Electromechanics and Materials Engineering College, Dalian Maritime University, Dalian 116026, Liaoning, China) Abstract: Serpentine heat-treated at 550, 900℃was analyzed by X-ray diffraction. The dispersive property of serpentine powder in the lubricating oil was investigated using different dispersants. The wear characteristics of the metal friction pairs were explored by an MM–200 friction and wear tester. Base oil and oil containing two kinds of heat-treated serpentine powder were used as lubricants respectively. The surface topographies and composition of the worn surface of 45steel rings lubricated with the three kinds of lubricating oil were analyzed by scanning electron microscopy and energy dispersive X-ray analysis spectrometer. The results indicate that the phase transformation of serpentine takes place after heating at 900℃. The serpentine powders modified by titanate coupling agent in the lubricant oil possess very good dispersibility. Both of the two kinds of heat-treated serpentine powder in lubricating oil can form protective film on the surface of the worn metals. The mechanism is that the rubbing action of the asperities between the two contact surfaces makes the surface of the powder generate a large number of dangling bonds during the friction. At the same time, an activated surface is formed on the contact metal friction pairs, these make the powder adhere to the worn metal surface. Under the action of shearing stress on the contact surfaces, the serpentine powder is spread on the worn metal surface to form self-repairing film.Key words: serpentine; heat treatment; additive; wear characteristics; self-repairingFriction and wear is a common phenomenon for mechanical equipment in operation. At present, there are four kinds of methods to reduce friction and wear: (a) Surface chemical treatment, such as nitriding and carburizing; (b) Covering the surfaces of metal parts with a wear-resistant protective film, such as diamond-like coating (DLC); [1] (c) Adding anti-wear additives such as nano-metals, nano-compounds and nano-rare-earth compounds to lubricant and reduce wear in the process of friction and wear;[2–5] (d) Applying self-repairing techniques to metals by adding self-repairing additives into lubricant oil, thereby achieving in situ self-repairing of the worn metal by creating metal-ceramic protective film during the operation of the equipment.Many researchers have studied self-repairing technology of the worn surface of metal friction pairs.[6–10] They收稿日期：2007–03–10。
修改稿收到日期：2007–04–02。
基金项目：国家自然科学基金(50472025)资助项目。
第一作者：陈文刚(1973—)，男，博士研究生。
通讯作者：高玉周(1967—)，男，副教授。
Received date:2007–03–10. Approved date: 2007–04–02.First author: CHEN Wengang (1973–), male, postgraduate student for doctor degree.E-mail: chenwengang999@Correspondent author: GAO Yuzhou (1967–), male, associate professor.第36卷第1期2008年1月硅酸盐学报JOURNAL OF THE CHINESE CERAMIC SOCIETYVol. 36，No. 1January，2008陈文刚等：经热处理的蛇纹石粉体对金属磨损特性的影响· 31 ·第36卷第1期reached the same conclusion that a self-repairing protective film can be formed on the worn surface by adding self-repairing additives into the lubricant. But for the film-forming mechanism, some researchers believe that the self-repairing additives directly participate in the formation of self-repairing film,[6–7] whereas others researchers believe that the self-repairing additives themselves promote the formation of the protective film instead of participating in the formation process.[8–9] In order to further investigate the change of the powder additives in the process of friction and the film formation mechanism, the present work includes the study of heat treatment properties of the serpentine powder under different temperatures and the influence of different surface modifiers on the powder dispersion in the lubricant. A high dispersivity lubricant containing self- repairing powder additives was prepared. We analyzed the effect of the powder heat-treated at different temperatures on the film characteristics of worn metal surfaces by friction and wear test, and clarified the surface film formation mechanism of the self-repairing powder on worn metal surfaces.1 Experimental procedure1.1 Preparation and heat-treatment of serpentinepowderThe serpentine was first pulverized into powder with sizes of less than 0.07mm, then uninterruptedly ground for 24h using a QM–BP planetary ball-milling machine. The resulting fine powder was smaller than 10µm. Afterwards, the serpentine powder was subjected to 3h heat treatment at 550℃ and 900℃. Finally, the phases of the original powder and the thermally activated powder were analyzed on a Rigaku Dmax–3A X-ray diffractometer.1.2 Dispersibility experimentSilane coupling agent KH550 and titanate coupling agent NXT–105 were added into acetone containing 2g of serpentine powder heat-treated at 550℃ and 900℃, respectively. The amount of surface modifier was 5%(in mass, the same below) of the powder. The attained mixed solutions were ultrasonically oscillated for 20min. Then surface-modified serpentine powder was obtained after the evaporation of acetone. Finally, the different surface-modified powder (2%) was added into the lubricant, in which the dispersibility of the powder was observed.1.3 Friction and wear testThe properties of wear and friction were evaluated on an MM–200 friction and wear tester under the load of 600N with the fixed upper sample and the lower sample rotating at 200r/min for 100h. The friction pairs were made of 45 steel quenched at 860℃and then tempered at 450℃ for 2h. The hardness of HRC 45–47 could be attained. The size of the upper sample was 10mm×10mm×16mm. The lower sample was 40mm in diameter and 10mm in thick. There were three kinds of lubricants: (1) pure lubricant, (2) lubricant containing the serpentine powder heat-treated at 550℃, and (3) lubricant containing the serpentine powder heat-treated at 900℃. For convenience, in this paper these are referred to as #1 oil, #2 oil and #3 oil, respectively. The amounts of the serpentine powder were 2% of the lubricating oil. We preferred titanate coupling agent NXT–105 as surface modifier. The experiment was interrupted at intervals of 10h. The lower sample was cleaned ultrasonically in acetone. Wear loss was measured at specific intervals by weighing the lower samples to an accuracy of 0.1mg by using electronic balance. The worn surface topography was examined using a Philips XL–30 scanning electronic microscope (SEM). The composition on the wear scars was analyzed using an energy dispersive X-ray analysis spectrometer (EDAX).2 Results and discussion2.1 Heat-treatment of serpentine powderFigure 1 shows the XRD patterns of serpentine powder. The phase of the powder heat-treated at 550℃shows no obvious difference compared with the original serpentine powder; only the intensities of various phases is weaker. When the heating temperature reaches 900℃, almost every diffraction peaks turns to forsterite peaks and enstatite peaks.[11] This variation can be expressed by the following chemical equation:Mg3Si2O5(OH)4→Mg2SiO4+MgSiO3+2H2O↑Fig.1 XRD patterns of serpentine powder heat-treated at different temperaturesThe apparent variation of the treated serpentine powder exhibits obvious regularity. The original serpentine powder is white. The color of the powder heat-treated at 550℃ changes to ocean-gray, and the powder heat硅酸盐学报· 32 ·2008年treated at 900℃ changes to dark brown. The results indicate that after heat treatment of 900℃, the structural water is stripped from the serpentine and new phases are formed.2.2 Dispersivity of serpentine powder in lubricantStatic lay-aside lubricants were observed, in which all of the serpentine powder heat-treated at 550℃ and 900℃ were respectively modified by two kinds of surface modifiers. The two kinds of the heat-treated powder modified by silane coupling agent KH550 surface modifier soon began to be deposited in the lubricant. When the lay aside time reached 12 h, all of the powder were deposited on the bottom and the lubricants became clear again. In contrast, the two kinds of the heat-treated powder modified by NXT–105 remained suspended in the lubricant after 200h static lay-aside. The results indicate that the NXT–105 can evidently improve the powder dispersivity in lubricant. The reason is that the NXT–105 has a hydrophobic group and a hydrophilic group, which can couple with the surface of the inorganic mineral powder, then obtain effective dispersive action.[12] Although the silane coupling agent KH550 has the same property, it has three hydrolytic groups and mainly produces an intermolecular condensation reaction; therefore, it does not produce good dispersibility of the powder.2.3 Friction and wear propertiyFigure 2 is the wear loss graph of the 45 steel rings lubricated with different lubricants for 100h. It can be seen that the wear loss of the sample lubricated with #1 oil gradually increases with the increase of test time; however, the wear loss of the samples lubricated with #2 oil reduces at 50, 70h and 100h, and sample lubricated with #3 oil reduces at 60, 90h and 100h, The wear losses of the latter two samples are less than that of the sample with #1 oil from the beginning. As the time is prolonged, the difference of the wear loss increases. When the friction times reaches 100h, the wear loss of the latter two samples are 1.1mg and 1.3mg, respectively, Obviously which are less than the w e a r l o s s o f t h e s a m p l e l u b r i c a t e dFig.2 Wear loss of steel ring vs. test duration in different lubricant systems with #1 oil. These results indicate that both of the two kinds of the heat-treated serpentine powder can create protective films on the sample surface to reduce the wear loss of the samples during the test process. The intermittent reduces of the wear loss during the test process means that the film formation is dynamic process, in which the wear of the sample and the formation of the protective film occur simultaneously. From the Fig.2, it can be seen that the wear-resistance property of the #2 oil is better than that of the #3 oil.Figure 3 shows the variations of friction coefficients of the friction pairs lubricated with different lubricating oil. It can be seen that the friction coefficient of the friction pairs lubricated with #1 oil gradually decrease with friction time increasing and eventually remains at 0.04. However, the friction coefficients of the samples lubricated with #2 and #3 oil decrease first, then increase rapidly with time increasing, finally basically remaining at around 0.106. During the friction process, the friction coefficients of the samples lubricated with lubricant containing serpentine powder are higher than that of the sample lubricated with base oil; namely, the addition of the serpentine powder makes the friction coefficient ofthe friction pairs increase.Fig.3 Friction coefficient vs. test duration for different lubricant oilsThe SEM photographs of wear topography of steel rings under the same load on the MM–200 friction and wear tester are given in Fig.4. Obviously, there are many deep scratches on the surface of the 45 steel ring lubricated with #1 oil. The scratches on the surface of the sample lubricated with #2 oil are shallower. For the sample lubricated with #3 oil, there are only a few scratches on the local area and the majority of its surface is very smooth. A large number of micro-pores can be observed on the surface of the two samples lubricated with #2 oil and #3 oil, respectively. The sample with #2 oil has more and larger micro-pores than the sample with #3 oil.According to the SEM photograph of the cross-section of the 45 steel ring lubricated with #2 oil (as shown in Fig.5), there is a white layer on the steel substrate edge, which adheres very closely to the steel substrate without陈文刚等：经热处理的蛇纹石粉体对金属磨损特性的影响· 33 ·第36卷第1期Fig.4 SEM micrographs of worn surface of 45 steel rings at various lubricant systemsFig.5 SEM photograph on the cross-section of 45 steel ring lubricated with #2 oil any obvious interface. This indicates that the porous surface formed on the samples as shown in Fig.4 is really a self-repairing film. The chemical compositions of surfaces after wear testing and subsequent ultrasonic cleaning in acetone were analyzed to study the possible chemical interactions between additives and steel, as shown in Fig.6. It can be found that the magnesium and silicon elements, which are the two characteristic elements of the serpentine, exist on the surfaces of the two samples. Although the 45 steel containing a certain amount of silicon element and the silicon element on the surface do not definitely confirm that the powder directly take part in the formation of the self-repairing film, but the appearance of the magnesium element does confirm that the powder is directly involved in the formation ofthe surface self-repairing film.Fig.6 EDAX spetra of worn surface of 45 steel lubricated with #2 oil and #3 oilBased on the results presented above, the following conclusion can be drawn about the mechanism of self-repairing film formed on the sample surface: the serpentine powder can be ground into smaller sizes in the friction process by the surface asperities of the two contacting surfaces. Serpentine minerals are Mg–rich trioctahedral 1:1 phyllosilicate. The ideal chemical formula of serpentine is Mg3Si2O5(OH)4. Sheets are bonded by van der Waals forces. There are unsaturated Si—O—Si, O—Si—O and magnesium bonds on硅酸盐学报· 34 ·2008年the fracture surface of the serpentine. Additionally, brucite laminae ruptures during the dissociation process of the powder. The powder has a large number of surface dangling bonds after ground. [13–14] At the same time, the friction produces fresh surfaces of the steel where the activity of friction pairs is enhanced. Therefore, it can be inferred that the powder can adhere to the fresh surfaces of the steel and then spread on the worn metal surface under the action of shearing stress on the contacting surfaces. The spreading phenomenon can be seen clearly in Fig.7. Thus, a self-repairing film forms on the worn surface.[15–16] As the self-repairing film forms in the friction process, the surfaces in contact with the friction pairs are transformed from 45 steel/45 steel to self-repairing film/self-repairing film, and the friction coefficient of the self-repairing film is higher, therefore it causes an increase in the friction coefficient for the friction pairs lubricated with #2 oil and #3 oil.Fig.7 SEM micrographs of worn surface of 45 steel rings lubricated with #2 oil and #3 oil3 Conclusions(1) In contrast to original serpentine, no phase transformation occurred on the powder treated at 550℃, which only discharged interlayer water and adsorptive water. 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