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Self-repairing properties of complex titanium grease containing hydroxyl silicate
Accepted Manuscript Self-repairing properties of complex titanium grease containing hydroxyl silicate Qu Jianjun, Qu Huajie, Wen Zhongpu, Luo Yunxia PII:
S0301-679X(19)30163-X
DOI:
https://doi.org/10.1016/j.triboint.2019.03.043
Reference:
JTRI 5685
To appear in:
Tribology International
Received Date: 21 November 2018 Revised Date:
17 March 2019
Accepted Date: 18 March 2019
Please cite this article as: Jianjun Q, Huajie Q, Zhongpu W, Yunxia L, Self-repairing properties of complex titanium grease containing hydroxyl silicate, Tribology International (2019), doi: https:// doi.org/10.1016/j.triboint.2019.03.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Self-Repairing Properties of Complex Titanium Grease Containing Hydroxyl Silicate Qu Jianjun, Qu Huajie, Wen Zhongpu, Luo Yunxia (School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, China，150001)
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Abstract: With magnesium hydroxyl silicate modifiers, a kind of complex titanium grease was prepared containing magnesium hydroxyl silicate modifier. Using four-ball friction and wear testing machine, The dosage of magnesium hydroxyl silicate modifier was studied on the influence of friction coefficient and wear spot diameter, the best dosage was obtained. By using M-200 wear testing machine, friction and wear test of complex titanium grease containing the best dosage of modifier was tested for 30 hours. The results show that the complex titanium grease containing magnesium hydroxyl silicate modifier has better anti-friction and anti-wear properties and self-repairing function. The surfaces of the steel sample testing for 10 hours, 20 hours and 30 hours are respectively analyzed based on SEM and X-ray Photoelectron Spectrometer. It is shown that the self-repairing film on the steel surface can be formed after 10~20 hours’ friction. The self-repairing mechanism of magnesium silicate modifier was explored. A kind of self-repairing model was proposed by adding magnesium hydroxyl silicate modifier to the complex titanium grease. It is concluded that the microscopic particles of magnesium hydroxyl silicate modifier generate adsorbent substance on the surface of steel substrate to repair the wear surface through a series of physical and chemical reaction. Key words: complex titanium grease; hydroxyl silicate; self-repairing; friction; wear
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As an important component of industrial lubricants, grease plays an important role in reducing friction and wear, reducing energy consumption and extending the service life of machines. With the enhancement of people's awareness of environmental protection, increasing demand for long service life and environment friendly grease, developing new high-performance lubrication products has become an inevitable trend. It is necessary to develop a kind of grease with self-repairing properties. Adding self-repairing additives to the grease is one of the effective methods to achieve this purpose [1]. Xi Xiang added montmorillonite to lithium grease to increase the anti-wear properties of grease [2]. MoYunhui filled soft metals to grease to make grease a certain self-repairing function [3]. Ilie Filip studied the anti-friction and anti-wear properties of MoS2。He considered MoS2 having a certain self-repairing function [4]. Since its discovery at the beginning of last century, Hydroxyl silicate has been found a kind of lubricant additives with self-repairing function, and applied in a lot of lubricating oil products which obtained the certain self-repairing effect. Liu Qing and his research group introduced the research progress of hydroxyl silicate in detail [5]. Rudenko Pavlo added magnesium hydroxyl silicate into lubricating oil and studied the influence of particle size [6]. Zhu Gongzhi studied the self-repairing mechanism of magnesium hydroxyl silicate modifier added into lubricating oil [7]. The addition of magnesium hydroxyl silicate modifier into lubricating oil did plays a certain role in reducing friction and self-repairing. However, there are still problems exist. The dispersed suspension, easiness of precipitate, blockage of oil circuit have been studying. Nevertheless, it is a worth studying to use the properties of special antifriction and self-repairing wear of hydroxyl silicate. Researchers have focused on the modification of hydroxyl silicate and the research of synthetic technology [8,9]. If it is added into the grease, the precipitation problem of the particles can be improved by the high viscosity property of the grease. And the well-dispersed and suspended grease containing hydroxyl silicate can be obtained. Adding some organic or inorganic particles in grease has been reported several times [10,11] to improve the friction and wear properties of grease. However, adding hydroxyl silicate to grease is seldom studied. Yu Helong added hydroxyl silicate to tank grease to improve the tribological properties of grease [12]. Recently, A. D. Brekia filled two hydroxyl silicates to multifunctional lithium grease to improve the wear and repair properties of lithium grease [13]. However, the addition of hydroxyl silicate to composite titanium grease has rarely been reported. 1
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Complex titanium grease is a new type of environment friendly lubricants [14,15]. But its tribological properties are not very ideal. Adding the modifier containing magnesium hydroxyl silicate into the grease, it is expected to improve its tribological properties and develop a new type of complex titanium grease with a self-repairing function. In this paper, the properties of friction and wear and self-repairing of complex titanium grease containing magnesium hydroxyl silicate modifier were studied by using four-ball tester and M-200 (AMSLER) wear tester respectively. By means of XPS and SEM images, the surface morphology of the grinding mark was studied, the relative atomic content of the surface was analyzed, and the self-repairing effect of magnesium hydroxyl silicate modifier was explored.
1. Experimental 1.1 Sample preparation and experiment equipment 1.1.1 Sample preparation
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The complex titanium grease and the grease containing magnesium hydroxyl silicate modifier used in this paper were prepared by the laboratory itself. The complex titanium grease is made from complex titanium soap, mineral base oil and additive, in which stearic acid, benzoic acid, tetraeisopropyl titanate and distilled water are the main materials for preparation of thickener[16]. The samples of pure complex titanium grease need to add diphenylamine antioxidants at the cooling stage, and the samples of self-repairing complex titanium grease need to add self-repairing modifier with magnesium hydroxyl silicate at cooling stage. Magnesium hydroxyl silicate is a kind of mixed powder modifier, which is composed of serpentine, nephrite, graphite and various catalysts. Its particle size is between 5μm-10μm. The properties of titanium grease containing magnesium hydroxyl silicate and the main components of self-repairing modifier of magnesium hydroxyl silicate are given in Tables 1 and 2, respectively. Table 1 Performances of composite titanium grease and the grease containing magnesium hydroxyl silicate Dropping
Cone
Initial cone
point after
penetration after
Pressure oil
Water
penetration/0.1mm
ten thousand
ten thousand
separation
resistance
shears/℃
shears/0.1mm
290
279.6
7.7
1.73
297
258.8
7.1
1.88
Items
dropping point/℃
Base grease
295
hydroxyl
302
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silicate grease
268.3
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Magnesium
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Table 2
250.2
Main composition of magnesium hydroxyl silicate additives
Chemical molecular formula
Mg6Si4O10（OH)8
Particle size
Composition /%
/µm
Magnesium oxide
Silicon dioxide
water
43.0
44.1
12.9
5-10
1.1.2 Experiment equipment and test methods Friction and wear and self-repairing experiments were carried respectively with a SQ-Ⅲ four-ball friction and wear tester and a M-200 (AMSLER) wear tester. The optimum dosage of self-repairing modifier was determined by four-ball friction and wear tester. Test conditions are: room temperature, rotation speed 1450r/min, the load of 392 N, the test time for 10 hours, weight fraction of self-repairing modifier is 1%, 2%, 3%, 4%, 5%, grade II GCr15 steel ball, diameter is 12.7 mm. The complex titanium grease containing the optimum content of magnesium hydroxyl silicate modifier was 2
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2 Results and discussion 2.1 Influence of concentration of self-repairing modifier
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selected. Using M-200 (AMSLER) wear tester, the friction pair of steel block to steel ring was used. The test speed is 200 r/min, specimen material is 45 steel, hardness is HRC43~47, surface roughness is Ra1.6μm. The upper sample block size is 20mm×6mm×7mm; The dimension of the lower sample ring isΦ40mm×17mm. Under fixed load, wear tests were carried out for 30 hours using grease lubrication, the wear weight loss of specimen was measured every 5 hours, the wear weight loss of specimen was used to evaluate the self-repairing effect of magnesium hydroxyl silicate modifier in the complex titanium grease. Every experiment was repeated three times in each group, and the average value was taken as the evaluation of friction and wear and self-repairing properties. After the ring/block wear test, the typical cuboid block wear specimen was selected. And the surface morphology and chemical composition were analyzed. Before the analysis, the obvious grease on samples were wiped off with degreased cotton. Then the samples were put into 95% ethanol solution using ultrasonic vibration to clean for 30 minutes. In the end, samples were dried with dry nitrogen and stored in a drying vessel. JSM-5600LV scanning electron microscopy (SEM) was used to observe the micro-morphology of the wear mark surface of steel specimen. At the same time, ESCALAB210 X-ray photoelectron spectrometer (XPS) was applied to analyze the chemical state of typical elements on the surface of wear marks, and the energy was 29.35 eV. Mg-Ka was used as excitation source, and the electronic binding energy of C1s polluted carbon was 284.60 eV as reference binding energy. The measurement accuracy of binding energy was about ±0.3eV.
1000 f
0.09
950
d
0.08
900
0.07
850
0.06
800
0.05
750
0.04
0
1
2
3
4
5
-3
0.10
wear dim. dX10 /mm
friction coefficient f
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Fig.1 shows variations of friction coefficient and steel ball worn diameter of complex titanium grease with the concentration of magnesium hydroxyl silicate modifier. From Fig.1, under four ball friction mode, as the amount of magnesium hydroxyl silicate modifier increases, the friction coefficient of complex titanium grease decreases. When adding 1% dosage, the friction coefficient reduces by about 24.9%, when the dosage of 3%, the friction coefficient of the grease is minimum, compared with the pure complex titanium grease, its friction coefficient reduced by about 27.5%, as the amount of further increase, the friction coefficient is slightly higher. It indicates that the antifriction effect is weakened when the dosage of self-repairing modifier is excessive. It can also be seen from the effect on the worn diameter of steel ball that the anti-wear effect becomes more and more obvious with the increase of the additive dosage. When the additive amount reaches 3%, the best value of anti-wear effect appears. With the further increase of the additive amount, the anti-wear effect becomes worse gradually. It is shown that the modifier concentration has an appropriate value. Under the experimental conditions, the complex titanium grease has the best anti-friction and anti-wear effect when it is about 3% of the weight fraction.
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Concentration of modifier wt% Fig.1 Variations of friction and wear properties of complex titanium grease with concentration of modifier 3
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2.2 Self-repairing properties of grease containing lubricating modifier
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Fig.2 shows variations of friction coefficient of complex titanium grease containing 3% weight of magnesium hydroxyl silicate modifier with time under 500N load. From Fig.2, under the friction mode of the ring block, the friction coefficient of complex titanium grease can be significantly reduced by using the modifier. At the beginning of the test, the initial friction coefficient of the two grease samples is basically the same. After 20 hours, the friction coefficient of the base grease changes very little, it stays basically at 0.095. The grease containing the self-repairing modifier drops to 0.022 after 7 hours and then stabilizes until 17 hours. After 17 hours, decreases to 0.019.
0.14
Pure Ti-grease
0.12
3%Self-Repairing additive
0.10 0.08
0.04 0.02 0.00
0
5
10
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0.16
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Fig. 2 Variation of friction coefficient of friction pair with time (500N)
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Fig.3 shows the variation of steel sample’s wear weight loss with time with composite titanium grease containing 3% magnesium hydroxyl silicate modifier and pure titanium grease under 500N load, respectively. As shown in Fig.3(a), the wear weight loss of composite titanium grease containing modifier increases with time with in 15 hours. After 15 hours, the wear weight loss of specimens decreased gradually with time. The weight loss of the specimens decreases and negative wear occurs, which indicates that there are new substances on the friction surface to fill the wear. As can be seen in Fig.3 (b), the wear weight loss of titanium grease without self-repairing agent increases slowly with time. At the same time, the wear weight loss of steel specimen is one order of magnitude larger than that of titanium grease containing self-repairing agent. The results show that the self-repairing agent can improve the wear resistance of composite titanium grease.
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2
0 100 0 80
5
10
15
20
25
30
20
25
30
Time/h
b
Pure Ti-grease
60 40 20 0
0
5
10
15
Time/h
Fig. 3 Variations of wear weight loss of specimen with time (500N) 4
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The surface wear of steel sample was observed every 5 hours with SEM. Fig. 4 is the wear morphology of steel specimens lubricated by pure titanium grease and titanium grease containing self-repairing agent at three time points. From Fig.4 (a), (b) and (e) the wear of steel specimens is serious under pure titanium grease lubrication from 10 hours to 30 hours. The wear marks on the surface are deeper and wider. In addition, there is a certain phenomenon of adhesive wear which can be seen in (e) according to appearance of abrasion marks at 30 hours. At this point, no repairing film appeared on the worn surface. However, under the lubrication of titanium grease containing self-repairing agent, the wear marks on the wear surface of steel specimens are more uniform with no deep stripes, and there is no adhesive wear phenomenon. When friction lasts for 10 hours, as shown in Fig. 4 (b), there are many scratches on the worn surface of the test sample. Although the scratches are shallow, they are clearly distinguishable. Most of them are through the scratches, and the self-repairing film can hardly be seen at the scratch edges. In Fig.4(d), there are still scratches on the worn surface of the test sample, as the friction goes to 20 hours. But self-repairing films begin to appear on the edges of some scratches. Some self-repairing films have covered the surface of the scratches which make the scratches intermittently. At 30 hours shown in Fig.4(f), there are more self-repairing films covering the wear marks on the surface of the wear marks. Compared with the morphology of friction at 20 hours, the wear marks become shallow and the attachments on the wear marks increase. Fig. 5 is the partial enlargement of the wear marks’ surface of Fig. 4 (d). Discontinuous repairing films have been observed on the wear surface of steel specimens after 20 hours of wear test.
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（a）10 hours pure titanium grease （b）10 hours self-repairing modifier
（c）20 hours pure titanium grease （d）20 hours self-repairing modifier
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（e）30 hours pure titanium grease （f）30 hours self-repairing modifier
SEM images of wear marks on steel samples lubricated with different greases
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Fig. 5 SEM image with high magnification of worn surface on steel sample after 20 hours
3 Self-repairing mechanism of modified complex titanium grease
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Table 3 and table 4 are the relative atomic contents of 1min and 3min sputtered on worn surface of steel specimen tested for 10 hours and 30 hours respectively. From Tab.3，it is shown that when the friction lasted for 10 hours, the characteristic element Ti of complex titanium grease appears on the surface of the grinding mark on the steel specimen, but there is no characteristic element of magnesium hydroxyl silicate modifier. After 3min of sputtering, a small amount of characteristic element Mg of magnesium hydroxyl silicate modifier is found in the region about 70~80Å away from the surface, while the characteristic element Ti of the complex titanium grease still exists. Table 3
C1s
O1s
Mg1s
Cr2p
Ti2p
Fe2p
0
72.71
23.64
0.00
0.48
0.34
2.82
180
15.28
12.59
0.32
1.00
0.23
70.57
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Relative atomic content of worn surface for 10 hours
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Relative atomic content of worn surface for 30 hours
Sputter times(s)
C1s
O1s
Mg1s
Si2p
Ti2p
Fe2p
0
52.87
33.79
2.21
3.57
0.69
6.97
60
22.30
47.02
4.98
4.71
0.83
20.16
180
14.42
46.49
6.51
4.33
0.99
27.27
From table 4, when the friction lasted for 30 hours, a large number of characteristic elements Ti , Mg and Si of magnesium hydroxyl silicate modifier are found on the surface of grinding mark and in the area about 70~80 Å away from the surface. This indicates that during the test period of 10 hours to 30 hours, the self-repairing film is gradually formed on the friction surface. In addition, from fig. 5, there is some self-repairing film on the surface 6
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1min 3min surface
2000
Intensity/cps
1600
1200 1000 800 600 400 275
280
285
290
295
300
305
310
Binding Energy/eV
1min 3min surface
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4000 3600 3200 2800 2400 2000
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1600 700
710
720
730
740
750
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（c）Fe2p
2500
2200
Intensity/cps
Intensity/cps
2300 2100 2000 1900 1800 1700 1600
100
105
110
115
Binding Energy/eV
（e）Si2p
535
540
545
550
555
11600 1min 11200 3min 10800 surface 10400 10000 9600 9200 8800 8400 8000 7600 7200 1295 1300 1305 1310 1315 1320 1325 1330
（d）Mg1s 1min 3min surface
2400
95
530
Binding Energy/eV
Brinding Energy/eV
1500 90
525
（b）O1s
4400
Intensity/cps
315
1min 3min surface
Binding Energy/eV
（a）C1s
1200 690
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Intensity/cps
1800
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of grinding mark. The XPS spectra of worn surface, sputtering for 1 minute and 3 minutes are shown in Figure 6. Figure 6 (a) is the C1s peaks of worn surface and being sputtered. The C1s peak at electron binding energy 284.6eV originates from environmental pollution and adsorption. O1s on the surface of grinding mark is characterized by multi-chemical state. As shown in Fig. 6 (b), the wide peak of O1s located at 531.2ev is due to the grease and environmental pollution and adsorption, while O1s located at 530.1ev corresponds to a small amount of oxygen in Fe2O3 and TiO2. Figure 6 (c) is the Fe2p3/2 peaks of worn surface and being sputtered, which is characterized by multi-chemical state, the higher binding energy of Fe2p3/2 on the surface of grinding marks is 711.6ev, which belongs to FeOOH. The Fe2p3/2 peak after being sputtered located at 706.50ev corresponds to element iron. The Fe2p3/2 peak located at 709.8ev corresponds to FeO. The Fe2p3/2 peak located at 710.6ev corresponds to oxide in Fe2O3 and Fe3O4, and iron oxides are the main ones. The Mg1s peaks located at 1304.35ev of worn surface is shown in figure 6(d), which belongs to a kind of magnesium salt. The Si2p peaks located at 102.1ev is shown in figure 6(e), which belongs to silicate. The Ti2p peaks located at 458.6ev is shown in figure 6(f), which belongs to TiO2 decomposed from complex titanium soap.
120
125
3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 445 450 455 460 465 470 475 480
1min 3min surface
485 490
Binding Energy/eV
（f）Ti2p
Fig. 6 XPS spectra of worn surface of steel samples for 30 hours According to the experimental results in the paper and the research in reference [17,18], the lubrication model of titanium grease containing magnesium hydroxyl silicate self-repairing modifier is presented in Fig.7. It can be seen that the friction surface of steel specimen is mainly composed of self-repairing film, matrix debris 7
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particles, self-repairing modifier particles and grease. In the process of friction, the particles of self-repairing modifier are subjected to high pressure of micro-convex body and shear action between two friction surfaces. After ultra-fine grinding of micro-convex body, a large number of chemical active groups and suspension bonds will appear and release on the surface during the crushing process. Previous studies show that [19-20], there are many active groups in magnesium hydroxyl silicate, namely unsaturated Si-O-Si bonds, OH-Mg-OH (O) bonds, O-Si-O bonds, O-H-O bonds and-OH bonds. Under mechanical force, friction heat and chemical action, these active groups react with the metal elements on the surface of the matrix and the metal elements that have formed abrasive debris as follows [13], Mg6[Si4O10](OH)8+Fe2O3+H2→4(MgFe)SiO4 +5H2O. Self-repairing particles penetrate into the metal surface. The oxide deposition film and new phase are gradually formed on the friction surface. Thus the composite film of metal and self-repairing agent is formed which is the self-repairing film. According to the energy spectrum analysis in tables 3 and 4 of the worn surface and the XPS spectra, it can be inferred that the self-repairing film is mainly composed of titanium dioxide formed by decomposition of composite titanium soap, iron oxides, self-repairing particles adsorbed on the worn surface and embedded in the micro-pits of the wear mark. After 30 hours of friction, the self-repairing modifier particles adsorbed on the friction surface reacted with Fe constantly. In addition, the self-repairing film and the matrix are continuously exchanged dynamically, and gradually combined with each other. The self-repairing film will gradually spread and eventually cover the whole friction surface. When the increase of the repairing layer reaches the balance with the wear amount, the friction surface will appear zero wear. According to the variations of macroscopic friction coefficient and wear weight loss with time（in Fig.2 and 3）, it can be seen that the self-repairing film formed by self-repairing modifier has good antifriction effect, which reduces the friction coefficient of composite titanium grease, and has good wear resistance as well. It can prevent the surface of metal matrix of friction pair from direct contact, reduce the wear amount of steel sample, and even produce negative wear phenomenon.
Fig.7 Lubrication model of titanium complex grease containing magnesium hydroxyl silicate modifier
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The hydroxyl magnesium silicate modifier can improve the frictional wear properties of the complex titanium grease. And the most suitable dosage is 3% of the weight fraction. The complex titanium grease containing hydroxyl magnesium silicate modifier has self-repairing function. The results of wear test by M-200 shows that the complex titanium grease containing hydroxyl magnesium silicate modifier has better friction reducing and anti-wear effect. And self-repairing protective film appeared after 20 hours.. A lubrication model of complex titanium grease containing hydroxyl magnesium silicate modifier is proposed. It is revealed that the self-repairing mechanism of hydroxyl magnesium silicate modifier belongs to a form of anti-wear, which is an inverse process of natural wear, a process from the old state to the new one. And the particles of hydroxyl magnesium silicate modifier are ground, refined and activated through physical and chemical reactions under the action of friction chemical force which are combined with the steel matrix or wear particles into the macro solid and adsorbed on the surface of the steel matrix to form a self-repairing film. Acknowledgements 8
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The authors acknowledge the financial supports from the National Basic Research Program of China (973 Program, No.2013CB632305) and National Natural Science Foundation of China (No.51175104).
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Highlights
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The hydroxyl magnesium silicate modifier can improve the friction and wear properties of the complex titanium grease, and the most suitable dosage is 3% in weight fraction. The complex titanium grease containing hydroxyl magnesium silicate modifier has self-repairing function. Under the condition of M-200 friction, its self-repairing protective film appeared after 15 hours. A lubrication model of complex titanium grease containing hydroxyl magnesium silicate modifier was proposed which revealed that the self-repairing mechanism of hydroxyl magnesium silicate modifier belongs to a form of anti-wear, which is an inverse process of natural wear.
1
S0301-679X(19)30163-X
DOI:
https://doi.org/10.1016/j.triboint.2019.03.043
Reference:
JTRI 5685
To appear in:
Tribology International
Received Date: 21 November 2018 Revised Date:
17 March 2019
Accepted Date: 18 March 2019
Please cite this article as: Jianjun Q, Huajie Q, Zhongpu W, Yunxia L, Self-repairing properties of complex titanium grease containing hydroxyl silicate, Tribology International (2019), doi: https:// doi.org/10.1016/j.triboint.2019.03.043. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Self-Repairing Properties of Complex Titanium Grease Containing Hydroxyl Silicate Qu Jianjun, Qu Huajie, Wen Zhongpu, Luo Yunxia (School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, China，150001)
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Abstract: With magnesium hydroxyl silicate modifiers, a kind of complex titanium grease was prepared containing magnesium hydroxyl silicate modifier. Using four-ball friction and wear testing machine, The dosage of magnesium hydroxyl silicate modifier was studied on the influence of friction coefficient and wear spot diameter, the best dosage was obtained. By using M-200 wear testing machine, friction and wear test of complex titanium grease containing the best dosage of modifier was tested for 30 hours. The results show that the complex titanium grease containing magnesium hydroxyl silicate modifier has better anti-friction and anti-wear properties and self-repairing function. The surfaces of the steel sample testing for 10 hours, 20 hours and 30 hours are respectively analyzed based on SEM and X-ray Photoelectron Spectrometer. It is shown that the self-repairing film on the steel surface can be formed after 10~20 hours’ friction. The self-repairing mechanism of magnesium silicate modifier was explored. A kind of self-repairing model was proposed by adding magnesium hydroxyl silicate modifier to the complex titanium grease. It is concluded that the microscopic particles of magnesium hydroxyl silicate modifier generate adsorbent substance on the surface of steel substrate to repair the wear surface through a series of physical and chemical reaction. Key words: complex titanium grease; hydroxyl silicate; self-repairing; friction; wear
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As an important component of industrial lubricants, grease plays an important role in reducing friction and wear, reducing energy consumption and extending the service life of machines. With the enhancement of people's awareness of environmental protection, increasing demand for long service life and environment friendly grease, developing new high-performance lubrication products has become an inevitable trend. It is necessary to develop a kind of grease with self-repairing properties. Adding self-repairing additives to the grease is one of the effective methods to achieve this purpose [1]. Xi Xiang added montmorillonite to lithium grease to increase the anti-wear properties of grease [2]. MoYunhui filled soft metals to grease to make grease a certain self-repairing function [3]. Ilie Filip studied the anti-friction and anti-wear properties of MoS2。He considered MoS2 having a certain self-repairing function [4]. Since its discovery at the beginning of last century, Hydroxyl silicate has been found a kind of lubricant additives with self-repairing function, and applied in a lot of lubricating oil products which obtained the certain self-repairing effect. Liu Qing and his research group introduced the research progress of hydroxyl silicate in detail [5]. Rudenko Pavlo added magnesium hydroxyl silicate into lubricating oil and studied the influence of particle size [6]. Zhu Gongzhi studied the self-repairing mechanism of magnesium hydroxyl silicate modifier added into lubricating oil [7]. The addition of magnesium hydroxyl silicate modifier into lubricating oil did plays a certain role in reducing friction and self-repairing. However, there are still problems exist. The dispersed suspension, easiness of precipitate, blockage of oil circuit have been studying. Nevertheless, it is a worth studying to use the properties of special antifriction and self-repairing wear of hydroxyl silicate. Researchers have focused on the modification of hydroxyl silicate and the research of synthetic technology [8,9]. If it is added into the grease, the precipitation problem of the particles can be improved by the high viscosity property of the grease. And the well-dispersed and suspended grease containing hydroxyl silicate can be obtained. Adding some organic or inorganic particles in grease has been reported several times [10,11] to improve the friction and wear properties of grease. However, adding hydroxyl silicate to grease is seldom studied. Yu Helong added hydroxyl silicate to tank grease to improve the tribological properties of grease [12]. Recently, A. D. Brekia filled two hydroxyl silicates to multifunctional lithium grease to improve the wear and repair properties of lithium grease [13]. However, the addition of hydroxyl silicate to composite titanium grease has rarely been reported. 1
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Complex titanium grease is a new type of environment friendly lubricants [14,15]. But its tribological properties are not very ideal. Adding the modifier containing magnesium hydroxyl silicate into the grease, it is expected to improve its tribological properties and develop a new type of complex titanium grease with a self-repairing function. In this paper, the properties of friction and wear and self-repairing of complex titanium grease containing magnesium hydroxyl silicate modifier were studied by using four-ball tester and M-200 (AMSLER) wear tester respectively. By means of XPS and SEM images, the surface morphology of the grinding mark was studied, the relative atomic content of the surface was analyzed, and the self-repairing effect of magnesium hydroxyl silicate modifier was explored.
1. Experimental 1.1 Sample preparation and experiment equipment 1.1.1 Sample preparation
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The complex titanium grease and the grease containing magnesium hydroxyl silicate modifier used in this paper were prepared by the laboratory itself. The complex titanium grease is made from complex titanium soap, mineral base oil and additive, in which stearic acid, benzoic acid, tetraeisopropyl titanate and distilled water are the main materials for preparation of thickener[16]. The samples of pure complex titanium grease need to add diphenylamine antioxidants at the cooling stage, and the samples of self-repairing complex titanium grease need to add self-repairing modifier with magnesium hydroxyl silicate at cooling stage. Magnesium hydroxyl silicate is a kind of mixed powder modifier, which is composed of serpentine, nephrite, graphite and various catalysts. Its particle size is between 5μm-10μm. The properties of titanium grease containing magnesium hydroxyl silicate and the main components of self-repairing modifier of magnesium hydroxyl silicate are given in Tables 1 and 2, respectively. Table 1 Performances of composite titanium grease and the grease containing magnesium hydroxyl silicate Dropping
Cone
Initial cone
point after
penetration after
Pressure oil
Water
penetration/0.1mm
ten thousand
ten thousand
separation
resistance
shears/℃
shears/0.1mm
290
279.6
7.7
1.73
297
258.8
7.1
1.88
Items
dropping point/℃
Base grease
295
hydroxyl
302
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268.3
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Magnesium
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Table 2
250.2
Main composition of magnesium hydroxyl silicate additives
Chemical molecular formula
Mg6Si4O10（OH)8
Particle size
Composition /%
/µm
Magnesium oxide
Silicon dioxide
water
43.0
44.1
12.9
5-10
1.1.2 Experiment equipment and test methods Friction and wear and self-repairing experiments were carried respectively with a SQ-Ⅲ four-ball friction and wear tester and a M-200 (AMSLER) wear tester. The optimum dosage of self-repairing modifier was determined by four-ball friction and wear tester. Test conditions are: room temperature, rotation speed 1450r/min, the load of 392 N, the test time for 10 hours, weight fraction of self-repairing modifier is 1%, 2%, 3%, 4%, 5%, grade II GCr15 steel ball, diameter is 12.7 mm. The complex titanium grease containing the optimum content of magnesium hydroxyl silicate modifier was 2
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2 Results and discussion 2.1 Influence of concentration of self-repairing modifier
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selected. Using M-200 (AMSLER) wear tester, the friction pair of steel block to steel ring was used. The test speed is 200 r/min, specimen material is 45 steel, hardness is HRC43~47, surface roughness is Ra1.6μm. The upper sample block size is 20mm×6mm×7mm; The dimension of the lower sample ring isΦ40mm×17mm. Under fixed load, wear tests were carried out for 30 hours using grease lubrication, the wear weight loss of specimen was measured every 5 hours, the wear weight loss of specimen was used to evaluate the self-repairing effect of magnesium hydroxyl silicate modifier in the complex titanium grease. Every experiment was repeated three times in each group, and the average value was taken as the evaluation of friction and wear and self-repairing properties. After the ring/block wear test, the typical cuboid block wear specimen was selected. And the surface morphology and chemical composition were analyzed. Before the analysis, the obvious grease on samples were wiped off with degreased cotton. Then the samples were put into 95% ethanol solution using ultrasonic vibration to clean for 30 minutes. In the end, samples were dried with dry nitrogen and stored in a drying vessel. JSM-5600LV scanning electron microscopy (SEM) was used to observe the micro-morphology of the wear mark surface of steel specimen. At the same time, ESCALAB210 X-ray photoelectron spectrometer (XPS) was applied to analyze the chemical state of typical elements on the surface of wear marks, and the energy was 29.35 eV. Mg-Ka was used as excitation source, and the electronic binding energy of C1s polluted carbon was 284.60 eV as reference binding energy. The measurement accuracy of binding energy was about ±0.3eV.
1000 f
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950
d
0.08
900
0.07
850
0.06
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0.05
750
0.04
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2
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4
5
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0.10
wear dim. dX10 /mm
friction coefficient f
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Fig.1 shows variations of friction coefficient and steel ball worn diameter of complex titanium grease with the concentration of magnesium hydroxyl silicate modifier. From Fig.1, under four ball friction mode, as the amount of magnesium hydroxyl silicate modifier increases, the friction coefficient of complex titanium grease decreases. When adding 1% dosage, the friction coefficient reduces by about 24.9%, when the dosage of 3%, the friction coefficient of the grease is minimum, compared with the pure complex titanium grease, its friction coefficient reduced by about 27.5%, as the amount of further increase, the friction coefficient is slightly higher. It indicates that the antifriction effect is weakened when the dosage of self-repairing modifier is excessive. It can also be seen from the effect on the worn diameter of steel ball that the anti-wear effect becomes more and more obvious with the increase of the additive dosage. When the additive amount reaches 3%, the best value of anti-wear effect appears. With the further increase of the additive amount, the anti-wear effect becomes worse gradually. It is shown that the modifier concentration has an appropriate value. Under the experimental conditions, the complex titanium grease has the best anti-friction and anti-wear effect when it is about 3% of the weight fraction.
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Concentration of modifier wt% Fig.1 Variations of friction and wear properties of complex titanium grease with concentration of modifier 3
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2.2 Self-repairing properties of grease containing lubricating modifier
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Fig.2 shows variations of friction coefficient of complex titanium grease containing 3% weight of magnesium hydroxyl silicate modifier with time under 500N load. From Fig.2, under the friction mode of the ring block, the friction coefficient of complex titanium grease can be significantly reduced by using the modifier. At the beginning of the test, the initial friction coefficient of the two grease samples is basically the same. After 20 hours, the friction coefficient of the base grease changes very little, it stays basically at 0.095. The grease containing the self-repairing modifier drops to 0.022 after 7 hours and then stabilizes until 17 hours. After 17 hours, decreases to 0.019.
0.14
Pure Ti-grease
0.12
3%Self-Repairing additive
0.10 0.08
0.04 0.02 0.00
0
5
10
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Fig. 2 Variation of friction coefficient of friction pair with time (500N)
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Fig.3 shows the variation of steel sample’s wear weight loss with time with composite titanium grease containing 3% magnesium hydroxyl silicate modifier and pure titanium grease under 500N load, respectively. As shown in Fig.3(a), the wear weight loss of composite titanium grease containing modifier increases with time with in 15 hours. After 15 hours, the wear weight loss of specimens decreased gradually with time. The weight loss of the specimens decreases and negative wear occurs, which indicates that there are new substances on the friction surface to fill the wear. As can be seen in Fig.3 (b), the wear weight loss of titanium grease without self-repairing agent increases slowly with time. At the same time, the wear weight loss of steel specimen is one order of magnitude larger than that of titanium grease containing self-repairing agent. The results show that the self-repairing agent can improve the wear resistance of composite titanium grease.
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5
10
15
20
25
30
20
25
30
Time/h
b
Pure Ti-grease
60 40 20 0
0
5
10
15
Time/h
Fig. 3 Variations of wear weight loss of specimen with time (500N) 4
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The surface wear of steel sample was observed every 5 hours with SEM. Fig. 4 is the wear morphology of steel specimens lubricated by pure titanium grease and titanium grease containing self-repairing agent at three time points. From Fig.4 (a), (b) and (e) the wear of steel specimens is serious under pure titanium grease lubrication from 10 hours to 30 hours. The wear marks on the surface are deeper and wider. In addition, there is a certain phenomenon of adhesive wear which can be seen in (e) according to appearance of abrasion marks at 30 hours. At this point, no repairing film appeared on the worn surface. However, under the lubrication of titanium grease containing self-repairing agent, the wear marks on the wear surface of steel specimens are more uniform with no deep stripes, and there is no adhesive wear phenomenon. When friction lasts for 10 hours, as shown in Fig. 4 (b), there are many scratches on the worn surface of the test sample. Although the scratches are shallow, they are clearly distinguishable. Most of them are through the scratches, and the self-repairing film can hardly be seen at the scratch edges. In Fig.4(d), there are still scratches on the worn surface of the test sample, as the friction goes to 20 hours. But self-repairing films begin to appear on the edges of some scratches. Some self-repairing films have covered the surface of the scratches which make the scratches intermittently. At 30 hours shown in Fig.4(f), there are more self-repairing films covering the wear marks on the surface of the wear marks. Compared with the morphology of friction at 20 hours, the wear marks become shallow and the attachments on the wear marks increase. Fig. 5 is the partial enlargement of the wear marks’ surface of Fig. 4 (d). Discontinuous repairing films have been observed on the wear surface of steel specimens after 20 hours of wear test.
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（a）10 hours pure titanium grease （b）10 hours self-repairing modifier
（c）20 hours pure titanium grease （d）20 hours self-repairing modifier
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（e）30 hours pure titanium grease （f）30 hours self-repairing modifier
SEM images of wear marks on steel samples lubricated with different greases
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Fig. 5 SEM image with high magnification of worn surface on steel sample after 20 hours
3 Self-repairing mechanism of modified complex titanium grease
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Table 3 and table 4 are the relative atomic contents of 1min and 3min sputtered on worn surface of steel specimen tested for 10 hours and 30 hours respectively. From Tab.3，it is shown that when the friction lasted for 10 hours, the characteristic element Ti of complex titanium grease appears on the surface of the grinding mark on the steel specimen, but there is no characteristic element of magnesium hydroxyl silicate modifier. After 3min of sputtering, a small amount of characteristic element Mg of magnesium hydroxyl silicate modifier is found in the region about 70~80Å away from the surface, while the characteristic element Ti of the complex titanium grease still exists. Table 3
C1s
O1s
Mg1s
Cr2p
Ti2p
Fe2p
0
72.71
23.64
0.00
0.48
0.34
2.82
180
15.28
12.59
0.32
1.00
0.23
70.57
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Relative atomic content of worn surface for 10 hours
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Relative atomic content of worn surface for 30 hours
Sputter times(s)
C1s
O1s
Mg1s
Si2p
Ti2p
Fe2p
0
52.87
33.79
2.21
3.57
0.69
6.97
60
22.30
47.02
4.98
4.71
0.83
20.16
180
14.42
46.49
6.51
4.33
0.99
27.27
From table 4, when the friction lasted for 30 hours, a large number of characteristic elements Ti , Mg and Si of magnesium hydroxyl silicate modifier are found on the surface of grinding mark and in the area about 70~80 Å away from the surface. This indicates that during the test period of 10 hours to 30 hours, the self-repairing film is gradually formed on the friction surface. In addition, from fig. 5, there is some self-repairing film on the surface 6
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1min 3min surface
2000
Intensity/cps
1600
1200 1000 800 600 400 275
280
285
290
295
300
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310
Binding Energy/eV
1min 3min surface
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Intensity/cps
2300 2100 2000 1900 1800 1700 1600
100
105
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Binding Energy/eV
（e）Si2p
535
540
545
550
555
11600 1min 11200 3min 10800 surface 10400 10000 9600 9200 8800 8400 8000 7600 7200 1295 1300 1305 1310 1315 1320 1325 1330
（d）Mg1s 1min 3min surface
2400
95
530
Binding Energy/eV
Brinding Energy/eV
1500 90
525
（b）O1s
4400
Intensity/cps
315
1min 3min surface
Binding Energy/eV
（a）C1s
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Intensity/cps
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of grinding mark. The XPS spectra of worn surface, sputtering for 1 minute and 3 minutes are shown in Figure 6. Figure 6 (a) is the C1s peaks of worn surface and being sputtered. The C1s peak at electron binding energy 284.6eV originates from environmental pollution and adsorption. O1s on the surface of grinding mark is characterized by multi-chemical state. As shown in Fig. 6 (b), the wide peak of O1s located at 531.2ev is due to the grease and environmental pollution and adsorption, while O1s located at 530.1ev corresponds to a small amount of oxygen in Fe2O3 and TiO2. Figure 6 (c) is the Fe2p3/2 peaks of worn surface and being sputtered, which is characterized by multi-chemical state, the higher binding energy of Fe2p3/2 on the surface of grinding marks is 711.6ev, which belongs to FeOOH. The Fe2p3/2 peak after being sputtered located at 706.50ev corresponds to element iron. The Fe2p3/2 peak located at 709.8ev corresponds to FeO. The Fe2p3/2 peak located at 710.6ev corresponds to oxide in Fe2O3 and Fe3O4, and iron oxides are the main ones. The Mg1s peaks located at 1304.35ev of worn surface is shown in figure 6(d), which belongs to a kind of magnesium salt. The Si2p peaks located at 102.1ev is shown in figure 6(e), which belongs to silicate. The Ti2p peaks located at 458.6ev is shown in figure 6(f), which belongs to TiO2 decomposed from complex titanium soap.
120
125
3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 445 450 455 460 465 470 475 480
1min 3min surface
485 490
Binding Energy/eV
（f）Ti2p
Fig. 6 XPS spectra of worn surface of steel samples for 30 hours According to the experimental results in the paper and the research in reference [17,18], the lubrication model of titanium grease containing magnesium hydroxyl silicate self-repairing modifier is presented in Fig.7. It can be seen that the friction surface of steel specimen is mainly composed of self-repairing film, matrix debris 7
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particles, self-repairing modifier particles and grease. In the process of friction, the particles of self-repairing modifier are subjected to high pressure of micro-convex body and shear action between two friction surfaces. After ultra-fine grinding of micro-convex body, a large number of chemical active groups and suspension bonds will appear and release on the surface during the crushing process. Previous studies show that [19-20], there are many active groups in magnesium hydroxyl silicate, namely unsaturated Si-O-Si bonds, OH-Mg-OH (O) bonds, O-Si-O bonds, O-H-O bonds and-OH bonds. Under mechanical force, friction heat and chemical action, these active groups react with the metal elements on the surface of the matrix and the metal elements that have formed abrasive debris as follows [13], Mg6[Si4O10](OH)8+Fe2O3+H2→4(MgFe)SiO4 +5H2O. Self-repairing particles penetrate into the metal surface. The oxide deposition film and new phase are gradually formed on the friction surface. Thus the composite film of metal and self-repairing agent is formed which is the self-repairing film. According to the energy spectrum analysis in tables 3 and 4 of the worn surface and the XPS spectra, it can be inferred that the self-repairing film is mainly composed of titanium dioxide formed by decomposition of composite titanium soap, iron oxides, self-repairing particles adsorbed on the worn surface and embedded in the micro-pits of the wear mark. After 30 hours of friction, the self-repairing modifier particles adsorbed on the friction surface reacted with Fe constantly. In addition, the self-repairing film and the matrix are continuously exchanged dynamically, and gradually combined with each other. The self-repairing film will gradually spread and eventually cover the whole friction surface. When the increase of the repairing layer reaches the balance with the wear amount, the friction surface will appear zero wear. According to the variations of macroscopic friction coefficient and wear weight loss with time（in Fig.2 and 3）, it can be seen that the self-repairing film formed by self-repairing modifier has good antifriction effect, which reduces the friction coefficient of composite titanium grease, and has good wear resistance as well. It can prevent the surface of metal matrix of friction pair from direct contact, reduce the wear amount of steel sample, and even produce negative wear phenomenon.
Fig.7 Lubrication model of titanium complex grease containing magnesium hydroxyl silicate modifier
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The hydroxyl magnesium silicate modifier can improve the frictional wear properties of the complex titanium grease. And the most suitable dosage is 3% of the weight fraction. The complex titanium grease containing hydroxyl magnesium silicate modifier has self-repairing function. The results of wear test by M-200 shows that the complex titanium grease containing hydroxyl magnesium silicate modifier has better friction reducing and anti-wear effect. And self-repairing protective film appeared after 20 hours.. A lubrication model of complex titanium grease containing hydroxyl magnesium silicate modifier is proposed. It is revealed that the self-repairing mechanism of hydroxyl magnesium silicate modifier belongs to a form of anti-wear, which is an inverse process of natural wear, a process from the old state to the new one. And the particles of hydroxyl magnesium silicate modifier are ground, refined and activated through physical and chemical reactions under the action of friction chemical force which are combined with the steel matrix or wear particles into the macro solid and adsorbed on the surface of the steel matrix to form a self-repairing film. Acknowledgements 8
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The authors acknowledge the financial supports from the National Basic Research Program of China (973 Program, No.2013CB632305) and National Natural Science Foundation of China (No.51175104).
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composite powder as a lubricating oil additive for steel-steel pair.Mocaxue Xuebao/Tribology.2012,32(2):183-188(Chinese). [8] Gao Kai, Chang Qiuying,Wang, Bin, et al.The tribological performances of modified magnesium silicate hydroxide as lubricant additive. Tribology International, 2018, (121):64-70.
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Highlights
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The hydroxyl magnesium silicate modifier can improve the friction and wear properties of the complex titanium grease, and the most suitable dosage is 3% in weight fraction. The complex titanium grease containing hydroxyl magnesium silicate modifier has self-repairing function. Under the condition of M-200 friction, its self-repairing protective film appeared after 15 hours. A lubrication model of complex titanium grease containing hydroxyl magnesium silicate modifier was proposed which revealed that the self-repairing mechanism of hydroxyl magnesium silicate modifier belongs to a form of anti-wear, which is an inverse process of natural wear.
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