Effect of Nano-coppers with Different Dimensions on Tribological Properties of L

Wang Jing; Guo Xiaochuan; Mi Hongying; Zhou Weigui; Yang Xin

(1. Institute of Military New Energy Technology, Beijing 102300;2. Clean Energy Research Institute, Shenzhen 518031;3. Army Logistics University of PLA, Chongqing 401311)

Abstract: As an excellent anti-wear and friction-reducing additive, nano-copper has attracted extensive researches and attention in tribology field. However, the existing researches are still limited to zero-dimensional nano-copper, that is, the effect of copper nanoparticles on the tribological properties of lubricants. In this paper, the effects of nano-coppers with different dimensions (copper nanoparticles, copper nanowires and copper nanoplates) on the anti-wear ability and frictionreducing performance of lithium grease were systematically investigated. And the mechanism of interaction between dimension and tribological properties was explained in detail. It is concluded that the tribological properties of lithium grease can be improved by the addition of nano-coppers due to the formation of the molten copper film and the CuO tribochemical reaction film on the worn surface. And the dimension of nano-copper has a great influence on its tribological property. The copper nanoparticles can play the role of nano-bearing to change the friction form from sliding friction to rolling friction, which would decrease the wear loss of friction pairs. The wear loss can be sharply decreased by addition of copper nanoplates with micron-scale diameter and lamellar structure which significantly prevents the direct contact of micro-protrusions on the surface of friction pairs and reduces the “welding” between micro-protrusions due to local high temperature caused by friction.

Key words: nano-copper; dimension; additive; tribology

1 Introduction

Lubrication is one of the main means to reduce the wear loss of equipment, improve its work reliability and extend its service life[1]. The lubricant forms a lubrication film on the contact surface of friction pair, which can prevent the direct contact of the friction pair, thus significantly reducing the friction coefficient and wear loss[2].However, with the development of advanced equipment,the working environment of lubricating parts is becoming increasingly harsh, which raises higher requirements for lubricant performance. It needs high-performance additives to further improve the anti-wear and frictionreducing performance and the extreme pressure performance of lubricants[3-4].

Thanks to its low melting point, low shear strength and high ductility, nano-copper has attracted extensive researches and attention as an excellent anti-wear and friction-reducing agent and self-repairing additive[5-7].

The researchers have systematically studied the effects of copper nanoparticles with different particle sizes on the tribological properties of lubricants[8]. A large number of studies have verified that nano-copper as an additive can significantly improve the tribological properties of lubricants, so that lubricants can meet the lubrication requirements of equipment to a certain extent[9-11]. At the same time, it is found that the tribological properties of lubricants can be further improved by surface modification of nano-copper[12-14].

However, the existing researches are still limited on the zero-dimensional nano-copper to study the effect of copper nanoparticles on the tribological properties of lubricants. The effects of nano-coppers with different dimensions such as copper nanoparticles, copper nanowires, and copper nanoplates on the tribological properties of lubricants have not been systematically investigated yet. It is well known that the structure and dimension of nanomaterials have a huge impact on their performance. Therefore, it is of great significance to realize the controllable preparation of nano-coppers with different dimensions and reveal the influence of their dimension on the tribological properties.

Based on the above considerations, the effects of nanocoppers (copper nanoparticles, copper nanowires, and copper nanoplates) with different concentrations on the anti-wear ability and friction-reducing performance of lithium grease were systematically investigated in this paper. The mechanism of interaction between the dimension and tribological properties was explained in detail, which could provide a theoretical support for further enriching and broadening the application of nanocopper lubricating additives.

2 Experimental

2.1 Preparation of nano-coppers with different dimensions

Nano-coppers with different dimensions were prepared in our laboratory. The copper nanoparticles and copper nanoplates were prepared via in-situ chemical reduction in liquid according to the methods referred to in the literature reports[15-16]. The copper nanowires were prepared by the hydrothermal method. The typical preparation procedure was described as follow: 0.17 g of CuCl2·2H2O were dissolved in the deionized water under magnetic stirring followed by addition of 0.391 g of glucose. Then 1.44 g of HDA was added slowly under stirring for 5 hours to obtain a blue emulsion. The emulsion was transferred to an 100-mL hydrothermal reactor to take part in reaction at a constant temperature of 120 °C for 24 hours to obtain a reddish brown emulsion.The reddish brown emulsion was washed three times with n-pentane, deionized water, and ethanol, respectively, to remove excess impurities. The resulting precipitates were freeze-dried to obtain a reddish brown powder which was the copper nanowires.

2.2 Characterization of copper nanowires

The morphology of prepared copper nanowires were observed by a JSM-7800F scanning electron microscope from JEOL (Japan) operating at a voltage of 10.0 kV.And structural characteristics of copper nanowires were analyzed by the transmission electron microscopy from FEI (USA) with an accelerating voltage of 200 kV.

2.3 Tribological measurements

The preparation of lithium greases with nano-coppers with different dimensions was conducted according to the methods referred to in the literature[17]. According to the ASTM D5707-11 method, the tribological tests were performed with a SRV-Ⅳ oscillating reciprocaing friction and wear tester from OPTIMOL, Germany. The Specific method was the same as shown in the literature[17].

2.4 Characterization of worn surface

After the tribological test, the friction pairs were ultrasonically cleaned in petroleum ether for 10 minutes.Then the morphology and elements distribution of worn surface were analyzed by the following methods. The morphology of the worn surfaces was observed by a white light interferometer (Bruker, Germany). Characterization of the elements distribution on worn surfaces were studied by an EDS spectrometer; The chemical state of elements on worn surfaces was analyzed by a Thermo ESCALab-250Xi X-ray photoelectron spectroscope (Thermo Fisher Scientific, USA) with the Al K radiation serving as the excitation source and the binding energy of contaminated carbon (C1s: 284.80 eV) serving as the reference.

3 Results and Discussion

3.1 Characterization of copper nanowires

As shown in Figure 1(a), the three characteristic diffraction peaks in the XRD spectrum are identified at 2θ of 43.4°, 50.5°, and 74.2°, respectively, corresponding to the (111), (200), and (220) crystal planes of the Cu(fcc), indicating the outcome for synthesis of copper nanowires. And there is no existence of peaks such as Cu(OH)2or Cu2O in the spectrum, indicating that the obtained copper nanowire is extremely pure. As shown in the SEM image (Figure 1(b)) and the TEM image (Figure 1(c)), the copper nanowires have a good aspect ratio with a diameter of 20 nm and a length of micron scale. And as illustrated by the HRTEM image, its lattice spacing is 0.22 nm, which corresponds to the (111) plane of Cu (fcc),further indicating the fact that the prepared materials are copper nanowires. Moreover, it can be seen from the selected area in the electron diffraction pattern that the prepared copper nanowires are polycrystalline materials.

3.2 Effect of nano-coppers with different dimensions on tribological properties of lithium grease

Figure 2 shows the effect of nano-coppers with different dimensions on the friction coefficient of lithium grease and the wear loss of friction pairs under a load of 200 N.It can be seen that when 1.0% of copper nanoparticles was added to the lithium grease, the wear loss of friction pairs was decreased by 78.5%. By adding 1.0% of copper nanowires, the wear loss of friction pairs decreased by 72.3%. When 0.5% of copper nanoplates were added,the wear loss of friction pairs was decreased by 82.2%.The wear loss of friction pairs could be dramatically reduced by the addition of nano-coppers with different dimensions, indicating that the anti-wear ability of lithium grease was significantly improved.

“Thanks, thanks, you heavenly little bird. I know you well. I banished you from my kingdom once, and yet you have charmed away the evil faces from my bed, and banished Death from my heart, with your sweet song. How can I reward you?”

At the same time, the concentration of nano-coppers with different dimensions has diverse effects on the wear loss of friction pairs. As the concentration of copper nanowires increases, the wear loss gradually decreases to a lowest value. After that the wear loss increases sharply with an increasing concentration of copper nanowires.By comparison, when the concentration of copper nanoparticles or copper nanoplates is 0.2%, the wear loss is reduced by more than 70%. At the same time, the wear loss of friction pairs did not further decrease significantly with an increasing concentration of copper nanoparticles or copper nanoplates.

Figure 1 (a) XRD spectrum, (b) SEM image, (c) TEM image, and (d) HRTEM image of the copper nanowires(the insert in (d) is SAED)

As shown in Figure 2 (b), the average friction coefficient of lithium grease containing copper nanoplates is lower than that with other two dimensions of nano-coppers. It can be seen from Figure 2 (c) that the friction coefficient of base grease rises sharply in the initial stage of friction process and then stabilizes at a high value until the lubricant film is broken down in the later stage of friction process. And then the friction coefficient begins to fluctuate sharply.

When 0.5% of copper nanowires are added, the friction coefficient of grease in the initial stage of friction process is low and fluctuates slightly. After having run for a period of time, the friction coefficient increases suddenly and then remains at friction coefficient a high level with obvious fluctuations. We suppose that this phenomenon re flects the initial stage of friction process, when copper nanowires are melted on the surface of friction pair to form a protective film, which can keep the friction coefficient at a low value. However, as the frictional process progresses, the molten protective film is destroyed and the friction coefficient rises abruptly. At the same time, the speed for formation of the molten protective film is less than its destruction speed, which is expressed by strong fluctuations of friction coefficient with the time that can reach a high value eventually.

When 0.5% of copper nanoparticles were added, the friction coefficient fluctuated continuously in a lower range. The continuous fluctuation can still re flect that the formation speed of molten protective film generated by the melt of copper nanoparticles under the condition of local high temperature caused by friction is less than the destruction speed of the molten film. However, the copper nanoparticles with a small particle size can play the role of nano-bearings during the friction process which makes its friction coefficient smaller than that of copper nanowires.It is interesting to observe the curve of friction coefficient changing with copper nanoplates. During the whole friction process, the friction coefficient remains at low value and changes steadily with time without obvious fluctuation. It only increases slowly in the later stage of friction process, showing a good friction-reducing performance. This phenomenon is determined by the structural characteristics of copper nanoplates. The copper nanoplates with a micron-scale diameter can be deposited on the surface of friction pairs to form a deposition film which can play a protective role in the initial stage of friction process. At the same time, the friction coefficient can be further reduced due to its lamellar structure,so that the friction-reducing performance of grease is significantly improved.

Figure 2 Effect of nano-copper concentration on wear loss(a), average friction coefficient (b), and friction coefficient (c)under a load of 200 N

3.3 Morphology of worn surface

Figure 3 illustrates the 3D pro files of the worn surface lubricated by grease samples with optimal addition of nano-coppers with different dimensions. It can be seen that there is a “furrow” phenomenon, which illustrates a serious abrasive wear on the worn surface lubricated by the base grease. The abrasive wear can be further confirmed by observing the 2D cross-section of worn surface (Figure 4). The scratches on the worn surface lubricated by the base grease are very deep, showing a obvious wear loss of the friction pair. The addition of copper nanowires greatly reduces the abrasive wear of friction pair. And there are only a small amount of scratches with a shallow depth in the worn surface. At the same time, the addition of copper nanoparticles or copper nanoplates can completely inhibit the abrasive wear. No obvious scratches are detected in the worn surface. And the smoothness and flatness of the worn surface are better than the worn surface lubricated by the base grease, which indicates a significant improvement of the anti-wear ability of the grease achieved by the addition of copper nanoparticles or copper nanoplates.

Figure 3 3D pro file of worn surface lubricated by grease with optimal addition of nano-coppers under a load of 200N

3.4 Distribution and chemical states of elements on worn surface

It can be seen that the main elements on the worn surface lubricated by the base grease are C, O, Fe and Cr, which are the components of friction pairs. The O element is produced by oxidation of friction pairs due to local high temperature generated during friction. The C element not only comes from the matrix of friction pairs, but can also originate from the deposition of C element of long chain hydrocarbons of grease on the matrix surface during friction.

Figure 4 2D cross section of worn surface lubricated by grease with optimal addition of nano-coppers under a load of 200 N

After addition of copper nanoparticles and copper nanoplates in the base grease, the EDS spectra of the worn surface show obvious diffraction peaks at near 0.93 keV, 8.04 keV,and 8.88 keV. These peaks correspond to L, Kα and Kβ peaks of copper, respectively, which can directly explain the existence of copper element on the worn surface.

It is explained that copper nanoparticles and copper nanoplates used as lubricating additives may participate in the tribochemical reaction on the worn surface during the friction process, forming tribochemical reaction films with C, O, Fe, Cr and Cu as the main elements, or forming a physical molten film of copper to significantly reduce the wear loss and further improve the anti-wear ability of lithium grease.

However, there is no characteristic peak of Cu element on the worn surface lubricated by the grease containing copper nanowires. This may occur because the amount of physical molten film or chemical reaction film formed on the worn surface is too small to be detected compared with the copper nanoparticles or copper nanoplates participating in the friction process. It may also be caused by the fact that the strength of protective film formed on the worn surface lubricated by the grease with copper nanowires is quite low and this protective film may be destroyed during friction. The difference in the EDS spectrum further re flects that the dimension of nano-copper plays a key role in the tribological performance of nano-copper. And the effects of nano-copper with different dimensions on the tribological performance of grease are different.

In order to further clarify the chemical state of Cu element on the worn surface, the XPS analysis was carried out. As shown in Figure 6, when the friction pairs are lubricated by the base grease, the peak of C1s is 284.6 eV, which corresponds to C-H or C-C of long chain hydrocarbons,indicating the existence of lubricating oil film on the worn surface. And the peaks of Fe2p1/2 and Fe2p3/2 are at 711.3 eV and 724.0 eV, respectively, which correspond to Fe2O3, illustrating the formation of a tribochemical film composed of Fe2O3on the worn surface during friction process. No peaks of Cu2p1/2 and Cu 2p3/2 can be found,indicating that there was no copper or its compounds on the worn surface. The peak of O1s is identified at 531.3 eV, corresponding to Li2O. And the peak of O1s at 529.8 eV corresponds to Fe2O3, which further explains the formation of a tribochemical film compoased of Fe2O3.

When copper nanoparticles are added, the peak of C1s remains unchanged at 284.6 eV, reflecting the presence of lubricating oil film. The new peak of Fe2p3/2 at 709.4 eV corresponds to FeO, illustrating that the tribochemical films of Fe2O3and FeO are formed on the worn surface.The low peak value and small peak area indicating a low content of FeO. It is worthy of mentioning that the peaks of Cu2p1/2 and Cu 2p3/2 are identified at 952.1 eV and 932.0 eV, respectively, corresponding to Cu. Meanwhile, the peaks of Cu 2p1/2 and Cu 2p3/2 at 953.7 eV and 934.0 eV,respectively, correspond to CuO. It can be concluded that a protective film composed of Cu and CuO is formed on the worn surface during friction process after the addition of copper nanoparticles. Cu is formed by the melted copper nanoparticles due to high temperature produced by friction.And CuO is caused by oxidation of a protective film composed of molten copper. And the peak value and area of CuO in the XPS spectrum are smaller than that of Cu, which indicates a lower content of CuO than that of Cu. Namely a protective film of molten copper is partially oxidized.Generally speaking, the XPS experiment clarifies the chemical states of elements on the worn surface and illustrates the formation of protective film consisting of the long-chain hydrocarbons, the molten copper film, and the Fe2O3, FeO, and CuO involved tribochemical reaction film on the worn surface during the friction process, which protects the friction pair and improves the anti-wear ability and friction reducing performance of lithium grease.

Figure 5 Elements distribution on worn surface lubricated by grease with optimal addition of nano-coppers under a load of 200N

Figure 6 XPS spectra of C1s, Fe2p, Cu2p and O1s of the worn surface ((a)(b)(c)(d) lubricated by base grease;(e)(f)(g)(h)lubricated by base grease+1.0% of copper nanoparticles

3.5 Tribology mechanism

The tribological properties of lithium grease can be improved by addition of nano-coppers with different dimensions. Due to the action of frictional heat, the nanocoppers with different dimensions are melted and spread on the worn surface during the friction process, which effectively reduces the surface roughness and prevents the direct contact of friction pair. At the same time, as a typical soft metal, the molten copper film with low strength is easily damaged during the friction process and plays a sacrificial role, thereby effectively reducing the exfoliation of worn surface caused by adhesive wear and the “furrow” phenomenon caused by abrasive wear to significantly reduce the wear loss of the friction pair. At the same time as the destruction takes place, the molten copper film continues to form due to the presence of nano-copper in the grease. This dynamic balance between the destruction and formation of surface film can result in improvement of the tribological properties of grease for a long time.

The wear loss of friction pairs are different upon adding nano-coppers with different dimensions, which is caused by the difference of their structures. The micro-pits in the worn surface can be filled by copper nanoparticles with small size, which can reduce the surface roughness.Moreover, the copper nanoparticles can play a role of nano-bearing to change the friction pattern from sliding friction to rolling friction. Meanwhile, the molten copper film produced by the melting of copper nanoparticles can prevent the direct contact of friction pairs. Therefore, the wear loss of friction pairs can be decreased sharply with the addition of copper nanoparticles.

However, copper nanowires can only be coated on the surface of friction pairs by melting at high temperature produced by friction. And the wear loss can be reduced via sacrifice of the molten film. Therefore, the wear rate of the friction pair also shows a very obvious dependence of the amount of added coppers nanowires. Namely, only when the amount of copper nanowires added is sufficient,the wear loss of the friction pair can be significantly reduced. At the same time, due to the long length of the copper nanowires, they can be easily agglomerated when they are added excessively, which lead to increase the wear loss of the friction pair.

The wear loss of the friction pair can be sharply decreased by addition of copper nanoplates with a micron-scale diameter and lamellar structure, which can significantly prevent the direct contact of microprotrusions on the surface of friction pairs and can reduce the “welding” between micro-protrusions due to local high temperature caused by friction. However,the copper nanoplates with excessive addition will be agglomerated because of large size, which can increases the wear loss and will deteriorate the anti-wear ability of lithium grease.

Figure 7 The diagram of lubrication mechanism of nano-=coppers with different dimensions

4 Conclusions

The antiwear ability and friction-reducing performance of lithium grease can be significantly improved by the addition of nano-coppers with different dimensions.The nano-coppers with different dimensions are melted to form a molten film on the worn surface during the friction process, which can effectively reduce the surface roughness and prevents the direct contact of friction pair. At the same time, as a typical soft metal, the molten copper film with low strength is easily damaged which plays a sacrificial role to protect friction pair and can significantly reduce the wear loss. Moreover, the wear loss of friction pair is different upon adding nanocoppers with different dimensions, which is caused by the difference of their structures.