Conductive grease synthesized using nanometer ATO as an additive

A new conductive grease was synthesized using a nanometer powder, i.e., Sb doped SnO2 (ATO), as an additive. The typical properties of this new conductive grease were investigated in detail. The results indicate that ATO can dramatically improve the dropping point and reduce contact resistance. The tribological properties of the new conductive grease were investigated using the MFT-R4000 reciprocating friction and wear tester. The tribol-test results indicate that ATO can dramatically improve the tribological properties of the grease. When the ATO concentration is 0.1wt%, the grease demonstrates the best friction reduction properties; when the concentration is 0.5wt%, the grease demonstrates the best anti-wear properties. The worn surfaces were observed and analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy, and the friction mechanisms for the new conductive grease are proposed. The excellent tribological properties of the new conductive grease are attributed to the mechanical effect of ATO, and the film formed by Sn and Sb elements or metallic oxide deposited on worn surfaces during the friction process.


Introduction
When lubricating grease is applied to the conductive parts of electrical equipment, such as electrical switches, integrated circuits, microelectronic mechanical systems, power machines, power transmission and transformation equipment [1−3], it not only reduces friction and wear in the mechanical systems but also plays a significant role in saving energy and reducing CO 2 emission by improving the lubrication efficiency, reducing the contact resistance, and prolonging service life; thus, the conductivity capacity and tribological properties of the grease become particularly important [4]. The conventional method of preparing conductive greases is by adding conductive additives to the base grease. However, existing conductive additives have many problems: While possessing good electrical conductivity, precious metal powders such as gold, silver, and nickel are too expensive to be suitable for largescale employment; copper powder is cheap but is easily oxidized; carbon series materials have good electrical conductivity, but their color is too dark and hard to disperse.
In this paper, a conductive grease was synthesized using polyethylene oxide polypropylene oxide tinbutadiene styrene ether 50HB660 (PAG) as the base oil; polytetrafluoroethylene (PTFE) as the thickener; and Cu powder, Ag powder, and ATO as additives.

Materials and preparation
The base oil, PAG, used in this study was purchased commercially from Dow Chemical (China) Investment Company. The grain size of the PTFE micro-powder (Dyneon TM TF9207) was about 4 μm and the density was 2.2 g/cm 3 . The ethanol (Sinopharm) was analytical grade, and all the chemical reagents used were without further purification. The typical properties of Cu powder (Sinopharm), Ag powder (Sinopharm) and ATO (Beijing DK nano) are shown in Table 1, and the transmission electron microscope (TEM) image of ATO is shown in Fig. 1.
The lubricating greases were synthesized according to the following procedure. Firstly, the base oil (with a certain amount of the Cu, Ag, and ATO additives, initially dispersed in ethanol) was added to the corresponding vessel and stirred. Secondly, PTFE was slowly added and stirred vigorously. When the base oil and PTFE were uniformly mixed, a specific amount of the ethanol was injected and stirred for about 30 min to ensure that the PTFE was completely dispersed in the base oil. After that, the mixture was heated to about 90 °C and maintained at this temperature for about 30 min to remove the ethanol. Finally, the mixture was cooled to room temperature, and the PAG grease and the conductive greases (namely the PAG+additives grease) were obtained after three separate fine grinding/homogenization steps in a threeroller mill.

Characterization of the physical properties of the grease
The dropping point, the penetration, and the copper strip tests of the prepared lubricating greases were characterized under national standards GB/T 3498, GB/T 269, and GB/T 7326; the conductivity and volume resistivity were ascertained using a DDSJ-308A conductivity tester and a GEST-121 volume surface resistance tester, respectively. The initial and final contact resistances were measured by HLY-200A circuit resistance tester, and the testing current and time were 100 ampere and 10 seconds, respectively.

Friction and wear tests
A MFT-R4000 reciprocating friction and wear tester ( Fig. 2) was employed to investigate the tribological performance of the conductive greases with a ballon-disc configuration. Contact between the frictional pairs was achieved by pressing the upper ball (diameter 5 mm, AISI 52100 steel, hardness 710 Hv) against the lower stationary steel discs (ø 24 mm × 7.9 mm, AISI 52100 steel, hardness 340-350 Hv). The upper ball slid reciprocally at an amplitude of 5 mm against the stationary lower steel discs. All the tests were conducted at room temperature and each lasted for 30 min. All the disc specimens and steel balls were cleaned in petroleum ether using an ultrasonic cleaner for 10 min, before and after each tribol-test. Prior to the friction and wear test, about 1 g grease was introduced to the ball-disc contact area and the coefficient of friction (COF) was recorded automatically by a computer connected to the MFT-R4000 tester. The wear widths of the lower discs were determined using an optical microscope. Three repeated measurements were performed, and the average values with error bars are reported in this article. The morphologies of the worn surfaces were analyzed using a JSM-5600LV SEM (JEOL, Tokyo, Japan), and the elements in the tribofilm were investigated using EDS.

Physical properties of the greases
Photo images of the prepared lubricating greases are shown in Fig. 3. Table 2 lists the typical properties of the lubricating greases. The PAG grease has a relatively low dropping point (281 °C ) and all lubricating greases have good corrosion resistance (copper corrosion 1a). ATO can dramatically improve the dropping point of the grease as ATO nanoparticles have a relatively high specific surface area (SSA) [13]; this presents an obstacle to the movement of the liquid molecules, attributed to surface force (similar to intermolecular force). Thus, ATO nanoparticles have relatively high adsorption [14], responsible for the improvement in the dropping point of the grease. Table 3 lists the conductive capacity of the lubricating greases. The stabilizing coefficient is the ratio of final  contact resistance and initial contact resistance. The mechanism of ATO increasing the conductivity is in accordance with percolation theory [15−19]. The ATO conductive greases have lower contact resistance than other prepared greases. The ATO can reduce the contact resistance, thus can decrease heat generation by resistance, reduce energy consumption, and make the current more stable.

Tribological test results
To investigate the tribological properties of ATO, three influence factors (additive concentration, load, and frequency) were investigated in this paper. Figure 4 demonstrates the evolution of COF with time and wear widths of the prepared lubricating greases at room temperature at 20 N and 5 Hz. The ATO greases exhibit better friction-reducing behavior than other greases. The COFs show little change between 0.108 and 0.115 with an increase in ATO concentration; slightly increased COFs might result from the slight degradation of mechanical properties occurring in the PAG+ATO grease [20]. When the ATO concentration is 0.1wt%, it exhibits the lowest COF (about 0.108) of the prepared greases. Moreover, the COF of ATO grease is much more stable than PAG grease. The wear widths are dramatically reduced as the ATO concentration increases, and a 0.5wt% concentration of ATO can generally improve the anti-wear property of the PAG grease by about 30%. When the additive concentration is higher than 0.5wt%, the wear width of all greases increases substantially, resulting from the surplus particles acting as abrasive particles during the sliding friction process. Figure 5 shows the average COFs and average wear widths of PAG grease containing 0.1wt% ATO and 0.5wt% ATO at different loads, at 5 Hz at room temperature. It is shown that the average COFs of PAG+ATO greases (about 0.108-0.113) are much lower than those of other greases (all above 0.115), and rise with increases in the load, to some extent. It is shown that ATO is capable of superior friction-reducing behavior at all loads. The PAG+ATO greases have relatively narrower wear widths than other greases, and the wear width of PAG+0.5wt% ATO grease remains considerably less than with other greases under test conditions, indicating the best anti-wear capability.

Effect of frequency
The COFs and wear widths of the lubricating greases at various frequencies are shown in Fig. 6. The COFs of the PAG+ATO greases decrease with frequency increases, to some extent, and are lower than those of other greases at concentrations of both 0.1wt% and 0.5wt%. The COF can be reduced by nearly 10% (plus 0.5wt% ATO at 50 N and 5 Hz), thus ATO has better friction-reducing properties. The results of the wear widths clearly demonstrate that the PAG+0.5wt% ATO grease has far better anti-wear performance than other greases during the 30 min test time at whole frequencies.

Surface analysis
To study the lubrication mechanism, SEM and EDS were used to analyze the worn surfaces. Figure 7 displays the SEM morphologies of the worn surfaces on steel discs lubricated by the prepared greases at room temperature. It can be clearly seen that the worn    surface of the steel disc lubricated by PAG grease (Figs. 7(a)-7(c)) shows much wider and deeper wear scars, with some deep and narrow grooves demonstrating that severe scuffing occurred in this instance.
In marked contrast, the worn steel surfaces lubricated by PAG grease with 0.1wt%ATO and 0.5wt%ATO (Figs. 7(d)-7(i)) show much smoother and smaller wear scars; only a little adhesive wear could be observed. Scuffing is greatly alleviated, as proof of the excellent tribological properties of ATO additives. Moreover, the worn surface of 0.5wt% is even smoother than that of 0.1wt% ATO, in agreement with wear width results. The results are also consistent with the aforementioned tribol-test data, again proving that ATO additives demonstrate better tribological performance.
EDS analysis was employed to clarify further the chemical component of several typical elements on the worn surfaces. Figure 8 gives the EDS of the typical elements on the worn surfaces lubricated by the PAG grease and PAG+0.5wt% ATO grease at 50 N and 5 Hz.
The low peaks of Sn and Sb elements [21,22] shown in Fig. 8(b) are attributed to the low ATO concentration in grease (only 0.5wt%). However, the element concentration confirms the existence of Sn and Sb at 0.79wt% and 0.43wt% on the worn surfaces, respectively. It is concluded that the Sn and Sb deposited metallic oxide during the sliding process, and the protecting film significantly contributed to the friction-reducing and anti-wear properties of the lubricating grease [23]. Thus, the ATO additives exhibited excellent tribological properties.
Schematic diagrams of friction mechanisms and the distribution of ATO particles in the grease are shown in Fig. 9. The improvement of the tribological properties of ATO can be explained by the following three aspects. First, with the lubricating greases stably and uniformly located between the two contact surfaces, the nanoparticles (ATO) served as spacers during sliding, preventing direct contact between them [24]. Second, the nanoparticles also had a rolling effect that changed sliding friction into rolling friction and had a protective effect [25]. Third, due to their small size, nanoparticles may serve as nuclear centers for developing continuous polymer regions, then connect with each other to form a tightly-structured network; and this may contribute to the improved anti-wear ability of the grease [26]. Moreover, Sn and Sb ions were deposited as a film on the protecting surface during the sliding process, serving to enhance it. Therefore, the COFs and wear widths were stable and narrow at room temperature, and the worn surfaces lubricated by ATO grease were relatively smooth.
Conductive grease containing ATO can be used in power transmission and transformation equipment such as cable clips for transmission lines (Fig. 10).

Conclusions
Nanometer ATO, a new kind of conductive additive, possesses a superior conductive capacity and a lower contact resistance in grease. It can be used in the conductive parts of electrical equipment. ATO conductive grease shows better friction-reducing and anti-wear properties. EDS analysis indicates that the Sn and Sb form a film or metallic oxide deposit on worn surfaces during the sliding process. The protecting film significantly contributes to the friction-reducing and anti-wear properties of the ATO conductive greases.