Current carrying tribological properties of multi arc ion plated titanium nitride doped silver coating

Sliding electrical contact materials play a crucial role in the transmission and conversion of electrical energy, but due to various factors such as force, electricity, and heat, the interface exhibits complex wear behavior. A single solid or liquid lubrication system can no longer meet the growing performance requirements of current carrying tribology. In this study, a TiN-Ag coating was prepared using multi arc ion plating technology, and a solid–liquid composite lubrication system was formed with ionic liquid and polyurea grease, respectively. Through current carrying friction and wear tests, their tribological properties, electrical contact resistance(ECR) values, and stability were tested, and compared with the results obtained during dry friction. The coating and worn surfaces were analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results indicated that compared with dry friction, TiN-Ag coatings lubricated with ionic liquids and polyurea grease showed higher friction reduction, wear resistance, and conductivity, especially the synergistic effect between ionic liquids and coatings is prominent. The behavior of ionic liquids under voltage was analyzed, and it was found that ionic liquids formed a physical adsorption film composed of a mixture of anions and cations on the worn surface. The ordered layered structure improved the tribological performance of the system.


Introduction
Sliding electrical contacts are widely found in the fields of electrified railways, high-speed multiple units, rocket launchers, industrial generators, and high-end transmission equipment, such as slip rings, electronic knobs, relays, and catenary wires [1,2].Electric contacts can transmit energy and signals between fixed and moving components, and their performance largely determines the reliability, stability, accuracy, and lifespan of the entire equipment and system [3,4].
The sliding electrical contact interface is constrained by both mechanical and electrical factors, and is influenced by multiple factors such as force, electricity, and heat [5].Therefore, the interface will exhibit complex wear behavior, which requires electrical contact materials to have excellent friction and wear resistance, low contact resistance, and good corrosion resistance.Research has shown that preparing appropriate coatings on the surface of metal materials can significantly improve the electrical contact performance of workpieces, such as carbon based coatings, nitride based coatings, oxide based coatings, etc [6][7][8][9].Among them, titanium nitride (TiN) coatings are widely used, with advantages such as high hardness, good toughness and chemical stability, as well as strong adhesion to the substrate [10][11][12].However, their conductivity and wear resistance and friction reduction performance are not ideal, and further improvement is needed.Researchers have found that by doping reinforcing elements into the coating, the comprehensive protective performance of the coating can be greatly improved [13,14].Metal/non-metal doping is a strategy that has good application effects in various materials, and can improve the performance of materials from different aspects [15][16][17][18].Metal elements are often doped in coatings to improve their properties.For example, Some scholars have studied Ti, Cu, and Mo doped DLC coatings, and found that appropriate element doping can improve the mechanical and tribological properties of DLC coatings [19][20][21].Al, Cr, and Zr were also doped into TiN coatings and achieved good expectations [22][23][24].Silver, which has excellent conductivity and lubrication properties, is often doped in electrical contact materials.Wang et al prepared silver coatings, silver graphite composite coatings, and silver graphene composite coatings on copper surfaces using electrodeposition methods.The test results showed that the Ag graphite composite coating had the lowest friction coefficient and relatively low contact resistance [25].Aida M et al found that the Ag TaN composite coating provides the best balance between mechanical properties, electrochemical activity, and silver release [26].
It is confirmed by some scholars that using suitable conductive lubricants in sliding electrical contacts can improve the friction, wear, and electrical properties of electrical contacts [27,28].Solid-liquid composite lubrication systems composed of coatings and lubricants can more effectively combine their advantages.The solid-liquid composite lubrication system can simultaneously leverage the advantages of a wide temperature range for solid lubricants, high load-bearing capacity of liquid lubricants, and long service life, while reducing the drawbacks of each component to a certain extent, thereby generating synergistic effects.Xia Yanqiu's team studied the synergistic effect of various coatings and various lubricants, and found that the tribochemical effect of coatings and lubricating grease, as well as the thermal effect generated by the current excitation at the contact, promoted the formation of the facial mask on the friction surface, thus improving the friction reducing and antiwear ability of the friction pair and reducing the contact resistance [29][30][31][32].
Ionic liquids (ILs) are excellent lubricants with higher decomposition temperatures and lower kinematic viscosity, making them more suitable for applications in various complex working conditions.Some scholars have introduced ILs into solid-liquid composite lubrication systems, but most of them are combined with DLC coatings [33].Jiang et al studied the effect of ionic liquid lubricants on the tribological properties of DLC coatings using molecular dynamics models.The results showed that ionic liquid films can prevent the formation of C-C bonds between sliding interfaces, thereby improving the tribological properties [34].Arshad et al studied the interaction between tungsten doped diamond like carbon (W-DLC) thin films and ILs lubricants under boundary lubrication conditions.The tribological test results showed that all three ILs achieved low friction (with a friction coefficient of 0.024) under 10 N and 100 °C conditions [35].The application of lubricating grease in solid-liquid composite lubrication systems is relatively limited, but research results show that the appropriate use of lubricating grease on the coating surface can also obtain friction surfaces with good friction reduction and wear resistance.Polyurea grease is a commonly used lubricating grease that is recognized for its high temperature resistance, wide range of temperature changes, long service life, and excellent extreme pressure and wear resistance.It is necessary to study its synergistic lubrication effect with coatings.
Based on the above considerations, this paper prepared TiN-Ag coatings with good mechanical properties on copper substrates using multi arc ion plating technology.The current carrying friction and wear properties and electrical properties of the coatings under dry friction, ILs lubrication and Polyurea grease lubrication were studied and compared.The worn surfaces were characterized, and the influence of current magnitude on synergistic lubrication effect was analyzed to explore the lubrication mechanism.

Experimental details 2.1. Lubrication agents
The ILs this article used is 1-octyl-3-methylimidazole bis (trifluoromethanesulfonyl) imine salt (OMImNTf2) synthesized by the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.Its physicochemical properties are shown in table 1. Poly α Olefin (PAO 40) purchased from ExxonMobil Corporation is used as the base oil to prepare polyurea lubricating grease according to the literature method.The drop point and cone penetration of lubricating grease were characterized according to national standards GB/T 3498 and GB/T 269, respectively.The corrosion resistance was tested according to GB/T 7326-87, and the volume surface resistance was measured using a volume surface resistance tester produced by Beijing Guance Precision Electrical Equipment Co., Ltd.The basic physical and chemical properties of polyurea grease (PAO) are shown in table 2. Coating deposition substrate with a size of 30 mm × 30 mm × 3 mm pure copper block with a purity of 99.9% and a hardness of 82 HV.The target materials are pure titanium targets and pure silver targets (purity 99.9%, diameter 8 cm) produced by Liaoning Beiyu Vacuum Technology Co., Ltd.

Deposition of the coating
Figure 2(a) displays the multifunctional vacuum coating equipment produced by Liaoning Beiyu Vacuum Technology Co., Ltd, which was employed to prepare the coat on pure copper substrates using the DG-2-ZY arc ion plating system.Before deposition, first polish the substrate surface with sandpaper and diamond polishing agent until the roughness is less than 0.05 μ m.Subsequently, the polished substrates were ultrasonically cleaned with alcohol and acetone (99.9%) for 10 min each, and then the substrates were removed and air dried for later use.As shown in figure 2(b), in the vacuum chamber of the coating equipment, the substrate is clamped on a specially made stainless steel trestle, and the titanium target and silver target are vertically arranged on the cavity wall with an interval of 10 cm, always maintaining a horizontal distance of 20 cm between the substrate and the target material.
Successively use mechanical pumps and molecular pumps to vacuum the cavity to 10 −3 Pa, and then argon gas (99.999%) is introduced to increase the vacuum to 0.3 Pa.Next, heat the substrate to stabilize it at 150 °C, set the pulse bias voltage to −800 V, and the duty cycle to 20%.Clean the substrate with Ar ions for 10 min.Then raise the substrate temperature to 300 °C, adjust the pulse bias voltage and duty cycle to 200 V and 40%, respectively.Finally, control the titanium target power to 1600 W and the silver target power to 1200 W, and prepare a titanium nitride silver doped coating TiN-Ag with a deposition time of 40 min.

Characterization and tribological test of the coating
The current carrying tribology test was carried out using the MFT-R4000 improved current carrying friction and wear tester produced by the State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.The friction pair of the testing machine adopted a ball-disc contact mode, and the upper sample was a pure copper ball with a diameter of 5 mm, which was clamped and fixed.The lower specimen was a copper block with deposited TiN-Ag coating, driven by a crank slider mechanism for reciprocating motion.The reciprocating stroke was 5 mm, the reciprocating frequency is 2 Hz, and the test temperature is room temperature.To ensure the stability of the load, the experiment adopts weight loading with a test load of 5 N.The test voltage was provided by a regulated power supply, set to 0.5 V, 1 V, and 1.5 V respectively.To eliminate the randomness of test results, each test condition was repeated three times, with each test lasting 30 min.Before and after each test, the copper balls and specimens were ultrasonically cleaned with petroleum ether for 10 min.The testing machine automatically recorded and saved one real-time friction coefficient and real-time current every second.During the test, the voltage kept constant, so the real-time contact resistance could be calculated using Ohm's law.The stability of contact resistance and friction behavior has a significant impact on the performance and service life of electrical contacts.Therefore, standard deviation (SD) is introduced to evaluate the stability of contact resistance and friction behavior.Equation (1) was used to calculate the standard deviation of contact resistance and friction coefficient.The smaller the standard deviation, the more stable it is.
where x i is the instantaneous value and n is the number of the instantaneous values.After friction test, the coatings were cleaned ultrasonically in the acetone for 10 min to remove the surface pollutants, and then measure the width of the wear scar using an optical microscope.

Surface analysis
The surface morphology and chemical composition of the coating and the wear scars were characterized by a scanning electron microscopy (SEM, EVO-18, Zeiss) and energy-dispersive x-ray spectroscopy (EDX, Bruker, Germany).

Results and discussion
3.1.Analysis of the coating Figure 3 (a) and (b) depict the SEM morphology of the surface and cross-section of the TiN-Ag coating.Observing the image of the surface, it can be seen that the surface of the coating is flat and smooth.The scattered small droplets on the surface of the coating are a normal phenomenon during the multi arc ion plating process [36,37].From the cross-sectional view, which could determine that the thickness of the coating is ∼14.29 μm, it can be observed that there are some porous structures in the cross-section of the coating, but the overall structure is relatively compact and tightly bonded to the substrate, indicating good coating quality.Figure 3 (c °, and 75.94 °.Therefore, it can be inferred that two phases, TiN and elemental Ag, were generated during the coating deposition process, indicating the successful preparation of the coating.

Tribological and conductive properties
Figure 4 gives the current carrying friction test results under three different conditions at a voltage of 0.5 V. Figure 4 (a) and (b) show the curves and the corresponding SD values of the friction coefficient and contact resistance over time.It can be seen that the friction coefficients under the lubrication of ionic liquids and polyurea grease are significantly lower than that under dry friction, and the curves are much smoother as the corresponding SD values are much smaller than that under dry friction, indicating that the solid-liquid composite lubrication system has excellent friction reduction performance.On the other hand, the contact resistance values under ionic liquid lubrication are slightly higher than those under polyurea grease lubrication, but they are all lower than dry friction, suggesting that the solid-liquid composite lubrication system can improve the conductivity of the material.From figure 4 (c), it can be observed that the width of wear marks under the lubrication of ionic liquids and polyurea grease is less than 25% of that under dry friction, indicating that the anti wear ability of the solid-liquid composite lubrication system is also strong.
When the voltage increases to 1 V, the results in figure 5 (a) indicate that the friction coefficient and contact resistance values have varying degrees of increase in all three cases, with a smaller change in friction coefficient and a larger change in contact resistance value, which may be caused by an increase in current density leading to an increase in temperature in the electrical contact area.However, the solid-liquid composite lubrication system still exhibits excellent ability to reduce friction coefficient and contact resistance values, and its numerical performance is more stable (figure 5 (b)), especially with the advantage of ionic liquid lubrication.However, from figure 5 (c), it can be seen that the width of wear marks in the two solid-liquid composite lubrication systems shows different trends.The wear width significantly increases when lubricated with PAO, but decreases obviously when lubricated with ionic liquids, reflecting the specialty of lubricating oil at elevated temperatures.As shown in figure 6 (a), when the voltage is 1.5 V, the COF values under the three conditions remain basically unchanged.The ECR values under dry friction and PAO lubrication decrease to close, while there is no significant fluctuation under ionic liquid lubrication.The data in figure 6 (b) indicates that the COF and ECR of the solid-liquid composite lubrication system are the most stable at this time, but the wear width has increased (figure 6 (c)).These trends that vary with voltage can be more clearly distinguished from figures 7 (a)-(c).The results show that the solid-liquid composite lubrication system has certain changes in anti wear, friction reduction, and conductivity under different voltages, but still improves the performance of the electrical contact pair.In current carrying friction, the resistance heat and arc generated by the current passing through the friction pair have a significant impact on its friction and wear characteristics.When dry friction occurs, as the   voltage increases, the current density through the friction pair increases.Under the coupling effect of electric heating and frictional heat, the micro convex bodies on the coating surface are more prone to damage, resulting in an increase in the wear width of the coating.When lubricated with polyurea grease, due to the thicker lubricating film and higher viscosity, the friction pair is more difficult to convection and dissipate heat during sliding, and the trend of increasing wear width with increasing voltage is more obvious.Ionic liquid lubrication mainly relies on the action of electric double-layer force to form an ordered double-layer structure.When the voltage is relatively small (0.5 V), the electric double-layer force is not large enough, and the double-layer structure is not formed.The wear width of the coating is similar to that of polyurea grease.When the voltage increases to 1 V, an ordered layered structure forms and firmly adheres to the energized surface, resulting in a obvious decrease in wear width.However, as the voltage continues to increase to 1.5 V, the electrostatic force of polar groups in the ionic liquid and the van der Waals force generated by alkyl chains also increase, to the point where the ordered structure of ions is disrupted, resulting in a significant increase in the wear width of the coating.

Analysis of the worn surface
It is easy to know that the solid-liquid composite lubrication system composed of TiN-Ag coating and ionic liquid exhibits better anti wear, friction reduction, and conductivity compared to the results of current carrying tribological tests.To further investigate the mechanism of action of the coating and ionic liquid solid-liquid composite lubrication system, the wear marks of this combination at 0.5 V and 1.5 V were selected for morphology and elemental analysis.
Figure 8 shows the overall view and local details of the wear marks.From figure 8 (a) (b), it can be seen that when the voltage is 0.5 V, the width of the wear marks is small, and the surface of the wear marks is relatively smooth, with only slight adhesion, shallow grooves, and small pits.When the voltage is 1.5 V (figure 8 (c) (d)), the wear marks widen, with deep pear grooves in the wear marks, and the coating shows obvious tearing and peeling, indicating severe wear on the material surface.The results indicate that Joule heat, an important factor in the current carrying friction process, closely affects the performance of electrical contact materials.
As shown in figures 9 (a), (b), from the EDS analysis results, it can be seen that there are no F, S, or O elements present on the worn surface, and a small amount of copper element should come from the wear of copper balls.Therefore, it can be inferred that the solid-liquid composite lubrication system composed of TiN-Ag coating and ionic liquid mainly functions by producing a physical adsorption film, without significant chemical reactions.This may be attributed to the relatively low silver content in this coating, high hardness and dense structure, effectively resisting the corrosion effect of ionic liquids [38,39].Ionic liquids with special molecular structures adsorb on the surface of the friction pair, forming a uniform and dense effective lubricating film on the surface of the coating, which plays an anti wear and friction reducing role [40,41].According to literature reports, as the schematic diagram provided in figure 10, the interaction between ionic liquids and contact surfaces is greatly affected by the applied current.Ionic liquid molecules transition from random arrangement to ordered arrangement, forming an interfacial adsorption film composed of a mixture of anions and cations [42][43][44].The ordered layer structure is believed to firmly adhere to the energized surface, with higher loadbearing capacity and thereby improving the wear resistance and friction reduction of the contact.
Another factor to consider is the frictional chemical reaction under the action of current carrying friction.From figure 7, it can be seen that the wear scar width of the coating under ionic liquid lubrication is 50 μm to 500  μm.Due to the small number of friction products on small-scale wear marks, it is very difficult to determine the friction products.Therefore, element F and element S cannot be detected on the worn surface.Nevertheless, this study acknowledges the role of frictional chemical products, and the combined effects of physical adsorption and frictional chemical products on tribological properties are more reasonable [45,46].

Conclusions
By studying the current carrying tribological properties of TiN-Ag coatings in solid-liquid composite lubrication systems composed of ionic liquids (OMIMNF2) and polyurea lubricants, the following conclusions were drawn: (1) Solid-liquid composite lubrication systems can significantly improve the friction reduction, wear resistance, and conductivity of electrical contact materials.
(2) At different voltages, due to changes in Joule heat, the indices of the two solid-liquid composite lubrication systems are different, but they can still improve the current carrying tribological properties of the electrical contact pair.
(3) The better synergistic effect between OMIMNF2 and the coating is mainly attributed to the ordered layered films formed by anions and cations in the ionic liquid on the worn surface, which firmly adhere to the electrified surface, improving the frictional and electrical properties of the friction pair.

Figure 1 (
a) gives the structural composition of ILs and figure 1(b) is the molecular structure formula of OMImNTf2.

Figure 2 .
Figure 2. (a) The vacuum coating equipment (b) Sketch of the vacuum chamber.

Figure 4 .
Figure 4. (a) COF and ECR as a function of time (b) SD values of COF and ECR (c) wear widths of the wear scars under a voltage of 0.5 V.

Figure 5 .
Figure 5. (a) COF and ECR as a function of time (b) SD values of COF and ECR (c) wear widths of the wear scars under a voltage of 1 V.

Figure 6 .
Figure 6.(a) COF and ECR as a function of time (b) SD values of COF and ECR (c) wear widths of the wear scars under a voltage of 1.5 V.

Figure 7 .
Figure 7. (a) Average COF (b) Average ECR (c) wear widths of the wear scars under different voltage.

Figure 8 .
Figure 8. SEM morphologies of the worn surfaces under a voltage of (a) and (b) 0.5 V, (c) and (d) 1.5 V lubricated by ILs.

Figure 9 .
Figure 9. EDX of the worn surfaces under a voltage of (a) 0.5 V, (b) 1.5 V lubricated by ILs.

Figure 10 .
Figure 10.Schematic diagram of wear mechanism of the coatings lubricated by ILs.

Table 1 .
Main properties of ionic liquid.

Table 2 .
Basic physical and chemical properties of PAO.