A Study on the Tribological Behaviors of a Pin Coated with Layer-by-Layer Gold/Nickel Materials within an Electrical Connector

Abstract

An electrical connector is an important component for achieving the interconnection of electric equipment. However, the degradation of contacting parts within the electrical connector under repetitive mechanical insertion and extraction operations causes a decrease in the contact reliability level, resulting in considerable safety hazards. The coating quality, determining the degree of degradation of contact pairs, is considered a critical factor in fabricating more reliable and safer electrical connectors. In this paper, a gold and nickel coating is deposited onto the surface of a pin within an electrical connector using magnetron sputtering and is compared to an electroplated pin, and the effects of different processing techniques on the microstructure, mechanical properties, and wear behavior are systematically investigated. The measurement results indicate that the surface quality (uniformity and defect density) and mechanical properties (hardness and elastic modulus) of the gold/nickel coating based on magnetron sputtering are significantly better than those achieved using electroplating, showing excellent wear properties and electrical contact stability after repetitive insertion–extraction operations. This study is critical for the development of advanced coatings using a novel deposition technique.

1. Introduction

Stable and low-contact resistance in the electrical contact field for electrical connectors is a critical issue for ensuring satisfactory electrical properties, thanks to its application in electric vehicles, communication equipment, and weapon systems [1]. The function of electrical connection and disconnection during mechanical insertion and extraction operations for electrical connectors is mainly determined through a large number of contacts integrated into the internal insulation. An unsuitable insertion/extraction force could lead to many failures, including, but not limited to, loose contact and instantaneous, even permanent, disconnection [2,3].

Electrical contacts are usually treated with various coatings to protect the copper alloy substrate from severe corrosion and wear, thereby maintaining stable insertion and extraction operations. Song et al. [4] discussed the effect of coating quality on electrical contact behaviors after repetitive insertion/extraction operations. As the degradation degree of a coating increases, the contact pair not only increases the coefficient of friction but also gives rise to a reduction in the stability of the electrical contact. Kolmer et al. [5] summarized various characterization methods to evaluate the degradation behavior of coatings within electrical connectors, thereby determining their failure mechanisms. Therefore, coating quality plays quite an important role in maintaining the stability of the electrical contact between contact pairs within an electrical connector. Among all of the coating materials, harmless gold and nickel [6], which act as protective coatings, are widely applied in various scenarios due to their superior surface properties like low resistivity, excellent anti-corrosion quality, and low propensity to form alien film on a contact’s surface. Several typical strategies for fabricating a gold/nickel coating, mainly including electroplating and magnetron sputtering, have universally attracted attention, and substantial research has been carried out [7,8].

Ren et al. [9] studied the degradation mechanism of contact pairs prepared via electroplating in electrical connectors under repetitive insertion and extraction operations in detail and demonstrated the importance of coating quality. Feng et al. [10] described the change in the law of force of an electroplated coating under repetitive insertion–extraction operations and found that a decrease in the maximum insertion force not only slows down the rushing phenomenon in the insertion process but can also reduce the wear rate of contacts under vibration conditions. It should be noted that the coating of electrical contacts generates severe wear due to the movement of the repeated insertion and extraction of electrical connectors. Once the protective coatings have been detached, the electrical contacts will fail. However, the realistic difficulty in the agglomeration of particles in plating electrolytes for electrodeposition to achieve a uniform coating, which increases the probability of coating defects, leading to mechanical failure during the deformation process, should be considered. Magnetron sputtering may be an alternative technique for the deposition of various coatings thanks to its high deposition rate and low contamination of deposits [11,12].

A Au/Ni-C coating was deposited onto a substrate through magnetron co-sputtering by Chen et al. [13], and it exhibited better hardness (400 HV) than a pure gold coating. Coutu et al. [14] prepared different gold-based contacts employing magnetron sputtering, and they exhibited various micro-contact models. Yang et al. [15] proposed the effect of the microstructure and mechanical properties of a Au/Ni coating on switch lifetime; that is, outstanding mechanical properties are conducive to the stability and service life of electrical contacts. However, an investigation of the effect of coatings prepared using different fabrication techniques (electroplating and magnetron sputtering) on the degradation law of dynamic waveforms of contact resistance and contact force and their correspondence is yet to be conducted. Consequently, the investigation of high-quality coatings using various fabrication techniques plays an important role in guaranteeing the lifetime and reliability of electrical connectors.

In this paper, a pin coated layer-by-layer with Au/Ni within an electrical connector is prepared using the magnetron sputtering technique and is compared to a commercial pin fabricated via electroplating according to their microstructure and mechanical properties. From this, the difference in the insertion and extraction performance of the electrical contacts is evaluated to determine the effect of coating quality on wear resistance, and the different wear mechanisms of the coatings are assessed to demonstrate variations in the insertion and extraction forces and contact resistance.

2. Experiment

2.1. Deposition of Au/Ni Layer-by-Layer Coating

Before the use of magnetron sputtering to prepare the coating, the vacuum chamber of the sputtering equipment needed to be cleaned. The main parts that required cleaning were the vacuum chamber wall, sample grips, and baffles. Absorbent cotton was dipped in absolute ethanol for wiping and drying in combination with blowing. After the above work was completed, the coating operation was carried out according to the following steps: (1) Sample glow cleaning: The pretreated pin specimen was installed in the middle of the stainless steel tube, and the specimen was glow-cleaned in the vacuum chamber for 5 min to further clean the surface of the substrate, increase the surface activity, and improve the adhesion between the substrate and the coating. (2) Sample installation: After glow cleaning, the sample was installed on the motor clamping device to ensure that the sample was coated evenly during sputtering. (3) Vacuuming: The air pressure of the sputtering chamber was first set to below 10 Pa; then, the forestage valve was opened to turn on the molecular pump, and the pump vacuum was set to below 1 × 10⁻⁵ Pa. (4) Substrate heating: The heating power supply was turned on, and the sample was heated to a predetermined temperature. (5) Adjusting the experimental parameters: The bias voltage, sputtering power, working air pressure, and other parameters were adjusted to the experimental values. (6) Start sputtering: The target baffle was closed and pre-sputtering was conducted for 2 min to remove impurities on the surface of the target. Then, the target baffle was opened to start sputtering, and the experimental information, such as time, was recorded. (7) End sputtering, sampling: After sputtering to the specified time, the equipment was turned off, and the sample was removed after cooling for more than 3 h; (8) Sample encapsulation: After removing the sample, it was placed in a plastic bag and vacuum-sealed, so as to prepare it for the various performance tests and analysis.

The pin was ultrasonically cleaned using acetone and alcohol solvents for 5 min, respectively, and then the pins that were mounted on the substrate base in the sputtering chamber were continuously cleaned in glowing discharge plasma to increase the surface activity. Subsequently, the Ni coating was deposited onto the substrate surface with a gas pressure, time, temperature, and power of 1.4 Pa, 20 min, 50 °C, and 500 W, respectively. Then, the Au coating was sputtered onto the Ni surface to form a layer-by-layer microstructure, corresponding to a gas pressure, time, temperature, and power of 1.4 Pa, 15 min, 50 °C, and 200 W, respectively.

2.2. Characterization Techniques

The surface morphologies of the pins were examined using a confocal microscope (CM) and scanning electron microscopy (Merlin Compact, SEM, Zeiss, Oberkochen, Germany). The compositions and surface chemicals were detected using an SEM-based energy-dispersive spectrometer (EDS) with an X-MAX detector (Oxford Instruments, Oxford, UK). Nano-indentation test measurements were made in compliance with the European Standard UNIEN 1071-3-2005 (Plzen, Czech Republic) using a CETR UMT-2 tester.

2.2.1. Thickness Test

The thickness of the sample was tested using the Nantong Fischer Instrumentation Ltd. (Shanghai, China) XDL240 X-ray fluorescence thickness gauge available in the laboratory. For each pin, five test points were taken evenly from the tail of the contact area of the pin towards the head of the pin. The thickness of the coating at different locations of the pins was tested to evaluate the uniformity of the coating growth at each location during the coating process. In addition, a plating control group with the same substrate pin was set up to evaluate the data.

2.2.2. Corrosion Resistance

To determine the corrosion resistance, the pickling method was used to corrode the plating, and then the test results were obtained via pouring inspection [16,17]. Since only nickel in the copper–nickel–gold three-layer coating reacts with hydrochloric acid at room temperature, the nickel layer was dissolved by pickling so that the chromogenic reagent came into contact with the copper substrate to form corrosion spots; finally, the porosity was calculated by resolving the spot diameter under an optical microscope. Spots with a diameter of less than 25 μm after corrosion were not included in the statistical range, and the number of pores was calculated by rounding the square of the diameter ratio for spots with an approximate diameter of more than 25 μm.

2.2.3. Thermal Stability

A qualitative test of the bonding strength of the two coatings was carried out using the thermal shock test method due to the limitation of their shapes. The plated specimen was soaked in glycerol (analytically pure) to minimize oxidation during heating. The sample was heated to 250 °C in an electric blast drying oven for 15 min and then taken out and cooled in room-temperature water immediately after heat preservation. Under an optical microscope, the coating was considered qualified if there was no bubbling, flaking, peeling, etc. After the thermal shock test was completed using the above method, an optical microscope was used to record whether any coating had peeled off or there were bubbles on the surface of the sample. For the specimens with no obvious failure on the surface of the coating after the thermal shock test, the bonding strength was considered to be qualified.

2.2.4. Insertion–Extraction Tests

The mechanical insertion–extraction characteristic test device used in this paper was composed of three parts: mechanical, measurement and control, and software. The mechanical part was composed of the following main components: a piezoelectric ceramic motor with high step resolution for driving the linear motion of the contacts, a two-dimensional slide table for adjusting the position of the motor, and various connectors for fixing and clamping the contacts. The measurement and control part was composed of the following components: millinewton force sensor and its supporting instrument for testing contact pressure; an AC micro-resistance tester for testing the contact resistance between contacts; a software part, using Visual Studio software (Visual Studio 2013) to write a WinForm program, through the form of a serial port and connected to the main measurement and control components for the setting of the experimental parameters before the test, the real-time display of data reading during the test, and the saving of the experimental results after the test. A schematic diagram of the structure of the mechanical insertion–extraction characteristic test device is shown in Figure 1.

3. Results and Discussion

The commercial pins fabricated via electroplating are denoted as PED, and the pins prepared using magnetron sputtering are denoted as PMS.

3.1. Surface Quality and Morphology

3.1.1. Macro Surface Quality

The thickness test results are shown in

Table 1. Thickness test results.

Test Points	1	2	3	4	5	AVG	Maximum Deviation
PED-Au	0.54	0.67	0.96	1.17	1.31	0.93	0.77
PED-Ni	0.48	0.64	0.78	0.83	1.03	0.752	0.55
PED	1.0	1.3	1.7	2.0	2.3	1.66	1.3
PMS-Au	1.12	1.14	1.12	1.11	1.13	1.124	0.03
PMS-Ni	0.85	0.86	1.02	1.03	1.06	0.964	0.21
PMS	2.0	2.0	2.1	2.1	2.2	2.08	0.2

.

From the corrosion resistance experimental results, it can be seen that the magnetron sputtering gold plating process can control the porosity of the coating and improve the corrosion resistance under the condition that the thickness of the coating is not much different. From the thermal stability experimental results, it can be seen that the thermal stability of the magnetron sputtering coating process is not significantly different from that of the electroplating coating in terms of the thickness of the coating.

The surface features of three parts (top-ending, middle, and bottom-ending) of 10 randomly sampled specimens prepared using magnetron sputtering were characterized using an optical microscope and were compared to the commercial pin fabricated via electroplating. As shown in Figure 2, the difference between both coatings is illustrated by the surface quality of the localized region. The PMS surface presents smooth characteristics without any distinct defects (Figure 2a–c,g), while some structural defects (pores and microcracks) are scattered on the surface of the PED (Figure 2d–f,h). Moreover, the structural integrity and continuity of the PMS are obviously better than that of the PED, which is ascribed to the better capacity of magnetron sputtering to control the defect density. The uniform thickness of the coating is also an important indicator that can be used to evaluate the coating quality, which is measured by randomly selecting ten points along the direction of the axial pin with a coating thickness gauge. In comparison, the PED (1 µm~2.3 µm) exhibits a wider range than that between 2.0 µm and 2.2 µm of the PMS, which further demonstrates that magnetron sputtering provides a continuous and uniform coating.

3.1.2. Microscopic Surface Quality

A representative SEM micrograph of the surface morphology of both coatings is depicted in Figure 3. The coating of the PMS, composed of the accumulation of granular-sized grains, shows a dense structure free of pores (Figure 3a–c), while large numbers of structural defects (microcracks, holes, and black spots) are scattered on the surface of the PED, which further indicates that the porosity of the PED is more serious than for the PMS. The EDS mapping images (Figure 3d–l) reveal that gold is uniformly distributed in the coatings, and a small amount of O and C is also distributed. Figure 4 shows the cross-sectional morphology of the Au-Ni composition coating of the PMS and PED, which have similar coating thicknesses of 2.3 µm. A clear edge between the coating and the substrate can be observed in Figure 4a, and there are no distinct cracks or peeling visible on the interface, demonstrating good adhesion force. The EDS results of the cross-sectional image of the coating show that the coating predominantly consisting of gold and carbon (C) is uniformly distributed across the entire surface, with a small amount of nitrogen (Ni), copper (Cu), and oxide (O), which further verify the formation of the composition coating on the substrate. Compared with the electroplating coating technique used in pin protection, the advantages of the magnetron sputtering method are the good surface quality and microstructure, as shown in Figure 2 and Figure 3, and good localized uniformity in the thickness of the coating, as shown in Figure 4 and Table 1.

3.2. Mechanical Properties of the Coating

3.2.1. Hardness and Elastic Modulus

The coating hardness can be evaluated using Equation (1).

$$H=\frac{P_{\max }}{S}$$

(1)

P_max—maximum load during the unloading process.

S—project area between the indenter and specimen at the action of load P_max.

We used the nano-indentation method to investigate the mechanical properties. The dependence of the depth on the load of the PED and PMS is depicted in Figure 5. The intensity changes in the maximum load F_m and residue penetration h_p can be clearly observed. The maximum hardness (H) and elastic modulus (E) of the PED are 0.986 GPa and 89.081 GPa, respectively, while the maximum H and E of the PMS reach 2.142 GPa and 100.223 GPa, respectively. To obtain further insight into the accurate values of H and E, four to five indentation tests on the surface of both coatings were performed to calculate the mean hardness and elastic modulus. The mean hardness and elastic modulus of the PED and PMS were 0.864 GPa, 104.644 Gpa, 1.278 GPa, and 64.028 GPa, respectively.

3.2.2. Analysis of Mechanical Properties

The improvement in H and E could be attributed to the combined effect of the high crystalline quality, the fine surface structure, and the low defect density. The ratio of H/E could represent the elastic–plastic property of coating materials, which represents load-carrying capacity and plastic deformation resistance [18]. The H/E of the PMS (~0.02) is higher than the PED (0.0083), indicating that the PMS has a larger load-carrying capacity and plastic deformation resistance. Thus, the mechanical properties of the PMS far exceed those of the PED.

3.3. Evaluation of Insertion–Extraction Behavior

The probability of contacting failure caused by the adaptability of the mechanical environment (including, but not limited to, mechanical vibration, impact, and constant acceleration test) in each batch of electrical connectors is largely determined by the coating quality and insertion and extraction features (degree of force and wear). To investigate the degradation process of contact pairs with different coatings, a repeating mechanical insertion and extraction experiment (500 times) involving 20 groups of randomly sampled pins was performed in the designed test rig. The displacement–force curve of the PED and PMS shows step–square–wave variation (Figure 6a,b). When contact pairs start to move toward each other, a contact force exists in the contact region. Then, the pin moves toward the mated contact, and there is an increase in the insertion force with the increasing depth of the plug into the mated contact [9]. The force decreases until the pin tip is completely inserted when passing through the maximum peak, followed by trending to a stable value. The reliability of the PMS (1.5 N) is obviously better than that of the PED (2 N) after 500 cycles, corresponding to the well-known insertion peak [10,19]. The insertion–extraction characteristic of the extracted 1st, 50th, 100th, 200th, and 500th maximum insertion forces for the PED and PMS was recorded to evaluate the difference in the insertion and extraction behaviors, as shown in Figure 6c,d. The insertion–extraction force of the PMS is more convergent than that of the PED, indicating that the resistance to wear with respect to the insertion/extraction behavior of the PMS exceeds that of the PED. As the times of insertion and extraction increase, the instability of the contact state between the contact pairs is revealed, which is attributed to the fatigue and stress relaxation between the contact pairs. Then, the worn residues increase after certain cycles, leading to a wider range of fluctuation in the overall trend of the insertion/extraction force. Compared to the PMS, the PED exhibits inferior convergence during the repeated insertion and extraction process, where the degree of dispersion gradually increases with an increase in the insertion and extraction times. This is due to the fact that the structure defects located in localized regions lead to uneven deformation and wear, thereby accelerating the coating failure. Compared with the electroplating coating technique used in pin protection, the advantages of the magnetron sputtering method are good mechanical properties like plastic–deformation resistance and good consistency in the depth variation from the insertion/extraction force over 500 cycles, as shown in Figure 6.

3.4. Wear Mechanism

3.4.1. Wear Phenomenon

The wear mechanism of PED and PMS was deeply studied following the insertion–extraction tests. Figure 7 presents micrographs of the wear surface for both coatings. The contact region of the PED and PMS experienced elastic–plastic deformation due to repeated wear from the mechanical insertion and extraction test with an increase in operation times, and the accumulated wear resulted in a distinct slope on the front of the pin. For the PED, there were visible peeling stripes on the wear track, accompanied by furrows and large microcracks, which are ascribed to the uneven hardness distribution, ranging from 0.2 GPa to 2.2 GPa, as shown in Figure 5. When contact pairs are in contact with each other, this defect causes large plastic deformation, and, in turn, the coating is extruded and peels off; simultaneously, a series of microcrack nucleation sources gather on the coating surface and are gradually interconnected and extended to the surface, forming fleck-like abrasive debris. The debris and oxides produced by the fractured coating form substantial micro-asperities in the slide region and act as scratch indenters under the action of a vertical-acting load.

3.4.2. Analysis of Wear Mechanism

The PED wear mechanism corresponds to a combination of adhesion and abrasive wear, as well as brittle fracture. As for the PMS, the wear bar is relatively shadowed and exhibits narrow furrows, including some debris and delamination. Figure 7c–h show that the PMS retains better structural integrity than that of the PED after 500 insertion–extraction cycles. On the one hand, high H, H/E, and H²/E³ could enhance the resistance of the coating to high contact force. On the other hand, the good coating quality achieved using magnetron sputtering provides the possibility to alleviate the expansion and propagation of the defects during the extraction–insertion process. Thus, the wear mechanism of the PMS involves plowing wear and slight adhesive wear.

3.5. Contact Resistance

We referred to the contact resistance detection method used in the previous study to explore the changes in contact resistance under the repetitive insertion and extraction behaviors [20,21]. Since the measuring device itself determines line resistance and connection resistance, we calculated the amount of change based on the first resistance. The different wear mechanisms in the two coatings caused a significant difference in the contact resistance, and the variance tendency can be considered as follows: (1) The contact force between the contact pairs decreased as the insertion and extraction wear increased and was unable to maintain a reliable contact state between the contact pairs, leading to the distinct Increase in the contact resistance. (2) The substrate metal was exposed to the environment and oxidized owing to the accumulation of repetitive wear. The oxide film adhered to the contact surface, resulting in an increase in the contact resistance. The formed oxide film was continuously destroyed under the repetitive insertion and extraction behaviors, which directly caused the fluctuation in the contact resistance. (3) The debris oxide particles and metal particles generated by the repetitive wear adhered to the contact surface and caused a decrease in the real area of the current conduction, thus increasing the contact resistance. The variation in the contact resistance of the PED was more obvious than for the PMS (Figure 8), owing to the wear of the PED coating being significantly more severe than that of the PMS. Compared with the electroplating coating technique used in pin protection, the advantages of the magnetron sputtering method are good stability on contact resistance over 500 cycles because of the anti-wear performance, as shown in Figure 8.

3.6. Future Studies

The adhesion and anti-acid erosion performance of the coating is also acceptable since it passed the thermal shock test as well as the erosion test according to the standard, which the electroplating coating should also pass. Alloy and compound coatings can also be easily sputtered using the multi-magnetron system, and their percentage can be easily controlled by adjusting the input power of each target, which means good flexibility.

The limitations associated with magnetron sputtering mainly relate to large-scale production and cost. During sputtering, all of the pins should roll around their axial on the platform, which is achieved through vibration. However, it is much easier to coat using electroplating, as the pins can roll in a plastic cage. Therefore, a better way to scale the sputtering batch should be developed. Another limitation is the cost of gold: particles are sputtered from the target, but only a portion of them attach to the pins, and the remaining particles are recycled through smelting. Currently, the chamber and the platform are covered using tin papers to collect the wasted gold, which is then sent to a target provider for smelting.

The challenges in the transition from laboratory to industry are as follows: (1) investment in the sputtering equipment, including the vacuum system, high-purity gas, high-purity gold target, and the gold recycling furnace, which are much more expensive than electroplating basins; (2) consistency in the uniformity of the coatings on each pin when production is scaled up to include larger numbers of pins.

In electric contacts, fretting wear studies are important in order to prove the suitability of a material for application. Therefore, fretting wear studies will be performed in the future.

4. Conclusions

In this work, the influence of a pin coated using electroplating and magnetron sputtering on the stability of electrical contacts was systematically examined in terms of the microstructure evolution, mechanical properties, and wear resistance. The main conclusions are as follows:

(1)

The coating prepared via magnetron sputtering had a more uniform and denser microstructure than that of the one produced using electroplating, thereby revealing excellent mechanical properties.

(2)

The magnetron sputtering process was better able to control the porosity of the coating and enhance its corrosion resistance compared to the electroplating technique.

(3)

In combination with microstructure evolution, stress relaxation, and fatigue failure, the main wear mechanism of the electroplated coating was a mixture of abrasive wear, adhesive wear, and localized brittle fracture, while the coating prepared using magnetron sputtering exhibited abrasive wear and localized brittle fracture.

(4)

The contact resistance of the pin coated via magnetron sputtering exhibited better stability than the electroplated pin after the repetitive insertion/extraction operations owing to the difference in the microstructure, mechanical properties, and tribological behavior.