Most mechanical systems (in particular, gear transmission system) undergo relative motion which results in increased friction phenomenon (friction coefficient, stresses, and wear rate) and thereby results in loss of efficiency. Mechanical parts undergo relative motion in different geometry configurations and orientations that induce a different state of stress as a result of friction. Till date, attempts are being made to minimize the friction with full sphere pin geometry configuration. The present work focused to reduce the frictional and wear rate, and experiments are conducted with tribo-pairs. i.e., nickel-coated pin surface slide against Al6061 alloy disc. The friction studies are carried out at different loads and geometries of pin surfaces (sphere and hemisphere configured at different orientations such as full sphere and hemisphere configured at 0°, 45°, and 90°) to induce different stress states with reference to sliding directions. Change in the geometry of EN8 pin material and their orientation with reference to sliding direction resulted in a different state of stress. The resulting stress levels were examined under the scanning electron microscope, which revealed the mechanisms of adhesion, abrasion, and extrusion. At a lower magnitude of orientation and load, the extent of asperity breaking lessens and material removal from pin surface decreases. Abrasion wear mechanism was observed corresponding to full sphere configuration on Al 6061 disc, whereas adhesive wear mechanisms are seen with hemisphere pins. The amount of aluminum transfer on pin surface with a hemisphere pin is comparatively more than that of full sphere configuration. At a lower magnitude of state of stress, the mechanism of sliding was dominated by the adhesion effect. At a higher level of state of stress, the mechanism of sliding was dominated by abrasion and extrusion.

1. Introduction

Interface quality in moving parts is of practical significance to ensure proper function in the mechanical system [1, 2]. The drag and frictional forces are minimized due to their longitudinal ribs of shark, and tree frogs grip even on the smooth surface at wet environment due to their surface features (hexagonal cell separated by gaps observed in microstructure) [3, 4]. These observations ensure the study of surface properties, and their design for better performance in moving parts is of industrial relevance [5, 6].

Al alloys are extensively used in many industrial and household applications [7, 8]. Owing to excellent properties (high corrosion rate, good formability, and mechanical properties) [9], Al 6061 finds major applications in automotive and marine applications [10]. Several attempts are being made to widespread the applications of Al6061 alloy by improving wear resistance with the addition of reinforcement materials [911]. The effect of reinforcement to Al6061 affects the machinability and formability and needs significant attention to bring back to optimal functioning conditions [12, 13]. Experimental study reveals that Al6061 composites exhibited better mechanical (tensile strength) and tribological (wear resistance) properties over Al7075 (high strength and high toughness) composites [10, 14]. Attempts are being made to enhance the tribological characteristics of two forge, and customary gear materials are evaluated [15]. Tests are conducted to examine the performance (friction coefficient, friction force, and wear rate) of planetary gear transmission (made of EN 24)-based CNC bending machine [16]. The tribological characteristic examination was performed on the pin-on-disc test rig against the different shaft materials (EN8 and EN24) with Al 6061 alloy under dry and wet conditions (SAE 20W50) [17]. The minimum coefficient of friction was observed with EN8-Al6061 tribo-pair [18]. It was observed that different geometric configuration (i.e., pin geometry: sphere-sphere, sphere-plane, block on ring, and piston ring-cylinder liner) induces different tribological characteristics (i.e., friction coefficient and wear rate) [18, 19]. Therefore, EN8-Al6061 tribo-pairs possess better tribological properties and require in-depth analysis (in terms of the state of stresses and friction developed, plastic deformation, and so on) at the different geometric configurations for further enhancement and widespread use of applications in industries.

A frictional phenomenon in moving parts brings loss in efficiency of the machine [20]. For example, spur gear efficiency was significantly affected by different frictional coefficients, load, velocity, and tooth profile [21]. Frictional parts subjected to periodic loading result in wear out of parts that causes increased clearances and losses of dimensional stability [22]. To minimize the frictional effects among moving parts, a series of experiments are conducted by distinguished researchers with material pairs, which are unlubricated, lubricated, and coatings [23, 24]. These techniques are basically employed to alter the displacement and velocity discontinuity across the contacting material pair [25, 26]. Therefore, significant attention is thus required to know the state of stress developed on sliding phenomenon for different tribo-pairs. The state of stress either a uniform or nonuniform is dependent on contacting surfaces (i.e., smooth surface results in steady state, whereas rough surface could result in the nonuniform state of stress) and load transfer between the mating element pairs [27]. It was confirmed that studying the different state of stress as a result of mating pairs are of practical significance. In spur gear applications, increased contact stresses are attributed to excessive load, which alter the gear tooth profile that causes wear on gear tooth [28, 29]. The contact stress can be minimized by altering the geometric parameters [30]. Excess wear on the tooth profile results in noise and vibration, which in turn reduces the efficiency [31, 32]. These phenomena are more common in mechanical transmission applications [28]. The effect of geometric parameters (gauge, gauge deficiency, cross-level, and longitudinal slope) on the wear phenomenon (lateral or vertical) of railway track was investigated [33]. It was observed that minimized wear with the right choice of geometric parametric conditions resulted in increased ride performance at reduced maintenance cost [34]. Creating artificial textures on the geometry of the cutting tools could help to minimize friction and wear [35]. These textures could help for better lubrication during the cutting phenomenon, thereby enhances the tool life and reduces workpiece surface roughness and power consumptions [20, 36]. Microtextures in the form of grooves, circle, rectangle, and hemisphere reduce the cutting temperature, friction, and wear that could enhance the tool life [37, 38]. The effect of pin geometries (flat and spherical) on the wear rate of low carbon steel material was investigated [39]. The results showed that the wear rate that corresponds to a flat pin is more than that of a spherical pin for a low load, whereas at higher loads, spherical pin quickly wears out. It was ascertained from the above literature review that changes in geometric parameters introduce different state of stress and help to control friction and wear. Therefore, the study of wear and friction phenomena as a result of geometric parameters and their orientations is of relative significance.

Coatings applied to substrate materials tend to improve the surface characteristics without affecting the properties of the parent material [40]. The application of coatings improves the hardness and surface integrity and reduces friction, wear, and corrosion [4144]. Experiments are performed with EN8 carbon steel mating pair with ductile material Al6061 to know the wear phenomenon [45]. The magnetron deposition method was employed to coat aluminum oxide (Al2O3) on the aluminum substrate [46]. Compared to aluminum substrate, the coated samples resulted in reduced friction coefficient with enhanced strength and hardness were obtained. The friction and wear behaviors of hot-dip Al-Si and electroplated Zn-Ni alloy-coated steel blanks were investigated [47]. The coated Zn-Ni alloy resulted in better wear resistance at a reduced friction coefficient. The physical vapor deposition (PVD) technique was applied to coat the iron (Fe) nanoparticles on the flat surface of the pin on EN8 pin material, and the results are tested at room- and high-temperature applications [48]. The hard oxide layers are formed after sliding against pin material, which could help to sustain higher loads at elevated temperatures. Experiments are performed with Ni-P/biocomposite coating applied on EN 8 steel, which was subjected to friction and wear studies [49]. The coated samples resulted in a lower friction coefficient and wear rate than bare substrate material. The Mo-Ni-Cr coatings are applied on super-duplex stainless steel that resulted in better wear resistance and higher hardness than uncoated substrate material [43]. Nickel coatings are applied to H13 steel with the laser cladding technique that resulted in enhanced wear resistance and thermal fatigue characteristics [50]. The resistance to wear and corrosion, ductility, and microstructure characteristics are improved with nickel-based coatings [51, 52]. Nickel coatings find potential applications in biomedical implants and automotive parts [53, 54]. Although coating technology improves the surface properties, the selection of the right choice of coating methods is often difficult as each technique possesses its own advantages and limitations. Electroplating is treated as a cost-effective technique compared to chemical vapor deposition and sputtering [55, 56]. In addition, electroplating coats materials in a single processing step (with faster, repeatable, control over the thickness of coating deposits and offers coating over a large surface area), which do not require costly equipment, materials, and nonhazardous method to deposit material on the substrates [57, 58]. There exists a significant scope to study the impact of electrodeposited nickel coatings on the substrate material for automotive applications. Most of the parts (prosthetic joint, crankshaft, connecting rod, brake pads, steering, chassis, axle housing, etc.) undergo either an individual or in the combination of sliding, reciprocating, and rotary actions [5961]. Furthermore, tool wear occurred on die surface possessing sharp edges may not yield appropriate results with spherical or flat pin surfaces sliding against discs [39]. It was confirmed from the above literature review that applications of coatings increase the wear resistance. The said applications undergo wear with a different state of stress based on their contact movements and edge effects. Furthermore, not much research efforts have been made to examine worn surface morphology with different pin geometric configurations at different angles and applied loads to detail the insights of surface morphologies (plastic deformation and wear mechanism, i.e., adhesion, abrasion, and extrusion) relationship with friction and wear phenomenon.

To date, no studies have yet reported to study the field condition, which is arbitrary in terms of the state of stress concern. In this study, a cost-effective electrodeposited coating technique (widely employed with a growth rate of 15% per annum compared to other methods) [62] was used to apply nickel coating on the EN 8 steel pin. Nickel coatings are selected based on their properties, wide range of applications, and ability to withstand reduced wear loss both in oxidizing and nonoxidizing environments. The geometric parameters (spherical and hemispherical) of pin tip surface oriented at different angles are investigated to know the wear behavior and state of stress induced. Therefore, this study gives detailed insight into a different state of stress developed, friction induced, worn-surface morphologies (in terms of plastic deformation, abrasion, abrasive, and extrusion), and wear rate caused by different geometries and angle of orientation of pin surfaces with and without coatings.

2. Materials and Methods

EN 8 steel used in many mechanical parts due to better strength and toughness characteristics resulted in distinguished applications (connecting rod, spindles, axles, and so on) of automotive parts [63, 64]. Al 6061 alloy is used in commercial engineering and automotive applications due to better mechanical properties and ductility [9, 65]. The chemical composition of Al 6061 alloy disc and EN8 steel pin is presented in Table 1.

2.1. Electrodeposition Coatings

Nickel coatings based on electrodeposition technique were applied on the substrate material, i.e., EN8 steel. The dimensions of the pin specimens were equal to 9 mm × 35 mm (diameter and height), subjected to a nickel sulfate bath solution for coating applications as depicted in Figure 1. Prior to coating applications, the pins are cleaned (to remove the organic matter, present if any) after immersing in a dilute HCL acid bath solution. This process is referred to as the pickling process. After the acid pickling process, the pins are dipped in dilute sulfuric acid so as to improve the microroughness. This could help in better adhesion characteristics of coating on the substrate material. Note that, nickel coating was carried out by holding the pin as cathode and pure nickel as anode material in nickel sulfate solutions. The thickness of the coatings is about 100 μm and is decided after adjusting the duration of coating.

Pure nickel was selected to coat the substrate EN8 material due to their distinguished properties such as low friction and higher hardness with reduced wear and corrosion resistance properties [62]. Nickel electrodeposits are carried out, wherein nickel ion concentrations are varied subjected to 50–250 g/L, after maintaining Na2SO4 and boric acid fixed at 80 g/L and 10 g/L in the bath solution. It was observed that the bright deposits (without burnt) are attributed that correspond to the current density maintained equal to 3 Adm−2. Note that during experimentation, the bath solution was maintained fixed that correspond to the potential of hydrogen equal to 3. Prior to electrodeposits, the solutions are subjected to agitation (say 500 rpm) for 24 hours at room temperature. From trial experimentations, it was clear that there is no presence of burnt appearance and uncoated regions when the pH value was maintained equal to 3. DC power source was used, and the depositions are carried out on substrate at the stagnant condition of bath solution. Schematic representation of electrodeposition technique used for coating nickel on EN8 substrate material is presented in Figures 2 and 3. Theoretically, the coating thickness was determined according to equations derived from the practical experiments.

2.1.1. XRD Characterization of Pin and Disc Materials

The XRD patterns of Al6061 are shown in Figure 4(a). The Al6061 XRD data show strong peaks at 2θ = ∼38.13°, ∼44.37°, followed by weaker peaks at ∼64.74° and ∼77.88°, and ∼82.11° due to the inherent Al content [66]. For Al 6061, the XRD pattern peak intensity is matched with JCPDS file card number 96-901-2004. In the case of Figure 4(b), i.e., EN8 steel pin (Peak 1), XRD data reveal sharp peaks at 2θ = ∼44.70°. Also, at 2θ = ∼65.09° and ∼82.45° dimmer peaks were detected owing to the intrinsic Fe content [66]. For EN8 steel pin, the XRD pattern peak intensity is matched with JCPDS file card number 96-900-6596. In the case of Figure 4(b), i.e., EN8 steel pin—Ni coated (Peak 2), the XRD data reveal sharp peaks at 2θ = ∼ 44.20°. The XRD pattern peak intensity of Ni-coated EN8 Pin is matched with JCPDS file card number no. 96-900-0658 and 96-901-2996. Similarly, at 2θ = ∼64.97° and ∼82.28° dimmer peaks were detected owing to the intrinsic Fe content. Also, the cause of a high broad peak at ∼44.20° may be due to Ni-P coating [67, 68].

2.2. Experimental Details

The kinematic pair is treated as the building block in any mechanical system. The aluminum alloys are lightweight and possess excellent aesthetic qualities in any environment for a prolonged duration. Hot and cold extrusions are treated as the secondary route for processing aluminum parts. Tribo-phenomenon (i.e., mechanical interaction among stock and die) is of practical significance, as it decides the quality of parts. In the efforts of this research, understanding the tribo-phenomenon between die and aluminum stock is proposed to conduct laboratory experiments using a pin-on-disc test rig. The Al 6061 disc was used to simulate the stock in extrusion equipment. Electrodeposition of nickel material was performed to coat EN8 steel pin to a thickness of 100 μm.

The spherical (half and full) pin surface held at different angles against the rotating Al6061 disc could help to alter changes in the state of stress. The mating pair element in the disc form was precipitate hardened Al6061 alloy. Turning followed by facing operations ensures the dimensions of pin specimen possessing diameter and height equal to 9 and 35 mm, respectively. During experimentation (before and after), the pin and disc surfaces are ultrasonically cleaned with acetone. Figure 5 shows the pin-on-disc wear test rig, wherein the nickel-coated pin surfaces are subjected to three test loads (0.5, 3, and 5 kg or 4.9, 29.4, and 49.1 N). The wear tests are conducted at room temperature. Mechanical parts, namely, bearings, rotate smoothly at the speed of 100–250 rpm [69], wherein bearing supports gear parts, shafts, and so on. The test loads (0.5–5 kg) are selected in accordance with gear applications carried out earlier by authors [70, 71]. The wear parameters considered for examination on friction coefficient and wear rate are presented in Table 2. Three samples are separately prepared for each loading condition (0.5, 3, and 9 kg), and average values of three wear rate and coefficient of frictions are recorded to conduct analysis. The specimens are held on an aluminum disc, which is oriented at different configured stress state (full sphere and hemisphere configured at 0°, 45°, and 90°) with reference to sliding directions subjected to wear parameters (Figure 6). This could result in a different state of stress. Note that, after performing wear examination the surface topography of, namely, Al6061 and nickel-coated EN 8 steel is captured. SEM was performed to know the insight of wear mechanisms. The dimensions corresponding to Al 6061 disc are presented in Figure 7. Prior to wear examination, the pin and disc samples are examined for surface roughness and hardness. The surface roughness that corresponds to the uncoated pin, nickel-coated pin, and disc surface was maintained equal to 0.2 μm. The hardness values that correspond to uncoated pin, nickel-coated pin, and disc surfaces were found equal to 217.7 HV, 261.5 HV, and 98.1 HV.

3. Results and Discussion

The results of velocity discontinuity and state of stress caused by the sliding pair contact surfaces are discussed based on sliding mechanics. The EN8 pin surfaces coated with nickel for altering velocity discontinuity against the Al 6061 disc were also discussed. The state of stress altered by different pin tip-shaped surfaces (i.e., spherical and half-spherical) configured pin at different angles with reference to sliding direction. The state of stress as a result of different normal loads is discussed. Wear behavior and its mechanisms are examined with the help of scanning electron microscopic images.

3.1. Full Sphere Coated with Nickel Sliding against Al 6061 Disc

Figure 8 shows the coefficient of friction obtained for different loads with reference to time constraints. At 0.5 kg load, the friction coefficient increases from 0 to 0.18 for the duration of 5 seconds, and thereafter, a steady increase was observed till 110 seconds. Increased coefficient of friction with load might occur due to the plowing effect as a result of increased dislocation density, plastic deformation, and penetration depth [72]. Similarly, the coefficient of friction value tends to increase from 0 to ∼0.5 till 10 seconds for 3 kg and 0 to ∼0.56 till 8 seconds for 5 kg. The coefficient of friction remains at an almost steady state after crossing 10 seconds of duration. It was noted that at all loads the friction coefficient curves showed a similar trend, wherein the transient state was seen during the initial stage followed by a steady state (refer to Figure 8). The rate of increase in friction coefficient values was found to be of lesser magnitude after 10 seconds at all loads, compared to the time period of 0–5 seconds. This occurs because at the beginning of the experiments or sliding phenomenon, the soft disc (i.e., Al 6061 alloy) was plowed by the hard asperities of coating surfaces that resulted in the highest friction peaks. Similar observations are seen on friction performance of phosphate-coated brake materials [73]. A steady state with friction coefficient curves became flattened with the increased number of revolutions or time duration against sliding tests. As sliding continues, the hard asperities of coated pin surfaces slowly covered with accumulated transferred material (i.e., plowing mechanisms of hard asperities reduce, and in turn, friction coefficient becomes steady state), which was later deformed, and curves became flattened. The observations are similar to the friction behavior studies of titanium nickel coatings [74]. The average coefficient of friction tested at different loading conditions is presented in Table 3. The minimum and maximum friction coefficients were found equal to 0.3 for 0.5 kg and 0.48 for 3 kg loads, respectively. Interesting to note that, negligible change in friction coefficient values was attained after crossing the load from 3 kg to 5 kg. This is because more plastic deformation occurs at the sliding pairs as a result of compressive and shear forces [75].

3.2. Pin with Tip of the Hemisphere Coated with Nickel Configured 90° to the Sliding Directions against the Aluminum Disc

Figures 9(a)9(c) show the variations of coefficient of friction for nickel-coated half-sphere configured at different orientations (0°, 45°, and 90°) sliding against Al6061 disc at different loads. For 0.5 kg load, the coefficient of friction rapidly increases from 0 to 0.33 up to 15 seconds at 90° orientation, 0 to 0.37 up to 35 seconds at 45° orientation, and 0 to 0.43 up to 8 seconds at 45° orientation, respectively. Similarly for 3 kg load, the coefficient of friction was found equal to 0.54 for 0°, 45°, and 90° orientations, respectively. However, for 5 kg load, the coefficient of friction steadily increases to 0.6 for 0° and 45°, whereas 0.63 for 90° orientation of pin sample. It was clearly noted that comparatively stabilized friction curves were observed for 45° orientation than largest scatter (this occurrence might be due to stick-slip phenomenon) in friction values for 0° and 90°. It was clearly noticed from the friction curves that a rapid increase in friction at the initial stage of sliding due to soft aluminum disc was plowed by hard asperities of coatings. Thereafter, the surfaces are significantly altered such that original topographical orientation features (smooth surface ensures friction equalized to similar influenced values) possess minor variations showing that the friction curves remain flattened or steady for all orientations tested against different loads. Hard asperities of coatings tend to repeatedly indent the soft aluminum disc surfaces under stick-slip conditions, which results in increased sliding forces above the steady-state value during the beginning of experiments at all loads and orientations [76]. From Figures 9(a)9(c), it was clear that the load at which both stick slips at start and amplitude oscillations are lower for 45° orientation than 0° and 90°. The average friction coefficients tested at different loads (0.5, 3, and 5 kg) against different pin configurations (0°, 45°, and 90°) are presented in Table 4. The coefficient of friction that increases with increasing load was clearly observed.

The surface topography orientation did not influence much on friction average friction coefficient for 0° and 90°, whereas comparatively lesser friction values were observed at 45° orientations. The plane strain conditions cause more amplitude of oscillations with higher frictions in the curves for 0° and 90° orientations. The reduced friction at 45° orientation is attributed to less continuous contact surface area. The behavior of pin geometry (full sphere and hemisphere) on frictional curves is presented in Figures 8 and 9. The mean values of coefficient of friction for spherical and hemispherical pins possess approximately similar magnitude values, with comparatively less friction for spherical pin surfaces (refer to Tables 3 and 4). The hemispherical pin friction curves showed that more fluctuations might be due to the damage that occurred at sliding or tribo-pairs, wherein the accumulated debris released from surface layers causes highly scattered variation in friction coefficients [77, 78]. However, full spherical pin surfaces are often smooth, which resulted in more stable friction. Similar observations are reported with different geometries of pin surfaces [79, 80]. Tables 3 and 4 clearly show that the nickel-coated pins resulted in a lesser coefficient of friction compared with uncoated or bare specimens. Increased hardness with nickel coating led to reducing the coefficient of friction values due to coating imparts, better oxide protection, and hot hardness properties. These observations are in good agreement with published literature [81].

3.3. Worn Surface Analysis of Al6061 Disc

Figure 10 depicts the wear surface morphology of Al6061 disc surfaces at different loads and orientations. From Figures 10(a)10(h), the arrow mark represents sliding direction and the numbering 1, 2, and 3 represent flecks, slender abrasive furrows, and dense abrasive furrows, respectively. Figures 10(a)10(b) show the SEM micrographs of track surface subjected to a load of 0.5 and 5 kg that correspond to the geometry of pin surface made of a full sphere. The track surface of a full sphere subjected to 5 kg load showed more abrasion than with 0.5 kg of load by pin material. Also, it is observed that a lesser extent of adhesions is observed on the track surfaces. Figures 10(c)10(d) show the SEM micrographs and show the track of the hemispherical pin configured at 90° against sliding direction subjected to different loading conditions. Both figures showed more adhesion effects than that of abrasion. It can also be observed that the abrasion effects are comparatively larger than that obtained for Figure 9(a). Figure 9(c) shows more pronounced adhesion for 0.5 kg load compared to that obtained for 5 kg of load.

Figure 10(d) clearly shows that there exists a larger extent of extrusion compared to the micrographs of Figures 10(a)10(c). Figure 10(e) shows the track surface that corresponds to a pin configured to 45° against sliding direction subjected to different loads. The said figures clearly show that there is an extrusion phenomenon coupled with traces of abrasion. Figure 10(g) shows the small extent of abrasion, and the amount of extrusion was more pronounced for 5 kg normal load than that obtained for 0.5 kg against sliding direction. Under dry sliding conditions, there is a significant level of iron layer transfer from the EN8 steel pins to the surface of the aluminum disc plate. The amount of transferred layer was also found to increase load from 0.5 kg to 5 kg. Analogous remarks were made by quite a few researchers [76, 80, 82].

3.4. Worn Surface Analysis of the Nickel-Coated Pin

Figure 11 depicts the wear surface morphology of EN8 steel pin coated with nickel at different loads and orientations. From Figures 11(a)11(h), the arrow mark represents cracks and the numbering 1, 2, and 3 represent slender abrasive furrows, material transfer area, and dense abrasive furrows, respectively. Figure 11(a) shows the SEM micrograph of pin surface for the full sphere at 0.5 kg load depicting the transfer of aluminum layer on the pin surface at a leading edge. Figure 11(b) presents the hemispherical pin configured to 90° that showed the aluminum layer transferred on the pin surface at the leading edge. Interesting to note that, the amount of aluminum transfer on the pin surface is comparatively more than that of the full sphere configuration seen in Figure 11(a). Similar observations were recorded for a hemispherical pin configured to 45° and 0° as shown in Figures 11(c) and 11(d). Figures 11(e)11(h) show the pin surface for all configurations at 5 kg load depicting aluminum transfer layer at the leading edge. Compared to 0.5 kg, the amount of aluminum transfer is quite large at 5 kg. Furthermore, as the steel pins slid across the aluminum plate, a few of the relatively weak asperities with poorer Al plate areas will indeed crack and then abide by the steel pin. The level of severity is such that these weaken the crack and relocate to the steel pin surface that is determined by oriented angle. While EN8 steel is slid tangential to the abrasion track rather than parallel to the sliding direction, the chances of breakdown of plate severities will surge. This is due to the asperities being aligned across the sliding direction when the oriented angle is 90° but along the sliding direction when the oriented angle is 0°. As the oriented angle and load are reduced, the extent of asperity breaking lessens, and thus, the extent of iron removal from the pin also decreases. Similar observations were made by several researchers [45, 76, 80, 83].

3.5. EDAX Analysis of the Nickel-Coated Pin

EDAX studies have been conducted for identifying elements present in worn surface and debris of nickel-coated spherical tipped pin surface at a load of 5 kg at different selected areas. The elemental analysis on transferred aluminum on pin surface was carried out at different locations and is shown in Figures 12(a)12(c), respectively. From Figure 12(a), it was revealed that aluminum is 94.19 in weight percentage and the transfer layer is aluminum. Figure 12(b) reveals that nickel is 87.31 in weight percentage. The nickel coating is found to be intact. Figure 12(a) reveals that nickel is 76.32 in weight percentage. The nickel coating is found to be intact. From Figures 12(a)12(c), the commonly observed elements were Al, Fe, Ni, and O. The occurrence of Fe implies that Fe has been transferred from the pin (EN8) to the disc surface, confirming the formation of an oxide layer on the disc, which protects the disc. The existence of thick oxides aids in the formation of wear protective layers.

4. Conclusions

The friction and wear behavior of tribo-pairs such as nickel-coated EN8 steel pin and Al6061 alloy disc was investigated at different loads and pin surface that configured to different geometries and orientations. The following conclusions that are drawn from this study are discussed in the following:(1)Electrodeposition technique was applied to coat the nickel material onto the EN8 steel pin surfaces to a thickness of 100 μm. To induce a different state of stress, the geometry of pin surfaces is configured to full sphere and hemisphere with a different orientation (0°, 45°, and 90°) and load (0.5, 3, and 5 kg load).(2)In full sphere configuration, the minimum and maximum friction coefficients were found equal to 0.3 for 0.5 kg, 0.48 for 3 kg loads, and 0.47 for 5 kg, respectively. The plastic deformation that occurs at sliding pairs initiates at 3 kg load (as a result of compressive and shear forces), which resulted in a negligible change in friction coefficient values.(3)The hemispherical pin friction curves showed that more fluctuations might be due to the damage that occurred at sliding or tribo-pairs, wherein the accumulated debris released from surface layers causes highly scattered variation in friction coefficients. However, full spherical pin surfaces are often smooth, which resulted in more stable friction.(4)At higher load (say 5 kg), the abrasion and extrusion were more pronounced than 0.5 kg against sliding direction on the wear tracks of Al6061 disc surfaces.(5)The amount of aluminum transfer on the hemisphere pin surface is comparatively more than that of full sphere configuration. As the oriented angle and load are reduced, the extent of asperity breaking lessens, and thus, the extent of iron removal from the pin also decreases.(6)From COF vs. time plot, the coating studies are described by the transient state seen during the initial stage, followed by a steady state at all loads.(7)Hard asperities of coatings tend to repeatedly indent the soft aluminum disc surfaces under stick-slip conditions, which results in increased sliding forces above the steady-state value during the beginning of experiments at all loads and orientations.(8)From the wear surface morphology and surfaces at different loads and orientations, it is observed that a lesser extent of adhesion effects is observed than that of abrasion on the track surface effects [84].

Data Availability

Data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.