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Effect of Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared by Powder Metallurgy

Effect of Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared... processes Article Effect of Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared by Powder Metallurgy 1 , 1 , 2 1 Ruoxuan Li *, Seiji Yamashita *, Katsumi Yoshida and Hideki Kita Department of Materials Process Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan; kita.hideki@material.nagoya-u.ac.jp Laboratory for Zero-Carbon Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan; k-yoshida@zc.iir.titech.ac.jp * Correspondence: li.ruoxuan.a4@s.mail.nagoya-u.ac.jp (R.L.); yamashita.seiji@material.nagoya-u.ac.jp (S.Y.) Abstract: The purpose of this study was to investigate the influence of different counterbodies against Cu/magnesium phosphate treated graphite (Cu-MgPG) composite materials to find the best material combination in terms of friction coefficient and specific wear amount. A Cu matrix composite reinforced with 10 vol% magnesium phosphate treated graphite and pure Cu powder were prepared by powder metallurgy techniques under the same consolidation processing condition. The friction and wear properties of the composites were investigated at 10 N using a pin-on-disc tribometer on Al O , SiC and SUJ2 bearing steel counterbodies. The Cu-MgPG/Al O pair showed 2 3 2 3 the lowest friction coefficient, but the specific wear rate tended to increase slightly when compared with Cu/Al O pair. On the other hands, the Cu-MgPG/SUJ2 pair showed about the same specific 2 3 wear rate as the Cu/SUJ2 pair, but the friction coefficient was significantly reduced. These phenomena are thought to be due to the fact that the added graphite acts as a solid lubricant during sliding and also suppresses the oxidation behavior of the sliding material. Citation: Li, R.; Yamashita, S.; Keywords: graphite; Magnesium Phosphate; sliding wear; copper-matrix composite; friction; wear; Yoshida, K.; Kita, H. Effect of counterbody Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared by Powder Metallurgy. Processes 2022, 10, 804. https://doi.org/10.3390/pr10050804 1. Introduction Graphite as a common solid lubricant that is often added to metal materials to make Academic Editor: Sung-Churl Choi new materials with self-lubricating properties [1–6]. Received: 24 March 2022 Copper-graphite composites possess both copper and graphite properties, including Accepted: 18 April 2022 excellent thermal and electrical conductivity, solid lubrication, low coefficient of thermal Published: 19 April 2022 expansion and so on, which makes these composite widely used as a brush and bearing Publisher’s Note: MDPI stays neutral material in many applications [3,7]. In fact, components in this type of application are often with regard to jurisdictional claims in exposed to high friction, temperature rise and environmental erosion [8]. The good friction published maps and institutional affil- properties of copper-graphite composites are often due to the fact that the graphite particles iations. exposed on the surface of the composites act as lubricants during the friction process [9–11]. Considering that graphite is prone to oxidation in high temperature environments, the operating temperature range of ordinary copper-graphite composites is also limited. In a previous work [12], it proposed a method to improve the oxidation resistance of Copyright: © 2022 by the authors. graphite while maintaining the lubricating properties of graphite. In this study, the mag- Licensee MDPI, Basel, Switzerland. nesium phosphate-treated graphite (MgPG) was used as a solid lubricating phase to mix This article is an open access article with copper powder, and used powder metallurgy technique to make copper/magnesium distributed under the terms and phosphate-treated graphite composite materials (Cu-MgPG). conditions of the Creative Commons In the preparation of copper-graphite composite materials, many studies [9,13–16] Attribution (CC BY) license (https:// have investigated the effect of the content of graphite added on the properties of the final creativecommons.org/licenses/by/ composites. While this work aims to study the friction properties of composites made 4.0/). Processes 2022, 10, 804. https://doi.org/10.3390/pr10050804 https://www.mdpi.com/journal/processes Processes 2022, 10, 804 2 of 12 of 10 vol% magnesium phosphate treated graphite and copper. Research [14] has shown that when the volume content of graphite is less than 50%, sintering temperature lies in 700 C–950 C to improve the metallurgy bonding between copper and copper in the material. At the same time, the raw material with treated graphite [12] used in this study was produced by sintered natural graphite with magnesium phosphate at 800 C. Based on the above data, the sintering temperature of the composite material selected in this study was 800 C. Many scholars have studied the tribological properties of copper-graphite composite materials under different conditions, and the results show that the tribological behavior of copper-graphite composite materials is complex and dependent on external factors. These important external factors include applied load, sliding velocity, ambient temperature, humidity, and counterbody material. However, there are few studies on the effect of counterbody materials on the friction properties of copper-graphite composite materials. In this study, powder metallurgy technique was used to prepare Cu-MgPG composite materials and a comparative sample of pure copper (Cu) materials prepared under the same conditions. Then we compared the physical properties, friction properties and surface microstructure of the pure copper materials and Cu-MgPG composites. In addition, in order to find the optimum tribopair combination of Cu and Cu-MgPG with the counterbody materials, the influence of the counterbody materials in terms of coefficient of friction and wear was investigated. 2. Materials and Methods 2.1. Preparation of the Samples The magnesium phosphate-treated graphite (MgPG) raw material used in this study was prepared according to the method of previous study [12]. The arithmetic average particle size of the magnesium phosphate-treated graphite (natural graphite powder, D50 = 93.16 m; Mg(H PO ) 4H O, MW:290.34, JUNSEIKAGAKU) used in this work 2 4 2 2 was 131.70 m. The composites were fabricated by powder metallurgy technique. 10 vol% MgPG powder and copper powder (Nilaco, Ginza, Tokyo, Japan) were dry-mixed with a bench kneader machine (Irie Shokai Co., Ltd., Tokyo, Japan) for 3 h. After mixing, the powder mixture was first cold pressed at 30 MPa in 35 mm  35 mm square mold, and then hot- pressing in nitrogen flow (2 L/min) at 800 C for 3 h with a uniaxial pressure of 40 kN. For comparison, parallel compacts made from pure copper powders were consolidated under the same conditions applied for the preparation of Cu-MgPG composites. All specimens were polished to get a mirror-effect and cleaned with acetone before every experiment. The polished surfaces were then observed by SEM (JEOL Ltd., JSM-7500F, Tokyo Japan). The typical SEM micrograph of the polished surface of Cu-MgPG composites and copper are Processes 2022, 10, 804 3 of 13 shown in Figure 1. The magnesium phosphate-treated graphite particles dispersed in the copper matrix can be observed in Figure 1A. Figure 1. A typical SEM micrograph of the polished surface of (A) copper/MgP-graphite composites Figure 1. A typical SEM micrograph of the polished surface of (A) copper/MgP-graphite composites and (B) copper. and (B) copper. 2.2. Physical Properties The densities of the sintered copper/MgP-graphite composite and copper block were measured by Archimedes’ method. The theoretical density was calculated by dividing the bulk density of the sintered composite by the theoretical density calculated from the rule 3 3 of mixtures using 8.95 g/cm , and 2.25 g/cm as densities for copper and magnesium phos- phate, respectively. The hardness was investigated using a Micro Vickers hardness tester (HMV-G, SHIMADZU Corp., Kyoto, Japan) under a load of 0.5 kg with a dwell time of 10 s. Each sample was measured five times, and the data obtained were the average value. The physical properties of the sintered composites are shown in Table 1. Table 1. Physical properties of the copper and the sintered composite blocks. Copper Block Cu-MgPG Composite Block 3 3 Bulk density (g/cm ) 8.89 Bulk density (g/cm ) 8.27 3 3 Theoretical density (g/cm ) 8.95 Theoretical density (g/cm ) 8.36 Relative density (%) 99.3 Relative density (%) 98.9 Vickers hardness (Hv) 40.73 Vickers hardness (Hv) 54.83 The density of sintered samples reached 99% of the theoretical density, which demonstrated the efficiency of the powder metallurgy technology in producing high-den- sity materials. From Table 1, it can also be noted that the addition of graphite increased the hardness of copper materials but decreased the apparent density of the sintered com- posites. 2.3. Friction Coefficient and Wear Test The friction coefficient and wear sliding tests was carried out in a pin-on-disc tribo- meter (T-18-0162, NANOVEA Corp., USA) in ambient conditions (20 ± 0.5 °C and 45% ± 5% RH). The tribological properties of the prepared composite materials were investi- gated in a dry sliding test. Counterbodies used in the sliding test are 8 mm diameter pol- ished balls (the average surface roughness is less than 0.01 μm) made from commercially available balls-SiC, Al2O3, SUJ2 bearing steel (Sato Tekkou Corp., Japan). The hardness values of SiC, Al2O3, and SUJ2 balls are 2400 HV, 1600 HV, and 770 HV, respectively, which are obtained from the supplier. Before test, both the composite materials and the counterbody were ultrasonically cleaned in an acetone bath for 10 min. Sliding friction and wear tests were performed with a circular sliding under a load of 10 N and the sliding velocity was 0.1 m/s, the wear track radius was 5 mm, and the total sliding distance was 2000 m. There are no other lubricants added in each sliding test. Figure 2 shows the sche- matic illustration of the sliding tester. Three tests were performed for each counterbody. The friction coefficients were continuously recorded and the wear volume on each sample Processes 2022, 10, 804 3 of 12 2.2. Physical Properties The densities of the sintered copper/MgP-graphite composite and copper block were measured by Archimedes’ method. The theoretical density was calculated by dividing the bulk density of the sintered composite by the theoretical density calculated from the 3 3 rule of mixtures using 8.95 g/cm , and 2.25 g/cm as densities for copper and magnesium phosphate, respectively. The hardness was investigated using a Micro Vickers hardness tester (HMV-G, SHIMADZU Corp., Kyoto, Japan) under a load of 0.5 kg with a dwell time of 10 s. Each sample was measured five times, and the data obtained were the average value. The physical properties of the sintered composites are shown in Table 1. Table 1. Physical properties of the copper and the sintered composite blocks. Copper Block Cu-MgPG Composite Block 3 3 Bulk density (g/cm ) 8.89 Bulk density (g/cm ) 8.27 3 3 Theoretical density (g/cm ) 8.95 Theoretical density (g/cm ) 8.36 Relative density (%) 99.3 Relative density (%) 98.9 Vickers hardness (Hv) 40.73 Vickers hardness (Hv) 54.83 The density of sintered samples reached 99% of the theoretical density, which demon- strated the efficiency of the powder metallurgy technology in producing high-density materials. From Table 1, it can also be noted that the addition of graphite increased the hardness of copper materials but decreased the apparent density of the sintered composites. 2.3. Friction Coefficient and Wear Test The friction coefficient and wear sliding tests was carried out in a pin-on-disc tribome- ter (T-18-0162, NANOVEA Corp., Irvine, CA, USA) in ambient conditions (20  0.5 C and 45  5% RH). The tribological properties of the prepared composite materials were investigated in a dry sliding test. Counterbodies used in the sliding test are 8 mm diameter polished balls (the average surface roughness is less than 0.01 m) made from commercially available balls-SiC, Al O , SUJ2 bearing steel (Sato Tekkou Corp., Oita, Japan). The hard- 2 3 ness values of SiC, Al O , and SUJ2 balls are 2400 HV, 1600 HV, and 770 HV, respectively, 2 3 which are obtained from the supplier. Before test, both the composite materials and the counterbody were ultrasonically cleaned in an acetone bath for 10 min. Sliding friction and wear tests were performed with a circular sliding under a load of 10 N and the sliding velocity was 0.1 m/s, the wear track radius was 5 mm, and the total sliding distance was 2000 m. There are no other lubricants added in each sliding test. Figure 2 shows the schematic illustration of the sliding tester. Three tests were performed for each counterbody. The friction coefficients were continuously recorded and the wear volume on each sample disc and counterbody was calculated from the surface profile traces across the wear track using a surface profilometer (Mitutoyo Corp., 178-570-01, Kawasaki, Japan). The worn volume of the disc (V , mm ) was calculated according to the following equation [17]: S + S + S + S 2 3 1 4 V = 2pr( ) where S (mm ) and r (mm) are the cross-sectional areas of the wear track, each at 90 with respect to the previous one, and sliding radius, respectively. The worn volume of the ball (V , mm ) was calculated according to the following equations [17]: V = ph (R ) 2 2 h = R R r w Processes 2022, 10, 804 4 of 13 disc and counterbody was calculated from the surface profile traces across the wear track using a surface profilometer (Mitutoyo Corp., 178-570-01, Japan). The worn volume of the disc ( , mm ) was calculated according to the following equation [17]: + + + = 2( ) where (mm ) and (mm) are the cross-sectional areas of the wear track, each at 90° with respect to the previous one, and sliding radius, respectively. The worn volume of the ball ( , mm ) was calculated according to the following equations [17]: = ℎ ( − ) Processes 2022, 10, 804 4 of 12 ℎ = − − where ℎ (mm) is the height of the removed material, (mm) is original radius of the ball, and wher e (mm) is t h (mm) he ra is thedius of heightwea of the r sca removed r on the ba material, ll. R (mm) is original radius of the ball, andThe r (mm) specif is ic the wea radius r rate of ( wear , mm scar /N· on m) the was ball. further calculated according to the fol- lowing eq Theuati specific on [17wear ]: rate (W R, mm /Nm) was further calculated according to the following equation [17]: W R = ∙ LS where (mm ) is the wear volume of the disc ( ) or ball ( ), (N) is the normal load, where V (mm ) is the wear volume of the disc (V ) or ball (V ), L (N) is the normal load, d b and (m) is the total sliding distance. and S (m) is the total sliding distance. The worn surfaces of sample disc after sliding test were studied by SEM and EDS to The worn surfaces of sample disc after sliding test were studied by SEM and EDS to identify wear mechanisms. identify wear mechanisms. Figure 2. Schematic illustration of the pin on disc tester. Figure 2. Schematic illustration of the pin on disc tester. 2.4. Evaluation Method of Worn Surface 2.4. Evaluation Method of Worn Surface The surface morphology and elemental distribution of the samples were character- The surface morphology and elemental distribution of the samples were character- ized using a scanning electron microscope (SEM, JSM-7500F, JEOL Corp., Tokyo, Japan) ized using a scanning electron microscope (SEM, JSM-7500F, JEOL Corp., Tokyo, Japan) equipped with energy dispersive X-ray detector (EDS, JEOL Corp., Tokyo, Japan). Worn equipped with energy dispersive X-ray detector (EDS, JEOL Corp., Tokyo, Japan). Worn surfaces of the samples were also investigated by an X-ray photoelectron spectroscope surfaces of the samples were also investigated by an X-ray photoelectron spectroscope (XPS) to understand wear-induced surface chemistry modification. (XPS) to understand wear-induced surface chemistry modification. 3. Results and Discussion 3. Results and Discussion 3.1. Friction Coefficient and Wear Rate 3.1. Friction Coefficient and Wear Rate Typical evolution of the friction coefficients of copper block sliding against SiC, Al O , 2 3 and SUJ2 counterbodies at 10 N are shown in Figure 3. All of the friction coefficient Typical evolution of the friction coefficients of copper block sliding against SiC, diagrams contain two stages such as Figure 3, named the (1) run-in and (2) steady state Al2O3, and SUJ2 counterbodies at 10N are shown in Figure 3. All of the friction coefficient stage. In the run-in stage, the friction coefficient shows a large and irregular increase and diagrams contain two stages such as Figure 3, named the (1) run-in and (2) steady state decrease for a period of time; in the steady-state stage, the friction coefficient oscillates stage. In the run-in stage, the friction coefficient shows a large and irregular increase and continuously within a certain range. The formation of the run-in state stage may be related to the increase of the contact area, the work hardening effect of wear and the accumulation of debris at the pin-disk interface, and the change of the wear mechanism from two-body wear to three-body wear [18]. The reported friction coefficient in this study is the average of these data in the steady stage. An initial run-in period followed by a steady-state period was observed in all of the tribopairs, while in the run-in stage, the friction coefficient of copper block fluctuates within certain range in a fluctuating manner against SiC, SuJ2, and Al O . The friction 2 3 coefficient of copper block changed in the range of 0.65–0.85 against SiC, Al O , and SUJ2 2 3 counterbodies. When sliding against different counterbodies, the Cu/Al O tribopair has 2 3 the lowest coefficient of friction. Cu/SiC tribopair showed the highest friction coefficient in the steady-state period. Processes 2022, 10, 804 5 of 13 decrease for a period of time; in the steady-state stage, the friction coefficient oscillates continuously within a certain range. The formation of the run-in state stage may be related to the increase of the contact area, the work hardening effect of wear and the accumulation of debris at the pin-disk interface, and the change of the wear mechanism from two-body wear to three-body wear [18]. The reported friction coefficient in this study is the average of these data in the steady stage. Processes 2022, 10, 804 5 of 13 decrease for a period of time; in the steady-state stage, the friction coefficient oscillates continuously within a certain range. The formation of the run-in state stage may be related to the increase of the contact area, the work hardening effect of wear and the accumulation of debris at the pin-disk interface, and the change of the wear mechanism from two-body Processes 2022, 10, 804 5 of 12 wear to three-body wear [18]. The reported friction coefficient in this study is the average of these data in the steady stage. Figure 3. Evolution of friction coefficients of copper block sliding against SiC, Al2O3, and SUJ2 coun- terbodies at 10N. An initial run-in period followed by a steady-state period was observed in all of the tribopairs, while in the run-in stage, the friction coefficient of copper block fluctuates within certain range in a fluctuating manner against SiC, SuJ2, and Al2O3. The friction coefficient of copper block changed in the range of 0.65–0.85 against SiC, Al2O3, and SUJ2 counterbodies. When sliding against different counterbodies, the Cu/Al2O3 tribopair has Figure 3. Evolution of friction coefficients of copper block sliding against SiC, Al O , and SUJ2 2 3 the lowest coefficient of friction. Cu/SiC tribopair showed the highest friction coefficient counterbodies at 10 N. in the steady-state period. Typica Figure 3 l . evolu Evoluttion ion of of fri the ction fr c iction oefficien coeff ts of icient copper s of block Cus -lid Mg ing PG agai cons mpo t SiC, sites Al2O slidi 3, and ng SU agai J2 coun- nst Typical evolution of the friction coefficients of Cu-MgPG composites sliding against terbodies at 10N. SiC, Al2O3, and SUJ2 counterbodies at 10N are shown in Figure 4. The average friction SiC, Al O , and SUJ2 counterbodies at 10 N are shown in Figure 4. The average friction 2 3 coefficients of all of the tribopairs in steady-state period are summarized in Figure 5. coefficients of all of the tribopairs in steady-state period are summarized in Figure 5. An initial run-in period followed by a steady-state period was observed in all of the tribopairs, while in the run-in stage, the friction coefficient of copper block fluctuates within certain range in a fluctuating manner against SiC, SuJ2, and Al2O3. The friction coefficient of copper block changed in the range of 0.65–0.85 against SiC, Al2O3, and SUJ2 counterbodies. When sliding against different counterbodies, the Cu/Al2O3 tribopair has the lowest coefficient of friction. Cu/SiC tribopair showed the highest friction coefficient in the steady-state period. Typical evolution of the friction coefficients of Cu-MgPG composites sliding against Processes 2022, 10, 804 6 of 13 SiC, Al2O3, and SUJ2 counterbodies at 10N are shown in Figure 4. The average friction coefficients of all of the tribopairs in steady-state period are summarized in Figure 5. The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material. The friction coefficient of Cu- MgPG composites changed in the range of 0.15–0.35 against SiC, Al2O3, and SUJ2 counter- bodies. Similar to the results of copper tribopair, Cu-MgPG/Al2O3 tribopair has the lowest Figure 4 coeff . Evoluti icient of f on of rictio friction n. How coefficient ever, sthe of copper/M Cu-MgPG/S gP-UJ graphi 2 trite bopair compshowed osites slithe hig ding aghest ainst fr SiC, ictio n Figure 4. Evolution of friction coefficients of copper/MgP-graphite composites sliding against SiC, Al2O3, acoeff nd SUJ2 count icient in the erbod stead ies a yt 1 -state 0N. pe riod. Al O , and SUJ2 counterbodies at 10 N. 2 3 Figure 4. Evolution of friction coefficients of copper/MgP-graphite composites sliding against SiC, Al2O3, and SUJ2 counterbodies at 10N. Figure 5. The Figu average re 5. T friction he avera coef ge fr ficients iction coefficient of tribopairs. s of tribopairs. From Figure 5, it can be noted that the addition of magnesium phosphate treated graphite decreased the friction coefficient of the sintered composites against each coun- terbody. The specific wear rate ( ) of the sintered disc is displayed in Figure 6, and the re- −6 −6 sults showed WR of sintered copper discs changed in the range of 3.13 × 10 – 6.27 × 10 mm /N·m against Al2O3, SiC, and SUJ2 counterbodies at a load of 10 N. Figure 6. Average specific wear rate of the sintered disc against SiC, Al2O3, SUJ2 counterbodies. For the sintered copper disc, the lowest wear rate disappeared when sliding against Al2O3 counterbody, which is similar to the relationship of the friction coefficient. How- ever, the highest WR disappeared when sliding against SUJ2 counterbody that is different Processes 2022, 10, 804 6 of 13 The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material. The friction coefficient of Cu- MgPG composites changed in the range of 0.15–0.35 against SiC, Al2O3, and SUJ2 counter- bodies. Similar to the results of copper tribopair, Cu-MgPG/Al2O3 tribopair has the lowest coefficient of friction. However, the Cu-MgPG/SUJ2 tribopair showed the highest friction coefficient in the steady-state period. Processes 2022, 10, 804 6 of 12 The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material. The friction coefficient of Figure 5. The average friction coefficients of tribopairs. Cu-MgPG composites changed in the range of 0.15–0.35 against SiC, Al O , and SUJ2 2 3 counterbodies. Similar to the results of copper tribopair, Cu-MgPG/Al O tribopair has the 2 3 From Figure 5, it can be noted that the addition of magnesium phosphate treated lowest coefficient of friction. However, the Cu-MgPG/SUJ2 tribopair showed the highest graphite decreased the friction coefficient of the sintered composites against each coun- friction coefficient in the steady-state period. terbody. From Figure 5, it can be noted that the addition of magnesium phosphate treated graphite decreased the friction coefficient of the sintered composites against each counterbody. The specific wear rate ( ) of the sintered disc is displayed in Figure 6, and the re- The specific wear rate (W R) of the sintered disc is displayed in Figure 6, and the −6 −6 sults showed WR of sintered copper discs changed in the range of 3.13 × 10 – 6.27 × 10 results showed WR of sintered copper discs changed in the range of 3.13  10 –6.27 mm /N·m against Al2O3, SiC, and SUJ2 counterbodies at a load of 10 N. 6 3 10 mm /Nm against Al O , SiC, and SUJ2 counterbodies at a load of 10 N. 2 3 Figure 6. Average specific wear rate of the sintered disc against SiC, Al O , SUJ2 counterbodies. Figure 6. Average specific wear rate of the sintered disc against SiC, Al2O 2 3, 3 SUJ2 counterbodies. For the sintered copper disc, the lowest wear rate disappeared when sliding against For the sintered copper disc, the lowest wear rate disappeared when sliding against Al O counterbody, which is similar to the relationship of the friction coefficient. However, 2 3 Al2O3 counterbody, which is similar to the relationship of the friction coefficient. How- the highest WR disappeared when sliding against SUJ2 counterbody that is different with ever, the highest WR disappeared when sliding against SUJ2 counterbody that is different the result of the friction coefficient. In addition, the WR results with sintered Cu-MgPG disc also shows that the lowest wear rate and highest wear rate disappeared when sliding against Al O and SUJ2 counterbody, respectively. Considering the data in Figures 5 and 6, 2 3 the specific wear rate of the sintered disc against SiC, Al O , SUJ2 counterbodies didn’t 2 3 show obvious dependence on their friction coefficients. At the same time, even though the average friction coefficient of the sintered Cu-MgPG composite materials decreased significantly with sintered copper materials, the specific wear rate increased instead when sliding against SiC and Al O ball. 2 3 The average specific wear rates of SiC, Al O , SUJ2 counterbodies with two sintered 2 3 discs are summarized in Figure 7. It can be found from Figure 7 that Al O ball exhibited lower specific wear rate when 2 3 sliding against two kinds of sintered disc compared with other counterbodies. The specific wear rate of SiC ball was the highest when sliding against sintered Cu-MgPG composite materials which is corresponding to the result shown in Figure 6. From the comparison between Figures 6 and 7, it is clear that the specific wear rates between the sintered disc and counterbodies were consistent. Processes 2022, 10, 804 7 of 13 with the result of the friction coefficient. In addition, the WR results with sintered Cu- MgPG disc also shows that the lowest wear rate and highest wear rate disappeared when sliding against Al2O3 and SUJ2 counterbody, respectively. Considering the data in Figures 5 and 6, the specific wear rate of the sintered disc against SiC, Al2O3, SUJ2 counterbodies didn’t show obvious dependence on their friction coefficients. At the same time, even though the average friction coefficient of the sintered Cu-MgPG composite materials de- creased significantly with sintered copper materials, the specific wear rate increased in- stead when sliding against SiC and Al2O3 ball. Processes 2022, 10, 804 7 of 12 The average specific wear rates of SiC, Al2O3, SUJ2 counterbodies with two sintered discs are summarized in Figure 7. Figu Figure re 7. 7. Av Aerage veragespecific specific wear wear r ra atte es s o of f SSiC, iC, A Al l O 2O3a a nnd d S SU UJ2 J2 co counte unterbro b d odies ies w iwith th di fdiffe ferenrent t sintsin eretd ered disc . 2 3 disc. 3.2. The Morphology Analysis of Worn Surface Processes 2022, 10, 804 8 of 13 It Figur can be foun e 8 shows d from the Fig morphologies ure 7 that Al of 2O worn 3 ball surfaces exhibited l ofow theer specific sintered discs wear after rate whe sliding n sliding tests with again dif st fer two ent kicounterbody nds of sintere . d The disc corr compar esponding ed witelemental h other counterb analysis odies. results The (EDS) specific ar e shown in Figure 9. wear rate of SiC ball was the highest when sliding against sintered Cu-MgPG composite materials which is corresponding to the result shown in Figure 6. From the comparison between Figures 6 and 7, it is clear that the specific wear rates between the sintered disc and counterbodies were consistent. 3.2. The Morphology Analysis of Worn Surface Figure 8 shows the morphologies of worn surfaces of the sintered discs after sliding tests with different counterbody. The corresponding elemental analysis results (EDS) are shown in Figure 9. Figure 8. SEM morphology of the worn surface of the sintered discs that sliding against different Figure 8. SEM morphology of the worn surface of the sintered discs that sliding against different counterbodies (A) Cu/Al O ; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al O ; (E) Cu-MgPG/SiC; 2 3 2 3 counterbodies (A) Cu/Al2O3; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al2O3; (E) Cu-MgPG/SiC; (F) (F) Cu-MgPG/SuJ2. Cu-MgPG/SuJ2. Figure 8A–C displayed the worn surface of the sintered copper sample. While Figure 8D–F displayed the worn surface of the sintered Cu-MgPG sample. It can be seen that after the sliding experiment, the worn surface left on the sintered Cu-MgPG sample is different from sintered copper sample. Clear grooves appear on the wear surface of the graphite-added sintered samples. From the analysis of the results of EDS (Figure 9), it can be inferred that the formation of these grooves is related to the oxides formed on the sur- face. In addition, SEM images of Figure 8 D, E showed that there was a partial detachment of graphite form the inner region of the pockets. For the samples with graphite addition, graphite particles agglomerate in regions between Cu matrix grains, stored in pockets with internal voids [19]. At the same time, it indicates that the reduced density and low theoretical density (as shown in Table 1) of the graphite-containing samples is associated with the internal voids in pockets of graphite present in the composites. Meanwhile, in Figure 8F, graphite particles staying on the worn surface are observed, which are presum- ably exposed from the graphite-storing voids during the friction process. Processes 2022, 10, 804 9 of 13 Processes 2022, 10, 804 8 of 12 Figure 9. EDS analysis of the worn surface of the sintered discs that sliding against different counterbodies (A) Cu/Al O ; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al O ; (E) Cu-MgPG/SiC; 2 3 2 3 Figure 9. EDS analysis of the worn surface of the sintered discs that sliding against different coun- (F) Cu-MgPG/SuJ2. terbodies (A) Cu/Al2O3; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al2O3; (E) Cu-MgPG/SiC; (F) Cu- MgPG/SuJ2. Processes 2022, 10, 804 10 of 13 Elemental analysis of all samples after sliding experiments showed the presence of a large amount of oxygen on the wear track. However, material shedding, plastic defor- mation and numerous grooves can be clearly seen on the wear track of the graphite added sintered samples. There are only a few grooves on the wear track surface of the sintered copper sample. This proves that different oxides are formed on the surface of the two samples. No obvious graphite film formation was observed in the EDS mapping in Figure 9D,E, but observed on the wear track formed by the Cu-MgPG/SuJ2 tribopair shown in Figure 9F, there are some linear tracks formed by the graphite element. Processes 2022, 10, 804 9 of 12 According to the results in Figure 5, it can be known that the lowest friction coeffi- cient of sintered disc appeared when sliding against Al2O3 counterbody, while the hard- Figure 8A–C displayed the worn surface of the sintered copper sample. While ness of the Al2O3 ball is in the middle of the other two counterbodies. It was proved that Figure 8D–F displayed the worn surface of the sintered Cu-MgPG sample. It can be seen the hardness of the counterbody is not the key to determining the friction coefficient. Com- that after the sliding experiment, the worn surface left on the sintered Cu-MgPG sample bined with the EDS results, it is assumed that the reduction in the average friction coeffi- is different from sintered copper sample. Clear grooves appear on the wear surface of cient of Cu/Al2O3 and Cu-MgPG/Al2O3 tribopairs should be related to the oxides produced the graphite-added sintered samples. From the analysis of the results of EDS (Figure 9), it can be inferred that the formation of these grooves is related to the oxides formed on on the worn surface during the friction process. the surface. XPS results in Figures 10 and 11 were also conducted on the sintered copper sample In addition, SEM images of Figure 8 D, E showed that there was a partial detachment and sintered Cu-MgPG sample surfaces after sliding against different counterbodies to of graphite form the inner region of the pockets. For the samples with graphite addition, confirm the oxidation in sliding. Results from XPS analysis for Al2p, O1s, C1s core levels graphite particles agglomerate in regions between Cu matrix grains, stored in pockets with obtained on the worinternal n surfa voids ce of [19 sin ]. tere At the d same samples time, s itlidi indicates ng again that st theAl reduced 2O3 counte density rbo and dy low are theor eti- cal density (as shown in Table 1) of the graphite-containing samples is associated with the provided in Figure 10. internal voids in pockets of graphite present in the composites. Meanwhile, in Figure 8F, As shown in Figure 10A,D, the Al2p peak(P1 + P2 + P3) fitted to the original peak is graphite particles staying on the worn surface are observed, which are presumably exposed composed of three peaks P1, P2, P3 (as revealed by Gaussian curve fitting), each of which from the graphite-storing voids during the friction process. are assigned to different bonds containing Al. The Al2p peak at 75 eV corresponds to the Elemental analysis of all samples after sliding experiments showed the presence of a large amount of oxygen on the wear track. However, material shedding, plastic deformation presence of Al2O3 [20]. Meanwhile, there are also two unknown peaks appeared in copper and numerous grooves can be clearly seen on the wear track of the graphite added sintered sample and Cu-MgPG sample. The fitted O1s peak was deconvoluted into three parts us- samples. There are only a few grooves on the wear track surface of the sintered copper ing Gaussian curve fitting. The two main O1s peak at 530.35 eV and 531.4 eV appeared in sample. This proves that different oxides are formed on the surface of the two samples. No Figure 10B correspond to the presence of CuO [21] and Al2O3 [22] or Al2CuO4 [23] respec- obvious graphite film formation was observed in the EDS mapping in Figure 9D,E, but tively. While as shown observed in Figure on the 10 wear E, the track O1s formed peak a by t 5 the 30.2 Cu-MgPG/SuJ2 eV corresponds tribopair to the shown prese innce Figure 9F, there are some linear tracks formed by the graphite element. of CuO2 [24], O1s peak at 532.0 eV belongs to Al2O3 [25]. The C element in the EDS results According to the results in Figure 5, it can be known that the lowest friction coefficient (Figure 9A) indicates that there is also carbon in the wear surface of the pure copper sam- of sintered disc appeared when sliding against Al O counterbody, while the hardness 2 3 ple. After the analysis of C1s by XPS, the fitted C1s peak was deconvoluted into three sep- of the Al O ball is in the middle of the other two counterbodies. It was proved that the 2 3 arate peaks using Gaussian curve fitting. C1s peak at 284.8 eV as shown in Figure 10C, and hardness of the counterbody is not the key to determining the friction coefficient. Combined with the EDS results, it is assumed that the reduction in the average friction coefficient of 284.9 eV as shown in Figure 10F corresponds to the presence of diamond structure [26,27]. Cu/Al O and Cu-MgPG/Al O tribopairs should be related to the oxides produced on 2 3 2 3 It can be speculated that there are some abrasive particles (diamond) remaining after the the worn surface during the friction process. copper sample is polished. In addition, the C1s peak at 285.7 eV in Figure 10F corresponds XPS results in Figures 10 and 11 were also conducted on the sintered copper sample to the presence of graphite [28]. This proves that during the friction process, there are and sintered Cu-MgPG sample surfaces after sliding against different counterbodies to graphite particles exp confirm osed fthe rom oxidation the grain phi sliding. te-stor Results ing vofr ids omaXPS nd stay analysis on t for he Al fricti , O on, cont C cor act e levels 2p 1s 1s obtained on the worn surface of sintered samples sliding against Al O counterbody are 2 3 surface. provided in Figure 10. Figure 10. XPS analysis of worn surfaces of sintered copper samples (A–C) and sintered Cu-MgPG Figure 10. XPS analysis of worn surfaces of sintered copper samples (A–C) and sintered Cu-MgPG samples (D–F) against Al O counterbody. 2 3 samples (D–F) against Al2O3 counterbody. Processes 2022, 10, 804 11 of 13 XPS analysis of worn surfaces of copper and Cu-MgPG composite materials against SuJ2 counterbody and SiC counterbody is shown in Figure 11. Compared with the analy- sis results of EDS (as shown in Figure 9C), the presence of Mn element was detected on the wear surface with XPS analysis (Figure 11A) after the copper sample sliding against SUJ2 counterbody. It can be speculated that the Mn element comes from the debris of the SUJ2 counterbody remaining on the disc surface. The Mn2p peak at 642.2 eV corresponds to the presence of MnO2 [29] and the peak at 650.0 eV corresponds to the presence of Mn [30]. However, the peak representing the Mn element is not significant on the Cu-MgPG sample. The Si2p peak at 103.1 eV corresponds to the presence of SiO2 [31]. The peak rep- Processes 2022, 10, 804 10 of 12 resenting the Si element is not significant on the Cu/MgPG sample. Figure 11. XPS analysis of worn surfaces of sintered copper and Cu-MgPG samples against (A) SUJ2 Figure 11. XPS analysis of worn surfaces of sintered copper and Cu-MgPG samples against (A) SUJ2 counterbody; (B) SiC counterbody. counterbody; (B) SiC counterbody. As shown in Figure 10A,D, the Al peak(P + P + P ) fitted to the original peak is 2p 1 2 3 Combined with the results of the Specific wear rate shown in Figures 6 and 7, it can composed of three peaks P1, P2, P3 (as revealed by Gaussian curve fitting), each of which are assigned to different bonds containing Al. The Al peak at 75 eV corresponds to the be speculated that the sliding test with Cu-MgPG sample against 2pSUJ2 counterbody can presence of Al O [20]. Meanwhile, there are also two unknown peaks appeared in copper 2 3 reduce the average friction coefficient while reducing wear. sample and Cu-MgPG sample. The fitted O peak was deconvoluted into three parts 1s using Gaussian curve fitting. The two main O peak at 530.35 eV and 531.4 eV appeared 1s 4. Conclusions in Figure 10B correspond to the presence of CuO [21] and Al O [22] or Al CuO [23] 2 3 2 4 respectively. While as shown in Figure 10E, the O peak at 530.2 eV corresponds to the 1s In this study, a Cu-MgPG material was prepared, and the experimental results presence of CuO [24], O peak at 532.0 eV belongs to Al O [25]. The C element in the 2 1s 2 3 proved that the Cu-MgPG material in this study has self-lubricating properties. In addi- EDS results (Figure 9A) indicates that there is also carbon in the wear surface of the pure tion, under fixed experimental conditions (fixed load 10N and sliding speed 0.1 m/s), the copper sample. After the analysis of C by XPS, the fitted C peak was deconvoluted 1s 1s optimum tribopair combination of Cu and Cu-MgPG with the counterbody material, the into three separate peaks using Gaussian curve fitting. C peak at 284.8 eV as shown in 1s Figure 10C, and 284.9 eV as shown in Figure 10F corresponds to the presence of diamond influence of the counterbody materials in terms of coefficient of friction and wear was structure [26,27]. It can be speculated that there are some abrasive particles (diamond) examined, and the following findings were obtained. remaining after the copper sample is polished. In addition, the C peak at 285.7 eV in 1s (1) The friction coefficient detected with the tribopairs of Cu-MgPG composite mate- Figure 10F corresponds to the presence of graphite [28]. This proves that during the friction rial is significantly lower than that of the pure copper material since the added graphite process, there are graphite particles exposed from the graphite-storing voids and stay on acts as a solid lubrica the nt.friction contact surface. XPS analysis of worn surfaces of copper and Cu-MgPG composite materials against (2) The Cu-MgPG/SUJ2 pair was found to decrease both the friction coefficient and SuJ2 counterbody and SiC counterbody is shown in Figure 11. Compared with the analysis specific wear amount compared to the Cu/SUJ2 pair. From the results of XPS and EDS results of EDS (as shown in Figure 9C), the presence of Mn element was detected on the analysis of the sliding surfaces, it is considered that the added MgPG acted as a solid lub- wear surface with XPS analysis (Figure 11A) after the copper sample sliding against SUJ2 ricant and suppressed the oxidation behavior of the material. On the other hand, the Cu- counterbody. It can be speculated that the Mn element comes from the debris of the SUJ2 counterbody remaining on the disc surface. The Mn peak at 642.2 eV corresponds to the MgPG/Al2O3 pair was found to have the lowest coefficient of frictio 2p n and specific wear of presence of MnO [29] and the peak at 650.0 eV corresponds to the presence of Mn [30]. all of the pairs. However, the peak representing the Mn element is not significant on the Cu-MgPG sample. In general, the results of this study have demonstrated the application potential of The Si peak at 103.1 eV corresponds to the presence of SiO [31]. The peak representing 2p 2 Cu/MgPG materials under specific circumstances. However, in the case of practical appli- the Si element is not significant on the Cu/MgPG sample. cation, there are changes Combined in loads with and the slid results ing of speed the Specific s, so the wear mater rate shown ials pre in pa Figur red es in 6 and this 7, it can be speculated that the sliding test with Cu-MgPG sample against SUJ2 counterbody can study cannot be immediately put into the application of brush or bearing materials. In reduce the average friction coefficient while reducing wear. future research, it is necessary to continue studying the effects of sliding loads and sliding speeds on the coefficient of friction and wear with different tribopairs. 4. Conclusions In this study, a Cu-MgPG material was prepared, and the experimental results proved Author Contributions: that data c theura Cu-MgPG tion, R.L. material ; inves in tig this atio study n, R.L.; has wr self-lubricating iting-originapr l draf operti t, R.L es. In .; m addition, ethod- under fixed experimental conditions (fixed load 10 N and sliding speed 0.1 m/s), the optimum ology, S.Y.; resources, H.K. and K.Y.; writing—review and editing, S.Y. and K.Y.; supervision, S.Y. tribopair combination of Cu and Cu-MgPG with the counterbody material, the influence of and H.K.; project administration, H.K.; funding acquisition, H.K. All authors have read and agreed the counterbody materials in terms of coefficient of friction and wear was examined, and to the published version of the manuscript. the following findings were obtained. Processes 2022, 10, 804 11 of 12 (1) The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material since the added graphite acts as a solid lubricant. (2) The Cu-MgPG/SUJ2 pair was found to decrease both the friction coefficient and specific wear amount compared to the Cu/SUJ2 pair. From the results of XPS and EDS analysis of the sliding surfaces, it is considered that the added MgPG acted as a solid lubricant and suppressed the oxidation behavior of the material. On the other hand, the Cu-MgPG/Al O pair was found to have the lowest coefficient of friction and specific wear 2 3 of all of the pairs. In general, the results of this study have demonstrated the application potential of Cu/MgPG materials under specific circumstances. However, in the case of practical application, there are changes in loads and sliding speeds, so the materials prepared in this study cannot be immediately put into the application of brush or bearing materials. In future research, it is necessary to continue studying the effects of sliding loads and sliding speeds on the coefficient of friction and wear with different tribopairs. Author Contributions: Data curation, R.L.; investigation, R.L.; writing-original draft, R.L.; methodol- ogy, S.Y.; resources, H.K. and K.Y.; writing—review and editing, S.Y. and K.Y.; supervision, S.Y. and H.K.; project administration, H.K.; funding acquisition, H.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in this article. Acknowledgments: This work was financially supported by JST SPRING, Grant Number JPMJSP2125. The author (Initial) would like to take this opportunity to thank the “Interdisciplinary Frontier Next- Generation Researcher Program of the Tokai Higher Education and Research System.” This work was partly supported by the DII Collaborative Graduate Program for Accelerating Innovation in Future Electronics, Nagoya University. Conflicts of Interest: The authors declare no conflict of interest. References 1. Kestursatya, M.; Kim, J.; Rohatgi, P. Wear performance of copper–graphite composite and a leaded copper alloy. Mater. Sci. Eng. A 2003, 339, 150–158. [CrossRef] 2. Kato, H.; Takama, M.; Iwai, Y.; Washida, K.; Sasaki, Y. Wear and mechanical properties of sintered copper–tin composites containing graphite or molybdenum disulfide. Wear 2003, 255, 573–578. [CrossRef] 3. Rohatgi, P.K.; Ray, S.; Liu, Y. Tribological properties of metal matrix-graphite particle composites. Int. Mater. 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Effect of Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared by Powder Metallurgy

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processes Article Effect of Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared by Powder Metallurgy 1 , 1 , 2 1 Ruoxuan Li *, Seiji Yamashita *, Katsumi Yoshida and Hideki Kita Department of Materials Process Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan; kita.hideki@material.nagoya-u.ac.jp Laboratory for Zero-Carbon Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8550, Japan; k-yoshida@zc.iir.titech.ac.jp * Correspondence: li.ruoxuan.a4@s.mail.nagoya-u.ac.jp (R.L.); yamashita.seiji@material.nagoya-u.ac.jp (S.Y.) Abstract: The purpose of this study was to investigate the influence of different counterbodies against Cu/magnesium phosphate treated graphite (Cu-MgPG) composite materials to find the best material combination in terms of friction coefficient and specific wear amount. A Cu matrix composite reinforced with 10 vol% magnesium phosphate treated graphite and pure Cu powder were prepared by powder metallurgy techniques under the same consolidation processing condition. The friction and wear properties of the composites were investigated at 10 N using a pin-on-disc tribometer on Al O , SiC and SUJ2 bearing steel counterbodies. The Cu-MgPG/Al O pair showed 2 3 2 3 the lowest friction coefficient, but the specific wear rate tended to increase slightly when compared with Cu/Al O pair. On the other hands, the Cu-MgPG/SUJ2 pair showed about the same specific 2 3 wear rate as the Cu/SUJ2 pair, but the friction coefficient was significantly reduced. These phenomena are thought to be due to the fact that the added graphite acts as a solid lubricant during sliding and also suppresses the oxidation behavior of the sliding material. Citation: Li, R.; Yamashita, S.; Keywords: graphite; Magnesium Phosphate; sliding wear; copper-matrix composite; friction; wear; Yoshida, K.; Kita, H. Effect of counterbody Counterbody on Friction and Wear Properties of Copper-MgP-Graphite Composites Prepared by Powder Metallurgy. Processes 2022, 10, 804. https://doi.org/10.3390/pr10050804 1. Introduction Graphite as a common solid lubricant that is often added to metal materials to make Academic Editor: Sung-Churl Choi new materials with self-lubricating properties [1–6]. Received: 24 March 2022 Copper-graphite composites possess both copper and graphite properties, including Accepted: 18 April 2022 excellent thermal and electrical conductivity, solid lubrication, low coefficient of thermal Published: 19 April 2022 expansion and so on, which makes these composite widely used as a brush and bearing Publisher’s Note: MDPI stays neutral material in many applications [3,7]. In fact, components in this type of application are often with regard to jurisdictional claims in exposed to high friction, temperature rise and environmental erosion [8]. The good friction published maps and institutional affil- properties of copper-graphite composites are often due to the fact that the graphite particles iations. exposed on the surface of the composites act as lubricants during the friction process [9–11]. Considering that graphite is prone to oxidation in high temperature environments, the operating temperature range of ordinary copper-graphite composites is also limited. In a previous work [12], it proposed a method to improve the oxidation resistance of Copyright: © 2022 by the authors. graphite while maintaining the lubricating properties of graphite. In this study, the mag- Licensee MDPI, Basel, Switzerland. nesium phosphate-treated graphite (MgPG) was used as a solid lubricating phase to mix This article is an open access article with copper powder, and used powder metallurgy technique to make copper/magnesium distributed under the terms and phosphate-treated graphite composite materials (Cu-MgPG). conditions of the Creative Commons In the preparation of copper-graphite composite materials, many studies [9,13–16] Attribution (CC BY) license (https:// have investigated the effect of the content of graphite added on the properties of the final creativecommons.org/licenses/by/ composites. While this work aims to study the friction properties of composites made 4.0/). Processes 2022, 10, 804. https://doi.org/10.3390/pr10050804 https://www.mdpi.com/journal/processes Processes 2022, 10, 804 2 of 12 of 10 vol% magnesium phosphate treated graphite and copper. Research [14] has shown that when the volume content of graphite is less than 50%, sintering temperature lies in 700 C–950 C to improve the metallurgy bonding between copper and copper in the material. At the same time, the raw material with treated graphite [12] used in this study was produced by sintered natural graphite with magnesium phosphate at 800 C. Based on the above data, the sintering temperature of the composite material selected in this study was 800 C. Many scholars have studied the tribological properties of copper-graphite composite materials under different conditions, and the results show that the tribological behavior of copper-graphite composite materials is complex and dependent on external factors. These important external factors include applied load, sliding velocity, ambient temperature, humidity, and counterbody material. However, there are few studies on the effect of counterbody materials on the friction properties of copper-graphite composite materials. In this study, powder metallurgy technique was used to prepare Cu-MgPG composite materials and a comparative sample of pure copper (Cu) materials prepared under the same conditions. Then we compared the physical properties, friction properties and surface microstructure of the pure copper materials and Cu-MgPG composites. In addition, in order to find the optimum tribopair combination of Cu and Cu-MgPG with the counterbody materials, the influence of the counterbody materials in terms of coefficient of friction and wear was investigated. 2. Materials and Methods 2.1. Preparation of the Samples The magnesium phosphate-treated graphite (MgPG) raw material used in this study was prepared according to the method of previous study [12]. The arithmetic average particle size of the magnesium phosphate-treated graphite (natural graphite powder, D50 = 93.16 m; Mg(H PO ) 4H O, MW:290.34, JUNSEIKAGAKU) used in this work 2 4 2 2 was 131.70 m. The composites were fabricated by powder metallurgy technique. 10 vol% MgPG powder and copper powder (Nilaco, Ginza, Tokyo, Japan) were dry-mixed with a bench kneader machine (Irie Shokai Co., Ltd., Tokyo, Japan) for 3 h. After mixing, the powder mixture was first cold pressed at 30 MPa in 35 mm  35 mm square mold, and then hot- pressing in nitrogen flow (2 L/min) at 800 C for 3 h with a uniaxial pressure of 40 kN. For comparison, parallel compacts made from pure copper powders were consolidated under the same conditions applied for the preparation of Cu-MgPG composites. All specimens were polished to get a mirror-effect and cleaned with acetone before every experiment. The polished surfaces were then observed by SEM (JEOL Ltd., JSM-7500F, Tokyo Japan). The typical SEM micrograph of the polished surface of Cu-MgPG composites and copper are Processes 2022, 10, 804 3 of 13 shown in Figure 1. The magnesium phosphate-treated graphite particles dispersed in the copper matrix can be observed in Figure 1A. Figure 1. A typical SEM micrograph of the polished surface of (A) copper/MgP-graphite composites Figure 1. A typical SEM micrograph of the polished surface of (A) copper/MgP-graphite composites and (B) copper. and (B) copper. 2.2. Physical Properties The densities of the sintered copper/MgP-graphite composite and copper block were measured by Archimedes’ method. The theoretical density was calculated by dividing the bulk density of the sintered composite by the theoretical density calculated from the rule 3 3 of mixtures using 8.95 g/cm , and 2.25 g/cm as densities for copper and magnesium phos- phate, respectively. The hardness was investigated using a Micro Vickers hardness tester (HMV-G, SHIMADZU Corp., Kyoto, Japan) under a load of 0.5 kg with a dwell time of 10 s. Each sample was measured five times, and the data obtained were the average value. The physical properties of the sintered composites are shown in Table 1. Table 1. Physical properties of the copper and the sintered composite blocks. Copper Block Cu-MgPG Composite Block 3 3 Bulk density (g/cm ) 8.89 Bulk density (g/cm ) 8.27 3 3 Theoretical density (g/cm ) 8.95 Theoretical density (g/cm ) 8.36 Relative density (%) 99.3 Relative density (%) 98.9 Vickers hardness (Hv) 40.73 Vickers hardness (Hv) 54.83 The density of sintered samples reached 99% of the theoretical density, which demonstrated the efficiency of the powder metallurgy technology in producing high-den- sity materials. From Table 1, it can also be noted that the addition of graphite increased the hardness of copper materials but decreased the apparent density of the sintered com- posites. 2.3. Friction Coefficient and Wear Test The friction coefficient and wear sliding tests was carried out in a pin-on-disc tribo- meter (T-18-0162, NANOVEA Corp., USA) in ambient conditions (20 ± 0.5 °C and 45% ± 5% RH). The tribological properties of the prepared composite materials were investi- gated in a dry sliding test. Counterbodies used in the sliding test are 8 mm diameter pol- ished balls (the average surface roughness is less than 0.01 μm) made from commercially available balls-SiC, Al2O3, SUJ2 bearing steel (Sato Tekkou Corp., Japan). The hardness values of SiC, Al2O3, and SUJ2 balls are 2400 HV, 1600 HV, and 770 HV, respectively, which are obtained from the supplier. Before test, both the composite materials and the counterbody were ultrasonically cleaned in an acetone bath for 10 min. Sliding friction and wear tests were performed with a circular sliding under a load of 10 N and the sliding velocity was 0.1 m/s, the wear track radius was 5 mm, and the total sliding distance was 2000 m. There are no other lubricants added in each sliding test. Figure 2 shows the sche- matic illustration of the sliding tester. Three tests were performed for each counterbody. The friction coefficients were continuously recorded and the wear volume on each sample Processes 2022, 10, 804 3 of 12 2.2. Physical Properties The densities of the sintered copper/MgP-graphite composite and copper block were measured by Archimedes’ method. The theoretical density was calculated by dividing the bulk density of the sintered composite by the theoretical density calculated from the 3 3 rule of mixtures using 8.95 g/cm , and 2.25 g/cm as densities for copper and magnesium phosphate, respectively. The hardness was investigated using a Micro Vickers hardness tester (HMV-G, SHIMADZU Corp., Kyoto, Japan) under a load of 0.5 kg with a dwell time of 10 s. Each sample was measured five times, and the data obtained were the average value. The physical properties of the sintered composites are shown in Table 1. Table 1. Physical properties of the copper and the sintered composite blocks. Copper Block Cu-MgPG Composite Block 3 3 Bulk density (g/cm ) 8.89 Bulk density (g/cm ) 8.27 3 3 Theoretical density (g/cm ) 8.95 Theoretical density (g/cm ) 8.36 Relative density (%) 99.3 Relative density (%) 98.9 Vickers hardness (Hv) 40.73 Vickers hardness (Hv) 54.83 The density of sintered samples reached 99% of the theoretical density, which demon- strated the efficiency of the powder metallurgy technology in producing high-density materials. From Table 1, it can also be noted that the addition of graphite increased the hardness of copper materials but decreased the apparent density of the sintered composites. 2.3. Friction Coefficient and Wear Test The friction coefficient and wear sliding tests was carried out in a pin-on-disc tribome- ter (T-18-0162, NANOVEA Corp., Irvine, CA, USA) in ambient conditions (20  0.5 C and 45  5% RH). The tribological properties of the prepared composite materials were investigated in a dry sliding test. Counterbodies used in the sliding test are 8 mm diameter polished balls (the average surface roughness is less than 0.01 m) made from commercially available balls-SiC, Al O , SUJ2 bearing steel (Sato Tekkou Corp., Oita, Japan). The hard- 2 3 ness values of SiC, Al O , and SUJ2 balls are 2400 HV, 1600 HV, and 770 HV, respectively, 2 3 which are obtained from the supplier. Before test, both the composite materials and the counterbody were ultrasonically cleaned in an acetone bath for 10 min. Sliding friction and wear tests were performed with a circular sliding under a load of 10 N and the sliding velocity was 0.1 m/s, the wear track radius was 5 mm, and the total sliding distance was 2000 m. There are no other lubricants added in each sliding test. Figure 2 shows the schematic illustration of the sliding tester. Three tests were performed for each counterbody. The friction coefficients were continuously recorded and the wear volume on each sample disc and counterbody was calculated from the surface profile traces across the wear track using a surface profilometer (Mitutoyo Corp., 178-570-01, Kawasaki, Japan). The worn volume of the disc (V , mm ) was calculated according to the following equation [17]: S + S + S + S 2 3 1 4 V = 2pr( ) where S (mm ) and r (mm) are the cross-sectional areas of the wear track, each at 90 with respect to the previous one, and sliding radius, respectively. The worn volume of the ball (V , mm ) was calculated according to the following equations [17]: V = ph (R ) 2 2 h = R R r w Processes 2022, 10, 804 4 of 13 disc and counterbody was calculated from the surface profile traces across the wear track using a surface profilometer (Mitutoyo Corp., 178-570-01, Japan). The worn volume of the disc ( , mm ) was calculated according to the following equation [17]: + + + = 2( ) where (mm ) and (mm) are the cross-sectional areas of the wear track, each at 90° with respect to the previous one, and sliding radius, respectively. The worn volume of the ball ( , mm ) was calculated according to the following equations [17]: = ℎ ( − ) Processes 2022, 10, 804 4 of 12 ℎ = − − where ℎ (mm) is the height of the removed material, (mm) is original radius of the ball, and wher e (mm) is t h (mm) he ra is thedius of heightwea of the r sca removed r on the ba material, ll. R (mm) is original radius of the ball, andThe r (mm) specif is ic the wea radius r rate of ( wear , mm scar /N· on m) the was ball. further calculated according to the fol- lowing eq Theuati specific on [17wear ]: rate (W R, mm /Nm) was further calculated according to the following equation [17]: W R = ∙ LS where (mm ) is the wear volume of the disc ( ) or ball ( ), (N) is the normal load, where V (mm ) is the wear volume of the disc (V ) or ball (V ), L (N) is the normal load, d b and (m) is the total sliding distance. and S (m) is the total sliding distance. The worn surfaces of sample disc after sliding test were studied by SEM and EDS to The worn surfaces of sample disc after sliding test were studied by SEM and EDS to identify wear mechanisms. identify wear mechanisms. Figure 2. Schematic illustration of the pin on disc tester. Figure 2. Schematic illustration of the pin on disc tester. 2.4. Evaluation Method of Worn Surface 2.4. Evaluation Method of Worn Surface The surface morphology and elemental distribution of the samples were character- The surface morphology and elemental distribution of the samples were character- ized using a scanning electron microscope (SEM, JSM-7500F, JEOL Corp., Tokyo, Japan) ized using a scanning electron microscope (SEM, JSM-7500F, JEOL Corp., Tokyo, Japan) equipped with energy dispersive X-ray detector (EDS, JEOL Corp., Tokyo, Japan). Worn equipped with energy dispersive X-ray detector (EDS, JEOL Corp., Tokyo, Japan). Worn surfaces of the samples were also investigated by an X-ray photoelectron spectroscope surfaces of the samples were also investigated by an X-ray photoelectron spectroscope (XPS) to understand wear-induced surface chemistry modification. (XPS) to understand wear-induced surface chemistry modification. 3. Results and Discussion 3. Results and Discussion 3.1. Friction Coefficient and Wear Rate 3.1. Friction Coefficient and Wear Rate Typical evolution of the friction coefficients of copper block sliding against SiC, Al O , 2 3 and SUJ2 counterbodies at 10 N are shown in Figure 3. All of the friction coefficient Typical evolution of the friction coefficients of copper block sliding against SiC, diagrams contain two stages such as Figure 3, named the (1) run-in and (2) steady state Al2O3, and SUJ2 counterbodies at 10N are shown in Figure 3. All of the friction coefficient stage. In the run-in stage, the friction coefficient shows a large and irregular increase and diagrams contain two stages such as Figure 3, named the (1) run-in and (2) steady state decrease for a period of time; in the steady-state stage, the friction coefficient oscillates stage. In the run-in stage, the friction coefficient shows a large and irregular increase and continuously within a certain range. The formation of the run-in state stage may be related to the increase of the contact area, the work hardening effect of wear and the accumulation of debris at the pin-disk interface, and the change of the wear mechanism from two-body wear to three-body wear [18]. The reported friction coefficient in this study is the average of these data in the steady stage. An initial run-in period followed by a steady-state period was observed in all of the tribopairs, while in the run-in stage, the friction coefficient of copper block fluctuates within certain range in a fluctuating manner against SiC, SuJ2, and Al O . The friction 2 3 coefficient of copper block changed in the range of 0.65–0.85 against SiC, Al O , and SUJ2 2 3 counterbodies. When sliding against different counterbodies, the Cu/Al O tribopair has 2 3 the lowest coefficient of friction. Cu/SiC tribopair showed the highest friction coefficient in the steady-state period. Processes 2022, 10, 804 5 of 13 decrease for a period of time; in the steady-state stage, the friction coefficient oscillates continuously within a certain range. The formation of the run-in state stage may be related to the increase of the contact area, the work hardening effect of wear and the accumulation of debris at the pin-disk interface, and the change of the wear mechanism from two-body wear to three-body wear [18]. The reported friction coefficient in this study is the average of these data in the steady stage. Processes 2022, 10, 804 5 of 13 decrease for a period of time; in the steady-state stage, the friction coefficient oscillates continuously within a certain range. The formation of the run-in state stage may be related to the increase of the contact area, the work hardening effect of wear and the accumulation of debris at the pin-disk interface, and the change of the wear mechanism from two-body Processes 2022, 10, 804 5 of 12 wear to three-body wear [18]. The reported friction coefficient in this study is the average of these data in the steady stage. Figure 3. Evolution of friction coefficients of copper block sliding against SiC, Al2O3, and SUJ2 coun- terbodies at 10N. An initial run-in period followed by a steady-state period was observed in all of the tribopairs, while in the run-in stage, the friction coefficient of copper block fluctuates within certain range in a fluctuating manner against SiC, SuJ2, and Al2O3. The friction coefficient of copper block changed in the range of 0.65–0.85 against SiC, Al2O3, and SUJ2 counterbodies. When sliding against different counterbodies, the Cu/Al2O3 tribopair has Figure 3. Evolution of friction coefficients of copper block sliding against SiC, Al O , and SUJ2 2 3 the lowest coefficient of friction. Cu/SiC tribopair showed the highest friction coefficient counterbodies at 10 N. in the steady-state period. Typica Figure 3 l . evolu Evoluttion ion of of fri the ction fr c iction oefficien coeff ts of icient copper s of block Cus -lid Mg ing PG agai cons mpo t SiC, sites Al2O slidi 3, and ng SU agai J2 coun- nst Typical evolution of the friction coefficients of Cu-MgPG composites sliding against terbodies at 10N. SiC, Al2O3, and SUJ2 counterbodies at 10N are shown in Figure 4. The average friction SiC, Al O , and SUJ2 counterbodies at 10 N are shown in Figure 4. The average friction 2 3 coefficients of all of the tribopairs in steady-state period are summarized in Figure 5. coefficients of all of the tribopairs in steady-state period are summarized in Figure 5. An initial run-in period followed by a steady-state period was observed in all of the tribopairs, while in the run-in stage, the friction coefficient of copper block fluctuates within certain range in a fluctuating manner against SiC, SuJ2, and Al2O3. The friction coefficient of copper block changed in the range of 0.65–0.85 against SiC, Al2O3, and SUJ2 counterbodies. When sliding against different counterbodies, the Cu/Al2O3 tribopair has the lowest coefficient of friction. Cu/SiC tribopair showed the highest friction coefficient in the steady-state period. Typical evolution of the friction coefficients of Cu-MgPG composites sliding against Processes 2022, 10, 804 6 of 13 SiC, Al2O3, and SUJ2 counterbodies at 10N are shown in Figure 4. The average friction coefficients of all of the tribopairs in steady-state period are summarized in Figure 5. The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material. The friction coefficient of Cu- MgPG composites changed in the range of 0.15–0.35 against SiC, Al2O3, and SUJ2 counter- bodies. Similar to the results of copper tribopair, Cu-MgPG/Al2O3 tribopair has the lowest Figure 4 coeff . Evoluti icient of f on of rictio friction n. How coefficient ever, sthe of copper/M Cu-MgPG/S gP-UJ graphi 2 trite bopair compshowed osites slithe hig ding aghest ainst fr SiC, ictio n Figure 4. Evolution of friction coefficients of copper/MgP-graphite composites sliding against SiC, Al2O3, acoeff nd SUJ2 count icient in the erbod stead ies a yt 1 -state 0N. pe riod. Al O , and SUJ2 counterbodies at 10 N. 2 3 Figure 4. Evolution of friction coefficients of copper/MgP-graphite composites sliding against SiC, Al2O3, and SUJ2 counterbodies at 10N. Figure 5. The Figu average re 5. T friction he avera coef ge fr ficients iction coefficient of tribopairs. s of tribopairs. From Figure 5, it can be noted that the addition of magnesium phosphate treated graphite decreased the friction coefficient of the sintered composites against each coun- terbody. The specific wear rate ( ) of the sintered disc is displayed in Figure 6, and the re- −6 −6 sults showed WR of sintered copper discs changed in the range of 3.13 × 10 – 6.27 × 10 mm /N·m against Al2O3, SiC, and SUJ2 counterbodies at a load of 10 N. Figure 6. Average specific wear rate of the sintered disc against SiC, Al2O3, SUJ2 counterbodies. For the sintered copper disc, the lowest wear rate disappeared when sliding against Al2O3 counterbody, which is similar to the relationship of the friction coefficient. How- ever, the highest WR disappeared when sliding against SUJ2 counterbody that is different Processes 2022, 10, 804 6 of 13 The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material. The friction coefficient of Cu- MgPG composites changed in the range of 0.15–0.35 against SiC, Al2O3, and SUJ2 counter- bodies. Similar to the results of copper tribopair, Cu-MgPG/Al2O3 tribopair has the lowest coefficient of friction. However, the Cu-MgPG/SUJ2 tribopair showed the highest friction coefficient in the steady-state period. Processes 2022, 10, 804 6 of 12 The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material. The friction coefficient of Figure 5. The average friction coefficients of tribopairs. Cu-MgPG composites changed in the range of 0.15–0.35 against SiC, Al O , and SUJ2 2 3 counterbodies. Similar to the results of copper tribopair, Cu-MgPG/Al O tribopair has the 2 3 From Figure 5, it can be noted that the addition of magnesium phosphate treated lowest coefficient of friction. However, the Cu-MgPG/SUJ2 tribopair showed the highest graphite decreased the friction coefficient of the sintered composites against each coun- friction coefficient in the steady-state period. terbody. From Figure 5, it can be noted that the addition of magnesium phosphate treated graphite decreased the friction coefficient of the sintered composites against each counterbody. The specific wear rate ( ) of the sintered disc is displayed in Figure 6, and the re- The specific wear rate (W R) of the sintered disc is displayed in Figure 6, and the −6 −6 sults showed WR of sintered copper discs changed in the range of 3.13 × 10 – 6.27 × 10 results showed WR of sintered copper discs changed in the range of 3.13  10 –6.27 mm /N·m against Al2O3, SiC, and SUJ2 counterbodies at a load of 10 N. 6 3 10 mm /Nm against Al O , SiC, and SUJ2 counterbodies at a load of 10 N. 2 3 Figure 6. Average specific wear rate of the sintered disc against SiC, Al O , SUJ2 counterbodies. Figure 6. Average specific wear rate of the sintered disc against SiC, Al2O 2 3, 3 SUJ2 counterbodies. For the sintered copper disc, the lowest wear rate disappeared when sliding against For the sintered copper disc, the lowest wear rate disappeared when sliding against Al O counterbody, which is similar to the relationship of the friction coefficient. However, 2 3 Al2O3 counterbody, which is similar to the relationship of the friction coefficient. How- the highest WR disappeared when sliding against SUJ2 counterbody that is different with ever, the highest WR disappeared when sliding against SUJ2 counterbody that is different the result of the friction coefficient. In addition, the WR results with sintered Cu-MgPG disc also shows that the lowest wear rate and highest wear rate disappeared when sliding against Al O and SUJ2 counterbody, respectively. Considering the data in Figures 5 and 6, 2 3 the specific wear rate of the sintered disc against SiC, Al O , SUJ2 counterbodies didn’t 2 3 show obvious dependence on their friction coefficients. At the same time, even though the average friction coefficient of the sintered Cu-MgPG composite materials decreased significantly with sintered copper materials, the specific wear rate increased instead when sliding against SiC and Al O ball. 2 3 The average specific wear rates of SiC, Al O , SUJ2 counterbodies with two sintered 2 3 discs are summarized in Figure 7. It can be found from Figure 7 that Al O ball exhibited lower specific wear rate when 2 3 sliding against two kinds of sintered disc compared with other counterbodies. The specific wear rate of SiC ball was the highest when sliding against sintered Cu-MgPG composite materials which is corresponding to the result shown in Figure 6. From the comparison between Figures 6 and 7, it is clear that the specific wear rates between the sintered disc and counterbodies were consistent. Processes 2022, 10, 804 7 of 13 with the result of the friction coefficient. In addition, the WR results with sintered Cu- MgPG disc also shows that the lowest wear rate and highest wear rate disappeared when sliding against Al2O3 and SUJ2 counterbody, respectively. Considering the data in Figures 5 and 6, the specific wear rate of the sintered disc against SiC, Al2O3, SUJ2 counterbodies didn’t show obvious dependence on their friction coefficients. At the same time, even though the average friction coefficient of the sintered Cu-MgPG composite materials de- creased significantly with sintered copper materials, the specific wear rate increased in- stead when sliding against SiC and Al2O3 ball. Processes 2022, 10, 804 7 of 12 The average specific wear rates of SiC, Al2O3, SUJ2 counterbodies with two sintered discs are summarized in Figure 7. Figu Figure re 7. 7. Av Aerage veragespecific specific wear wear r ra atte es s o of f SSiC, iC, A Al l O 2O3a a nnd d S SU UJ2 J2 co counte unterbro b d odies ies w iwith th di fdiffe ferenrent t sintsin eretd ered disc . 2 3 disc. 3.2. The Morphology Analysis of Worn Surface Processes 2022, 10, 804 8 of 13 It Figur can be foun e 8 shows d from the Fig morphologies ure 7 that Al of 2O worn 3 ball surfaces exhibited l ofow theer specific sintered discs wear after rate whe sliding n sliding tests with again dif st fer two ent kicounterbody nds of sintere . d The disc corr compar esponding ed witelemental h other counterb analysis odies. results The (EDS) specific ar e shown in Figure 9. wear rate of SiC ball was the highest when sliding against sintered Cu-MgPG composite materials which is corresponding to the result shown in Figure 6. From the comparison between Figures 6 and 7, it is clear that the specific wear rates between the sintered disc and counterbodies were consistent. 3.2. The Morphology Analysis of Worn Surface Figure 8 shows the morphologies of worn surfaces of the sintered discs after sliding tests with different counterbody. The corresponding elemental analysis results (EDS) are shown in Figure 9. Figure 8. SEM morphology of the worn surface of the sintered discs that sliding against different Figure 8. SEM morphology of the worn surface of the sintered discs that sliding against different counterbodies (A) Cu/Al O ; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al O ; (E) Cu-MgPG/SiC; 2 3 2 3 counterbodies (A) Cu/Al2O3; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al2O3; (E) Cu-MgPG/SiC; (F) (F) Cu-MgPG/SuJ2. Cu-MgPG/SuJ2. Figure 8A–C displayed the worn surface of the sintered copper sample. While Figure 8D–F displayed the worn surface of the sintered Cu-MgPG sample. It can be seen that after the sliding experiment, the worn surface left on the sintered Cu-MgPG sample is different from sintered copper sample. Clear grooves appear on the wear surface of the graphite-added sintered samples. From the analysis of the results of EDS (Figure 9), it can be inferred that the formation of these grooves is related to the oxides formed on the sur- face. In addition, SEM images of Figure 8 D, E showed that there was a partial detachment of graphite form the inner region of the pockets. For the samples with graphite addition, graphite particles agglomerate in regions between Cu matrix grains, stored in pockets with internal voids [19]. At the same time, it indicates that the reduced density and low theoretical density (as shown in Table 1) of the graphite-containing samples is associated with the internal voids in pockets of graphite present in the composites. Meanwhile, in Figure 8F, graphite particles staying on the worn surface are observed, which are presum- ably exposed from the graphite-storing voids during the friction process. Processes 2022, 10, 804 9 of 13 Processes 2022, 10, 804 8 of 12 Figure 9. EDS analysis of the worn surface of the sintered discs that sliding against different counterbodies (A) Cu/Al O ; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al O ; (E) Cu-MgPG/SiC; 2 3 2 3 Figure 9. EDS analysis of the worn surface of the sintered discs that sliding against different coun- (F) Cu-MgPG/SuJ2. terbodies (A) Cu/Al2O3; (B) Cu/SiC; (C) Cu/SuJ2; (D) Cu-MgPG/Al2O3; (E) Cu-MgPG/SiC; (F) Cu- MgPG/SuJ2. Processes 2022, 10, 804 10 of 13 Elemental analysis of all samples after sliding experiments showed the presence of a large amount of oxygen on the wear track. However, material shedding, plastic defor- mation and numerous grooves can be clearly seen on the wear track of the graphite added sintered samples. There are only a few grooves on the wear track surface of the sintered copper sample. This proves that different oxides are formed on the surface of the two samples. No obvious graphite film formation was observed in the EDS mapping in Figure 9D,E, but observed on the wear track formed by the Cu-MgPG/SuJ2 tribopair shown in Figure 9F, there are some linear tracks formed by the graphite element. Processes 2022, 10, 804 9 of 12 According to the results in Figure 5, it can be known that the lowest friction coeffi- cient of sintered disc appeared when sliding against Al2O3 counterbody, while the hard- Figure 8A–C displayed the worn surface of the sintered copper sample. While ness of the Al2O3 ball is in the middle of the other two counterbodies. It was proved that Figure 8D–F displayed the worn surface of the sintered Cu-MgPG sample. It can be seen the hardness of the counterbody is not the key to determining the friction coefficient. Com- that after the sliding experiment, the worn surface left on the sintered Cu-MgPG sample bined with the EDS results, it is assumed that the reduction in the average friction coeffi- is different from sintered copper sample. Clear grooves appear on the wear surface of cient of Cu/Al2O3 and Cu-MgPG/Al2O3 tribopairs should be related to the oxides produced the graphite-added sintered samples. From the analysis of the results of EDS (Figure 9), it can be inferred that the formation of these grooves is related to the oxides formed on on the worn surface during the friction process. the surface. XPS results in Figures 10 and 11 were also conducted on the sintered copper sample In addition, SEM images of Figure 8 D, E showed that there was a partial detachment and sintered Cu-MgPG sample surfaces after sliding against different counterbodies to of graphite form the inner region of the pockets. For the samples with graphite addition, confirm the oxidation in sliding. Results from XPS analysis for Al2p, O1s, C1s core levels graphite particles agglomerate in regions between Cu matrix grains, stored in pockets with obtained on the worinternal n surfa voids ce of [19 sin ]. tere At the d same samples time, s itlidi indicates ng again that st theAl reduced 2O3 counte density rbo and dy low are theor eti- cal density (as shown in Table 1) of the graphite-containing samples is associated with the provided in Figure 10. internal voids in pockets of graphite present in the composites. Meanwhile, in Figure 8F, As shown in Figure 10A,D, the Al2p peak(P1 + P2 + P3) fitted to the original peak is graphite particles staying on the worn surface are observed, which are presumably exposed composed of three peaks P1, P2, P3 (as revealed by Gaussian curve fitting), each of which from the graphite-storing voids during the friction process. are assigned to different bonds containing Al. The Al2p peak at 75 eV corresponds to the Elemental analysis of all samples after sliding experiments showed the presence of a large amount of oxygen on the wear track. However, material shedding, plastic deformation presence of Al2O3 [20]. Meanwhile, there are also two unknown peaks appeared in copper and numerous grooves can be clearly seen on the wear track of the graphite added sintered sample and Cu-MgPG sample. The fitted O1s peak was deconvoluted into three parts us- samples. There are only a few grooves on the wear track surface of the sintered copper ing Gaussian curve fitting. The two main O1s peak at 530.35 eV and 531.4 eV appeared in sample. This proves that different oxides are formed on the surface of the two samples. No Figure 10B correspond to the presence of CuO [21] and Al2O3 [22] or Al2CuO4 [23] respec- obvious graphite film formation was observed in the EDS mapping in Figure 9D,E, but tively. While as shown observed in Figure on the 10 wear E, the track O1s formed peak a by t 5 the 30.2 Cu-MgPG/SuJ2 eV corresponds tribopair to the shown prese innce Figure 9F, there are some linear tracks formed by the graphite element. of CuO2 [24], O1s peak at 532.0 eV belongs to Al2O3 [25]. The C element in the EDS results According to the results in Figure 5, it can be known that the lowest friction coefficient (Figure 9A) indicates that there is also carbon in the wear surface of the pure copper sam- of sintered disc appeared when sliding against Al O counterbody, while the hardness 2 3 ple. After the analysis of C1s by XPS, the fitted C1s peak was deconvoluted into three sep- of the Al O ball is in the middle of the other two counterbodies. It was proved that the 2 3 arate peaks using Gaussian curve fitting. C1s peak at 284.8 eV as shown in Figure 10C, and hardness of the counterbody is not the key to determining the friction coefficient. Combined with the EDS results, it is assumed that the reduction in the average friction coefficient of 284.9 eV as shown in Figure 10F corresponds to the presence of diamond structure [26,27]. Cu/Al O and Cu-MgPG/Al O tribopairs should be related to the oxides produced on 2 3 2 3 It can be speculated that there are some abrasive particles (diamond) remaining after the the worn surface during the friction process. copper sample is polished. In addition, the C1s peak at 285.7 eV in Figure 10F corresponds XPS results in Figures 10 and 11 were also conducted on the sintered copper sample to the presence of graphite [28]. This proves that during the friction process, there are and sintered Cu-MgPG sample surfaces after sliding against different counterbodies to graphite particles exp confirm osed fthe rom oxidation the grain phi sliding. te-stor Results ing vofr ids omaXPS nd stay analysis on t for he Al fricti , O on, cont C cor act e levels 2p 1s 1s obtained on the worn surface of sintered samples sliding against Al O counterbody are 2 3 surface. provided in Figure 10. Figure 10. XPS analysis of worn surfaces of sintered copper samples (A–C) and sintered Cu-MgPG Figure 10. XPS analysis of worn surfaces of sintered copper samples (A–C) and sintered Cu-MgPG samples (D–F) against Al O counterbody. 2 3 samples (D–F) against Al2O3 counterbody. Processes 2022, 10, 804 11 of 13 XPS analysis of worn surfaces of copper and Cu-MgPG composite materials against SuJ2 counterbody and SiC counterbody is shown in Figure 11. Compared with the analy- sis results of EDS (as shown in Figure 9C), the presence of Mn element was detected on the wear surface with XPS analysis (Figure 11A) after the copper sample sliding against SUJ2 counterbody. It can be speculated that the Mn element comes from the debris of the SUJ2 counterbody remaining on the disc surface. The Mn2p peak at 642.2 eV corresponds to the presence of MnO2 [29] and the peak at 650.0 eV corresponds to the presence of Mn [30]. However, the peak representing the Mn element is not significant on the Cu-MgPG sample. The Si2p peak at 103.1 eV corresponds to the presence of SiO2 [31]. The peak rep- Processes 2022, 10, 804 10 of 12 resenting the Si element is not significant on the Cu/MgPG sample. Figure 11. XPS analysis of worn surfaces of sintered copper and Cu-MgPG samples against (A) SUJ2 Figure 11. XPS analysis of worn surfaces of sintered copper and Cu-MgPG samples against (A) SUJ2 counterbody; (B) SiC counterbody. counterbody; (B) SiC counterbody. As shown in Figure 10A,D, the Al peak(P + P + P ) fitted to the original peak is 2p 1 2 3 Combined with the results of the Specific wear rate shown in Figures 6 and 7, it can composed of three peaks P1, P2, P3 (as revealed by Gaussian curve fitting), each of which are assigned to different bonds containing Al. The Al peak at 75 eV corresponds to the be speculated that the sliding test with Cu-MgPG sample against 2pSUJ2 counterbody can presence of Al O [20]. Meanwhile, there are also two unknown peaks appeared in copper 2 3 reduce the average friction coefficient while reducing wear. sample and Cu-MgPG sample. The fitted O peak was deconvoluted into three parts 1s using Gaussian curve fitting. The two main O peak at 530.35 eV and 531.4 eV appeared 1s 4. Conclusions in Figure 10B correspond to the presence of CuO [21] and Al O [22] or Al CuO [23] 2 3 2 4 respectively. While as shown in Figure 10E, the O peak at 530.2 eV corresponds to the 1s In this study, a Cu-MgPG material was prepared, and the experimental results presence of CuO [24], O peak at 532.0 eV belongs to Al O [25]. The C element in the 2 1s 2 3 proved that the Cu-MgPG material in this study has self-lubricating properties. In addi- EDS results (Figure 9A) indicates that there is also carbon in the wear surface of the pure tion, under fixed experimental conditions (fixed load 10N and sliding speed 0.1 m/s), the copper sample. After the analysis of C by XPS, the fitted C peak was deconvoluted 1s 1s optimum tribopair combination of Cu and Cu-MgPG with the counterbody material, the into three separate peaks using Gaussian curve fitting. C peak at 284.8 eV as shown in 1s Figure 10C, and 284.9 eV as shown in Figure 10F corresponds to the presence of diamond influence of the counterbody materials in terms of coefficient of friction and wear was structure [26,27]. It can be speculated that there are some abrasive particles (diamond) examined, and the following findings were obtained. remaining after the copper sample is polished. In addition, the C peak at 285.7 eV in 1s (1) The friction coefficient detected with the tribopairs of Cu-MgPG composite mate- Figure 10F corresponds to the presence of graphite [28]. This proves that during the friction rial is significantly lower than that of the pure copper material since the added graphite process, there are graphite particles exposed from the graphite-storing voids and stay on acts as a solid lubrica the nt.friction contact surface. XPS analysis of worn surfaces of copper and Cu-MgPG composite materials against (2) The Cu-MgPG/SUJ2 pair was found to decrease both the friction coefficient and SuJ2 counterbody and SiC counterbody is shown in Figure 11. Compared with the analysis specific wear amount compared to the Cu/SUJ2 pair. From the results of XPS and EDS results of EDS (as shown in Figure 9C), the presence of Mn element was detected on the analysis of the sliding surfaces, it is considered that the added MgPG acted as a solid lub- wear surface with XPS analysis (Figure 11A) after the copper sample sliding against SUJ2 ricant and suppressed the oxidation behavior of the material. On the other hand, the Cu- counterbody. It can be speculated that the Mn element comes from the debris of the SUJ2 counterbody remaining on the disc surface. The Mn peak at 642.2 eV corresponds to the MgPG/Al2O3 pair was found to have the lowest coefficient of frictio 2p n and specific wear of presence of MnO [29] and the peak at 650.0 eV corresponds to the presence of Mn [30]. all of the pairs. However, the peak representing the Mn element is not significant on the Cu-MgPG sample. In general, the results of this study have demonstrated the application potential of The Si peak at 103.1 eV corresponds to the presence of SiO [31]. The peak representing 2p 2 Cu/MgPG materials under specific circumstances. However, in the case of practical appli- the Si element is not significant on the Cu/MgPG sample. cation, there are changes Combined in loads with and the slid results ing of speed the Specific s, so the wear mater rate shown ials pre in pa Figur red es in 6 and this 7, it can be speculated that the sliding test with Cu-MgPG sample against SUJ2 counterbody can study cannot be immediately put into the application of brush or bearing materials. In reduce the average friction coefficient while reducing wear. future research, it is necessary to continue studying the effects of sliding loads and sliding speeds on the coefficient of friction and wear with different tribopairs. 4. Conclusions In this study, a Cu-MgPG material was prepared, and the experimental results proved Author Contributions: that data c theura Cu-MgPG tion, R.L. material ; inves in tig this atio study n, R.L.; has wr self-lubricating iting-originapr l draf operti t, R.L es. In .; m addition, ethod- under fixed experimental conditions (fixed load 10 N and sliding speed 0.1 m/s), the optimum ology, S.Y.; resources, H.K. and K.Y.; writing—review and editing, S.Y. and K.Y.; supervision, S.Y. tribopair combination of Cu and Cu-MgPG with the counterbody material, the influence of and H.K.; project administration, H.K.; funding acquisition, H.K. All authors have read and agreed the counterbody materials in terms of coefficient of friction and wear was examined, and to the published version of the manuscript. the following findings were obtained. Processes 2022, 10, 804 11 of 12 (1) The friction coefficient detected with the tribopairs of Cu-MgPG composite material is significantly lower than that of the pure copper material since the added graphite acts as a solid lubricant. (2) The Cu-MgPG/SUJ2 pair was found to decrease both the friction coefficient and specific wear amount compared to the Cu/SUJ2 pair. From the results of XPS and EDS analysis of the sliding surfaces, it is considered that the added MgPG acted as a solid lubricant and suppressed the oxidation behavior of the material. On the other hand, the Cu-MgPG/Al O pair was found to have the lowest coefficient of friction and specific wear 2 3 of all of the pairs. In general, the results of this study have demonstrated the application potential of Cu/MgPG materials under specific circumstances. However, in the case of practical application, there are changes in loads and sliding speeds, so the materials prepared in this study cannot be immediately put into the application of brush or bearing materials. In future research, it is necessary to continue studying the effects of sliding loads and sliding speeds on the coefficient of friction and wear with different tribopairs. Author Contributions: Data curation, R.L.; investigation, R.L.; writing-original draft, R.L.; methodol- ogy, S.Y.; resources, H.K. and K.Y.; writing—review and editing, S.Y. and K.Y.; supervision, S.Y. and H.K.; project administration, H.K.; funding acquisition, H.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in this article. Acknowledgments: This work was financially supported by JST SPRING, Grant Number JPMJSP2125. The author (Initial) would like to take this opportunity to thank the “Interdisciplinary Frontier Next- Generation Researcher Program of the Tokai Higher Education and Research System.” This work was partly supported by the DII Collaborative Graduate Program for Accelerating Innovation in Future Electronics, Nagoya University. Conflicts of Interest: The authors declare no conflict of interest. References 1. Kestursatya, M.; Kim, J.; Rohatgi, P. Wear performance of copper–graphite composite and a leaded copper alloy. Mater. Sci. Eng. A 2003, 339, 150–158. [CrossRef] 2. Kato, H.; Takama, M.; Iwai, Y.; Washida, K.; Sasaki, Y. Wear and mechanical properties of sintered copper–tin composites containing graphite or molybdenum disulfide. Wear 2003, 255, 573–578. [CrossRef] 3. Rohatgi, P.K.; Ray, S.; Liu, Y. Tribological properties of metal matrix-graphite particle composites. Int. Mater. 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Journal

Processes – Multidisciplinary Digital Publishing Institute

Published: Apr 19, 2022

Keywords: graphite; Magnesium Phosphate; sliding wear; copper-matrix composite; friction; wear; counterbody

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