Preparing and Wear-Resisting Property of Al2O3/Cu Composite Material Enhanced Using Novel In Situ Generated Al2O3 Nanoparticles

Al2O3/Cu composite material (ACCM) are highly suitable for various advanced applications owing to its excellent properties. In the present work, a combination of the solution combustion synthesis and hydrogen reduction method was first employed to prepare Al2O3/Cu composite powder (ACCP), and subsequently ACCM was prepared by employing spark plasma sintering (SPS) technique. The effect of Al2O3 contents and SPS temperatures on the properties (relative density, hardness, friction coefficient, and electrical conductivity, et al.) of ACCM were investigated in detail. The results indicated that ACCM was very dense, and microstructure was consisted of fine Al2O3 particles evenly distributed in the Cu matrix. With the increase of SPS temperature, the relative density and hardness of ACCM had first increased and then decreased. At 775 °C, the relative density and hardness had attained the maximum values of 98.19% and 121.4 HV, respectively. With the increase of Al2O3 content, although the relative density of ACCM had gradually decreased, nevertheless, its friction coefficient had increased. Moreover, with the increase of Al2O3 contents, the hardness of ACCM first increased and then decreased, and reached the maximum value (121.4 HV) with 3 wt.% addition. On the contrary, the wear rate of ACCM had first decreased and then increased with the increase of Al2O3 contents, and attained the minimum (2.32 × 10−5 mm3/(N.m)) with 3 wt.% addition.


Introduction
During recent years, Cu matrix composites (CuMC) are found to be highly suitable for various advanced applications such as integrated circuit wire frame and spot welding electrode owing to its high thermal and electrical conductivity, superior mechanical properties, and excellent wear resistance [1][2][3]. Scholars, all over the world, have achieved significant results by investigating various aspects of CuMC [4][5][6]. CuMC can be divided into three types such as fiber reinforced, whisker reinforced, and particle reinforced. Among these three composite materials, the CuMC using particle-reinforced have found broader applications owing to its better isotropic properties, simpler preparation processes, and good processing properties. Common reinforcing particles include oxide ceramics (Al 2 O 3, ZrO 2 , SiO 2 [7][8][9], etc.) and non-oxide ceramics (AlN, SiC, TiB 2 [10][11][12], etc.). Out of these reinforcing, Al 2 O 3 with its superior properties can significantly improve the properties of ACCM [13,14]. These properties include high melting point and hardness, stable chemical properties and strong wear resistance. Moreover, abundant natural resources of Al 2 O 3 make its industrial production cost effective. Therefore, the prepared ACCM exhibits

Synthesis Procedure
The preparation of ACCP was comprised of following two steps. Firstly, the preparation of (Al 2 O 3 + CuO) precursor was carried out by SCS method according to the procedure employed in our previous documents [28,29]. Subsequently, the (Al 2 O 3 + CuO) precursor was subjected to the hydrogen reduction reaction in a flowing H 2 at 500 • C for 1 h in a tube furnace to prepare ACCP. The flow rate, employed for hydrogen gas, was 0.5 L/min. Finally, the ACCP was put into the graphite mold and performed the SPS process in vacuum environment to obtain ACCM. The heating rate was 100 • C/min with the vacuum was below 0.01 Pa. The sintering pressure was 30 MPa, and the soaking time was 5 min. The effect of various SPS temperatures (675 • C, 725 • C, 775 • C, 825 • C, 875 • C) on the properties of the ACCM was studied in detail.

Characterization
The specific surface area (SSA) of simple was determined by the BET method by using an Automated Surface Area & Pore Size Analyzer (QUADRASORB SI-MP, Quantachrome Instruments, Florida, and American). X-ray diffraction study of samples was carried out in an X-ray diffractometer using CuKa radiation (XRD, D/max-RB12, Rigaku, Tokyo, Japan). The morphology and particle size of samples were observed by a scanning electron microscope (SEM, JSM-6380LV, JEOL, Wuhan, China) with an energy dispersive X-ray spectrometer (EDS, OXFORD Link-ISIS-300, JEOL, Wuhan, China). The relative density and hardness of five SPS samples were measured by the Archimedes method and Vickers hardness tester (HV-30, Shanghai Daling Optical Instrument Co., LTD., Shanghai, China), respectively. The Vickers hardness was performed with a load of 49.0 N force for 20 s and taking an average over five separate measurements. The electrical conductivity of five SPS samples was tested by digital eddy current metal conductor (Sigma2008, Beijing Staavo Technology Co., LTD., Beijing, China), during testing, it needs to pay attention to the parallel of upper and lower end surface, after testing 12 values, remove the maximum and minimum values and then take the average value as the final electrical conductivity value of this sample. The wear resistance of six SPS samples with a size of 2 mm × 10 mm × 10 mm was tested by the high-speed reciprocating friction wear testing machine with a ball-ondisc tribometer (HSR-2M, Lanzhou Zhongke Kaihua Technology Development Co., LTD., Lanzhou, China). The working condition of friction and wear is: a GCr15 steel grinding ball with a diameter of 4 mm and a hardness of 60 HRC, load of 10 N, rotating speed of 500 r/min, friction time of 20 min, and single stroke of 5 mm, test temperature of 23 • C and humidity of 30%.

Preparation of Al 2 O 3 /Cu Composite Powder
ACCP was prepared by combining the SCS and hydrogen reduction methods. Figure 1 depicts XRD pattern and EDS results of the precursor (a,c) and the reduction products (b,d).
Only the CuO peak is found in Figure 1a and no obvious Al 2 O 3 diffraction peak appears due to the small Al 2 O 3 contents (1 wt.%) or the possibility of Al 2 O 3 in an amorphous state in the precursor [19]. Moreover, the EDS result ( Figure 1c) confirms that the precursor powders only consist of Cu, Al, and O elements. It implies that the precursor is comprised of alumina with cupric oxide (Al 2 O 3 + CuO). Subsequently, the (Al 2 O 3 + CuO) precursor is subjected to the hydrogen reduction reaction to prepare ACCP according to procedure. as mentioned in Section 2.2. Figure 1b,d reveals the XRD and EDS results of the reduction products. The absence of CuO and Al 2 O 3 diffraction peaks in Figure 1b indicates the complete transformation of CuO into Cu. Furthermore, the EDS result ( Figure 1d) confirms that the reduction product only consists of Cu, Al, and O elements. Therefore, it is deduced that the absence of Al 2 O 3 in Figure 1b is still ascribed to the small Al 2 O 3 content or the possibility of Al 2 O 3 in an amorphous state in the precursor [19]. Figure 2 shows SEM images of ACCP with various Al 2 O 3 contents. Compared with the sample without adding Al 2 O 3 (Figure 2a), it is obvious that other samples containing various Al 2 O 3 contents exhibit finer particles and more uniform distribution of particle size, which is accordant with their SSA values (Table 1). It is obvious that the SSA of the sample without adding Al 2 O 3 is the lowest (3.6 m 2 /g). This is because the wettability of Al 2 O 3 with the Cu matrix is very poor [30], which hinders its effective binding with Cu particles. However, The particle size of the sample with 3 wt.% Al 2 O 3 content (Figure 2d) is significantly smaller than that of other samples, indicating that dispersion of this sample is the best among all the samples. It is obvious that the sample with 3 wt.% Al 2 O 3 content is the highest (6.2 m 2 /g) among six samples. With the Al 2 O 3 content beyond 3 wt.% (such as 4 wt.% and 5 wt.%), the particle size of these samples increases (Figure 2e,f), while the dispersion of these samples decrease caused by the increase of agglomeration of Al 2 O 3 particles, which is also agree to their SSA trend (see Table 1). This phenomenon also adversely effects the dispersion of Cu particles resulting in their enhanced agglomeration.   (Figure 2a), it is obvious that other samples containing various Al2O3 contents exhibit finer particles and more uniform distribution of particle size, which is accordant with their SSA values (Table 1). It is obvious that the SSA of the sample without adding Al2O3 is the lowest (3.6 m 2 /g). This is because the wettability of Al2O3 with the Cu matrix is very poor [30], which hinders its effective binding with Cu particles. However, The particle size of the sample with 3 wt.% Al2O3 content ( Figure 2d) is significantly smaller than that of other samples, indicating that dispersion of this sample is the best among all the samples. It is obvious that the sample with 3 wt.% Al2O3 content is the highest (6.2 m 2 /g) among six samples. With the Al2O3 content beyond 3 wt.% (such as 4 wt.% and 5 wt.%), the particle size of these samples increases (Figure 2e,f), while the dispersion of these samples decrease caused by the increase of agglomeration of Al2O3 particles, which is also agree to their SSA trend (see Table 1). This phenomenon also adversely effects the dispersion of Cu particles resulting in their enhanced agglomeration.  6, x FOR PEER REVIEW 5 of 15

Effect of SPS Temperature on the Density and Hardness of ACCM
It is always difficult to fabricate high density ACCM by pressureless sintering technique due to much difference in the melting points of Al2O3 and Cu. Moreover, grain growth will also occur during liquid phase sintering at high temperature, which will deteriorate the performance of ACCM. The SPS is a new and efficient sintering method which has the various advantages over other techniques. These include fast heating/cooling rates, short sintering time, high density, fine grain growth, short preparation cycle, energy saving, and so on [31,32]. Therefore, the SPS method is utilized to prepare ACCM in the present work. Figure 3 shows the relative density and hardness of ACCM with 3 wt.% Al2O3 prepared at various SPS temperatures. It is obvious that with the increase of SPS temperature from 675 °C to 775 °C, the relative density of ACCM increases from 82.49% to 98.19%. At lower SPS temperatures (such as 675 °C), the diffusion rate and atomic movement are slow. With the increase of SPS temperature, the sintering neck grows faster interatomic distance decreases resulting in an increase in the relative density of ACCM. However, the relative density of ACCM decreases slightly with the constant increase of SPS temperatures (such as 775 °C to 875 °C). The higher SPS temperaturse result in the growth of grains (see Figure 4), which reduces overall the relative density of the material. Moreover, at higher SPS temperatures such as 775 °C, the gas in the sintering mass void expands at high temperatures, which also deteriorates the density of ACCM.

Effect of SPS Temperature on the Density and Hardness of ACCM
It is always difficult to fabricate high density ACCM by pressureless sintering technique due to much difference in the melting points of Al 2 O 3 and Cu. Moreover, grain growth will also occur during liquid phase sintering at high temperature, which will deteriorate the performance of ACCM. The SPS is a new and efficient sintering method which has the various advantages over other techniques. These include fast heating/cooling rates, short sintering time, high density, fine grain growth, short preparation cycle, energy saving, and so on [31,32]. Therefore, the SPS method is utilized to prepare ACCM in the present work. Figure 3 shows the relative density and hardness of ACCM with 3 wt.% Al 2 O 3 prepared at various SPS temperatures. It is obvious that with the increase of SPS temperature from 675 • C to 775 • C, the relative density of ACCM increases from 82.49% to 98.19%. At lower SPS temperatures (such as 675 • C), the diffusion rate and atomic movement are slow. With the increase of SPS temperature, the sintering neck grows faster interatomic distance decreases resulting in an increase in the relative density of ACCM. However, the relative density of ACCM decreases slightly with the constant increase of SPS temperatures (such as 775 • C to 875 • C). The higher SPS temperaturse result in the growth of grains (see Figure 4), which reduces overall the relative density of the material. Moreover, at higher SPS temperatures such as 775 • C, the gas in the sintering mass void expands at high temperatures, which also deteriorates the density of ACCM.
It is also obvious in Figure 3 that the hardness of ACCM first increases and then decreases with the increase of SPS temperature. The hardness value reaches at the highest at 775 • C (121.4 HV). Such behavior occurs because with the increase of SPS temperature upto 775 • C, the internal gap defects of ACCM decreases and the relative density of ACCM increases, resulting an increase in the hardness of ACCM. With the SPS temperature continues to rise above 775 • C (such as 825 • C), the hardness of ACCM decreases due to grain coarsening effect. Materials 2023, 16, x FOR PEER REVIEW 6 of 15 It is also obvious in Figure 3 that the hardness of ACCM first increases and then decreases with the increase of SPS temperature. The hardness value reaches at the highest at 775 °C (121.4 HV). Such behavior occurs because with the increase of SPS temperature upto 775 °C, the internal gap defects of ACCM decreases and the relative density of ACCM increases, resulting an increase in the hardness of ACCM. With the SPS temperature continues to rise above 775 °C (such as 825 °C), the hardness of ACCM decreases due to grain coarsening effect. Figure 4 shows the fracture morphology of ACCM with 3 wt.% Al2O3 contents at various SPS temperatures. It is obvious that at lower SPS temperature of 675 °C, the sample exhibits a flat fracture surface with the grains exhibiting a strong cubic effect. The sample also exhibits a very clear grain boundary with hole at the fracture surface due to the low sintering temperature and short sintering time [33]. However, with SPS temperature reaching at 725 °C, the grain boundary becomes faded with less number of holes at grain boundary. This sample exhibits the highest value of relative density (see Figure 3). At higher SPS temperature range of 775 °C to 875 °C, a large number of dimples appear at the fracture surface, indicating the strong deformation at fracture surface and the weak bonding among grains in the ACCM composite.   Figure 5 shows the relative density and hardness of ACCM with various Al2O3 contents prepared by SPS method at 775 °C. With the Al2O3 contents increasing from 0 wt.% to 5 wt.%, the relative density of ACCM decreases from 99.89% to 95.32%. This phenome-  Figure 4 shows the fracture morphology of ACCM with 3 wt.% Al 2 O 3 contents at various SPS temperatures. It is obvious that at lower SPS temperature of 675 • C, the sample exhibits a flat fracture surface with the grains exhibiting a strong cubic effect. The sample also exhibits a very clear grain boundary with hole at the fracture surface due to the low sintering temperature and short sintering time [33]. However, with SPS temperature reaching at 725 • C, the grain boundary becomes faded with less number of holes at grain boundary. This sample exhibits the highest value of relative density (see Figure 3). At higher SPS temperature range of 775 • C to 875 • C, a large number of dimples appear at the fracture surface, indicating the strong deformation at fracture surface and the weak bonding among grains in the ACCM composite. Figure 5 shows the relative density and hardness of ACCM with various Al 2 O 3 contents prepared by SPS method at 775 • C. With the Al 2 O 3 contents increasing from 0 wt.% to 5 wt.%, the relative density of ACCM decreases from 99.89% to 95.32%. This phenomenon can be ascribed to various reasons. Firstly, the presence of higher Al 2 O 3 contents will reduce the compressibility of Cu particles resulting in the lowering of the density [33]. Secondly, the diffusion of Cu particles is hindered by the distribution of Al 2 O 3 particles at the Cu crystal boundary, thus further deteriorating the compressibility of the ACCM [34]. Thirdly, the sinterability is also deteriorated by the of increase in interface area of ACCM caused by the addition of Al 2 O 3 contents. The hardness of ACCM first increases with the increase of Al2O3 conte decreases. With the Al2O3 contents increases from 0 wt.% to 3 wt.%, the hardne increases from 78.3 HV to 121.4 HV. However, with the Al2O3 contents abov hardness of ACCM slightly decreases from 121.4 HV (3 wt.%) to 106.4 HV (5 the increasing Al2O3 contents. It is worth noting that the hardness of ACCM Al2O3 contents is still higher than that of the sintered pure Cu sample (0 wt.% that the increase of Al2O3 particles can significantly improve the hardness of phenomenon can be ascribed to following reasons. Firstly, Al2O3 exists as fin inforcement in ACCM, reducing the expansion of the strain zone inside ACCM improving the ability of ACCM to resist plastic deformation [35]. Secondly, o ness of ACCM improves due to the higher hardness of Al2O3 than that o Thirdly, the addition of Al2O3 increases the crystal boundary and the hardne by preventing Cu grains from growing up [36]. Fourthly, the addition of Al2 It is worth noting that the hardness of ACCM with various Al 2 O 3 contents is still higher than that of the sintered pure Cu sample (0 wt.%), indicating that the increase of Al 2 O 3 particles can significantly improve the hardness of ACCM. This phenomenon can be ascribed to following reasons. Firstly, Al 2 O 3 exists as fine particle reinforcement in ACCM, reducing the expansion of the strain zone inside ACCM, and hence improving the ability of ACCM to resist plastic deformation [35]. Secondly, overall hardness of ACCM improves due to the higher hardness of Al 2 O 3 than that of Cu matrix. Thirdly, the addition of Al 2 O 3 increases the crystal boundary and the hardness of ACCM by preventing Cu grains from growing up [36]. Fourthly, the addition of Al 2 O 3 beyond 3 wt.% reduces the density of composite by increasing voids and defects in it. This reduction in density leads to slight decrease in ACCM hardness with the continuous increase in Al 2 O 3 contents.

Effect of Al 2 O 3 Content on the Relative Density and Hardness of ACCM
According to the sintering theory, the sintering process involves various mechanisms such as gas overflow in the block, the reduction of pores, the formation of the sintering neck and the process of interparticle locking. Figure 6 shows the fracture morphology of ACCM prepared with various Al 2 O 3 contents. With the Al 2 O 3 contents in the range of 0 wt.% to 3 wt.%, the fracture mode of ACCM is ductile rupture with the formation of the dimples due to their good plasticity [37]. With the Al 2 O 3 contents in the range of 4-5 wt.%, the fracture mode is mainly ductile rupture with the appearance of crystal fracture phenomenon. It is also worth noting that with the Al 2 O 3 contents ranging from 0 wt.% to 2 wt.%, the dimple size of ACCM is large and evenly distributed (Figure 6a-c). With the Al 2 O 3 contents in the range of 3-5 wt.%, the number of the dimples of ACCM significantly reduces, and the average size of dimple is small and all dimples are evenly distributed (Figure 6d-f). In addition, there are some holes appear in the ACCM. The concave and convex shaped fracture appears, with cracks on it. However, the relatively clean interface between Al 2 O 3 particles and the Cu matrix exhibit some gaps at the interface, indicating that the binding strength between Al 2 O 3 particles and copper matrix is poor. Moreover, it also indicates that ACCM with Al 2 O 3 contents in the range of 1-3 wt.% exhibits higher strength than that containing higher Al 2 O 3 contents such as 4 wt.% and 5 wt.%. Moreover, it can be seen clearly that Al 2 O 3 particles in samples with higher Al 2 O 3 contents (such as 4 wt.% or 5 wt.%) agglomerate at the crystal boundary resulting in weak interface between Al 2 O 3 and Cu. Therefore, it is concluded that with higher Al 2 O 3 contents such as 4 wt.% or 5 wt.%, the Al 2 O 3 is prone to agglomeration, which decreases the density of ACCM, and renders the plastic deformation capacity of ACCM poor. Figure 7 shows the electrical conductivity of ACCM with various Al 2 O 3 contents. It is obvious from Figure 7 that with the increase of Al 2 O 3 contents from 0 wt.% to 5 wt.%, the electrical conductivity of ACCM decreases from 98.24% IACS to 56.66% IACS. However, 80% of total electrical conductivity of ACCM is achieved with Al 2 O 3 additions up to 3 wt.%, which meets the index requirements of high conductivity and high strength material. Moreover, with the increase of Al 2 O 3 contents beyond 3 wt.% (such as 4 wt.% or 5 wt.%), the electrical conductivity of ACCM decreases rapidly. Therefore, the optimum reinforcement amount of Al 2 O 3 lies in the range of 0 wt.% to 3 wt.% in the present work.

Effect of Al 2 O 3 Content on the Electrical Conductivity of ACCM
There are various advantages of using Al 2 O 3 as reinforcement in Cu matrix composites. Firstly, Al 2 O 3 is a nonconducting ceramic, its effect on scattering electrons enhances with the increase in its concentration [38]. Secondly, the addition of Al 2 O 3 limits the diffusion of Cu by causing dispersion strengthening in ACCM. This phenomenon increases the crystal boundary by refining Cu grains, and hence improves the effect of crystal boundary on scattering during electron transmission [39]. Thirdly, the large internal stress will occur during the SPS process due to mismatch of the thermal expansion coefficients of Al 2 O 3 and Cu, which further deteriorates the density of ACCM and hence decreases its electrical conductivity.  Figure 7 shows the electrical conductivity of ACCM with various Al2O3 contents. It is obvious from Figure 7 that with the increase of Al2O3 contents from 0 wt.% to 5 wt.%, the electrical conductivity of ACCM decreases from 98.24% IACS to 56.66% IACS. However, 80% of total electrical conductivity of ACCM is achieved with Al2O3 additions up to 3 wt.%, which meets the index requirements of high conductivity and high strength material. Moreover, with the increase of Al2O3 contents beyond 3 wt.% (such as 4 wt.% or 5 wt.%), the electrical conductivity of ACCM decreases rapidly. Therefore, the optimum reinforcement amount of Al2O3 lies in the range of 0 wt.% to 3 wt.% in the present work.

Effect of Al2O3 Content on the Electrical Conductivity of ACCM
There are various advantages of using Al2O3 as reinforcement in Cu matrix composites. Firstly, Al2O3 is a nonconducting ceramic, its effect on scattering electrons enhances with the increase in its concentration [38]. Secondly, the addition of Al2O3 limits the diffusion of Cu by causing dispersion strengthening in ACCM. This phenomenon increases the crystal boundary by refining Cu grains, and hence improves the effect of crystal boundary on scattering during electron transmission [39]. Thirdly, the large internal stress will occur during the SPS process due to mismatch of the thermal expansion coefficients of Al2O3 and Cu, which further deteriorates the density of ACCM and hence decreases its electrical conductivity.  Figure 7 shows the electrical conductivity of ACCM with various Al2O3 contents. It is obvious from Figure 7 that with the increase of Al2O3 contents from 0 wt.% to 5 wt.%, the electrical conductivity of ACCM decreases from 98.24% IACS to 56.66% IACS. However, 80% of total electrical conductivity of ACCM is achieved with Al2O3 additions up to 3 wt.%, which meets the index requirements of high conductivity and high strength material. Moreover, with the increase of Al2O3 contents beyond 3 wt.% (such as 4 wt.% or 5 wt.%), the electrical conductivity of ACCM decreases rapidly. Therefore, the optimum reinforcement amount of Al2O3 lies in the range of 0 wt.% to 3 wt.% in the present work.

Effect of Al2O3 Content on the Electrical Conductivity of ACCM
There are various advantages of using Al2O3 as reinforcement in Cu matrix composites. Firstly, Al2O3 is a nonconducting ceramic, its effect on scattering electrons enhances with the increase in its concentration [38]. Secondly, the addition of Al2O3 limits the diffusion of Cu by causing dispersion strengthening in ACCM. This phenomenon increases the crystal boundary by refining Cu grains, and hence improves the effect of crystal boundary on scattering during electron transmission [39]. Thirdly, the large internal stress will occur during the SPS process due to mismatch of the thermal expansion coefficients of Al2O3 and Cu, which further deteriorates the density of ACCM and hence decreases its electrical conductivity.   Figure 8 shows a relation between friction coefficient and time period for ACCM prepared with various Al 2 O 3 contents. It is obvious that the friction coefficient of ACCM increases with the increase of Al 2 O 3 contents. However, with the Al 2 O 3 contents in the range of 0-2 wt.%, the fluctuation of the friction coefficient is gentle and the run-in time is short. When the Al 2 O 3 contents are increased to 3-4 wt.%, the fluctuation time increases to 3 min, and this fluctuation range further increases and the run-in time is extended to 5.2 min with the Al 2 O 3 contents reaching up to 5 wt.%. There are various reasons which are ascribed to above phenomenon. Firstly, Cu matrix is soft, while Al 2 O 3 is a wear-resistant material, therefore, the friction coefficient of ACCM increases with the increase of Al 2 O 3 contents [40]. Secondly, during the early stages of the wear test, Cu matrix is worn and peeled off, resulting in the dispersion of Al 2 O 3 particles on the worn surface in ACCM leading to their high hardness. Therefore, the whole friction process involves relatively large frictional force, which leads to the relatively high friction coefficient of ACCM. Thirdly, Al 2 O 3 particles are prone to agglomeration in ACCM with the increase of Al 2 O 3 contents, thus Al 2 O 3 will be subjected to exfoliation during friction wear experiment, which leads to the larger fluctuation in the friction coefficient value. Figure 8 shows a relation between friction coefficient and time period f pared with various Al2O3 contents. It is obvious that the friction coefficien creases with the increase of Al2O3 contents. However, with the Al2O3 conten of 0-2 wt.%, the fluctuation of the friction coefficient is gentle and the run-in When the Al2O3 contents are increased to 3-4 wt.%, the fluctuation time incr and this fluctuation range further increases and the run-in time is extend with the Al2O3 contents reaching up to 5 wt.%. There are various reasons cribed to above phenomenon. Firstly, Cu matrix is soft, while Al2O3 is a material, therefore, the friction coefficient of ACCM increases with the inc contents [40]. Secondly, during the early stages of the wear test, Cu matri peeled off, resulting in the dispersion of Al2O3 particles on the worn sur leading to their high hardness. Therefore, the whole friction process invo large frictional force, which leads to the relatively high friction coefficie Thirdly, Al2O3 particles are prone to agglomeration in ACCM with the inc contents, thus Al2O3 will be subjected to exfoliation during friction wea which leads to the larger fluctuation in the friction coefficient value.   Figure 9 shows polishing scratch depths and wear rates of ACCM prepared with various Al 2 O 3 contents. It is obvious from Figure 9a that polishing scratch depth first decreases with the increase of Al 2 O 3 contents from 0 wt.% to 3 wt.%, and arrives at a minimum value of 3 wt.%. It is obvious that the polishing scratch depth increases quickly with the increase in Al 2 O 3 contents from 3 wt.% to 5 wt.%. Moreover, it is evident in Figure 9a that when the Al 2 O 3 contents are in the range of 0-1 wt.%, the scratches are relatively gentle. However, with the Al 2 O 3 contents in the range of 2-5 wt.%, the size of scratches increases, indicating that the furrow appears during friction wear experiment and the wear mechanism is mainly grinding abrasion. It is concluded that ACCM with 3 wt.% Al 2 O 3 exhibits the minimum wear deformation area under the same conditions. Figure 9b shows the wear rate of ACCM with various Al 2 O 3 contents. It is clear that the wear rate of samples decreases from 5.679 × 10 −5 mm 3 /(N·m) to 2.32 × 10 −5 mm 3 /(N·m) as Al 2 O 3 contents are increased from 0 wt.% to 3 wt.%. However, with the Al 2 O 3 contents exceeding 3 wt.% (such as 4 wt.% to 5 wt.%), the wear rate of ACCM increases quickly from 7.732 × 10 −5 mm 3 /(N·m) to 9.957 × 10 −5 mm 3 /(N·m). It is also obvious that the wear resistance of ACCM with the range of 1 wt.% to 3 wt.% is higher than that of pure copper (0 wt.%). Moreover, ACCM with 3 wt.% Al 2 O 3 exhibits the optimum composition in terms of wear resistance. As the Al 2 O 3 contents are increased up to 3 wt.%, the ground ball exhibits large number of support dots showing that the sample can bear a large payload, and hence reducing the wear of Cu matrix. This behavior can be ascribed to following reasons. Firstly, Al 2 O 3 is an excellent wear-resistant material, thus, the increase of Al 2 O 3 content can improve the wear resistance of ACCM [34]. Secondly, during the friction and wear process, the addition of Al 2 O 3 particle transforms adhesive wear mechanism to grinding grain wear mechanism, which effectively reduces the overall wear of ACCM. Thirdly, a composite lubrication film is formed by very high activity of Al 2 O 3 particles on the polishing scratch surface under the effect of the load and friction force. This lubrication film will strengthen the self-lubrication effect and improve the wear resistance [41]. Fourthly, although the addition of Al 2 O 3 in higher amounts (such as 4 wt.% and 5 wt.%) decreases the hardness and strength by adversely affecting the compaction during the sintering process, however, it increases the wear resistance of ACCM by decreasing the wear rate of ACCM. Fifthly, the addition of more Al 2 O 3 contents (such as 4 wt.% or 5 wt.%) will facilitate more peeling off during friction and wear, leading to the friction wear mechanism of three-body friction pattern among Al 2 O 3 particles, grinding ball, and ACCM. This phenomenon leads to the complex frictional behavior resulting in an increase in the wear rate of ACCM. Therefore, it is obvious that the ACCM with 3 wt.% Al 2 O 3 additions exhibit the best wear resistance under the similar conditions. Figure 10 exhibits the wear morphology of ACCM with various Al 2 O 3 contents. It is obvious from Figure 10 that the wear mechanism of two samples with small Al 2 O 3 contents (such as 0 wt.% and 1 wt.%) is mainly adhesive wear, while the wear mechanism of samples with high Al 2 O 3 contents (such as 2-5 wt.%) is mainly grinding abrasion. It is obvious from Figure 10a that the friction surface of pure copper sample (0 wt.%) exhibits significant exfoliation. This happens due to large difference in hardness of Cu and steel balls, rendering the friction surface under severe plastic deformation resulting in adhesive wear [42].

Effect of Al2O3 Content on the Friction and Wear Performance of ACCM
tle. However, with the Al2O3 contents in the range of 2-5 wt.%, the size of scratches increases, indicating that the furrow appears during friction wear experiment and the wear mechanism is mainly grinding abrasion. It is concluded that ACCM with 3 wt.% Al2O3 exhibits the minimum wear deformation area under the same conditions. Figure 9b shows the wear rate of ACCM with various Al2O3 contents. It is clear that the wear rate of samples decreases from 5.679 × 10 −5 mm 3 /(N·m) to 2.32 × 10 −5 mm 3 /(N·m) as Al2O3 contents are increased from 0 wt.% to 3 wt.%. However, with the Al2O3 contents exceeding 3 wt.% (such as 4 wt.% to 5 wt.%), the wear rate of ACCM increases quickly from 7.732 × 10 −5 mm 3 /(N·m) to 9.957 × 10 −5 mm 3 /(N·m). It is also obvious that the wear resistance of ACCM with the range of 1 wt.% to 3 wt.% is higher than that of pure copper (0 wt.%). Moreover, ACCM with 3 wt.% Al2O3 exhibits the optimum composition in terms of wear resistance. As the Al2O3 contents are increased up to 3 wt.%, the ground ball exhibits large number of support dots showing that the sample can bear a large payload, and hence reducing the wear of Cu matrix. This behavior can be ascribed to following reasons. Firstly, Al2O3 is an excellent wear-resistant material, thus, the increase of Al2O3 content can improve the wear resistance of ACCM [34]. Secondly, during the friction and wear process, the addition of Al2O3 particle transforms adhesive wear mechanism to grinding grain wear mechanism, which effectively reduces the overall wear of ACCM. Thirdly, a composite lubrication film is formed by very high activity of Al2O3 particles on the polishing scratch surface under the effect of the load and friction force. This lubrication film will strengthen the self-lubrication effect and improve the wear resistance [41]. Fourthly, although the addition of Al2O3 in higher amounts (such as 4 wt.% and 5 wt.%) decreases the hardness and strength by adversely affecting the compaction during the sintering process, however, it increases the wear resistance of ACCM by decreasing the wear rate of ACCM. Fifthly, the addition of more Al2O3 contents (such as 4 wt.% or 5 wt.%) will facilitate more peeling off during friction and wear, leading to the friction wear mechanism of three-body friction pattern among Al2O3 particles, grinding ball, and ACCM. This phenomenon leads to the complex frictional behavior resulting in an increase in the wear rate of ACCM. Therefore, it is obvious that the ACCM with 3 wt.% Al2O3 additions exhibit the best wear resistance under the similar conditions.  obvious from Figure 10 that the wear mechanism of two samples with small Al2O3 contents (such as 0 wt.% and 1 wt.%) is mainly adhesive wear, while the wear mechanism of samples with high Al2O3 contents (such as 2-5 wt.%) is mainly grinding abrasion. It is obvious from Figure 10a that the friction surface of pure copper sample (0 wt.%) exhibits significant exfoliation. This happens due to large difference in hardness of Cu and steel balls, rendering the friction surface under severe plastic deformation resulting in adhesive wear [42]. It is observed that the wear mechanism of ACCM is mainly adhesive wear with small Al2O3 contents of 1 wt.% (Figure 10b). Nevertheless, with such small Al2O3 contents, the wear surface is less peeled off, and some holes appear in the wear surface formed by peeling off some Al2O3 particles during the wear process. When the Al2O3 contents are 2 wt.% (Figure 10c), furrows with larger gaps appear at the wear surface. Moreover, a slight sheet peeling phenomenon can also be observed, indicating that the main wear mechanism is grinding grain wear and slight adhesive wear. In Figure 10d clearly indicates that ACCM composite with 3 wt.% Al2O3 contents exhibits the furrows with small gaps at friction surface. The debris shown in Figure 10d (indicated by arrow) are the abrasive dust formed It is observed that the wear mechanism of ACCM is mainly adhesive wear with small Al 2 O 3 contents of 1 wt.% (Figure 10b). Nevertheless, with such small Al 2 O 3 contents, the wear surface is less peeled off, and some holes appear in the wear surface formed by peeling off some Al 2 O 3 particles during the wear process. When the Al 2 O 3 contents are 2 wt.% (Figure 10c), furrows with larger gaps appear at the wear surface. Moreover, a slight sheet peeling phenomenon can also be observed, indicating that the main wear mechanism is grinding grain wear and slight adhesive wear. In Figure 10d clearly indicates that ACCM composite with 3 wt.% Al 2 O 3 contents exhibits the furrows with small gaps at friction surface. The debris shown in Figure 10d (indicated by arrow) are the abrasive dust formed during friction wear process, and the wear mechanism is main grinding grain wear. Figure 10e clearly indicates that with the Al 2 O 3 contents of 4 wt.%, some deep plows with cracking phenomenon and peeling effects appear at the friction surface, and the wear mechanism is mainly grinding grain wear and slight adhesive wear. In Figure 10f it is obvious that for the composites with 5 wt.% Al 2 O 3 contents, a deep furrow forms at the friction surface, and significant exfoliation occurs with some holes form at the middle area of the frictional part. In this case, it is obvious that the wear mechanism is a combination of grinding grain wear and adhesive wear. Above results indicate that with the increase of Al 2 O 3 contents, the binding of Al 2 O 3 and Cu matrix weakens. This is because Al 2 O 3 particles are prone to agglomeration with the increase of Al 2 O 3 contents, which decreases the density of ACCM and consequently increases the friction coefficient of ACCM by increasing the friction resistance caused by increase in roughness of the friction surface. Above phenomenon can be further explained by various reasons. Firstly, severe plastic deformation occurs at surface due to large difference in the hardness of Cu and steel ball with high hardness, leading to sticky wear and tear. Secondly, the addition of Al 2 O 3 can act as a particle reinforcement, which improves the shear deformation ability of ACCM through the synergistic action. However, it is found that the increase in strength and hardness of ACCM resulting from the increase of Al 2 O 3 contents is actually caused by a reduction in the size of debris (Figure 10d-f). This similar phenomenon is also explained by previous documents [43,44]. Thirdly, when the Al 2 O 3 contents are too high (such as 4 wt.% and 5 wt.%), the Al 2 O 3 particles are separated and detached from ACCM during wearing process, which makes the Al 2 O 3 particles directly to lose its wear resistance capability. Therefore, cracks appear at the ACCM surface causing surface peeling phenomenon.

Conclusions
The ACCP is prepared by combining solution combustion synthesis and hydrogen reduction method. Subsequently, the ACCM is prepared by the SPS method. Moreover, the effect of adding various Al 2 O 3 contents on particle size and morphology of ACCP is studied. Furthermore, the effects of SPS temperature and Al 2 O 3 contents on the mechanical property of ACCM have also been studied in detail. The following conclusions are drawn.
(1) The samples with the addition of various Al 2 O 3 contents exhibit better dispersity than the samples without the addition of Al 2 O 3 . The ACCP with 3 wt.% Al 2 O 3 content exhibits the smallest particle size and the best particle dispersity.