The effect of cold and hot pressing on mechanical properties and tribological behavior of Mg-Al2O3 nanocomposites

In this research, pure powder of Mg was mixed with 0, 1.5, 3, 5%vol. of Aluminium oxide in a planetary mill. Next, the powder mixture was poured in a mold and pressed in two diverse conditions of (1) hot pressing at 600 MPa pressure and 450 °C temperature for 25 min and (2) cold pressing at 600 MPa pressure in the room temperature and samples sintered in a furnace under Argon gas at 450 °C temperature for 2 h. Density and mechanical properties, e.g., microhardness, and wear properties of the produced samples were assessed. Also, metallographic photography and SEM analysis were done on the samples to investigate their microstructure properties and analyze their worn surfaces. The results revealed that with an increase in the volume of the reinforcement particles, the experimental density and microhardness soared, on the contrary, the relative density showed a decreasing trend. Moreover, the results of the microhardness analysis for the produced samples via hot pressing method were achieved better than those of cold pressing, as the highest hardness 81HV was achieved for %5 vol. Al2O3 containing samples produced through the hot pressing method, which was about %18 more than that of the %5 vol. Al2O3 containing samples produced via the cold pressing method and was about %85 more than of the pure Mg samples produced via the hot pressing method. The results of the samples’ wear properties also signified the improvement of wear resistance and decrease of mass loss with an increase in the volume fraction of the reinforcement particles. The lowest mass loss of 2.5 g was obtained for the sample containing %5 vol. of the reinforcement particle which was produced via the hot pressing method. This value was less about %40 and %80 compared to pure Mg samples produced via hot and cold pressing methods, respectively.


Introduction
Due to the lightness, Magnesium is one of the most applied metals in the automotive and aerospace industries. Mechanical and wear properties of Mg and its alloys can be enhanced through the addition of reinforcement materials. Nanocomposites are composite materials formed of two phases. The first phase is called the 'base' or the 'matrix' and can be polymeric, metallic, or ceramic. The second phase is comprised of nanoparticles which are added to the first phase as 'reinforcement' to improve the strength, wear resistance, electrical conductivity, etc of the composite. Mg is one of the most applied and lightest metals which is used as the base material in the production of nanocomposites. Till now, many researches have been done in the field of production of Mgbased nanocomposites with ceramic particles, such as B 4 C, TiN, Al 2 O 3 , and SiC as the reinforcement, to improve the mechanical properties. The role of reinforcement depends on the structure of the base material. In reinforced metal-based nanocomposites by nanoparticles, the base material is the main part to endure the applied load, and the role of the reinforcement is to enhance the strength and hardness to avoid plastic deformation of the sample [1]. Metal-based nanocomposites have drawn attention to engineering applications owing to their lightness, strength, high resistance against fatigue and corrosion, and dimensional stability [2]. There are various methods for the production of Mg reinforced by nanoparticles, such as powder metallurgy [3,4], dynamic compaction [5,6], and cold pressing along with normal sintering [7]. Production of nanocomposites through the powder metallurgy process possesses many advantages as compared to the other methods, and this has led to its wide use relative to the other methods [8]. The wear phenomenon is one of the problems that the industry has faced for a long time. Wear is about erosion or mechanical decay of material from the surface of a subject through its connection with a surface or another subject. Despite the mechanical nature of the erosion mechanism, this phenomenon is sometimes followed by a chemical reaction which leads to corrosion [9]. Different kinds of wear in the industry are as follows: (1) abrasive wear, whose one of signs is the presence of parallel grooves on the wear surface, (2) adhesive wear, which normally occurs at low speed and high pressure between the surfaces which are in slip contact, and its dominant mechanism is based on the removal of the particles from the surface followed by plastic cut [10], (3) layered wear, which, at low slip speeds, includes germination and subsurface cracks and their growth in parallel to the surface [3], and (4) fatigue wear, which occurs when one surface is stagnant and the other surface is moving [9]. The effective factors on wear are metallurgies, operational, and environmental variables, the amount of applied force, and the slip distance. Also, for nanocomposites, there are other parameters, e.g., type, shape, size, the volume fraction of reinforcing particles, and type of thermal operations which have a profound influence on their wear properties [11]. There are different reports on the impact of volume fraction and size of the reinforcement on mechanical properties and wear resistance of metal-based nanocomposites. Jiang. Q, et al [12] investigated the advantages of production Mg-based nanocomposites via the powder metallurgy method. They developed an affordable, simple method based on powder metallurgy. The researchers compared the experimental results of mechanical and microstructure analysis to find the best percentage of the nanoparticle. One of the most important parameters involved in the production of materials via powder metallurgy method is the working temperature of the production process [13]. Rahmani. K, et al [14,15] and Majzoobi, G, et al [16,17] investigated the impact of temperature on the production of Mg-based nanocomposites. Francis, E. et al [18] assessed the effect of sintering in the production process of Mg-based nanocomposites with Al 2 O 3 reinforcing nanoparticles. They performed mechanical experiments on the produced samples and realized that by adding nanoparticles, the mechanical properties improved dramatically. Thakur, S K, et al [19] investigated the weight ratio of ceramic reinforcing particles in an Mg base. Also, diverse research has been done on the impact of Al 2 O 3 nanoparticles; for instance, Prasad, Y, et al [8] appraised the effect of the nanoparticles on mechanical properties of Mg/Al 2 O 3 nanocomposite which they produced via powder metallurgy and extrusion. Leong Eugene, et al [20] produced Mg-based nanocomposites with Cu, Al 2 O 3, and SiC reinforcing nanoparticles via the method of mixing, pressing, and sintering by microwave. They investigated the impact of particle size and mechanical properties of the reinforcing particles on Mg and reported the improvement of mechanical properties. Hassan, S, et al [21] appraised the mechanical properties of Mg/Al 2 O 3 nanocomposites via the mechanical alloy method. In this research, they experienced improvement of nanocomposites' mechanical properties with an increase in the percentage of the reinforcement, and the most improved mechanical properties were finally obtained in the Mg/1.11Al 2 O 3 sample. Also, Lu, D [22] and Umeda, J [23] reported the production of hybrid nanocomposites reinforced by CNTs and SiO2 nanoparticles via the FSP method, and the result was an increase of wear resistance and decrease of the coefficient of friction. Finally, they reported the lowest wear rate of the Mg/Al 2 O 3 sample relative to the Mg/CNTs sample, and they were lower than those of both nanocomposites in hybrid form.
It should be noted that until now, only a few studies have been performed on comparing mechanical and wear properties of nanocomposites by the hot press (HP) and cold press (CP). The main objective of this study is to experimental investigation of HP and CP effect on the mechanical and wear properties of Mg reinforcement by Al 2 O 3 nanoparticles. In this research, Mg base nanocomposite samples reinforced by Al 2 O 3 nanoparticles were produced. The reinforcing nanoparticles were added to the Mg base material in different fraction volume (0, 1.5, 3, and 5%). The samples were produced via two methods. One method included hot pressing followed by simultaneous applying of pressure and temperature, and another method included cold pressing and sintering in a furnace. The applied pressure and temperature values were the same in both methods. Theoretical and experimental density, relative density, and the porosity of the produced samples were calculated. Also, microhardness and mass loss tests were done on the produced samples. Finally, to assess the microstructure properties and the worn surfaces of the samples, metallographic photography and SEM analysis were carried out.

Materials and devices
In this research, the pure powder of Mg with a mean particle size of 40micron, an irregular spherical morphology and purity of %99 was used as the base material, and Al 2 O 3 nanoparticles with the purity of %99 and the size less than 100 nm with spherical morphology were used as the reinforcing material. Figures 1 and 2 show the scanning electron microscope (SEM) images of Mg and Al 2 O 3 at two different magnifications.
To produce the nanocomposite materials, at first, different fraction volume (0, 1.5, 3, 5%vol.) of the Al 2 O 3 reinforcing powder were mixed with Mg powder in a planetary mill with a ball to powder weight ratio of 12:1 and rotation speed of 200 rpm for 1 h. To avoid agglomeration of the particles as well as the formation of MgO composition, stearic acid (0.5%vol.) was used, and Argon gas was applied to the mill to impede the oxidation of the powders. The mixture of the powders was pressed in a mold encompassing the upper mold, cap, mandrel, and two hot-working steel discs (figure 3). The samples were fabricated via two diverse methods. The first method included hot pressing at 600 MPa pressure and 450°C temperature for 25 min [24], and the second method included cold pressing followed by pressing the powders at room temperature at 600 MPa pressure and sintering them in a furnace under Argon gas at 450°C temperature for 2 h. Measuring the samples' density was done by the Archimedes method based on the standard ASTM B962 [25]. Relative density and porosity percentage of each sample were obtained separately. The microstructural examination was done using a VEGA Field Emission Scanning Electron Microscope (FESEM). X-ray diffraction (XRD) analysis was performed under Cu K radiation of wavelength k=1.540 56 Å with a scan speed of 2/min. The Vickers hardness of the samples was done based on the standard ISO-6507 with the force of 10 N at 10 s [26]. To investigate the wear resistance of the samples, the pin on disc test was performed based on the standard ASTM-G99 [27]. The erosive pin was made of AISI 52100 steel with a diameter of 1.5 mm, and the disc was made of nanocomposite samples with a diameter of 2 cm. The pin was moving at the speed of 0.09 m s −1 and force of 20 N in a 200 m distance to apply wear on the samples. To assess the morphology of the powders, the microstructure of the samples, and the worn surfaces, optic microscope and scattering electron microscope (SEM) were used.    (1), the total density, ρ tot , of each material is multiplied by the value of fraction volume, in which r r v v . . . . m r m r are fraction volumes of mg and Al 2 O 3 and density of Mg and Al 2 O 3 , respectively, and their summation results in the value of theoretical density. The resulted number was the same for an equal fraction volume of every reinforcing nanoparticle and was not dependent on the process of the production samples. The experimental density, r , Exp and relative density, r , relative were also calculated based on the Archimedes method and equation (2), respectively, and the results are shown in table 1. Moreover, the porosity percentage of the samples was calculated according to equation (3) in terms of theoretical density, r , The and relative densities. In this research, from now on, the samples produced via hot pressing are referred to as 'HP', the samples pressed via cold pressing and then sintered in the furnace are referred to as 'CP', and the percentage of the reinforcement is written along with them. Figure 4 shows the produced Mg nanocomposite samples. It should be noticed that the diameter and height of the produced green sample are equal to 15 mm and 12 mm, respectively.
The Exp The * ( ) () Figure 5 shows the diagrams of the experimental density and theoretical density of the nanocomposites produced via two methods of the production samples As seen, with the percentage of the nanoparticles increased, theoretical density showed an increasing trend, while the experimental density showed an increasing trend which revealed that addition of Al 2 O 3 nanoparticles gave rise to abatement of density owing to the hardness and agglomeration rendered among Mg particles, so the number of pores augmented in the samples [28]. Also, it can be seen that for the entire fraction volume of the reinforcing nanoparticles, the experimental density of the samples produced via hot pressing method was more than that of those produced via the cold  pressing method. This was because of simultaneous applying of pressure and temperature during the sintering process which led to better bonding between the nanoparticles and the base material [29]. The highest density equal to 1.68 g cm −3 was obtained for HP-%5 vol. sample, which was respectively %5 and %1 more than those of two CP-%0 vol. and HP-%0 vol. samples, respectively.

XRD analysis
The XRD analysis was done on the produced specimens to find and realize the structure of the composition after and while milling [30]. Figure 6 illustrates the XRD patterns of Mg and Al 2 O 3 powders after 1 h ball milling at a different percentage of reinforcement. Since the time of mixing was short, no new phases were recognized. On the other hand, MgO has not been produced because of mixing time and using stearate acid as well as Argon gas. Also, figures 7 and 8 illustrate the XRD patterns of produced Mg-Al 2 O 3 nanocomposites samples via HP and CP methods, respectively. No new phases were identified after the compaction and sintering process. It can be considered that temperature effects on grain growth under uncontrolled thermal conditions [31]. In this regard, W-H approach [32] was employed to decrease the crystallite size of the Mg matrix due to the changing peaks' intensity and as a result of thermo-mechanical deformation of Mg in the vicinity of Al 2 O 3 nanoparticles. Rashad et al [33] also reached the grain size reduction of Mg matrix after hot extrusion as a result of recrystallization phenomena in their investigation. Similar reports and results can be found in [34].

Microstructural analysis
To investigate and validate density results, metallographic photography was done for the entire samples. This photography test is useful to assess the grain boundaries, bonds, and pores among the particles which all have an impact on density results. As seen in SEM images, the more the percentage of the reinforcing particles, the more the number of the pores and the more the discontinuity among the particles. Also, it can be seen that due to a b  simultaneous applying of pressure and temperature, the bond between the particles noticeably improved for the samples fabricated via hot pressing method relative to those fabricated via the cold pressing method [35]. Figure 9 shows the produced samples via the cold pressing method, and the periphery of the samples are marked by circles. Figure 10 exhibits the metallographic photographs of the produced sample via the hot pressing method. As seen, these samples possessed fewer pores and in discontinuity as compared to those produced via the cold pressing method. Moreover, SEM analysis was done to have a higher resolution for investigating the microstructure of the samples' surface and, especially, the effect of the hot pressing process on the quality and mechanical properties of the samples. SEM images showed more surface coherency and smoothness for the samples containing less amount of nanoparticles and produced through simultaneous of applying pressure and temperature in comparison with the cold pressing process. Figures 11 and 12 show the produced samples via CP and HP methods.

Microhardness
The microhardness analysis was done on three points of the produced nanocomposites. The Microhardness results have been listed in table 2, and the average microhardness results of the samples are shown in figure 13 for better investigation and evaluation.  Microhardness results revealed that with an increase in the percentage of the reinforcement, the value of hardness increased. This was due to the presence of the reinforcing nanoparticles and their proper distribution and sufficient bond with the base material, which allowed the nanocomposite samples to resist against the plastic flow [36]. Upon applying the force on these samples, the nanoparticles played the main role, relative to the base material, to endure the force. Also, the obtained result of the hardness of the produced samples via the hot pressing method was better than that of those produced via the cold pressing method. This resulted from the production method and the way of applying pressure and temperature. The highest hardness equal to 81HV was achieved for the HP-%5 vol. sample, which was an improvement of about %85 relative to the pure Mg sample      produced via the hot pressing method. The hardness of this sample was also %18 and %87 more than that of both CP-%5 vol. and CP-%0 vol. samples, respectively.

Wear resistance behavior
Produced nanocomposites via different methods, based on the specified parameters in section 2, underwent the wear analysis. The weight of the samples was calculated before and after the wear test, and the mass change of each sample was separately determined. Also, the mean coefficient of friction of the samples was calculated and presented. Also, the results of the wear analysis, in terms of wear loss and coefficient of friction, have been listed in table 3 and shown in figures 14 and 15 for better investigation and comparison. Based on figure 14, mass loss of the reinforced samples decreased with an increase in the percentage of the reinforcing nanoparticles, and the reason was the presence of Al 2 O 3 nanoparticles and their good distribution and bonding in the base material which endured the main load upon applying the force and thus led to increasing of wear resistance and also, to the strong bonding between the nano reinforcements and Mg matrix   that facilitates the load transfer from the matrix to the hard particles. Also, it was previously mentioned that the hardness of the samples produced via hot pressing method was more than that of those produced via the cold pressing method which could be the reason for the increase of wear resistance of the samples produced via simultaneous applying of pressure and temperature as compared to the samples produced via the cold pressing method. The highest mass loss was obtained for pure Mg sample, and the lowest was for HP-%5 vol. sample equal to 2.5 g was achieved which was %32 lower than that of CP-%5 vol. sample and %44 lower than that of CP-%0 vol. sample which had the highest mass loss. Another parameter related to wear properties is nanocomposites' coefficient of friction. Figure 15 shows the mean coefficient of friction versus the percentage of the reinforcing nanocomposite for the produced samples. As seen, the highest coefficient of friction was for the pure Mg sample. But through the addition of reinforcing nanoparticles, coefficient of friction curtailed. This was owing to the presence of the tough, wear-resistant particles of Al 2 O 3 . The results also revealed that the production process of hot pressing, relative to cold pressing, resulted in a decrease in the coefficient of friction. The lower coefficient of friction for produced samples by HP method can be explained due to the strong bonding between Mg and hard Al 2 O 3 nanoparticles, and a lower tendency to adhesive friction during wear. Generally, harder surfaces lead to the smaller contact area between the pin and the sample surface, consequently, reduction of coefficient friction [5]. The highest coefficient of friction equal to 0.0248 was obtained for CP-%0 vol. sample and the lowest coefficient of friction equal to 0.021 was obtained for HP-%5 vol. sample, which signified an improvement of about %18. This sample also reached an improvement of about %9 relative to HP-%5 vol. sample. The total wear loss is proportional to the coefficient of friction. It means that the uniform distribution of Al 2 O 3 nanoparticles is effective in improving the tribological properties of the Mg-Al 2 O 3 nanocomposite by decreasing the coefficient friction and wear loss during sliding [5].
To validate the results of wear analysis of the samples' worn surfaces, SEM analysis was done. Figures 16 and  17 show the worn surfaces of the produced samples in terms of the increase of reinforcing nanoparticles' percentage in two cold and hot pressing conditions. It is obvious in the figures the reinforcing content increases, the wear track becomes smaller and the width of the grooves decreases. Given the low number and shallowness of the appeared grooves, which are marked in the obtained images, increase of wear resistance, as compared to pure Mg sample, was concluded owing to the presence of Al 2 O 3 nanoparticles. The SEM images of the produced samples via the hot pressing method revealed that their worn surfaces were in a better surficial condition relative to the samples produced via the cold pressing method, and less plastic deformation occurred for them. Moreover, the grooves on the samples fabricated by HP method are narrower and shallower compared with CP method that indicates an adhesive friction mechanism [33]. Besides, the samples were produced via HP method have less plastic deformation and delaminations on their surfaces which means, these samples have more strength and hardness, as well as stronger bonding between Mg and Al 2 O 3 particles, compare to the produced samples by CP method [5]. Actually, for the samples with lower microhardness, the counter faces between the pin and the sample surface increase, therefore more materials are detached from the sample's surface. This detachment may be due to adhesive friction that consequently increases the mass loss. Lower mass loss in the samples produced by HP is traced back to the mild wear with abrasive wear mechanism and shows improved condition for wear behavior [37]. Higher surface hardness due to nano reinforcement or obtained by HP method reduces adhesion features and converts the wear mechanism to abrasive a comparison CP method.
In these samples, the parallel and continuous grooves were signs of abrasive wear, and the obtained mass loss value also supported this result and indicated the increase of wear resistance [38]. Moreover, the images of the samples produced via the hot pressing method indicated that an increase of the volume fraction of the Al 2 O 3 nanoparticles led to increase in wear resistance and decrease of plastic deformation, and abrasive wear was blatantly obvious for HP-%5 vol. sample; also, the mass loss of this sample was the lowest among the entire samples. Better bond and junction during simultaneous applying of pressure and temperature allowed the produced sample to have better bonding between the base material and the reinforcing nanoparticles which increased resistance against wear. Indeed, the presence of Al 2 O 3 nanoparticles led to an enhancement of strength and stability of the base phase and increased the resistance of the base phase against plastic deformation [39,40]. Besides, SEM images showed the worn surfaces of the samples produced via cold pressing, so that they exhibited severe wear, and their plastic deformation was extremely high. Severe plastic deformation and the appearance of lateral cracks on the surface of the samples indicated adhesive wear. The higher mass loss of these samples, relative to that of those fabricated via the hot pressing method, supported this result.

Conclusion
In this research, the physical, mechanical, and wear properties of the Mg-based nanocomposite reinforced with %0, 1.5, 3, 5 of Al 2 O 3 were investigated. The samples were produced via two methods of hot pressing and cold pressing. Summary of research results is as follows: 1. The relative density of the samples decreased with an increase in the percentage of the reinforcing nanoparticles. The relative density of the samples produced via the hot pressing method was more than that of the samples produced via the cold pressing method. The highest obtained relative density equal to 1.68 g cm −3 was for the sample containing %5 of Al 2 O 3 produced via hot pressing, which was respectively % 5 and %1 more than that of two CP-%0 vol. and HP-%0 vol. samples.
2. The hardness of the samples increased with an increase in the percentage of the reinforcing nanoparticles. The highest hardness equal to 81HV was obtained for HP-%5 vol. sample indicating an improvement of about %85 relative to the pure Mg sample produced via the hot pressing method. Also, this sample had an improvement of about %18 and %87 relative to that of CP-%5 vol. and CP-%0 vol. samples, respectively.
3. The increase of the percentage of reinforcing nanoparticles also led to increase in wear resistance and a decrease in mass loss also, the samples produced via hot pressing method had a lower mass loss relative to the samples produced via the cold pressing method. The lowest was for HP-%5 vol. sample equal to 2.5 g was achieved which was %32 lower than that of CP-%5 vol. sample and %44 lower than that of CP-%0 vol. sample which had the highest mass loss.
4. SEM analysis of the worn surface of samples revealed that by increasing of reinforcing nanoparticles' percentage the less number and shallowness were achieved and increased wear resistance. Less plastic deformation occurred in the worn surfaces and it is a sign of abrasive wear mechanism in the samples produced via HP method compared to the adhesive wear mechanism in the samples produced by CP method.
5. Similar to mass loss, the coefficient of friction of the nanocomposites experienced a decreasing trend with the increase of reinforcing nanoparticles. The highest coefficient of friction equal to 0.0248 was obtained for CP-%0 vol. sample and the lowest coefficient of friction equal to 0.021 was obtained for HP-%5 vol. sample which reached an improvement of about %18. This sample also reached an improvement of about %9 relative to HP-%5 vol. sample.