Microstructure and Strengthening Effect of Coated Diamond Particles on the Porous Aluminum Composites

In this work, porous Al alloy-based composites with varying Ti-coated diamond contents (0, 4, 6, 12 and 15 wt.%) were prepared, employing the powder metallurgy route and using a fixed amount (25 wt.%) of polymethylmethacrylate (PMMA) as a space holder. The effects of the varying wt.% of diamond particles on the microstructure, porosities, densities and compressive behaviors were systematically evaluated. The microstructure study revealed that the porous composites exhibited a well-defined and uniform porous structure with good interfacial bonding between the Al alloy matrix and diamond particles. The porosities ranged from 18% to 35%, with an increase in the diamond content. The maximum value of plateau stress of 31.51 MPa and an energy absorption capacity of 7.46 MJ/m3 were acquired for a composite with 12 wt.% of Ti-coated diamond content; beyond this wt.%, the properties declined. Thus, the presence of diamond particles, especially in the cell walls of porous composites, strengthened their cell walls and improved their compressive properties.


Introduction
Porous materials are increasingly demanded materials for aerospace and automobile applications due to their structural and functional advantages. These materials possess high specific strength, high specific stiffness and outstanding energy absorption capabilities [1,2]. As a result, porous materials have received extensive interest. Among all metals, aluminum (Al) is the most commonly used for the development of porous materials due to its low weight-to-strength ratio [3,4]. However, during their development, it was found that their cell structure was irregular, and the cell size was non uniform as a result of their low viscous metal melts [5]. In addition, they also exhibited comparatively lower compressive strength [6,7], ultimately limiting their applications. Thus, to increase the viscosity of metal melts and enhance the strength of porous Al, the metal additives and reinforcement addition were considered as a good approach. Several researchers reported the influence of metal additives and reinforcement on both the foaming as well as compressive behaviors of the resultant composites. The introduction of the alloying elements such as tin (Sn), boron (B), copper (Cu), magnesium (Mg) or chromium (Cr) into the Al matrix has been found to enhance the properties of porous composites [8][9][10][11][12]. The effects of the addition of boron carbide (B 4 C), silicon carbide (SiC), alumina (Al 2 O 3 ) and carbon nanotubes (CNTs) to Al or their alloys on the compressive properties have been extensively studied [13][14][15][16][17]. There are limited reports on the diamond particles in porous composites due to their lower chemical activity, making it difficult to achieve good interfacial bonding with the metal matrix during composite preparation [18]. Similarly, diamond has extremely low wettability with the Al matrix, resulting in poor interfacial bonding [19]. To overcome these problems, the surface modification of diamond particles was preferred, which involves depositing elements

Materials
The powders of magnesium (Mg), tin (Sn), copper (Cu) and boron (B) were mixed with aluminum (Al) powder, as per the wt.% shown in Table 1, to form an alloy matrix mix. These powders were procured from Nova Scientific Resources Sdn Bhd, Malaysia, and their addition was meant to facilitate liquid sintering and disrupt the oxide layer present on the surface of the Al particles. Further, titanium-coated diamond particles of an average particle size (approximately 45 µm) at varying contents (0, 4, 8, 12 and 15% by weight) were used as reinforcements. PMMA particles obtained from Sigma Aldrich Malaysia were employed as space holders at a fixed amount of 25 wt.% to achieve the controlled porosity.

Preparation of Diamond-Reinforced Porous Al Composites
The powder metallurgy technique was used to develop porous composites. It involved a mixing, compaction, and sintering process. The mixing process was accomplished in three steps. First, the powders of Al, Mg, Sn, Cu and B were mixed for 24 h at 300 rpm using a horizontal ball mill. The powder-to-ball ratio was set at one-tenth. Further, the alloy matrix mix was mixed with the Ti-coated diamond particles using an oscillatory mixer for 2 h at 800 rpm, followed by mixing the entire mixture with PMMA particles using an oscillatory mixer for 2 h at 800 rpm in the last step. Then, the compaction process was carried out by compressing the powder mix in a cylindrical die with a 10 mm diameter using a hydraulic press at 350 MPa pressure to obtain the compacts. The compacted specimens were then heat-treated at 450 • C for 1 h to remove the PMMA particles, followed by being sintered at 590 • C for 1.5 h in an argon atmosphere in a carbolite tube furnace.

Characterization and Testing
The morphology of the starting powder (Al, Mg, Sn, Cu, B, Ti-coated diamond, PMMA particles and composite powder mixture) and the microstructure of the developed porous composites were examined by using a scanning electron microscope (SEM) (JEOL JSM-6300F). The porosity and density were measured via the Archimedes principle. The relative foam density of the samples was measured as per ASTM D3575 by using the equation given below.
where P s is the density of the porous Al composite sample in terms of g/cm 3 , and S d is the total density of solid raw materials (2.7 g/cm 3 ).
To identify phase transformations, the X-ray diffraction patterns of sintered porous Al composites were acquired using (XRD, PAN analytical empyrean 1032) and Cu K radiation. The XRD patterns were achieved in the range of 20 • to 80 • . The compression testing was carried out using a uniaxial compression testing machine (Dartec model 3500 universal testing machine) at a constant crosshead speed of 0.5 mm/min and a load cell of 30 kN. The energy absorption capacity (W) of the resulting porous composites was calculated using the stress-strain curve and the following equation [42].
where σ and ε are the compressive stress and strain, respectively.  Figure 1b-e. Sn was incorporated in the Al matrix to enhance the fluidity of the porous Al composite during the sintering process. Additionally, to improve the sintering properties of Al, it is critical to break down the stable oxide film that has formed on the surface of the Al particles. For this purpose, Mg was added to the Al matrix [43]. Further, Cu and B were added to improve the strength of the Al matrix [10,44,45]. Additionally, these elements impact the various characteristics of molten Al, such as the melting point during the formation of a foam structure, the surface tension, and the viscosity. The melting point and the surface tension greatly influence the relative density and the cell size of Al foams made with molten alloys [46,47]. [44,45]. Additionally, these elements impact the various characteristics of molten Al, such as the melting point during the formation of a foam structure, the surface tension, and the viscosity. The melting point and the surface tension greatly influence the relative density and the cell size of Al foams made with molten alloys [46,47]. Further the three-step mixing resulted in regular and lamellar particle shapes of powders, structured with a smaller particle size (average particle size of 20 µm), as illustrated in Figure 2a. Mixing the powder for a long time resulted in the strain hardening of the powder particles. This results in brittleness, causing the fragmentation and formation of more equiaxed and finer particles [48]. Additionally, mixing PMMA particles with the binder prior to mixing with the metallic mix results in the sticking of metallic powders to the PMMA surface, as shown in Figure 2b. Additionally, metallic particles adhere to the surface of diamond particles, as shown in Figure 2c.  Further the three-step mixing resulted in regular and lamellar particle shapes of powders, structured with a smaller particle size (average particle size of 20 µm), as illustrated in Figure 2a. Mixing the powder for a long time resulted in the strain hardening of the powder particles. This results in brittleness, causing the fragmentation and formation of more equiaxed and finer particles [48]. Additionally, mixing PMMA particles with the binder prior to mixing with the metallic mix results in the sticking of metallic powders to the PMMA surface, as shown in Figure 2b. Additionally, metallic particles adhere to the surface of diamond particles, as shown in Figure 2c.  [44,45]. Additionally, these elements impact the various characteristics of molten Al, such as the melting point during the formation of a foam structure, the surface tension, and the viscosity. The melting point and the surface tension greatly influence the relative density and the cell size of Al foams made with molten alloys [46,47]. Further the three-step mixing resulted in regular and lamellar particle shapes of powders, structured with a smaller particle size (average particle size of 20 µm), as illustrated in Figure 2a. Mixing the powder for a long time resulted in the strain hardening of the powder particles. This results in brittleness, causing the fragmentation and formation of more equiaxed and finer particles [48]. Additionally, mixing PMMA particles with the binder prior to mixing with the metallic mix results in the sticking of metallic powders to the PMMA surface, as shown in Figure 2b. Additionally, metallic particles adhere to the surface of diamond particles, as shown in Figure 2c.  Finally, the SEM micrographs of the porous Al composites, as shown in Figure 3, revealed the closed macro-porous structure with an average macro-pore size ranging from 160 µm to 175 µm, which resembles the morphology of the as-received PMMA particle size and shape. These macro-pores are distributed uniformly and are separated from each other by a unique cell wall. It is vital to obtain pores that imitate the morphology of the starting space holder material, indicating that the pore structure can be tailored, depending on the space holder shape, size and content. Similar observations were made in steel foams developed using carbamide particles as space holders [49]. Moreover, the spherical-shaped pores have fewer edges and corners, thus reducing the surface roughness and decreasing the local stress concentrations and the inconsistent deformation during compression. This enhances the strength of the porous Al composite [50].

Microstructural Analysis
Finally, the SEM micrographs of the porous Al composites, as shown in Figure 3, revealed the closed macro-porous structure with an average macro-pore size ranging from 160 µm to 175 µm, which resembles the morphology of the as-received PMMA particle size and shape. These macro-pores are distributed uniformly and are separated from each other by a unique cell wall. It is vital to obtain pores that imitate the morphology of the starting space holder material, indicating that the pore structure can be tailored, depending on the space holder shape, size and content. Similar observations were made in steel foams developed using carbamide particles as space holders [49]. Moreover, the sphericalshaped pores have fewer edges and corners, thus reducing the surface roughness and decreasing the local stress concentrations and the inconsistent deformation during compression. This enhances the strength of the porous Al composite [50]. Additionally, Figure 3 reveals a well-defined shape and uniform distribution of pores in the case of composites with 0 and 12 wt.% of diamond particles as compared to 15 wt.% of diamond particles, which reveals the presence of distorted pores. These distorted pores are due to the agglomeration of diamond particles compressing the PMMA particles during compaction, distorting their shape [27]. Moreover, the micro porosities exhibited by the porous composites are mainly due to the insufficient availability of the matrix required to fill the gaps or pores between diamond particles [51]. As evident from the porosity and relative density values of porous composites shown in Table 2, the porosity increased from 18% to 35%, whereas their relative density decreased from 71% to 60% with an increase in the diamond content. Similar results were acquired in one of our recent works [52].   Additionally, Figure 3 reveals a well-defined shape and uniform distribution of pores in the case of composites with 0 and 12 wt.% of diamond particles as compared to 15 wt.% of diamond particles, which reveals the presence of distorted pores. These distorted pores are due to the agglomeration of diamond particles compressing the PMMA particles during compaction, distorting their shape [27]. Moreover, the micro porosities exhibited by the porous composites are mainly due to the insufficient availability of the matrix required to fill the gaps or pores between diamond particles [51]. As evident from the porosity and relative density values of porous composites shown in Table 2, the porosity increased from 18% to 35%, whereas their relative density decreased from 71% to 60% with an increase in the diamond content. Similar results were acquired in one of our recent works [52]. A remarkable improvement in the wettability of the Al alloy matrix and Ti-coated diamond particles was also observed, and these diamond particles were mostly found in the cell walls of the porous composites, as evident in Figure 4a,d. The well-bonded diamond particles in cell walls contribute to the enhancement of the cell wall strength, especially up to a 12 wt.% diamond particle content. A significant change in the porosity level and relative density for a 12 wt.% diamond content can be observed in Table 2. However, beyond this, the diamond particles bonding in the cell walls of porous composites reduces with the further increase in the diamond particle content due to the unavailability of a sufficient liquid matrix for wetting diamond particles. Thus, the higher wt.% of diamond composites exhibited lower relative densities and higher porosities due to the presence of cracks and voids in the interfaces of Al and diamond particles. A similar effect was reported by the researchers using diamond or CNT reinforcements in the Al matrix [21,53]. Further, this was found to impair the strength of the porous Al composites [54]. A remarkable improvement in the wettability of the Al alloy matrix and Ti-coated diamond particles was also observed, and these diamond particles were mostly found in the cell walls of the porous composites, as evident in Figure 4a,d. The well-bonded diamond particles in cell walls contribute to the enhancement of the cell wall strength, especially up to a 12 wt.% diamond particle content. A significant change in the porosity level and relative density for a 12 wt.% diamond content can be observed in Table 2. However, beyond this, the diamond particles bonding in the cell walls of porous composites reduces with the further increase in the diamond particle content due to the unavailability of a sufficient liquid matrix for wetting diamond particles. Thus, the higher wt.% of diamond composites exhibited lower relative densities and higher porosities due to the presence of cracks and voids in the interfaces of Al and diamond particles. A similar effect was reported by the researchers using diamond or CNT reinforcements in the Al matrix [21,53]. Further, this was found to impair the strength of the porous Al composites [54]. The bonding can be attributed to the presence of sintering additives, and these additives form low-temperature intermetallic phases. The presence of such phases was revealed by the XRD analysis in Figure 5, the intermetallic Al-rich phases represented by the (111), (200), (220) and (311) diffraction peaks at 2 θ of 38.54°, 44.78°, 65.11° and 78.28°, respectively, and the Al12Mg17, Cu5Sn6, AlB2 and Al3Ti phases in the porous Al composites. These phases are formed during sintering because of a partial reaction between constituents. Upon the addition of Sn and Cu to Al, the solid solubility of Sn in Cu occurs, thus producing Cu5Sn6 phases. As copper has higher affinity for Sn, Sn melts first and forms Cu-Sn phases in the vicinity of Cu [55]. This solid-solid transformation occurs around the temperature of 250 °C [56]. Further, when Mg is added to an Al matrix, the Al primarily precipitates along a grain boundary in the form of the Al12Mg17 phase at temperatures below 430 °C [57]. Additionally, boron tends to enrich at the interface due to the pull of chemical bond forces. Finally, the interface-enriched B reacts spontaneously with the Al The bonding can be attributed to the presence of sintering additives, and these additives form low-temperature intermetallic phases. The presence of such phases was revealed by the XRD analysis in Figure 5, the intermetallic Al-rich phases represented by the (111), (200), (220) and (311) diffraction peaks at 2 θ of 38.54 • , 44.78 • , 65.11 • and 78.28 • , respectively, and the Al 12 Mg 17, Cu 5 Sn 6 , AlB 2 and Al 3 Ti phases in the porous Al composites. These phases are formed during sintering because of a partial reaction between constituents. Upon the addition of Sn and Cu to Al, the solid solubility of Sn in Cu occurs, thus producing Cu 5 Sn 6 phases. As copper has higher affinity for Sn, Sn melts first and forms Cu-Sn phases in the vicinity of Cu [55]. This solid-solid transformation occurs around the temperature of 250 • C [56]. Further, when Mg is added to an Al matrix, the Al primarily precipitates along a grain boundary in the form of the Al 12 Mg 17 phase at temperatures below 430 • C [57]. Additionally, boron tends to enrich at the interface due to the pull of chemical bond forces. Finally, the interface-enriched B reacts spontaneously with the Al alloy matrix around 590 • C to form AlB 2 [58,59]. The formation of an intermetallic is found to improve the properties [60]. alloy matrix around 590 °C to form AlB2 [58,59]. The formation of an intermetallic is found to improve the properties [60]. Additionally, due to the presence of Ti coating on diamond particles, during the coating process, the carbon atoms of diamond diffuse into the Ti-coated layer, occupying the octahedral interstitial positions of its lattices in Ti crystal-forming δ-TiC. This δ-TiC transition layer combines metallurgically with diamond, and the outer α-Ti layer remains on the outer surface. Upon the addition of Ti-coated diamond to the Al alloy matrix, the outer α-Ti later enables the wetting of the Al alloy matrix [61]. As a result, the Al3Ti phase was formed around the temperature of 590 °C [62,63], as evident from the XRD peaks at 38.46° and 64.56°, as shown in Figure 5; similar peaks were observed in other research works [58]. Additionally, an improvement in the peak reflection of the Al12Mg17 can be seen in Figure 5, indicating that either the amount of (Al12Mg17) increases or the crystallinity of the (Al12Mg17) phase increases with the increase in the Ti-coated diamond content.

Compressive Behavior
The porous Al composites under compression revealed an accurate classical deformation pattern, which can be classified into three phases: (a) The initial phase, known as the linear elastic region, where cell wall bending and face stretching take place; (b) The second phase, called the plateau region, where plastic deformation occurs at constant flow stress; and (c) The third phase, also called the densification region, characterized by a region of a sudden increase in flow stress [64,65], as evident in Figure 6. Also in Figure 6, it can be observed that there was an improvement in the plateau stress of porous Al composites upon the addition of Ti-coated diamond as a reinforcement. This can be attributed to the presence of harder reinforcement in the cell walls of the Al alloy matrix, which strengthens the Al alloy matrix by the Orowan mechanism, thus increasing their plateau stress. In addition, the increase in compressive properties is due to the solid solution Additionally, due to the presence of Ti coating on diamond particles, during the coating process, the carbon atoms of diamond diffuse into the Ti-coated layer, occupying the octahedral interstitial positions of its lattices in Ti crystal-forming δ-TiC. This δ-TiC transition layer combines metallurgically with diamond, and the outer α-Ti layer remains on the outer surface. Upon the addition of Ti-coated diamond to the Al alloy matrix, the outer α-Ti later enables the wetting of the Al alloy matrix [61]. As a result, the Al 3 Ti phase was formed around the temperature of 590 • C [62,63], as evident from the XRD peaks at 38.46 • and 64.56 • , as shown in Figure 5; similar peaks were observed in other research works [58]. Additionally, an improvement in the peak reflection of the Al 12 Mg 17 can be seen in Figure 5, indicating that either the amount of (Al 12 Mg 17 ) increases or the crystallinity of the (Al 12 Mg 17 ) phase increases with the increase in the Ti-coated diamond content.

Compressive Behavior
The porous Al composites under compression revealed an accurate classical deformation pattern, which can be classified into three phases: (a) The initial phase, known as the linear elastic region, where cell wall bending and face stretching take place; (b) The second phase, called the plateau region, where plastic deformation occurs at constant flow stress; and (c) The third phase, also called the densification region, characterized by a region of a sudden increase in flow stress [64,65], as evident in Figure 6. Also in Figure 6, it can be observed that there was an improvement in the plateau stress of porous Al composites upon the addition of Ti-coated diamond as a reinforcement. This can be attributed to the presence of harder reinforcement in the cell walls of the Al alloy matrix, which strengthens the Al alloy matrix by the Orowan mechanism, thus increasing their plateau stress. In addition, the increase in compressive properties is due to the solid solution strengthening that occurs as a result of the addition of alloying elements in the Al matrix [66]. However, it increased up to 12 wt.%; beyond this, the strength declines, mainly due to the presence of higher porosities, making elastic deformation easier. Additionally, this is due to the cell wall brittleness resulting from the weak adhesion force between the Al alloy matrix and diamond reinforcement as a result of the availability of the insufficient matrix for wetting diamond particles. Consequently, the porous specimen is unable to sustain the load applied and causes the collapse of the cell walls at the weakest point with the lowest density, a high stress distribution and the initiation of cracks, thereby reducing the plateau stress considerably [16].
strengthening that occurs as a result of the addition of alloying elements in the Al matrix [66]. However, it increased up to 12 wt.%; beyond this, the strength declines, mainly due to the presence of higher porosities, making elastic deformation easier. Additionally, this is due to the cell wall brittleness resulting from the weak adhesion force between the Al alloy matrix and diamond reinforcement as a result of the availability of the insufficient matrix for wetting diamond particles. Consequently, the porous specimen is unable to sustain the load applied and causes the collapse of the cell walls at the weakest point with the lowest density, a high stress distribution and the initiation of cracks, thereby reducing the plateau stress considerably [16]. The porous Al composite with (12 wt.%) diamond exhibited the highest value of plateau stress, as shown in Table 3. This is due to the presence of denser cell walls due to the presence of well-bonded diamond particles in cell walls, as evident in Figure 4, resulting in higher bending and buckling deformation resistance. The same behavior was observed in CNT-reinforced Al matrix foam [67]. In addition, the crystallinity of intermetallic phases was found to increase ( Figure 5) with an increase in the Ti-coated diamond content, thereby assisting in the improvement of the compressive properties. Further, from the area under stress-strain curves shown in Figure 5, the energy absorption capacity of the porous composites was measured. As evident in Figure 5, the area under stress-strain curve is larger for the porous Al composites with a 12 wt.% diamond The porous Al composite with (12 wt.%) diamond exhibited the highest value of plateau stress, as shown in Table 3. This is due to the presence of denser cell walls due to the presence of well-bonded diamond particles in cell walls, as evident in Figure 4, resulting in higher bending and buckling deformation resistance. The same behavior was observed in CNT-reinforced Al matrix foam [67]. In addition, the crystallinity of intermetallic phases was found to increase ( Figure 5) with an increase in the Ti-coated diamond content, thereby assisting in the improvement of the compressive properties. Further, from the area under stress-strain curves shown in Figure 5, the energy absorption capacity of the porous composites was measured. As evident in Figure 5, the area under stress-strain curve is larger for the porous Al composites with a 12 wt.% diamond content and was recorded to be 7.46 MJ/m 3 as compared to the previous study, where the energy absorption capacity was reported to be 1.41 MJ/m 3 in the resultant porous Al (30 wt.% of PMMA) without diamond particle reinforcement [64]. Additionally, due to the presence of good interfacial bonding between the diamond and Al alloy matrix as a result of Ti-coating, there occurs load transfer between them, resulting in stress concentration, thereby leading to improved strength. With the increase in the diamond content, the compressive strain remains nearly constant; however, the compressive stress levels decrease, thereby decreasing the energy absorption capacity. This is probably due to the presence of a more homogeneous pore structure of porous composites up to 12 wt.%, as shown in Figure 3, thus demonstrating better compressive strength and energy absorption capacity. However, beyond this wt.%, the weak bonding of the diamond particle in the Al alloy matrix leads to poor energy absorption capacity as a result of weak cell structures that can hardly bear higher compressive loading prior to fractures [17]. Therefore, this elucidates that the introduction of diamond as a reinforcement can effectively increase the compressive properties of the porous Al composites.

Conclusions
Porous Al composites with varying Ti-coated diamond particle contents (0, 4, 8, 12 and 15 wt.%) and PMMA (25 wt.%) as a space holder were successfully developed using the powder metallurgy technique. The key findings of this study are summarized as follows: The uniform distribution of particles was obtained after a three-step mixing process, and the resultant composite mix consisted of a regular and lamellar structure with a smaller particle size.
(1) The microstructure of the porous Al composites revealed a uniformly distributed porous structure with less formation of micro-pores and cracks. The pore morphology resembled that of space holders and can thus be controlled. The spherical porosities improve the properties. The porous composites up to 12 wt.% exhibited a homogeneous distribution of pores and diamond particles. (2) The porosity and relative densities were found to be maximum for porous Al composites with a 12 wt.% diamond content due to the better interfacial bonding between the Al alloy matrix and diamond particles as a result of the intermetallic and Ti-coating on diamond particles. (3) The maximum plateau stress and energy absorption capacity were found to depend on the relative density, and their values were 31.51 MPa and 7.46 MJ/m 3 , respectively, for 12 wt.% diamond. (4) Therefore, better microstructural and compressive behavior was obtained for porous Al composites with 12 wt.% of diamond particles; beyond this, the agglomeration of diamond particles occurred.