Structural and Mechanical Properties of Al-SiC-ZrO 2 Nanocomposites Fabricated by Microwave Sintering Technique

: In the present study, Al-SiC-ZrO 2 nanocomposites were developed and characterized. Towards this direction, the aluminum (Al) matrix was reinforced with nano-sized silicon carbide (SiC) and zirconium dioxide (ZrO 2 ), and the mixture was blended using ball milling technique. The blended powder was compacted and sintered in a microwave sintering furnace at 550 ◦ C with a heating rate of 10 ◦ C / min and a dwell time of 30 min. The amount of SiC reinforcement was ﬁxed to 5 wt.%, while the concentration of ZrO 2 was varied from 3 to 9 wt.% to elucidate its e ﬀ ect on the microstructural and mechanical properties of the developed nanocomposites. Microstructural analysis revealed the presence and uniform distribution of reinforcements into the Al matrix without any signiﬁcant agglomeration. The mechanical properties of Al-SiC-ZrO 2 nanocomposites (microhardness and compressive strength) were observed to increase with the increase in the concentration of ZrO 2 nanoparticles into the matrix. Al-SiC-ZrO 2 nanocomposites containing 9 wt.% of ZrO 2 nanoparticles demonstrated superior hardness (67 ± 4 Hv), yield strength (103 ± 5 MPa), and compressive strength (355 ± 5 MPa) when compared to pure Al and other compositions of the synthesized composites. Al-SiC-ZrO 2 nanocomposites exhibited the shear mode of fracture under compression loadings, and the degree of deformation was restricted due to the work hardening e ﬀ ect. The appealing properties of Al-SiC-ZrO 2 nanocomposites make them attractive for industrial applications.

Recently, Al-based nanocomposites have been considered to be better substitutes for the conventional Al alloys and AMMCs because of their multifunctionality and superior properties [14,15].

Materials and Methods
Pure Al (with 99.5% purity, 10 µm average particle size, Alfa Aesar, Tewksbury, MA, USA) was used as the matrix material. SiC (45-55 nm particle size, purity >99%, Alfa Aesar, Tewksbury, MA, USA) and ZrO 2 (15 nm particle size, 99.7% purity, Alfa Aesar, Tewksbury, MA, USA) particles were used as reinforcing materials. The compositions of pure Al and reinforced materials are shown in Table 1. The samples of nanocomposites materials were fabricated by powder metallurgy method involving ball milling and the microwave sintering process. The stoichiometric amounts of the matrix (Al) and reinforcements (SiC, ZrO 2 ) were weighed carefully using an analytical balance (Sartorius, ENTRIS64-1S, Lower Saxony, Germany). The weighed powders were blended at a speed of 200 rpm for 12 0min using a planetary ball mill PM200 (RETSCH 20.640.0001, Haan, Germany). The blended powder weighing~1.0 g was compacted into cylindrical pellets at an applied pressure of 50 MPa for 60 sec. Then, the green pellets were sintered using a microwave sintering furnace (VB ceramic furnace, VBCC/MF/1600 • C/14/15, Chennai, India). The furnace has a designed maximum temperature of 1600 • C with high-grade alumina insulation. This type of insulation helps the furnace have a fast rate of heating and serving for severe thermal shock heating cycles. The compacted cylindrical pellets were sintered in a microwave sintering furnace at 550 • C with a heating rate of 10 • C/min and a dwell time of 30 min. Following the pre-determined time in the microwave, the billet was allowed to cool to room temperature without any holding time under ambient atmospheric conditions. The sintering temperature of the billets was determined using a non-contact IR in the microwave sintering furnace. Figure 1 shows the schematic representation of the experimental procedure of the developed nanocomposites.
used as the matrix material. SiC (45-55 nm particle size, purity >99%, Alfa Aesar, Tewksbury, MA, USA) and ZrO2 (15 nm particle size, 99.7% purity, Alfa Aesar, Tewksbury, MA, USA) particles were used as reinforcing materials. The compositions of pure Al and reinforced materials are shown in Table 1. The samples of nanocomposites materials were fabricated by powder metallurgy method involving ball milling and the microwave sintering process. The stoichiometric amounts of the matrix (Al) and reinforcements (SiC, ZrO2) were weighed carefully using an analytical balance (Sartorius, ENTRIS64-1S, Lower Saxony, Germany). The weighed powders were blended at a speed of 200 rpm for 12 0min using a planetary ball mill PM200 (RETSCH 20.640.0001, Haan, Germany). The blended powder weighing ⁓1.0 g was compacted into cylindrical pellets at an applied pressure of 50 MPa for 60 sec. Then, the green pellets were sintered using a microwave sintering furnace (VB ceramic furnace, VBCC/MF/1600 °C/14/15, Chennai, India). The furnace has a designed maximum temperature of 1600 °C with high-grade alumina insulation. This type of insulation helps the furnace have a fast rate of heating and serving for severe thermal shock heating cycles. The compacted cylindrical pellets were sintered in a microwave sintering furnace at 550 °C with a heating rate of 10 °C /min and a dwell time of 30 min. Following the predetermined time in the microwave, the billet was allowed to cool to room temperature without any holding time under ambient atmospheric conditions. The sintering temperature of the billets was determined using a non-contact IR in the microwave sintering furnace. Figure 1 shows the schematic representation of the experimental procedure of the developed nanocomposites.
The XRD (X-ray diffraction) analysis was performed on the microwave sintered samples (PANalytical X'pert pro, PANalytical B.V., Almelo, The Netherlands), with a scanning rate of 1.5°/min and step size of 0.02° recorded in the 2θ range of 20-80°. The morphological analysis was performed using Field-Emission scanning electron microscope (SEM-FEI Nova NanoSEM 450 FE-SEM, Hillsboro, OR, USA) analysis. The compositional analysis was conducted with energydispersive X-ray spectroscopy (Bruker SDD-EDS, Coventry, UK). The surface morphology of the developed Al-SiC-ZrO2 nanocomposites was studied by using an atomic force microscope (AFM, model MFP-3D, Asylum Research, Abingdon, Oxford, UK). The XRD (X-ray diffraction) analysis was performed on the microwave sintered samples (PANalytical X'pert pro, PANalytical B.V., Almelo, The Netherlands), with a scanning rate of 1.5 • /min and step size of 0.02 • recorded in the 2θ range of 20-80 • . The morphological analysis was performed using Field-Emission scanning electron microscope (SEM-FEI Nova NanoSEM 450 FE-SEM, Hillsboro, OR, USA) analysis. The compositional analysis was conducted with energy-dispersive X-ray spectroscopy (Bruker SDD-EDS, Coventry, UK). The surface morphology of the developed Al-SiC-ZrO 2 nanocomposites was studied by using an atomic force microscope (AFM, model MFP-3D, Asylum Research, Abingdon, Oxford, UK). The relative density of the microwave sintered samples was measured using Archimedes' principle. An analytical balance with density determination equipment (Sartorius YDK03, Lower Saxony, Germany) with an accuracy of ±0.0001 g was used. The microhardness of the Al-SiC-ZrO 2 nanocomposites was determined by using a Vickers microhardness tester (FM-ARS9000, MKV-h21, Tokyo, Japan) with an applied load of 25 gf for 10 sec. The experiment was conducted at room temperature and reported values are an average of five successive indentations for each sample.
The compressive strength of Al-SiC-ZrO 2 nanocomposites analysis was determined at room temperature using a universal testing machine (Lloyd, USA-LR50Kplus, Sussex, UK) applying an engineering strain rate of 10 −4 s −1 under uniaxial compression loadings. The reported values are an average of three successive values of test results. The fractographic analysis was carried out to study the deformation behavior of Al-SiC-ZrO 2 nanocomposites using a field emission scanning electron microscope (SEM-FEI Nova NanoSEM 450 FE-SEM, Hillsboro, OR, USA). Figure 2 shows the X-ray diffraction patterns of the microwave sintered pure Al and the developed nanocomposites containing various concentrations of ZrO 2 . The results confirm the presence of Al (high-intensity peaks) and reinforcements (SiC and ZrO 2 ) in the nanocomposites. The intensity of the reinforcement peaks is minimal as compared to the matrix due to their small volume fraction and may be below the detectable limit of the XRD technique [39]. However, with a further increase in the concentration of the reinforcement, an increment in the peak intensities can be noticed, enabling its appearance in the XRD spectra. For more clarity, the enlarged XRD pattern of A5 nanocomposites is also presented in Figure 2b, which confirms the presence of Al, SiC, and ZrO 2 in the matrix, as represented by different symbols [36,40]. These XRD patterns demonstrate that no other phases or impurities are present, which confirms the high purity of the fabricated nanocomposites. The relative density of the microwave sintered samples was measured using Archimedes' principle. An analytical balance with density determination equipment (Sartorius YDK03, Lower Saxony, Germany) with an accuracy of ±0.0001 g was used. The microhardness of the Al-SiC-ZrO2 nanocomposites was determined by using a Vickers microhardness tester (FM-ARS9000, MKV-h21, Tokyo, Japan) with an applied load of 25 gf for 10 sec. The experiment was conducted at room temperature and reported values are an average of five successive indentations for each sample.

XRD Analysis
The compressive strength of Al-SiC-ZrO2 nanocomposites analysis was determined at room temperature using a universal testing machine (Lloyd, USA-LR50Kplus, Sussex, UK) applying an engineering strain rate of 10 -4 s -1 under uniaxial compression loadings. The reported values are an average of three successive values of test results. The fractographic analysis was carried out to study the deformation behavior of Al-SiC-ZrO2 nanocomposites using a field emission scanning electron microscope (SEM-FEI Nova NanoSEM 450 FE-SEM, Hillsboro, OR, USA). Figure 2 shows the X-ray diffraction patterns of the microwave sintered pure Al and the developed nanocomposites containing various concentrations of ZrO2. The results confirm the presence of Al (high-intensity peaks) and reinforcements (SiC and ZrO2) in the nanocomposites. The intensity of the reinforcement peaks is minimal as compared to the matrix due to their small volume fraction and may be below the detectable limit of the XRD technique [39]. However, with a further increase in the concentration of the reinforcement, an increment in the peak intensities can be noticed, enabling its appearance in the XRD spectra. For more clarity, the enlarged XRD pattern of A5 nanocomposites is also presented in Figure 2b, which confirms the presence of Al, SiC, and ZrO2 in the matrix, as represented by different symbols [36,40]. These XRD patterns demonstrate that no other phases or impurities are present, which confirms the high purity of the fabricated nanocomposites.      confirms the presence of reinforcement. The FE-SEM results are consistent with our XRD results presented in Figure 2. The energy-dispersive X-ray (EDX) analysis and elemental mapping of the Al-SiC-ZrO2 nanocomposites (A3, A4, and A5) are presented in Figure 4. The EDX and elemental mapping results confirm the uniform distribution of Si, C, Zr, and O elements in the Al matrix. The EDX spectra in Figure 4 also proves the presence of Al, Si, C, Zr, and O elements in the Al-SiC-ZrO2 nanocomposites.

AFM Analysis
Atomic force microscopy (AFM) was utilized to measure the surface properties such as the morphology and surface roughness of the developed nanocomposites. Figure 5 presents the 2D images and X-profile of the developed nanocomposites, as well as the corresponding measured surface roughness. The surface roughness was measured in terms of average roughness (R a ) and root-mean-square (RMS) roughness. The Ra of pure Al is 21.793 nm, however, by adding SiC and ZrO 2 nanoparticles, a gradual increase in the surface roughness was observed in the fabricated nanocomposites, reaching its terminal value at 51.565 nm in A5. The increase in the roughness of Al-SiC-ZrO 2 nanocomposites may be associated with the incorporation of hard ceramic nanoparticles of SiC and ZrO 2 , as observed in FE-SEM analysis presented in Figure 3.

AFM Analysis
Atomic force microscopy (AFM) was utilized to measure the surface properties such as the morphology and surface roughness of the developed nanocomposites. Figure 5 presents the 2D images and X-profile of the developed nanocomposites, as well as the corresponding measured surface roughness. The surface roughness was measured in terms of average roughness (Ra) and rootmean-square (RMS) roughness. The Ra of pure Al is 21.793 nm, however, by adding SiC and ZrO2 nanoparticles, a gradual increase in the surface roughness was observed in the fabricated nanocomposites, reaching its terminal value at 51.565 nm in A5. The increase in the roughness of Al-SiC-ZrO2 nanocomposites may be associated with the incorporation of hard ceramic nanoparticles of SiC and ZrO2, as observed in FE-SEM analysis presented in Figure 3.   Figure 6 shows the relative density and microhardness of the microwave developed Al-SiC-ZrO 2 nanocomposites with different reinforcement contents. It was observed that the relative densities of the nanocomposites increased with an increasing amount of ZrO 2 particles in the matrix. The nanocomposites showed higher relative density than that of their base matrix due to the higher density of the SiC (3.21 g/cm 3 ) and ZrO 2 (5.68 g/cm 3 ) as compared to the Al matrix (2.7 g/cm 3 ). The increasing relative density corresponds to the presence of hard reinforcements, which allows the densification of the reinforcements in the nanocomposites [34]. Figure 6 also shows the variation of the microhardness of the nanocomposites with increases in the ZrO 2 content in the Al matrix, and the corresponding calculated values are tabulated in Table 2. The microhardness results show a decent increase in the microhardness of the developed nanocomposites. The observed microhardness of pure Al is 36 ± 3 HV, which reaches the maximum value of 67± 4 HV for the A5 nanocomposite.

Relative Density and Hardness
Crystals 2020, 10, x FOR PEER REVIEW 7 of 12 Figure 6 shows the relative density and microhardness of the microwave developed Al-SiC-ZrO2 nanocomposites with different reinforcement contents. It was observed that the relative densities of the nanocomposites increased with an increasing amount of ZrO2 particles in the matrix. The nanocomposites showed higher relative density than that of their base matrix due to the higher density of the SiC (3.21 g/cm 3 ) and ZrO2 (5.68 g/cm 3 ) as compared to the Al matrix (2.7 g/cm 3 ). The increasing relative density corresponds to the presence of hard reinforcements, which allows the densification of the reinforcements in the nanocomposites [34]. Figure 6 also shows the variation of the microhardness of the nanocomposites with increases in the ZrO2 content in the Al matrix, and the corresponding calculated values are tabulated in Table 2. The microhardness results show a decent increase in the microhardness of the developed nanocomposites. The observed microhardness of pure Al is 36 ± 3 HV, which reaches the maximum value of 67± 4 HV for the A5 nanocomposite.
This increase in the hardness value of nanocomposites with the increasing amount of ZrO2 concentration in the Al-matrix can be associated with (i) the presence of hard ceramic particles (SiC and ZrO2) and their homogenous distribution in Al matrix, (ii) the well-known dispersion hardening effect, (iii) improved densification, which contributes to the increase in hardness, (iv) an improved load-bearing capacity, and (v) strengthening due to the presence of hard particles following the rule of mixture. A similar behavior of the hardness has been reported in previous research [23,41].

Compressive Analysis
The mechanical properties of the developed nanocomposites were further evaluated by conducting a compressive test at room temperature under uniaxial compressive loading. The engineering stress/strain curves, compressive yield strength (CYS), and ultimate compressive strength (UCS) of pure Al and the developed nanocomposites are presented in Figure 7. Figure 7a indicates the engineering stress/strain curves of the developed nanocomposites. Figure 7b shows the calculated yield strength and compressive strength of the developed nanocomposites from the corresponding engineering stress/strain curves. The calculated mechanical properties of the developed nanocomposites are summarized in Table 2. It can be noticed that the addition of ZrO2 nanoparticles into the Al matrix leads to increase in the yield strength, and ultimate compression strength of the nanocomposites. As a comparison, nanocomposites demonstrated improved mechanical properties when compared with Al matrix. Moreover, as shown in Table 2, the A5 sample exhibited the maximum compressive strength of 355 ± 5 MPa with a yield strength of 103 ± 5 MPa at a uniform failure strain of 0.62. This increase in the hardness value of nanocomposites with the increasing amount of ZrO 2 concentration in the Al-matrix can be associated with (i) the presence of hard ceramic particles (SiC and ZrO 2 ) and their homogenous distribution in Al matrix, (ii) the well-known dispersion hardening effect, (iii) improved densification, which contributes to the increase in hardness, (iv) an improved load-bearing capacity, and (v) strengthening due to the presence of hard particles following the rule of mixture. A similar behavior of the hardness has been reported in previous research [23,41].

Compressive Analysis
The mechanical properties of the developed nanocomposites were further evaluated by conducting a compressive test at room temperature under uniaxial compressive loading. The engineering stress/strain curves, compressive yield strength (CYS), and ultimate compressive strength (UCS) of pure Al and the developed nanocomposites are presented in Figure 7. Figure 7a indicates the engineering stress/strain curves of the developed nanocomposites. Figure 7b shows the calculated yield strength and compressive strength of the developed nanocomposites from the corresponding engineering stress/strain curves. The calculated mechanical properties of the developed nanocomposites are summarized in Table 2. It can be noticed that the addition of ZrO 2 nanoparticles into the Al matrix leads to increase in the yield strength, and ultimate compression strength of the nanocomposites. As a comparison, nanocomposites demonstrated improved mechanical properties when compared with Al matrix. Moreover, as shown in Table 2 This improvement in the compressive strength of the nanocomposites compared to the Al matrix can be associated with the coupled effect of the presence of hard ceramic nanoparticles, their uniform distribution in the matrix, and the enrichment of the dislocation density due to dispersion hardening effect [11]. There are many reported strengthening mechanisms/theories to explicate the strengthening mechanism in metal matrix composites. In the present study, the strengthening of the fabricated nanocomposites can be explained by considering the most active strengthening mechanism called the dispersion hardening mechanism. Dispersion hardening is also known as Orowan strengthening, which is caused by the hard second phase dispersed in the matrix material. The strengthening effect in the composite materials can be estimated as given by the Orowan-Ashby equation [1,7], which indicates that the smaller particle size of the reinforcement and its large volume fraction will improve the strength of the composite material when keeping all other variables as constant, as is observed in our nanocomposites. A similar observation has been reported in much earlier reports [6].
where λ is interparticle spacing, G is the shear modulus of Al, b is the Burgers vector of Al, and r is the particle radius of the nanoparticle. The interparticle spacing is given by the following equation [42]: where f is the volume fraction of the reinforcement particles.  This improvement in the compressive strength of the nanocomposites compared to the Al matrix can be associated with the coupled effect of the presence of hard ceramic nanoparticles, their uniform distribution in the matrix, and the enrichment of the dislocation density due to dispersion hardening effect [11]. There are many reported strengthening mechanisms/theories to explicate the strengthening mechanism in metal matrix composites. In the present study, the strengthening of the fabricated nanocomposites can be explained by considering the most active strengthening mechanism called the dispersion hardening mechanism. Dispersion hardening is also known as Orowan strengthening, which is caused by the hard second phase dispersed in the matrix material. The strengthening effect in the composite materials can be estimated as given by the Orowan-Ashby equation [1,7], which indicates that the smaller particle size of the reinforcement and its large volume fraction will improve the strength of the composite material when keeping all other variables as constant, as is observed in our nanocomposites. A similar observation has been reported in much earlier reports [6].
where λ is interparticle spacing, G is the shear modulus of Al, b is the Burgers vector of Al, and r is the particle radius of the nanoparticle. The interparticle spacing is given by the following equation [42]: where f is the volume fraction of the reinforcement particles. 3.6. Fractography Figure 8 illustrates the fractographic images of pure Al and Al-SiC-ZrO 2 nanocomposites under compressive loading. The shear mode structure can be observed in both the pure Al and nanocomposites. It can be observed that the occurrence of shear mode fractures in the Al matrix was less compared to its reinforced nanocomposites [49]. Furthermore, due to the work hardening behavior, the degree of compressive deformation that occurred in the pure Al and the nanocomposites is different. The plastic deformation is restricted because of the presence of secondary phases of the reinforcement in the nanocomposites [50]. Al6061-7Al2O3-20SiC --300 - [48] 3.6. Fractography Figure 8 illustrates the fractographic images of pure Al and Al-SiC-ZrO2 nanocomposites under compressive loading. The shear mode structure can be observed in both the pure Al and nanocomposites. It can be observed that the occurrence of shear mode fractures in the Al matrix was less compared to its reinforced nanocomposites [49]. Furthermore, due to the work hardening behavior, the degree of compressive deformation that occurred in the pure Al and the nanocomposites is different. The plastic deformation is restricted because of the presence of secondary phases of the reinforcement in the nanocomposites [50].

Conclusions
In this study, Al-SiC-ZrO2 nanocomposites containing a fixed amount of SiC (5 wt.%) and varying concentrations of ZrO2 (3, 6, and 9 wt.%) were fabricated successfully by powder metallurgy route using the microwave sintering technique. A comparison of mechanical properties indicated that the fabricated nanocomposites demonstrated superior properties when compared to monolithic aluminum (Al). The mechanical properties of the nanocomposites (hardness, yield strength, and

Conclusions
In this study, Al-SiC-ZrO 2 nanocomposites containing a fixed amount of SiC (5 wt.%) and varying concentrations of ZrO 2 (3, 6, and 9 wt.%) were fabricated successfully by powder metallurgy route using the microwave sintering technique. A comparison of mechanical properties indicated that the fabricated nanocomposites demonstrated superior properties when compared to monolithic aluminum (Al). The mechanical properties of the nanocomposites (hardness, yield strength, and compressive strength) increased with an increasing amount of ZrO 2 . Al-5SiC-9ZrO 2 showed 119%, and 56% increases in yield strength compared to pure Al and Al-5SiC nanocomposites, respectively. This improvement in mechanical behavior can be attributed to the uniform of hard ceramic nanoparticles (SiC and ZrO 2 ) and their presence, which activates the dispersion hardening phenomenon.