Article Structural and Mechanical Properties of Amorphous Si3N4 Reinforced Al Matrix Composites Prepared by Microwave Sintering

Manohar Reddy Mattli 1, Penchal Reddy Matli 2, Abdul Shakoor 1,* and Adel Mohamed Amer Mohamed 3

1 Center for Advanced Materials, Qatar University, Doha 2713, Qatar; [email protected] 2 Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore; [email protected] 3 Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Suez 43721, Egypt; [email protected] * Correspondence: [email protected]; Tel.: +974-44036867

 Received: 10 January 2019; Accepted: 13 February 2019; Published: 14 February 2019 

Abstract: The present study focuses on the synthesis and characterization of amorphous (Si3N4) reinforced aluminum matrix nanocomposites through the microwave sintering process. The effect of Si3N4 (0, 1, 2 and 3 wt.%) nanoparticles addition to the microstructure and mechanical properties of the Al-Si3N4 nanocomposites were investigated. The of Al-Si3N4 nanocomposites increased with increased Si3N4 content, while porosity decreased. X-ray diffraction (XRD) analysis reveals the presence of Si3N4 nanoparticles in Al matrix. Microstructural investigation of the nanocomposites shows the uniform distribution of Si3N4 nanoparticles in the aluminum matrix. Mechanical properties of the composites were found to increase with an increasing volume fraction of amorphous Si3N4 reinforcement particles. Al-Si3N4 nanocomposites exhibits higher hardness, yield strength and enhanced compressive performance than the pure Al matrix. A maximum increase of approximately 72% and 37% in ultimate compressive strength and 0.2% yield strength are achieved. Among the synthesized nanocomposites, Al-3wt.% Si3N4 nanocomposites displayed the maximum hardness (77 ± 2 Hv) and compressive strength (364 ± 2 MPa) with minimum porosity level of 1.1%.

Keywords: aluminum; amorphous silicon nitride; microwave sintering; microstructural; mechanical properties

1. Introduction Aluminum metal matrix composites (AMMCs) have been widely used and have vast applications in automotive and aerospace industries due to their light weight, low density, high strength to weight ratio and attractive properties [1–3]. The process of choosing the matrix and reinforcement is complex, and by adding suitable reinforcements to the matrix material, the properties of the composites can be tailored to meet the specific requirements. The main aim to develop the aluminum metal matrix composites is to produce materials that are lightweight, high strength, and lower cost when compared to the aluminum and traditional aluminum alloys. Aluminum is the most commonly used matrix material in metal matrix composites. Ceramics that are generally used as the reinforcements in metal matrix composites are SiC, B4C, Si3N4, Al2O3, Y2O3, and TiC. These reinforcements provide better microstructure and mechanical properties when combined with the metal matrix materials [4]. When compared to monolithic aluminum, Al-based metal matrix composites shows the significant improvement in mechanical properties in terms of

Ceramics 2019, 2, 126–134; doi:10.3390/ceramics2010012 www.mdpi.com/journal/ceramics Ceramics 2019, 2 FOR PEER REVIEW 2 Ceramics 2019, 2 127 enhanced strength and stiffness, good fatigue and wear resistance, less weight and high-temperature stability [5–8]. enhanced strength and stiffness, good fatigue and wear resistance, less weight and high-temperature There are different methods available to fabricate the aluminum metal matrix composites, such stability [5–8]. as powder metallurgy, forging and stir casting routs [9–11]. Among these, powder metallurgy (PM) There are different methods available to fabricate the aluminum metal matrix composites, such technique, involving blending, compaction and sintering, is one of the common methods used in the as powder metallurgy, forging and stir casting routs [9–11]. Among these, powder metallurgy (PM) synthesis of metal matrix composites. In the blending process, planetary ball mill was used for technique, involving blending, compaction and sintering, is one of the common methods used in blending the matrix and reinforcements powders in order to get uniform distribution of the synthesis of metal matrix composites. In the blending process, planetary ball mill was used for nanoparticles in the matrix phase. The sintering process has the ability to develop the blending the matrix and reinforcements powders in order to get uniform distribution of nanoparticles microstructural and mechanical properties of the final synthesized product. There are many heating in the matrix phase. The sintering process has the ability to develop the microstructural and mechanical processes in the sintering method, such as microwave, vacuum, spark plasma, and the conventional properties of the final synthesized product. There are many heating processes in the sintering method, sintering process [12–14]. In the microwave sintering process, heat is generated within the samples such as microwave, vacuum, spark plasma, and the conventional sintering process [12–14]. In the by rapid oscillation of dipoles at microwave frequencies. Additionally, uniform volumetric heating microwave sintering process, heat is generated within the samples by rapid oscillation of dipoles at of the microwave sintering results in significant improvement in the microstructure and mechanical microwave frequencies. Additionally, uniform volumetric heating of the microwave sintering results properties. in significant improvement in the microstructure and mechanical properties. So far, few materials are used as the reinforcing materials like SiC, B4C, Si3N4, Al2O3, So far, few ceramic materials are used as the reinforcing materials like SiC, B4C, Si3N4, Y2O3, TiC and TiB2, and so on, in metal matrix composites [15–18]. Among these ceramic Al2O3,Y2O3, TiC and TiB2, and so on, in metal matrix composites [15–18]. Among these ceramic reinforcements, amorphous silicon nitride (Si3N4) is selected as the reinforcement used in the present reinforcements, amorphous silicon nitride (Si N ) is selected as the reinforcement used in the present study owing to its good mechanical, thermal 3and4 electrical properties, superior corrosion and good study owing to its good mechanical, thermal and electrical properties, superior corrosion and good thermal stability [19–21]. To the best of authors knowledge, there has been no study conducted on thermal stability [19–21]. To the best of authors knowledge, there has been no study conducted on the the compressive behavior of the amorphous silicon nitride (Si3N4) reinforced Al-matrix compressive behavior of the amorphous silicon nitride (Si N ) reinforced Al-matrix nanocomposites. nanocomposites. 3 4 The present research work deals with the fabrication and characterization of Al-Si3N4 The present research work deals with the fabrication and characterization of Al-Si3N4 nanocomposites using microwave sintering technique (MWS) and the effect of varying contents of nanocomposites using microwave sintering technique (MWS) and the effect of varying contents of amorphous Si3N4 nanoparticles on the microstructure and compressive performance of the synthesized amorphous Si3N4 nanoparticles on the microstructure and compressive performance of the nanocomposites were investigated in detail. synthesized nanocomposites were investigated in detail. 2. Materials and Methods 2. Materials and Methods Pure Al (99.5% purity, ~10 µm, Alfa Aesar, Tewksbury, MA, USA) and amorphous Si3N4 Pure Al (99.5% purity, ~10 µm, Alfa Aesar, Tewksbury, MA, USA) and amorphous Si3N4 nanoparticles (98.5+% purity, ~15–30 nm, Alfa Aesar, USA) were used as raw materials for the synthesis nanoparticles (98.5+% purity, ~15–30 nm, Alfa Aesar, USA) were used as raw materials for the of Al-Si3N4 nanocomposites. Aluminum powder was mixed with 0, 1, 2 and 3wt.% of amorphous synthesis of Al-Si3N4 nanocomposites. Aluminum powder was mixed with 0, 1, 2 and 3wt.% of Si3N4 nanoparticles. The blending of two mixtures was carried out at room temperature in a planetary amorphous Si3N4 nanoparticles. The blending of two mixtures was carried out at room temperature ball mill (PM 200) for 2 hours with the 200 rpm. No balls were used during the blending of powders. in a planetary ball mill (PM 200) for 2 hours with the 200 rpm. No balls were used during the The blended powder was compacted into cylindrical pellets by applying uniaxial pressure of 50 MPa blending of powders. The blended powder was compacted into cylindrical pellets by applying with holding time of 1 min. The compacted cylindrical pellets were sintered in a microwave sintering uniaxial pressure of 50 MPa with holding time of 1 min. The compacted cylindrical pellets were furnace at 550 ◦C with a heating rate of 10 ◦C/min for 30 min. The compacted pellets were placed at sintered in a microwave sintering furnace at 550 °C with a heating rate of 10 °C/min for 30 min. The the center of the cavity and sintering was performed inside the multimode cavity [22]. The schematic compacted pellets were placed at the center of the cavity and sintering was performed inside the of the experimental procedure is presented in Figure1. multimode cavity [22]. The schematic of the experimental procedure is presented in Figure 1.

Specimen

Figure 1. Schematic diagram of Si3N4 -reinforced Al nanocomposites. Figure 1. Schematic diagram of Si3N4 nanoparticle-reinforced Al nanocomposites.

TheThe densitydensity ofof the the microwave microwave sintered sintered samples samples was calculatedwas calculated by using by theusing Archimedes’ the Archimedes’ principle. principle.The X-ray The diffraction X-ray diff (XRD,raction PANalytical (XRD, PANalytical X’pert pro) X’pert analysis pro) was analysis performed was performed to identify to the identify phases present in Al-Si3N4 nanocomposites and to identify the presence of any impurities in the developed the phases present in Al-Si3N4 nanocomposites and to identify the presence of any impurities in the

Ceramics 2019, 2 FOR PEER REVIEW 3 Ceramics 2019, 2 128 developed composites. The XRD analysis was performed in the 2θ range of 30–90° with a scanning ratecomposites. of 1.5°/min. The The XRD morphology analysis of was the performed sintered composites in the 2θ wasrange observed of 30–90 using◦ with scanning a scanning electron rate microscopyof 1.5◦/min. (SEM, The JeolNeoscope morphology ofJSM6000, the sintered Peabod compositesy, MA, USA) was equipped observed with using energy scanning dispersive electron X-raymicroscopy spectroscopy (SEM, JeolNeoscope(EDX). JSM6000, Peabody, MA, USA) equipped with energy dispersive X-ray spectroscopyThe microhardness (EDX). of the Al-Si3N4 nanocomposites was determined by using Vickers MicrohardnessThe microhardness tester (MKV-h21) of the with Al-Si an3N applied4 nanocomposites load of 25 gf was and determineda dwell time by of using10 s, for Vickers each sampleMicrohardness with an average tester (MKV-h21) of 5 successive with indentations. an applied load of 25 gf and a dwell time of 10 s, for each sampleCompression with an average analysis of of 5 successivethe nanocomposites indentations. was performed at room temperature by using the universalCompression testing machine analysis (UTM-Lloyd), of the nanocomposites under an wasengineering performed strain at room rate temperatureof 10−4/s. The by respective using the datauniversal of each testing sample machine was obtained (UTM-Lloyd), by an underaverage an of engineering three successive strain ratevalues of 10 of− 4test/s. Theresults. respective Yield strengthdata ofeach (YS), sample ultimate was obtainedcompressive by an averagestrength of three(UCS) successive and failure values strain of test (FS) results. values Yield strengthof the nanocomposites(YS), ultimate compressive were calculated strength by (UCS)the obtained and failure -strain strain (FS) curves. values Fractographic of the nanocomposites analysis werewas performedcalculated byto thestudy obtained the fractu stress-strainred surfaces curves. of Fractographicthe Al-Si3N4 nanocomposites analysis was performed by using to the study field the emissionfractured scanning surfaces electron of the Al-Simicroscope3N4 nanocomposites (Hitachi FESEM-S4300, by using Tokyo, the field Japan). emission scanning electron microscope (Hitachi FESEM-S4300, Tokyo, Japan). 3. Experimental Analysis 3. Experimental Analysis 3.1. Density and Porosity of Al-Si3N4 Nanocomposites 3.1. Density and Porosity of Al-Si3N4 Nanocomposites Figure 2 shows the variation of density and porosity of the microwave sintered Al-Si3N4 nanocompositesFigure2 shows with thevarying variation Si3N4 content. of density It can and be porosity observed of from the Figure microwave 2 that sintered the density Al-Si of the3N4 compositesnanocomposites increased with with varying the increasing Si3N4 content. amorphous It can Si be3N observed4 content, from due Figureto the 2higher that the density density of Siof3N the4 phase composites [23]. The increased higherwith density the increasingof the sintered amorphous nanocomposites Si3N4 content, influences due to the highermicrostructural density of andSi3N mechanical4 phase [23 properties]. The higher of the density synthesized of the sinteredAl composite nanocomposites materials. The influences porosity the of the microstructural composites hasand shown mechanical a decreasing properties trend of with the synthesized the increasing Al compositeamount of materials.amorphous The Si3 porosityN4 content. of theThe composites nature of thehas reinforcement shown a decreasing content trend and withvolumetric the increasing heating amountphenomenon of amorphous is one of Sithe3N main4 content. reasons The for nature the lowof the porosity. reinforcement content and volumetric heating phenomenon is one of the main reasons for the low porosity.

2.730 0.50 Density Porosity 2.720 (a) 0.40

(g/cc) (b) 2.710 0.30 2.700

2.690 0.20 Porosity (%) 2.680 0.10 2.670 Experimental density

2.660 0.00 0123 0123 Content of Si3N4 (wt.%) Content of Si3N4 (wt.%)

Figure 2. Variation of experimental density (a), and porosity (b) with Si3N4 content. Figure 2. Variation of experimental density (a), and porosity (b) with Si3N4 content.

3.2. XRD Analysis of Al-Si3N4 Nanocomposites 3.2. XRD Analysis of Al-Si3N4 Nanocomposites Figure3 shows the XRD patterns of the microwave sintered Al-Si 3N4 nanocomposites. Figure3b Figure 3 shows the XRD patterns of the microwave sintered Al-Si3N4 nanocomposites. Figure shows the enlarged pattern of the Al-3wt.% Si3N4 nanocomposite. The XRD patterns clearly indicate 3(b) shows the enlarged pattern of the Al-3wt.% Si3N4 nanocomposite. The XRD patterns clearly that the presence of Al and amorphous Si3N4 in Al-Si3N4 nanocomposites and also shows that no other indicate that the presence of Al and amorphous Si3N4 in Al-Si3N4 nanocomposites and also shows phases and impurities are present in Al composites [24,25]. Due to the presence of a small percentage that no other phases and impurities are present in Al composites [24,25]. Due to the presence of a of the reinforcement content, the reinforcement phase appears smaller in intensity as compared to the small percentage of the reinforcement content, the reinforcement phase appears smaller in intensity peaks of the Al matrix phase. The intensity of the amorphous Si3N4 diffraction peak increased with as compared to the peaks of the Al matrix phase. The intensity of the amorphous Si3N4 diffraction the increasing amount of reinforcement content. peak increased with the increasing amount of reinforcement content.

Ceramics 2019, 2 129 Ceramics 2019, 2 FOR PEER REVIEW 4

(a)(a) (b)

Figure 3. (a) X-ray diffraction (XRD) pattern of Al-Si3N4 nanocomposite, (b) Al-3wt.% of Si3N4 Figure 3. (a) X-ray diffraction (XRD) pattern of Al-Si3N4 nanocomposite, (b) Al-3wt.% of Si3N4 nanocomposites.

3.3. SEM Analysis of Al-Si3N4 Nanocomposites

Figure4 shows SEM images of microwave sintered Al-Si 3N4 nanocomposites with different Si3N4 Ceramics 2019, 2 FOR PEER REVIEW 4 contents. It can be observed that clearly the amorphous Si3N4 nanoparticles are homogeneously distributed in the Al matrix. 3.3. SEM Analysis of Al-Si3N4 Nanocomposites

(a(a()) ) (b)

(c) (d)

(e)

Figure 4. Scanning electron microscopy (SEM) images of the (a) pure Al (b) Al-1wt.% Si3N4 (c) Al-2wt.%Si3N4 (d) Al-3wt.% Si3N4 nanocomposites and (e) Energy dispersive X-ray spectroscopy (EDX) spectrum analysis of Al-2% Si3N4 nanocomposites. Figure 4. Scanning electron microscopy (SEM) images of the (a) pure Al (b) Al-1wt.% Si3N4 (c) Al-2wt.%Si3N4 (d) Al-3wt.% Si3N4 nanocomposites and (e) Energy dispersive X-ray spectroscopy (EDX) spectrum analysis of Al-2% Si3N4 nanocomposites.

Figure 4 shows SEM images of microwave sintered Al-Si3N4 nanocomposites with different Si3N4 contents. It can be observed that clearly the amorphous Si3N4 nanoparticles are homogeneously distributed in the Al matrix. The white color particles represent the amorphous silicon nitride and a gray phase represents the aluminum matrix. The EDS analysis was employed to identify the elemental distribution in the Al matrix. Figure 4(e) shows the EDX spectrum of Al-2wt.% Si3N4 nanocomposite was composed of mainly Al, Si and N elements. The homogeneous distribution and an excess amount of amorphous Ceramics 2019, 2 130

The white color particles represent the amorphous silicon nitride and a gray phase represents the aluminumCeramics 2019 matrix., 2 FOR PEER The EDSREVIEW analysis was employed to identify the elemental distribution in the 6 Al matrix. Figure4e shows the EDX spectrum of Al-2wt.% Si 3N4 nanocomposite was composed of mainlysilicon Al, Si nitride and N in elements. aluminum The matrix homogeneous influence on distribution the microstructure and an excess and mechanical amount of amorphousproperties of the siliconAl- nitride Si3N4 innanocomposites. aluminum matrix influence on the microstructure and mechanical properties of the Al- Si3N4 nanocomposites. 3.4. Microhardness of Al-Si3N4 Nanocomposites 3.4. Microhardness of Al-Si N Nanocomposites Hardness is a 3useful4 mechanical property that provides valuable insight into the overall Hardnessmechanical is behaviour a useful mechanicalof composites. property Generally, that se providesveral factors valuable would insight affect intothe microhardness the overall of mechanicalthe composites behaviour such of as composites. particle size, Generally, amount of several reinforcement factors wouldand method affect of the preparation. microhardness The effect of theof composites Si3N4 nanoparticles such as particle in Al composites size, amount is of shown reinforcement in Figure and 5 and method Table of 1. preparation. The effect of Si3N4 nanoparticles in Al composites is shown in Figure5 and Table1.

80

70

60

50

40

30

Microhardness (Hv) 20

10

0 0123

Content of Si3N4 (wt.%)

Figure 5. Microhardness of Al-Si N nanocomposites versus Si N content. 3 4 3 4

Figure 5. Microhardness of Al-Si3N4 nanocomposites versus Si3N4 content. It can be clearly seen that the microhardness of the Al-Si3N4 nanocomposites gradually increased with the increase in amorphous Si3N4 content [26]. The Al-3wt.% Si3N4 composite has the highest hardness valueIt can (77 be± clearly4) when seen compared that the with microhardness that of pure Al. of This the higher Al-Si value3N4 nanocomposites of the hardness ofgradually the nanocompositesincreased with can bethe associated increase in with amorphous the presence Si3N of4 hardcontent amorphous [26]. The silicon Al-3wt.% nitride Si3N nanoparticles4 composite has in the the aluminumhighest hardness matrix and value the dispersion(77 ± 4) when hardening compared effect [with27,28 ].that The of microhardness pure Al. This of higher the microwave value of the sinteredhardness samples of inthe this nanocomposites study was found ca ton be higherassociated than with the vacuum the presence sintering of hard samples amorphous [29]. silicon nitride nanoparticles in the aluminum matrix and the dispersion hardening effect [27,28]. The The presence of hard amorphous Si3N4 nanoparticles improves the hardness of the composites materialsmicrohardness as explained of bythe the microwave rule of mixtures sintered [30 samples]. in this study was found to be higher than the vacuum sintering samples [29].

3 4 The presence of hard amorphousHc =Si HNmV nanoparticlesm + HrVr improves the hardness of the composites materials as explained by the rule of mixtures [30]. where Hc represents the hardness of the composite, Hm and Vm represents the hardness and Hc = HmVm + HrVr volume fraction of the matrix and Hr and Vr represents the hardness and volume fraction of the reinforcements,Where respectively. Hc represents the hardness of the composite, Hm and Vm represents the hardness and volume fraction of the matrix and Hr and Vr represents the hardness and volume fraction of the 3.5. Compressivereinforcements, Analysis respectively. of Al-Si3N 4 Nanocomposites

Figure6a presents the engineering stress-strain curves of the microwave sintered Al-Si 3N4 nanocomposites and their corresponding mechanical data presented in Figure6b and Table1.

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Ceramics 2019, 2 131 3.5. Compressive Analysis of Al-Si3N4 Nanocomposites

140 370 Yield strength 130 Ultimate Compression strength 360

120 350 (a) (b) 110 340

100 330

90 320

Yield strength (MPa) strength Yield 80 310

70 300 Ultimate compression strength (MPa) 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Content of Si3N4 (wt.%)

Figure 6. (a) The compression stress-strain curves and (b) mechanical data of Al-Si3N4 nanocomposites.

Figure 6. (a) The compression stress-strain curves and (b) mechanical data of Al-Si3N4 nanocomposites.It can be seen that the yield strength (YS) and ultimate compressive strength (UCS) significantly increase with the addition of amorphous Si3N4 content. The value of yield strength (YS) obtained Figure 6(a) presents the engineering stress-strain curves of the microwave sintered Al-Si3N4 for Al-3wt.% Si3N4 composite was 127 ± 4 MPa, with an ultimate compressive strength (UCS) of nanocomposites364 ± 2 MPa at and a uniform their corresponding failure strain mechanical of 7.4%. The data increase presented in in yield Figure strength 6(b) and and Table compressive 1. strengthIt can ofbe theseen nanocomposites that the yield strength can be (YS) attributed and ultimate to the strengtheningcompressive strength mechanisms (UCS) because significantly of the increaseeffective with load the transfer addition between of amorphous the matrix Si and3N4 reinforcementcontent. The value particles, of yield dispersion strength hardening (YS) obtained effect for and Al-3wt.%uniform Si distribution3N4 composite of reinforcement was 127 ± 4 MPa, particles with in an the ultimate matrix compressive [31,32]. strength (UCS) of 364 ± 2 MPa at a uniform failure strain of 7.4%. The increase in yield strength and compressive strength of the nanocompositesTable 1. Microhardness, can be attributed yield strength to andthe ultimatestrengthening compressive mechanisms strength ofbecause Al-Si3N of4 nanocomposites. the effective load transfer between the matrix and reinforcement particles, dispersion hardening effect and uniform distributionComposition of reinforcement particles Microhardness in the (Hv) matrix [31,32]. CYS (MPa) UCS (MPa) Failure Strain (%) Pure Al 38 ± 3 70 ± 4 305 ± 3 >7 Al-1wt.% Si N 43 ± 5 86 ± 5 325 ± 5 >7 Table 1. Microhardness,3 4 yield strength and ultimate compressive strength of Al-Si3N4 Al-2wt.% Si3N4 58 ± 3 104 ± 3 349 ± 4 >7 nanocomposites. Al-3wt.% Si3N4 77 ± 2 127 ± 4 364 ± 2 >7 Al-3wt.% Si N [29] 57 ± 2 198 ± 21 292 ± 18 – 3 4 CYS UCS Failure Strain Al-9wt.%Composition Si3N4 [33] 59.5Microhardness – (Hv) – – Al alloy-3wt.% Si3N4 [34] 82 —(MPa) — (MPa) – (%) Pure Al 38 ± 3 70 ± 4 305 ± 3 >7 ThereAl-1wt.% are several Si strengthening3N4 mechanisms 43 and ± 5 theories to 86 enhance ± 5 the 325 mechanical± 5 properties >7 of the compositeAl-2wt.% materials. Si3N4 In the present study, 58 the± 3 main strengthening 104 ± 3 349 mechanism ± 4 is dispersion >7 hardening.Al-3wt.% Dispersion Si3 hardening,N4 also known as 77 Orowan ± 2 strengthening, 127 ± 4 is 364 caused ± 2 by the >7dispersed second phase.Al-3wt.% The Si Orowan3N4 [29] strengthening mechanism 57 ± 2 is given by 198 the ± Orowan-Ashby21 292 ± 18 equation -- [35]. Al-9wt.% Si3N4 [33] 59.5 ------Al alloy-3wt.% Si3N4 [34] 0.13 82 Gb d ------σOrowan = ln (1) λ 2b whereThereG isare the several shear strengthening modulus of Al,mechanismsb is the Burgersand theories vector to ofenhance Al, d isthe the mechanical average diameterproperties of ofnanoparticles, the composite and materials.λ is the In interparticulate the present study, distance the main between strengthening the reinforcement mechanism particles, is dispersion which is hardening.given by the Dispersion following hardening, equation [36also]. known as Orowan strengthening, is caused by the dispersed second phase. The Orowan strengthening mechanism is given by the Orowan-Ashby equation [35]. " #  1 1/3 λ = d 0.13Gb− 1 d (2) σ = 2 f ln (1) Orowan λ 2b where,Wheref is G the is volumethe shear fraction modulus of the of reinforcementAl, b is the Burgers particles. vector of Al, d is the average diameter of nanoparticles, and λ is the interparticulate distance between the reinforcement particles, which is given by the following equation [36].

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13 1 λ =−d 1 (2) 2 f Ceramics 2019, 2  132

Where, f is the volume fraction of the reinforcement particles.

3.6. Fractography of Al-Si3N4 Nanocomposites 3.5. Fractography of Al-Si3N4 Nanocomposites The fracture morphology analysis of the microwave sintered pure Al and Al-Si3N4 nanocompositesThe fracture tested morphology under compression analysis of loading the microwave is shown insintered Figure7 .pure A typical Al and shear Al-Si mode3N4 fracturenanocomposites can be observed tested under in the nanocompositescompression loading and the is fractureshown surfacesin Figure show 7. A the typical cracks shear at 45o modeto the compressionfracture can be loading observed axis. in The the compressive nanocomposites deformations and the fracture obtained surfaces in Al composites show the cracks are indifferent. at 45o to Thisthe compression is due to the workloading hardening axis. The behavior. compressive The plastic deformations deformations obtained of the composites in Al composites are restricted are byindifferent. the presence Thisof is adue dispersion to the work of the hardening second phase behavior. in the The matrix plastic [37]. deformations of the composites are restricted by the presence of a dispersion of the second phase in the matrix [37].

Figure 7. (a–d) Compression fracture surface images of Al-Si3N4 nanocomposites.

4. ConclusionsFigure 7. (a–d) Compression fracture surface images of Al-Si3N4 nanocomposites.

4. ConclusionsIn this study, amorphous Si3N4 nanoparticle reinforced Al matrix nanocomposites were successfully synthesized using microwave sintering process. The effect of amorphous Si3N4 content In this study, amorphous Si3N4 nanoparticle reinforced Al matrix nanocomposites were on the microstructural and mechanical of the developed Al composites are investigated. Experimental successfully synthesized using microwave sintering process. The effect of amorphous Si3N4 content density of the synthesized composites increased with the increase in amorphous Si N content, on the microstructural and mechanical of the developed Al composites are investigated.3 4 while porosity slightly decreased. The XRD analysis reveals the presence of amorphous Si3N4 in Experimental density of the synthesized composites increased with the increase in amorphous Si3N4 Al matrix. SEM and EDX analyses show the uniform distribution of amorphous Si N nanoparticles content, while porosity slightly decreased. The XRD analysis reveals the presence3 4of amorphous in the aluminum matrix. Hardness, Yield strength and compressive strengths of the nanocomposites Si3N4 in Al matrix. SEM and EDX analyses show the uniform distribution of amorphous Si3N4 increased with an increasing silicon nitride content, hence, Al-3wt.%Si N nanocomposite exhibited nanoparticles in the aluminum matrix. Hardness, Yield strength and 3compressive4 strengths of the enhanced hardness (77 ± 2 Hv), yield strength (27 ± 4 MPa) and UCS (364 ± 2 MPa). nanocomposites increased with an increasing silicon nitride content, hence, Al-3wt.%Si3N4 Authornanocomposite Contributions: exhibitedA.S. andenhanced A.M.A.M. hardness proposed (77 the ± original2 Hv), yield project strength and supervised (27 ± 4 the MPa) investigation. and UCS M.R.M.(364 ± and2 MPa). P.R.M. performed the experiments, analyzed the data, and wrote the paper with assistance from all authors. All authors contributed to the discussions in the manuscript. Author Contributions: A.S. and A.M.A.M. proposed the original project and supervised the investigation. Funding: This publication was made possible by NPRP Grant 7-159-2-076 from the Qatar National Research Fund (aM.R.M. member and of P.R.M. the Qatar performed Foundation). the experi Statementsments, analyzed made herein the data, are solely and wrote the responsibility the paper with of the assistance authors. from all authors. All authors contributed to the discussions in the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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