A. Influence on the strength properties of the MMC's

Aluminum MMCs are produced by adding harder ceramic particles such as SiC and Al₂O₃ into monolithic aluminum alloys to obtain improved strength properties and heat treating the MMC's will further help in improving specific properties such as compressive strength, ductility, and microstructural refinements. Commonly, tensile strength of MMC's prepared by casting method is about 1.5–2 times of the samples produced by powder metallurgy route. The powder metallurgy route had comparable compressive strength and hardness data as samples prepared by casting route. Tensile and compressive strength were decreased and hardness is increased by increasing the ceramic particulate amount. ^{Reference Ünlü8} In addition, as particulate amount increased, notch impact strength decreased in casting materials and bending strength decreased in powder metallurgy processed samples. ^{Reference Ünlü8} The increase in the volume percentage of SiC particulates refines the structure of the matrix and gives a finer grain size when samples produced through squeeze casting technique. ^{Reference Bekheet, Gadelrab, Salah and El-azim9} The highest hardness of MMC's is marginally higher than that of 2024 monolithic alloy, which was achieved by cold deformation process after adding SiC particles. The time required to reach the highest hardness is apparently affected by the presence and amount of SiC particulates. ^{Reference Bekheet, Gadelrab, Salah and El-azim9} The addition of SiC particulates and artificial aging of the aluminum alloy with copper presence greatly improves the fatigue limit of the MMC's. Roughly 100% increase in the fatigue limit has been accomplished by addition of 5 wt% SiC particulates to the monolithic aluminum alloy. ^{Reference Bekheet, Gadelrab, Salah and El-azim9}

Age hardening is an inexpensive way to improve the mechanical properties of MMC's compared to monolithic aluminum alloys. During the powder metallurgy fabrication of aluminum alloy 2014 which are heat treatable, age-hardening of aluminum alloy MMC's occur in three different steps; solution treating at (510 °C), quenching in water medium and aging at (20, 150, 200 °C), which increases strength and hardness by maximum 80% and minimum 40%. ^{Reference Durmuş and Meriç10} The Vickers hardness values have increased during aging duration accounts for roughly 60% of the overall hardness increase witnessed at the high hardness condition for sintered at 600 °C and natural aged sample. At both temperatures, rapid increase in hardness is witnessed within aging duration of 20 h for sintering temperature at 600 °C artificial aged and natural aged sample, supreme hardness value was ensured in samples sintered at 600 °C and samples that are artificial aged at 150 °C found with hardness value of (120 HV), in these samples the microstructure was uniformly dispersed with Al₂Cu precipitates and over aging of the composites caused the degradation of the mechanical properties because of larger amount of precipitates formed and dispersed in the matrix when processed by powder metallurgy route and hardness increased with the increase in artificial aging time. ^{Reference Durmuş and Meriç10} A direct correlation exists between the density and hardness, where density reduces with increase in the reinforcement percentage, the hardness increased and vice versa. All natural aged samples are observed to have lesser hardness than artificial aged MMC samples. ^{Reference Durmuş and Meriç10} Peak hardness was observed for natural aged and sintered MMC's sample at 620 °C. The micro hardness of natural aged samples decreased with increasing the sintering temperature. There was an increase in theoretical density with increase in the sintering temperature of the MMC's. Peak theoretical density values are noticed for MMC samples that are sintered at 620 °C. ^{Reference Durmuş and Meriç10}

Compared to conventional sintering methods discussed earlier, mechanical properties of aluminum alloys can be improved by hybrid microwave sintering process conducted near to melting temperature of the monolithic parent metal, where diffusion process intensifies because of microwave processing of the composites and improved grain distribution can be attained. ^{Reference Wai, Eugene and Gupta11–Reference Singh, Gupta, Jain and Sharma13} The micro hardness of microwave processed composite samples was 1064 HV, and percentage of porosity was considerably reduced to ∼0.89%. Improvements in the field of microwave processing of composites have improved strength when compared to conventional heating techniques. ^{Reference Singh, Gupta, Jain and Sharma13} For samples with monolithic aluminum alloys, an observed development in MMC's hardness, 0.2% yield strength, UTS and work of fracture was noticed when compared with hybrid microwave sintered samples with conventionally sintered one's. ^{Reference Wai, Eugene and Gupta11} Aluminum with TiN as reinforcement with various fractions of 10, 20, and 30 wt% MMC's were manufactured by powder metallurgy route and microwave processed. Experimental data propose that the microwave sintering for 2 min is satisfactory and melting of composite pellet was observed while increasing the sintering time. Increasing the reinforcement of TiN from 10 to 30 wt% exhibited higher hardness and strength properties when compared to aluminum with TiN composites prepared by conventional hot pressing. This is attributed to good interfacial bonding noticed in the matrix microstructure due to excellent diffusion mechanism assisted by microwave sintering technique. ^{Reference Venkateswarlu, Saurabh, Rajinikanth, Sahu and Ray14} Considering ceramics reinforcements like SiC and Al₂O₃, alumina has a better reflection on the mechanical properties of the composite as shown in Fig. 2. However wear behavior of the composites might be diminished. ^{Reference Senthilvelan, Gopalakannan, Vishnuvarthan and Keerthivaran15,Reference Kang and Chan16} Heat treating the aluminum alloy 2xxx, the precipitation phases found in the MMC's in both as processed and solutionized conditions were CuAl₂ and CuMgAl₂ phases. The MMC's samples solution treated for 1 h has a larger amount of these undissolved precipitates in the matrix microstructure than those solutionized for 2 h and 3 h. ^{Reference Mousavi Abarghouie and Reihani17,Reference Dhadsanadhep, Luangvaranunt, Umeda and Kondoh18} . The MMC's reached its max hardness in lesser time compared with the unreinforced monolithic aluminum alloy on solution treatment for 2 h and 3 h, but reached it in longer time on solution treatment for 1 h, where considerable amount of uniformly dispersed strengthening precipitates are observed in the matrix (Fig. 3). ^{Reference Mousavi Abarghouie and Reihani17}

FIG. 2.

Tensile properties of the aluminum and composites and yield strength for nano-Al₂O₃/Al composites compared with experimental results. ^{Reference Kang and Chan16}

FIG. 3.

Effect of alumina particle size and weight percentage on the strength parameters of the composites. ^{Reference Rahimian, Parvin and Ehsani25}

Current trend in powder metallurgy is fabrication of hybrid aluminum alloy composites by adding two or more reinforcements to the matrix. The increase in weight percentage of glass and SiC particulates as reinforcement in the aluminum matrix increases the strength characteristics of the MMC's, because of decrease in pore size or improvement in relative density of the samples were observed when fabricated by powder metallurgy route. In general, the strain hardening index value increases with increasing weight percentage of glass and SiC particles as the pore size decreases. This intern decreases the geometric work hardening and increases the metal matrix work hardening. The strength coefficient value increases with higher weight percentage addition of glass and SiC because of better densification of powder mixtures processed by powder metallurgy. ^{Reference Persson, Weiland, Hultman and Wahlstro4,Reference Kumar, Narayanasamy and Loganathan19} The coarser particulate of size (150 µm) of glass and SiC added in hybrid MMCs shows an increase in strain hardening index and increase in strength coefficient characteristics because of improved load transfer rate of glass and SiC is observed in the aluminum matrix compared to fine particulate sizes. ^{Reference Persson, Weiland, Hultman and Wahlstro4,Reference Kumar, Narayanasamy and Loganathan19,Reference Purohit, Rana and Verma20}

The improved strain index and improved formability stress index are again noticed because SiC ceramic particles are harder than glass. Al-4% glass with various volume percentage of SiC particulate hybrid MMC's shows improved formability stress index characteristics when compared to Al-4% SiC with various percentage of glass hybrid MMC's, which actually infers that the amount of ceramic content positively influence the strength behavior and the microstructural behavior of the composites. ^{Reference Kumar, Narayanasamy and Loganathan19} Varying the particle size of the ceramic content also has better effect on the strength properties of the composites. Pure monolithic aluminum alloy and its MMCs reinforced with 10 vol% SiC particulates of average particle size 3.5, 10, and 20 µm were fabricated by powder metallurgy route. The result obtained indicates that both monolithic pure aluminum and SiC-particulates-reinforced powder metallurgy processed composites exhibit improved characteristics of apparent-stress exponent of 14.3 ± 26.1 and superficial activation energy (253 ± 261 kJ/mol). ^{Reference Tjong and Ma21} The creep rates of aluminum composites reinforced with lesser SiC particles size of 3.5 µm are 2 times lesser in magnitude and creep resistance with larger particulates of size 10 and 20 µm are nearly to those of monolithic aluminum alloys. ^{Reference Tjong and Ma21} A diffusion or super solidus liquid state sintering method was used to enhance desired properties of powder metallurgy based aluminum alloy 2014 with additions of Ag or Sn. But by using such diffusion and liquid state sintering technique, hardness and tensile properties of highly aged powder metallurgy processed aluminum alloy 2014 were superior to the wrought alloys. As well, the powder metallurgy processed composites exhibited better corrosion resistance. ^{Reference Bishop, Cahoon, Chaturvedi, Kipouros and Caley22} The passive oxidation performance of SiC particulates at elevated temperatures, interfacial response in aluminum alloy 2014 reinforced with SiC particles after prolonged high thermal exposure was characterized for the samples fabricated by liquid infiltration technique. According to the oxidation characteristics of SiC particulates, the oxidation constraints and the degree of oxidation for SiC particulates can be efficiently controlled. ^{Reference Shi, Yang, Lee, Zhang, Lee and Wu23} Observation on the interfacial response between the oxidized SiC particulates and aluminum alloy (AA) 2014 metal matrix displayed that nano-particles MgO can form primarily but will ultimately turn into a thermo stable MgAl₂O₄ crystals. The SiO₂ layer which exhibit good bonding adherence with both SiC and 2014 aluminum matrix metal; however, it can be exhausted by the interfacial response after prolonged high thermal exposure. ^{Reference Shi, Yang, Lee, Zhang, Lee and Wu23} Apart from preprocessing of ceramic reinforcements with techniques like oxidation and ceramic coatings, mechanical alloying of the matrix metal improves the composites characteristics by the increased grade of deformation, increased density of displacements and strengthening by oxide and carbide scattering. The addition of ceramic reinforcement particulates drives the mechanical alloying technique due to the improved deformation enforced on the matrix metal phases. ^{Reference Purohit, Rana and Verma20,Reference Fogagnolo, Velasco, Robert and Torralba24} In a study, the investigation of 0–20 wt% alumina (Al₂O₃) with average particle sizes 48, 12, and 3 µm and aluminum matrix of average particle size of 30 µm fabricated by powder metallurgy route on the mechanical properties like relative density, hardness, yield strength, compressive strength, and elongation. Axial compaction pressure of 440 MPa and sintering temperature of 550 °C for 45 min was used in the fabrication of this MMC's. As the amount of Al₂O₃ particles increases, the aluminum MMC's relative density reduces.

As the Al₂O₃ particle size increases, the relative density increases at first, and then reduces. ^{Reference Rahimian, Parvin and Ehsani25,Reference Pournaderi, Mahdavi and Akhlaghi26} Reducing the alumina (Al₂O₃) particulate size increases the hardness and raising the amount of Al₂O₃ from 10 to 20 wt% with 12 µm particle size gives rise to an increase in hardness from 62 to 74 Brinell hardness number (HBN), with peak hardness of 79 HBN for 20 wt% 3 µm sized Al₂O₃ particulates. ^{Reference Rahimian, Parvin and Ehsani25,Reference Pournaderi, Mahdavi and Akhlaghi26} The finer particulate size of Al₂O₃ offers superior yield strength and compressive strength of the composites which is because of decrease in the particle size and at lesser weight fraction of the ceramic content, grain size and alumina particle distribution homogeneity was observed to increase in the microstructure of the MMC's. By raising the amount of Al₂O₃ up to 10 wt%, yield strength and compressive strength raises, where peak was 307 MPa for particulate size of 3 µm, conversely, raising the amount of Al₂O₃ from 10 to 20 wt% exhibited a reverse trend. ^{Reference Rahimian, Parvin and Ehsani25,Reference Kok27} Existence of finer Al₂O₃ in MMC's affects peak elongation. Addition of 20 wt% of 12 µm Al₂O₃ to the monolithic aluminum alloys, results in reduction of elongation percentage by 32% because at lesser particle size and with more weight percentage the number of particles count increase which leads to agglomeration of the ceramic reinforcements. ^{Reference Kok27} The reduction in the Al₂O₃ particle size in a stable quantity results to a raise in the elongation. ^{Reference Rahimian, Parvin and Ehsani25,Reference Kok27}

The fabrication of macro aluminum composites reinforced with 1.0 to 5.0 wt% percentages of boron nitride nano tube (BNNT) were produced by two different techniques of powder metallurgy, namely spark plasma sintering (SPS) and high pressure torsion (HPT). The characteristics for aluminum with BNNT 3.0 wt% MMC's was observed to be double (190 MPa) when compared to monolithic aluminum–HPT compacts (90 MPa). And the ultimate tensile strength of aluminum with BNNT MMC's comprising 3.0 wt% BNNT (∼300 MPa) ∼1.5 times higher than that of a nano tube free HPT–aluminum material(∼200 MPa). ^{Reference Yamaguchi, Meng, Firestein, Tsuchiya and Golberg28} Remarkable grain refinement was observed in HPT samples compared to SPS ones which leads to an overall denser microstructure with a minor amount of micro pores between the sintered aluminum grains. ^{Reference Yamaguchi, Meng, Firestein, Tsuchiya and Golberg28}

Generally in heat treatable aluminum alloys, failures occur by dimple type fracture from inclusions or second phase particulates where inter granular, volumetric shear, and delamination type fractures were observed. Fracture of precipitate phases of intermetallic compounds like CuAl₂ and Al₂CuMg is noticed in the size range of 5–8 µm and their strength is valued approximately 710 MPa and decreasing with increasing particulate sizes. ^{Reference Kobayashi29} Fractures of metal ceramic compositions like Al₃Zr or Al₃Ti particulates were not observed in MMC's. Besides, it is understood that delamination happens when the normal stress against the grain boundary inter phase reaches 116 and 113 MPa when synthesized at liquid helium and room temperature, respectively. ^{Reference Kobayashi29}

An examination on the forgeability and strength properties of powder metallurgy processed and extruded, co-sprayed SiC reinforced 2014 and 2618 MMCs were studied. The results examined leads to a conclusion that, segregated bands, holding high and low concentration of SiC particulates, formed an exceptional metallurgical array in the co-sprayed reinforced matrix material only. ^{Reference See and Dean30} The forgeability of powder metallurgy processed AA 2014 MMC is superior, but strength properties are inferior compared to MMC's manufactured by the co-spray deposition method. ^{Reference See and Dean30} The forgeability characteristics of AA 2618 MMC's are less than one half of it characteristics when compared to AA 2618 monolithic alloy.

The lesser ductility characteristics of the MMC's containing more SiC ceramic particles may confirm the fact that higher volume fractions generally give a nonhomogenous mixture and grouping of the ceramic particles, reducing the mechanical properties of the MMC's i.e., increasing the volume fraction of 16 µm SiC particulates reduces the strain to fracture characteristics as shown in Fig. 4. ^{Reference Marahleh5,Reference Hassani, Bagherpour and Qods31} MMC's with higher amount of SiC particles or with lower particle size, the total metal matrix–ceramic interfaces is longer because of which the finer reinforcement particulates cause an raise in strength and a reduction in ductility and formability of the MMC's (Fig. 5). ^{Reference Hassani, Bagherpour and Qods31}

FIG. 4.

Stress strain behavior of aluminum material reinforced with various percentage of SiC and Al₂O₃ ceramic particulates. ^{Reference Marahleh5}

FIG. 5.

Variation of hardness versus aging time for 2024Al-20 vol% SiC composite and 2024 Al alloy aged at 191 °C and solutionized at 495 °C for: (a) 1 h; (b) 2 h; (c) 3 h. ^{Reference Mousavi Abarghouie and Reihani17}

Powder metallurgy technique was used to fabricate a dense AA-4.0 Cu (wt%) MC's reinforced with 3–15 µm-65 vol% Si particulates which were uniformly dispersed where, larger volume fractions of silicon has been the auspicious material for lightweight and low-thermal-expansion characteristics. ^{Reference Liu, Wei, Fan, Ma and Zuo32} Tensile strength and flexural strength of the MMC's were recorded as 282 and 455 MPa, respectively. The observed fracture toughness of the MMC's was 4.90 MPa √m. The enhanced strength of 65%Si-AA was qualified to the optimized particle behavior and metal matrix characteristics. ^{Reference Liu, Wei, Fan, Ma and Zuo32}

The validation of various volume percentage of β phase-Al₃Mg₂ intermetallic compounds as strengthening agents in powder metallurgy processed MMCs were studied, results outcome showed that compression tests at room temperatures exposes that the adding of the β phase-Al₃Mg₂ intermetallic compounds curiously increases the strength properties of monolithic aluminum alloy. ^{Reference Scudino, Liu, Sakaliyska, Surreddi and Eckert33} In specific, the MMC's with 20 and 40 vol% of reinforcement shows yield strength and compressive strengths exceeding the monolithic aluminum alloy by a multiple of 2–3, however holding considerable plastic deformation characteristic ranging from 15% to 45% to . ^{Reference Scudino, Liu, Sakaliyska, Surreddi and Eckert33} The strength of the MMC's is additionally increased for the samples reinforced with 60 and 80 vol% of β phase-Al₃Mg₂ intermetallic phases; however, the MMC's exhibit negligible plastic deformation. Additionally, adding the low-density β phase-Al₃Mg₂ particles decrease the density of the MMC's compared to that of monolithic aluminum alloy, noticeably increasing the specific strength of the MMC's. ^{Reference Scudino, Liu, Sakaliyska, Surreddi and Eckert33}

As discussed earlier, improvement on the interfacial strength of powder metallurgy processed aluminum–ceramic (SiC) MMC's were studied by coating electroless nickel on the ceramic particle SiC. It was visualized that ultimate failure load for electroless nickel coated ceramic (SiC) reinforcement has shown an increasing trend by 40% with respect to uncoated one. ^{Reference Umasankar, Karthikeyan and Xavior34} The improvement in interfacial bonding strength was witnessed for coated ceramics (SiC) reinforcement in aluminum matrix. The coating improved diffusion of ceramics reinforcement (SiC) into the aluminum matrix as shown in Fig. 13. ^{Reference Umasankar, Karthikeyan and Xavior34}

Investigating the effect of Al₂O₃ of particle size 12 and 48 µm, sintering temperature of (500–600 °C) and sintering time of (30–90 min) on the properties of Al–Al₂O₃ composite were studied. The peak strength of 318 MPa, maximum hardness of 76 HB was observed in samples having average particulate size of 3 µm and conventionally sintered at 600 °C for duration of 45 min and increasing in sintering time to 90 min results in a decreasing in hardness by 59 HB, which is of 33% reduction in hardness values in prolonged sintering time. ^{Reference Rahimian, Ehsani, Parvin and Baharvandi35} For prolonged sintering times and smaller the particle sizes of Al₂O₃, larger the compressive strength and improved elongation percentage of the MMC's were noticed. ^{Reference Rahimian, Ehsani, Parvin and Baharvandi35} Peak elongation was 61.8% in MMC's samples having the average particulate size of 3 µm because at lesser particle size of ceramic contents the porosity levels in the matrix micrographs are observed to be less and leads to higher relative densities of the composites, in turn improved mechanical properties. ^{Reference Rahimian, Ehsani, Parvin and Baharvandi35}

For powder metallurgy processed aluminum composites tensile strength properties of artificially under aged and over aged MMC's were observed to be merely same. Notched bend results showed that artificially under aged MMC's fracture at higher bending stresses than did the artificially over aged MMC's. Fracture in artificially under aged MMC's initiated through ceramic SiC particles fracture ahead of the crack tip point by following linkage through failure in the monolithic matrix metal. ^{Reference Chennakesava Reddy and Kotiveerachari36} Crack diverging and crack bridging were observed in the artificially under aged MMC's, while the over aged MMC's displayed a planar crack with a more liking for fracture near the ceramic SiC metal matrix interface. ^{Reference Chennakesava Reddy and Kotiveerachari36}

MMC's of aluminum alloy 2900 metal matrix reinforced with 10%, 15%, and 20% wt% of SiC, Al₂O₃, and graphene were prepared by powder metallurgy route. The brittleness was observed in the sample when the hardness test was conducted after duration of 2 months letting the samples to natural age, which had lead to formation of intermetallic compounds such as Al₂Cu in larger amount in the matrix that dispersed which degrades the strength of MMC's. ^{Reference Ashwath and Xavior37} The results were obtained when the samples are microwave sintered at rate of heating 20 °C per minute because of carbide formation in the samples considering the rate of heating factor. At 2 °C per minute heating which is almost equal to conventional sintering type will eliminate such issues. ^{Reference Ashwath and Xavior37}

Conventional powder metallurgy method was used to fabricate nanometric particulates reinforced aluminum MMC's. It is obvious from the effort that the nanometric particulates enhance the mechanical properties in the monolithic aluminum alloy. The major strengthening mechanism was observed to be the Orowan strengthening by these ceramics particulates. ^{Reference Kang and Chan16} However, as the particulate volume percentage increased, the degree of agglomerations of the ceramic particulates increased. The nano-particulate amount in the MMC's when increased more than 4 vol%, the agglomerations were observed to reduce. ^{Reference Kang and Chan16}

B. Influence on the tribological properties of the MMC's

Among all the ceramic reinforcements such as SiC whiskers, TiC particles, boron, carbon and Al₂O₃, SiC particulates are widely used due to their low cost, varied choices of available grades, mostly more stable and chemically compatible SiC and Al₂O₃ particles addition improves the resistance to wear of the monolithic aluminum alloys. The experimental data verified that addition of the 20 wt% of SiC particulate to the liquid metallurgy processed aluminum alloy steered to an intense improvement in resistance to wear property of the monolithic aluminum alloy. The wear resistance of MMCs was found favorable and observed to be better than those of ductile iron, its alloy and heat-treated specimens, tested at the 70 µm size of abrasive sheets. The resistance to wear increased with increasing wt% ceramic particles added to the MMCs. ^{Reference Sahin and Kilicli38} When composites tested against 70 µm sized abrasive sheets, it was noticed that the amount of wear rate in the MMC's was less than that of the aluminum alloys irrespective of all applied loads with, micrographs observations such as small quantity of wear craters, reattachment of debris particles due to abrasion of fractured particles in MMC's. ^{Reference Sahin and Kilicli38} Considering the MMC's, the harder reinforcing SiC particulates might be resisted against the micro material removal action of abrasives, and the least material removal was observed for the 20 wt% SiC particles reinforced MMC's than that of the 10 wt% SiC particulate reinforced MMC's, but no substantial variances were noticed between them because of comparable mechanism throughout abrasive wear. ^{Reference Sahin and Kilicli38}

Testing powder metallurgy processed aluminum alloy with ASTM standard, resistance to wear was observed to reduce as sintering time was increased. This behavior is a result of micro structural coarsening. ^{Reference Bishop, Li, Tandon and Caley39} Out of samples that were Ag reformed, and Sn reformed aluminum alloy and wrought 2014, Sn reformed powder metallurgy processed specimens showed the peak resistance to wear. ^{Reference Bishop, Li, Tandon and Caley39} Hybrid MMCs based on Al/SiC/Gr have been effectively fabricated by powder metallurgy route tested for various loadings and sliding velocities examined, MMC's have displayed improved resistance to wear and improved coefficient of friction when compared with monolithic alloy matrix. The fusion of ceramic SiC particle reinforcement and addition of graphene to AA 2024 matrix raises the resistance to wear of the MMC's. The resistance to wear property and coefficient of friction of the MMC's samples increases with increasing the reinforcement percentage as shown in Fig. 6. ^{Reference Ravindran, Manisekar, Rathika and Narayanasamy40} From Fig 15, the existence of harder SiC particulates in higher amount had influence on the wear behavior for which SEM micrographs clearly displays the existence of deep permanent grooves, micro cutting, grain pullouts and fracture of the oxide debris, which have lead to increase in wear loss. ^{Reference Ravindran, Manisekar, Rathika and Narayanasamy41} The wear mechanism driving in hybrid MMC's with 20 wt% SiC and 5% graphite hybrid MMC's was delamination and oxidative wear (Fig. 7). ^{Reference Ravindran, Manisekar, Rathika and Narayanasamy40,Reference Ravindran, Manisekar, Rathika and Narayanasamy41}

FIG. 6.

Variation of COF and wear loss with SiC content. ^{Reference Ravindran, Manisekar, Rathika and Narayanasamy40}

FIG. 7.

Effect of matrix alloy on (a) wear rate; (b) maximum temperature; (c) coefficient of friction; (d) seizure pressure; sliding velocity: 3.35 m/s. ^{Reference Rao and Das46}

A study on heat treatable aluminum alloy 2024 with Al₂O₃ processed by liquid metallurgy route does not reflect a valuable role of alumina (Al₂O₃) particles during the wear process for MMC's in extruded condition. The as extruded aluminum alloy 2024 comprises fine precipitate particulates and alumina particles aligned along extrusion direction. ^{Reference Narayan and Bai42} Moreover, unlike SiC particulates which are recognized as a good load bearing material and resist surface plastic flow, the alumina (Al₂O₃) particles have the tendency to form fragments and disperse. ^{Reference Narayan and Bai42} Heat treating the MMC's with alumina exhibited lesser surface cracks and failure where the precipitates are observed to be uniformly dispersed. Alumina with finely dispersed precipitates in the matrix material enhances the tribological property of the composites even at higher loading conditions. ^{Reference Narayan and Bai42}

Results on wear study of powder metallurgy processed Al-MMC's reinforced with 15 wt% ceramic SiC particulates using pin on disc wear testing machine against several SiC particle abrasive grades showed that, the abrasive grain size applied the highest effect on the wear rate of MMC's, followed by the improved hardness properties. ^{Reference Şahin43} In consideration, bigger particles in the composites lead to more resistance to wear than those of smaller particle sized reinforcements in the composites, where the matrix microstructure with bigger particles ceramic content was observed to be agglomeration free compared with lesser particle sized reinforcement. This wear behavior might be because of fabrication technique of powder metallurgy, particulate sizes, the interaction effects, and testing conditions of the experiments planed. ^{Reference Şahin43}

Dry sliding wear experiments on cast samples were conducted by a pin-on-disk wear testing machine where the disks materials were aluminum alloy MMCs reinforced with various ceramic reinforcements such as SiC-whiskers, Al₂O₃-fibers and SiC-particles, and the pin materials were made of 0.45% carbon steel. The amount of enhancement on resistance to wear of MMCs powerfully relies on the type of ceramic reinforcement and its volume fractions. Firstly severe wear rate was not observed for MMCs for volume fractions V _fp = 5%, V _ff = 10%, and V _fw = 22%. ^{Reference Miyajima and Iwai44} It is understandable that the ceramic reinforcement effectively prevents the initial severe wear rate and both fibers type and particulates type ceramics reinforcement was more operative in this respect than whiskers type once.

The steady-state wear of the composites were observed to decrease with increase in volume fraction of the ceramic reinforcement V _f. ^{Reference Miyajima and Iwai44} The volume percentages at which the steady-state wear attained the least value were 2, 10, and 29% for MMC with particulates, MMC's with fibers and MMC's with whiskers, respectively. Ceramic whisker fragments developed in the wear process were smaller in size, resulting in a lesser destructive action on the counter face of the rubbing material. ^{Reference Miyajima and Iwai44} Moreover, fiber and particulate fragments were large in their size and angular in their shape, so that they encouraged abrasion on the counter face at higher volume fractions V _f. particulate type ceramic reinforcements are better in case of improving wear resistances and frictional coefficient as a focused property as shown in Fig. 14. ^{Reference Miyajima and Iwai44}

Aluminum alloy matrix composite reinforced with low amount (0.2–0.6 wt%) of graphene have been effectively fabricated through powder metallurgy technique and it was noticed that change in the hardness data on addition of graphene is inadequate. The wear studies obviously prove the optimistic effect of graphene addition where it suppresses the friction coefficient and wear rate (weight loss). Especially, graphene addition to the matrix acts as a film which improves the tribological property. ^{Reference Kumar and Xavior45} However, it progresses the surface roughness constraints by diminishing the wear grooves and least value is seen for 0.4 weight percentage of graphene in the aluminum matrix alloy. ^{Reference Kumar and Xavior45}

Sliding wear characteristics of the aluminum alloy composites examined for various weight fractions with response constraints such as amount of wear rate, seizure pressure, temperature rise, and frictional coefficient were studied. The wear resistance of the alloy is improved significantly due to ceramic particle addition. The wear rate is observed to decrease with increase in SiC content irrespective of matrix metal. At early wear period, the temperature rise in the aluminum alloy sample is noticed to be least. ^{Reference Rao and Das46}

Amongst the MMC's samples, rise in temperature is noticed to be high in AA-25 wt% SiC MMC's and less in the case of AA-10 wt% SiC reinforced MMC's, this behavior is addressed that, at higher amount of SiC the large number of particles count in the matrix is observed to be dispersed, which when wear tested will create high temperature in the contact interface where more particles come into contact with the mating disc. 2024 aluminum alloy system suffers from maximum material removal rate, where as the 7010 aluminum alloy system showed least material removal rate regardless of SiC amount and processing conditions. Aluminum alloy and ceramic SiC reinforcement plays a significant role in governing the wear behavior of the MMC's. ^{Reference Rao and Das46} In a nut cell growth of seizure temperature because of varying SiC amount from 0 to 25 wt% is noted to be around 25 °C. An average seizure temperature increase is observed at the factor of 1 °C per wt% SiC particles. The coefficient of friction values increases at a factor of 0.123 per wt% of SiC ceramic particles. ^{Reference Rao and Das46}

Seizure pressure during wear varies with various aluminum alloy systems, 7010 displayed highest seizure pressure, while 2024 displayed least seizure pressure irrespective of processing condition and SiC ceramic content. 7010 alloy exhibited good tribological characteristics because of the higher amount of alloying elements added which leads to formation of intermetallic precipitates in larger amount and dispersed evenly when compared to 2024 and 7009. The material removal coefficient is observed to decrease with increase in the applied load reaching to a least value and then again observe to increase when the applied load reached near to the seizure of the samples. ^{Reference Rao and Das46,Reference Asif, Chandra and Misra47} It is motivating to notice that the seizure load of MMC's barely affected when SiC ceramic content increase from 10 to 15 wt%. This was attributed to the fact that higher temperature flowability of the MMC's systems cannot be changed when SiC ceramic amount varied from 10 to 15 wt% weight percentage. ^{Reference Rao and Das46}

The hardness values and density measurements of the hybrid MMC's were observed to decrease with an increase in graphite amount. The MMC's with 5 wt% graphite showed both exceptional resistance to wear and frictional coefficients. The addition of solid lubricant particulates as the second phase reinforcement in the metal matrix successfully increases frictional coefficients and wear behavior. ^{Reference Asif, Chandra and Misra47,Reference Ravindran, Manisekar, Narayanasamy and Narayanasamy48} A consistent graphite film on top of the worn surface helped to reduce both the wear loss and friction coefficient avoiding severe wear in the Al/5% SiC/5% Gr composites. Inclusion of graphite as primary reinforcing material increases the resistance to wear characteristics of MMC's by developing a defensive layer between mating pin and counter face of the disk and the addition of alumina (Al₂O₃) as a secondary ceramic reinforcement also had an important effect on the wear behavior of the MMC's. ^{Reference Radhika, Subramanian and Prasat49,Reference Iacob, Popescu and Buzatu50}

The effects of the hardness characteristics of counter face on the wear rate of ceramic SiC whisker or SiC particle reinforced aluminum MMCs were examined with pin-on-disk wear testing machine in which MMC were coupled by pins or disks. Concerning MMC's pin against steel disk mating specimens, the wear rates of MMC's of pins were persistent below a given hardness values, but varying inversely proportional with increasing hardness values of the mating steel counter face. ^{Reference Miyajima, Sasayama, Honda, Fuwa and Iwai51}

Looking on the other hand, the amount of wear on the steel disk material, varying inversely proportional with their hardness values. Concerning MMC's disk against steel pin samples, the amount of wear of MMC's whiskers was observed to increase marginally but those of MMC's particles displayed almost persistent values with increase in counter face hardness values. The amount of wear of the steel counter faces was curiously less and observed to increase for elevated hardness. ^{Reference Miyajima, Sasayama, Honda, Fuwa and Iwai51}

For the MMC fabricated by stir casting technique comprising the equal amount of B₄C particulate reinforcements, the specific wear rates were observed to decrease with increase in amount of B₄C particle. Plastic deformation (micro-cutting and micro-ploughing) was identified as the main wear mechanism operating on the worn surfaces of the composites that is investigated. Plastic plowing and cutting parameters are affected and shows a decreasing trend by adding B₄C particulates. ^{Reference Canakci and Arslan52}

The penetration and amount of trenches in MMC's are decreasing with increasing volume percentage and particle size of B₄C particles. ^{Reference Canakci and Arslan52} The surface roughness of the MMC's specimens increased rapidly with sliding duration up to 670 s and then increased gradually or practically stranded with the sliding duration for all specimens after 670 s. The surface roughness of the MMC's is observed to be decreasing normally with increasing volume percentage and particle size of ceramic reinforcement (Fig. 8). ^{Reference Canakci and Arslan52}

FIG. 8.

Effect of wear loss, initial wear rate and steady state wear wrt sliding distance and volume percentage for three different type of reinforcement. ^{Reference Miyajima and Iwai44}

C. Influence on the microstructural properties of the MMC's

Powder metallurgy processing cover a major area of green compact sintering out of which there are various types available for sintering the MMC's. Those are conventional sintering electrical furnace type; microwave sintering, hybrid microwave, and SPS. Among researchers, there is a misconception that all metallic materials reflect microwave or cause plasma creation, and hence cannot be microwave heated except exhibiting surface heating due to limited diffusion of the microwave emission. Many commercial powder metallurgy components of various alloy material compositions including iron and steel, copper alloys, aluminum alloys, nickel alloys, Mo, Co, Ti, W, Sn, etc., have been microwave processed to attain better properties than their conventional counterparts. The properties of the materials are determined by their microstructures. ^{Reference Oghbaei and Mirzaee12,Reference Singh, Gupta, Jain and Sharma13} It is usually agreed that a decisive concern in microstructural development is the relationship between relative densification and grain coarsening. To control this microstructural growth, parameters such as sintering temperature, compaction pressure, sintering duration, and rate of heating should be optimized. ^{Reference Oghbaei and Mirzaee12,Reference Singh, Gupta, Jain and Sharma13} Out of all these parameters, fast heating rates have seemingly created helpful properties such as improved final sintered density for a given grain size or fine microstructure compared with conventional type processing techniques. Microstructural improvement, improved finer grain size on an average and excellent relative density achieved in products by microwave sintering, result in superior mechanical properties with respect to conventionally processed samples (Fig. 9). ^{Reference Singh, Gupta, Jain and Sharma13}

FIG. 9.

Normalized wear rate of MMCs as function of V _f. (a) Whisker reinforcement MMC, (b) fiber reinforced MMC and (c) particle reinforced MMC. ^{Reference Miyajima and Iwai44}

Current activated, pressure activated infiltration technique was observed to be the effective way of producing ceramic metal composites where the ceramic foam Ti₂AlC was infiltrated with molten aluminum alloy 6061. From Fig. 10 it is understood that 97% of the pores in the foam was infiltrated by molten aluminium. ^{Reference Hu, Kothalkar, O'Neil, Karaman and Radovic53} Nearly all the residual pores observed in the samples are closed pores and are formed due to incomplete sintering during fabricating of the preform samples. This novel method leads to improvement in the mechanical property of specific compressive strength of the composite by 1.5 times of the specific yield strength of peak aged aluminum alloy 6061. ^{Reference Hu, Kothalkar, O'Neil, Karaman and Radovic53} Small reaction was between the ceramic foam and the molten aluminum, which suggests that the current activated, pressure activated infiltration method a highly reactive ceramic–metal composite processing method which could not be achieved by using powder metallurgy and conventional infiltration methods. ^{Reference Hu, Kothalkar, O'Neil, Karaman and Radovic53}

FIG. 10.

(a) A picture of a porous Ti₂AlC preform, and SEM images of (b) showing open pores and struts, (c) showing both closed pores and open pores. (d) A picture of an as infiltrated AA 6061/Ti₂AlC composite, and SEM images of (e) showing AA 6061 (dark gray) infiltrated into the open pores, (f) enlarged view on the strut showing Ti₂AlC (1, light), titanium aluminide (2, gray), and Al₂O₃ (3, dark). ^{Reference Hu, Kothalkar, O'Neil, Karaman and Radovic53}

MMC's with larger quantity of ceramics reinforcement are marked with arrows in SEM images of Fig. 11 which shows little amount of porosities in the sintered MMC's specimens. By decreasing the particulate size from 16 to 12 µm, the amount of porosity was decreased by a factor of about 3.7%. In the MMC's with larger quantity of ceramic reinforcements or lesser particulate size, the total matrix–ceramic interfaces are observed to be longer. As a result the paths for diffusion, is one of the mechanisms for pore elimination during sintering process in the MMC's. As a result, the reduction of porosity is accelerated. The finer particle size of ceramic reinforcement leads to a huge number of minor pores after sintering of MMC's. The microstructure properties and mechanical properties of a MMC, is directly related with the volume fraction and particle size of its ceramic reinforcements.

FIG. 11.

SEM images of sintered MMC's (a) Al-10% SiC-12 µm and (b) Al-10% SiC-16 µm. Some porosity is indicated by the arrows. ^{Reference Hassani, Bagherpour and Qods31}

Figure 12 shows the optical micrographs which indicate the microstructure of the specimens containing 5–20 wt% of alumina (Al₂O₃) with average particulate size of 12 µm. It can be observed that the increase in the volume percentage of alumina (Al₂O₃) leads to a finer grain microstructure which have a favorable effect on the hardness values of the composites fabricated by powder metallurgy route. ^{Reference Rahimian, Parvin and Ehsani25,Reference Rahimian, Ehsani, Parvin and Baharvandi35} As the alumina (Al₂O₃) amount decreases in the MMC's, alumina (Al₂O₃) particulate dispersion homogeneity increases. This is because alumina (Al₂O₃) particulates perform as a barrier against the grain boundary sliding during sintering process. This quality of ceramic behavior in the MMCs leads to improved mechanical properties. The ceramic particulate dispersion in the matrix containing 20 wt% (Al₂O₃) will decrease because of larger tendency for clustering and agglomeration with increased amount of ceramic reinforcements. ^{Reference Rahimian, Parvin and Ehsani25,Reference Rahimian, Ehsani, Parvin and Baharvandi35}

FIG. 12.

SEM micrographs showing the effect of 12 µm Al₂O₃ particle content on the microstructure (LHF a, b, c) and the effect of Al₂O₃ particle size on the microstructure (RHF a, b, c). ^{Reference Rahimian, Ehsani, Parvin and Baharvandi35}

FIG. 13.

SEM images of Ni coated Al-SiC composites. ^{Reference Umasankar, Karthikeyan and Xavior34}

In aluminum alloy based MMCs, particle to particle bondability is reduced because of the formation of oxide layer on aluminum surfaces in the matrix. Nevertheless the development of metalized Nickel layer on SiC particulates helps the interface bonding. ^{Reference Umasankar, Karthikeyan and Xavior34} The coverage of aluminum alloy powders by metalized SiC is observed to be nonuniform in nature. This may be because, emanation of aluminum alloy starts at 500 °C sintering temperature.

Addition of the ceramic particulates reduced the grain size in the MMC's. The biggest reduction was noticed at early additions of the ceramic particulates, i.e., at 5 wt% irrespective of the aluminum matrix metal, then the reduction was noticed at 20 wt% ceramic particulate additions the grain sizes were more or less even, which is reflecting in the strain characteristics as shown in Fig. 4. From Fig. 4 the lesser ceramic content which leads to better strain rate is compared to other percentage of ceramic reinforcements. ^{Reference Marahleh5} The MMC’s grain sizes are finer than that of the monolithic aluminum matrix. This modification is caused by the ceramic reinforcement particulates performing as nuclei for the grain construction during the process of solidification and similarly these ceramic particulates would restrain the process of the grain growth. ^{Reference Marahleh5} The amount of ceramic addition had a positive influence on the stress strain behavior of the composites where the lesser amount of ceramic addition lead to considerable stress strain behavior equal to the monolithic matrix metal as shown in Fig. 4. ^{Reference Marahleh5}

The maximum effect in refining the grain size was in case of the SiC reinforced MMC's. Similar observation was seen where 20 wt% Al₂O₃ addition produced 90% decrease in the grain size growth of the MMC's. ^{Reference Marahleh5} Considering the wear micrographs of the MMC's reinforced with ceramic contents, at minimum volume fraction V _f [Figs. 14(a) and 14(c) and (e)], the worn out surfaces were significantly dissimilar for every MMC's. MMC with whiskers and MMC's with fiber type formed bigger wear scars caused by adhesion of the ceramics to the matrix. ^{Reference Miyajima and Iwai44} The damages by adhesion were heavier for MMC with fibre type than that of MMC with whiskers type. Only deep grooves were noticed for MMC with particle type. For higher volume fractions V _f [Figs. 14(b), 14(d) and 14(f)], the bonding area and huge grooves vanished on all composite samples. MMC with fiber-type displayed a brittle worn out surfaces with undersized grooves, but MMC with particle-type had a leveled and soft surfaces. ^{Reference Miyajima and Iwai44} Comparing the three types of reinforcements used, the microstructural observation reveals that particle type ceramic reinforcements gives better microstructural characteristics without compromising the tribological properties of the MMCs as discussed earlier.

FIG. 14.

SEM photographs of the worn out surfaces of MMCs in steady-state wear regime (sliding distance 1000 m). (a) SiC-whisker, V _fw = 5%; (b) SiC-whisker, V _fw = 29%; (c) Al₂O₃-fiber, V _ff = 5%; (d) Al₂O₃-fiber, V _ff = 19%; (e) SiC-particle, V _fp = 5% and (f) SiC-particle, V _fp = 10%. ^{Reference Miyajima and Iwai44}

Figure 15 shows the worn out surfaces of the aluminum alloy 2024 matrix (Figs. 15a and 15b) which clearly displays the existence of deep eternal grooves, micro cutting, particle pullouts, and fracture of the oxide fragments, which might have caused the amplification of the wear loss. This morphology shows that the aluminum alloy 2024 matrix has undergone considerable rigorous plastic deformation. ^{Reference Ravindran, Manisekar, Rathika and Narayanasamy40}

FIG. 15.

SEM morphologies of the worn out surface of Al 2024 matrix at applied load of 20 N. (a) Low-magnification micrograph and (b) high-magnification micrograph. ^{Reference Ravindran, Manisekar, Rathika and Narayanasamy41}