Dry sliding wear response of aluminium matrix composites (AMCs): a critical review

Researchers were compelled to create composites as alternatives to the already used engineering materials due to the industrial desire for fresh, promising materials with superior mechanical and tribological properties. Due to their superior characteristics, aluminium matrix composites (AMCs) with the appropriate class of particulate/particle reinforcements have been shown to have a wide range of tribological applications. A thorough evaluation of the sliding wear response of aluminium matrix composites (AMCs) in a dry environment using a pin-on-disc wear tester has been attempted in this review study. A discussion regarding wear performance of Al monolithic alloy and its composites has been made with respect to varying process parameters (e.g. normal load, sliding distance, and speed) and the concentration of different particle reinforcements incorporated in the production of aluminium matrix composites. The existing paper provides a synergic presentation of the effects of various intrinsic and extrinsic variables on wear characteristics, leading to the novelty and uniqueness of this review article.


Introduction
Researchers have switched from monolithic to composite materials as a result of the numerous efforts made over the past few years to create materials which can substitute traditional materials with widespread characteristics [1,2]. The term 'composite' refers to a synergetic material system having defined interfacial boundaries between discrete 'reinforcement' constituents incorporated in the 'matrix' phase [3], as shown in figure 1. In the combined form, both constituents retain their physical and chemical identities, however, they prevail in a combination of properties that cannot be attained with either of the constituents [4,5]. Depending on the nature of the constituents used (i.e., matrix and reinforcement) [6], composite materials can be categorized. In the recent few years, considering the reports that were made regarding tough air pollution regulations and increased demands by consumers for improved and complex automobile interiors with electronic parts and multimedia gadgets imposing extra overall weight, the need for lightweight materials for the production of various automobile components can be overemphasized [7][8][9]. This led to the emergence of light-weight aluminium (Al) based composites, an advanced material posing improved physical and mechanical properties such as enhanced strength-to-weight ratio, better stiffness, improved elevated temperature properties, superior resistance to abrasion and corrosion, controlled damping capabilities and coefficient of thermal expansion, control of mass (mainly in reciprocating parts), improved and tailored electrical performance, etc [10][11][12]. Depending on the major alloying elements, there are several aluminium alloy groups (series) that offer greater flexibility for the selection of matrix material [7,13,14]. Among these alloy groups, medium strength (Al 5000 and 6000 series) and high strength (Al 2000 and 7000 series) alloys are commonly employed as the matrix material [15][16][17][18]. Table 1 lists the most common matrix materials used in the manufacture of Al-based composites. On the other hand, reinforcing materials used for the development of AMCs are generally hard ceramic particulates (e.g., TiC, SiC, TiB 2 , ZrO 2 ), continuous fibers (e.g., graphite or boron fibers), or discontinuous whiskers [19][20][21][22][23][24][25][26][27]. In addition, mechanically alloyed particles [28][29][30][31], and some agricultural wastes (e.g., rice husk ash, groundnut shell ash, sugarcane bagasse ash,) and industrial wastes (e.g. red mud, fly ash, bottom ash, cenosphere) have also gained attraction as alternatives for the aforementioned traditional reinforcing materials [30, 32, 33]. Al-based composites are produced utilizing both liquid and solid fabrication procedures, each of which has advantages and limitations of its own, depending on the primary treatment done for the matrix [34][35][36][37] [3,45,46] are effective strategies for addressing the wettability issue, researchers must opt for another production technique because of their criticality and high cost [47,48]. Today, aluminium matrix composites (AMCs) are largely employed in various sectors such as aerospace and automotive industries, railway and defense sectors, structural and marine applications [8,[49][50][51][52][53], e.g. Al-Si alloy (A356 or Al 6061) composites embedded with particulate SiC or combination of Al 2 O 3 and ZrO 2 particulates have been employed in automotive components such as cylinder heads and liners, connecting rods, brake rotors, pistons, and piston rings [51,52,[54][55][56]. In the present review article, a critical assessment is made regarding the dry sliding wear response of aluminium matrix composites highlighting the influence of single or hybrid reinforcements and varying test parameters such as normal load, sliding speed, and sliding distance on wear attributes.

Dry sliding wear response
Wear is the steady material loss caused by surfaces coming into contact with one another when they are being loaded [76]. A better understanding of various wears, including sliding [77], abrasive [78], corrosive [79], and fatigue [80], can result in the creation of wear reduction techniques that are more efficient. Al alloys used for sliding and abrasive wear are primarily based on an Al-Si alloy and are typically utilized in industrial sceneries [81]. Hardness is the mechanical property that has the biggest impact on wear [82,83]. To boost the hardness and wear and scuff resistance of Al alloys, many processes, including solid solution strengthening [84], dispersion strengthening [85,86], and precipitation hardening [87,88], can be used. To strengthen a soft matrix by dispersion, hard particles [89] or a combination of hard and soft particles [31, 90] might be added. The necessity to link and fix the structural components using nuts and bolts or riveting is constant in loadbearing applications [67]. The efficiency of these joints is largely dependent on the material's resistance to fretting wear [91,92], which is the sort of wear that the components in these locations are subjected to. The contact surface's wear, which is in a smaller-amplitude oscillation, is known as fretting wear. As a result, the resistance to fretting wear and the resistance to sliding wear may be connected [93]. Enhanced fretting wear resistance may be achieved by using materials with stronger sliding wear resistance [94]. Addressing the sliding wear properties of structural materials is therefore necessary. The current review article is a modest effort in that direction that provides a critical overview of the influence of varying test parameters and different particle reinforcing agents on the wear attributes of aluminium matrix composites (AMCs). The schematic diagram of the pin on disc wear tester has been illustrated in figure 2.

Influence of particle reinforcement
The studies [34, [95][96][97][98] reported that the wear characteristics of the Al-based composites greatly rely on the class, geometry and architecture, and concentration of the reinforcing elements. In a statistical analysis of wear parameters, Suresh et al [99] stated that amongst all the wear parameters, wt% of reinforcement had the highest influence of around 29% on the wear properties of Al6063-based composites. They found normal load and sliding distance as the other most affecting factors with the contribution of around 7% and 4%, respectively. Ravindranath et al [90] investigated the sliding wear performance of a hybrid Al2219/Gr p /B 4 C p composite and discovered that with the increased load, speed, and traveling distance, the wear in all the materials got raised. However, the hybridized composite was more resistant to wear, which was most likely due to the action of the ceramic particle reinforcements that were present. The particles provided significant resistance to the abrasive's micro-cutting of the composite, resulting in a reduction in the rate at which material was removed from the composites surface. The wear response of an Al430-based composite containing MgO and SiC was studied by Kumar et al [100]. They found that by increasing the amount of reinforcement content, the material wear increased. Weight loss was decreased by 40% when reinforcement concentration was kept at 2.5% at the speed of 600 rpm. Moreover, at the increased reinforcement concentration of 5% and 7.5%, the wear reduction was noticed by 45% and 91%, respectively. At various loads and speeds, the composite wear rate was also lower than those of the base alloy. Sharma et al [101] investigated wear in a composite reinforced with Al/Fly-ash. They discovered that the tribo-counterparts of the disc and composite surfaces had the lowest frictional coefficient and material damage of 0.12 and 0.32 g at 6% of fly ash particulates content. The wear behavior of aluminium containing dual particles of SiC and Cu was investigated by Vedrtnam et al [102]. According to the findings, the weight percentage of the reinforcements was the largest predominating factor in the overall material wear. The sequence of dominance placed load second and sliding speed third, respectively, with sliding distance having the least influence. In a dry environment and at various sliding distances, Singh et al [103] examined the material wear and friction characteristics of an Al-alloy composite materials embedded with SiC dispersoids. The research led to the conclusion that the SiC dispersed Al composites had a 30%-40% lower wear rate than the aluminium matrix alloy. This observation is consistent with those made in a different investigation on the tribological characteristics of Al MMC-SiC p composites by Walczak et al [104]. Their research findings demonstrated that the SiC-reinforced aluminium composite had a wear resistance that was approximately 14% higher than that of the aluminium alloy. Kumar et al [105] used powder metallurgy to create the hybrid composite reinforced with rutile (TiO 2p ), a naturally occurring mineral. Researchers looked at how the rutile type of reinforcing element affected the composite microhardness and wear parameters. Figure 3 depicts the research findings. According to the study, the hybrid Al-SiC p -TiO 2p composite had improved resistance against material damage than that of Al-composite embedded with sole SiC and the monolithic base alloy. It was discovered that the oxide compounds evolved due to the occurrence of TiO 2p , which in the meantime caused the reduction in material wear. These phases resisted the abrasive micromachining. In addition, it was also identified that delaminating type of material damage mode and adhesion kind of wear factor were the other main wear modes. Sarada et al [106] studied a hybrid Al MMC's wear and hardness characteristics. According to the study, hybrid reinforcement increased the composite hardness and reduced wear loss in contrast with sole dispersoid. For a span of 300 s, hybrid reinforced LM25 composite with mica and activated carbon contents exhibited 5% and 10% lower material damage than that of the composite with either active carbon or mica. According to reports [76,99,[107][108][109], Al-based MMCs with reinforcements have a restricted plastic deformation flow. This is such that the counter faces in the composites and the abrasive opposing material are separated by a layer of protecting material [110][111][112]. Gopinath et al [113] noticed the highest enhanced wear resistance in AA6061%-30%BN-10%Al 2 O 3 -5%Gr hybrid composite over all other materials regardless of the test conditions. According to a study by Sharma et al [114], sillimanite-reinforced composite offered enhanced wear resistance over the monolithic alloy. Up to a certain point, increasing the concentration of particulate reinforcement sustained to improve the resistance against material loss; however, beyond that point, fine particle agglomeration caused wear resistance to start to decline. Hillary et al [115] reported superior resistance to wear for AA6061/5%SiC/x%TiB 2 composite when the concentration of the second reinforcement was increased. They suggested that the improvement in the wear resistance was around 72% at miner load and 47% at greater load applications, respectively. Phanibhushana et al [116] observed the wear properties of Al MMC reinforced with hematite. According to the study, adding reinforcement made the composite more resistant to wear and had better mechanical properties including hardness and ultimate tensile strength. A study of the wear behavior of AA7075/Si 3 N 4p MMC was done by Mistry et al [117]. They suggested Si 3 N 4 reinforcing particles as load-carrying elements and observed reduced weight loss in the composites when the percentage of Si 3 N 4 content was increased.

Influence of normal load
In a study, Sharma et al [114] showed that when the applied normal load increased, it consequences greater material loss because of the intensive frictional heating between counter surfaces. The load may have contributed up to 85% of the overall wear of the material under study [118]. This might be due to rise in the bulk material temperature, which results in the loss in material hardness and, eventually, increased wear in the material. Butola et al [3] obtained greater material wear with increased applied load irrespective of the material composition. They also claimed an improved wear resistance in SiC-reinforced composite over Al7075 alloy due to the incorporation of reinforcement. The wear behavior of a composite reinforced with garnet particle was studied by Kumar et al [119]. The investigation demonstrated that as the load increased, material loss rate for both the monolithic material and composites increased. The material damage for the monolithic alloy, however, happened more rapidly, and 200 N served as the load at which the wear mechanism abruptly switched from mild wear to severe wear. The presence of ceramic reinforcement in the composite samples caused a delay in their transition values. The resulting graphic in figure 4 makes this analysis clear. The findings also supported the idea that metal matrix composite reinforcements improve wear resistance at lower loads. Al-AlB 2 composites showed higher wear with applied load in a study performed by Dayanand et al [120]. They also noted that unreinforced alloy had higher volumetric wear loss. According to the study of Saravanakumar et al [121], there was around 41% contribution of applied load on the wear rate of the Al2219-based composites reinforced with Gr particles. In addition, they noticed that irrespective of the reinforcement concentration in the matrix material or the running speed, material wear was increasing when the applied load was increased. The increased material wear of SiC-reinforced Al-based composite due to increased applied load can be seen in figure 5, in a different investigation by Kaushika et al [122]. However, in all cases of applied normal loads, the wear rate was increasing with increased concentration of SiC reinforcement. When researching Al/B 4 C composite, Nieto et al [78] and Celik et al [123] achieved a comparable finding. Herein, the creation of the B 2 O 3 layer decreased the overall frictional coefficient, and greater material loss under higher normally applied loading were common findings by both investigations. According to research by Murthy et al [124], wear in the AA6063 alloy was noticed to be dramatically increased when tested at increased applied load conditions. However, the material loss was less with increasing reinforcement TiB 2 concentration. Singh et al [125] examined the influence of varying applied pressure (0.5 to 2.125 MPa) dry sliding wear behavior of Al alloy composites reinforced with 2%-4% ZrO 2 particle and found that the wear rate initially increased with increasing applied pressure and eventually increased suddenly to a very high value when the materials got seized at higher pressures. The variation in the wear rate of materials in different conditions can be seen in figure 6.

Influence of sliding distance
In a study, Singla et al [126] claimed that in an analysis of the wear properties of Al-SiC composites, the wear rate rose linearly as the sliding distance increased at a fixed sliding velocity. The trend was attributed to the gathering of SiC reinforcement and improper unification of the elements. Radhika et al [127] characterized Aluminium/ Alumina/Graphite composites for tribological behavior and noted a fairly unexpected outcome. Their research revealed that the wear rate and coefficient of friction reduced with increasing sliding distance. The inverse connection was attributed to reduced wear caused by a layer of graphite that was generated at the mating area of the opposing material surfaces and improved resistance against abrasive action enabled through the existing hard ceramic Al 2 O 3 particles. According to Saraswat et al [128] in a different investigation of the Al-B 4 C composite, material loss volume rose along with advancements in the travelling distance. Increased temperature due to amplifies frictional heating at higher sliding travels softening the matrix materials have been linked to this increased wear volume [129]. Figure 7 demonstrates the role of gliding distance on the wear characteristics of an Al-based composite materials containing 4% ZrO 2 particles studied by Singh et al [130]. They found that the wear rate was increased with advancing sliding distance. Furthermore, the alloy, in cast condition, seizure after traveling 4000 m. However, the rest of the materials had not attained seizure even after sliding travel of 5000 m. A study on the impact of sillimanite particulate reinforcement and traveling distance on the sliding wear performance of Al-based composites was conducted by Sharma et al [114]. Because of mechanical joining/weld  at the region where asperities of the opposing surfaces intermingle, it has been revealed that material wear was raised with advancing traveling distance. Moreover, when the oxidized coating established on the pin sample, it was found that the rate of wear decreased as the distance increased. The presence of this coating served as a protective deposit, minimizing the overall mating area existing between the counteracting sides. The third zone occurred wherein the evolution and disintegration of the mechanically mixed layer (MML) turned out to be synchronous, causing consistent material damage with the advancement in the sliding distances. Figure 8 displays a plot that represents the trend. Pramanik [131] investigated the wear properties of an Al6061/Al 2 O 3p composite and found a more or less proportionate relationship between material wear with respect to a traveling distance of 2 km that meticulously followed the Archard theorem.

Influence of sliding speed
In their research, Basavarajappa et al [132] examined the subsurface deformation level in SiC-reinforced Al2219 composite with varying sliding speeds. They revealed that the sublayer experienced higher deformation when the sliding speed was increased in the mild wear zone. Moreover, the variation of wear rate in the materials with different sliding speeds can be seen in figure 9. Marigoudar et al [133] found that under a persistent loading of 40 N, the material wear rose as the sliding speed did. However, as the amount of reinforcement grew, less material  was lost owing to wear. Figure 10 shows the pattern in detail. However, as the tool rotated faster at much higher speeds, there was an increase in heat input that helped the composite's flow characteristics improve and reduced tool wear. In a different investigation for dry sliding wear of AA5083 alloy [134], the material loss rate was higher at low speeds. Also, it was noted that the material loss rate decreased when running velocity raised. This pattern was seen because there was the development of an oxide-coated layer at the mating zone of the opposing surfaces and the coefficient of friction reduced as sliding wear continued. The wear rate decreased as a result of this. However, it was discovered that with larger load applications and greater running distances, the oxide film was lost, leading to a faster material removal from the composite sample. Owing to the crack-resistant feature of AlBr 2 particulate reinforcement, the consequence of varying sliding speeds obeys a more or less linear pattern as noted by Dayanand et al [120].

Wear mechanism
Al-based MMCs are vulnerable to delamination, adhesive, abrasive, and fretting wear modes. Here, it is discussed how surface morphology relates to each of the mechanisms. In a study, Rubesh et al [135] reported  abrasion and delaminating type of wear modes as the leading wear mechanisms during the tribological study of Al-Si alloy/Si 3 N 4 composites. Moreover, these wear mechanisms led to the formation of micro grooves onto the material surface. Delamination is defined as excessive material breakage, which manifests as flake-like debris [119], Figure 11. Schematic illustration of delamination type of wear [147,148]. as well as deep channels, pits, and craters [136]. In delamination wear, as noted by Zhou et al [137], a network of linked cracks forms as a result of weak particle bonding, surface contaminant, and oxidation, as shown in figure 11. Acilar et al [138] noticed the delamination type of wear mechanism exclusively when the test was carried out at higher normal loads. Desai et al [82] characterized the wear surfaces of Al alloy and fly ash-reinforced Al composites and found that crater wear, surface cracks, and adhesion were the dominating wear mechanisms, as can be seen in figure 12. Pits and prows [139] and plastic deformation [140] are indicators of adhesive wear, respectively. Compared to delamination, adhesive wear pits are often smaller [136]. Abrasive wear is defined as having longitudinal or parallel grooves as evidence of micro-cutting or micro-plowing actions [141]. The [142][143][144] reported shallower grooves during the abrasive type of wear than that formed during the delamination in the material. Moreover, fretting type of wear mode is characterized via appearance of minute scrapes and slack oxide waste pieces. Typically, it is brought on by the cyclic stress that results from sliding between two surfaces [105]. While characterizing the worn surface of Al-10% SiC composite, Alagarsamy et al [145] noticed plows and deep groves as the prime wear modes, as shown in figure 13. In a different study by Gupta et al [146], wear debris was observed in the wear surface LM27 alloy composites. They also claimed that grooves and surface delamination were the leading surface wear approaches, as can be seen in figure 14. Table 2 gives an overview of distinct modes of wear in different Al-based composites.

Conclusions
The wear performance of AMCs is improved with hard reinforcing particles. The wear resistance of AMCs rises with an increase in reinforcing particles. Reinforcement particles aid in wear resistance by trying to resist the micro-cutting effect of the abrasion and limiting plastic deformation caused by the shielding oxidized coating of established at the interacting surfaces. The expanse of normal loading determines how quickly materials are removed during the dry sliding wear of AMCs. The effects of sliding distance and sliding speed on the wear rates of AMCs appear to follow predictable patterns. In this aspect, further research needs to be done. AMCs may experience abrasive, adhesive, delamination, and fretting wear processes. While abrasion is more likely in lowload wear situations in reinforced composites, delamination predominates under high loads and in base alloys.
In view of the above-discussed sections, this review paper concludes that the concentration of the reinforcing particles added to the Al-based matrix alloy and the test variables such as normally applied load, sliding speed and sliding distance played critical role in the wear performance of the aluminium matrix composites (AMCs). Moreover, this review paper also states that the wear mechanisms or modes may differ depending on to the compositions of AMCs and other wear test parameters. Therefore, this review paper synergically offers a comprehensive understanding of the effects of various variables on wear performance of AMCs, which makes it novel for the readers.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Funding
None.

Conflict of interests
None.