Fabrication and Investigation of Abrasive Wear Behavior of AZ31-WC-Graphite Hybrid Nanocomposites

Abstract

In current investigation, effects of incorporation of varying amount of graphite nanoparticles on abrasive wear behavior of AZ31-WC nanocomposites are examined. AZ31-WC-Gr hybrid nanocomposites are developed using ultrasonic vibration associated stir casting technique. Developed hybrid nanocomposites are characterized using optical microscope (OM) and scanning electron microscope (SEM). Compositions of hybrid nanocomposites are investigated by energy dispersive spectroscopy (EDS). Characterization results disclose that reinforcement particles are uniformly distributed in AZ31 matrix. Compositional analysis confirms fortification of reinforcements. Microhardness values of developed hybrid nanocomposites are examined through microhardness tests. Abrasive wear behavior of AZ31 alloy and AZ31-WC-Graphite nanocomposites are investigated for varying sliding distance and varying abrasive grit size in a pin-on-disc tribotester. Abrasive wear tests disclose that incorporation of only 1 wt. % of graphite nanoparticles enhances wear resistance significantly. AZ31-2WC-1Graphite nanocomposite is found to be the most wear resistant material for all experimental conditions. Worn surfaces are scrutinized under SEM and EDS to reveal worn morphology. Investigation of worn surfaces discloses that abrasion and oxidation are main wear mechanism for AZ31-2WC-1Graphite nanocomposite tested at 50 mm track diameter and 800 grit.

1. Introduction

Nowadays, research fraternity is highly emphasized on metal matrix composites due to its attractive specific properties. Basically, fortification of secondary ceramic reinforcements in base matrix is an impressive mechanism to achieve desirable properties, which may not be achieved by matrix metals or single phase materials. Recently, metal matrix composites (MMCs) have become a favorable choice for automobile, electronics, aerospace, and chemical industries. Therefore, scientific community is continuously trying to meet their demands by incorporating different ceramic reinforcements in different matrices. In this regard, Mg is favorable as a matrix metal due to the impressive strength to weight ratio, excellent castability, good machinability (50% better than Al), better manufacturability, high damping capacity, etc. [1]. Even Mg based materials are taking over different ferrous components (pistons, brake components, steering shaft, and transmission case) of automobile sectors. Accordingly, Mg/Mg alloys have become a favorable choice as matrix metal among researchers. Recently, different micron level reinforcements, i.e., BN, Al₂O₃, Y₂O₃, TiC, B₄C, SiC, ZnO, etc. were fortified in Mg matrix [2,3,4,5,6,7,8]. Moreover, the literature also reveals that the particle size of reinforcements has great importance on properties of MMCs. Dey and Pandey [9] have observed that particle size of reinforcement always play an important role to control properties. Huang et al. [10] observed noticeable effect of particle size on tribological characteristics of Mg-MMCs. Gopal et al. [11] have noticed a significant contribution of particle size and amount (wt. %) of CRT panel glass on hardness and the tribological behavior of Mg-CRT-BN composites. Moreover, the recent development in nanoscience has also instigated the scientific community toward nanolevel reinforcements. The available literature also noticed that fortification of approximately 2 wt. % nanoparticles is sufficient to attain desirable properties [12]. Accordingly, different nanoparticles (TiC, SiC, BN, Al₂O₃, B₄C, ZnO, WC, etc.) have been incorporated in Mg matrix by the scientific community [13,14,15,16,17,18,19,20,21,22,23]. Nowadays, WC nanoparticles are being emphasized by researchers due to different excellent properties, such as superior hardness, high melting point, higher elastic modulus, excellent oxidation resistance, etc. [24]. Recently, Banerjee et al. [20,21,22] investigated the contribution of fortified WC nanoparticles on mechanical, tribological, and corrosion behavior. It was reported that elastic modulus and microhardness were enhanced by 170% and 53%, respectively, by incorporating only 2 wt. % of WC. Moreover, tribological properties have also enhanced significantly compared to AZ31 alloy due to incorporation of the same amount of nanoparticles. Karuppusamy et al. [25,26] developed AZ91-1.5WC nanocomposite and observed excellent wear behavior at cryogenic treated condition.

However, fabrication of nanocomposites is a very challenging job as nanoparticles have agglomeration and wettability issues, whereas magnesium has a propensity to oxidation. Hence, picking a proper fabrication method is very important. Until now, researchers have followed various fabrication techniques to develop Mg-MMNCs. Among those processes, liquid metallurgy based processes are widely used because of their economical potentiality, simplicity, and excellent productivity. Ultrasonic treatment method, disintegrated melt deposition and stir casting are the most common liquid metallurgy based methods. Recently, researchers have been fascinated with ultrasonic vibration assisted stir casting method because of its high productivity and simple function. In this method, powerful ultrasonic cavitation, high temperature, and acoustic streaming are generated to de-agglomerate clusters of nanoparticles. Ultrasonic waves generate a large number of microscopic bubbles. Implosions of these bubbles produce acoustic streaming and high temperature. Consequently, nanoparticles are distributed homogeneously [27]. Recently, Banerjee et al. [20,28] synthesized AZ31-WC nanocomposites through ultrasonic vibration associated stir casting technique and achieved homogeneous distribution of nanoparticles. Casati and Vedani [12] disclosed that ultrasonic vibration associated stir casting technique is well accepted by researchers to synthesize Mg-MMNCs. Erman et al. [15] have synthesized Mg-SiC composites using ultrasonic treatment. Accordingly, the ultrasonic vibration assisted stir casting method is considered in this study as a fabrication method.

Researchers also found a positive effect of incorporation of carbon based reinforcements in Mg matrix. Al-maamari et al. [29] observed an affirmative response of adding graphite particles in Mg matrix on tribological and mechanical characteristics. Similarly, Narayansamy et al. [3] investigated the role of fortification of graphite/MoS₂ particles in Mg matrix and disclosed a noticeable enhancement in mechanical as well as wear behavior. Attempts are also made to enhance properties through surface coatings [30]. However, with ever increasing demands, it becomes arduous to meet all imperative needs. Accordingly, the concept of hybrid nanocomposite having one hard reinforcement and another soft reinforcement becomes a field of interest for the scientific community. Recently, Girish et al. [31] incorporated SiC and Graphite particles in the AZ91 matrix and examined wear characteristics. It is observed that AZ91-SiC-Gr composites yield better wear characteristics at all experimental conditions. Banerjee et al. [32] have studied the mechanical and corrosion characteristics of AZ31-WC-Graphite hybrid composites and reported enhancement in hardness and elastic modulus up to a transition limit of graphite incorporation. Prakash et al. [33] reported that Mg-SiC-Gr hybrid composite possesses three times better microhardness and an approximately 80% less wear rate than the base matrix. Khatkar et al. [34] have reviewed the effect of hybridization and reported that graphite possesses noticeable quality as a secondary reinforcement to control mechanical and tribological characteristics. It was also observed that this positive impact of incorporating graphite is obtained up to a transition limit. Babu et al. [35] have investigated the mechanical properties of Mg-Al₂O₃-Gr composites and have reported the superior mechanical properties of hybrid nanocomposites.

Typically, available literature on tribological behavior of Mg-MMNCs is mainly on dry sliding and high temperature conditions, while 63% of total wear are abrasive wear. Researchers such as Kaviti et al. [18], Banerjee et al. [20,21], Zhang et al. [36], Nguyen et al. [13], and Labib et al. [37] have thoroughly examined dry sliding and elevated temperature wear behavior of different composites. Basically, abrasive wear is a function of abrasive grit properties (shape and size of abrasive grit), load and material properties (elastic modulus, hardness, fracture strain, fracture toughness, etc.). It is well known that study on the abrasive wear behavior of materials is very important in applications where continuous contact with foreign particles is involved. Recently, Banerjee et al. [28] examined the abrasive wear characteristics of AZ31-WC nanocomposites and observed that nanocomposites acquire admirable wear resistance compared to AZ31 alloy within an experimental range. Extensive review of the available literature reveals that reports on investigations of the abrasive wear characteristics of magnesium based hybrid nanocomposites having WC and graphite as reinforcements are scanty. The current study aims to remit that lacuna of literature by studying the abrasive wear behavior of AZ31-WC-Graphite nanocomposites at different experimental conditions. The effect of wt. % of Graphite, abrasive grit size, and sliding distance on wear and friction behavior are examined. Wear morphology is also investigated using 3D surface plot, SEM micrographs, and EDAX spectra.

2. Materials and Methods

In this investigation, the widely used AZ31 (Mg-3Al-Zn) magnesium alloy is considered as a base matrix. Compositional details (in wt. %) of AZ31 alloy are as follows: Al-3.2%, Mn-0.28%, Si-0.10%, Fe-0.005%, Zn-1.2%, and Mg-balance. In the current investigation, a ceramic based hard WC nanoparticle is considered as a primary reinforcement and the soft graphite (Gr) nanoparticle is considered as a secondary reinforcement. The SEM micrograph of Graphite nanoparticles are depicted in Figure 1. Details of reinforcements are tabulated in

Table 1. Details of Gr and WC nanoparticles.

Reinforcement	Supplier	Purity	Average Article Size	Form	Characteristics
WC	Hongwu International Group Ltd.	99.9%	80 nm	Hexagonal	Surface area: 60 m2/g;, Melting point: 2870 °C;
Graphite	Hongwu International Group Ltd.	99.95%	80 nm	Flake	Surface area: 50–60 m2/g; Melting point: 3652–3697 °C

. Different amounts (wt. %) of graphite nanoparticles are added with AZ31-1WC and AZ31-2WC nanocomposites. Details of fabricated materials are listed in

Table 2. Details of fabricated nanocomposites.

Material	AZ31-1WC-1Gr	AZ31-1WC-2Gr	AZ31-2WC-1Gr	AZ31-2WC-2Gr
WC (wt. %)	1	1	2	2
Graphite (wt. %)	1	2	1	2
AZ31 (wt. %)	Balance	Balance	Balance	Balance

.

It is well acknowledged that synthesis of magnesium hybrid nanocomposites is a very troublesome job due to the oxidation vulnerability of magnesium along with agglomeration issues and the wettability problem of nanoparticles. These problems are encountered by using liquid metallurgy related ultrasonic vibration associated stir casting technique. In the current investigation, a specially designed furnace (SwamEquip, Chennai, India) with an advanced mechanical stirrer (Blade angle 45°), a melt temperature monitoring system (K-type thermocouple), a powder preheating and pouring arrangement, a split type die (Φ 30 × 300 mm), and die preheating set up and ultrasonification arrangement (20 kHz, 2 KW) is employed. Pictorial representation of the furnace along with ultrasonification unit is illustrated in Figure 2. Structural outline of the fabrication method is also shown in Figure 3. At first, the required amount of AZ31 alloy ingots are put inside the main furnace to become liquefied. The heating temperature of the furnace is kept at 750 °C and melt temperature is monitored by placing a K-type thermocouple. Judicious selection of melt temperature is needed to avoid impermissible byproducts and control fluidity of melt. Concurrently, the required proportion of reinforcement particles are pre-heated in the particle pre-heating furnace at 300 °C so that different issues (wettability, particle burn out, and presence of moisture content) of nanoparticles can be resolved. A split-type cast iron die is used for this casting. Initially, the inner surface of the die is coated using a thin layer of graphite so that eventual wear as well as erosion can be eliminated. That die is also preheated at 300 °C to avoid segregation. Afterwards, a mechanical stirrer is put inside the main furnace and the melt is stirred at 500 rpm to form a vortex. Then, particles are injected into the vortex of the melt and the speed of the mechanical stirrer is increased to 600 rpm and continues for another 6–8 min. After completion of mechanical stirring, the ultrasonic horn is dipped inside the melt at 2–3rd depth and ultrasonic vibration (20 kHz, 2 KW) is employed in the melt for 3–5 min. Ultrasonic vibration helps to generate acoustic streaming, millions of shock waves and high temperature which helps to break the clusters of particles and disperse uniformly. Then, the ultrasonic horn is removed from the furnace and molten slurry is poured in the permanent die via a special pouring arrangement at high vacuum conditions (10⁻² mbar). A composite bar is then taken out by disengaging the split die. To eradicate oxidation issues, the entire fabrication process is conducted in an inert gas (Ar:SF₆ = 9:1) atmosphere. All of the parameters of the fabrication method are selected from extensive review of the literature and past experience [20,21,22,23,24]. The same process is followed to synthesize all the samples. Eventually, cast bars are duly machined to prepare samples for various tests.

Optical microscope (LEICA) and scanning electron microscope (SEM, JEOL, Japan & ZEISS) are used to conduct typical characterization of prepared samples. Additionally, energy dispersive spectroscopy (EDS, JSM-6360, Japan) is employed to scrutinize compositional details of the samples. Primarily, machining of cast bars are performed to prepare samples of desired dimensions. Then, samples are etched properly with acetic-picral solution to perform microstructural characterizations. Finally, worn surfaces are examined under SEM and EDS to thoroughly investigate possible wear mechanisms.

ASTM E384-99 is followed to study the microhardness values of developed composites. Microhardness values are obtained by using a UHL micro-hardness tester (VMHT MOT, Technische Mikroskopie). A diamond indenter is employed to create indentation. Microhardness tests are conducted considering the load of 50 gf and dwell time of 10 s. Microhardness of five different locations are examined for each experiment to check repeatability. Indentation marks are captured in a computerized system and scrutinized to calculate hardness values.

In present investigation, abrasive wear tests are executed in a DUCOM, India (TR-208-M2) made tribotester (pin-on-disc) following ASTM G99-05. Experiments are conducted at room temperature condition (30 °C, 84% RH). Initially, different grit abrasive papers of proper size are attached on the surface of counter disc. Pin samples are properly cleaned with acetone and attached vertically in the sample holder on the EN8 counter-disc. Three different grit of abrasive SiC papers (400, 600, and 800) and three different track diameters (30, 40, and 50 mm) are considered for the present study. A change in the track diameter refers to a change in the sliding distance. Throughout the study, sliding distances are designated by track diameters. In the current study, sliding speed, duration and applied load are kept constant at 100 rpm, 5 min, and 30 N, respectively. Experimental parameters are supervised with the help of a connected computerized system and controller. The dead weight is put on the loading pan which is connected to the loading lever. The loading lever is coupled with a load sensor to appraise the proper load. A load cell (beam type, capacity 1000 N) is utilized to compute frictional force. Wear of the samples are computed by measuring weight loss of samples and a digital balance (Afcoset) is used for this purpose. Friction forces are directly obtained from the controller and coefficient of friction values are computed by comparing that friction force with the applied load.

Roughness profiles of worn samples are thoroughly investigated with the help of optical profilometer (non-contact type, Contour X-Bruker). Arithmetic mean roughness (R_a) is considered here as the surface roughness parameter of the worn surface. Five different locations are examined for each experiment and surface plots (2D and 3D) are observed with the help of a computerized set up. Afterwards, those samples are scrutinized under SEM.

3. Results

3.1. Characterization

Optical images of AZ31-WC-Graphite nanocomposites are presented in Figure 4a–d. Optical images contain α-magnesium and β-Mg₁₇Al₁₂ phases. Fortification of particles forms interconnecting chains, which are blackish in nature. It is also observed in Figure 4 that blackish frameworks generate grain boundaries and concentration of these frameworks increases proportionately with wt. % of graphite incorporation. Typically, fortification of WC particles generates spherical structures, whereas graphite forms flake type structures. SEM images of AZ31-WC-Graphite nanocomposites are depicted in Figure 5. Figure 5 illustrates the uniform distribution of WC and Graphite nanoparticles without any noticeable cluster formation. Effects of fortification of WC nanoparticles have already been discussed in previous studies where the presence of equiaxed grains along with superior interfacial bonding was observed [20,21,22,23,24]. Furthermore, the details of composition of AZ31-WC-Graphite hybrid nanocomposites are examined using EDS spectra. EDS spectra of AZ31-WC-Graphite nanocomposites are presented in Figure 6. It is observed in Figure 6 that all EDS spectra consist of peaks of Al, Zn, Fe, C, O, W, and Mg. EDS spectra confirms inclusion of reinforcements. However, the exact wt. % of each element is not matched as the focused area is at a microscopic level and the quantity of reinforcements are very small. Detailed discussions of characterization of hybrid nanocomposites are present elsewhere [38]. However, characterization details confirm incorporation of reinforcements in AZ31 matrix.

3.2. Microhardmess

Microhardness values of AZ31-WC-Graphite hybrid nanocomposites are evaluated by considering 50 gf load and 10 s dwell time. In earlier studies, it was observed that microhardness enhances continuously with incorporation of wt. % of WC particles. With 2 wt. % of WC, microhardness enhances by approximately 53% compared to AZ31 alloy. The effect of incorporation of graphite nanoparticles with AZ31-WC nanocomposites is depicted in Figure 7. Figure 7 illustrates that incorporation of graphite nanoparticles possesses significant effect on the microhardness value. It is noticed that inoculation of 1 wt. % of Graphite nanoparticle secures a positive impact on hardness values of both AZ31-1WC and AZ31-2WC nanocomposites. Yet, further addition of graphite nanoparticles (2 wt. %) shows a detrimental effect. This detrimental effect is mainly due to the softer nature of graphite particles. Hardness decreases as the quantity of Gr increases due to its flake-like lamellar structure. Similar observations are also made by Prakash et al. [33]. However, AZ31-2WC-1Gr possesses the highest microhardness among tested materials. The microhardness value of AZ31-2WC-1Gr hybrid nanocomposites enhances by approximately 78% compared to AZ31 alloy.

3.3. Abrasive Wear Behavior

Abrasive wear characteristics of AZ31-WC-Graphite hybrid nanocomposites are evaluated under varying sliding distances (30, 40, and 50 mm track dia.) and varying abrasive grits (400, 600, and 800 grit). All experiments are carried out at a 100 rpm sliding speed for a duration of 5 min. The effect of incorporation of varying amounts (wt. %) of graphite nanoparticles on abrasive wear behavior is illustrated in Figure 8. Figure 8 shows experimental results against 800 grit SiC abrasive paper and 30N load. It is obvious from Figure 8 that AZ31 alloy has the maximum wear rate among all tested materials. Maximum wear rate of AZ31 alloy may be due to its lowest hardness. Discussion on effect of incorporation of WC nanoparticles on abrasive wear is already studied by Banerjee et al. [28]. Banerjee et al. [28] observed that the wear rate decreases linearly with increase in wt. % of WC and AZ31-2WC possesses the best abrasive wear behavior. AZ31-2WC presented the highest wear resistance due to its highest hardness. However, it is distinct from Figure 8 that incorporation of only 1 wt. % of graphite nanoparticles with AZ31-WC nanocomposites significantly enhances wear resistance. However, wear resistance decreases when the amount of graphite increases from 1 wt. % to 2 wt. %. It is also observed that the wear rate continuously enhances with the increase in track diameter, i.e., sliding distance. Figure 8 also illustrates that AZ31-2WC-1Gr yields the best wear resistance among tested materials for all experimental conditions. Typically, existence of good microstructural integrity between matrix and reinforcements provide strength to the matrix–particle interface, resulting in enhanced hardness as well as load bearing capability. In Figure 7 it was observed that incorporation of only 1 wt. % of graphite nanoparticles with AZ31-WC nanocomposites enhances microhardness values which in turn enhance wear resistance. This finding is in line with Archard’s wear law. Typically, penetration depth of base matrix is mainly regulated by effect of load, hardness and sliding parameters on base matrix whereas the same for nanocomposites are mainly regulated by those parameters of reinforcement phases [39,40]. Hence, load generated during sliding is mainly countered by reinforcement phases (WC and Gr nanoparticles). Being ceramic based particle, WC enhances load bearing capacity, while nano-Gr acts as solid lubricant and generate a tribo-layer between pin samples and counter surface at the time of contact. Accordingly, hybrid nano-composite possesses better wear behavior. However, further addition of nano-Gr decreases microhardness and consequently enhances wear rate as brittle fracture comes into play because of a greater amount (wt. %) of graphite particles. This result is in line with the literature [33]. During sliding, frictional heating generated oxidative layer, which is mixed with graphite nanoparticles and forms a protective tribolayer between the contact surface of the pin and the counter-face. In contrast, enhancement in the sliding distance causes repetitive loading, which hinders the formation of the protective layer and converts oxidation into delamination. As a result, the wear rate enhances with the increase in sliding distance.

Figure 9 depicts the effect of abrasive grit size on the wear behavior of developed AZ31-WC-Graphite nanocomposites. It is observed that AZ31 alloy possesses maximum wear rate among all tested materials for all abrasive grit size. The wear rate of all nanocomposites reduces as the abrasive grit size (400 grit to 800 grit) increases. Initially the wear rate of each sample reduces noticeably when the grit size changes from 400 grit to 600 grit, while moderate change in wear rate is noticed for the range of 600 grit to 800 grit. Basically, a lower grit size denotes coarse grains, while an increase in grit size denotes smoother grains. Hence, SiC grit paper having a lower grit size may cut faster and deeper. For the AZ31 alloy, asperities of abrasive paper come into direct contact with a base matrix and ruptures the sample surface effortlessly compared to nanocomposites. However, for nanocomposites, asperities of abrasive papers initially come into contact with nanoparticles, which helps to protect the sample surface due to their load bearing capacity and self-lubricating ability. Initially, the wear rate decreases because of addition of WC nanoparticles. It is also observed that wear rate further decreases when 1 wt. % of graphite nanoparticles are incorporated in AZ31-WC nanocomposites and sliding against different papers at 30 N load and 50 mm track diameter. However, wear rate increases when the amount of nano-graphite is increased to 2 wt. %. This result can be co-related with the microhardness values of the base alloy and AZ31-WC-Graphite nanocomposites. It was observed (Figure 7) that incorporation of only 1 wt. % of graphite nanoparticles with AZ31-WC nanocomposites enhances microhardness values, whereas incorporation of 2 wt. % of nano-graphite shows a detrimental effect. Accordingly, wear resistance enhances due to incorporation of 1 wt. % of nano-graphite with AZ31-WC nanocomposites. This result is in line with the literature [33]. Initially graphite nanoparticles behave differently to enhance abrasive wear behavior. When soft graphite nanoparticles come into contact with a hard counter surface, they are sheared out and then generate a protective layer between the sample and the counter surface due to self-lubricating property of nano-graphite. This protective layer helps to bring down the stress concentration at asperity contact and hinder the cutting activities of abrasive particles. It is also mentioned in literature that the effectiveness of abrasive particles can be minimized due to shelling, clogging, or capping when they are in contact with composites [41]. Hence, detailed investigation of the worn surface morphology is very much required to find actual reasons.

Furthermore, investigations of surface roughness of worn surfaces are also required to understand the actual condition worn surface. For brevity, surface roughness plot of AZ31-2WC-1Gr nanocomposite tested at 50 mm track diameter and 800 grit paper is shown in Figure 10. It is observed that roughness of the worn surface of the base alloy experimented at 50 mm track diameter and 800 grit is maximum, whereas the same for AZ31-2WC-1Graphite is minimum. It is also noticed that pin surfaces experimented under lower grit (400/600 grit) possess higher roughness values compared to pin sample (same material) tested under higher grit (800 grit) abrasive paper. Lower roughness of a worn surface yields less wear [40]. These roughness results can be correlated with the findings of Figure 8 and Figure 9.

Result of Figure 10 implies that a comparatively smoother surface is present for AZ31-2WC-1Graphite than other samples. This result clearly indicates that some sort of protective tribolayer has surely formed at different experimental conditions. To confirm and reveal the details of formation mechanisms of those tribolayers, samples are needed to be studied under SEM and EDS. In this regard, SEM micrographs of worn surfaces of samples (Base alloy, AZ31-2WC-1Gr and AZ31-2WC-2Gr) experimented under 50 mm track diameter and 800 grit are shown in Figure 11. Figure 11 depicts that surface damage of the base alloy is greater compared to other nanocomposites. Figure 11a reveals that the worn surface of the AZ31 alloy contains signs of extensive plastic deformation, signs of cracks, deeper grooves, and press-in particles. Typically, abrasive grits can easily penetrate the matrix surface, resulting in deeper grooves due to its softer nature. Signs of plastic deformation signify the presence of thermal softening which may have formed due to repeated loading. Moreover signs of cracks and torn grooves describe the presence of delamination. Therefore abrasion, plastic deformation, and delamination are decisive mechanisms for AZ31 alloy. Figure 11b describes the worn surface of AZ31-2WC-1Graphite. The worn surface of AZ31-2WC-1Graphite contains abrasive grooves only and out of them very few are deeper. Trace of oxidized debris is also visible on the worn surface. Hence, abrasion and oxidation are the main wear mechanisms for AZ31-2WC-1Graphite experimented at 50 mm track diameter and 800 grit. Figure 11c depicts the worn surface of the AZ31-2WC-2Graphite nanocomposite. It is observed that deeper grooves, welded debris, and delamination are mainly present in the worn surface. A sign of initiation of plastic deformation is also visible. Hence, abrasion and delamination are the main mechanisms for AZ31-2WC-2Graphite nanocomposites. Additionally, contribution of abrasive grit needs to be investigated. Consequently, SEM micrograph of the worn surface of AZ31-2WC-1Graphite nanocomposite tested at 50 mm track diameter and 400 grit is presented in Figure 12. A comparison between Figure 11b and Figure 12 helps to understand the effect of abrasive grit size on the worn surface as Figure 11b depicts the worn surface of AZ31-2WC-1Graphite tested at 50 mm track diameter and 800 grit. Figure 12 clearly shows abrasive grooves, surface cracks, delaminated flakes, and welded debris on the sample surface. Deeper grooves are generated due to contact with coarser grits. Deeper grooves signify the presence of abrasion [10,21]. Even the contact of the pin surface and coarse grits generate a higher amount of heat, which helps to form welded debris. The presence of cracks yields occurrence of delamination wear [21,28]. Hence, abrasion, adhesion, and delamination are the dominant wear morphology for this condition.

Beside SEM micrograph, investigations of elemental details of worn surfaces are also essential to clearly understand wear mechanisms. Figure 13 depicts EDS spectra of worn surface of the base alloy, AZ31-2WC-1Gr and AZ31-2WC-2Gr, nanocomposites tested at 50 mm track diameter and 800 grit. Figure 13a shows that the basic content of the base alloy are present along with a higher percentage of silicon. The percentage of silicon basically helps to understand the effect of SiC abrasive grits. A greater percentage of silicon suggests more ruptured SiC particles from the SiC grit paper.

The presence of a ruptured foreign particle will definitely damage the pin surface at the time of repetitive loading. Even these foreign particles may produce more heat due to frictional heating. Heat can cause thermal softening and plastic deformation of the sample surface. Then, sharp edges of abrasive particles can remove material from the soft pin surface. Hence, the amount of silicon in the worn surface indicates the condition of the worn surface. In EDS spectra, the amount of silicon has reduced for nanocomposites and the amount of silicon is minimum for AZ31-2WC-1Graphite (Figure 13b). It is also observed in Figure 13b and Figure 13c that the percentage of oxygen has also increased and that is maximum AZ31-2WC-1Graphite. The presence of a strong oxygen peak yields formation of an oxidized tribolayer, which helps to protect the sample surface. Accordingly, AZ31-2WC-1Graphite possesses a minimum wear rate among all tested samples.

3.4. Friction Behavior

Friction behavior of the base alloy and AZ31-WC-Graphite nanocomposites is depicted in Figure 14. Figure 14a reveals the effect of incorporation of the graphite particle with AZ31-WC nanocomposites on friction behavior. It is observed in Figure 14a that AZ31 possesses the highest coefficient of friction (COF) in all experimental conditions. Initially, COF values decreased due to incorporation of WC nanoparticles. COF values decrease continuously with an increasing percentage of WC reinforcement and AZ31-2WC possesses the lowest COF among the base alloy and AZ31-WC nanocomposites. Typically, the real contact area is less for AZ31-WC nanocomposites due to the presence of discontinuous reinforcing phases in the surface, which protrude from the matrix. Even the number of asperity contact is less in composites compared to base matrix. As a result, less force is required to shear off the asperities in composites leading to low coefficient of friction in composites.It is also noticed that COF values initially decrease due to incorporation of 1 wt. % of graphite nanoparticles but further incorporation shows a detrimental effect. Figure 14a also shows that COF values continuously enhance with the increase in track diameter. This result can be correlated with the hardness of the tested samples. For the base alloy, abrasive grits can directly affect the soft matrix surface. For nanocomposites, tips of abrasive grits will come into contact with WC and graphite particles. Due to the harder nature of WC particles and the self-lubricating nature of graphite particles, abrasive grits will face hindrance to penetrate the sample surface. However, that hindrance will be affected when graphite percentage is beyond a transition limit and after that limit COF values again start to increase. Furthermore, enhancement in sliding distance enhances frictional heating and creates local hot spots. As a result, abrasive grit can penetrate easily and penetration depth also enhances. Accordingly, COF value enhances. The effects of abrasive grits on the friction behavior of AZ31 alloy and AZ31-WC-Graphite nanocomposites at 50 mm track diameter are depicted in Figure 14b. It is noticed that COF values decreases with incorporation of 1 wt. % of graphite nanoparticles with AZ31-WC nanocomposites, while COF value tends to increase when 2 wt. % of graphite nanoparticles are fortified with AZ31-WC nanocomposites. The higher the abrasive grit size, the finer the abrasive particles. Hence, interaction of finer particles with harder composites produces less frictional force compared to coarser particles. As a result, AZ31-2WC-1Graphite possesses the least COF in all experimental conditions.

4. Conclusions

In the current study, varying amounts (1 and 2 wt. %) of graphite nanoparticles are incorporated with AZ31-1WC and AZ31-2WC nanocomposites using ultrasonic vibration associated stir casting technique. Abrasive wear characteristics of the base alloy and AZ31-WC-Graphite nanocomposites are scrutinized for varying sliding distances (track dia. 30, 40, and 50 mm) and varying abrasive grit sizes (400, 600, and 800 grits). From this study, the following conclusions can be drawn:

• Basic characterization of developed hybrid nanocomposites yields that graphite and WC particles are homogeneously distributed in AZ31 matrix;
• The microhardness study reveals that the microhardness value has significantly enhanced due to incorporation of 1 wt. % of graphite with AZ31-2WC nanocomposite;
• AZ31 alloy has a maximum wear rate among tested materials. It is noticed that incorporation of only 1 wt. % of graphite nanoparticles with AZ31-WC nanocomposites enhances wear resistance significantly. However, wear resistance decreases when the amount of graphite increases from 1 wt. % to 2 wt. %. It is also observed that wear rate enhances continuously with an increase in track diameter, i.e., sliding distance. AZ31-2WC-1Gr yields best wear resistance among tested materials for all experimental conditions. The wear rate of all nanocomposites decreases with the increase in abrasive grit size;
• It is also noticed that COF values initially decrease due to incorporation of 1 wt. % of graphite nanoparticles but further incorporation shows a detrimental effect;
• Examination of worn surface depicts that abrasion, plastic deformation, and delamination are decisive mechanisms for AZ31 alloy. Abrasion and oxidation are the main wear mechanisms for AZ31-2WC-1Graphite nanocomposite;
• The SEM micrograph of the worn surface of the AZ31-2WC-1Graphite nanocomposite experimented at 50 mm track diameter and 400 grit shows that abrasive grooves, surface cracks, delaminated flakes, and welded debris are present on the sample surface.