Investigation on dry sliding wear behavior of AA5083/nano-Al₂O₃ metal matrix composites

2.1. Fabrication of Nano-MMCs

Al 5083 was used as matrix material in the present work and its composition is indicated in Table 1. Nano-Al₂O₃ particles supplied by Sigma-Aldrich (India) was used as reinforcement and their size was 40 nm. The packets of approximately 8 gm of particles were prepared using aluminum foils. During the processing of packets, packed particles in aluminum foils were preheated up to 580 °C for 2 hours to eliminate the absorbed moisture. The stir casting method was employed for the fabrication of Al5083/Al₂O₃ nanocomposites. This method has proved to be very successful in the preparation of nano-MMCs at relatively lower cost and simple method (Ekka et al., 2015Ekka, K.K., Chauhan, S.R., Varun (2015). Dry Sliding Wear Characteristics of SiC and Al₂O₃ Nanoparticulate Aluminium Matrix Composite Using Taguchi Technique. Arab. J. Sci. Eng. 40, 571-581. https://doi.org/10.1007/s13369-014-1528-2.). The method used for processing both alloy and composite. The packets were added to melt during the stir casting process. The stirrer is rotated before and after the addition of particles to achieve uniform distribution in the melt. During stirring, the addition of a small amount of magnesium (0.5 wt.-%) strips was done to enhance the wettability of particles. Later, the molten mix was poured into a preheated mold box (heated to 800 °C) and castings were removed after solidification to obtain the cylindrical specimen. The percentage of nano-Al₂O₃ particle reinforcement was varied as 2 wt.-%, 4 wt.-%, 6 wt.-% and 8 wt.-% to prepare 4 different Al/nano-Al₂O₃ MMCs.

2.2. Wear test

The dry sliding wear behavior of prepared composites was studied with the help of a pin-on-disc test rig (Manufactured by DUCOM®) under ambient conditions. Specimens were carefully mounted on a steel disc and adjusted to ensure perpendicularity. The counter-face disc rotated against specimens was EN 32 grade steel with 65 HRC. The samples were machined into cylindrical pins of 8 mm diameter and 30 mm length with ground ends as per the ASTM G99 (2017)ASTM G99 (2017). Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus. ASTM International, West Conshohocken, PA, USA.. The wear was recorded through the data acquisition system as a function of pin height reduction. The sliding wear test was conducted at a constant sliding speed of 13.33 m·s^-1, while other parameters are shown in Table 2. The possible wear mechanism was studied using worn surface morphology through field emission scanning electron microscopy (FESEM) images.

2.3. Microstructure analysis and hardness

The particulate dispersion study and microstructural analysis of prepared specimens were carried out using FESEM images. Standard metallographic techniques were followed to prepare specimens for microstructure and hardness study. Figure 1 to 4Figure 1, 2, 3, 4 presents SEM micrographs of AA5083 reinforced with 2, 4, 6 and 8 wt.-% of nano-Al₂O₃ respectively. The agglomeration phenomenon of nano-particles in prepared composites was minimum in 2, 4 and 6 wt.-% of nano-Al₂O₃ incorporated Al composites, while relatively notable amount of agglomeration was observed in 8% nano-Al₂O₃ incorporated Al MMCs (Fig. 5). Microstructure images depict fine precipitates of nano-Al₂O₃ dispersed along grain boundary in the Al matrix. Thus, nearly homogeneous dispersion of nano-particles was attained. The Micro-hardness test was carried out on the Brinell hardness tester as per ASTM E10 (2018)ASTM E10 (2018). Standard Test Method for Brinell Hardness of Metallic Materials. ASTM International, West Conshohocken, PA, USA. at a load of 31.25 kgf. The average of 5 trials was considered to determine the hardness value of each specimen and results are illustrated in Table 3. There was an increase in hardness with an increased percentage of Al₂O₃ till 6 wt.-% and decreased marginally for 8 wt.-%. The hard Al₂O₃ particles perhaps caused a barrier for initial stresses. Besides, nearly uniform dispersion of Al₂O₃ particles in the matrix has decreased the inter-spatial gap, resulting in pile-up of dislocations. The work hardening of the matrix alloy increases due to Al₂O₃ reinforcement thereby increasing the load required for the nucleation and propagation of voids. Hence, an increase in hardness was observed. In addition, increase in hardness owes to Hall-Petch hardening effect (Jenczyk et al., 2021Jenczyk, P., Grzywacz, H., Milczarek, M., Jarzabek, D.M. (2021) Mechanical and Tribological Properties of Co-Electrodeposited Particulate-Reinforced Metal Matrix Composites: A Critical Review with Interfacial Aspects. Materials 14 (12), 3181. https://doi.org/10.3390/ma14123181.). Further increase in Al₂O₃ caused a marginal decrease in hardness. It may be because an increase in nano-particles leads to increased particle to particle affinity and greater surface to volume ratio. It has caused the agglomeration of particles, resulting in uneven dispersion of reinforcement in the matrix. Similar observations were made in available research (Ma et al., 2017Ma, P., Jia, Y., Gokuldoss, P. konda, Yu, Z., Yang, S., Zhao, J., Li, C. (2017). Effect of Al₂O₃ nanoparticles as reinforcement on the tensile behavior of Al-12Si composites. Metals 7 (9), 359. https://doi.org/10.3390/met7090359.). Figure 6 shows the EDS spectrum of Al 5083 alloy reinforced with, 6% nano-Al₂O₃, from the figure it was evident that Al and O are present, which confirms the presence of nano-Al₂O₃ in the Al5083 matrix. Also, the weight percentage of each element in the composition is shown in Table 4.

2.4. Design of experiments

Taguchi-based experimental design has been employed to acquire experimental data systematically. Taguchi’s L25 orthogonal array was chosen for investigation since it allows operating three parameters each at five levels (Table 5). The obtained results were analyzed using Analysis of Variance (ANOVA) to study the significance of each parameter. The response studied was wear, whose experimental average was computed for three repetitions to achieve the minimum experimental error. ANOVA was carried out for one response and three variables using smaller the better quality characteristic since the objective of the investigation was to minimize wear. MINITAB-17®, a statistical package was used to perform statistical analysis. The linear regression model was developed to correlate studied parameters with wear. Subsequently, the presented model helps to predict wear within the studied range of parameters. The main effect plots and rank analysis were employed to analyze the influence of each parameter on the response.

3.1. Effect of parameters on wear

Table 5 illustrates results obtained for various combinations of parameter levels as per the L25 orthogonal array for wear and specific wear rate. ANOVA was used to find out the effect of each input parameter on wear characteristics of prepared nanocomposites. It aids to determine significant parameters and percentage contributions of the individual parameter on wear behavior. The analysis of performance criteria based on ANOVA during wear test and percentage contribution of parameters were evaluated for a significance level of 0.05 or confidence interval of 95%. It implies that a source having less than 0.05 P value was considered a statistically significant parameter. ANOVA results for wear and specific wear rate are shown in Table 6 and 7 respectively. It was observed that the P-value for all sources was less than 0.05 suggesting the statistical significance of each studied parameter.

ANOVA analysis and rank analysis (Table 8 and 9) suggests that the sliding distance was the highest influencing factor on wear followed by the applied load. The percentage of reinforcement was the least influencing factor. On contrary, the applied load and percentage of reinforcement was the most significant factor. While, sliding distance was the least significant influencing factor on the specific wear rate. Besides, it can be observed that the studied parameters were statistically significant factors and affects the wear rate of developed composites. According to the main effect plot for means, the least value of the mean of means for the respective parameter will result in a minimum rate of wear which was the objective of the study. Figure 6b and Fig. 7a illustrates the main effect plot for wear and specific wear rate. Wear loss was found to increase with the increase in the reinforcement percentage, while a decrease in specific wear rate has observed. It was observed that wear is minimum at level 4% of reinforcement i.e. 6 wt.-%, while at level 1 for Load and sliding distance i.e., 4.905 N and 1000 m respectively. The higher hardness was obtained for the 6% reinforced MMC. The higher hardness corresponds to the higher wear resistance of the composite sample with additions of Al₂O₃ reinforcement (Siddesh Kumar et al., 2016Siddesh Kumar, N.G., Ram Prabhu, T., Shiva Shankar, G.S., Basavarajappa, S. (2016). Dry sliding wear properties of unhybrid and hybrid Al alloy based nanocomposites. Tribol. - Mater. Surf. Interfaces 10 (3), 138-149. https://doi.org/10.1080/17515831.2016.1247132.). Thus, the minimum wear was obtained for the Al/6%-nanoAl₂O₃ composite specimen. The values of wear increased with an increase in the applied load and wear rate was found to decrease with the increase in load. The reinforcement possesses the ability to withstand an applied load to a certain level within which wear is attributed only to mild wear. At a higher level of load, the transition of mild to severe wear has occurred. Perhaps, a higher load resulted in an increased of the high surface stresses and led to severe material loss. The wear of the specimen was increased with an increase in the sliding distance significantly. The wear rises sharply above 2000 m due to increased interaction between counterparts. During the initial condition, the matrix with strongly bonded particles is in contact with a hard rotating disc resisting the wear loss. In the later stage, particles were removed along with the matrix resulting in increased of the wear loss. Thus, severe wear was observed at higher levels of sliding distance. While, sliding distance has increase specific wear rate initially owing to matrix removal. The further increase in sliding distance has resulted in decrease of specific wear rate perhaps revealing the exposure of reinforcement particles.

3.2. Regression model

The obtained results were fitted as a linear regression model, which aids to correlate wear with studied parameters viz. percentage of reinforcement, applied load and sliding distance. The model was developed using Minitab® 17 statistical package and is presented as Eq. (1).

where P is the percentage of reinforcement of nano-Al₂O₃, L is the applied load and D is the sliding distance. The feasibility of any developed models can be understood and studied through R² and R² (adj) and validation is attained with the help of confirmation tests. The R² and R² (adj) values obtained for developed equations are 91.58% and 90.38%. Thus, the presented equation is adequate and feasible to predict wear within the studied range of parameters. Further, to validate the model, confirmation tests were conducted at the random value of parameters within the studied range viz. the sliding distance of 4500 m and 19.62 N applied load for the alloy and the different MMC specimens as shown in Table 10. The experimental and regression equation predicted values of wear were compared and corresponding error percentage values are illustrated. The computed percentage error for wear was less than 10%. Thus, it can be concluded that the presented wear model is adequate and can be successfully used to predict wear satisfactorily.

3.3. Worn surface morphology

Figure 8 (a-e) shows SEM images of worn surfaces for various materials studied at 4500 m and 19.62 N of sliding distance and load, respectively. The wear behavior of MMCs reinforced with nano-Al₂O₃ particles has been investigated in the present study. The analysis has revealed that an increase in the load and the sliding distance has increased wear. At low load conditions, sliding wear occurs with minimum loss due to low contact stress. While, the increase in load is attributed to an increase in contact stress, which leads to the rapid growth of wear. In addition, increased high load condition results in an increase of the temperature at the pin-disc interface, perhaps due to the induced large contact stress and friction. It causes delamination of the surface layer and transfer of pin material to counterface occurs (Basavarajappa et al., 2007Basavarajappa, S., Chandramohan, G., Paulo Davim, J. (2007). Application of Taguchi techniques to study dry sliding wear behaviour of metal matrix composites. Mater. Design 28 (4), 1393-1398. https://doi.org/10.1016/j.matdes.2006.01.006.). Hence, the transition of mild to severe wear was observed with variation of the load from a low to a high value during the study.

The wear was increased drastically with an increase in the sliding distance. During the initial stages, the surface layer of the pin was in contact with the counter-face. The irregularities existing on the pin surface undergo plastic deformation and detach progressively from its parent metal. It results in the formation of a protective mechanical mixed layer between two counterparts. Further, hard reinforcement particles present in tested MMCs resist deformation and arrest dislocation movement. While an increase in the sliding distance increases the interaction between counterparts and the applied load has further supported for disruption of the protective layer. Besides, incorporated particles in the MMCs come out of pins due to wear and were entrapped between counterparts. The hard asperities have caused the transformation of sliding wear into three-body abrasive wear. Thus, grooves were observed in a sliding direction on worn surface images (Fig. 8). Hence, greater wear of the specimen was observed at higher sliding distance conditions. The obtained results agree with the Archard model, which states that wear is directly proportional to the load and the sliding distance (Archard, 1953Archard, J.F. (1953). Contact and Rubbing of Flat Surfaces. J. Appl. Phys. 24, 981-988. https://doi.org/10.1063/1.1721448.).

The results obtained indicated that there exists a threshold value beyond which the percentage of reinforcement leads to a deterioration of the tribological property. The particulate incorporation into Al alloy has enhanced its wear resistance. It may be due to the fact that, particulate incorporated MMCs possess high surface energy and acts as a barrier for the penetration of wear debris during sliding phenomenon. Further, burrs formed due to the plowing of counter surface asperities get detached from the worn surface. The detached debris of worn surface exists as loose debris between pin-disc interface. Also, they possess a hard and abrasive nature. The debris acts as a plowing agent under contact force and accelerates wear.

The SEM image for the worn surface of AA5083 is shown in Fig. 8a. Deep and wider grooves are present in the direction of sliding. It could well correspond to the characteristics of severe abrasive wear due to hard asperities or wear debris between surfaces in contact (Yao et al., 2015Yao, Y.T., Jiang, L., Fu, G.F., Chen, L.Q. (2015). Wear behavior and mechanism of B₄C reinforced Mg-matrix composites fabricated by metal-assisted pressureless infiltration technique. Trans. Nonferrous Met. Soci. China 25 (8), 2543-2548. https://doi.org/10.1016/S1003-6326(15)63873-0.). Plastic flow during abrasion generates high shear stresses on the surface. Subsequently, frictional heat gives rise to delamination wear, thus increasing the depth of the groove (Joshua et al., 2018Joshua, K.J., Vijay, S.J., Selvaraj, D.P. (2018). Effect of nano TiO₂ particles on microhardness and microstructural behavior of AA7068 metal matrix composites. Ceram. Int. 44 (17), 20774-20781. https://doi.org/10.1016/j.ceramint.2018.08.077.). In addition, the resistance offered against the wear debris penetration was relatively inadequate in the neat alloy. Hence, deterioration of the surface was observed in a larger area. Fig. 8b shows the worn surface of AA5083+2wt.-%Al₂O₃. Deep grooves persist which can be attributed to abrasive wear. Besides, the incorporation of nanoparticles means a relatively enhanced surface barrier for the penetration of worn-out particles. Subsequently, combined wear mechanisms of plastic deformation and progressive removal of embedded particles has occurred, resulting in wear.

Figure 8 (c-d) correspond to AA5083+4wt.-%Al₂O₃ and AA5083+6wt.-%Al₂O₃ worn surface images. They reveal the presence of shallow grooves, indicating the occurrence of abrasive wear, because of micro plowing and cutting, while plastic deformation was least observed. It may be because an increase in the reinforcement percentage has resulted in an increased in hardness. Subsequently, surface energy required for the detachment of the surface layer has increased drastically (Shivakumar et al., 2015Shivakumar, N., Vasu, V., Narasaiah, N., Kumar, S. (2015). Synthesis and Characterization of Nano-sized Al₂O₃ Particle Reinforced ZA-27 Metal Matrix Composites. Procedia Materials Science 10, 159-167. https://doi.org/10.1016/j.mspro.2015.06.037.). Thus, sliding wear resistance has enhanced; yet, the entrapped hard asperities of worn debris present between counterparts has caused abrasive wear. Hence, the dominant wear mechanism observed for 4% and 6% nanoparticles reinforced MMCs was a three-body abrasive wear mechanism, instead of sliding wear. Besides, the dark spots in Fig. 8c indicate the occurrence of oxidation of the tested specimen. Further, it suggests the dominant phenomenon of oxidative wear mechanism in 4% Al₂O₃ incorporated MMC.

Figure 8e shows the worn surface of AA5083+8wt.-%Al₂O₃. Similar wear pattern comprising thin grooves can be seen on the surface with slightly wider grooves for 8 wt.-%. The high content of nano-Al₂O₃ and high dislocation density of deformed planes results in mild abrasion, thereby reducing the loss of material (Kumar and Rajadurai, 2016Kumar, C.A.V., Rajadurai, J.S. (2016). Influence of rutile (TiO₂) content on wear and microhardness characteristics of aluminium-based hybrid composites synthesized by powder metallurgy. Trans. Nonferrous Met. Soc. China 26 (1), 63-73. https://doi.org/10.1016/S1003-6326(16)64089-X.). Further, having a higher reinforcement percentage resulted in poor boding strength and poor resistance to plastic deformation due to the decrement of hardness. Therefore, a higher number of worn-out lose particles was observed compared to 6% nanoparticles reinforced MMC. Further, a small amount of metal fold indicates plastic deformation and grooves, suggesting abrasive wear is present on the work surface. Hence, both types of wear mechanisms were observed in the 8% nanoparticles reinforced MMC. However, abrasive wear was dominant due to a large number of entrapped worn-out particles between contact surfaces.