Performance studies on hybrid nano-metal matrix composites for wear and surface quality

Aluminium-alumina hybrid metal matrix composite (AHMCs) was produced by the stir-squeeze casting method. The stir casting process was chosen because of its lower cost, better particle matrix bonding and many controllable processing parameters [11]. The SAAW was the matrix material and was reinforced with varying combinations of reinforcement ie., 1, 2, wt.% and 5.5, 7 wt.% of micro-sized alumina (Al₂O₃)m and nano-sized alumina (Al₂O₃)n particle respectively.

According to ASTM G99, wear samples of AMMCs were prepared with dimensions of 10mm diameter and 30mm height. Before loading the specimen into the wear test apparatus, surface preparation was done by using fine disc sandpaper (320 grit) and thoroughly cleaned using alcohol. The pin-on-disc test experiments were performed in DUCOM make, TR201LE model capable of friction and wear measurements along with data acquisition system. A disc material of EN31 with a hardness of 63HRC was used. Table 1 presents-the testing parameters for wear, including material specifications. The composites after the wear tests were analyzed using a Field Emission Scanning Electron Microscope (FESEM) of Shimadzu make and model: UV-1700 with Energy Dispersive Spectroscopy (EDS). The tribological tests were conducted in triplicate for each experimental condition. This was done to ensure the reliability and repeatability of the results. The repeated tests helped to minimize any experimental variations and provide a more robust analysis of the tribological behavior of the hybrid nano-metal matrix composites. The pins were made of EN31 hardened bearing steel with a specified hardness of 63 HRC (Rockwell hardness scale). Regarding the surface roughness of the pins, they were initially prepared with a smooth surface finish, with a roughness value (Ra) within the range of 0.20-0.25 μm. The contact between the pin and the disc surface was ensured by applying a sufficient normal load during the wear tests. The normal load exerted on the pin creates contact pressure, allowing for contact between the entire pin surface and the disc surface. The applied normal load ensures adequate contact and facilitates the initiation of wear between the pin and the disc.

Many researchers have used the Taguchi process effectively for the DOE approach in wear analysis of composite materials [12]. In the current studies, an L₉ OA was chosen based on degrees of freedom. Table 2 presents the control factors and their levels. In the present investigation,reinforcement composition, the influence of sliding load, sliding velocity and temperature on AMMCs has been studied and evaluated. Table 3 displaysthe experimental parameter detail for the wear tests. The composite samples selected in this study are based on our recent publication [11], where again, an L9 OA was used but to optimize the stir-squeeze parameters that can produce composites with lower porosity and better mechanical and wear properties.

A multi-criteria method of decision analysis for evaluating a set of alternatives by identifying weights for each criterion and standardizing scores for each criterion is considered for this study. A multi-criteria-based decision-making system called Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) was developed in the 1980s. TOPSIS selects the alternative with the largest distance from the negative ideal solution and the lowest Euclidean distance from the ideal solution.In this approach, a set of alternatives are compared by specifying weights for each criterion and standardizing the scores for each criterion [13, 14].

The normalization of the decision matrix, weight normalization, positive ideal solution negative ideal solution, variance of each alternative and closest relative are evaluated using the equation (1) to (7) 1aij=pij∑i=1bp2ij,i=1,2…bj=1,2…a$$\matrix{ {{a_{ij}} = {{{p_{ij}}} \over {\sqrt {\sum\nolimits_{i = 1}^b {{p_{2ij}}} } }},} \hfill & {i = 1,2 \ldots {b_j} = 1,2 \ldots a} \hfill \cr } $$ 2Dij=ωj×aij$$Dij = {\omega _j} \times {a_{ij}}$$ 3E∗={ D1∗,D2∗…,Di }={ ((maxDij′i∈I)j,(minDij″i∈I)j) }$$\matrix{ {{E^ * }} \hfill & = \hfill & {\left\{ {D_1^ * ,D_2^ * \ldots ,Di} \right\}} \hfill \cr {} \hfill & = \hfill & {\left\{ {\left( {{{\left( {{{\max Dij'} \over {i \in I}}} \right)} \over j},{{\left( {{{\min Dij''} \over {i \in I}}} \right)} \over j}} \right)} \right\}} \hfill \cr } $$ 4E−={ D1−,D2−…,D¯i }={ ((minDij′i∈I)j,(maxDij″i∈I)j) }$$\matrix{ {{E^ - }} \hfill & = \hfill & {\left\{ {D_1^ - ,D_2^ - \ldots ,\bar Di} \right\}} \hfill \cr {} \hfill & = \hfill & {\left\{ {\left( {{{\left( {{{\min Dij'} \over {i \in I}}} \right)} \over j},{{\left( {{{\max Dij''} \over {i \in I}}} \right)} \over j}} \right)} \right\}} \hfill \cr } $$ where benefit and cost criteria indicated with I′ and I′′ respectively 5Sj∗=∑i=1n(vij−v∗),j=1,2…b$$\matrix{ {S_j^ * = \sum\limits_{i = 1}^n {\left( {{v_{ij}} - {v^ * }} \right),} } \hfill & {j = 1,2 \ldots b} \hfill \cr } $$ 6Sj−=∑i=1n(vij−v∗)2,j=1,2…b$$\matrix{ {S_j^ - = \sum\limits_{i = 1}^n {{{\left( {{v_{ij}} - {v^ * }} \right)}^2},} } \hfill & {j = 1,2 \ldots b} \hfill \cr } $$ 7Fj=Gj∗Gj∗−Gj−j=1,2…b$$\matrix{ {Fj = {{{G_{{j^ * }}}} \over {{G_{{j^ * }}} - {G_{{j^ - }}}}}} \hfill & {j = 1,2 \ldots b} \hfill \cr } $$ where Fj’s index value is 0 to 1.

Ranking must be done according to Fj’s descending values; larger values mean higher ratings and better alternative. Table 4 shows the summary of test results (wear loss, COF, surface roughness) and TOPSIS ranking.

Figure 1 shows the mean effect plot for the factors, levels and preference values. It is evident that the composition of reinforcement wt.% shows the significant influence on the output performance namely COF, wear and surface roughness. The 2% nano Al₂O₃ + 7% micro Al₂O₃ composite shows low COF, wear and surface roughness.The incorporation Al₂O₃ in the matrix (aluminium), improves the hardness, and subsequently reduces the COF and wear resistance of the composites. The results are in line with the findings in which Al alloy composite reinforced with more amount of Al₂O₃ demonstrated greater wear resistance [15]. At 2% nano Al₂O₃ + 5.5% micro Al₂O₃ the composite shows high COF, wear and surface roughness. The lesser (5.5%) Al₂O₃ increase the plastic deformation of the matrix attributing for higher COF, wear and surface roughness.

Fig. 1.

An increase in sliding load leads to higher COF, wear, and surface roughness values. The presence of 2% nano Al₂O₃ in composites resist the plastic deformation of the matrix and takes up the forces created during sliding between the Al₂O₃ reinforcement. The COF of A206/silica sand containing composites is lower than the matrix alloy, as reported by Rohatgi et al. [16]. Figure 2(a)-(d) shows the composite surface with wear tracks and craters. The modification of applied load match to the penetration of the counter surface’s. Hard projections into the softer pin surface, attributes for increase in deformation and fracturing of the soft surface asperities. The material is removed from the worn pin surface in the form of platelet-shaped particles that resultin the formation of a shallow crater. Sliding grooves are also observed around the surface of the shallow crater, as shown in figure. The propagation of cracks through the surface of the worn pins is witnessed.When loads are low, worn surfaces of composite specimens normally have wear marks and shallow craters in all cases [17]. The detailed wear inspection revealed that the weardebris was loose and scattered over the surface. A detailed investigation of the worn surfaces and subsurface expose that tribo-layers emerge under a variety of loads and temperatures, and it never disappear until severe wear arises [17]. The tribo-layer is a mechanically mixed layer (MML) consisting of tribooxides, wearing alloy fragments, and Fe transferred from the counter-face.MML thickness increases slowly with increase in temperature and load. Figure 2(b) shows the layer of material moved by the shear force due to the sliding effect.Abrasive reinforcing particles erode the steel surface and cause iron transfer and MML production. The MML does not only consist of matrix alloy and spent composite reinforcements, it also shows the transfer of iron from the face of the steel disk. The formation of the MML is attributed to the turbulent plastic flow due to the onset of shear instability at a critical depth below the worn surface [18]. Figure 3(a) and (c) depicts the FESEM image of the worn-out surface of the experiment run 4 and experiment run 7. Figure 3(b) and (d) indicates the spot/area where EDS was performed. The EDS spectrum in Figure 3(e) and (f) exhibited iron-oxygen rich MMLs. It is evident that the transfer of alumina and iron particles took place during sliding action.

Fig. 2.

Fig. 3.

Figure 3(e) and (f) furthermore corroborate the presence of an oxide layer. Most metals naturally oxidize in the air, forming an oxide film within minutes which plays dominant role in the attribute of friction. At regular loading, the oxide films separate the two metals, and COF is low because of the low shearing strength and ductility. The presence of an oxide layer minimizes the risk of immediate metal contact. However, at higher loads, surface films distort and fracture, causing a metallic contact, leading to a deformation of the surface which is avoided by the presence of Al₂O₃.

It is inferred from Figure 1 that the COF, wear and surface roughness increases with sliding velocity and then decreases with increase in sliding velocity. Strong lubrication offered by the alumina for the sliding rate ranging from 1-2m/s credits for less COF. The normal load determines the actual contact area, which individually regards the impact of normal and tangential loads [19]. The wear rate is increased due to delamination, at the sliding speed range of 1-2 m/s. The platelet-shaped delaminated wear debris attributes rigorousdamage of the worn surface of the pin and increasing friction. The development of MML reduces COF for the mono composite above the sliding speedof 2 m/s. For sliding velocity of 1 to 2 m/s the increase in COF is witnessed due to friction created by the delaminated debris trapped between the contact surfaces of the tribo-couple. To shear over this area of contact, a tangential force is needed, and if normal load increases, then the real contact area with tangential force increases, contributing toan increase in COF. Increasing of COF with load contributes to the plowing effect and micro-cutting at the time of wear leads to elevated frictional forces.

Figure 1 shows that increase in temperature increases the COF, wear and surface roughness. Figure 4(a&b) shows the correlation of temperature relatedwear loss and COF for all the nine specimens. As the temperature of the pin increases, it becomes softer, and wear resistance is liable to decrease, which attributes for more material removal from the contact surface [20].

Fig. 4.

The optical profilometer is used to obtain the surface roughness of worn-out samples of the AMMCs and is presented in Table 5. A Zygo New View optical profilometer was used for surface roughness measurements. The average surface roughness (Ra) values of the firstthree TOPSIS ranked composites are 0.322μm, 0.405μm, and 0.604μm, respectively. The surface roughness of worn-out AMMCs depends on a surface features-such as pore size and density, pores (deep or shallow), micro-cracks and pinholes which is formed during the solidification and wear testing process.

The 3D surface profiles of the top ranked composites are shown in Figure 5(a-c). Among the 3D surface profiles of composites, 2% nano Al₂O₃ 7% micro Al₂O₃ shows a smooth surface, which is also evident from the depth profile shown in Figure 6(a). The depth profiles of first three TOPSIS ranked composites are shown in Figure 6(a-c).

Fig. 5.

Fig. 6.