Journal Archive

Johnson Matthey Technol. Rev., 2022, 66, (2), 186
doi: 10.1595/205651321X16238564889537

Towards the Enhanced Mechanical and Tribological Properties and Microstructural Characteristics of Boron Carbide Particles Reinforced Aluminium Composites: A Short Overview

Improved properties compared to silicon carbide or alumina reinforced composites

  • V. V. Monikandan*§
  • MatRICS Technological Solutions, 3/4A, Aanavalli, Vellimalai, Kalpadi Post, Kanyakumari District, Tamil Nadu – 629 204, India
  • §Present address: School of Minerals, Metallurgy and Materials Engineering, Indian Institute of Technology Bhubaneswar, Jatni Road Argul, Bhubaneswar, Odisha – 752 050, India
  • K. Pratheesh
  • Department of Mechanical Engineering, Mangalam College of Engineering, Mangalam Hills, Ettumanoor, Kottayam, Kerala – 686 631, India
  • P. K. Rajendrakumar, M. A. Joseph
  • Department of Mechanical Engineering, National Institute of Technology Calicut, Kozhikode, Kerala – 673 601, India
  • *Email: saai.manikandan@gmail.com

PEER REVIEWED
Received 22nd March 2021; Revised 21st May 2021; Accepted 15th June 2021; Online 16th June 2021

Article Synopsis

This paper overviews the fabrication, microstructural characteristics, mechanical properties and tribological behaviour of B4C reinforced aluminium metal matrix composites (AMMCs). The stir casting procedure and parameters used to fabricate the Al-B4C composites are discussed. The influence of physical parameters such as applied load, sliding speed and sliding distance on tribological behaviour is analysed. The role of the mechanically mixed layer (MML) and wear mechanisms on the wear behaviour and friction coefficient are emphasised. The overview of tribological behaviour revealed that the Al-B4C composites possess excellent abrasion resistance and the ability to operate over a wide range of physical parameters. The Al-B4C composites exhibited better tribological behaviour when compared with the composites reinforced with conventional reinforcement particles (SiC).

1. Introduction

Metal matrix composites (MMCs) are systematic combinations of two or more materials (one of the materials is a metal) engineered to achieve tailored properties (1). Thus, engineered MMCs have two or more chemically and physically distinct phases that are suitably distributed to provide properties not attainable with either of the individual phases (2). AMMCs exhibit better mechanical and physical properties than the aluminium-matrix alloy (3–5). The hardness and strength of AMMCs are significantly higher than that of the aluminium-matrix alloy, leading to improved wear resistance (6). AMMCs have found applications in aerospace, automotive, nuclear, telecommunications (7) and marine industries (6). The applications of AMMCs in the automotive and aerospace industry sectors can reduce fuel usage by replacing steel and cast-iron parts with lighter AMMCs. Some of these applications include pistons, piston rings, cylinder liners, connecting rods (1), cylinder blocks, driveshafts and brake drums (6). The tribological behaviour of particle reinforced AMMCs has been regularly reported. However, most of the studies have analysed the tribological behaviour of composites reinforced with SiC and Al2O3 particles. Besides these conventional reinforcement particles, aluminium alloys also can be reinforced with h-BN (8), TiC (9), TiO2 (10), ZrO2 (10) and B4C (11) to impart wear resistance. Studies on AMMCs reinforced with B4C particles have been limited mainly due to the higher cost of B4C particles than SiC and Al2O3 particles (12). Al-B4C composites are commonly used in automotive, sports (7) and neutron shielding applications (13).

B4C possesses excellent properties such as high hardness, low density, high melting point, chemical inertness and wear resistance, making it suitable for many high-performance applications (14). The hardness of B4C (Vickers Hardness under the load of 0.981 N (HV0.1) = 3200) is far superior to the hardness of conventional reinforcement particles, SiC (HV0.1 = 2500) and Al2O3 particles (HV0.1 = 1900) (15). The density of B4C (2.52 g cm–3) (16) is less than the density of solid aluminium (2.70 g cm–3) (17), which significantly improves specific properties. The densities of SiC, Al2O3 and B4C are 3.21 g cm–3, 3.92 g cm–3 and 2.52 g cm–3, respectively. The density of molten aluminium is 2.38 g cm–3 (17). Hence, it is evident that the difference in density between molten aluminium and B4C is lower when compared to the difference in density between molten aluminium and conventional reinforcement phases (SiC and Al2O3). This phenomenon minimises the sedimentation of B4C particles at the crucible bottom during stir casting (12). The abrasive resistance of B4C (0.4–0.422 (expressed in arbitrary units)) is higher than that of SiC (0.314 (expressed in arbitrary units)) due to its high hardness and strength (18).

This overview aims to discuss the microstructural characteristics, mechanical properties and tribological behaviour of Al-B4C composites. The different properties and microstructural characteristics of Al-B4C, Al-SiC and Al-Al2O3 composites are compared. Furthermore, the statistical significance of physical parameters (applied load, sliding speed and sliding distance) on the tribological behaviour of the composites is analysed. However, the literature that compares the microstructural characteristics, mechanical properties and tribological behaviour of Al-B4C, Al-SiC and Al-Al2O3 composites are insufficient. The literature on the statistical analysis of the tribological behaviour of Al-B4C composites is also sparse. Despite these shortcomings, this overview discusses the mechanical and tribological properties of the composites mentioned above. Section 2 gives a brief insight into the fabrication of Al-B4C composites through the stir casting technique. Section 3 compares the microstructural characteristics of Al-B4C, Al-SiC and Al-Al2O3 composites. Section 4 analyses the tribological behaviour of Al-B4C composites. The tribological properties of Al-B4C and Al-SiC composites are also compared in Section 4.

2. Fabrication of Aluminium-Boron Carbide Composites

Many methods are available to fabricate MMCs, and the commonly used two primary processes are: (a) solid-state processes; and (b) liquid state processes (6). Liquid state processes include infiltration techniques (pressure infiltration and squeeze casting) and dispersion techniques (stir casting and compocasting). The stir casting (vortex addition) technique has been the most studied method for producing AMMCs due to its simplicity, flexibility, commercial viability and ease of processing (19, 20). The core requirement of the stir casting of MMCs is close contact and bonding between the ceramic phase and the molten alloy. The wettability of the ceramic particles to molten melt is inherently weak. Thus, intimate contact and bonding between them are enhanced by artificially inducing wettability or using an external force to weaken the thermodynamic surface energy barrier. One of the commonly used methods to incorporate, wet and uniformly distribute the ceramic particles is to add the particles to a vigorously stirred molten melt. The stirring action (external force) enhances wetting and ensures homogenous dispersion of reinforcement particles through the matrix. Wettability is also induced artificially by modifying the chemical composition of the matrix alloy: small quantities of reactive elements, such as magnesium, calcium, lithium or sodium, are added (20). The addition of magnesium improves the wettability of Al2O3 and SiC particles to the alloy matrix, which increases the wear resistance of Al-Al2O3-SiC hybrid composites (21).

Lashgari et al. (22) reported that during stir casting, the addition of magnesium improved wettability between the matrix (A356) and reinforcement particles (B4C). The reinforcement particles were preheated to enhance the wettability of the ceramic particles with the metal matrix. Details of the stir casting technique and the particle size of B4C particles are shown in Table I. Mahesh et al. (23) preheated the reinforcement particles to remove impurities and to enhance the wetting characteristics. Canakci et al. (24) observed that the vortex formed due to stirring holds the reinforcement particles dispersed in the melt, which ensured their uniform distribution. After particle addition, the composite melt is poured into a permanent mould. Kalaiselvan et al. (25) fabricated AA6061-B4C composites reinforced with 4 wt%, 6 wt%, 8 wt%, 10 wt% and 12 wt% B4C particles through the stir casting process. Uniform distribution of reinforcement particles was observed at all weight percent additions. Furthermore, X-ray diffraction (XRD) analysis of the composites revealed that there is no reaction of the AA6061 matrix with the B4C particles. This phenomenon shows the thermodynamic stability of B4C particles at the temperature (920°C) used for the stir casting of AA6061-B4C composites. The parameters used by Lashgari et al. (22), Mahesh et al. (23), Canakci et al. (24), Kalaiselvan et al. (25), Toptan et al. (26), Mazahery and Shabani (27), Toptan et al. (28) and Baradeswaran and Perumal (29) for the stir casting of Al-B4C composites are listed in Table I.

Table I

Details of Stir Casting Technique and Particle Size of Boron Carbide Particles

Parameters of stir casting and particle size of B4C Literature
Lashgari et al. (22) Mahesh et al. (23) Canakci et al. (24) Kalaiselvan et al. (25) Toptan et al. (26) Mazahery and Shabani (27) Toptan et al. (28) Baradeswaran and Perumal (29)
Composite type A356-B4C AA6061-B4C AA2014-B4C AA6061-B4C AA1070-B4C and AA6063-B4C A356-B4C AlSi-CuMg-B4C AA7075-B4C
Temperature of melt, °C 730 – 700 920 850 750 850 850
Stirring speed, rpm 720 600–700 450 and 350a 300 500 600 1000 500
Stirring time, min 20 – 3 and 4a 5 5 – – 4
Pouring Temperature, °C 730 730 680 – 850 – 900 850
Particle size 65 μm (APSb) 20 μm (APS) 85 μm (APS) 10 μm (mesh size) 32 μm (APS) 1–5 μm 32 μm (APS) 16–20 μm
Particle preheat temperature, °C 250 250–600 400 400 850 – – –
Melting environment Argon Room Argon Room Room Argon Vacuum Room

a Before and after particles addition

b APS = average particle size

3. Microstructural Characteristics and Mechanical Properties

Shorowordi et al. (30) studied the matrix-reinforcement interface of Al-20 vol% SiC (Figure 1(a)), Al-20 vol% Al2O3 (Figure 1(b)) and Al-13 vol% B4C (Figure 1(c)) composites produced through the stir casting technique.

Fig. 1.

Scanning electron microscopy (SEM) micrographs of the matrix-reinforcement interface: (a) Al-20 vol% SiC composite; (b) Al-20 vol% Al2O3 composite; and (c) Al-13 vol% B4C composite. Reprinted from (30), Copyright (2003), with permission from Elsevier

The microstructure and interfacial characteristics of Al-SiC, Al-Al2O3 and Al-B4C composite are extensively reported in this study. The interfacial reaction product is not observed for the Al-B4C composite, unlike the Al-SiC composite, which revealed an apparent interfacial reaction. Furthermore, it was observed from the fracture surfaces that Al-B4C composite exhibited the strongest bonding at the matrix-reinforcement interface, and the bonding of Al-SiC composite is weak due to the low adherence of aluminium matrix to the SiC particles. In Al-Al2O3 composites, voids and microvoids are observed at the interface, indicating poor bonding. Moreover, particle distribution is found to be better for Al-B4C composite when compared to Al-SiC and Al-Al2O3 composites.

The mechanical properties of spray-cast Al 6061-15 vol% B4C and aluminium 6061-15 vol% SiC composites have been reported (31). The B4C reinforced composite exhibited significantly greater strength, strain to failure in tension and strain hardening compared to the SiC reinforced ones, due to strong bonding at the Al-6061-B4C interface (31). The strong bonding at the interface is ascribed to the chemical stability of B4C particles, the absence of interfacial reaction products and the excellent wetting of the particles by the matrix alloy. The wetting characteristics of the Al-6061-SiC composite are weaker than that of the Al-6061-B4C composite.

3.1 Influence of Boron Carbide Particles Addition on Hardness

Kalaiselvan et al. (25) studied the relationship between the weight percent addition of B4C particles and the hardness of the composites. Al-B4C composites were reinforced with 4 wt%, 6 wt%, 8 wt%, 10 wt% and 12 wt% B4C particles and fabricated through the stir-casting method. It can be observed from Figure 2 that both the micro- and macrohardness of the Al-B4C composites increase linearly with the increase in weight percent addition of B4C particles. This observation agrees with that of Hynes et al. (32), who reported that the microhardness of the aluminium-matrix composites increased with an increase in B4C particles addition of 5 wt%, 10 wt% and 15 wt%. Furthermore, almost unvaryingly, the microhardness of materials is higher compared to its standard macrohardness (33).

Fig. 2.

Effect of weight percent addition of B4C particles on the hardness of AA6061-B4C composites. Reprinted from (25), Copyright (2011), with permission from Elsevier

During hardness testing, the pressure induced by the indenter is partially accommodated by the plastic flow of the matrix but mainly by localised increase in the weight percent addition of hard reinforcement particles (34).

It has been reported that hard reinforcement particles inherently exhibit considerable resistance to indentation by the hardness tester. Hence the increase in weight percent addition of reinforcement particles leads to an increase in hardness. Furthermore, it has been reported that bonding between the matrix and reinforcement particles and the matrix-reinforcement interface plays a significant role in the hardness of the composites. The strong bonding between the matrix and reinforcement and their interface, which is free of reaction products, improves the capability of the matrix to transfer the indentation load to reinforcement particles. This phenomenon, in turn, leads to an increase in the hardness of the composites (25).

4. Tribological Properties of Boron Carbide Reinforced Aluminium Matrix Composites

An overview of the literature on the tribological properties of Al-B4C composites is provided in the following subsections. The tribological properties are controlled by the physical parameters (applied load, sliding speed and sliding distance) and material parameters (the type of reinforcement and volume fraction) (35). Hence, the overview is focused on analysing the influence of physical and material parameters on the dry sliding tribological behaviour of the composites. The relevant details of sliding wear studies are shown in Table II.

Table II

Details of Sliding Wear Studies of Boron Carbide Reinforced Aluminium Matrix Composites

Features of interest Literature
Lashgari et al. (36) Tang et al. (37) Sharifi et al. (38) Shorowordi et al. (39) Shorowordi et al. (40) Toptan et al. (28)
Process route Stir casting Powder metallurgy Powder metallurgy Stir casting Stir casting Stir casting
Particle size 65 μm (APS) – 10–60 nm 40 μm 40 μm 32 μm (APS)
Weight or volume fraction of reinforcement particles 10 vol% B4C 5 wt% and 10 wt% B4C 5 wt%, 10 wt% and 15 wt% nano-B4C 13 vol% SiC and 13 vol% B4C 13 vol% SiC and 13 vol% B4C 15 vol% and 19 vol% B4C
Secondary process Heat treatment Hot rolling – Hot extrusion Hot extrusion –
Type of tribo-couple A356-B4C and DIN 100Cr6 steel disc AA5083-B4C and 45 carbon steel disc AISI 52100 steel and Al-B4C disc Al-SiC, Al-B4C and phenolic brake pad (disc) Al-SiC, Al-B4C and phenolic brake pad (disc) AISI 4140 steel and AlSi9Cu3Mg-B4C disc
Type of tribometer Pin-on-disc Pin size: 5 mm × 15 mm Pin-on-disc Pin diameter: 4 mm Pin-on-disc Disc diameter: 50 mm Pin-on-disc Pin size: 5 mm × 12 mm Disc size: 65 mm × 10 mm Pin-on-disc Pin size: 5 mm × 12 mm Disc size: 65 mm × 10 mm Pin-on-disc Pin diameter: 5 mm
Test parametersa L: 20 N, 40 N and 60 N S: 0.5 m s–1 D: 1000 m L: 50 N, 65 N and 80 N S: 0.6 m s–1, 0.8 m s–1 and 1.25 m s–1 D: up to 3000 m Mass loss measurement interval: 500 m L: 20 N S: 0.08 m s–1 D: varied up to 600 m Mass loss measurement interval: 25 m L: 15 N S: 1.62 m s–1 and 4.17 m s–1 D: 5832 m L: 15 N, 30 N, 44 N and 60 N S: 1.62 m s–1 D: varied up to 6000 m Total test duration: 1 h L: 20 N and 40 N S: 0.02 m s–1 and 0.03 m s–1 D: 200 m and 400 m
Wear mechanisms Delamination Abrasion and adhesion – Delamination and abrasion Delamination and abrasion Abrasion, delamination and adhesion

a L = applied load; S = sliding speed; D = sliding distance

4.1 Effect of Variation of Applied Load

Table II gives information regarding the materials, fabrication route, secondary process and tribological test parameters used in the study of Lashgari et al. (36). It is observed from Figure 3 that the wear resistance of heat-treated A356-10 vol% B4C composites decreased with an increase in applied load from 20 N to 60 N, due to the induction of different wear mechanisms. At 20 N applied load, long and continuous grooves (Figure 4(a)) are observed on the worn surface. The formation of these grooves is attributed to the induction of abrasive (cutting and ploughing) wear mechanisms.

Fig. 3.

Variation of wear resistance with applied load for a sliding speed 0.5 m s–1 and sliding distance 1000 m (not heat treated A356 alloys, heat treated A356 alloys, not heat treated A356-10 vol% B4C composites and heat treated A356-10 vol% B4C composites). Reprinted from (36), Copyright (2010), with permission from Elsevier

Fig. 4.

SEM micrographs of worn surfaces of heat treated A356-10 vol% B4C composites: (a) Long and continuous grooves at 20 N; (b) cracks at 60 N (sliding direction is indicated as SD). Reprinted from (36), Copyright (2010), with permission from Elsevier

Furthermore, the investigators observed that at applied loads of 20 N and 40 N, the B4C particles remained unfractured and carried the surface load, which resulted in a relatively undamaged worn surface. However, as the applied load was increased to 60 N, the worn surface underwent cracking parallel to the sliding direction (Figure 4(b)), and the primary wear mechanism induced was delamination.

4.2 Effect of Variation of Sliding Distance and Sliding Speed

Table II gives information regarding the materials, fabrication route, secondary process and tribological test parameters used in Tang et al. (37). The variation of AA5083-5 wt% B4C composite pin length is plotted against sliding distance, as shown in Figure 5. Low wear rate is observed up to 1000 m for the different applied load and sliding speed combinations tested. However, a significant increase in wear rate is observed from 1000 m to 3000 m. Abrasion operated until 1000 m sliding distance, and adhesion is induced as the sliding distance increased to 3000 m. The induction of an adhesion wear mechanism increases wear as a chunk of matrix material gets transferred to the counterface.

Fig. 5.

AA5083-5 wt% B4C composite: variation of pin length with sliding distance for different test combinations. Reprinted from (37), Copyright (2008), with permission from Elsevier

Figure 6 shows the variation of pin length reduction rate (average) and friction coefficient of AA5083-B4C composites against sliding speed when the applied load is 65 N (37). The AA5083-B4C composites are reinforced with 5 wt% and 10 wt% B4C particles. It is inferred from the plot (Figure 6) that the pin length reduction rate (average) increased with an increase in sliding speed.

Fig. 6.

AA5083-B4C composites reinforced with 5 wt% and 10 wt% B4C particles: variation of composite pin length reduction rate (average) and friction coefficient with sliding speed. Reprinted from (37), Copyright (2008), with permission from Elsevier

Meanwhile, the friction coefficient decreased with an increase in sliding speed for both the AA5083-5 wt% B4C and AA5083-10 wt% B4C composites. Furthermore, it is observed that the wear rate exhibited by AA5083-10 wt% B4C composite is 40% lower than that of the AA5083-5 wt% B4C composite (37). This phenomenon suggested the significance of B4C particles concentration on the wear resistance of the composites. The increase in the concentration of B4C particles leads to their effective resistance to the abrasion imparted by work-hardened wear debris and hard counterface asperities (37).

4.3 Influence of Mechanically Mixed Layer

The importance of MML in reducing the wear rate of aluminium-matrix composites reinforced with conventional reinforcement particles has frequently been reported (41–45). In the case of Al-B4C composites, Sharifi et al. (38) explained MML formation using cross-sectional scanning electron microscopy (SEM) images. The investigators also discussed the influence of MML on the wear rate of Al-B4C composites. Figure 7 shows that the wear rate decreased with 5 wt% (A5), 10 wt% (A10) and 15 wt% (A15) addition of nano-B4C particles. SEM and energy-dispersive X-ray spectroscopy (EDS) analysis of the worn surface revealed the formation of a dark layer which is chemically composed of aluminium, oxygen and iron. The presence of oxygen indicated an oxidation reaction, and the presence of iron indicated the transfer of steel debris from the counterface. The mechanical mixing of tribo-couple debris between two solid surfaces led to the formation of the MML. SEM cross-sectional micrographs of the MML (white layer (marked with arrow)) formed on 5 wt% (A5), 10 wt% (A10) and 15 wt% (A15) nano-B4C composite worn surfaces are shown in Figures 8(a), 8(b) and 8(c), respectively. The composites were tested at a sliding speed of 0.08 m s–1, applied load of 20 N and sliding distance of 25 m. Information regarding the materials, fabrication route and tribological test parameters used in Sharifi et al. (38) is shown in Table II. Furthermore, Monikandan et al. (46, 47) reported that the increase in applied load leads to the destruction of the MML, while the increase in sliding speed is conducive for its formation.

Fig. 7.

Variation of wear rate with 5 wt% (A5), 10 wt% (A10) and 15 wt% (A15) addition of nano B4C particles for a sliding speed 0.08 m s–1, applied load 20 N and sliding distance 25 m. Reprinted from (38), Copyright (2011), with permission from Elsevier

Fig. 8.

Cross-sectional SEM micrographs of worn surfaces showing the MML (marked with arrow): (a) 5 wt% nano B4C composite (A5); (b) 10 wt% nano B4C composite (A10); and (c) 15 wt% nano B4C composite (A15) (sliding speed 0.08 m s–1, applied load 20 N and sliding distance 25 m). Reprinted from (38), Copyright (2011), with permission from Elsevier

4.4 Beneficial Effects of Boron Carbide Particles Addition

Shorowordi et al. (39) compared the tribological properties of pure aluminium, Al-13 vol% B4C, and Al-13 vol% SiC composites at two different sliding speeds (1.62 m s–1 and 4.17 m s–1) and an applied load of 15 N. The investigators reported that pure aluminium experienced a higher wear rate than the composite at the sliding speed of 1.62 m s–1. At 4.17 m s–1, the wear rate of pure aluminium is very high, which led to the termination of the test at 1000 m before completing the selected test distance (5832 m). SEM analysis of the worn surface of the Al-B4C composite at 4.17 m s–1 revealed finely polished B4C particles and no sliding striations (Figure 9(a)). Meanwhile, at 4.17 m s–1, sliding striations were observed on the worn surface of the pure aluminium, which indicated ploughing of the ductile matrix by the hard counterface material (the ploughed region is marked with dotted lines in Figure 9(b)). It is evident that the worn surface of the aluminium-matrix was severely damaged, while the worn surface of the Al-B4C composite was damaged only mildly. After sliding for some duration, the tribo-contact was made of B4C particles and the counterface. The B4C imparted resistance against abrasion induced by the asperities of the counterface (18). Hence there was no ploughing of the composite. Moreover, in the case of composites, B4C particles bore a significant fraction of applied load during sliding; thus extending the applied load or sliding speed at which severe wear is induced. However, the unreinforced aluminium-matrix undergoes severe wear at much lower applied load or sliding speed than the Al-B4C composite. The information regarding the materials, fabrication route, secondary process and tribological test parameters used in the study is shown in Table II (39).

Fig. 9.

SEM micrographs of the worn surfaces at applied load 15 N and sliding speed 4.17 m s–1: (a) Al-13 vol% B4C composite (sliding distance 5832 m); (b) ploughed region (marked with dotted lines) of pure aluminium (sliding distance 1000 m). Reprinted from (39), Copyright (2004), with permission from Elsevier

4.5 Comparison of Tribological Properties of Aluminium-Boron Carbide and Aluminium-Silicon Carbide Composites

It is inferred from the bar chart shown in Figure 10(a) that the Al-B4C composite in Shorowordi et al. (39) exhibited a lower wear rate than the Al-SiC composite at a sliding speed of 1.62 m s–1. The composites were tested for the applied load of 15 N and sliding distance of 5832 m. Figure 10(b) shows the steady-state friction coefficient of Al-B4C composites and Al-SiC composites. At the sliding speed of 1.62 m s–1, the Al-B4C composite exhibited a slightly lower steady-state friction coefficient than the Al-SiC composite. However, as the sliding speed increased to 4.17 m s–1, both composites appeared to attain similar steady-state friction coefficient values. It is reported that the friction coefficient of both the composites reached a steady-state value at a sliding distance between 500–600 m (39).

Fig. 10.

Bar charts of pure Al-13 vol% SiC and pure Al-13 vol% B4C composites: (a) wear rate; (b) friction coefficient (sliding speeds of 1.62 m s–1 and 4.17 m s–1, applied load of 15 N and sliding distance of 5832 m). Reprinted from (39), Copyright (2004), with permission from Elsevier

In related work, Shorowordi et al. (40) compared the tribological properties of the same tribo-couple by varying the applied load and sliding distance. Information regarding the materials, fabrication route, secondary process and tribological test parameters used in the study is shown in Table II. The wear rate of Al-SiC composite is higher than that of Al-B4C composite at high applied loads, which is attributed to the formation of cracks at the Al-SiC interface and the pullout of SiC particles from the worn surface (40). The presence of a brittle phase at the Al-SiC interface might be the reason for the formation of cracks and pullout of SiC particles (30). However, in the case of Al-B4C composite, particle pullout is not observed. It is to be noted that the interface of the Al-B4C composite is seemingly less brittle than that of the Al-SiC composite. The hardness of the B4C particle is also higher than that of the SiC particle, leading to the low wear rate of Al-B4C composite. The friction coefficient of the Al-B4C composite is slightly lower than that of Al-SiC composite, which is attributed to the presence of boron in the oxidised state on the worn surface of the Al-B4C composite.

4.6 Inferences Obtained from the Statistical Analysis

Statistical analysis is useful in the initial stages of the experimental findings. It aids in assessing the preliminary change in the trend of the responses (wear and friction coefficient) (48–50). Toptan et al. (28) studied the tribological behaviour of AlSi9Cu3Mg-B4C composites reinforced with 15 vol% and 19 vol% B4C particles. Information regarding the materials, fabrication route and tribological test parameters used in the study is shown in Table II. A statistical method (24 full factorial design) was used to design the experiments; the four parameters varied for two levels are volume percent addition of B4C particles, applied load, sliding speed and sliding distance (28). Figures 11(a) and 11(b) show the normal probability plots of the wear rate and friction coefficient, respectively.

Fig. 11.

Normal probability plots of AlSi9Cu3Mg-B4C composites: (a) wear rate; (b) friction coefficient. Reprinted from (28), Copyright (2012), with permission from Elsevier

The normal probability plots reveal that the residuals lie very close to the normal probability line, which indicates that the residuals are fitted convincingly to the normal distribution (28). The normal distribution and lack of outlier residuals and absence of change in the slope of the normal probability line confirm that all relevant physical and material factors that influence the tribological behaviour were considered in the experimental study (51). Figures 12(a) and 12(b) show the main effects of the wear rate and friction coefficient, respectively (28). It is observed from the main effects plot (Figure 12(a)) that the wear rate increased with an increase in B4C particles addition, applied load and sliding distance. However, the wear rate decreased with an increase in sliding speed. Meanwhile, the friction coefficient increased with an increase in B4C particles addition and sliding distance (Figure 12(b)). The friction coefficient decreased with an increase in sliding speed and applied load.

Fig. 12.

Main effects plots of AlSi9Cu3Mg-B4C composites: (a) wear rate; (b) friction coefficient. Reprinted from (28), Copyright (2012), with permission from Elsevier

The analysis of variance (ANOVA) technique analyses experimental data to give vital inferences: the impact of physical and material factors on the responses and the impact of interaction of physical and material factors on the responses (52, 53). ANOVA analysis by Toptan et al. (28) revealed that applied load, volume percent of B4C particles and interaction of sliding speed and applied load had statistically and physically significant influence on wear rate. The sliding distance and interaction of other physical parameters were not statistically or physically significant to influence the wear rate. The ANOVA analysis of the friction coefficient revealed that volume percent of B4C particles and applied load provided statistical and physical significance on the friction coefficient. Meanwhile, the sliding speed, sliding distance and interaction of physical parameters did not provide statistical and physical significance on the friction coefficient (28).

5. Summary

The fabrication and tribological properties of Al-B4C composites are discussed in this overview. The Al-B4C composites exhibited better particle distribution than Al-SiC or Al-Al2O3 composites. The bonding at the matrix-reinforcement interface is also strong, and the interface is free of the interfacial reaction product, which is not the case with Al-SiC and Al-Al2O3 composites. The presence of a brittle phase at the matrix-reinforcement interface reduced the wear resistance of Al-SiC composites. The friction coefficient of Al-B4C composites is lower than that of Al-SiC composites due to the presence of oxidised boron on the contact surfaces. The better tribological properties of Al-B4C composites compared to those of pure aluminium are due to the abrasion resistance imparted by the B4C particles. The wear mechanisms induced during wear studies of Al-B4C composites are plastic deformation, adhesion, abrasion and delamination. Statistical analysis revealed that the influence of physical and material factors and their interaction on the tribological behaviour is statistically significant.

To summarise, Al-B4C composites exhibit better microstructural characteristics than aluminium-matrix composites reinforced with SiC and Al2O3 particles. The tribological properties of Al-B4C composites are better than those of aluminium and Al-SiC composites; thus, these composites may be considered as a potential candidate for different tribologically crucial applications.

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Acknowledgements

The corresponding author expresses sincere thanks to the Ministry of Human Resources Development, Government of India, for providing the fellowship to conduct his doctoral research. Furthermore, the authors sincerely thank the reviewers for their useful suggestions, and the Editor Ms Sara Coles and Editorial Assistant Mrs Yasmin Stephens for prompt responses and brilliant editing work.

Supplementary information

Materials Research and Innovation Centric Solutions LINK https://www.matricstech.com/

The Authors

V. V. Monikandan is a Postdoctoral Researcher with the School of Minerals, Metallurgy and Materials Engineering, Indian Institute of Technology Bhubaneswar, India. Formerly, he was with Materials Research and Innovation Centric Solutions, India as a research associate. He received his PhD in tribological behaviour of aluminium matrix composites from the National Institute of Technology Calicut, India. He specialises in additive manufacturing of MMC coatings and synthesis of smart composites through pressureless infiltration process and biodegradable lubricants.

K. Pratheesh is a Professor of Mechanical Engineering and affiliated with Mangalam College of Engineering, Kottayam, Kerala, India. He received his PhD in grain size modification of aluminium-silicon alloys from the National Institute of Technology Calicut. His research interests include fabrication of as-cast alloys using liquid metallurgy technique, synthesis of grain modifier mixtures for non-ferrous alloy castings and solidification of castings.

P. K. Rajendrakumar is a Professor (HAG) of the Department of Mechanical Engineering, National Institute of Technology Calicut. His research interests include tribology, biomechanics and product design.

M. A. Joseph is a Professor (HAG) of the Department of Mechanical Engineering, National Institute of Technology Calicut. His research interests include MMCs, polymer materials and non-ferrous alloys.

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