1 Introduction

Low hardness and poor tribological properties represent major limitations of aluminum and its alloys [1]. Despite these limitations, aluminum is characterized by its low density, high thermal conductivity and high toughness. Also, it has high corrosion resistance which extends its uses to a wide variety of environmental conditions [2].

Attempts were made to expend the use of aluminum and its alloys by overcoming drawbacks that restrict its use by incorporating various ceramic hard phases such as SiC [3], eggshells (Es) [4], Al2O3 [5] and BN [6]. Further, hybrid reinforcements, e.g., fly ash–alumina [7], MoS2-WC [8], SiC-Es [9], were used to improve mechanical and tribological properties of aluminum alloys.

Eggshell is a ceramic material with low density, low cost and high thermal stability. Further, it is considered as an eco-friendly renewable material [10,11,12,13,14]. Moreover, the good corrosion resistance of graphite encourages the researchers to use as a reinforcement material in aluminum matrix composite, especially that graphite is considered as a good solid lubricant because of the weak Van der Waals bond between layers of graphite and a strong interlayer covalent bond between carbon atoms [15, 16].

Dispersion of discontinuous reinforcement in the metal matrix is carried out by stir casting or powder metallurgy (PM) technique. In stir casting, the distribution of reinforcement throughout the matrix results in less homogenous products as compared to powder metallurgy in which a highly uniform distribution of reinforcement in the matrix can be attained [17, 18].

The positive effect of graphite (up to a certain limit) on wear resistance was presented in many studies [19,20,21,22]. On the other hand, several studies reported that eggshells can improve mechanical properties [9, 23, 24].

To our knowledge, nobody had studied the effect of hybrid green reinforcement (Es–Gr) on the mechanical and tribological properties of the aluminum matrix. Therefore, the current research suggests the use of two kinds of reinforcement, eggshells and graphite, to combine their advantages in one filler material that may overcome the limitations of the aluminum matrix.

2 Experimental details

2.1 Characterization of raw and mixture powders

In this research, composites of aluminum alloy matrix reinforced with two kinds of reinforcements, eggshell and graphite, are prepared by powder metallurgy route. The matrix material was aluminum with a fixed amount of tin (Sn) and magnesium (Mg). The size, shape and density of the materials used in the composites have important effects on the mechanical, tribological and thermal properties of the produced composites [25]. Firstly, powder of commercially pure aluminum of 99% purity provided by Acros Organics, USA, having a density of 2.7 g/cm3 and particle size of about 74 µm was mixed with a fixed amount of Sn and Mg to form the matrix alloy of the developed composites.

The compressibility of aluminum powders is restricted by the presence of alumina (Al2O3) layer surrounding aluminum particles, which prevents particle to particle contact during sintering. Thus, magnesium was used as a sintering agent to react with Al2O3 and form crystalline spinel (3MgAl2O4) and/or (3MgO) which can be easily bonded [26, 27]. Moreover, tin was also used as a sintering agent because of its low melting temperature which prompts phase sintering, leading to denser compact material [28]. Mg and Sn were used with similar and fixed amounts to that used by Macaskill et al. [27]. Accordingly, the matrix alloy composition is Al–1.5Sn–1.5 Mg, and the designation Al matrix will be used to refer to this composition everywhere in this article for simplicity.

Eggshells (Es) powder was used as primary reinforcement material and prepared by collecting raw eggshells from the nearby bakery, cleaning them by water, followed by drying in the sun to remove impurities and unpleasant odors. Plastic hammer was used to convert large shells into flakes that were pulverized by a blender. Next, the carbonization process was performed at 500 °C for 3 h to remove organic materials. During the sintering process, organic materials vaporized and burned off. Figure 1a–c shows raw, carbonized and uncarbonized eggshells, respectively. Finally, the sieving process was carried out by Retsch sieve shaker and the particle size of eggshells was less than 50 µm.

Fig. 1
figure 1

Eggshell samples: a uncarbonized (unground); b uncarbonized (ground); c carbonized (ground)

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Figure 2a shows SEM images for aluminum particles having elongated sub-angular shape. SEM image shown in Fig. 2b displays the used carbonized eggshell particles having irregular sizes and shapes because of the shear action of the blender blade during preparation. Graphite particles have flake-like shape shown in Fig. 2c. The densities of the reinforcements were determined by Archimedes rule as 2.5 g/cm3 and 2.1 g/cm3, for the carbonized eggshell and flakes graphite, respectively. Seven trials were conducted to get more accurate results. Energy-dispersive X-ray spectroscopy (EDX) test was conducted on the eggshells particles to investigate the elemental composition of the composites.

Fig. 2
figure 2

SEM micrograph of a elongated sub-angular aluminum matrix, b irregular shapes and sizes of carbonized eggshells and c flake graphite

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2.2 Densification of mixture powders and processing of hybrid composites

The mixed homogenous powders were compacted at room temperature in a single-action hardened steel die (Ф 8 mm × 20 mm) using a universal testing machine (Fig. 3) (model WAW-2000H) at 475 MPa as concluded from the densification curve shown in Fig. 4 which represents the relationship between the relative density and the applied pressure [29]. All produced samples were sintered at 630 °C for 2 h. Sintering was carried out in an electrical furnace. Sintering at high temperature can give enhanced physical and mechanical properties, as the powder compact goes through more densification with reduced porosity content and better particle bonding through progressive pores closure and neck formation between powder particles. The pores more likely become toward circular and smooth shape instead of having irregular and angular shape [30]. Zinc stearate lubricant, purchased from Sigma-Aldrich (USA), was coated on the internal surface of the mold cavity to facilitate sample removal after the compaction process. This step was performed as O’Donnell and Looney [31] suggested. The sample designation and reinforcement weight ratio are presented in Table 1.

Fig. 3
figure 3

Compaction process using universal testing machine

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Fig. 4
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Densification curves for Al matrix–6Es–3Gr composite

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Table 1 Sample designation and reinforcement weight ratio
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2.3 Density and porosity calculations

It is essential to study the main factors of the produced hybrid metal matrix composites namely density and volume porosity percentage to illustrate the properties of composite materials. The theoretical density of the samples was calculated using the rule of mixtures as follows [32, 33]:

$$ \rho_{{ {\text{Composite}}}} = \frac{100 }{{\frac{w1}{\rho 1} + \frac{w2}{\rho 2} + \frac{w3}{\rho 3}}} $$
(1)

where ρ1, ρ2 and ρ3 represent the density of the three material powders, while w1, w2 and w3 represent the weight percentages of those powders.

In the present research, the volume porosity percentage of hybrid metal matrix composites has been examined based on the determination of the density of hybrid composites. Using Archimedes principle, experimental values of the density of the hybrid composites have been evaluated, and using the rule of mixtures, theoretical values of the density of the composites have been accomplished. Experimentally, the volume porosity percentage of the composite materials has been determined as follows [34]:

$$ {\text{Volume }}\;{\text{porosity}} \left ( \% \right) = 100 {-}\left( {\frac{{{\text{sintered }}\;{\text{density}}}}{{{\text{theoretical }}\;{\text{density}}}}} \right) \times 100 $$
(2)

2.4 Mechanical, tribological and microstructural characterizations

The compression test was performed by Quasar 100 universal testing machine at a crosshead speed of 2 mm/min. The aspect ratio of investigated samples ranged from 1.9 to 1.96. Reducing the barreling shape effect was done by coating the machine bench and crosshead with a thin layer of grease. Micro-Vickers hardness testing machine (model MHT1, Tokyo, Japan) was set at a load of 100 gf for a dwell time of 10 s. Seven readings were taken from each sample. The surface of the samples was polished by 0.3 μm diamond paste (alumina suspension, PRESI) to clarify the pyramid indentation, thereby showing more accurate results. The wear test was carried out according to ASTM G 99–95a [35] using a pin-on-disk apparatus (model TM 200, Gunt, Hamburg). The parameters of the test were set at a load of 25 N and a speed of 100 RPM. Al matrix, Al matrix–3Gr and Al matrix–6Es–3Gr composites were analyzed. Scanning electron microscope (Quanta™ 450 FEG SEM) supplied with energy-dispersive spectroscopy (EDS) was used to investigate reinforcement distribution, cracks, reinforcement–matrix interface and the type of wear mechanism.

3 Results and discussion

3.1 Density and porosity analysis

Table 2 shows the theoretical density, experimental density and volume porosity percentage of the produced composites. For easier interpretation, the volume porosity percentage in Table 2 is represented in Fig. 5. As shown in the figure, increasing Gr percentages caused the porosity to increase. Adding 4.5 wt% of Gr gave the highest amount of porosity regardless of the level of Es. Meanwhile, the addition of 1.5 wt% of Gr causes a slight increase in porosity volume percentage as compared to Al matrix.

Table 2 Theoretical, experimental and relative density and porosity volume percentage of all produced samples
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Fig. 5
figure 5

Porosity volume percentage of all fabricated composites

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3.2 The effect of graphite percentage and sliding time on wear Loss

Figure 6 shows the change in mass loss of the Al matrix–(X)Gr composites with sliding time at different weight fractions of graphite. Graphite content of 1.5 wt% and 3 wt% causes only a slight increase in wear loss as the graphite lubricant forms a thin layer that prevents metal to metal contact. This is similar to findings by previous researchers [21, 22, 36]. However, at 4.5 wt% graphite, a significant incremental increase in the mass loss is observed, due to the high amount of porosity similar behavior was reported by other researchers [37,38,39].

Fig. 6
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Variation of cumulative mass loss of Al matrix–(X)Gr composites with sliding time at different graphite percentages

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3.3 The effect of hybrid green reinforcement percentage and sliding time on wear loss

For all the examined composites, keeping the eggshell weight percentages constant and changing the graphite contents will result in an increase in the mass loss. As an example, Fig. 7 shows the variation of the mass loss with sliding time for composites having 3 wt% eggshell and different weight fractions of graphite (1.5, 3, 4.5). The increased mass loss with the increase in graphite percentages is due to the increase in porosity which is attributed to poor Al–Gr interface and clustering. On the other hand, constant slope indicates that the wear loss is approximately uniform throughout the test.

Fig. 7
figure 7

Variation of cumulative mass loss of Al matrix–3Es–(X)Gr composites with varying graphite percentage and sliding time

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3.4 The effect of porosity percentage on wear loss

Figure 8 shows the cumulative mass loss (after 60 min) versus porosity volume percentages for the examined Al matrix–(X)Gr composites at different graphite weight percentages. The figure showed that the mass loss increased with the increase in graphite content and thus porosity percentage. This behavior was also observed by Sinha and Farhat [34].

Fig. 8
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Cumulative mass loss after 60 min versus porosity vol% for Al matrix–(X)Gr composite

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3.5 The effect of graphite percentage on micro-Vickers hardness

Figure 9 displays the effect of graphite addition on the hardness of Al matrix. The addition of 1.5 Gr wt% increased the hardness from 30.3 to 31.9, i.e., which is about 5%. This improvement may be attributed to the presence of magnesium carbonate (magnesite, MgCO3) hard phase. The presence of this hard phase reduced the amount of plastic deformation during the penetration of the indenter; therefore, the hardness will be increased. These results are consistent with the results presented by Hanna et al. [40]. However, the use of higher graphite content caused the hardness to decrease, where the hardness improvement decreased from 5% to 1.6% and 1.9% at 3 and 4.5 wt% Gr, respectively. The reduction in the hardness value can be attributed to the softness nature of graphite [19] and the increase in the porosity percentage with increasing graphite content. Similar results were presented in [15].

Fig. 9
figure 9

Hardness values against graphite wt% in Al matrix–(X)Gr composites

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3.6 The effect of hybrid green reinforcement percentage on micro-Vickers hardness

Figure 10 depicts the effect of hybrid green reinforcement on the hardness of Al matrix. It can be seen from the figure that increasing graphite percentages, regardless of the constant amount of Es, always lead to a decrease in the hardness except the sample that contains 6 Es wt% and 3 Gr wt%. This increase may be reasoned to the penetration of the indenter directly in presented hard phases in the sample. All composites that contain 3 wt% Es with different percentages of Gr are characterized by higher values compared to Al matrix. However, Al matrix–9Es–4.5Gr composite has the lowest HV numbers. This is due to the presence of the highest amount of reinforcement which accompanied by a high amount of porosity. Hardness behavior in the present composites gives an initial indication of the mass loss behavior. In other words, the samples that have high hardness have lower mass loss according to the Archard equation, Eq. 3 [41]. Many researchers clarify the inverse relationship between wear rate and hardness [42, 43].

$$ Q = \left( {\frac{W}{H}} \right) $$
(3)

Q represents the wear rate. K, W and H represent the wear coefficient constant, the normal load and the hardness, respectively.

Fig. 10
figure 10

Hardness values against hybrid green content in Al matrix–(X)Es–(X)Gr composites

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3.7 The effect of graphite percentage on compressive strength

Figure 11 illustrates the relationship between graphite percentages and compressive strength. The addition of flake graphite up to 1.5 wt% increased compressive strength by 23%, and similar result is obtained by Swamy et al. [44]. Beyond 1.5 wt% Gr, the compressive strength decreased again. At 1.5 Gr wt%, the amount of porosity was low; thus, the graphite, in turn, acts as a dislocation barrier improving compressive strength. Further, forming MgCO3 may help in compressive strength improvement [40]. Meanwhile, at 3 and 4.5 Gr wt%, the compressive strength decreased due to the presence of the high amount of porosity that causes a reduction in the cross-sectional area, resulting in high stress concentration. Furthermore, the porosity at matrix–particle interface reduces the amount of transmitted load to reinforcement particles, where transmission load from the matrix to reinforcement largely depends on interfacial bonding; therefore, the strength of overall composite will be decreased [45].

Fig. 11
figure 11

Graph of compressive strength of Al matrix–(X)Gr composite

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3.8 The effect of hybrid green reinforcement percentages on compressive strength

According to Fig. 12, generally, it can be observed that the best hybrid green reinforcement is the combination of 3 Es wt% with different Gr wt%. In contrast, the worst hybrid green reinforcement is a combination of 9 wt% Es with different Gr wt%. This refers to the high amount of reinforcement which is accompanied by porosity which causes the lower strength of the material as discussed earlier [46]. The combination of 6 wt% Es with different percentages of Gr takes values between 3 and 9 wt% Es. It can be said that the wear, compressive strength and hardness results are in good agreement.

Fig. 12
figure 12

Graph of compressive strength of Al matrix–(X)Es–(X)Gr composite

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3.9 Microstructure and surface morphology

Although the graphite is a solid lubricant, its usefulness depends on several factors such as porosity, surface finish and the interface between matrix and particles [37]. In any case, a comparison of composite structures containing different graphite wt% (0, 1.5 and 3) was made. The micrograph of the SEM of Al matrix is displayed in Fig. 13. It is noticed from the figure that there is a homogenous distribution of sintering aids (Mg and Sn) in the Al matrix. Further, the shape of tin particles was changed during the sintering process which means these particles were melting during the process. However, Fig. 15 (at 0 wt% of Gr) presents a lower amount of porosity when compared to Figs. 14 and 15 (at 1.5 and 4.5 wt% of Gr, respectively). In the case of Al matrix, it was observed that even the porosity that existed was shallow as shown in Fig. 13. Yellow large squares in Fig. 15 display higher magnification for reds mall squares.

Fig. 13
figure 13

SEM micrograph of Al matrix surface displaying low amount of porosity. Blue arrows (bright phase) indicate Sn particles, red arrows (dark phase) indicate Mg particles and white arrows indicate scratches (color figure online)

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Fig. 14
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SEM micrographs of polished surface of Al matrix–1.5Gr showing agglomeration, deep pores at matrix–particle interface and higher amount of porosity

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Fig. 15
figure 15

SEM micrographs of a polished surface of Al matrix–4.5Gr of 2 locations. The first location a shows poor wettability, large pores, cracks and interconnected porosity at matrix–particle interface. The second location b also shows interconnected porosity at matrix–particle interface. Note the amount of porosity is presented in Al matrix–4.5Gr

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It is worthwhile to highlight here that the amount of porosity increased with increasing percentages of graphite flakes. This is clear in Figs. 14 and 15. As shown in Figs. 14, 15 and 16, the regions that include graphite flakes contain pores due to the poor matrix–particle interface. These pores cause crack initiation and therefore interconnected porosity. Figure 14 shows the agglomeration of graphite flakes which causes defects in the composites due to the presence of pores. These agglomerates form weak points in the composite leading to undesirable properties. A similar result was concluded by Ahmad et al. [47]. Figure 15a clearly depicts poor wettability between flake graphite and aluminum matrix, and it shows higher interconnected porosity compared to Fig. 14 that contains a lower percentage of graphite (1.5 Gr wt%). Furthermore, Fig. 15b shows the interconnected porosity in another location of the same sample which contains 4.5 Gr wt%. Higher focusing on poor matrix–particle interface is shown in Fig. 16.

Fig. 16
figure 16

SEM micrograph of Al matrix–1.5Gr composite showing particle matrix interface

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For ease and convenience, the sample containing 6 wt% of eggshells and 3 wt% of graphite (Al matrix–6Es–3Gr) was taken to reflect the rest of the samples containing hybrid green reinforcement of Es and Gr. Figure 17 shows Es, Gr, Sn and Mg particles. Some clusters of these fine particles are observed. Also, we can observe that Es particles have irregular shapes and sizes due to shear force from blender blades as mentioned before. However, the porosity at the matrix–graphite interface is observed. Also, the porosity at the Es cluster has appeared.

Fig. 17
figure 17

SEM micrograph of Al matrix–6Es–3Gr composite showing Es clusters accompanied with porosity. Also, Gr, Sn and Mg are appeared

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3.10 Analysis of worn-out surfaces using SEM and EDX

The morphology of the worn-out surface is crucial for determining the wear mechanism in the composites. For the base monolithic matrix alloy, SEM micrographs shown in Fig. 18a conclude that different wear mechanisms were involved, i.e., adhesive and delaminated wear. Abrasive wear mechanism is characterized by the grooves on the surface. Platelike debris is clues for adhesive wear occurrence, and shallow cavities on the surface are result of delamination wear. Figure 18b presents the acquired EDS spectrum for the worn-out surface, which consists of peaks for oxygen, chromium and iron. The oxygen peak is a sign for the oxide formation at the interfacial sample surface and steel disk, where friction can cause an increase in the temperature and thus promote formation of oxide. The presence of iron and chromium peaks can be attributed to the transfer of steel disk asperities to the worn-out surface.

Fig. 18
figure 18

a SEM micrograph of worn-out surfaces of Al matrix alloy showing three types of wear: abrasion, adhesion and delamination wear, b EDX analysis of worn-out surface of Al matrix alloy

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Composites containing 1.5 wt% graphite have similar features of that observed in the aluminum matrix. Moreover, fragmentation of graphite flakes laid on the delaminated regions is shown in Fig. 19. SEM images show the occurrence of alteration of the wear mechanisms of the composites containing 4.5 wt% graphite, where the adhesive wear becomes more dominant as shown in Fig. 20. In addition, some pores appeared on the surface as a result of the graphite that is pulled out of the surface. The magnified image of one of these pores shows fragmentations of graphite flakes.

Fig. 19
figure 19

SEM micrograph of worn-out surfaces of Al matrix–1.5Gr composite showing the three types of wear and agglomeration, pullout and breakage of graphite flakes

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Fig. 20
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SEM micrograph of worn-out surfaces of Al matrix–4.5Gr composite showing the same three types of wear, agglomeration, pullout and breakage of graphite flakes

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4 Conclusions

The goal of this research is to examine the effect of incorporating both graphite and eggshell as reinforcement particles on mechanical (hardness, compressive strength) and tribological properties of the powder aluminum matrix. The main points that can be drawn as follows:

  1. 1.

    An enhancement of the compressive strength and hardness of the matrix alloy was attained by adding 1.5 wt% graphite as a result of the formation of MgCO3. However, the use of 3 wt% and 4.5 wt% of graphite decrease both the compressive strength and hardness of the composite as a result of the increase in porosity percentage and thus reduction in the transmitted load to the matrix.

  2. 2.

    Adding graphite to the aluminum matrix composite containing eggshell has a positive impact on the tribological properties of the composite to a certain limit (1.5 wt%). However, the additional increase in graphite content has an adverse effect.

  3. 3.

    Hybrid composites with 3 wt% eggshell show the best compressive strength and hardness, whereas hybrid composite with 9 wt% eggshell has the lowest compressive strength and hardness.

  4. 4.

    The mass loss of the hybrid composite increases with the increase in the graphite weight percentages, regardless of the eggshell weight percentages. This was attributed to the increase in clustering and porosity content with the increase in graphite content and the poor interface of aluminum graphite interface.

  5. 5.

    Composites with higher porosity contents have lower hardness and compressive strength, but higher wear rate as compared with composites having lower porosity contents.

  6. 6.

    The combination of SEM micrographs and EDX showed signs for four wear mechanisms: abrasive, adhesive, delaminated and oxidative wear in the examined composites.