Introduction

The use of aluminum metal matrix composites (AMCs) has steadily increased since last two decades.[1–3] The enhanced properties of the metal matrix composite is attributed to the ability of a light metal such as aluminum to blend with high strength and high modulus ceramic particles.[4,5] The strengthening mechanism for the composite is due to dispersion hardening as explained in Orowan’s model.[6] Shi and Arsenault[7,8] analyzed various contributing factors for strengthening. However, mechanical properties largely depend on the chemical constituents of the particles, their size, shape, and distribution in the matrix. Poor toughness in the materials due to incompatibility between the matrix and particulates has also been observed by various workers.[9–14] The interrelationship among all these in respect to property development has been emphasized by many workers.[13,14] The objective of development of such materials is of interest, particularly due to the replacement of the number of iron base alloys in the automobile industry, thereby increasing the fuel efficiency by the reduction of the weight.[15,16] Presently, these composites are produced by the addition of SiC/Al2O3 particulates into molten aluminum or its alloys to make it cost effective.[17–19] However, it is observed that while preparing the composite through melting routes, there is a loss of costly particulates due to poor wettability of particulates in the melt and the density difference between the particulates and the matrix, making the product even costlier.[20,21] Thus, cost plays a key role outweighing the weight saving vis-a-vis fuel saving. Again, the recyclability of the composite has not been found very promising. In order to make the process cost effective, a novel technique is adapted to produce in situ ceramic composites consisting of Al2O3, SiC, and C, developed indigenously from waste obtained during mining of coal from underground mines.

The ceramic composite so developed is thought to be a useful material as a reinforcing agent in molten aluminum and its alloys. The essential constituents of colliery shale are SiO2, Al2O3, and C. By treating these materials thermally at elevated temperatures in an inert atmosphere, it is expected that there will be an in situ transformation of SiO2 to SiC in the neighborhood of carbon and Al2O3, thus producing an in situ composite from CS material.[22] The excess carbon present in the shale material after full conversion will remain in the composite in the form of graphite, a crystalline phase. The composite synthesized from colliery shale has the role of enhancing mechanical and tribological properties as envisaged in the present study.

In the first phase of the work, experiments are conducted to develop an in situ ceramic composite from colliery shale in a plasma reactor as well as in a high vacuum (10−6 Torr) furnace. Before addition of these materials to the aluminum melt, full characterization of the new novel material is necessary. In the second phase of the work, an attempt is made to produce AMC using the new in situ ceramic composite. For comparison purposes, properties of the AMC prepared using the developed in situ material are compared with the AMC prepared using Al2O3 particulates.

The microstructural integrity largely depends on the continuity of the interface between the particulates with different sizes and shapes and the matrix. Generally, ceramic particulates provide incoherent high energy interfaces and thereby induce rapid diffusion by providing heavy sinks. The discontinuity will, thus, lead to poor fracture toughness of the material. By inducing mechanical working such as forging and rolling, mechanical bonding is established between the two, i.e., the matrix and particulates. In a recent study, it was observed that there is an enhancement in the mechanical properties of the AMCs due to mechanical bonding of the particulates with the matrix by hot working.[23] The improvement of mechanical properties by hot working is attributed to the enhancement of continuity in the interface between the matrix and particulates as is observed during the investigation.

Several authors have worked on the behavior of the aluminum-based discontinuously reinforced composite.[24–28] Hot working results in recrystallization of the cast composite. Owing to the limited plasticity of the particulate-reinforced MMC at room temperature, they are amenable to hot deformation. Due to enhanced plasticity in the matrix, building up of localized stress is prevented around the particulates and thus reduces the extent of particulate damage due to cracking or decohesion of interface. In the present investigation, the aluminum composite prepared with an in situ ceramic composite developed in the laboratory is hot forged and rolled. Similar treatment is given to the aluminum composite prepared from alumina particulates for comparison purposes. Properties such as frictional force and wear resistance in unlubricated conditions are of particular importance for the manufacture of brakes, engine pistons, etc.[29–32] The present study will, therefore, also report the tribological behavior of Al-CS and Al-Al2O3 composites both in cast and hot-worked conditions. The matrix plasticity of the composite is controlled by the presence of hard and brittle phases dispersed in the matrix. Therefore, the tensile properties are thought to be an important proposition since the stress–strain curve gives a measure of the toughness of the material. The materials are characterized microstructurally at different stages using an optical microscope, scanning electron microscope (SEM) fitted with energy dispersive X-ray (EDX), and X-ray diffractometry (XRD).

Experimental Procedure

Preparation of In Situ Ceramic Composite

The waste colliery shale material, obtained from mines in the form of lumps, was crushed and ground to a size <200 mesh. It was then treated in a wet magnetic separator using 10A current for the removal of Fe2O3. The purified powder was heated thermally in a high temperature plasma reactor in argon atmosphere and in a tubular furnace at 1673 K (1400 °C) under vacuum with a view to convert the material to a useful in situ ceramic composite. Finally, the material was furnace cooled in the atmosphere of argon and was characterized by XRD and SEM-EDX.

Preparation of AMC by Stir Casting Using Ceramic Composite

Aluminum of 99.6 pct purity was used for the preparation of the composite. Al2O3 and freshly prepared ceramic composite from colliery shale were used as reinforcing particulates. The melting furnace used was a pit type with a bottom pouring facility. CS material of an appropriate amount was preheated to the temperature of 873 K ± 5 K (600 °C ± 5 °C) with a view of enhancing the wettability of the particles before being added to the melt. During the melting process, argon atmosphere was maintained in the furnace throughout the process. During the melting, the temperature was controlled at 1073 K ± 5 K (800 °C ± 5 °C). The molten metal was stirred with a rotor and the speed of the rotor was controlled to 600 rpm. 0.5 pct Mg was added to the melt to enhance the wettability of the particulates. A vortex was created in the melt due to stirring where preheated Al2O3 or the new in situ material was poured centrally into the vortex. The rotor was pushed down slowly by keeping a clearance of 12 mm from the bottom. The rotor was then pushed back slowly to its initial position. The pouring temperature of the liquid was maintained at 973 K ± 5 K (700 °C ± 5 °C). The melt was then cast into a mold (100 × 20 × 40 mm3).

Optical Micrographic Studies

Metallographic samples were cut from the central part of the ingot. The samples were examined under an optical microscope. Quantitative phase analysis was done using the image processor (XJL-17). Hardness tests were done using a microhardness tester (DHV-1000). The cast Al-SiC samples were taken for examination in SEM (Hitachi, S 3400 N) attached with EDX facilities.

Mechanical Working Studies

Mechanical working was pursued with the composites. In order to hot work, the cast material (30 mm thick) was slowly heated in a muffle furnace to a temperature of ~723 K (450 °C) for 4 hours. Prolonged soaking was followed by hot forging in a pneumatic hammer followed by hot rolling in a rolling mill. Deformations were carried out slowly in different stages. In each stage, the amount of reduction was restricted to 10 pct. In between the two stages, intermittent annealing was done for 30 minutes. The deformation by forging was continued from 30 to 15 mm. Following the forging, hot rolling was continued from 15 to 6 mm. Thus, the total deformation carried out was 80 pct.

The cast as well as forged materials of Al-Al2O3 composite were tested for abrasion resistance by a pin-on-disk wear test machine (Ducom, TR-201LE). The test samples of dimension 40 × 8 × 8 mm3 (shown in Figure 1) were fabricated from as-cast as well as from forged composite samples and made to slide against a low alloy steel disk (material: 103 Cri-Eng-31HRS60W61, equivalent to AISI 4340) of diameter 215 mm and hardness 62 Rc. Tangential force and hence the coefficient of friction were measured continuously with an electronic sensor attached to the machine and recorded. Frictional force in N and cumulative wear loss in μm were measured from the sensor output as a function of time. The wear test was conducted with a track diameter of 50 mm and rotational speed of 600 rpm for a period of 30 minutes. The contact surfaces of the pins and disk were polished to a roughness of Ra = 0.1 μm before wear testing.

Fig. 1
figure 1

Geometry and photograph of Al-CS, Al2O3, and pure Al test samples used for abrasion tests

Full size image

The tensile samples were cut from the hot-worked Al-CS and Al-Al2O3 composites. The geometry of the tensile sample is shown in Figure 2. Tensile testing was done in a 20 Ton UTM machine (model: M-UTE 20).

Fig. 2
figure 2

Geometry of tensile sample

Full size image

Results and Discussion

The untreated colliery shale material contains 4 to 5 pct of Fe2O3 which has a detrimental effect on the AMC, particularly at higher temperature. Wet magnetic separation has enabled a reduction of the Fe2O3 content in the shale material to one-tenth of its value as indicated in the analysis of the shale material after the magnetic separation (Table I). All other impurities’ contents are negligible. The volatile material escaped subsequently during the thermal treatment of the colliery shale material. The XRD patterns of the materials before and after magnetic separation are shown in Figure 3. There is a reduction in the peak height of Fe2O3 in the XRD pattern of the sample after magnetic separation, corroborating the data shown in the chemical analysis (Table I). Colliery shale is basically alumino-silicate. At higher temperature in plasma or in a tubular furnace, the material dissociates into alumina and silicon carbide and carbon. Possible chemical reactions that occurred during carbothermal reaction in the plasma reactor/tubular furnace are shown in Eqs. [1] through [3].

Table I Chemical Composition of Colliery Shale Material Before and After Magnetic Separation
Full size table
Fig. 3
figure 3

XRD patterns of colliery shell materials; (a) as-received condition and (b) after magnetic separation

Full size image
$$ {\text{C}}\left( {\text{s}} \right) + {\text{ SiO}}_{2}\left( {\text{s}} \right) \, \to {\text{ SiO}}\left( {\text{g}} \right) \, + {\text{ CO}}\left( {\text{g}} \right) $$
(1)
$$ 2 {\text{C}}\left( {\text{s}} \right) + {\text{ SiO}}\left( {\text{g}} \right) \, \to {\text{ SiC}}\left( {\text{s}} \right) \, + {\text{CO}}\left( {\text{g}} \right) $$
(2)

The overall reaction is given as

$$ {\text{SiO}}_{2} + {\text{ 3C }} \to \, {\text{SiC }} + {\text{ 2CO}} $$
(3)

The stoichiometric requirement of carbon for full conversion of silica to SiC in the shale material is around three-fifth of the weight of SiO2. The present composition suggests that at a very high temperature under neutral atmosphere, the entire SiO2 converts to SiC during plasma synthesis, with some amount of carbon left in the product. This is evidenced by XRD, obtained from the reacted shale material in plasma (Figure 4(a)). The second set of experiments is done where the shale material is heated to a temperature of 1673 K (1400 °C) under high vacuum in a tubular furnace. The XRD pattern from the second product shows similar results as has been noticed in the plasma-treated shale material (Figure 4(b)). The in situ composite thus prepared has three major constituents, i.e., SiC-Al2O3-C. However, the SiO2 peak is not observed in the XRD, confirming nearly complete conversion of SiO2 to SiC. The presence of sharp carbon peaks in both the XRD patterns indicates the presence of unreacted carbon in the form of a crystalline phase, presumably in the form of graphite. As a result of chemical mixing during the formation of in situ phases such as SiC, Al2O3, and graphite by thermal treatment, the physico-mechanical properties are expected to improve. SiC and Al2O3 are known additives for the improvement of strength and abrasion resistance of the aluminum-based metal matrix composite (MMC). It is envisaged that the in situ occurrence of these two phases together with graphite combined in aluminum matrix will result in an enhancement in performance of the AMC.

Fig. 4
figure 4

XRD patterns of colliery shell materials after treatment in (a) plasma and (b) vacuum furnace

Full size image

The sizes of the in situ ceramic composite particulates range between 0.05 and 200 μm (Figure 5). The SEM micrograph with EDX analysis revealed the presence of C, Al2O3, and SiC in the plasma-treated colliery shale material (Figures 6 and 7). The presence of C, Al2O3, and SiC in the treated material is also confirmed through XRD analysis (Figure 3). The powder samples have different sizes and shapes ranging from 50 nm to 200 μm. The facets of silicon carbide can be seen in Figure 6 (vide spot analysis taken from region “A”). Figure 7 shows the presence of carbon (vide spot analysis in the region “B”). The aluminum metal matrix composite is then made using Al2O3 powder and also newly developed in situ ceramic composites. In the present investigation, it was intended that both the composites (i.e., Al-CS and Al-Al2O3) have the same vol. pct of the dispersed phases. However, within the experimental conditions, it was possible to have 12 and 15 vol. pct of particulates in the two composites.

Fig. 5
figure 5

Size analysis of in situ CS composite

Full size image
Fig. 6
figure 6

SEM micrograph and EDX analysis of colliery shale powder samples (plasma treated). Analysis done in the region marked “A”

Full size image
Fig. 7
figure 7

SEM micrograph and EDX analysis of colliery shale powder samples (plasma treated). Analysis done in the region marked “B”

Full size image

Microstructural Studies

Figures 8(a) and (b) show optical micrographs of Al-CS and Al2O3 composites (unetched), respectively, in the as-cast condition. Particulates are seen to segregate along the grain boundaries with some inside the bodies. Volume fraction is measured to be 12 vol. pct for the former composite in contrast to 15 vol. pct measured for the latter. Cast microstructures are presumably dendritic. Figures 8(c) and (d) show optical micrographs for hot-forged Al-CS and Al2O3 composites, respectively. Both the microstructures show elongated grains transverse to the direction of forging. The elongated grain boundaries have provided opportunities for enhancing nucleation sites for the formation of equiaxed small recrystallized grains. Grains are also not large owing to pining of grains at the boundaries by incoherent particulates.[24–28] The situation is analogous to the controlled rolling or forging behavior of microalloyed steels.[33] The strengthening behavior of hot-forged material may, thus, be attributed to the reasons like fine recrystallized grain size, sub-structural strengthening, dispersion strengthening, and enhanced mechanical bonding at the interface between the particulate and matrix. Figure 9 shows an SEM micrograph of the Al-CS composite. The size and shapes of the particulates comprising Al2O3, SiC, and C can clearly be seen. There is considerable variation in shapes, i.e., spherical to rod shapes. The sizes are measured to vary between 50 nm and 200 μm. The EDX analysis of the particulates has confirmed the presence of SiC, Al2O3, and C again as shown earlier in Figures 6 and 7.

Fig. 8
figure 8

Optical micrographs of composites; (a) Al-CS in as-cast condition, (b) Al-Al2O3 in as-cast condition, (c) Al-CS in forged condition, (d) Al-Al2O3 in forged condition

Full size image
Fig. 9
figure 9

SEM micrograph of Al-CS composite

Full size image

Mechanical Properties of Composites

The cast composite is hot forged and hot rolled with a view of enhancing mechanical bonding between the particulates and the matrix. Mechanical properties of the hot-worked composite are measured in the form of stress vs strain curves (Figure 10). For comparison purposes, curves for Al-Al2O3 and Al-CS composite are superimposed.

Fig. 10
figure 10

Stress–strain curves of Al-CS and Al-Al2O3 composites

Full size image

The stress–strain curve gives a good account of performances of the two types of Al-matrix composites. Slopes of the two curves differ widely as shown in Figure 10. It is seen that the Al-CS material has greater resilience (~1.5 times) in comparison to the Al-Al2O3 composite. There is no necking in the Al-CS material with higher uniform elongations and therefore, it will have a higher value of work hardening index. The Al-Al2O3 composite necks appreciably beyond the maximum load. Thus, the formability of the former composite is greater than the latter. The total percent elongation is higher for the Al-CS composite than the Al-Al2O3 composite. Both yield strength and tensile strength are superior for the colliery shale-reinforced composite in comparison to the Al2O3-reinforced composite. Even though the Al-CS composite has lesser vol. pct of particulates (12 pct) than the Al-Al2O3 composite (15 pct), Al-CS has higher toughness than the Al-Al2O3 composite, thereby qualitatively suggesting greater fracture toughness of the former material.

Fractographs taken from a broken tensile sample of Al-CS and Al-Al2O3 materials (Figure 11) show clearly marked differences in their features. For the Al-CS composite, the dimples are sheared and small in size, whereas the Al-Al2O3 composite shows larger and shallow dimples. Some facets are also observed in the latter composite. The fractographs clearly explain the differences in tensile properties for the two composites (Table II).

Fig. 11
figure 11

Fractographs obtained from broken tensile sample of composites (a) Al-CS and (b) Al-Al2O3

Full size image
Table II Tensile Properties of Al-CS & Al-Al2O3 Composites
Full size table

Wear Resistance Properties of Al-CS and Al-Al2O3 Composite

The abrasion resistance for composites with the colliery shale material has shown superior properties in forged as well as in cast conditions as compared to the Al-Al2O3 composite. Figure 12(a) represents the plot for cumulative wear loss in μm as a function of sliding distance in meter, which was obtained during the dry wear test in air without any lubrication, for as-cast pure Al, Al-Al2O3, and Al-CS composite pins sliding against a hardened disk at a load of 50 N. The tests were carried out with samples having an 8 × 8 mm2 area of cross section. After an initial transient period, the wear loss for each material is seen to be increasing linearly with increasing sliding distance, i.e., with increasing time. The Al-CS material has shown the best wear resistance among the three materials. The cumulative wear loss-sliding distance behavior of the composites after forging has shown almost similar results (Figure 12(b)) as compared to the as-cast condition. The specific wear rate (defined as wear loss per unit sliding distance) is simply given by the slope of the straight line fitted through the data points. The specific wear rate increases in the order Al-CS composite, Al-Al2O3 composite, Al. The wear loss in the case of the pure Al sample was distinctly higher than that of the as-cast and forged composites at that particular load. It is also observed that both the composites have shown greater abrasion resistance in the forged condition than their corresponding cast condition (Figure 12). The greater abrasion resistance of forged composites is due to modification in the interface region between the composite particulates and matrix. The modification of the interface by hot forging of the composite helps to improve the bonding between the two, thereby improving the abrasion resistance of the material. The average value of the coefficient of friction (μ) of test samples at a load of 50 N is 0.49, 0.47, and 0.45 for pure Al, as-cast Al-CS, and Al-Al2O3 composites, respectively. The representative curve of coefficient as a function of sliding distance for the as-cast Al-CS composite is shown in Figure 13. The average value of the coefficient of friction (μ) for forged Al-CS and Al-Al2O3 composites is 0.43 and 0.42, respectively. The data for coefficient of friction confirm the wearing of Al, whereas the composite materials in either forged or as-cast condition show minimum abrasion. However, it is worth mentioning that a quantitative comparison between the literature data and the results of the present study is not possible owing to the various conditions such as test materials, test configurations, nominal contact stress, sliding velocity, etc. Therefore, a qualitative comparison is given here. In the present work, the addition of reinforcing particulates to Al has significantly reduced the μ value. This is in agreement with Roy et al.[34] and Hosking et al.[35] But, Sato and Mehrabian[36] had reported an increase in μ value with the addition of reinforcing particulates in Al alloys. The reasons for such observations are not clear. The SEM studies were made for worn-out surfaces of the Al-CS composite in the cast and forged condition. Figure 14 clearly shows that the Al-CS composite has worn out appreciably more in the cast conditions (Figure 14(a)) than the forged condition (Figure 14(b)). The worn-out surface of the forged composite shows fine scoring marks (Figure 14(b)). The plastic deformation is less and the resistance offered by particulates to the plastic deformation is appreciable due to the recrystallized structure of the Al matrix, owing to high amount of deformation at hot working conditions. The mechanical bonding established between the particulates and the matrix results in better continuity and less crack initiation in the matrix. Evidence of extensive plastic flow lines has been observed in the Al-CS composite.

Fig. 12
figure 12

Cumulative wear loss as a function of sliding distance of pure Al, cast Al-CS, and Al2O3 composites at 50 N load; (a) as-cast condition and (b) forged condition

Full size image
Fig. 13
figure 13

The variation of friction coefficient of the Al-CS composite as a function of sliding distance

Full size image
Fig. 14
figure 14

SEM micrographs of worn-out surfaces of Al-CS composite; (a) as-cast and (b) forged condition

Full size image

Conclusions

In the present study, a novel in situ composite consisting of Al2O3-SiC-C has successfully been prepared from waste colliery shale material obtained during the mining of coal. The X-ray and EDAX studies have confirmed nearly complete conversion of SiO2 to SiC under the experimental conditions. A fine particle size ranging between a few nanometers and 200 μm with different shapes and aspect ratios is observed. A cost-effective AMC has been prepared with the novel in situ ceramic composite by the vortex melt technique. Stiffness, tensile strength, and pct elongation of the Al-CS composite are found to be better when compared with the Al-Al2O3 composite. The area under the stress–strain curve is more (~1.5 times) for the Al-CS composite in comparison to the Al-Al2O3 composite, thereby suggesting that the former AMC has better toughness than the latter. Fractographs from tensile samples agreed well with test results. The wear resistance offered by the Al-CS composite is greater than the Al-Al2O3 composite and aluminum both in the cast as well as in the forged condition. Both the composites, however, have shown better abrasion resistance in their forged condition than their corresponding cast condition.