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Vol. 8. Issue 6.
Pages 5443-5455 (November - December 2019)
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Vol. 8. Issue 6.
Pages 5443-5455 (November - December 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.09.012
Open Access
The microstructure and wear behaviour of garnet particle reinforced Al matrix composites
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393
Suresh Kumara, Anju Sharmab, Rama Arorab, O.P. Pandeyc,
Corresponding author
oppandey@thapar.edu

Corresponding author at: School of Physics and Material Science, Thapar Institute of Engineering and Technology, Patiala 147004, India.
a Terminal Ballistics Research Laboratory (TBRL), Defence Research and Development Organization (DRDO), India
b Department of Physics, PGCG, Sector-11, Chandigarh 160011, India
c Metallurgical Research Lab, SPMS, Thapar Institute of Engineering and Technology, Patiala 147004, India
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Tables (3)
Table 1. Chemical composition of LM13 alloy.
Table 2. Chemical composition of garnet Particle.
Table 3. Chemical composition of garnet Particle.
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Abstract

Composites containing different size range of garnet reinforced in LM13 alloy were prepared by liquid metallurgy route. Wear tests of composites were done at a constant sliding velocity of 1.6 m/s for a sliding distance of 3000 m at different loads varying from 9.8 to 49 N. The particle size variation of garnet mineral and its content used as reinforcement significantly influenced the microstructure, microhardness and wear behaviour of the composites. The wear rate of fine size garnet mineral reinforced composites is low as compared to composites reinforced with coarse size garnet mineral particles at all loads. The wear rate of 15 wt.% fine size garnet mineral reinforced composite is minimum even at higher load (49 N) when compared to the cast iron used in brake drum of vehicles.

Keywords:
Metal matrix composites, garnet mineral
Optical microstructure
Hardness
Wear
Wear debris
SEM.
Full Text
1Introduction

Metal matrix composites (MMCs) are now extensively used to replace other cast alloys [1,2]. The main reason behind this is the blend of properties consisting of high strength and ductility that can be achieved through MMCs [3,4]. In order to achieve better mechanical and wear properties efforts are being made to develop varieties of aluminium matrix composites (AMCs) by reinforcing with different ceramic particles [5–7]. This includes oxides [7], carbides [8], nitrides [9], borides [10] and most recent natural minerals [11]. Since the mineral reinforced composites exhibit better properties in terms of strength and wear resistance, efforts are now focused on such composites [12,13]. These composites are fabricated through powder metallurgy (P/M) route for small parts [14,15] or through liquid metallurgy route for bigger parts [16]. In order to optimize, the processing parameters of these composites for achieving a desired property, theoretical modelling at different boundary conditions has been done [17–21]. Currently, minerals have become a good substitute to replace high purity particulates as these minerals are economical and hence developed composite are cost-effective [22,23].

Garnet mineral is lighter having a low density as compared to other minerals like rutile [24], zircon [25] and sillimanite [26]. Therefore, it will further contribute to lightweight composite. Since most of the garnet contains aluminium metal so it will help in bonding with matrix through interfacial diffusion. Apart from this, garnet exhibit excellent chemical and thermal stability among the available natural minerals [9]. Aluminium garnet mineral reinforced composites can be fabricated with the simplest and cheapest stir casting route. The stirring process under optimized conditions may provide uniform distribution of reinforced particles and it does not allow the growth of coarse dendrites. In stirring process, dendrite arms are detached from the main dendrite trunk, and provide its uniform distribution throughout the cast component. In recent years, limited wear study is reported on reinforcement of garnet mineral particles in the zinc-based alloy (ZA-27) [27,28] and in aluminium base alloy (Al-6061) [29,30]. Moreover, the variation in size and amount of garnet mineral reinforced in the LM13 alloy matrix composite has not been studied so far, so its study will be highly useful for designing of engineering component. Keeping in view of these facts, LM13-garnet composites have been developed to study their wear property. This work aims to analyse the microstructure of the developed composites and study their wear behaviour.

2Experimental details2.1Raw materials

For the preparation of composites, LM13 alloy is used as matrix material. The LM13 alloy used as piston alloy was obtained from Emmes Metals Private Limited, Mumbai, India in the form of ingots. This LM13 alloy has better casting performance and reasonable strength as a base alloy. The chemical composition of LM13 alloy used in the present study is given in Table 1. Garnet was used as a reinforcing material for the preparation of the composites. Garnet is a silicate, abundantly available in nature and having the hardness of (6.5–7.5) mho. The amount of garnet was taken as 5 wt.%, 10 wt.% and 15 wt.% in the particles size range of low (50–75 µm) and high (106–125 µm). Garnet composition is given in Table 2.

Table 1.

Chemical composition of LM13 alloy.

Composition of LM-13 Alloy  Si  Fe  Cu  Mn  Mg  Zn  Ti  Ni  Pb  Sn  Al 
XRF  11.16  0.332  1.247  0.598  0.849  0.25  0.026  0.868  0.001  0.002  Bal. 
Chemical analysis  11.80  0.365  1.230  0.411  0.940  0.21  0.025  0.940  0.028  0.005  Bal. 
Table 2.

Chemical composition of garnet Particle.

Al2O3  FeO  SiO2  TiO2  CaO  MgO  Cr2O3 
22.78  7.16  42.45  0.18  4.78  21.27  1.27 
2.2Preparation of composite

Composites were prepared by the stir casting process. The required amount of LM13 alloy in the form of small pieces was taken in a graphite crucible and heated up to 750 °C in an electric furnace. The molten mass was stirred with the help of impeller (graphite) at a speed of 630 rpm. At this optimized speed, vortex was created in the melt. To get the uniform distribution of the reinforced particles in the matrix, it should be charged in the vortex [24,25]. Garnet particles were charged from the side of the vortex with the help of funnel kept on the top centre of the vortex. Prior to addition, the garnet particles were heated at 450 °C to remove the moisture. The rotation of the molten metal was continued for a few minutes even after the complete addition of garnet particles into the molten melt. Finally, the slurry was cast into a metallic mould. During the stir casting process, stirring supports in two ways (a) distribution of the reinforced particles into the molten metal, and (b) maintaining the particles in a state of suspension. The distribution of reinforced particles into matrix throughout the molten mass occurs because of the pressure difference between the inner and the outer surface of the melt around the vortex. During casting of the composites, the amount of LM13 alloy, the position of the stirrer in the crucible and stirring time were kept constant to minimize the contribution of variables related to stirring on the distribution of reinforcement phase as these phases affect the mechanical properties of cast composites [25].

2.3Materials characterization

X-ray diffraction patterns of base LM13 alloy and its composites (LM13/garnet) were recorded by means of Panalytical X’pert PRO MPD, Netherland using Cu-Kα radiation (λ = 1.54 A˚). The collected data was matched with reference data for identification of different phases present in the base LM13 alloy and its composites. For wear test, cylindrical specimens of diameter 14 mm and height 25 mm were machined from the cast composites. For microstructure study, the specimen surface was ground with 400, 800 and 1200 grit silicon carbide paper and then polished using 02 µm diamond paste to obtain a good surface finish. The specimens were then washed in distilled water, followed by etching with Keller’s reagent to obtain the better contrast of surface. The surface morphology of each sample of cast composite was studied with the help of an optical microscope (Eclipse MA-100, Nikon). Microhardness values at different phases were measured using a Vickers hardness testing machine (model: MVK_HO, Mitutoyo, Japan). Each value of hardness is an average of the five separate values taken from the different places of the samples. Cylindrical pins were subjected to sliding wear test under the dry sliding condition at ambient temperatures (25–30 °C). Tests were conducted on pin shaped specimen cut from each set of composites at a constant sliding velocity of 1.6 m/sec, using a pin on disc wear monitor (Model TR-20CH-400, Ducon, Bangalore, INDIA). Pin-shaped samples were made to slide against the hardened steel disc. The wear tests were measured as a function of sliding distance at different loads (9.8 to 49 N). The worn surfaces and debris were analysed by scanning electron microscopy (SEM) model JSM-6510 LV make of JOEL, Japan.

The density of developed composite was measured using the Archimedes principle. The variation of density of the developed composite is given in Table 3.

Table 3.

Chemical composition of garnet Particle.

Alloy/Composite  Particle size  Density (g/cm3
LM-13  –  2.65 
LM-13 5 wt% garnet  CoarseFine  2.702.65 
LM-13 10 wt% garnet  CoarseFine  2.742.70 
LM-13 15 wt% garnet  CoarseFine  2.802.74 
3Results and discussion

In this section, the influence of size variation of garnet mineral particles on microstructure, wear behaviour, structure of worn out pin and wear debris collected after the wear test are analysed. The microstructural evolution and wear mechanism involved in the material removal of the composites during dry slinging wear test are analysed.

3.1XRD analysis

The X-ray diffraction (XRD) patterns of LM13 alloy and its composite with 15 wt.% garnet reinforced particles is shown in Fig. 1(a) and (b) respectively. Fig. 1a shows the presence of Al and Si in LM13 alloy. The presence of garnet along with Al and Si elements is detected in the XRD pattern of LM13 alloy base matrix composite (Fig. 1b). In addition to these phases, Al8.05Fe1.05Mg3.15O20Si1.75 and MgNiSi2O6 were also present in the LM13 alloy base matrix composite. During the manufacturing of composite, it is quite possible that these phases have formed at the alloy-particle interface because of the interfacial reaction of mineral garnet particle and LM13 alloy during mixing and casting.

Fig. 1.

XRD patterns of composite with 15 wt.% garnet reinforced particles showing the presence of different phases.

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3.2Microstructure analysis

The most important factor in achieving the homogeneous property of discontinuously garnet mineral reinforced composites material is the nearly uniform dispersion of the reinforcement particles inside the matrix. The rotating impeller in the molten metal not only distributed the particles homogeneously but also delays the particle settling prior to solidification [25,26]. The microstructure analysis could give an insight into the quality of the composite material. The optical micrographs of the cast LM13 garnet mineral reinforced composites are shown in Fig. 2–5. Fig. 2(a) and (b) reveals the microstructure of 5 wt% reinforcement particles with coarse and fine size, respectively. Fig. 2a shows the microstructure of coarse size garnet mineral reinforced composite where the garnet mineral particles are embedded in the α-Al matrix. However, in case of fine size reinforced composite, agglomeration of garnet mineral particles occurs as encircled in Fig. 2b. The optical micrograph of composites reinforced with 10% coarse and fine garnet mineral particles are shown in Fig. 3(a–b). Fig. 3(a) shows the homogeneous distribution of coarse garnet mineral particles in the matrix phase whereas fine garnet mineral particles exhibit the tendency of agglomeration as shown in Fig. 3b. The optical micrographs containing 15 wt. % coarse and fine garnet particles are shown in Fig. 4(a–b). The coarse garnet mineral particles exhibit fairly uniform distribution as shown in Fig. 4a, whereas clustering of garnet particles at some places is observed for fine size reinforced composite. It is clear from the structural analysis that clustering of fine size particles increases as its amount is increased from 5 to 15 wt% in the matrix. The possibility of clustering of garnet particles increases with respect to decrement of particles size. Clustering of fine size particles may also occur due to the formation of many small size sub-vortexes on the main vortex. However, in fine size particle containing composites, porosity is also observed. The porosity may be attributed to the dissolved gases, and air bubbles sucked into the molten melt while adding the garnet mineral particles to the melt via the vortex during the mechanical stirring [25,26,31,32]. However, the agglomeration and porosity were created more in the composite containing 15 wt.% fine size garnet mineral reinforced particles. Garnet mineral is distributed in the aluminium matrix, which can effectively stop the dislocation movement in the LM13 alloy matrix. With the increase of the garnet mineral weight fraction in the matrix phase, the interspaces between the reinforced particles decrease in matrix [33]. It causes hindrance to dislocation movement and hence higher hardness of composites [31]. As shown in Fig. 4 (a–b), the eutectic Si grows into finer and nucleates near garnet mineral particle as clusters. Apart from this, the matrix also gets modified exhibiting cellular type and different dendritic morphology because of interference offered by coarse and fine particles to the growing solid-liquid interface as shown in Fig. 4. The growth of dendrites depends upon many factors like the presence of particle as a hindrance to its growth, cooling rate etc. Faster cooling rate leads to the formation of smaller dendritic structure and increases the ductility of the composite [26]. Solidification rate is affected by the addition of garnet mineral in the matrix because garnet mineral particles work as a heat insulator, which slows down the cooling rate of the composite. However, it also provides a nucleation site, where the normal dendritic structure changes to different forms as marked in the microstructures (Figs. 2–4). For a given volume fraction of garnet mineral reinforcement, fine size mineral particle in matrix provide the larger surface area in comparison to coarse size garnet mineral particle. Because of this, more nucleation site is available for the growth of α-dendrites hence the nucleated dendrites are thinner as compared to coarse size reinforced LM13 composites. Good bonding between garnet mineral reinforced and LM13 alloy matrix phase is observed as shown in Fig. 5. The performance of a smooth interface between reinforced and matrix provides better mechanical and tribological properties as the transfer of load occurs through the interface [34]. It is expected that the contact between reinforced particles and alloy melt would result in an interaction layer, which provides good wetting between the two constituents [35]. For the same amount of reinforcement, the presence of the fine size particle in the matrix leads to more interfacial reaction sites in the matrix due to higher heat content at the interface in comparison to coarse size particle in the matrix. This heat content provides better interfacial reaction due to thermal mismatch between particle and matrix. The type of interaction layer depends on the elements present at the interface during processing [36]. The higher magnification micrograph of composites indicated a good interfacial bonding between the garnet particles and LM13 alloy matrix (Fig. 5a). No interfacial decohesion, even for the fine size garnet particles, was observed as shown in Fig. 5b.

Fig. 2.

The optical micrograph of composites with 5 wt. % reinforced (a) coarse (black) and (b) fine particles. Particle positions have been highlighted with a circle.

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Fig. 4.

The optical micrograph of composites with 15 wt.% reinforced (a) coarse (black) and (b) fine particles. Position of the particles have been highlighted using circles.

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Fig. 5.

The optical micrograph at higher magnification of composites with 15 wt.% (a) coarse (black) and (b) fine particles reinforced.

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Fig. 3.

The optical micrograph of composites with 10 wt.% reinforced (a) coarse (black) and (b) fine particles. Circled have been used to highlight the position of the particles.

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3.3Microhardness

A hardness of composite is influenced by type and amount of reinforcement. Natural minerals tend to get itself bonded with matrix through interfacial reaction leading to the formation of the reactant phase at the interface. It also helps in the formation of the strong bond to keep the reinforced particles intact within the matrix. Microhardness measurement done at particle, interface, and matrix shows variation as given in Fig. 6 (a–b). It exhibits higher hardness on reinforced particles. There is a sharp decrease in hardness from particle to matrix. The high microhardness observed at reinforced/matrix interface compared to matrix phase is because of good interfacial bonding via formation of reacted phase. The new reactant phase observed in the XRD pattern of composite not only provides good bonding between the garnet mineral phase and Al matrix phase but also provides a lot of nucleation centre for precipitation of Si. The high hardness observed for 15 wt.% fine size reinforced composite is because of the higher number of particles and thus higher content of fine Si particles at the interface, as can be seen in Fig. 5. The increase in hardness of the cast composite is due to the refinement of the microstructure leading to rapid solidification effect [37]. Moreover, it also provides good interfacial bonding. According to Das et al. [34], an increase in micro-hardness of the composites results from the higher dislocation density in the matrix generated due to the difference in coefficient of thermal expansion of the aluminium matrix alloy and reinforcement.

Fig. 6.

Bar chart showing Vickers microhardness of the LM13 alloy composites with (a) coarse size and (b) fine size reinforced.

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3.4Wear characteristics3.4.1Effect of sliding distance on wear rate

Wear test of all the fabricated composites was done at different loads. The observed variation in wear rate, when a pin is in contact with rotating disc to a certain distance, is shown in Figs. (7–9). The nature of the obtained graph exhibits two different features. One, corresponding to the steep rise in the initial stage, represents higher wear and is known as run in wear. At the later stage, it acquires nearly constant wear known as steady-state wear. As the load is increased the run-in wear and steady-state wear rate both increase. The results observed here are analogous to those reported by Chaudhary et al. [37] for Al-2Mg-11TiO2 composites, and Kumar et al. [25] for zircon sand reinforced composites. It is observed that a composite containing fine size reinforcement exhibits lower wear rate compared to coarse size reinforcement at all loads tested during the investigation. This may be due to the higher surface area of reinforced particle providing minor roughness, as shown in Fig. 7(a–b). Similar results with improved wear rate were also observed for the composites reinforced with 10 and 15 wt.% garnet particles and are shown in Fig. (8–9), respectively. It is possible that clustering observed in smaller particles have caused porosity devoiding the particles to make a bond with matrix and causing its detachment during wear. The results are on similar lines as has been reported for other composites [24,28,31]. Wear curves indicate that steady-state wear is achieved after 1500 m sliding distance for composites containing coarse size particles (Fig. 7a). However, for composites containing fine size garnet mineral, it is delayed and is observed after a sliding distance of 2000 and 2500 m at low and higher loads, respectively. The amount of heat generated is higher with increasing sliding distance because of the higher frictional force offered by rubbing surfaces [38]. The overall results indicate that coarse garnet mineral composites show higher wear rate as compared to fine garnet mineral composites [24,25]. Composites containing finer size have more interfacial area compared to coarse size particles and are close to each other. This helps in transferring the load from matrix to particle early. However, due to clustering, their wettability is poor. Distribution of different size garnet particles in the alloy matrix may lead to development asperities on the surface of the composite. The variation in distance between these asperities governs the surface roughness, which depends on the particle size of the reinforcement. Composites containing fine particles may possess less roughness compared to composites having coarse particles. Thus, fine particles arranged in the matrix having lesser interparticle distance will offer higher wear resistance over the coarse one. During sliding, the surface undergoes deformation. With increasing load, the matrix material undergoes elastic to plastic transition as increased surface roughness brings the matrix material in direct contact with a rotating disc. During run-in wear state, the real contact area (Areal), at the surface is less than the nominal contact area (Anom), but in steady state condition, the real contact area (Areal) is increased. The increase in contact area of composite depends on the applied load, size and amount of reinforced ceramic particles that exist on the surface. With increased load, asperities undergo deformation as load-bearing capacity decreases and separation between counterface surface and pin decreases, thus making softer asperities to deform easily till the next layer containing hard asperities is developed [39]. With time as the sliding distance increases, this phenomenon of plastic deformation with the creation of more new surfaces having different surface conditions which may contain fine to blunt asperities with time is created, and loss of material occurs [39].

Fig. 7.

Wear rate against the sliding distance of the composites with 5 wt.% (a) coarse and (b) fine garnet particle reinforced.

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Fig. 8.

Wear rate against the sliding distance of the composites with 10 wt.% (a) coarse and (b) fine garnet particle reinforced.

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Fig. 9.

Wear rate against the sliding distance of the composites with 15 wt.% (a) coarse and (b) fine garnet particle reinforced.

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3.4.2Influence of load on wear rate

At constant sliding speed, the wear rate of the composites increases with an increase in load. During run-in wear, this variation is more for same sliding distance. However, as sliding distance increases, this variation becomes narrow. Similar observations were also reported by Chaudhary et al. [37]. Das et al. [40] have compared the wear properties of alumina, and zircon sand reinforced AMCs and found that the decrease in particle size of reinforcement improves the wear resistance of the composite. The strong interfacial bonding of reinforcement (garnet) with the matrix phase and higher microhardness, as possessed by fine size reinforced composite, results in better wear behaviour. The wear rate of all composites increases with the increase in the applied load. The wear performance depends on the real contact area between composite and rotating disk. With an increase in the load, the subsurface deformation increases as the material undergo plastic deformation. In this process, the reinforced particle cracks due to shearing stress and gets detached from the matrix. The detached particles re-entre into the soften aluminium matrix with increased load. These protruded particles finally wear out in due course of time as the shearing force becomes higher because of reduction in the area [9]. The frictional heat on the wear surface is increased with increasing applied load. This facilitates the formation of the oxide layer, which is harder and helps to reduce the wear rate. During the repeated dry sliding contacts, the work hardening on the wear surface may also occur. This creates hindrance in dislocation motion and enhances the wear resistance of the composites. The asperity deforms and starts blunting towards the higher radius. Its radius continues to increase with increased contact pressure and becomes flattened with a larger radius. The asperity with the smaller radius of fine size reinforced composite undergoes more plastic deformation as the smaller asperities deform plastically. When the asperity radius of coarse size reinforced composite is increased, the deformation rate of asperity is more in the elastic-plastic regime. The contact pressure at the higher load (49 N) between the asperities of the materials is higher and may undergo elastic to plastic deformation but at the same time, the protruded garnet particle will oppose the deformation of the matrix phase and act as load-bearing constituent and thus safeguards the matrix from severe wear losses [41].

At high load, the fine size garnet particles will be crushed and trapped between the grooves during sliding and thus reduces the wear losses. This will be counterbalanced by the increase in effective asperities contact area with applied load. The large active surface area of the fine particles in the matrix is also responsible for the better wear resistance in comparison to coarse size particles reinforced composites [42]. The shape of the garnet particles also plays a vital role in the wear behaviour of the composites [9,40]. The sharp edge particles (fine) under the applied load may also get inserted easily in the matrix as compared to the coarse particles. Since the coarse particles under these conditions are protruded, so they may get fractured and increase the wear rate of the material. Material removal in composites is due to the indentation and ploughing action of the sliding indenters (reinforced particles).

3.4.3Influence of reinforcement content and their size range on wear rate

The amount and particle size of reinforcement were varied from 5 to 15 wt.%, and the corresponding wear rate of the composite was monitored, as shown in Fig. 7–9. In Fig. 7(a) and (b), it is observed that the performance of composites having coarse particles is less than the fine particles. For the same wt.%, composite containing fine particles exhibit higher hardness compared to composite reinforced with coarse particles. Since the interparticle distance for fine particles decreases, so hardness is increased. As per Archard’s law, wear rate decreases with an increase in overall hardness. However, due to porosity in fine particle reinforced composites, the variation observed is not much. This variation is observed for all composites studied here. Wear rate is also observed to decrease with increase in garnet content as reinforcement. The observed wear rate is minimum for 15 wt.% fine size garnet particles reinforced composites. The study indicates that composites containing fine particles with proper distribution in the matrix will exhibit higher wear resistance, which has been reported by other workers also [23,37]. Apart from the higher content of reinforcement, good interfacial bonding will devoid the particles to be pulled out from the matrix during sliding [25]. However, because of variation in the thermal expansion coefficient of the matrix, and reinforced particles, the interfacial temperature during sliding will increase, as the matrix is the good conductor of heat, and particles are a bad conductor. This causes the creation of a void with an increase in sliding distance. This facilitates the pull out of particles from a matrix. Moreover, the matrix becomes soft and get oxidized easily. Under these circumstances, material loss is rapid. The phenomenon of cracking followed by the creation of grooves having variation in radius of asperities due to blunting causes loss of materials through delamination at higher loads.

3.4.4Morphological features of worn surface and debris

The worn surfaces of pins and debris provide clues to the wear mechanisms operative during dry sliding against the load. Since the composites developed are for the industrial application, only the feature at higher load are presented here. The SEM micrographs of wear track and debris for composites containing coarse and fine size garnet particles tested at higher loads of 49 N at a speed of 1.6 m/s are presented in Fig. 10–13. The standard features observed for all composites are the appearance of grooves and ridges running parallel to the sliding direction.

Fig. 10.

Worn surface of the composites with 5 wt.% (a) coarse and (b) fine garnet particle reinforced at 49 N load.

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Fig. 11.

Worn surface of the composites with 10 wt.% (a) coarse and (b) fine garnet particle reinforced at 49 N load.

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Fig. 12.

Worn surface of the composites with 15 wt.% (a) coarse and (b) fine garnet particle reinforced at 49 N load.

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Fig. 13.

EDS analysis of worn surface of the composites with 15 wt.% fine size garnet particle reinforced at 49 N load.

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SEM images of the worn surface subjected to 49 N load for 5 wt.% reinforcement is shown in Fig. 10(a–b). The overall features observed on the worn surface exhibit abrasion, adhesion, adhesion joints and the formation of cracks as shown in Fig. 10(a–b). The propagating cracks get interlinked further causing deformation and delamination of the matrix locally. At higher load, the pull out of particles leads to the formation of a void. These voids become a centre for crack initiation. With time cracks join with each other and lead to the removal of material as has been observed in Fig. 11. However, the intact particles do resist the crack propagation as shown in Fig. 11b. At higher applied load, delamination of sub-surface is formed by the coalescence of these wear cracks as shown in Fig. 12a. The various local delamination events are interlinked to form large craters on the worn surface. Formation of a mechanical mixed layer (MML) is also revealed from the worn surface analysis, as fine size debris of fractured MML is adherent within the cavity created by the delaminated layer [39]. Beyond that, oxidation facilitates the formation of adherent aluminium oxide (Al2O3) at the surface by temperature rise due to frictional heating (Fig. 12b). The formation of the oxide layer increases with increment in the amount of reinforcement because its increment is responsible for exposing more surface area. The adherent oxide layer acts as a solid lubricant that prevents the rubbing surfaces being in direct contact [36] and considerably reduces the wear rate.

The formation of oxide in the worn surface of the composite after sliding 2000 m is evidenced by the identification of O, Mg, Al, Si and Fe peaks in the selected zone (spectrum 1) in the energy dispersive spectroscopic analysis as shown in Fig. 13. Presence of the oxygen and iron in the spectra supports the formation of the oxide layer during the sliding at this load. This behaviour of low wear rate for fine size garnet reinforced particle is also reported by Das et al. [34] and Kumar et al. [25] on different composites.

Fig. 14a shows the plate-like debris of the alloy matrix and the debonded garnet particles embedded in molten debris. The fractured particles are attached with the larger size delaminated debris. The detachment of debris through ploughing action is observed. Microcracks on the surface of debris indicate the resistance offered by the embedded particles; once detached leaves crack in the surface. During rubbing action some of the debris get welded with the iron disc and when detached shows shearing features on the surface [41]. These observed morphological features of debris indicate higher wear rate of the composite. The pull-out of ductile material having thread-type morphology is also seen in Fig. 14b.

Fig. 14.

Worn debris of the composites with 5 wt.% (a) coarse and (b) fine garnet particle reinforced at 49 N load.

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The debris particles (Fig. 15a) are likely to act as the third-body abrasive particles and could be responsible for the higher wear rate also. Loose debris particles trapped between the specimen and the counterface causes a micro plowing on the contact surface of the composite. Majority of flakes have the number of cracks due to repetitive stress occurred during sliding under high load [25,39]. The hard oxide layer (Fig. 15b) observed indicate severe wear symptoms that dominate at higher load conditions. Corrugated structure and crashed debris are also seen in Fig. 15b, which is due to the continuous rubbing of worn out debris during sliding wear.

Fig. 15.

Worn debris of the composites with 10 wt.% (a) coarse and (b) fine garnet particle reinforced at 49 N load.

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Fig. 16a shows that the size of flakes becomes more substantial, which indicates that the matrix becomes softer with higher load and causes the transition from mild to severe wear. These wear debris indicate that the adhesive wear dominates in the sliding direction during wear. Due to the adhesive nature at higher load, metal is chipped out in the form of flakes as debris. Fig. 16b shows the presence of round shape oxide (Al2O3) debris. These oxide layers (Al2O3/Fe2O3) help to reduce the wear rate of the composite at higher load, as they keep on rotating inside the grooves [25,36]. Spherically shaped wear debris arises from several sources including entrapment of wear debris in the grooves of surfaces. These may also arise from base metal, local melting and air born contamination [37]. These spherical shapes arise during frictional heating at higher loads. During continuous rubbing process, the temperature of the entrapped metallic debris becomes higher. Since the surface area of this trapped debris is more so, it melts. The appearance of fine thread type debris along with spherical to elliptical shape indicate that metallic threads undergo incipient fusion and acquire nearly spherical shape. The smaller spherical molten debris (Fig. 16b) on the surface of the bigger one is due to the attachment of smaller molten mass to a bigger one [38]. At this condition, the wear rate does not increase much due to the formation of the adherent massive tribo-oxides layer under higher frictional heat and associated temperature rise.

Fig. 16.

Worn debris of the composites with 15 wt.% (a) coarse (b) fine garnet particle reinforced and (c) EDS analysis of wear debris generated from composite with fine size particle at 49 N load.

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EDS analysis of these debris indicates the presence of C, O, Al, Si, and Fe elements, as shown in Fig. 16c. The oxygen-rich wear debris help in reducing the wear rate of the composite by acting as in situ solid lubricants (Al2O3 tribo-oxides layer). The EDS results indicate the presence of Fe on wear debris, which corresponds to the transfer of material from the counterpart iron disc to the composite material. This also indicates that debris is generated by the rupture of a mechanically mixed layer [41]. The transfer of steel inclusions from counterface surfaces to the composite wear surfaces is another mechanism that contributes to an increase in the wear resistance of the composites. Here inclusions may act as additional reinforcement at the wear surface of the composite.

3.4.5Formation of the delamination layer by ceramic particles

At higher loads, the presence of an oxide layer on the surface reduces the chance of direct metallic contact. It provides a protective layer for further wear. However, with an increase in sliding distance, the temperature of the protective layer increases due to surface frictional heating. Ultimately, it results in the formation of a thin molten layer at asperity contacts in the case of low melting alloys. At this condition, the bonding between the ceramic particle and soften matrix becomes weak. The poor wetting or de-bonding initiates void formation around the garnet mineral particles. During the sliding, tensile force acts in the void, and coalescence occurs (Fig. 17a). The voids coalescence is responsible for breaking the interfacial bonding. Once the interfacial bond is broken, the physical gap between matrix and particles is created. Moreover, with continuous sliding, the work hardening at the particle-matrix interface will occur because of the continuous impact of the loosened particle on the grooved surface. This leads to crack initiation from the particle-matrix interface followed by its propagation to neighbouring particles. At higher load, the propagation of cracks is at a faster rate. Void formation followed by interfacial work hardening, and crack initiation will continue with time (Fig. 17b). The propagating cracks get interlinked in due course of time and finally, delamination of the matrix occurs. The reinforced particles are the main load-bearer and resist the wear. On the other hand, they are also acting as crack initiation sites, which causes delamination and an increase in the wear rate. The formation of grooves may lead to a removal of metal in the form of ligament or wire in the initial stage. However, when these grooves get interconnected through cracks, it may cause the removal of material in the form of delaminated chunk, as shown in Fig.17b.

Fig. 17.

The formation of delamination layer by ceramic particles during sliding wear test by pin on disc machine.

(0.13MB).
3.4.6Comparison of the developed composite with existing industrial product

The current study has been undertaken to develop lightweight components for industrial use. In order to compare the wear property of the developed composite with the cast iron used in brake drum in automobile industries, wear study of cast iron is done. Cast iron pins were tested under similar conditions, i.e., 1.6 m/s rotation speed at 49 N load. The wear rate of 15 wt.% fine size garnet mineral reinforced composite at the applied load 49 N and also for cast iron is shown in Fig. 18. The study indicates that the wear rate of the composite is comparable to that of the cast iron. This further confirms that the developed composite can be used to replace high density cast iron parts in automobile industries.

Fig. 18.

Comparison wear rate of the 15 wt.% fine size garnet reinforced composites with cast iron at 49 N load.

(0.14MB).
4Conclusions

The stir casting process was developed to fabricate LM13-garnet composites containing a different range of particle size. The wear properties of the developed composites were studied. Performance and mechanism of size variation of garnet mineral particles on the wear behaviour of metal-matrix composites were studied at different load conditions. From these studies, the following conclusions are drawn.

  • 1

    Microstructural analysis of the developed composites indicates that coarse size garnet mineral particles are uniformly distributed in the matrix of LM13 alloy compared to fine size particles. However, particle clustering is observed for fine size reinforced composites.

  • 2

    Garnet minerals influence the microstructure. It creates a hindrance for dendrite growth of primary aluminum phase. Garnet particles also provide the nucleation site. Depending upon the content of mineral particles and their size, different morphology is observed.

  • 3

    The variation observed in the hardness of the matrix phase, interface and on particles indicate good interfacial bonding of particles with the matrix.

  • 4

    Wear rate of all the composites is less compared to the base metal. The variation observed is due to poor interfacial bonding.

  • 5

    In the initial stage of wear, metallic wear dominates. However, at higher load, sever wear is observed. Due to frictional heating MML forms causing delamination of the surface at higher loads.

  • 6

    Metallic debris undergoes melting at higher loads causing spherical to elliptical shape debris. The delaminated debris is faceted type indicating its origin from MML.

  • 7

    The in-situ formed Al2O3/Fe2O3 layer on the surface of pin acts as a solid lubricant and helps in reducing the wear at higher loads.

  • 8

    The performance of the wear rate of the composite is comparable to that of the cast iron material at the same testing conditions. This can be a better substitute for the automobile component used by industries.

Acknowledgments

The authors are thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing financial support under the letter no. 22(0769)/18/EMR-II for this study.

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