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Zr composite at elevated temperatures
M A TE RI A L S CH A RACT ER IZ A TI O N 75 (2 0 1 3 ) 2 0 0–2 1 3
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Analysis of wear track and debris of stir cast LM13/Zr composite at elevated temperatures Ranvir Singh Panwar, O.P. Pandey⁎ School of Physics and Materials Science, Thapar University, Patiala, 147004, India
AR TIC LE D ATA
ABSTR ACT
Article history:
Particulate reinforced aluminum metal matrix composite is in high demand in automobile
Received 24 April 2012
industry where the operational conditions vary from low to high temperature. In order to
Received in revised form
understand the wear mode at elevated temperature, this study was planned. For this
3 November 2012
purpose we developed a metal matrix composite containing aluminum alloy (LM13) as
Accepted 6 November 2012
matrix and zircon sand as particulate reinforcement by stir casting process. Different amounts of zircon sand (5, 10, 15 and 20 wt.%) were incorporated in the matrix to study the
Keywords:
effect of reinforcement on the wear resistance. Dispersion of zircon sand particles in the
Metal matrix composite
matrix was confirmed by using optical microscopy. Sliding wear tests were done to study
Stir casting
the durability of the composite with respect to the base alloy. The effects of load and
Sliding wear
temperature on wear behavior from room temperature to 300 °C were studied to
Scanning electron microscope
understand the wear mechanism deeply. Surface morphology of the worn surfaces after
Debris
the wear tests as well as wear debris was observed under scanning electron microscope. Mild to severe wear transition was noticed in tests at high temperature and high load. However, there is interesting change in wear behavior of the composite near the critical temperature of the composite. All the observed behavior has been explained with reference to the observed microstructure of the wear track and debris. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
At the present time, durability of materials, especially those of tribological importance, is of great interest for motor vehicles and other engineering applications. Utility of Al–Si alloys in automobile industry has received considerable attention due to lightweight, good machinability and easy casting characteristics [1–3]. Clarke and Sarkar [4] suggested that presence of Si in Al–Si alloy of near eutectic composition improves the wear resistance of alloy. In similar investigations it was also found that particulate reinforced aluminum matrix composite (PAMCs) shows great improvement in wear resistance [5–9]. However, for better tribological and mechanical properties of the composites, selection and volume fractions of particulates, their size, shape and distribution in matrix are still being studied. Present work is an effort in this direction.
Among the available techniques to develop the PAMCs, the stir casting process is widely accepted in industries due to the lower production cost, simplicity, flexibility and applicability to large quantity production [10,11]. High hardness, high modulus of elasticity and good thermal stability of zircon sand particles attracted the researchers to use these particles as reinforcement in AMCs. Das et al. [12] have also reported that zircon sand reinforced composite shows better wear resistance than alumina reinforced composite due to its superior particle–matrix bonding. However, until now wear behavior and particularly the surface analysis of worn surfaces of Al–Si/Zr composites at elevated temperatures is not available. Considering this fact, in the present work we studied about the surface morphology of worn pins of Al–Si/Zr PAMCs at different loads and temperatures. Different amount of zircon sand was also added to the metal matrix to study the effect of
⁎ Corresponding author. Tel.: + 91 175 2393116; fax: + 91 175 2393020. E-mail addresses: [email protected] (R.S. Panwar), [email protected] (O.P. Pandey). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.11.003
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zircon sand in LM13/Zr composites. The effect of temperature under oxidative environment on wear for LM13/Zr composite is studied at different temperatures (50 °C to 300 °C) and loads (9.8 N and 49 N). LM13/zircon sand composite may be useful in automotive and defense areas as presence of zircon sand improves the thermal stability as well as hardness and wear resistivity of the composite as compared to aluminum alloy.
2.
Materials and Experimental Details
2.1.
Materials
Melting and Casting
Metallography and X-ray Diffraction
For metallographic study base alloy (LM13) and composite samples were ground, mechanically polished and etched with Keller's reagent. The surface morphology of etched sample was analyzed under optical microscope (Eclipse MA-100, Nikon). After wear test, worn surfaces and debris were analyzed under scanning electron microscope (JOEL, JSM-6510LV). To study the presence of elements in the sample, energy-dispersive X-ray spectroscopy (oxford INCA) with model no. 51-ADD 0076 was used. PANalytical X'pert PRO X-ray diffractometer with CuKα radiation (λ = 1.54 Å) was used for X-ray diffraction patterns of debris of composites collected after wear tests.
2.4.
ZrO2 (+HfO2)
SiO2
TiO2
Fe2O3
% in bulk
65.30
32.80
0.27
0.12
2.5.
The details of materials and casting procedure are mentioned elsewhere [13]. Composite containing 5, 10, 15 and 20 wt.% zircon sand were prepared by stir casting route. The alloy was melted and held at 750 °C. This melt was stirred with a graphite stirrer. Preheated sand particles were added slowly to this melt during stirring. After completing the mixing of sand particles, the melt was poured in steel molds and allowed to cool. Specimens for testing were cut from the cast composites.
2.3.
Elements
1/16″ diameter steel ball indenter under 100 Kgf at B scale. A fine manufacturing industry, India make Rockwell hardness tester (Model: TRSND) was used for carrying out the hardness testing.
In the present work a commercial grade piston alloy is used. Commercial aluminum alloy LM13 containing 12 wt.% Si was chosen as the matrix and commercial grade zirconium silicate (ZrSiO4), known as zircon sand, as reinforcements. Zircon sand was obtained from Indian rare earths limited, Chatrapur, Orissa, India. Before experiments the zircon sand was sieved and particles of 106–125 μm size were used in the present work. The chemical compositions of LM13 alloy and zircon sand are given in Tables 1 and 2, respectively.
2.2.
Table 2 – Chemical composition of zircon sand particles.
Hardness
The bulk hardness of the LM13/Zircon sand composite and LM13 alloy was measured by Rockwell hardness tester using
Table 1 – Chemical compositions of LM13 aluminum alloy. Elements
Si
Fe
Cu
Mn
Mg
Zn
Ni
Al
Chemical analysis (wt.%)
12.0
0.4
1.2
0.4
1.00
0.2
1.0
Bal.
Wear Testing
For wear tests a cylindrical pin of 8 mm in diameter was prepared from cast composite. Wear testing was performed on wear testing machine (wear and friction monitor, TR-20 CH-400, Ducom Instruments, Bangalore India) under dry sliding conditions in ambient air at controlled temperature (Fig. 1). Wear tests were conducted at different temperatures (50–300 °C) at low (9.8 N) and high loads (9.8 N). All the samples of base alloy (LM13) and LM13/Zr composites were tested against EN 32 steel disc having 65 HRC hardness and surface roughness (Ra) of 0.5 μm. To get an average value of wear rate, each test was run three times. Before each test the track was cleaned with acetone. For each test a new track was used to get similar test conditions. Throughout the experiment a constant sliding velocity of 1.6 m s−1 was maintained. Sliding distance covered during the experiment was about 3000 m. Wear rate was determined by measuring specimen height change using a linear variable displacement transducer (LVDT). To study the wear behavior, wear rate was calculated by using the formula, [W (mm3/m)= height change (mm) × pin area (mm2) / sliding distance (m)]. Cylindrical shaped pins with flat end were used as samples for wear testing. Test pins were ground to a smooth finish on 600 grit SiC paper. The worn surface regions (wear tracks) and collected debris after the dry sliding wear tests were also examined under scanning electron microscope. All the wear tests were conducted at constant temperature. A programmable furnace consisting of three band heaters enclosing the rotating counterface with proper insulation, as shown in Fig. 1, was used for this purpose.
3.
Results and Discussion
3.1.
Morphological Study
Fig. 2(a & b) shows the optical micrographs of base alloy (LM13). The dendritic growth of primary α-Al and eutectic phase in between the dendritic region (Fig. 2a) has been observed in LM13 base alloy; this is clearer at higher magnification in Fig. 2(b). LM13 alloy used in present investigation contains 12 wt.% Si. This composition is hypo eutectic but near to eutectic composition. High silicon content increases the corrosion and wear resistance and also improves casting and machining characteristics of the alloy [14]. When Al–Si alloy solidifies, the primary aluminum forms and grows as dendrites, whereas silicon solidifies as needle like structure. The Al–Si alloy usually has some other coexisting elements such as copper, magnesium,
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Fig. 1 – Images of pin on disc setup for wear test (Ducom-TR-20CH-400).
manganese, zinc and iron. Alloying elements can form dendritic intermetallic during solidification [14].
3.2.
Particle Distribution
Size of the particles and their volume fraction influences the structure of composite to greater extent. Fig. 3(a–d) shows the microstructure of composite reinforced with zircon sand
Fig. 2 – Optical micrographs of LM13 alloy (a) low magnification (b) at higher magnification.
particulates in the LM13 alloy matrix for 5, 10, 15 and 20 wt.% reinforcement respectively. In the present study we have selected the reinforcement of coarse particles (106–125 μm), but during stirring some of these particles got broken to smaller size as can be seen as the encircled ones in the micrographs shown in Fig. 3(a–d). Some fine carbon particles from stirrer may also come inside the melt. It is observed that fine particles have higher tendency of clustering as compared to coarse ones. As the solid– liquid interface moves during solidification, coarse particles are engulfed by the moving interface (Fig. 3e) and finer particles are pushed away from the interface (Fig. 3d, f). This leads to segregation of finer particles. These segregated particles are loosely bonded with matrix as they overlap on each other at certain places (Fig. 3f) and are devoid of matrix area to bond them properly. However, in general the reinforced particles are observed to be randomly distributed in the entire mass. The strengthening zircon sand particles occupy the eutectic zone. They contribute to the restricted growth phenomenon. The presence of ceramic particulates in the melt slows down the velocity of solidification front, causing delay in solidification. As the solidification time increases, more nuclei form leading to grain refinement [15]. Microstructural examination of metal matrix composites reveals an interfacial zone between the metal matrix and the reinforcement material, as can be seen in higher magnification micrograph (Fig. 3e). This zone between these two phases is one of the essential parts of MMCs. Interfacial bonds in the zone are developed from physical or chemical interaction of the particles with matrix. This causes interfacial frictional and thermal stresses due to difference between coefficient of thermal expansion of particulate and matrix [16]. The dendritic morphology changes to cellular one in composite as reinforced particles restrict the solute transport processes by diffusion and flow [17,18]. However, dendrites are visible in areas away from the reinforced particles which can be seen in Fig. 3(a & b). The bigger black spots correspond to the area where clustering of small particles have occurred due to relatively fast pushing up phenomenon at the interface (Fig. 3f). These particles have chipped out during grinding and polishing as they were loosely bonded as compared to single particle as has been marked by arrow in Fig. 3d. Moreover, the particles are distributed
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Fig. 3 – Optical micrographs of (a) LM13/5%Zr composite, (b) LM13/10%Zr composite, (c) LM13/15%Zr composite, (d) LM13/20%Zr composite and (e) LM13/10%Zr composite at higher magnification, and (f) SEM image taken from black zone in LM13/20%Zr composite at higher magnification showing clustering of particles.
throughout the casting though their volume fraction is observed to vary in the entire casting.
3.3.
Hardness
Fig. 4 shows the variation of bulk hardness of the LM13 base alloy and reinforced composites. An average of ten hardness tests of each samples was taken to get statistically significant values. Standard errors of the hardness values are incorporated
in the graph. From graph it is observed that hardness of composites increases with increasing concentrations of the zircon sand in matrix. However, at higher concentration (20 wt.%), hardness of the composite decreased slightly. It is due to agglomeration of the particles during stir casting. Due to agglomeration of the particles, the area affected by the particles/matrix interface in the matrix is reduced. These agglomerated particles overlap on each other and are not strongly bonded with the matrix, causing a decrease in bulk
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Fig. 4 – Hardness of the base alloy (LM13) and composites.
hardness of composite; this has also been reported in literature [17,14]. Hence overall hardness of the composite decreases slightly.
3.4.
Wear Behavior
3.4.1.
Effect of Load on the Wear Rate
A progressive loss of material from the operating surfaces as a result of relative motion is known as wear. In order to get idea about the durability of the materials under different conditions (particularly load and temperature), prepared samples were tested under dry sliding wear conditions. Table 3 shows the wear rate of base alloy and composites at room temperature. This is also depicted in Fig. 5. Fig. 5 clearly indicates that wear rate decreases with an increase in reinforcement for all the loads. Moreover, composites show better wear resistance in comparison to the matrix. It was also observed that with an increase in the amount of reinforcement in matrix the wear resistance of the composites improves. However, at higher concentration (20 wt.% Zr) wear rate of the composite decreased slightly in comparison to composite with 15 wt.% reinforcement. This is due to the presence of agglomerated particles. As was discussed in 3.2 above, agglomeration also decreases the hardness of the material, which also plays an important role in wear of that material.
3.4.2.
Effect of Temperature on the Wear Rate
To study the effects of high temperature on wear behavior of the matrix and composites, samples were tested in air at
Table 3 – Wear rate of base alloy and composite tested at different loads. Load (Newtons, N) 9.8 19.6 29.4 39.2 49
Wear rate × 10−3 (mm3/m) Matrix
5% Zr
10% Zr
15% Zr
20% Zr
4.20 7.31 9.46 12.75 15.94
3.53 6.53 7.40 10.24 13.65
3.35 4.21 5.08 6.3 7.90
1.98 3.84 4.82 5.56 6.67
2.23 3.56 4.18 5.10 6.45
different temperatures. Wear tests were carried out from 50 °C to 300 °C at high load (49 N); and the results are given in Table 4 and shown in Fig. 6. In the entire study we have kept the wear length of pin constant (2 mm) to have relative data. This indicates that a decrease in wear rate was observed after 150 °C, which is a critical temperature. The critical transition temperature in an alloy such as LM13 corresponds to 0.4 Tm (Tm = 923 K melting temperature for LM13 alloy, which is equal to 96.2 °C) [19]. Zhang et al. [19] suggested that at this critical temperature, thermally activated deformation processes are expected to become active and lead to softening of the material adjacent to the contact surfaces. As shown in Fig. 6, the wear rate of the matrix increases sharply with increase in temperature from 50 °C to 150 °C. At much higher temperature, above 200 °C, the matrix was ‘worn out’. In the ‘worn out’ condition, there is massive surface damage and large scale material transfer to the counterface that occurred. It indicates that around 200 °C (near critical temperature), the matrix becomes softer and leads to worn out conditions. The matrix samples were worn out at around 1100 m, 150 m and 50 m for 200 °C, 250 °C and 300 °C respectively. At higher temperature (above 200 °C) and high load (49 N) a transition from mild to severe wear was observed for the matrix (LM13). Addition of 5 wt.% of zircon sand to the matrix clearly brought about an improvement in wear rate of the composite (Fig. 5). However, composite containing 15 wt.% of reinforcement shows slightly better wear resistance than the composite having 20 wt.% zircon sand (Fig. 6). The nature of the reinforcement in the matrix plays an important role. Uniform and non-segregated particles stick to matrix and oppose wear. A very interesting behavior of wear rate is observed with increasing temperature for 10%, 15% and 20 wt.% reinforced composite. Wear behavior of the composite containing 10 wt.%, 15 wt.% and 20 wt.% zircon sand have been shown in the inset in Fig. 6 for the significant presentation of error bars. The wear rate for all composites increases slightly with an increase in temperature up to 150 °C. However, near 200 °C wear rate decreases. This decrease in wear rate is due to formation of thick oxide layer between the sample and counter surface, as will be described later. Wilson et al. [20] suggested that formation of oxide layer at high temperature (around 200 °C) reduces the wear rate by avoiding direct metal to metal contact. As the temperature is increased further up to 250 °C wear rate of the composites was observed to increase. At this critical temperature (near 250 °C) composites show transition from mild to severe wear. Rajaram et al. [21] suggested that the formation of oxide films at high temperature reduces the wear rate by avoiding direct metal to metal contact. The oxide film may be removed due to continuous sliding action which results in direct metal to metal contact and exposing new areas to environment at high temperature. Tearing and formation of oxide layer is a continuous process which results in decrement of wear rate when the temperature is increased. Wilson and Alpas [20] have observed similar behavior around 300 °C for an Al matrix composite. They suggested that this may occur due to debris generation within the steel disc at high temperature and high load. These debris can transfer to the opposing wear surface of the composite specimen, and this would lead to negative or minimum wear on the composites.
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Fig. 5 – Wear rate of the matrix and composites with variation of loads.
3.5. Topographical Examination of Worn Surfaces and Wear Debris 3.5.1.
Study of Worn Surfaces
Topographical studies of the worn surfaces and wear debris of the material can provide better platform to understand the wear mechanism of the material. Scanning electron microscopy was used for the morphological analysis of LM13 alloy and LM13/Zr composites. Fig. 7(a–d) shows the SEM images of the worn pin surface of LM13 alloy at high load (49 N) with variation in temperature. The worn surfaces of the base alloy pins tested at 49 N load were analyzed for specimens tested at 50 to 150 °C, as at higher temperature (above 200 °C) the sample pins were worn out. Fig. 7a shows the worn surface of LM13 alloy tested at 50 °C, where formation of continuous wear grooves along with some damaged regions can be seen. Flow of materials along the sliding direction, generation of cavities due to delamination of surface material and tearing of surface materials can be seen easily as shown in Fig. 7(a). Fig. 7b shows the wear track of matrix tested at 100 °C with greater depth of grooves and more delaminated material in comparison to the wear track of matrix tested at 50 °C; this indicates that the matrix shows higher wear rate at higher temperature The presence of oxide layers can be clearly seen in the SEM micrograph (Fig. 7c) of LM13 alloy at 150 °C. Fig. 7c indicates the simultaneous occurrence of formation and delamination of oxide layers on the worn surface. The onset of transition of mild to severe wear is observed at high temperature (200 °C), as shown in Fig. 7d. High degree of flow of materials along the sliding direction, generation of cavities due to
Table 4 – Wear rate for matrix and composites tested at different temperatures at 49 N load. Temperature (°C) 50 100 150 200 250 300
Wear rate × 10−3(mm3/m) Matrix
5% Zr
10% Zr
15% Zr
20% Zr
15.97 17.43 22.68 – – –
10.35 12.67 14.99 11.67 – –
6.82 6.93 7.23 5.44 6.27 5.46
4.24 4.52 4.73 3.19 4.94 3.37
4.65 4.85 4.84 3.44 5.41 3.89
delamination of surface materials and tearing of surface materials are indications of the transition from mild to severe wear at high temperature. Mondal et al. [22] suggested that flow of material in wavy form (serration) and large cavities due to delamination indicate the greater degree of softening of surface materials, partial melting of worn surface and localized adhesion between specimen surface and counter body. The SEM images (Fig. 7) also provide evidence that the wear rate of the alloy increases with an increase in temperature. A transition from mild to severe wear is also observed at higher temperature (above 150 °C). However, as shown in Fig. 6, the wear resistance of the alloy is improved with addition of zircon sand. Graphs plotted in Fig. 6 show that LM13/Zr composites have same type of wear behavior with variation of temperature. Hence for simplicity wear tracks of only one composite (LM13/15%Zr), having better wear resistance in comparison to other composites were taken for topographical analysis. Moreover, the features observed after tests at high load also exist for low loads, though they are of less severity. Fig. 8(a–f) represents the worn surface of the LM13/15%Zr composite after tests at high load (49 N) with temperatures ranging from 50 °C to 300 °C. Fig. 8(g and h) shows the SEM image of the wear track and the corresponding EDS spectra of the wear track of the composite tested at 49 N load and at 200 °C. Fig. 8a shows the worn surface of the LM13/15%Zr composite tested at high (49 N) load and at 50 °C temperature. The formation of grooves and small delaminations is observed in Fig. 8a. Depth of grooves and amount of delamination in the LM13/15%Zr composite are smaller in comparison to the matrix at same temperature and load, which indicates the better wear resistance of the composite with respect to alloy. Fractured zircon sand particles were also observed in the grooves. These trapped particles in grooves help to prevent the delamination of matrix. As the temperature is increased from 50 °C to 300 °C, the composite also shows increment in delaminated area, with presence of oxide layers. But if we compare these SEM images (Fig. 8) with those of the matrix (Fig. 7), we can easily observe the improved wear resistance of the composite in comparison with that of the alloy, particularly at high temperature. This improvement in wear resistance of material is due to presence of reinforced zircon sand. Fig. 8(a–h) reveals that both adhesive and abrasive wear mechanisms contribute to wear of samples. Zahmatkesh et al. [23] have also discussed their results based
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Fig. 6 – Effect of temperature on wear rate of base alloy and composites at 49 N load (inset is for the composite containing 10 wt.%, 15 wt.% and 20 wt.% zircon sand).
on SEM images which showed metal flow related to the adhesive wear mechanism and plowed grooves that are indicative of abrasive wear mechanisms. The parallel and continuous
scratches suggest abrasive wear as characterized by the penetration of the hard zircon sand particles into the softer surface, which is an important contribution to the wear behavior
Fig. 7 – SEM images of worn surfaces of LM13 alloy tested at 49 N load with variation temperatures of (a) 50 °C, (b) 100 °C, (c) 150 °C and (d) 200 °C.
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Fig. 8 – SEM images of wear tracks of LM13/15% Zr composite tested at 5 kg load with variation of temperature (a) 50 °C, (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C and (f) 300 °C, (g) SEM images of the wear track at higher magnification (1000×) showing microcracks and mechanically mixed layer. The corresponding EDS spectra taken from the marked rectangular area are also shown; (h) EDS of wear track (at 100×) of composite at 200 °C showing delaminated area. The corresponding EDS spectra taken from the marked rectangular area are also shown.
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of LM13/zircon composites. The groove on the worn surface indicates the removal of thick oxide layers. SEM image of wear track at higher magnification (Fig. 8g) shows the presence of microcracks, which are developed during wear test. These microcracks are the main source of delamination or removal of the material during the wear test. In the area around the cracks where delamination has occurred, a ‘mechanically mixed layer’ containing Fe from the counterface as well as oxides and other elements from the test pins can be seen clearly in Fig. 8g. The delaminated area shown in the SEM micrograph in Fig. 8h indicates the presence of an oxide layer on the worn surface; this supports the formation of a wear protective oxide layer on the load bearing surfaces at higher temperature. The oxide layer poses a greater resistance to sliding wear and friction, as oxide debris reduces the extent of direct metal to metal contact. The formation of chemically reacted oxide layer is result of the oxidation of the exposed metal surface during relative motion [16]. It was also observed that with increasing concentrations of zircon sand in LM13 base alloy, grooves become larger with more depth along the direction of sliding. As the concentration of reinforced particles increases, the amount of debris/breaking of sand particles is greater. Banerji et. al. [14] have also reported that the zircon sand particles fracture during the wear process at higher load. Fractured particles thereby lose their effectiveness as load-bearing components and the removal of material progresses by the fracture of the particles. However, these fractured particles are trapped in the grooves (Fig. 8d & f). Larsen Basse and Premaratne [24] have also stated that craters are formed on a material's surface under conditions where the grit was trapped (but could still rotate) between the counter face and the specimen. Trapped grit particles that could not rotate resulted in micro grooving of the material's surface. Same type of structural features of microgrooves can be easily seen in SEM micrographs of the composite at high temperature (Fig. 8f). In order to see the presence of different elements on the worn surface, X-ray dot mapping was also carried out. The presence of different elements on the worn surface is shown in Fig. 9. The dot mapping clearly indicates the presence of an oxide layer (Al2O3/Fe2O3) in the worn out area in which delamination had occurred. Moreover, Si is uniformly distributed throughout the matrix. The presence of Fe confirms the formation of a mechanically mixed layer on the surface. It also confirms the good bonding of the zircon sand particles with matrix, as these particles are seen inside the mapped area.
3.5.2.
Study of Wear Debris
Wear products (debris) collected during the wear test yield valuable information about the wear mechanism of the specimen. SEM images of the collected debris generated from LM13 base alloy and LM13/15%Zr composite at 49 N load and at different temperatures from 50 °C to 300 °C are shown in Figs. 10(a–f) and 11(a–f) respectively. For the entire presentation in this section, the area marked in all sets of micrographs represents the features as; A for delaminated debris, B for corrugated structure, C for thread type feature, D for molten state, E for shear morphology and F for fractured zircon sand particles. Fig. 10a represents the SEM micrographs of debris of base alloy at 50 °C for 49 N load. Debris (flake-like shape, marked as ‘A’) collected after wear test of the materials are of smaller
size. These wear debris (flakes) indicate that adhesive wear dominates in the sliding direction during the test. Due to adhesive nature at high load, metal is removed in the form of flakes as debris. Small flakes are generated by crushing of longer flakes at high load. Corrugated structure (marked as ‘B’) was observed in some debris, which may occur due to constant rubbing between contacting surfaces of sample and counterface. As the temperature is increased to 100 °C, the size of flakes becomes larger (Fig. 10b), which indicates that as temperature is increased, matrix becomes soft and wear resistance of the matrix is lowered. An increase in the amount of corrugated structured debris (marked as ‘B’) was observed in debris gathered in tests at 100 °C temperature. The elongated delaminated flakes (marked as ‘A’) in debris show grooves that indicate that they come out due to shearing of layer in the direction of applied force. At a much higher temperature, around 150 °C or higher, the alloy shows an onset of transition from mild to severe wear. The size and amount of debris collected are much greater than at lower temperature. At high temperature (Fig. 10c–f), thread type morphology (marked as ‘C’) is found, along with the presence of delaminated plate like debris with microcracks. Thread type debris are created during the pulling out of aluminum matrix in tests at high load and high temperature. The size of debris at high temperature, more than 150 °C (Fig. 10d–f), indicates that severe wear dominates for the alloy at this condition (high temperature and high load). In Fig. 10d some debris that solidified from the molten state (marked as ‘D’) was also observed at higher temperature (200 °C). Its amount and also the amount of delaminated debris was increased at 250 °C and 300 °C temperatures (Fig. 10e–f). Some thread like debris corresponding to α Al are trapped in between the grooves and acquire round shape. Due to continuous rubbing action, these get heated and reach the melting point of metal. This can be seen in Fig. 10 (e and f) (marked as ‘D’). At high temperature, the presence of partially molten state of debris may contribute to an increase in the wear rate of the composite. Fig. 11(a–f) shows the surface morphology of debris collected from LM13/15%Zr composite tested at 49 N load at different temperature from 50 °C to 300 °C. Fig. 11 indicates that the amount and size of wear debris of composite reduced in comparison to that of the matrix specimens (Fig. 10) at most temperatures. This is due to the improvement in the wear resistance of the composite after addition of zircon sand as shown in Fig. 6. As the zircon sand particles have low coefficient of thermal expansion as compared to LM13 alloy, their presence in matrix improves the overall thermal stability of composite [25]. Hence composites containing a higher amount of zircon sand in the matrix will show better thermal resistance in comparison to base materials. Among the studied composites, LM13/15% Zr composite shows better wear and thermal resistance. SEM image of debris collected after the wear test as shown in Fig. 11a gives idea about the wear mechanism of LM13/15%Zr composite at 50 °C with 49 N load. The reduced size of debris of the composite as compared to the base alloy is due to higher hardness of composite, which improves the wear resistance of material. Delamination, multiple plowing action and shearing features (marked as ‘E’) were observed as shown in Fig. 11a. Fragmentation of debris was observed, which may be due to continuous rubbing of delaminated flakes between the contacting surfaces. Debris trapped in wear tracks leads to a
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Fig. 9 – Dot X-ray mapping of wear track of the LM13/15%Zr composite tested at 200 °C with 49 N Load.
corrugated structure. Some fractured zircon sand particles were also observed (marked as ‘F’) as shown in Fig. 11(a–f). Dwivedi [26] suggested that a wear particle is detached due to tangential forces acting on surfaces causing nucleation of cracks at surface, which propagate in due course of time causing exposure of fresh surface followed by delamination. Fracture of particulates may arise from either strain transfer to the reinforcement interface from the matrix undergoing plastic deformation in the wear zone or from cracks initiated when the hard particulates are ground against each other and crushed by striking asperities on the opposing steel counterface. When temperature was increased to 100 °C and 150 °C, the size of debris was increased in comparison to previous ones. Fractured zircon sand particles (marked as ‘F’) and corrugated structured debris (marked as ‘B’) with delaminated flakes were also observed as shown in Fig. 11(b & c). SEM images of debris collected at much higher temperature (Fig. 11c–f) indicate smaller size of debris in presence of fractured particles and
molten state of portions of the matrix. At elevated temperatures, cavities or voids created by particulates fracturing in the subsurface wear zone are liable to be healed by extrusion of the matrix alloy into the region formed between newly fractured particulates [27,20]. The combined process of particulate fracture and matrix extrusion in the subsurface wear region is responsible for the formation of the characteristic comminuted particulate transfer layers (mechanically mixed layer), which are highly beneficial to matrix composite wear. The X-ray dot mapping confirms such type of transition (Fig. 9). The step clearly observed (marked as B) in Fig. 11(e & f), may be due to slip to the Al and zircon sand interfaces due to constant rubbing between the wear surfaces [28]. It is also observed that these debris particles undergo multiple plowing action, causing cracking in them, which can be seen in delaminated debris marked as A in Fig. 11e. Fig. 12(a and b) shows the EDS analysis of debris collected after the wear testing of the LM13/15% Zr composite at 250 °C
210 M A TE RI A L S CH A RACT ER IZ A TI O N 75 (2 0 1 3 ) 2 0 0–2 1 3
Fig. 10 – SEM micrographs of wear debris of LM13 alloy collected in tests at high (49 N) load with variation of temperature (a) 50 ºC, (b) 100 ºC, (c) 150 ºC, (d) 200 ºC, (e) 250 ºC and (f) 300 ºC.
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 2 0 0–2 1 3
Fig. 11 – SEM images of wear debris of LM13/15% Zr composite collected in tests at 49 N load with variation temperatures of (a) 50 °C, (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C and (f) 300 °C.
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Fig. 12 – EDS analysis of the wear debris LM13/15%Zr composite tested at high load (49 N).
temperature. EDS analysis of these debris indicate the presence of Al, Si, Fe and O elements. Fig. 12a shows the EDS spectra of the delaminated debris and Fig. 12b shows the EDS spectra of spherical debris which is trapped inside the grooves and got melted due to multiple rubbing action during the wear test. Fig. 12a also shows the presence of Fe in the debris collected after wear testing. The presence of Fe in the debris may be due to the transfer of steel counterface material to the composite surface. The transfer of steel inclusions from counterface surfaces to the composite wear surfaces is another mechanism which contributes to increase in wear resistance of the composites. Here inclusions may act as additional reinforcement at the wearing surface of the composite [20]. Moreover, at higher temperature, the delaminated debris also get welded to the surface as shown in Fig. 8e, causing negative or low wear. The X-ray diffraction pattern of debris collected after the wear test conducted at 49 N load and 300 °C is shown in Fig. 13. Fig. 13 also indicates the presence of Al2SiO5, Si and ZrSiO4 phase along with Fe. This indicates that apart from base metal, zircon is also coming out from the surface.
4.
behavior under different temperature. Microstructural analysis and wear tests were conducted on the specimens. From the experimental results the following conclusions were drawn: (1) A fairly uniform distribution of reinforcing particles was observed in the microstructural study. Good interfacial
Conclusion
Stir cast LM13 alloy and zircon sand particles reinforced composites were studied to get idea about the dry sliding wear
Fig. 13 – XRD pattern of debris of LM13/15% Zr composite tested at 49 N load and 300 °C.
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 2 0 0–2 1 3
(2) (3)
(4) (5)
(6)
(7)
bonding between zircon sand particles and matrix was also observed. Hardness of the composite material increased with an increase in amount of reinforced zircon sand. Wear resistance of the composite materials increases with an increase in reinforcement of zircon sand, although an increase in zircon sand content from 15 wt.% to 20 wt.% does not result in an appreciable change in wear rate. Wear rate of the both matrix and composite increased with an increase in applied load. The effect of temperature on wear was also studied at different loads. Composites with higher concentration of zircon sand show better wear resistance and thermal stability. An improvement in critical transition temperature of alloy from 100 °C to 200 °C for composites was observed with addition of zircon sand. The mircostructral study of specimen after wear test concludes that both adhesive and abrasive wear mechanisms contribute for wear of composites.
Acknowledgment The authors are thankful to Armament Research Board (ARMREB), Defence Research and Development Organization (DRDO), India for providing financial support under letter no. ARMREB/MAA/2008/105 for this study. Authors are also thankful to the reviewer for his critical comments and helping us to improve the manuscript.
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[8] Maity PC, Chakraborty PN, Panigrahi SC. Formation of spinel (MgAl2O4) MgO and pure Cu particles in Al–2Mg alloy–CuO particle composites. J Mater Sci 1996;31:6377-82. [9] Hosking FM, Folgar FP, Wunderlin R, Meharbian R. Composites of aluminium alloys: fabrication and wear behavior. J Mater Sci 1998;217:477. [10] Włodarczyk-Fligier A, Dobrzański LA, Adamiak M. Wear resistance of PM composite materials reinforced with the Ti(C, N) ceramic particles. J Achiev Mater Manuf Eng 2008;30(2):147-50. [11] Deuis RL, Subramaian C, Yellup JM. Dry sliding wear of aluminum composites — a review. Compos Sci Technol 1997;57:415-35. [12] Das Sanjeev, Udarabanu V, Das S, Das K. Synthesis and characterization of zircon sand/Al–4.5 wt% Cu composite produced by stir casting route. J Mater Sci 2006;41:4668-77. [13] Sharma Vipin, Kumar Suresh, Panwar Ranvir Singh, Pandey OP. Microstructural and wear behavior of dual reinforced particle (DRP) aluminum alloy composite. J Mater Sci 2012;47: 6633-46. [14] Banerji A, Surappa MK, Rohatgi PK. Cast aluminum alloys containing dispersions of zircon particles. Metall Mater Trans B 1983;14:273. [15] Kumar Suresh, Sharma Vipin, Panwar Ranvir Singh, Pandey OP. Wear Behavior of Dual Particle Size (DPS) Zircon Sand Reinforced Aluminum Alloy. Tribol Lett 2012;47:231-51. [16] Kaur Kamalpreet, Pandey OP. Dry sliding behavior of zircon sand reinforced Al–Si alloy. Tribol Lett 2010;38:377. [17] Das Sanjeev, Das Siddhartha, Das Karabi. Abrasive wear of zircon sand and lumina reinforced Al–4.5 wt%Cu alloy matrix composites — a comparative study. Compos Sci Technol 2007;67:746. [18] Kaur K, Pandey OP. Wear and microstructural characteristics of spray atomized zircon sand reinforced LM13 alloy. Materialwiss Werkstofftech 2010;41(7):568-74. [19] Zhang J, Alpas AT. Transition between mild and severe wear in aluminum alloys. Acta Mater 1997;45:513. [20] Wilson S, Alpas AT. Effect of temperature on the sliding wear performance of Al alloys and Al matrix composites. Wear 1996;196:270-8. [21] Rajaram G, Kumaran S, Srinivasa RT. High temperature tensile and wear behaviour of aluminum silicon alloy. Mater Sci Eng A 2010;528:247-53. [22] Mondal DP, Das S, Rao RN, Singh M. Effect of SiC addition and running-in-wear on the sliding wear behavior of Al–Zn–Mg aluminium alloy. Mater Sci Eng A 2005;402:307. [23] Zahmatkesh B, Enayati MH, Karimzadeh F. Tribological and microstructural evaluation of friction stir processed Al2024 alloy. Mater Des 2010;31:4891-6. [24] Larsen-Basse J, Premaratne B. Proc. Wear of materials conf. New York: ASME; 1983. p. 161. [25] Ejiofor JU, Reddy RG. Interaface and properties of Al–Si alloy zircon sand particulate composites. J Mater Res 1998;13: 3093-9. [26] Dwivedi DK. Adhesive wear behavior of cast aluminium–silicon alloys: overview. Mater Des 2010;31:2517. [27] Brechet Y, Newell J, Tao S, Embury JD. A note on particlecomminution at large plastic starins in Al–SiC composites. Scr Metall Mater 1993:2847. [28] bakshi Srnivasa R, Di Wang, Timthy Price, Deen Zhang, Keshri AK, Yao Chen, et al. Microsturcure and wear properties of aluminum/aluminum–silicon composite coating prepared by cold spraying. Surf Coat Technol 2009;204:503.
Available online at www.sciencedirect.com
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Analysis of wear track and debris of stir cast LM13/Zr composite at elevated temperatures Ranvir Singh Panwar, O.P. Pandey⁎ School of Physics and Materials Science, Thapar University, Patiala, 147004, India
AR TIC LE D ATA
ABSTR ACT
Article history:
Particulate reinforced aluminum metal matrix composite is in high demand in automobile
Received 24 April 2012
industry where the operational conditions vary from low to high temperature. In order to
Received in revised form
understand the wear mode at elevated temperature, this study was planned. For this
3 November 2012
purpose we developed a metal matrix composite containing aluminum alloy (LM13) as
Accepted 6 November 2012
matrix and zircon sand as particulate reinforcement by stir casting process. Different amounts of zircon sand (5, 10, 15 and 20 wt.%) were incorporated in the matrix to study the
Keywords:
effect of reinforcement on the wear resistance. Dispersion of zircon sand particles in the
Metal matrix composite
matrix was confirmed by using optical microscopy. Sliding wear tests were done to study
Stir casting
the durability of the composite with respect to the base alloy. The effects of load and
Sliding wear
temperature on wear behavior from room temperature to 300 °C were studied to
Scanning electron microscope
understand the wear mechanism deeply. Surface morphology of the worn surfaces after
Debris
the wear tests as well as wear debris was observed under scanning electron microscope. Mild to severe wear transition was noticed in tests at high temperature and high load. However, there is interesting change in wear behavior of the composite near the critical temperature of the composite. All the observed behavior has been explained with reference to the observed microstructure of the wear track and debris. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
At the present time, durability of materials, especially those of tribological importance, is of great interest for motor vehicles and other engineering applications. Utility of Al–Si alloys in automobile industry has received considerable attention due to lightweight, good machinability and easy casting characteristics [1–3]. Clarke and Sarkar [4] suggested that presence of Si in Al–Si alloy of near eutectic composition improves the wear resistance of alloy. In similar investigations it was also found that particulate reinforced aluminum matrix composite (PAMCs) shows great improvement in wear resistance [5–9]. However, for better tribological and mechanical properties of the composites, selection and volume fractions of particulates, their size, shape and distribution in matrix are still being studied. Present work is an effort in this direction.
Among the available techniques to develop the PAMCs, the stir casting process is widely accepted in industries due to the lower production cost, simplicity, flexibility and applicability to large quantity production [10,11]. High hardness, high modulus of elasticity and good thermal stability of zircon sand particles attracted the researchers to use these particles as reinforcement in AMCs. Das et al. [12] have also reported that zircon sand reinforced composite shows better wear resistance than alumina reinforced composite due to its superior particle–matrix bonding. However, until now wear behavior and particularly the surface analysis of worn surfaces of Al–Si/Zr composites at elevated temperatures is not available. Considering this fact, in the present work we studied about the surface morphology of worn pins of Al–Si/Zr PAMCs at different loads and temperatures. Different amount of zircon sand was also added to the metal matrix to study the effect of
⁎ Corresponding author. Tel.: + 91 175 2393116; fax: + 91 175 2393020. E-mail addresses: [email protected] (R.S. Panwar), [email protected] (O.P. Pandey). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.11.003
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zircon sand in LM13/Zr composites. The effect of temperature under oxidative environment on wear for LM13/Zr composite is studied at different temperatures (50 °C to 300 °C) and loads (9.8 N and 49 N). LM13/zircon sand composite may be useful in automotive and defense areas as presence of zircon sand improves the thermal stability as well as hardness and wear resistivity of the composite as compared to aluminum alloy.
2.
Materials and Experimental Details
2.1.
Materials
Melting and Casting
Metallography and X-ray Diffraction
For metallographic study base alloy (LM13) and composite samples were ground, mechanically polished and etched with Keller's reagent. The surface morphology of etched sample was analyzed under optical microscope (Eclipse MA-100, Nikon). After wear test, worn surfaces and debris were analyzed under scanning electron microscope (JOEL, JSM-6510LV). To study the presence of elements in the sample, energy-dispersive X-ray spectroscopy (oxford INCA) with model no. 51-ADD 0076 was used. PANalytical X'pert PRO X-ray diffractometer with CuKα radiation (λ = 1.54 Å) was used for X-ray diffraction patterns of debris of composites collected after wear tests.
2.4.
ZrO2 (+HfO2)
SiO2
TiO2
Fe2O3
% in bulk
65.30
32.80
0.27
0.12
2.5.
The details of materials and casting procedure are mentioned elsewhere [13]. Composite containing 5, 10, 15 and 20 wt.% zircon sand were prepared by stir casting route. The alloy was melted and held at 750 °C. This melt was stirred with a graphite stirrer. Preheated sand particles were added slowly to this melt during stirring. After completing the mixing of sand particles, the melt was poured in steel molds and allowed to cool. Specimens for testing were cut from the cast composites.
2.3.
Elements
1/16″ diameter steel ball indenter under 100 Kgf at B scale. A fine manufacturing industry, India make Rockwell hardness tester (Model: TRSND) was used for carrying out the hardness testing.
In the present work a commercial grade piston alloy is used. Commercial aluminum alloy LM13 containing 12 wt.% Si was chosen as the matrix and commercial grade zirconium silicate (ZrSiO4), known as zircon sand, as reinforcements. Zircon sand was obtained from Indian rare earths limited, Chatrapur, Orissa, India. Before experiments the zircon sand was sieved and particles of 106–125 μm size were used in the present work. The chemical compositions of LM13 alloy and zircon sand are given in Tables 1 and 2, respectively.
2.2.
Table 2 – Chemical composition of zircon sand particles.
Hardness
The bulk hardness of the LM13/Zircon sand composite and LM13 alloy was measured by Rockwell hardness tester using
Table 1 – Chemical compositions of LM13 aluminum alloy. Elements
Si
Fe
Cu
Mn
Mg
Zn
Ni
Al
Chemical analysis (wt.%)
12.0
0.4
1.2
0.4
1.00
0.2
1.0
Bal.
Wear Testing
For wear tests a cylindrical pin of 8 mm in diameter was prepared from cast composite. Wear testing was performed on wear testing machine (wear and friction monitor, TR-20 CH-400, Ducom Instruments, Bangalore India) under dry sliding conditions in ambient air at controlled temperature (Fig. 1). Wear tests were conducted at different temperatures (50–300 °C) at low (9.8 N) and high loads (9.8 N). All the samples of base alloy (LM13) and LM13/Zr composites were tested against EN 32 steel disc having 65 HRC hardness and surface roughness (Ra) of 0.5 μm. To get an average value of wear rate, each test was run three times. Before each test the track was cleaned with acetone. For each test a new track was used to get similar test conditions. Throughout the experiment a constant sliding velocity of 1.6 m s−1 was maintained. Sliding distance covered during the experiment was about 3000 m. Wear rate was determined by measuring specimen height change using a linear variable displacement transducer (LVDT). To study the wear behavior, wear rate was calculated by using the formula, [W (mm3/m)= height change (mm) × pin area (mm2) / sliding distance (m)]. Cylindrical shaped pins with flat end were used as samples for wear testing. Test pins were ground to a smooth finish on 600 grit SiC paper. The worn surface regions (wear tracks) and collected debris after the dry sliding wear tests were also examined under scanning electron microscope. All the wear tests were conducted at constant temperature. A programmable furnace consisting of three band heaters enclosing the rotating counterface with proper insulation, as shown in Fig. 1, was used for this purpose.
3.
Results and Discussion
3.1.
Morphological Study
Fig. 2(a & b) shows the optical micrographs of base alloy (LM13). The dendritic growth of primary α-Al and eutectic phase in between the dendritic region (Fig. 2a) has been observed in LM13 base alloy; this is clearer at higher magnification in Fig. 2(b). LM13 alloy used in present investigation contains 12 wt.% Si. This composition is hypo eutectic but near to eutectic composition. High silicon content increases the corrosion and wear resistance and also improves casting and machining characteristics of the alloy [14]. When Al–Si alloy solidifies, the primary aluminum forms and grows as dendrites, whereas silicon solidifies as needle like structure. The Al–Si alloy usually has some other coexisting elements such as copper, magnesium,
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Fig. 1 – Images of pin on disc setup for wear test (Ducom-TR-20CH-400).
manganese, zinc and iron. Alloying elements can form dendritic intermetallic during solidification [14].
3.2.
Particle Distribution
Size of the particles and their volume fraction influences the structure of composite to greater extent. Fig. 3(a–d) shows the microstructure of composite reinforced with zircon sand
Fig. 2 – Optical micrographs of LM13 alloy (a) low magnification (b) at higher magnification.
particulates in the LM13 alloy matrix for 5, 10, 15 and 20 wt.% reinforcement respectively. In the present study we have selected the reinforcement of coarse particles (106–125 μm), but during stirring some of these particles got broken to smaller size as can be seen as the encircled ones in the micrographs shown in Fig. 3(a–d). Some fine carbon particles from stirrer may also come inside the melt. It is observed that fine particles have higher tendency of clustering as compared to coarse ones. As the solid– liquid interface moves during solidification, coarse particles are engulfed by the moving interface (Fig. 3e) and finer particles are pushed away from the interface (Fig. 3d, f). This leads to segregation of finer particles. These segregated particles are loosely bonded with matrix as they overlap on each other at certain places (Fig. 3f) and are devoid of matrix area to bond them properly. However, in general the reinforced particles are observed to be randomly distributed in the entire mass. The strengthening zircon sand particles occupy the eutectic zone. They contribute to the restricted growth phenomenon. The presence of ceramic particulates in the melt slows down the velocity of solidification front, causing delay in solidification. As the solidification time increases, more nuclei form leading to grain refinement [15]. Microstructural examination of metal matrix composites reveals an interfacial zone between the metal matrix and the reinforcement material, as can be seen in higher magnification micrograph (Fig. 3e). This zone between these two phases is one of the essential parts of MMCs. Interfacial bonds in the zone are developed from physical or chemical interaction of the particles with matrix. This causes interfacial frictional and thermal stresses due to difference between coefficient of thermal expansion of particulate and matrix [16]. The dendritic morphology changes to cellular one in composite as reinforced particles restrict the solute transport processes by diffusion and flow [17,18]. However, dendrites are visible in areas away from the reinforced particles which can be seen in Fig. 3(a & b). The bigger black spots correspond to the area where clustering of small particles have occurred due to relatively fast pushing up phenomenon at the interface (Fig. 3f). These particles have chipped out during grinding and polishing as they were loosely bonded as compared to single particle as has been marked by arrow in Fig. 3d. Moreover, the particles are distributed
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Fig. 3 – Optical micrographs of (a) LM13/5%Zr composite, (b) LM13/10%Zr composite, (c) LM13/15%Zr composite, (d) LM13/20%Zr composite and (e) LM13/10%Zr composite at higher magnification, and (f) SEM image taken from black zone in LM13/20%Zr composite at higher magnification showing clustering of particles.
throughout the casting though their volume fraction is observed to vary in the entire casting.
3.3.
Hardness
Fig. 4 shows the variation of bulk hardness of the LM13 base alloy and reinforced composites. An average of ten hardness tests of each samples was taken to get statistically significant values. Standard errors of the hardness values are incorporated
in the graph. From graph it is observed that hardness of composites increases with increasing concentrations of the zircon sand in matrix. However, at higher concentration (20 wt.%), hardness of the composite decreased slightly. It is due to agglomeration of the particles during stir casting. Due to agglomeration of the particles, the area affected by the particles/matrix interface in the matrix is reduced. These agglomerated particles overlap on each other and are not strongly bonded with the matrix, causing a decrease in bulk
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Fig. 4 – Hardness of the base alloy (LM13) and composites.
hardness of composite; this has also been reported in literature [17,14]. Hence overall hardness of the composite decreases slightly.
3.4.
Wear Behavior
3.4.1.
Effect of Load on the Wear Rate
A progressive loss of material from the operating surfaces as a result of relative motion is known as wear. In order to get idea about the durability of the materials under different conditions (particularly load and temperature), prepared samples were tested under dry sliding wear conditions. Table 3 shows the wear rate of base alloy and composites at room temperature. This is also depicted in Fig. 5. Fig. 5 clearly indicates that wear rate decreases with an increase in reinforcement for all the loads. Moreover, composites show better wear resistance in comparison to the matrix. It was also observed that with an increase in the amount of reinforcement in matrix the wear resistance of the composites improves. However, at higher concentration (20 wt.% Zr) wear rate of the composite decreased slightly in comparison to composite with 15 wt.% reinforcement. This is due to the presence of agglomerated particles. As was discussed in 3.2 above, agglomeration also decreases the hardness of the material, which also plays an important role in wear of that material.
3.4.2.
Effect of Temperature on the Wear Rate
To study the effects of high temperature on wear behavior of the matrix and composites, samples were tested in air at
Table 3 – Wear rate of base alloy and composite tested at different loads. Load (Newtons, N) 9.8 19.6 29.4 39.2 49
Wear rate × 10−3 (mm3/m) Matrix
5% Zr
10% Zr
15% Zr
20% Zr
4.20 7.31 9.46 12.75 15.94
3.53 6.53 7.40 10.24 13.65
3.35 4.21 5.08 6.3 7.90
1.98 3.84 4.82 5.56 6.67
2.23 3.56 4.18 5.10 6.45
different temperatures. Wear tests were carried out from 50 °C to 300 °C at high load (49 N); and the results are given in Table 4 and shown in Fig. 6. In the entire study we have kept the wear length of pin constant (2 mm) to have relative data. This indicates that a decrease in wear rate was observed after 150 °C, which is a critical temperature. The critical transition temperature in an alloy such as LM13 corresponds to 0.4 Tm (Tm = 923 K melting temperature for LM13 alloy, which is equal to 96.2 °C) [19]. Zhang et al. [19] suggested that at this critical temperature, thermally activated deformation processes are expected to become active and lead to softening of the material adjacent to the contact surfaces. As shown in Fig. 6, the wear rate of the matrix increases sharply with increase in temperature from 50 °C to 150 °C. At much higher temperature, above 200 °C, the matrix was ‘worn out’. In the ‘worn out’ condition, there is massive surface damage and large scale material transfer to the counterface that occurred. It indicates that around 200 °C (near critical temperature), the matrix becomes softer and leads to worn out conditions. The matrix samples were worn out at around 1100 m, 150 m and 50 m for 200 °C, 250 °C and 300 °C respectively. At higher temperature (above 200 °C) and high load (49 N) a transition from mild to severe wear was observed for the matrix (LM13). Addition of 5 wt.% of zircon sand to the matrix clearly brought about an improvement in wear rate of the composite (Fig. 5). However, composite containing 15 wt.% of reinforcement shows slightly better wear resistance than the composite having 20 wt.% zircon sand (Fig. 6). The nature of the reinforcement in the matrix plays an important role. Uniform and non-segregated particles stick to matrix and oppose wear. A very interesting behavior of wear rate is observed with increasing temperature for 10%, 15% and 20 wt.% reinforced composite. Wear behavior of the composite containing 10 wt.%, 15 wt.% and 20 wt.% zircon sand have been shown in the inset in Fig. 6 for the significant presentation of error bars. The wear rate for all composites increases slightly with an increase in temperature up to 150 °C. However, near 200 °C wear rate decreases. This decrease in wear rate is due to formation of thick oxide layer between the sample and counter surface, as will be described later. Wilson et al. [20] suggested that formation of oxide layer at high temperature (around 200 °C) reduces the wear rate by avoiding direct metal to metal contact. As the temperature is increased further up to 250 °C wear rate of the composites was observed to increase. At this critical temperature (near 250 °C) composites show transition from mild to severe wear. Rajaram et al. [21] suggested that the formation of oxide films at high temperature reduces the wear rate by avoiding direct metal to metal contact. The oxide film may be removed due to continuous sliding action which results in direct metal to metal contact and exposing new areas to environment at high temperature. Tearing and formation of oxide layer is a continuous process which results in decrement of wear rate when the temperature is increased. Wilson and Alpas [20] have observed similar behavior around 300 °C for an Al matrix composite. They suggested that this may occur due to debris generation within the steel disc at high temperature and high load. These debris can transfer to the opposing wear surface of the composite specimen, and this would lead to negative or minimum wear on the composites.
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205
Fig. 5 – Wear rate of the matrix and composites with variation of loads.
3.5. Topographical Examination of Worn Surfaces and Wear Debris 3.5.1.
Study of Worn Surfaces
Topographical studies of the worn surfaces and wear debris of the material can provide better platform to understand the wear mechanism of the material. Scanning electron microscopy was used for the morphological analysis of LM13 alloy and LM13/Zr composites. Fig. 7(a–d) shows the SEM images of the worn pin surface of LM13 alloy at high load (49 N) with variation in temperature. The worn surfaces of the base alloy pins tested at 49 N load were analyzed for specimens tested at 50 to 150 °C, as at higher temperature (above 200 °C) the sample pins were worn out. Fig. 7a shows the worn surface of LM13 alloy tested at 50 °C, where formation of continuous wear grooves along with some damaged regions can be seen. Flow of materials along the sliding direction, generation of cavities due to delamination of surface material and tearing of surface materials can be seen easily as shown in Fig. 7(a). Fig. 7b shows the wear track of matrix tested at 100 °C with greater depth of grooves and more delaminated material in comparison to the wear track of matrix tested at 50 °C; this indicates that the matrix shows higher wear rate at higher temperature The presence of oxide layers can be clearly seen in the SEM micrograph (Fig. 7c) of LM13 alloy at 150 °C. Fig. 7c indicates the simultaneous occurrence of formation and delamination of oxide layers on the worn surface. The onset of transition of mild to severe wear is observed at high temperature (200 °C), as shown in Fig. 7d. High degree of flow of materials along the sliding direction, generation of cavities due to
Table 4 – Wear rate for matrix and composites tested at different temperatures at 49 N load. Temperature (°C) 50 100 150 200 250 300
Wear rate × 10−3(mm3/m) Matrix
5% Zr
10% Zr
15% Zr
20% Zr
15.97 17.43 22.68 – – –
10.35 12.67 14.99 11.67 – –
6.82 6.93 7.23 5.44 6.27 5.46
4.24 4.52 4.73 3.19 4.94 3.37
4.65 4.85 4.84 3.44 5.41 3.89
delamination of surface materials and tearing of surface materials are indications of the transition from mild to severe wear at high temperature. Mondal et al. [22] suggested that flow of material in wavy form (serration) and large cavities due to delamination indicate the greater degree of softening of surface materials, partial melting of worn surface and localized adhesion between specimen surface and counter body. The SEM images (Fig. 7) also provide evidence that the wear rate of the alloy increases with an increase in temperature. A transition from mild to severe wear is also observed at higher temperature (above 150 °C). However, as shown in Fig. 6, the wear resistance of the alloy is improved with addition of zircon sand. Graphs plotted in Fig. 6 show that LM13/Zr composites have same type of wear behavior with variation of temperature. Hence for simplicity wear tracks of only one composite (LM13/15%Zr), having better wear resistance in comparison to other composites were taken for topographical analysis. Moreover, the features observed after tests at high load also exist for low loads, though they are of less severity. Fig. 8(a–f) represents the worn surface of the LM13/15%Zr composite after tests at high load (49 N) with temperatures ranging from 50 °C to 300 °C. Fig. 8(g and h) shows the SEM image of the wear track and the corresponding EDS spectra of the wear track of the composite tested at 49 N load and at 200 °C. Fig. 8a shows the worn surface of the LM13/15%Zr composite tested at high (49 N) load and at 50 °C temperature. The formation of grooves and small delaminations is observed in Fig. 8a. Depth of grooves and amount of delamination in the LM13/15%Zr composite are smaller in comparison to the matrix at same temperature and load, which indicates the better wear resistance of the composite with respect to alloy. Fractured zircon sand particles were also observed in the grooves. These trapped particles in grooves help to prevent the delamination of matrix. As the temperature is increased from 50 °C to 300 °C, the composite also shows increment in delaminated area, with presence of oxide layers. But if we compare these SEM images (Fig. 8) with those of the matrix (Fig. 7), we can easily observe the improved wear resistance of the composite in comparison with that of the alloy, particularly at high temperature. This improvement in wear resistance of material is due to presence of reinforced zircon sand. Fig. 8(a–h) reveals that both adhesive and abrasive wear mechanisms contribute to wear of samples. Zahmatkesh et al. [23] have also discussed their results based
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Fig. 6 – Effect of temperature on wear rate of base alloy and composites at 49 N load (inset is for the composite containing 10 wt.%, 15 wt.% and 20 wt.% zircon sand).
on SEM images which showed metal flow related to the adhesive wear mechanism and plowed grooves that are indicative of abrasive wear mechanisms. The parallel and continuous
scratches suggest abrasive wear as characterized by the penetration of the hard zircon sand particles into the softer surface, which is an important contribution to the wear behavior
Fig. 7 – SEM images of worn surfaces of LM13 alloy tested at 49 N load with variation temperatures of (a) 50 °C, (b) 100 °C, (c) 150 °C and (d) 200 °C.
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Fig. 8 – SEM images of wear tracks of LM13/15% Zr composite tested at 5 kg load with variation of temperature (a) 50 °C, (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C and (f) 300 °C, (g) SEM images of the wear track at higher magnification (1000×) showing microcracks and mechanically mixed layer. The corresponding EDS spectra taken from the marked rectangular area are also shown; (h) EDS of wear track (at 100×) of composite at 200 °C showing delaminated area. The corresponding EDS spectra taken from the marked rectangular area are also shown.
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of LM13/zircon composites. The groove on the worn surface indicates the removal of thick oxide layers. SEM image of wear track at higher magnification (Fig. 8g) shows the presence of microcracks, which are developed during wear test. These microcracks are the main source of delamination or removal of the material during the wear test. In the area around the cracks where delamination has occurred, a ‘mechanically mixed layer’ containing Fe from the counterface as well as oxides and other elements from the test pins can be seen clearly in Fig. 8g. The delaminated area shown in the SEM micrograph in Fig. 8h indicates the presence of an oxide layer on the worn surface; this supports the formation of a wear protective oxide layer on the load bearing surfaces at higher temperature. The oxide layer poses a greater resistance to sliding wear and friction, as oxide debris reduces the extent of direct metal to metal contact. The formation of chemically reacted oxide layer is result of the oxidation of the exposed metal surface during relative motion [16]. It was also observed that with increasing concentrations of zircon sand in LM13 base alloy, grooves become larger with more depth along the direction of sliding. As the concentration of reinforced particles increases, the amount of debris/breaking of sand particles is greater. Banerji et. al. [14] have also reported that the zircon sand particles fracture during the wear process at higher load. Fractured particles thereby lose their effectiveness as load-bearing components and the removal of material progresses by the fracture of the particles. However, these fractured particles are trapped in the grooves (Fig. 8d & f). Larsen Basse and Premaratne [24] have also stated that craters are formed on a material's surface under conditions where the grit was trapped (but could still rotate) between the counter face and the specimen. Trapped grit particles that could not rotate resulted in micro grooving of the material's surface. Same type of structural features of microgrooves can be easily seen in SEM micrographs of the composite at high temperature (Fig. 8f). In order to see the presence of different elements on the worn surface, X-ray dot mapping was also carried out. The presence of different elements on the worn surface is shown in Fig. 9. The dot mapping clearly indicates the presence of an oxide layer (Al2O3/Fe2O3) in the worn out area in which delamination had occurred. Moreover, Si is uniformly distributed throughout the matrix. The presence of Fe confirms the formation of a mechanically mixed layer on the surface. It also confirms the good bonding of the zircon sand particles with matrix, as these particles are seen inside the mapped area.
3.5.2.
Study of Wear Debris
Wear products (debris) collected during the wear test yield valuable information about the wear mechanism of the specimen. SEM images of the collected debris generated from LM13 base alloy and LM13/15%Zr composite at 49 N load and at different temperatures from 50 °C to 300 °C are shown in Figs. 10(a–f) and 11(a–f) respectively. For the entire presentation in this section, the area marked in all sets of micrographs represents the features as; A for delaminated debris, B for corrugated structure, C for thread type feature, D for molten state, E for shear morphology and F for fractured zircon sand particles. Fig. 10a represents the SEM micrographs of debris of base alloy at 50 °C for 49 N load. Debris (flake-like shape, marked as ‘A’) collected after wear test of the materials are of smaller
size. These wear debris (flakes) indicate that adhesive wear dominates in the sliding direction during the test. Due to adhesive nature at high load, metal is removed in the form of flakes as debris. Small flakes are generated by crushing of longer flakes at high load. Corrugated structure (marked as ‘B’) was observed in some debris, which may occur due to constant rubbing between contacting surfaces of sample and counterface. As the temperature is increased to 100 °C, the size of flakes becomes larger (Fig. 10b), which indicates that as temperature is increased, matrix becomes soft and wear resistance of the matrix is lowered. An increase in the amount of corrugated structured debris (marked as ‘B’) was observed in debris gathered in tests at 100 °C temperature. The elongated delaminated flakes (marked as ‘A’) in debris show grooves that indicate that they come out due to shearing of layer in the direction of applied force. At a much higher temperature, around 150 °C or higher, the alloy shows an onset of transition from mild to severe wear. The size and amount of debris collected are much greater than at lower temperature. At high temperature (Fig. 10c–f), thread type morphology (marked as ‘C’) is found, along with the presence of delaminated plate like debris with microcracks. Thread type debris are created during the pulling out of aluminum matrix in tests at high load and high temperature. The size of debris at high temperature, more than 150 °C (Fig. 10d–f), indicates that severe wear dominates for the alloy at this condition (high temperature and high load). In Fig. 10d some debris that solidified from the molten state (marked as ‘D’) was also observed at higher temperature (200 °C). Its amount and also the amount of delaminated debris was increased at 250 °C and 300 °C temperatures (Fig. 10e–f). Some thread like debris corresponding to α Al are trapped in between the grooves and acquire round shape. Due to continuous rubbing action, these get heated and reach the melting point of metal. This can be seen in Fig. 10 (e and f) (marked as ‘D’). At high temperature, the presence of partially molten state of debris may contribute to an increase in the wear rate of the composite. Fig. 11(a–f) shows the surface morphology of debris collected from LM13/15%Zr composite tested at 49 N load at different temperature from 50 °C to 300 °C. Fig. 11 indicates that the amount and size of wear debris of composite reduced in comparison to that of the matrix specimens (Fig. 10) at most temperatures. This is due to the improvement in the wear resistance of the composite after addition of zircon sand as shown in Fig. 6. As the zircon sand particles have low coefficient of thermal expansion as compared to LM13 alloy, their presence in matrix improves the overall thermal stability of composite [25]. Hence composites containing a higher amount of zircon sand in the matrix will show better thermal resistance in comparison to base materials. Among the studied composites, LM13/15% Zr composite shows better wear and thermal resistance. SEM image of debris collected after the wear test as shown in Fig. 11a gives idea about the wear mechanism of LM13/15%Zr composite at 50 °C with 49 N load. The reduced size of debris of the composite as compared to the base alloy is due to higher hardness of composite, which improves the wear resistance of material. Delamination, multiple plowing action and shearing features (marked as ‘E’) were observed as shown in Fig. 11a. Fragmentation of debris was observed, which may be due to continuous rubbing of delaminated flakes between the contacting surfaces. Debris trapped in wear tracks leads to a
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Fig. 9 – Dot X-ray mapping of wear track of the LM13/15%Zr composite tested at 200 °C with 49 N Load.
corrugated structure. Some fractured zircon sand particles were also observed (marked as ‘F’) as shown in Fig. 11(a–f). Dwivedi [26] suggested that a wear particle is detached due to tangential forces acting on surfaces causing nucleation of cracks at surface, which propagate in due course of time causing exposure of fresh surface followed by delamination. Fracture of particulates may arise from either strain transfer to the reinforcement interface from the matrix undergoing plastic deformation in the wear zone or from cracks initiated when the hard particulates are ground against each other and crushed by striking asperities on the opposing steel counterface. When temperature was increased to 100 °C and 150 °C, the size of debris was increased in comparison to previous ones. Fractured zircon sand particles (marked as ‘F’) and corrugated structured debris (marked as ‘B’) with delaminated flakes were also observed as shown in Fig. 11(b & c). SEM images of debris collected at much higher temperature (Fig. 11c–f) indicate smaller size of debris in presence of fractured particles and
molten state of portions of the matrix. At elevated temperatures, cavities or voids created by particulates fracturing in the subsurface wear zone are liable to be healed by extrusion of the matrix alloy into the region formed between newly fractured particulates [27,20]. The combined process of particulate fracture and matrix extrusion in the subsurface wear region is responsible for the formation of the characteristic comminuted particulate transfer layers (mechanically mixed layer), which are highly beneficial to matrix composite wear. The X-ray dot mapping confirms such type of transition (Fig. 9). The step clearly observed (marked as B) in Fig. 11(e & f), may be due to slip to the Al and zircon sand interfaces due to constant rubbing between the wear surfaces [28]. It is also observed that these debris particles undergo multiple plowing action, causing cracking in them, which can be seen in delaminated debris marked as A in Fig. 11e. Fig. 12(a and b) shows the EDS analysis of debris collected after the wear testing of the LM13/15% Zr composite at 250 °C
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Fig. 10 – SEM micrographs of wear debris of LM13 alloy collected in tests at high (49 N) load with variation of temperature (a) 50 ºC, (b) 100 ºC, (c) 150 ºC, (d) 200 ºC, (e) 250 ºC and (f) 300 ºC.
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 2 0 0–2 1 3
Fig. 11 – SEM images of wear debris of LM13/15% Zr composite collected in tests at 49 N load with variation temperatures of (a) 50 °C, (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C and (f) 300 °C.
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Fig. 12 – EDS analysis of the wear debris LM13/15%Zr composite tested at high load (49 N).
temperature. EDS analysis of these debris indicate the presence of Al, Si, Fe and O elements. Fig. 12a shows the EDS spectra of the delaminated debris and Fig. 12b shows the EDS spectra of spherical debris which is trapped inside the grooves and got melted due to multiple rubbing action during the wear test. Fig. 12a also shows the presence of Fe in the debris collected after wear testing. The presence of Fe in the debris may be due to the transfer of steel counterface material to the composite surface. The transfer of steel inclusions from counterface surfaces to the composite wear surfaces is another mechanism which contributes to increase in wear resistance of the composites. Here inclusions may act as additional reinforcement at the wearing surface of the composite [20]. Moreover, at higher temperature, the delaminated debris also get welded to the surface as shown in Fig. 8e, causing negative or low wear. The X-ray diffraction pattern of debris collected after the wear test conducted at 49 N load and 300 °C is shown in Fig. 13. Fig. 13 also indicates the presence of Al2SiO5, Si and ZrSiO4 phase along with Fe. This indicates that apart from base metal, zircon is also coming out from the surface.
4.
behavior under different temperature. Microstructural analysis and wear tests were conducted on the specimens. From the experimental results the following conclusions were drawn: (1) A fairly uniform distribution of reinforcing particles was observed in the microstructural study. Good interfacial
Conclusion
Stir cast LM13 alloy and zircon sand particles reinforced composites were studied to get idea about the dry sliding wear
Fig. 13 – XRD pattern of debris of LM13/15% Zr composite tested at 49 N load and 300 °C.
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(2) (3)
(4) (5)
(6)
(7)
bonding between zircon sand particles and matrix was also observed. Hardness of the composite material increased with an increase in amount of reinforced zircon sand. Wear resistance of the composite materials increases with an increase in reinforcement of zircon sand, although an increase in zircon sand content from 15 wt.% to 20 wt.% does not result in an appreciable change in wear rate. Wear rate of the both matrix and composite increased with an increase in applied load. The effect of temperature on wear was also studied at different loads. Composites with higher concentration of zircon sand show better wear resistance and thermal stability. An improvement in critical transition temperature of alloy from 100 °C to 200 °C for composites was observed with addition of zircon sand. The mircostructral study of specimen after wear test concludes that both adhesive and abrasive wear mechanisms contribute for wear of composites.
Acknowledgment The authors are thankful to Armament Research Board (ARMREB), Defence Research and Development Organization (DRDO), India for providing financial support under letter no. ARMREB/MAA/2008/105 for this study. Authors are also thankful to the reviewer for his critical comments and helping us to improve the manuscript.
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