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Influence of graphite content on the dry sliding and oil impregnated sliding wear behavior of Al 2024–graphite composites produced by in situ powder metallurgy method
Wear 266 (2009) 37–45
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Influence of graphite content on the dry sliding and oil impregnated sliding wear behavior of Al 2024–graphite composites produced by in situ powder metallurgy method F. Akhlaghi ∗ , A. Zare-Bidaki School of Metallurgy and Materials Engineering, Faculty of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
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Article history: Received 9 July 2007 Received in revised form 6 May 2008 Accepted 27 May 2008 Available online 23 July 2008 Keywords: Aluminum–graphite composite In situ powder metallurgy Graphite content Dry sliding Oil impregnated sliding Transition load
a b s t r a c t The influence of graphite content on the dry sliding and oil impregnated sliding wear characteristics of sintered aluminum 2024 alloy–graphite (Al/Gr) composite materials has been assessed using a pin-ondisc wear test. The composites with 5–20 wt.% flake graphite particles were processed by in situ powder metallurgy technique. For comparison, compacts of the base alloy were made under the same consolidation processing applied for Al/Gr composites. The hardness of the sintered materials was measured using Brinell hardness tester and their bending strength was measured by three-point bending tests. Scanning electron microscopy (SEM) was used to analyze the debris, wear surfaces and fracture surfaces of samples. It was found that an increase in graphite content reduced the coefficient of friction for both dry and oil impregnated sliding, but this effect was more pronounced in dry sliding. Hardness and fracture toughness of composites decreased with increasing graphite content. In dry sliding, a marked transition from mild to severe wear was identified for the base alloy and composites. The transition load increased with graphite content due to the increased amount of released graphite detected on the wear surfaces. The wear rates for both dry and oil impregnated sliding were dependent upon graphite content in the alloy. In both cases, Al/Gr composites containing 5 wt.% graphite exhibited superior wear properties over the base alloy, whereas at higher graphite addition levels a complete reversal in the wear behavior was observed. The wear rate of the oil impregnated Al/Gr composites containing 10 wt.% or more graphite particles were higher than that of the base alloy. These observations were rationalized in terms of the graphite content in the Al/Gr composites which resulted in the variations of the mechanical properties together with formation and retention of the solid lubricating film on the dry and/or oil impregnated sliding surfaces. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloys are promising materials in high technology fields owing to their excellent specific mechanical properties. However, their low resistance to wear under poor lubricating conditions and their severe seizure under boundary lubrication conditions are the main obstacles for their high performance tribological applications. In view of this, aluminum alloy–graphite (Al/Gr) particulate composites are being explored for tribological applications. These self-lubricating composites have received attention because of their low friction and wear [1–11], reduced temperature rise at the wearing contact surface [6,12], improved machinability [7], excellent antiseizure effects [8,11,13,14], low thermal expansion and high
∗ Corresponding author. Tel.: +98 912 3728739; fax: +98 21 88006076. E-mail address: [email protected] (F. Akhlaghi). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.05.013
damping capacity [15–17]. Many authors have reported that during dry sliding the metal/Gr composites, a continuous layer of solid lubricant forms on the tribosurface [6,10,11,18–22]. This lubricating film is formed as a result of shearing graphite particles located immediately below the sliding surface of the composite. This graphite-rich lubricant film helps to reduce the magnitude of shear stress transferred to the material underneath the contact area, alleviates the plastic deformation in the subsurface region, prevents a metal-to-metal contact and acts as a solid lubricant between two sliding surfaces. Therefore, it helps in reducing friction and wear and improves seizure resistance of the composite. The formation and retention of this tribolayer on the sliding surface as well as its composition, area fraction, thickness and hardness are important factors in controlling the wear behavior of the material and depend on the nature of the sliding surface, the test condition, environment and the graphite content in the composite. It has been found that [22] by increasing the graphite content in Al/Gr
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composites, richer graphite lubricating film forms on the lubricating surface which result in lower wear rate. However, some reports indicate that with increasing the graphite content, the wear rate increases due to the decreased fracture toughness and hardness of composites [9,11,13,23–26]. Therefore, it is obvious that for any specific Al/Gr composite and test condition, there exist a range of optimal value of graphite particulate content on the basis of minimum friction coefficient as well as minimum wear rates of the specimens. Another advantage of the addition of graphite particles to the aluminum alloys is impeding the transition to higher loads and speeds, thereby delaying the transition to severe wear. Although this beneficial effect of graphite particles in Al/Gr composites has been reported by some investigators [21], but to the knowledge of the authors, no systematic study has been reported on the effect of graphite content on the mild to severe wear rate transition for these composites. The porosity of sintered materials is advantageous in withholding lubricant. The porous structure of these materials, when impregnated with lubricating oil, functions as a reservoir from which the gap between the contact surfaces is filled with lubricant. For example, when these materials are used as journal bearings, in the stationary position, shaft and bush are in contact with each other. When the shaft starts to rotate, it causes friction which increases temperature. Therefore, the lubrication oil within the bush weeps out and creates an oil film between the two surfaces. Once the shaft has stopped, the oil is sucked back into the pores of the bush due to capillary action. Many authors have studied the dry sliding wear behavior of sintered Al/Gr composites, but only a few investigators [27] have studied the sliding characteristics of these composites under lubricated conditions. In the present work, for the first time a new processing technique, termed “in situ powder metallurgy” was used for consolidating the aluminum 2024–graphite particle composites containing 5–20 wt.% graphite particles. In this method, the stir casting and the P/M synthesizing processes were combined into an integrated net shape forming process. It is well known that the production method has a strong influence on the mechanical and tribological properties of such composites via its effects on the matrix grain size, porosity, the distribution of graphite particles and the interfacial properties of the Al/Gr couple. This work aims to evaluate the effect of graphite content on the tribological behavior of Al 2024 composites, made by in situ powder metallurgy method in terms of wear rate and friction coefficient under both dry and oil impregnated wear conditions. In dry sliding, the effect of graphite content on the mild to severe wear rate transition for these composites has also been reported.
Fig. 1. The schematic view of the wear test apparatus used in this study.
constant at 750 ◦ C for 5 min and then it was lowered to 611 ◦ C in 12 min while stirring was continued at the same speed. The non-wettability of un-coated graphite particles with molten aluminum alloy together with the shear forces induced by the impeller result in the melt disintegration and formation of molten droplets distributed among the graphite particles. Finally the charge was evacuated from the crucible into a steel container and the alloy was allowed to solidify at ambient temperature and atmosphere resulting in a mixture of graphite and aluminum powder particles. More details about the in situ powder metallurgy technique are discussed elsewhere [28]. The volume percentage of graphite particles for powder mixtures containing 5, 10, 15 and 20 wt.% graphite particles was calculated as 6.20, 12.27, 18.18 and 23.93%, respectively. The powder particles of 2024 Al alloy were produced by “solid assisted melt disintegration” (SAMD) technique [29,30]. The SAMD technique is basically similar to the in situ powder metallurgy method but instead of graphite, coarse alumina particles are used for melt disintegration. The alumina particles were sieved out from aluminum alloy–alumina powder mixture and the Al powder particles were used for making compacts of the base alloy for the purpose of comparison. The size distribution of the resultant mixture of graphite and aluminum powder particles was determined by laser particle sizing technique. Aluminum–graphite powder mixtures containing different amounts of graphite as well as the base alloy powders were cold pressed at 650 MPa and at a speed of 300 mm/min in a rigid steel die on a single acting 45 t hydraulic
2. Materials and experimental procedures The experimental Al/Gr composites were produced from bulk 2024 aluminum alloy and graphite powder by in situ powder metallurgy technique [28]. The chemical composition of 2024 alloy was analyzed as: 4.62Cu, 1.74Mg, 0.65Mn, 0.27Fe, 0.23Si and balanced Al in weight percents. The liquidus and solidus temperatures of this alloy quantified by thermal analyzing technique are 638 and 502 ◦ C, respectively. The in situ powder metallurgy technique involved melting weighted amounts of the alloy in a clay-bonded graphite crucible of 2-kg capacity using an electrical resistance furnace. Then the temperature of the melt was raised to about 750 ◦ C and specific quantities of un-coated graphite particles (with a purity of 98.7%) were added to the melt. The melt was subsequently stirred at 400 rpm using a graphite impeller attached to a variable speed motor. The temperature of the furnace was kept
Fig. 2. The size distribution of the as-received graphite particles.
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Fig. 3. Typical SEM micrograph of the as produced mixture of aluminum and 15 wt.% graphite powder particles within the size range of 106–150 m.
press resulting in cylinders with 25 mm in diameter, 15 mm in height and 92–96% of the theoretical density. The green compacts were sintered in a tubular furnace in a nitrogen atmosphere to provide protection against oxidation of the powder. It was found that the optimum sintering conditions depended on the graphite content in the compact. Therefore, compacts with a graphite content of 15 wt.% or less, were sintered at 600 ◦ C for 30 min and those with 20 wt.% graphite were sintered at 610 ◦ C for 60 min. The samples were solution treated at 495 ◦ C for 3 h prior to cold water quenching and artificially aged at 170 ◦ C for 5 h before air cooling to room temperature. Composite samples were polished according to standard metallographic techniques for microstructural characterization. The density of the base alloy compact and Al/Gr composites with different graphite contents was determined using Archimedes’ principle. The measured density was compared to the value obtained using Rule-of-Mixtures so as to determine the volume fraction of porosity. The densities of graphite and 2024 Al alloy were considered to be 2.2 g/cm3 and 2.79 g/cm3 , respectively. The samples were precision weighed in an electronic balance to an accuracy of 0.1 mg. Hardness measurements were carried out on a Brinell hardness testing machine, using a load of 300 N, and the mean values of at least five measurements conducted on different areas of each sample was considered. The specimens for three-point bending test were prepared from Al/Gr powder mixtures containing various amounts of graphite as well as the monolithic 2024 alloy powders by cold pressing in a steel die having the internal dimensions of 3 mm × 4 mm × 45 mm at 650 MPa. These compacts were sintered and heat treated with the same procedures as used for cylindrical samples. The bending tests were carried out on the as-produced samples (without machining), at room temperature on a 10 t Instron machine according to ASTM C1161-94 standard method. In order to ascertain reproducibility, at least three measurements were typically made for each type of sample. Dry sliding pin-on-disc wear tests were carried out in a laboratory atmosphere at 50–60% relative humidity and the temperature around 25 ◦ C on the heat treated composite and un-reinforced samples. The schematic view of the wear test apparatus is shown in
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Fig. 1. The rotating test material in the form of discs of diameter 25 mm and height 10 mm were slid against a steel pin (1.5Cr, 1C, 0.35Mn, 0.25Si) with the hardness of 64HRC having the diameter of 8 mm and height of 12 mm. Wear tests were undertaken under the normal load of 50 N (resulting in a normal pressure of 1 MPa), the sliding velocity of 0.5 ms−1 and the total sliding distance of 1000 m. The wear tests were carried out in dry and lubricated conditions using a wear track diameter of 180 mm. The track diameter was kept constant for all the experiments to eliminate this as a further variable of the rubbing system. The mass of each specimen was measured using an electronic balance having a resolution of 0.1 mg, before and after each wear test and the mass difference was then converted to volumetric wear rates using the measured density of each material and the total sliding distance. Friction coefficient measurements were made using a transducer to measure the deflection of the pin holder caused by the disc rotation. The system was calibrated by applying known tangential loads and noting pin deflection. For the oil impregnated tests, SAE 10 engine oil having kinematic viscosity of 175 mm2 s−1 and 14.5 mm2 s−1 at 40 and 100 ◦ C, respectively was incorporated into the porous samples by submerging them into 250 ml of oil in a beaker for a fixed time without circulation, heating or pressurizing. The oil had the following chemical composition (in wt.%): N 0.45, Ca 0.11, P 0.03 and Zn 0.03 with a sulfated ash content of 0.4 wt.%. To obtain the oil content of the impregnated samples, they were weighted before and after specific hours of remaining in oil. The impregnated samples were wiped off to remove any trace of lubricant left externally. This procedure was repeated in 4 h intervals and it was noted that the weight gain of the samples ceased after 24 h. The volume of the impregnated oil was calculated by using the mass and measured density of oil (0.76 g cm−3 ). By considering the porosity of each sample and the volume of impregnated oil, it was revealed that more than 90% of the pores were filled with oil indicating the interconnectivity of the majority of the pores. The morphology of the pores were determined by optical microscopy of the polished surfaces of composites. The oil impregnated samples were also subjected to wear test under the same conditions used for dry sliding tests. In order to investigate the effect of graphite content on the mild to severe wear rate transition for these composites, compacts of the base alloy as well as the Al/Gr composites containing different amounts of graphite particles were subjected to dry sliding wear tests under identical conditions as mentioned before, but varying loads. A marked transition from a mild wear regime to a severe wear regime was detected for each sample at a certain load. The severe wear was accompanied by a sudden increase in wear rate, heavy noise and vibration during testing. Such tests were of very
Fig. 4. The cumulative size distribution of Al + graphite mixtures containing different weight percentages of graphite particles.
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Fig. 5. Typical micrographs of Al–15 wt.% Gr composite: (a) SEM of polished surface, (b) optical micrograph of polished surface, and (c) SEM of fracture surface.
short duration, not exceeding 1 min, due to immediate wearing of the samples. Scanning electron microscopy (SEM) was used to analyze the morphology of the powder mixture as well as the polished and fractured surfaces of composites. Also for determination of wear behavior of the 2024 aluminum matrix and the composites, the worn surfaces and debris particles were examined by SEM, where the specimens were gold coated for 2 min before examination. The wear debris created by oil impregnated sliding was difficult to collect because of the very small amount produced. 3. Results and discussion The size distribution of as-received graphite particles as measured by a laser diffraction method is shown in Fig. 2 and exhibits
the average size (D50 ) of 55 m and maximum size of 160 m. A typical SEM micrograph of the as produced mixture of aluminum and 15 wt.% graphite powder particles within the size range of 106–150 m, is shown in Fig. 3. It can be seen that the graphite particles are well distributed within the aluminum powder particles and no aggregates of the graphite particles can be seen in the mixture. The cumulative size distribution of Al + graphite mixtures containing different weight percentages of graphite particles are graphically presented in Fig. 4. Fig. 4 shows that the average size of the aluminum–graphite powder mixture containing 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% graphite particles varies in the range 80–190 m. Fig. 5(a) and (b) are the typical SEM and optical micrographs of the polished surfaces of Al–15 wt.% Gr composite, respectively. The dark regimes shown in Fig. 5(a) represent the pores or voids which were left behind by evacuation of graphite
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Fig. 6. The variation of the porosity of the samples with the weight percent of graphite particles.
particles from the surfaces during the polishing process. Fig. 5(c) shows a typical micrograph of the fracture surface of Al–15 wt.% Gr composite. It can be seen that the graphite particles are distributed uniformly within the matrix alloy. This uniform distribution of the second phase within the matrix alloy is a characteristic of the in situ powder metallurgy method [28], and improves the mechanical and tribological properties of the composite due to lack of the graphite clusters. The variation of the porosity of the samples with the weight percent of graphite particles is graphically presented in Fig. 6. Fig. 6 shows that the porosity of the aluminum–graphite composites containing different amounts of graphite particles varies in the range 3–8%. The decreased hardness with the weight percent of graphite in the composites is shown in Fig. 7. The effect of the graphite addition on the fracture energy of the composites obtained from three-point bending tests as shown in Fig. 8 indicates that the bending strength decreases as the graphite content increases. Generally, addition of graphite to aluminum alloys is known to decrease the strength [23–25], fracture energy [11], ductility [23–25] and hardness [9,13,26,31] of the material. The increased amount of the brittle graphite particles together with the increased tendency of crack initiation and propagation at the graphite/metal interface are responsible for these effects. The variation in the measured wear rate and coefficient of friction with the weight percent of graphite in the composites for both dry sliding and oil impregnated sliding are shown in Figs. 9 and 10, respectively. It can be seen that the dry sliding wear rate of the Al 2024–5 wt.% graphite is about 10 times lower than that for the base alloy. However, for composites with 10 wt.% or more graphite particles addition, the wear rate increases. These results are consistent with the trends reported by some investigators [32–34] who found that Al/Gr composites containing small amounts of graphite (2–5 wt.%) posses superior wear
Fig. 7. The variation of hardness with the weight percent of graphite in the composites.
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Fig. 8. The effect of the graphite addition on the fracture energy of the composites obtained from three-point bending tests.
Fig. 9. The variation in the measured wear rate with the weight percent of graphite in the composites for both dry sliding and oil impregnated sliding.
properties over the base alloy, whereas at higher graphite addition levels a complete reversal in the wear behavior was observed. This dropping of the wear rate for a certain amount of graphite addition may have been caused by the following two competing factors. First, the beneficial effect of the graphite addition in reducing the wear of the composites due to formation of a thin lubricating graphite rich film on the tribosurface [7,10,11,35]; and secondly, the adverse effects of graphite addition in formation of porosity and cracks [19] as well as the deterioration of mechanical properties [12,19,23,24,34] resulting in enhanced delamination [19]. It must be noted that some reports [8,9,11] indicate that the
Fig. 10. The variation in the measured coefficient of friction with the weight percent of graphite in the composites for both dry sliding and oil impregnated sliding.
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Fig. 11. The worn surface morphology of (a) the base alloy and composites containing (b) 5 wt.% and (c) 15 wt.% of graphite particles tested under dry sliding conditions. The arrows show the direction of sliding.
wear rate of Al/Gr composites initially increases as the amount of graphite addition increases up to certain levels (i.e. 5 vol.%), then drops to lower values. However, the decreased wear rate with increased graphite content (0–15 wt.%) was observed in our previous research [28] for A356 aluminum/graphite composites and has also been reported by others [2–5,7]. These conflicting results on the dry wear characteristics of Al/Gr composites may have been caused by this fact that the efficiency of solid lubrication by graphite in Al/Gr composites depends on a number of factors such as wear testing conditions [5,32,36] and especially the applied pressure [5,19,37]. Also the morphology, shape, size [5,38,39] and the distribution of graphite particles as well as the nature of the particle matrix interfacial bonding together with the matrix microstructure [3,18,35] influence the wear characteristics of the composite.
As shown in Fig. 10, for dry sliding, the friction coefficients starts off at 0.35 for the base alloy and decreases with increased graphite content reaching to a final value of about 0.12 for composites containing 15 wt.% graphite which is about one-third that of the base alloy. However, Fig. 10 shows that there is no significant difference in friction coefficient between composites with 15 wt.% and that with 20 wt.% graphite addition. The reason for this reduction in coefficient of friction could also be attributed to the presence of the smeared graphite layer at the sliding surface of the wear sample which acts as a solid lubricant. This lubricant film prevents direct contact of the two surfaces. Fig. 11 exhibits the worn surface morphology of the base alloy and composites containing different amounts of graphite particles tested under dry sliding conditions. During wear, the base alloy surface is subjected to severe plastic deformation and abrasive
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Fig. 12. SEM micrographs of the collected debris from (a) base alloy and (b) A1–5 wt.% graphite particles generated under dry contacts.
wear (Fig. 11a). However, in contrast to the surface morphology of the base alloy, the worn surface of Al 2024–5 wt.% graphite is covered with a black film and the grooves are smaller in comparison to the un-reinforced alloy and are filled with debris particles. Fig. 11c clearly shows that for Al 15 wt.% graphite, the smeared layer becomes thicker and denser due to the increased graphite content. In fact, after wear test of this composite, most of the worn surface is covered uniformly by the graphite lubricating film which can prevent direct contact between the pin and the counterface. This layer effectively reduces the friction coefficient. However, with increasing the amount of graphite content to 20 wt.%, the area fraction and/or thickness of the lubricating film has not changed significantly so that the friction coefficient has remained unchanged. In fact, for composites with 15 wt.% graphite, the coefficient of friction about 0.12 under dry sliding conditions is comparable to and
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even lower than that of graphite of about 0.2 and cannot be further decreased by increased amount of graphite content. It has been found that the thickness of the graphite rich layer at the sliding surface plays an important role in the wear behavior of composites [40]. As the amount of graphite addition increases, more graphite is released to the wear surface during the wear process. This film is formed as a result of shearing of graphite particles located immediately below the contact surface. As shown in Fig. 11, richer graphite lubricating film can be noted for higher graphite content of the composites. Also an optimum amount of graphite is needed to keep the film continuous. This may be attributed to variations in the area covered by smeared graphite during sliding. The decreased coefficient of friction in Al/Gr composites due to increased addition of graphite particles has also been observed by many workers [1,4,5,7,8,10,11,15,20,22,37,41]. The investigation of collected debris from base alloy samples generated under dry contacts revealed very large, irregular profiles and unequal dimensions as those shown in Fig. 12(a). The generation of this kind of debris can be attributed to an abrasive micro-cutting effect. However, the wear debris becomes smaller for Al/Gr composites as shown in Fig. 12(b). This decrease in the size of the debris is consistent with those reported by other researchers [9,11,22,27] and is mainly a result of the increase in the brittleness of the composites together with the decreased probabilities of direct contacts of two worn surfaces which decreases the severity of micro-cutting effects. Fig. 13 shows the effect of graphite content on the transition load for onset of severe wear regime for the un-reinforced alloy and composites. In the case of un-reinforced alloy, severe wear regime occurred at loads above 0.5 MPa. However, incorporation of graphite particles into 2024 Al alloy significantly served to suppress the transition to a severe wear rate regime and impeded the transition to higher loads. Fig. 13 shows that the transition load increases with increasing graphite content leading to a sixfold increase for Al–20 wt.% graphite compared to the base alloy. Graphite particles assist in the formation and retention of the solid lubricating film on the composite sliding surface which prevent metal-to-metal contact and keep wear behavior within the mild regime. As was mentioned previously, with increasing the amount of graphite content, the thickness and the area of the lubricating film covered by smeared graphite during dry sliding also increases. This in turn lowers the coefficient of friction in composites and reduces the shear stress transmitted to the bulk subsurface material underneath the tribolayer. Therefore, the ability of the composite to withstand higher applied normal loads during wear is also increased. These results confirm that the high content graphite composites are clearly superior to the base alloy in delaying the transition to severe wear and hence provide the best seizure resistance against steel.
Fig. 13. The effect of graphite content on the transition load for onset of severe wear regime for the un-reinforced alloy and composites.
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It is evident from Figs. 9 and 10 that the wear rate and friction coefficient of the oil impregnated base alloy are considerably lower than those of the dry sliding. The wear rate of the oil impregnated base alloy is approximately four times lower than that for the dry sliding. These obvious results are generally expected from the oil impregnated sliding and can be attributed to formation of an oil film between the two contacting surfaces. The effect of 5 wt.% graphite particulate addition into the Al alloy was a slight decrease in the wear rate in the oil impregnated sliding. Again, similar to dry sliding, the wear weight loss increased with increasing content of graphite particles in oil impregnated sliding. The occurrence of a minimum in the wear rate for the oil impregnated samples are similar to those observed for dry sliding and can also be attributed to formation of an effective tribolayer for 5 wt.% graphite addition and deterioration of mechanical properties for increased amount of graphite content. It is interesting to note that except for the base alloy, the wear rates of Al/Gr composites containing different amounts of graphite are always higher for oil impregnated sliding as compared with dry contacts. Similar results have been reported by Prasad [42] who investigated the presence of oil plus graphite lubricants on the wear behavior of a zinc-based alloy reinforced with SiC particles and examined the effect of different amounts of graphite as a solid lubricant on the performance of the oil lubricant towards controlling the wear characteristics of the base alloy and composite material. For conducting lubricated wear tests, he prepared a series of SAE 40 oil plus graphite lubricant mixtures by thoroughly mixing the graphite particles (size 50–100 m) in varying concentrations ranging from 0 to 10 wt.%. He found that the benefits of graphite addition to the lubricating oil in decreasing friction coefficient of the tested materials could be realized up to its specific concentration only (i.e. 4 wt.%) and at higher concentrations of graphite in the lubricant mixture, he observed the reversed trend. He attributed these results to a mixed influence of the graphite content on the frictional heating of the contact surface. Therefore he concluded that although increasing content of graphite in the oil decreased the severity of frictional heating initially, but an opposite trend could be noticed at higher graphite contents. Other investigators [43] have also reported about the destructive effects of solid particles on the lubricated wear resistance of aluminum MMCs under lubricated sliding. However, in our experiments the graphite particles were not initially mixed with the lubricating oil but they were protruded out the polished surface of the composite and contribute to the weight loss. In addition, it was observed that the presence of oil in the contact area prohibited the formation and retention of the graphite lubricating film and the graphite particles released from the composite material were displaced from the contact area and were pushed to a corner on the sliding surface. Therefore, instead of formation of a relatively thick and dense graphite layer on the contact area, the removal of graphite particles from the body of the composite material continued resulting in increased wear rate as compared to dry sliding. In fact, the lubrication performance of graphite is considerably different for dry sliding and oil impregnated sliding. As was mentioned before, in dry sliding, the reason for the decreased wear rate and coefficient of friction of Al/Gr composites as compared with the base alloy is attributable to the presence of the smeared graphite layer at the sliding surface of the wear sample which acts as a solid lubricant. However, the mechanism of the wear and friction of oil impregnated Al/Gr composites is more complicated than that of dry sliding. The increased amount of graphite particles in the composites and thereby the increased graphite particles suspended in the lubricating oil at the sliding surface may be responsible for the slight decrease in the coefficient of friction of the samples containing more graphite particles. Fig. 10 shows that for the base alloy and Al/Gr composites containing various graphite additions, the coefficient of friction is lower
in the oil impregnated sliding as compared to that of dry sliding. However, in oil impregnated sliding, the use of graphite was not significantly beneficial in lowering the coefficient of friction. As shown in Fig. 9, for both dry sliding and oil impregnated sliding, the wear rate of the Al–5 wt.% graphite composite is lower than that of the base material and all the other composites. However, the oil impregnated Al–5 wt.% composite may exhibit superior tribological properties as compared to dry sliding since the debit in frictional properties is larger than the very small benefit in wear rate reduction. 4. Conclusion (1) The “in situ powder metallurgy” technique can be used to produce 2024 aluminum alloy/5–20 wt.% flake graphite particulate composites in which the graphite particles are distributed uniformly within the matrix alloy. The hardness and bending strength of these composites decreased with increasing graphite content attributable to the increased amount of the brittle graphite particles together with the increased tendency of crack initiation and propagation at the graphite/metal interface. (2) The Al/Gr composites exhibited a lower frictional coefficient and wear rate than the base alloy for both dry and oil impregnated sliding. However, for the base alloy and Al/Gr composites containing various graphite additions, the coefficient of friction in the oil impregnated sliding was not affected by the amount of graphite and was lower as compared to that of dry sliding attributable to formation of an oil film between the two contacting surfaces. (3) For both dry sliding and oil impregnated sliding, at low addition levels (5 wt.%) graphite improved the wear resistance. However, for composites with 10 wt.% or more graphite particles addition, the wear rate increased. These observations were explained in terms of two competing factors. First, the beneficial effect of the graphite addition due to formation of a thin lubricating graphite rich film on the tribosurface; and secondly, the adverse effects in formation of cracks as well as the deterioration of mechanical properties resulting in enhanced delamination. (4) In contrast to the base alloy, the wear rates of Al/Gr composites containing different amounts of graphite were always higher for oil impregnated sliding as compared with dry contacts. These results are attributed to prohibited formation and retention of the graphite lubricating film at the presence of oil in the contact area due to displacement and pushing of the suspended graphite particles on the sliding surface. (5) In dry sliding, the increased transition loads observed for the increased graphite contents was attributed to the formation and retention of thicker solid lubricating film covering more area on the composite sliding surface. This tribolayer prevented metalto-metal contact, lowered the coefficient of friction and reduced the shear stress transmitted to the bulk subsurface material. References [1] P.R. Gibson, A.J. Clegg, A.A. Das, Wear of cast Al–Si alloys containing graphite, Wear 95 (2) (1984) 193–198. [2] P.K. Rohatgi, Y. Liu, M. Yin, T.L. Barr, A surface analytical study of triboformed aluminum alloy 319–10 vol.% graphite particle composite, Mater. Sci. Eng. 123A (1990) 213–218. [3] S. Das, S.V. Prasad, T.R. Ramachandran, Tribology of Al–Si alloy–graphite composites: triboinduced graphite films and the role of silicon morphology, Mater. Sci. Eng. 138A (1991) 123–132. [4] Y.B. Liu, S.C. Lim, S. Ray, P.K. Rohatgi, Friction and wear of aluminum–graphite composites: the smearing process of graphite during sliding, Wear 159 (1992) 201–205. [5] P.K. Rohatgi, S. Ray, Y. Liu, Tribological properties of metal matrix–graphite particle composites, Int. Mater. Rev. 37 (3) (1992) 129–149.
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[26] B.P. Krishan, M.K. Surappa, P.K. Rohatgi, The UPAL process: a direct method of preparing cast aluminum alloy–graphite particle composites, J. Mater. Sci. 16 (1981) 1209–1216. [27] H.Y. Chu, J.F. Lin, Experimental analysis of the tribological behavior of electroless nickel-coated graphite particles in aluminum matrix composites under reciprocating motion, Wear 239 (2000) 126–142. [28] F. Akhlaghi, S.A. Pelaseyyed, Characterization of aluminum/graphite particulate composites synthesized using a novel method termed “in-situ powder metallurgy”, Mater. Sci. Eng. A 385 (2004) 258–266. [29] F. Akhlaghi, H. Esfandiari, Solid-assisted melt disintegration (SAMD), a novel technique for metal powder production, Mater. Sci. Eng. A 452/453 (2007) 70–77. [30] F. Akhlaghi, H. Esfandiari, Aluminium powder particles produced by SAMD technique: typical characteristics and correlations between processing conditions and powder size, Mater. Sci. Technol. 23 (6) (2007) 646– 652. [31] A.K. Jha, S.V. Prasad, G.S. Upadhyaya, Preparation and properties of 6061 aluminum alloy/graphite composites by PM route, Powder Metall. 32 (4) (1989) 309–312. [32] S.F. Mustafa, Casting of graphite Al–Si base composites, Metall. Q. 33 (3) (1994) 259–264. [33] J.U. Ejiofor, R.G. Reddy, Developments in the processing and properties of particulate Al–Si composites, J. Miner. 49 (11) (1997) 31–37. [34] B.P. Krishan, P.K. Rohatgi, Modification of Al–Si alloy melts containing graphite particle dispersions, Met. Technol. 11 (1984) 41–44. [35] B.K. Prasad, S. Das, The significance of the matrix microstructure on the solid lubrication characteristics of graphite aluminum alloys, Mater. Sci. Eng. A 144 (1991) 229–235. [36] S.K. Biswas, B.N.P. Bai, Dry wear of Al–graphite particle composites, Wear 68 (1981) 347–358. [37] S.C. Lim, M. Gupta, W.B. Ng, Friction and wear characteristics of Al–Cu/C composites synthesized using partial liquid phase casting process, Mater. Des. 18 (3) (1997) 161–166. [38] U.T.S. Pillai, B.C. Pai, K.G. Satyanarayana, A.D. Damodaran, Fracture behaviour of pressure die-cast aluminum–graphite composites, J. Mater. Sci. 30 (1995) 1455–1461. [39] K. Komuro, M. Suwa, K. Sotene, M. Obesawa, US Patent 4,383,970 (1980). [40] B. Venkataraman, G. Sundararajan, Correlation between the characteristics of the mechanically mixed layer and wear behaviour in aluminium Al-7075 alloy and Al-MMCs, Wear 245 (2000) 22–39. [41] P.K. Rohatgi, Y. Liu, T.L. Barr, Tribological behavior and surface analysis of triboformed Al alloy-50 pct graphite particle composites, Metall. Trans. 22A (1991) 1435–1441. [42] B.K. Prasad, Investigation into sliding wear performance of zinc-based alloy reinforced with SiC particles in dry and lubricated conditions, Wear 262 (2007) 262–273. [43] H.H. Fu, K.S. Han, J.I. Song, Wear properties of Saffil/Al, Saffil/Al2 O3 /Al and Saffil/SiC/Al hybrid metal matrix composites, Wear 256 (2004) 705– 713.
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Influence of graphite content on the dry sliding and oil impregnated sliding wear behavior of Al 2024–graphite composites produced by in situ powder metallurgy method F. Akhlaghi ∗ , A. Zare-Bidaki School of Metallurgy and Materials Engineering, Faculty of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
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Article history: Received 9 July 2007 Received in revised form 6 May 2008 Accepted 27 May 2008 Available online 23 July 2008 Keywords: Aluminum–graphite composite In situ powder metallurgy Graphite content Dry sliding Oil impregnated sliding Transition load
a b s t r a c t The influence of graphite content on the dry sliding and oil impregnated sliding wear characteristics of sintered aluminum 2024 alloy–graphite (Al/Gr) composite materials has been assessed using a pin-ondisc wear test. The composites with 5–20 wt.% flake graphite particles were processed by in situ powder metallurgy technique. For comparison, compacts of the base alloy were made under the same consolidation processing applied for Al/Gr composites. The hardness of the sintered materials was measured using Brinell hardness tester and their bending strength was measured by three-point bending tests. Scanning electron microscopy (SEM) was used to analyze the debris, wear surfaces and fracture surfaces of samples. It was found that an increase in graphite content reduced the coefficient of friction for both dry and oil impregnated sliding, but this effect was more pronounced in dry sliding. Hardness and fracture toughness of composites decreased with increasing graphite content. In dry sliding, a marked transition from mild to severe wear was identified for the base alloy and composites. The transition load increased with graphite content due to the increased amount of released graphite detected on the wear surfaces. The wear rates for both dry and oil impregnated sliding were dependent upon graphite content in the alloy. In both cases, Al/Gr composites containing 5 wt.% graphite exhibited superior wear properties over the base alloy, whereas at higher graphite addition levels a complete reversal in the wear behavior was observed. The wear rate of the oil impregnated Al/Gr composites containing 10 wt.% or more graphite particles were higher than that of the base alloy. These observations were rationalized in terms of the graphite content in the Al/Gr composites which resulted in the variations of the mechanical properties together with formation and retention of the solid lubricating film on the dry and/or oil impregnated sliding surfaces. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Aluminum alloys are promising materials in high technology fields owing to their excellent specific mechanical properties. However, their low resistance to wear under poor lubricating conditions and their severe seizure under boundary lubrication conditions are the main obstacles for their high performance tribological applications. In view of this, aluminum alloy–graphite (Al/Gr) particulate composites are being explored for tribological applications. These self-lubricating composites have received attention because of their low friction and wear [1–11], reduced temperature rise at the wearing contact surface [6,12], improved machinability [7], excellent antiseizure effects [8,11,13,14], low thermal expansion and high
∗ Corresponding author. Tel.: +98 912 3728739; fax: +98 21 88006076. E-mail address: [email protected] (F. Akhlaghi). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.05.013
damping capacity [15–17]. Many authors have reported that during dry sliding the metal/Gr composites, a continuous layer of solid lubricant forms on the tribosurface [6,10,11,18–22]. This lubricating film is formed as a result of shearing graphite particles located immediately below the sliding surface of the composite. This graphite-rich lubricant film helps to reduce the magnitude of shear stress transferred to the material underneath the contact area, alleviates the plastic deformation in the subsurface region, prevents a metal-to-metal contact and acts as a solid lubricant between two sliding surfaces. Therefore, it helps in reducing friction and wear and improves seizure resistance of the composite. The formation and retention of this tribolayer on the sliding surface as well as its composition, area fraction, thickness and hardness are important factors in controlling the wear behavior of the material and depend on the nature of the sliding surface, the test condition, environment and the graphite content in the composite. It has been found that [22] by increasing the graphite content in Al/Gr
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composites, richer graphite lubricating film forms on the lubricating surface which result in lower wear rate. However, some reports indicate that with increasing the graphite content, the wear rate increases due to the decreased fracture toughness and hardness of composites [9,11,13,23–26]. Therefore, it is obvious that for any specific Al/Gr composite and test condition, there exist a range of optimal value of graphite particulate content on the basis of minimum friction coefficient as well as minimum wear rates of the specimens. Another advantage of the addition of graphite particles to the aluminum alloys is impeding the transition to higher loads and speeds, thereby delaying the transition to severe wear. Although this beneficial effect of graphite particles in Al/Gr composites has been reported by some investigators [21], but to the knowledge of the authors, no systematic study has been reported on the effect of graphite content on the mild to severe wear rate transition for these composites. The porosity of sintered materials is advantageous in withholding lubricant. The porous structure of these materials, when impregnated with lubricating oil, functions as a reservoir from which the gap between the contact surfaces is filled with lubricant. For example, when these materials are used as journal bearings, in the stationary position, shaft and bush are in contact with each other. When the shaft starts to rotate, it causes friction which increases temperature. Therefore, the lubrication oil within the bush weeps out and creates an oil film between the two surfaces. Once the shaft has stopped, the oil is sucked back into the pores of the bush due to capillary action. Many authors have studied the dry sliding wear behavior of sintered Al/Gr composites, but only a few investigators [27] have studied the sliding characteristics of these composites under lubricated conditions. In the present work, for the first time a new processing technique, termed “in situ powder metallurgy” was used for consolidating the aluminum 2024–graphite particle composites containing 5–20 wt.% graphite particles. In this method, the stir casting and the P/M synthesizing processes were combined into an integrated net shape forming process. It is well known that the production method has a strong influence on the mechanical and tribological properties of such composites via its effects on the matrix grain size, porosity, the distribution of graphite particles and the interfacial properties of the Al/Gr couple. This work aims to evaluate the effect of graphite content on the tribological behavior of Al 2024 composites, made by in situ powder metallurgy method in terms of wear rate and friction coefficient under both dry and oil impregnated wear conditions. In dry sliding, the effect of graphite content on the mild to severe wear rate transition for these composites has also been reported.
Fig. 1. The schematic view of the wear test apparatus used in this study.
constant at 750 ◦ C for 5 min and then it was lowered to 611 ◦ C in 12 min while stirring was continued at the same speed. The non-wettability of un-coated graphite particles with molten aluminum alloy together with the shear forces induced by the impeller result in the melt disintegration and formation of molten droplets distributed among the graphite particles. Finally the charge was evacuated from the crucible into a steel container and the alloy was allowed to solidify at ambient temperature and atmosphere resulting in a mixture of graphite and aluminum powder particles. More details about the in situ powder metallurgy technique are discussed elsewhere [28]. The volume percentage of graphite particles for powder mixtures containing 5, 10, 15 and 20 wt.% graphite particles was calculated as 6.20, 12.27, 18.18 and 23.93%, respectively. The powder particles of 2024 Al alloy were produced by “solid assisted melt disintegration” (SAMD) technique [29,30]. The SAMD technique is basically similar to the in situ powder metallurgy method but instead of graphite, coarse alumina particles are used for melt disintegration. The alumina particles were sieved out from aluminum alloy–alumina powder mixture and the Al powder particles were used for making compacts of the base alloy for the purpose of comparison. The size distribution of the resultant mixture of graphite and aluminum powder particles was determined by laser particle sizing technique. Aluminum–graphite powder mixtures containing different amounts of graphite as well as the base alloy powders were cold pressed at 650 MPa and at a speed of 300 mm/min in a rigid steel die on a single acting 45 t hydraulic
2. Materials and experimental procedures The experimental Al/Gr composites were produced from bulk 2024 aluminum alloy and graphite powder by in situ powder metallurgy technique [28]. The chemical composition of 2024 alloy was analyzed as: 4.62Cu, 1.74Mg, 0.65Mn, 0.27Fe, 0.23Si and balanced Al in weight percents. The liquidus and solidus temperatures of this alloy quantified by thermal analyzing technique are 638 and 502 ◦ C, respectively. The in situ powder metallurgy technique involved melting weighted amounts of the alloy in a clay-bonded graphite crucible of 2-kg capacity using an electrical resistance furnace. Then the temperature of the melt was raised to about 750 ◦ C and specific quantities of un-coated graphite particles (with a purity of 98.7%) were added to the melt. The melt was subsequently stirred at 400 rpm using a graphite impeller attached to a variable speed motor. The temperature of the furnace was kept
Fig. 2. The size distribution of the as-received graphite particles.
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Fig. 3. Typical SEM micrograph of the as produced mixture of aluminum and 15 wt.% graphite powder particles within the size range of 106–150 m.
press resulting in cylinders with 25 mm in diameter, 15 mm in height and 92–96% of the theoretical density. The green compacts were sintered in a tubular furnace in a nitrogen atmosphere to provide protection against oxidation of the powder. It was found that the optimum sintering conditions depended on the graphite content in the compact. Therefore, compacts with a graphite content of 15 wt.% or less, were sintered at 600 ◦ C for 30 min and those with 20 wt.% graphite were sintered at 610 ◦ C for 60 min. The samples were solution treated at 495 ◦ C for 3 h prior to cold water quenching and artificially aged at 170 ◦ C for 5 h before air cooling to room temperature. Composite samples were polished according to standard metallographic techniques for microstructural characterization. The density of the base alloy compact and Al/Gr composites with different graphite contents was determined using Archimedes’ principle. The measured density was compared to the value obtained using Rule-of-Mixtures so as to determine the volume fraction of porosity. The densities of graphite and 2024 Al alloy were considered to be 2.2 g/cm3 and 2.79 g/cm3 , respectively. The samples were precision weighed in an electronic balance to an accuracy of 0.1 mg. Hardness measurements were carried out on a Brinell hardness testing machine, using a load of 300 N, and the mean values of at least five measurements conducted on different areas of each sample was considered. The specimens for three-point bending test were prepared from Al/Gr powder mixtures containing various amounts of graphite as well as the monolithic 2024 alloy powders by cold pressing in a steel die having the internal dimensions of 3 mm × 4 mm × 45 mm at 650 MPa. These compacts were sintered and heat treated with the same procedures as used for cylindrical samples. The bending tests were carried out on the as-produced samples (without machining), at room temperature on a 10 t Instron machine according to ASTM C1161-94 standard method. In order to ascertain reproducibility, at least three measurements were typically made for each type of sample. Dry sliding pin-on-disc wear tests were carried out in a laboratory atmosphere at 50–60% relative humidity and the temperature around 25 ◦ C on the heat treated composite and un-reinforced samples. The schematic view of the wear test apparatus is shown in
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Fig. 1. The rotating test material in the form of discs of diameter 25 mm and height 10 mm were slid against a steel pin (1.5Cr, 1C, 0.35Mn, 0.25Si) with the hardness of 64HRC having the diameter of 8 mm and height of 12 mm. Wear tests were undertaken under the normal load of 50 N (resulting in a normal pressure of 1 MPa), the sliding velocity of 0.5 ms−1 and the total sliding distance of 1000 m. The wear tests were carried out in dry and lubricated conditions using a wear track diameter of 180 mm. The track diameter was kept constant for all the experiments to eliminate this as a further variable of the rubbing system. The mass of each specimen was measured using an electronic balance having a resolution of 0.1 mg, before and after each wear test and the mass difference was then converted to volumetric wear rates using the measured density of each material and the total sliding distance. Friction coefficient measurements were made using a transducer to measure the deflection of the pin holder caused by the disc rotation. The system was calibrated by applying known tangential loads and noting pin deflection. For the oil impregnated tests, SAE 10 engine oil having kinematic viscosity of 175 mm2 s−1 and 14.5 mm2 s−1 at 40 and 100 ◦ C, respectively was incorporated into the porous samples by submerging them into 250 ml of oil in a beaker for a fixed time without circulation, heating or pressurizing. The oil had the following chemical composition (in wt.%): N 0.45, Ca 0.11, P 0.03 and Zn 0.03 with a sulfated ash content of 0.4 wt.%. To obtain the oil content of the impregnated samples, they were weighted before and after specific hours of remaining in oil. The impregnated samples were wiped off to remove any trace of lubricant left externally. This procedure was repeated in 4 h intervals and it was noted that the weight gain of the samples ceased after 24 h. The volume of the impregnated oil was calculated by using the mass and measured density of oil (0.76 g cm−3 ). By considering the porosity of each sample and the volume of impregnated oil, it was revealed that more than 90% of the pores were filled with oil indicating the interconnectivity of the majority of the pores. The morphology of the pores were determined by optical microscopy of the polished surfaces of composites. The oil impregnated samples were also subjected to wear test under the same conditions used for dry sliding tests. In order to investigate the effect of graphite content on the mild to severe wear rate transition for these composites, compacts of the base alloy as well as the Al/Gr composites containing different amounts of graphite particles were subjected to dry sliding wear tests under identical conditions as mentioned before, but varying loads. A marked transition from a mild wear regime to a severe wear regime was detected for each sample at a certain load. The severe wear was accompanied by a sudden increase in wear rate, heavy noise and vibration during testing. Such tests were of very
Fig. 4. The cumulative size distribution of Al + graphite mixtures containing different weight percentages of graphite particles.
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Fig. 5. Typical micrographs of Al–15 wt.% Gr composite: (a) SEM of polished surface, (b) optical micrograph of polished surface, and (c) SEM of fracture surface.
short duration, not exceeding 1 min, due to immediate wearing of the samples. Scanning electron microscopy (SEM) was used to analyze the morphology of the powder mixture as well as the polished and fractured surfaces of composites. Also for determination of wear behavior of the 2024 aluminum matrix and the composites, the worn surfaces and debris particles were examined by SEM, where the specimens were gold coated for 2 min before examination. The wear debris created by oil impregnated sliding was difficult to collect because of the very small amount produced. 3. Results and discussion The size distribution of as-received graphite particles as measured by a laser diffraction method is shown in Fig. 2 and exhibits
the average size (D50 ) of 55 m and maximum size of 160 m. A typical SEM micrograph of the as produced mixture of aluminum and 15 wt.% graphite powder particles within the size range of 106–150 m, is shown in Fig. 3. It can be seen that the graphite particles are well distributed within the aluminum powder particles and no aggregates of the graphite particles can be seen in the mixture. The cumulative size distribution of Al + graphite mixtures containing different weight percentages of graphite particles are graphically presented in Fig. 4. Fig. 4 shows that the average size of the aluminum–graphite powder mixture containing 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% graphite particles varies in the range 80–190 m. Fig. 5(a) and (b) are the typical SEM and optical micrographs of the polished surfaces of Al–15 wt.% Gr composite, respectively. The dark regimes shown in Fig. 5(a) represent the pores or voids which were left behind by evacuation of graphite
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Fig. 6. The variation of the porosity of the samples with the weight percent of graphite particles.
particles from the surfaces during the polishing process. Fig. 5(c) shows a typical micrograph of the fracture surface of Al–15 wt.% Gr composite. It can be seen that the graphite particles are distributed uniformly within the matrix alloy. This uniform distribution of the second phase within the matrix alloy is a characteristic of the in situ powder metallurgy method [28], and improves the mechanical and tribological properties of the composite due to lack of the graphite clusters. The variation of the porosity of the samples with the weight percent of graphite particles is graphically presented in Fig. 6. Fig. 6 shows that the porosity of the aluminum–graphite composites containing different amounts of graphite particles varies in the range 3–8%. The decreased hardness with the weight percent of graphite in the composites is shown in Fig. 7. The effect of the graphite addition on the fracture energy of the composites obtained from three-point bending tests as shown in Fig. 8 indicates that the bending strength decreases as the graphite content increases. Generally, addition of graphite to aluminum alloys is known to decrease the strength [23–25], fracture energy [11], ductility [23–25] and hardness [9,13,26,31] of the material. The increased amount of the brittle graphite particles together with the increased tendency of crack initiation and propagation at the graphite/metal interface are responsible for these effects. The variation in the measured wear rate and coefficient of friction with the weight percent of graphite in the composites for both dry sliding and oil impregnated sliding are shown in Figs. 9 and 10, respectively. It can be seen that the dry sliding wear rate of the Al 2024–5 wt.% graphite is about 10 times lower than that for the base alloy. However, for composites with 10 wt.% or more graphite particles addition, the wear rate increases. These results are consistent with the trends reported by some investigators [32–34] who found that Al/Gr composites containing small amounts of graphite (2–5 wt.%) posses superior wear
Fig. 7. The variation of hardness with the weight percent of graphite in the composites.
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Fig. 8. The effect of the graphite addition on the fracture energy of the composites obtained from three-point bending tests.
Fig. 9. The variation in the measured wear rate with the weight percent of graphite in the composites for both dry sliding and oil impregnated sliding.
properties over the base alloy, whereas at higher graphite addition levels a complete reversal in the wear behavior was observed. This dropping of the wear rate for a certain amount of graphite addition may have been caused by the following two competing factors. First, the beneficial effect of the graphite addition in reducing the wear of the composites due to formation of a thin lubricating graphite rich film on the tribosurface [7,10,11,35]; and secondly, the adverse effects of graphite addition in formation of porosity and cracks [19] as well as the deterioration of mechanical properties [12,19,23,24,34] resulting in enhanced delamination [19]. It must be noted that some reports [8,9,11] indicate that the
Fig. 10. The variation in the measured coefficient of friction with the weight percent of graphite in the composites for both dry sliding and oil impregnated sliding.
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Fig. 11. The worn surface morphology of (a) the base alloy and composites containing (b) 5 wt.% and (c) 15 wt.% of graphite particles tested under dry sliding conditions. The arrows show the direction of sliding.
wear rate of Al/Gr composites initially increases as the amount of graphite addition increases up to certain levels (i.e. 5 vol.%), then drops to lower values. However, the decreased wear rate with increased graphite content (0–15 wt.%) was observed in our previous research [28] for A356 aluminum/graphite composites and has also been reported by others [2–5,7]. These conflicting results on the dry wear characteristics of Al/Gr composites may have been caused by this fact that the efficiency of solid lubrication by graphite in Al/Gr composites depends on a number of factors such as wear testing conditions [5,32,36] and especially the applied pressure [5,19,37]. Also the morphology, shape, size [5,38,39] and the distribution of graphite particles as well as the nature of the particle matrix interfacial bonding together with the matrix microstructure [3,18,35] influence the wear characteristics of the composite.
As shown in Fig. 10, for dry sliding, the friction coefficients starts off at 0.35 for the base alloy and decreases with increased graphite content reaching to a final value of about 0.12 for composites containing 15 wt.% graphite which is about one-third that of the base alloy. However, Fig. 10 shows that there is no significant difference in friction coefficient between composites with 15 wt.% and that with 20 wt.% graphite addition. The reason for this reduction in coefficient of friction could also be attributed to the presence of the smeared graphite layer at the sliding surface of the wear sample which acts as a solid lubricant. This lubricant film prevents direct contact of the two surfaces. Fig. 11 exhibits the worn surface morphology of the base alloy and composites containing different amounts of graphite particles tested under dry sliding conditions. During wear, the base alloy surface is subjected to severe plastic deformation and abrasive
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Fig. 12. SEM micrographs of the collected debris from (a) base alloy and (b) A1–5 wt.% graphite particles generated under dry contacts.
wear (Fig. 11a). However, in contrast to the surface morphology of the base alloy, the worn surface of Al 2024–5 wt.% graphite is covered with a black film and the grooves are smaller in comparison to the un-reinforced alloy and are filled with debris particles. Fig. 11c clearly shows that for Al 15 wt.% graphite, the smeared layer becomes thicker and denser due to the increased graphite content. In fact, after wear test of this composite, most of the worn surface is covered uniformly by the graphite lubricating film which can prevent direct contact between the pin and the counterface. This layer effectively reduces the friction coefficient. However, with increasing the amount of graphite content to 20 wt.%, the area fraction and/or thickness of the lubricating film has not changed significantly so that the friction coefficient has remained unchanged. In fact, for composites with 15 wt.% graphite, the coefficient of friction about 0.12 under dry sliding conditions is comparable to and
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even lower than that of graphite of about 0.2 and cannot be further decreased by increased amount of graphite content. It has been found that the thickness of the graphite rich layer at the sliding surface plays an important role in the wear behavior of composites [40]. As the amount of graphite addition increases, more graphite is released to the wear surface during the wear process. This film is formed as a result of shearing of graphite particles located immediately below the contact surface. As shown in Fig. 11, richer graphite lubricating film can be noted for higher graphite content of the composites. Also an optimum amount of graphite is needed to keep the film continuous. This may be attributed to variations in the area covered by smeared graphite during sliding. The decreased coefficient of friction in Al/Gr composites due to increased addition of graphite particles has also been observed by many workers [1,4,5,7,8,10,11,15,20,22,37,41]. The investigation of collected debris from base alloy samples generated under dry contacts revealed very large, irregular profiles and unequal dimensions as those shown in Fig. 12(a). The generation of this kind of debris can be attributed to an abrasive micro-cutting effect. However, the wear debris becomes smaller for Al/Gr composites as shown in Fig. 12(b). This decrease in the size of the debris is consistent with those reported by other researchers [9,11,22,27] and is mainly a result of the increase in the brittleness of the composites together with the decreased probabilities of direct contacts of two worn surfaces which decreases the severity of micro-cutting effects. Fig. 13 shows the effect of graphite content on the transition load for onset of severe wear regime for the un-reinforced alloy and composites. In the case of un-reinforced alloy, severe wear regime occurred at loads above 0.5 MPa. However, incorporation of graphite particles into 2024 Al alloy significantly served to suppress the transition to a severe wear rate regime and impeded the transition to higher loads. Fig. 13 shows that the transition load increases with increasing graphite content leading to a sixfold increase for Al–20 wt.% graphite compared to the base alloy. Graphite particles assist in the formation and retention of the solid lubricating film on the composite sliding surface which prevent metal-to-metal contact and keep wear behavior within the mild regime. As was mentioned previously, with increasing the amount of graphite content, the thickness and the area of the lubricating film covered by smeared graphite during dry sliding also increases. This in turn lowers the coefficient of friction in composites and reduces the shear stress transmitted to the bulk subsurface material underneath the tribolayer. Therefore, the ability of the composite to withstand higher applied normal loads during wear is also increased. These results confirm that the high content graphite composites are clearly superior to the base alloy in delaying the transition to severe wear and hence provide the best seizure resistance against steel.
Fig. 13. The effect of graphite content on the transition load for onset of severe wear regime for the un-reinforced alloy and composites.
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It is evident from Figs. 9 and 10 that the wear rate and friction coefficient of the oil impregnated base alloy are considerably lower than those of the dry sliding. The wear rate of the oil impregnated base alloy is approximately four times lower than that for the dry sliding. These obvious results are generally expected from the oil impregnated sliding and can be attributed to formation of an oil film between the two contacting surfaces. The effect of 5 wt.% graphite particulate addition into the Al alloy was a slight decrease in the wear rate in the oil impregnated sliding. Again, similar to dry sliding, the wear weight loss increased with increasing content of graphite particles in oil impregnated sliding. The occurrence of a minimum in the wear rate for the oil impregnated samples are similar to those observed for dry sliding and can also be attributed to formation of an effective tribolayer for 5 wt.% graphite addition and deterioration of mechanical properties for increased amount of graphite content. It is interesting to note that except for the base alloy, the wear rates of Al/Gr composites containing different amounts of graphite are always higher for oil impregnated sliding as compared with dry contacts. Similar results have been reported by Prasad [42] who investigated the presence of oil plus graphite lubricants on the wear behavior of a zinc-based alloy reinforced with SiC particles and examined the effect of different amounts of graphite as a solid lubricant on the performance of the oil lubricant towards controlling the wear characteristics of the base alloy and composite material. For conducting lubricated wear tests, he prepared a series of SAE 40 oil plus graphite lubricant mixtures by thoroughly mixing the graphite particles (size 50–100 m) in varying concentrations ranging from 0 to 10 wt.%. He found that the benefits of graphite addition to the lubricating oil in decreasing friction coefficient of the tested materials could be realized up to its specific concentration only (i.e. 4 wt.%) and at higher concentrations of graphite in the lubricant mixture, he observed the reversed trend. He attributed these results to a mixed influence of the graphite content on the frictional heating of the contact surface. Therefore he concluded that although increasing content of graphite in the oil decreased the severity of frictional heating initially, but an opposite trend could be noticed at higher graphite contents. Other investigators [43] have also reported about the destructive effects of solid particles on the lubricated wear resistance of aluminum MMCs under lubricated sliding. However, in our experiments the graphite particles were not initially mixed with the lubricating oil but they were protruded out the polished surface of the composite and contribute to the weight loss. In addition, it was observed that the presence of oil in the contact area prohibited the formation and retention of the graphite lubricating film and the graphite particles released from the composite material were displaced from the contact area and were pushed to a corner on the sliding surface. Therefore, instead of formation of a relatively thick and dense graphite layer on the contact area, the removal of graphite particles from the body of the composite material continued resulting in increased wear rate as compared to dry sliding. In fact, the lubrication performance of graphite is considerably different for dry sliding and oil impregnated sliding. As was mentioned before, in dry sliding, the reason for the decreased wear rate and coefficient of friction of Al/Gr composites as compared with the base alloy is attributable to the presence of the smeared graphite layer at the sliding surface of the wear sample which acts as a solid lubricant. However, the mechanism of the wear and friction of oil impregnated Al/Gr composites is more complicated than that of dry sliding. The increased amount of graphite particles in the composites and thereby the increased graphite particles suspended in the lubricating oil at the sliding surface may be responsible for the slight decrease in the coefficient of friction of the samples containing more graphite particles. Fig. 10 shows that for the base alloy and Al/Gr composites containing various graphite additions, the coefficient of friction is lower
in the oil impregnated sliding as compared to that of dry sliding. However, in oil impregnated sliding, the use of graphite was not significantly beneficial in lowering the coefficient of friction. As shown in Fig. 9, for both dry sliding and oil impregnated sliding, the wear rate of the Al–5 wt.% graphite composite is lower than that of the base material and all the other composites. However, the oil impregnated Al–5 wt.% composite may exhibit superior tribological properties as compared to dry sliding since the debit in frictional properties is larger than the very small benefit in wear rate reduction. 4. Conclusion (1) The “in situ powder metallurgy” technique can be used to produce 2024 aluminum alloy/5–20 wt.% flake graphite particulate composites in which the graphite particles are distributed uniformly within the matrix alloy. The hardness and bending strength of these composites decreased with increasing graphite content attributable to the increased amount of the brittle graphite particles together with the increased tendency of crack initiation and propagation at the graphite/metal interface. (2) The Al/Gr composites exhibited a lower frictional coefficient and wear rate than the base alloy for both dry and oil impregnated sliding. However, for the base alloy and Al/Gr composites containing various graphite additions, the coefficient of friction in the oil impregnated sliding was not affected by the amount of graphite and was lower as compared to that of dry sliding attributable to formation of an oil film between the two contacting surfaces. (3) For both dry sliding and oil impregnated sliding, at low addition levels (5 wt.%) graphite improved the wear resistance. However, for composites with 10 wt.% or more graphite particles addition, the wear rate increased. These observations were explained in terms of two competing factors. First, the beneficial effect of the graphite addition due to formation of a thin lubricating graphite rich film on the tribosurface; and secondly, the adverse effects in formation of cracks as well as the deterioration of mechanical properties resulting in enhanced delamination. (4) In contrast to the base alloy, the wear rates of Al/Gr composites containing different amounts of graphite were always higher for oil impregnated sliding as compared with dry contacts. These results are attributed to prohibited formation and retention of the graphite lubricating film at the presence of oil in the contact area due to displacement and pushing of the suspended graphite particles on the sliding surface. (5) In dry sliding, the increased transition loads observed for the increased graphite contents was attributed to the formation and retention of thicker solid lubricating film covering more area on the composite sliding surface. This tribolayer prevented metalto-metal contact, lowered the coefficient of friction and reduced the shear stress transmitted to the bulk subsurface material. References [1] P.R. Gibson, A.J. Clegg, A.A. Das, Wear of cast Al–Si alloys containing graphite, Wear 95 (2) (1984) 193–198. [2] P.K. Rohatgi, Y. Liu, M. Yin, T.L. Barr, A surface analytical study of triboformed aluminum alloy 319–10 vol.% graphite particle composite, Mater. Sci. Eng. 123A (1990) 213–218. [3] S. Das, S.V. Prasad, T.R. Ramachandran, Tribology of Al–Si alloy–graphite composites: triboinduced graphite films and the role of silicon morphology, Mater. Sci. Eng. 138A (1991) 123–132. [4] Y.B. Liu, S.C. Lim, S. Ray, P.K. Rohatgi, Friction and wear of aluminum–graphite composites: the smearing process of graphite during sliding, Wear 159 (1992) 201–205. [5] P.K. Rohatgi, S. Ray, Y. Liu, Tribological properties of metal matrix–graphite particle composites, Int. Mater. Rev. 37 (3) (1992) 129–149.
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