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aluminium metal matrix composites
Wear 251 (2001) 1408–1413
Dry sliding wear of garnet reinforced zinc/aluminium metal matrix composites G. Ranganath a,∗ , S.C. Sharma b , M. Krishna b a
b
Adhiyamaan College of Engineering, Hosur 635 109, Tamil Nadu, India Department of Mechanical Engineering, R.V. College of Engineering, Bangalore 560059, India
Abstract The unlubricated sliding wear behavior of ZA-27 alloy composites reinforced with garnet particles of size 30–50 m was evaluated. The content of garnet in the alloy was varied from 2 to 6% in steps of 2 wt.%. Liquid metallurgy technique was used to fabricate the composites. A pin-on-disc wear-testing machine was used to evaluate the wear rate, in which an EN24 steel disc was used as the counterface. Results indicated that the wear rates of the composites were lower than that of the matrix alloy and further decreased with the increase in garnet content. However, in both unreinforced alloy and reinforced composites, the wear rates increased with the increase in load and the sliding speed. Increase in the applied load increased the wear severity by changing the wear mechanism from abrasion to particle cracking induced delamination wear. It was found that with the increase in garnet content, the wear resistance increased monotonically. The observations have been explained using scanning electron microscopy (SEM) analysis of the worn surfaces and the subsurface of the composites. The debris size is of the order of millimeters at higher load while at the lower load, it is of the order of a few hundred micrometers. On the basis of the above experimental observation, the sequence of micromechnical events, which lead to the generation of wear debris, has been surmized. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Composite; ZA-27; Garnet; Sliding wear; Wear rate
1. Introduction Metal matrix composites (MMCs) offer designers benefits, they are particularly suited for applications requiring good strength at high temperature, good structural rigidity, dimensional stability, and light weight [1–5]. The trend is towards safe usage of the MMC parts in the automobile engine, which work particularly at high temperature and pressure environments [6,7]. Particle reinforced MMCs have been the most popular over the last two decades. The modern trend for potential applications, is to optimize the mechanical properties and heat treatment of MMCs. Zinc–base alloys are characterized by low initial cost, excellent foundry castability and fluidity, good mechanical properties, good machining properties, compared to many ferrous and nonferrous casting alloys [8]. In Europe and North America, the zinc–aluminum alloys have been applied for decades to bearing and many other parts of customarily high performance. However, limited attempts were made in the past to synthesize zinc matrix composites reinforced with particles by using the compocasting method [9–13]
∗
Corresponding author.
and also few studies were devoted to the heat treatment of ZA alloy MMCs [14,15]. In view of the above, an attempt has been made to evaluate the dry sliding wear behaviour of garnet particle reinforced MMCs over a range of loads and sliding speeds. The unreinforced ZA-27 was tested has a reference material. The operating wear mechanism causing material removal in all the cases has been examined.
2. Materials and experimental procedure In the present study, ZA-27 alloy having the chemical composition as per the ASTM B669-82 ingot specification given in Table 1 was used as the base matrix alloy. Garnet particles of size 30–50 m were used as reinforcement. The percentage of garnet was varied from 2–6% steps of 2wt.%. The liquid metallurgy technique was used to prepare the composite specimens, which is similar to the one used by Sharma et al. [10]. In this process, the matrix alloy (ZA-27) was first super-heated above its melting temperature and stirring was initiated to homogenize the temperature. The temperature was then lowered gradually until the alloy reached a semi-solid state. At this temperature (440◦ C), the preheated
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 7 8 1 - 5
G. Ranganath et al. / Wear 251 (2001) 1408–1413
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Table 1 Chemical composition of ZA-27 alloy (ASTM B669-82) Al (%)
Cu (%)
Mg (%)
Zn
25–28
1–2
0.01–0.02
Balance
garnet particles were introduce into the slurry. The temperature and stirring were continued until particle and matrix wetting occurred. The melt was then superheated above its liquidus temperature of 500◦ C and finally poured into the lower die-half of the press and the top die was brought down to solidify the composite by applying high pressure. Chemical analysis was carried out which showed that a significant part of the particles were retained in the melt. The wear specimen was cut from ZA-27/garnet particulate composite sample. The dimensions of the pin specimen measured 8 mm in diameter and 20 mm height. A pin-on-disc wear test configuration was used following the method of Sharma et al. [10], in which surface roughness of the specimen was R a = 2 m and was rubbed against a hardened steel disc (EN24 disc material, hardness H v = 0.5 kgf = 345 BHN), which had a better surface roughness of R a = 0.2 m. All the specimens followed a single-track 115 mm in diameter, with the tangential force (frictional force) during sliding being continuously monitored by a force transducer attached to the specimen holder. The ratio of the tangential to the normal force gave the coefficient of friction (µ). The test was conducted with a load range of 20–50 N at sliding velocities of 7.35–31.5 m s−1 and with sliding distance of 2 km; specific gravity measurements were conducted according to ASTM Standard C127-88. At each load, volume losses from the surface of specimens were determined as a function of sliding distance, sliding velocity, and applied load. The volume loss was calculated from the difference in weight of the specimen measured before and after the tests to the nearest 0.1 mg, using an electronic balance. All these tests were conducted at room temperature 27◦ C and relative humidity of 48%.
Fig. 1. Graph showing wear rate vs. applied load of the ZA-27 alloy composites at velocities of (a) 7.35; (b) 12.6; (c) 21; and (d) 31.5 m s−1 .
increase in the sliding velocity. Note that the composite specimens exhibited significantly lower wear rates than the base alloy specimens. The wear rate of each composite specimen reduced with the increase in garnet content (Fig. 3).
3. Results The incorporation of garnet particles to the ZA-27 alloy improves the sliding wear resistance in comparison with the unreinforced alloy. The effects of both applied load and speed were investigated as a function of garnet wt.%. The effect of the applied load on the wear rate at an different sliding distance is presented in Fig. 1(a)–(d) which indicate that the wear rate of the matrix alloy as well as the composite specimens increases with the applied load at sliding speeds of 7.35, 12.6, 21 and 31.5 m s−1 , respectively. Fig. 2(a)–(d) are the graphs representing the wear rates of the composites as well as the base alloy specimens as a function of the sliding velocity at applied load of 20, 30, 40 and 50 N, respectively. The wear rate of both the unreinforced alloy as well as the composite specimens increases with the
Fig. 2. Graph showing wear rate vs. sliding velocity of the ZA-27 alloy composites at applied load of (a) 20; (b) 30; (c) 40; and (d) 50 N.
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At 40 N (12.6 m s−1 ), the unreinforced alloy seizes, while the composites had good wear resistance. Similarly at high load of 50 N (31.5 m s−1 ), only the 6% garnet reinforced composite had good wear resistance, while the unreinforced as well as the 2 and 4% reinforced composites seize, thus, confirming the positive effect of the reinforcing garnet particles as well as its content in reducing wear rate of materials. A similar trend has been observed at speeds of 21 and 31.5 m s−1 .
tested. Similar micrographs were also generated for the composites of other compositions as well. For brevity and convenience, the micrographs of only 6% garnet composites at all the loads tested at 7.35 m s−1 have been presented. However, the explanation holds good even for the composites with 2 and 4% reinforcement as well. In all the specimens, rough wear surfaces were produced. The SEM examination of the worn surfaces shows areas from where material has been removed. As the load increases from low to higher values, the morphology of the worn surface gradually changes from fine scratches to distinct grooves and damaged spots in the form of craters can be seen. At low loads of 20 and 40 N, the composite specimens show a mixed abrasion-plastic deformation mechanism as is evident from Fig. 4(a) and (b). Abrasion is evident from the scoring grooves visible on the worn surfaces in the direction of sliding. It is possible that the scored grooves might have been formed due to the action of the wear-hardened deposits on the disk track. But at high loads of 40 and 50 N (Fig. 4(c) and (d)), the locally damaged and even fractured spots are observed. These are indications of severe deformation and fracture resulting in high wears rate. Surface fracture is clearly evident in SEM micrograph shown in Fig. 4(e). Fig. 4(c), which shows the morphology of the worn surfaces at an intermediate load of 30 N, is also provided for comparison. Under high loads, the protective layer of the reinforcing particles can no longer remain stable under the ploughing action, and the wear strips formed are more distinct. Material removal during the process is in the form of small pieces resulting in the formation of flake-type debris. The shear strains induced in the process are transmitted to the matrix alloy and the wear mechanism proceeds by a subsurface crack propagation caused the delamination wear. Das et al. [15] are of the opinion that in delamination wear, the subsurface cracks, which may either exist earlier or get nucleated due to the stresses, propagate during the course of wear. When such subsurface cracks join the wear surface, delamination is the dominant wear mechanism. Suh [16] has described the delamination wear of metal/materials and states that the large plastic strain in the deformed layers give rise to void nucleation and subsurface crack initiation and propagation. The surface material is removed and cracks get nearer to the surface and the shear strain is increased, thus causing the removal of the surface layers by delamination. These observations suggest that the main wear mechanism at high loads is delamination wear causing excessive fracture of the reinforcement and the matrix, resulting in deterioration of the wear resistance of the composite.
5. Morphology of the worn surfaces
6. Discussion
The SEM micrographs of a typical worn surfaces of the 6% garnet reinforced composite are presented in Fig. 4 which show the wear track morphology of the specimens
It is found from the all graphs that the wear rate of each ZA-27/garnet reinforced composite specimen reduces with the increase in garnet content in the specimens in dry sliding
Fig. 3. Graph showing wear rate vs. wt.% of garnet reinforced ZA-27 alloy composites at applied load of sliding velocity 7.35.
The experimental data suggest that there is a certain applied load, i.e. a transition phenomena at which there is a sudden increase in the wear rate of both reinforced as well as unreinforced materials. However, the transition loads for the composites were very much higher than that observed for the unreinforced alloy, and also the transition load increases with the increase in garnet particle content.
4. Effect of reinforcement on the wear resistance
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Fig. 4. (a) and (b) SEM showing the worn surface of the 6% garnet reinforced composites at low load of 20 and 40 N; (c) and (d) SEM showing the worn surface of the unreinforced matrix alloy at load of 30 and 40 N; (e) SEM showing the worn surface of the unreinforced matrix alloy at load of 50 N (delamination).
wear tests. The result obtained and the observations made are similar to that made by Alpas and Embury [17], who reported that the composite possesses better wear resistance than a matrix alloy under dry sliding conditions. The observed wear mechanisms involve abrasion wear in the ductile unreinforced alloys and delamination wear due to the subsurface
cracks in the less ductile composites. The increase in the reinforcing particle content decreased the abrasive wear, but enhanced delamination wear. Venkataraman and Sundarajan [18] reported in their research work, that the addition of 10% SiC to Al prevents the transition from mild to severe wear.
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At a speed of 21 m s−1 , the unreinforced matrix alloy showed a transition from mild to severe wear at a load of 30 N, while the 2 and 4% particle reinforced composites showed a transition at 40 N, and the 6% of garnet reinforced composites was showed no transition as shown in Fig. 1(a)–(c). This observation indicates that the presence of garnet reinforcement delays the transition from mild to severe wear, and increases the transition load of the 6% reinforced composite by almost two or more times with respect to the unreinforced alloy. This is clearly evident from the graph shown in Fig. 3. It follows from the results obtained that at lower loads, comparatively low wear rates exist indicating the regime of mild wear. In this regime of mild wear, the composites demonstrate significant wear resistance than the alloy counterpart. At higher loads, the materials exhibit rapid increase in wear rate. At loads greater than the transition load, severe wear occurs leading to seizure of the materials. The severe wear manifested itself by a rapid rate of material removal in the form of generation of coarse metallic debris, and also by massive surface deformation and material transfer to the counterface. Zhang and Alpas [19] pointed out that the transition from mild to severe wear was realized when the contact surface temperature exceeded a critical temperature which corresponds to about 0.4 times of the melting temperature of aluminium alloy. The material removal increased due to crack formation in the reinforcing particles as well as in the particle–matrix interface. The graphs shown in Fig. 2(a)–(d) reveal that the unreinforced alloy shows a transition at 30 N when tested at speeds of 21 m s−1 , while the same is observed at 40 N in case of 12.6 m s−1 test. Similarly, the composite with 2 and 4% garnet shows a transition at 30 N when tested at speeds of 21 m s−1 , while the same is observed at 50 N in case of 12.6 m s−1 test. The composite with 6% garnet shows no transition at all loads when tested at all speeds. The above observation clearly indicated that the sliding speeds employed have a significant effect on the wear rate transition of the materials. The transition in wear rate decreases with the increase in speed in all the materials. The results obtained are similar with those obtained by Lee et al. [20] who have reported that the wear mechanisms are strongly dependent upon the sliding speeds. At low speed, the wear rate is lower since the damage caused to the tribolayer formed between the mating surfaces is less, while under high speeds, there is a breakdown of the tribolayer leading to excessive wear. It is found that the mild wear of the alloy is oxidationdominated wear at low sliding speeds and loads. This kind of wear is maintained until higher loads are employed, at which the wear mechanism transforms from mild wear to severe wear. The morphologies of wear surfaces of 6% garnet reinforced composites at low loads of 20 and 40 N are shown in Fig. 4(a) and (b), respectively. The wear tracks show typical abrasive wear. The worn surfaces are smooth and the ploughing strips are seen on the surface, which are very shallow.
At higher loads, wear proceeds by a surface of the specimen (Fig. 4(e)) delamination process and because of the low ductility of the composite, the reinforcement does not provide a beneficial effect in improving the wear resistance of the composite at high loads. In addition to promoting surface cracking, wear debris also cause abrasion of the matrix. Under higher loads at which transition occurs from mild to severe wear, the wear rate quickly increases markedly. Due to the high loads employed, the friction and wear increased obviously, leading to severe wear. The morphology of the wear surface unreinforced matrix alloy at the intermediate load of 30 N is shown in Fig. 4(c), while the same at high loads of 40 and 50 N are shown in Fig. 4(d) and (e), respectively. Under high load condition, the wear surfaces are characterized by severe plastic deformation and extensive damage in the form of local cracks/cavities and delaminated surface layers. The presence of crack/cavities on the worn surface is clearly evident from the SEM micrograph shown in Fig. 4(e). Seizure may occur with the increase in the load applied. This improvement in wear resistance is attributed to the changes in the wear mechanism induced by the presence of the reinforcing particles. The mechanism of material removal during the wear process of the unreinforced alloy is by plastic deformation and gouging. In the composites, the operating mechanism in addition to these, involved fracture of the garnet particles leading to the formation of a thin layer at the interface, thereby providing protection to the matrix. The subsurface deformed layer beneath the worn surfaces of MMC pins is composed of a number of distinct layers like mechanically mixed surface (MMS) layers [21]. Moreover, the hard iron and aluminium oxide layer of matrix has very limited ductility and has the ability to withstand stress without plastic deformation or fracture under low load conditions. It is well established by investigators [22] that the wear rate and surface damage can be minimized if the plastic deformation of the material at the contact interface is prevented. The MMS layer withstands high stresses without plastic deformation or fracture and is very effective in reducing the wear rate in the case of composites. Hence, it can be concluded that the ability of the sheared reinforcement layers to adhere to the disc sliding surface decides the effectiveness of the garnet particles in reducing the wear rate of the composite materials. The layer formed at the surface of the specimen may have high hardness [23], but at the expense of low fracture toughness. Consequently, fracture and delamination becomes a dominant failure mechanism. At high loads, the garnet particle fractures and loses its ability to support the load, as a result the counterface consequently comes in direct contact with the matrix alloy in which high strains are developed causing removal of the surface layers by delamination. Thus, it follows from the above observation that the most significant feature of severe wear is that the garnet particles should remain intact during wear in order to support
G. Ranganath et al. / Wear 251 (2001) 1408–1413
the applied load and act as effective abrasive elements. This observation is in consistence with the work done by Chung and Hwang [23]. In their work on effect of particle size and content on the dry sliding wear of SiC particles reinforced aluminium composites, they have reported that for a SiC size fraction, the wear resistance increased with increasing SiC content. They have also reported that for a constant SiC content, the composite containing coarser particles exhibited higher wears resistance. The coarser particles were found to provide greater resistance to the propagation of the subsurface cracks as compared to the finer SiC particles.
7. Conclusions 1. Garnet particle reinforced composites exhibited reduced wear rate than the unreinforced alloy specimens. The wear rate decreased with increasing garnet content. The wear rate of the composites as well as the matrix alloy increased with the increase in load applied. 2. The composite specimens exhibited abrasion wear at low loads, while at high loads delamination wear was dominant. SEM micrographs of the composite specimens tested at high loads revealed subsurface cracks which nucleate due to the stresses, propagate during the course of the wear, join the wear surface to cause delamination wear. 3. The wear rate increased abruptly above a critical load. The transition to high wear rate regime was induced by massive surface damage and material transfer to the counterface. The reinforcing garnet particles help delay the transition to the severe wear regime. References [1] S. Guldberg, H. Westengen, D.L. Albright, Properties of squeeze cast magnesium-based composites, SAE Trans. Section 5 (1991) 813–816. [2] K.R. Baldwin, D.J. Bray, G.D. Howard, R.W. Gardiner, Corrosion behaviour of some vapour deposited magnesium alloys, Mater. Sci. Techonol. 12 (1996) 937–943. [3] R. Oakle, R.F. Cochrane, R. Stevens, Recent development in magnesium matrix composites, Key Eng. Mater. 104-107 (1995) 387–416.
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[4] H. Baker, Magnesium races ahead, Advanced Mater. Process. 4 (1989) 35–42. [5] G.B. Evans, Textbook of applications of magnesium in aerospace, Magnesium Technology, Institute of Metals, 1986, pp. 103–109. [6] Y. Kagawa, E. Nakata, Some mechanical properties of carbon fiber-reinforced magnesium matrix composite fabricated by squeeze casting, J. Mater. Sci. Lett. 11 (1992) 176–178. [7] N. Llorca, A. Bloyee, T.M. Yue, Fatigue behaviour of short alumina fibre reinforced AZ91 magnesium alloy metal matrix composite, Mater. Sci. Eng. 135A (1991) 247–252. [8] K.J. Altorfer, Zinc alloys compete with bronze in bearing and bushings, Metal Progress (1982) 29–31. [9] K.H.W. Seah, S.C. Sharma, B.M. Girish, R. Kamath, B.M. Satish, Effect of artificial aging on tensile strength of ZA-27/short glass fiber-reinforced composites, J. Inst. Eng. 38 (4) (1998) 21–25. [10] S.C. Sharma, K.H.W. Seah, B.M. Girish, R. Kamath, B.M. Satish, Hardness of aged ZA-27/short glass fiber-reinforced composites, Mater. Design 18 (3) (1997) 155–159. [11] S.C. Sharma, B.M. Girish, R. Kamath, B.M. Satish, Aging characteristics of short glass fiber-reinforced ZA-27 alloy composite materials, J. Mater. Eng. Performance 8 (3) (1999) 309–314. [12] S.C. Sharma, B.M. Girish, D.R. Somashekar, R. Kamath, B.M. Satish, Mechanical properties and fractography of zircon particles reinforced ZA-27 alloy composites materials, Composites Sci. Technol. 59 (1999) 1805–1812. [13] R.J. Barnhurst, Guideline for designing zinc alloy bearings — a technical manual, SAE Trans. J. Mater. 97 (1988) 164–170. [14] S.C. Sharma, B.M. Girish, D.R. Somashekar, B.M. Satish, R. Kamath, Sliding wear behaviour of zircon particles reinforced ZA-27 alloy composite materials, Wear 224 (1999) 89–94. [15] S. Das, S.V. Prasad, T.R. Ramachandran, Microstructure and wear of (Al–Si alloy) graphite composites, Wear 133 (1998) 187–194. [16] N.P. Suh, An overview of the delamination theory of wear, Wear 44 (1977) 1–8. [17] A.T. Alpas, J.D. Embury, Sliding and abrasive wear behaviour of an aluminium (2014)-SiC particle reinforced composites, Scripta Metall. 24 (1990) 931–935. [18] B. Venkataraman, G. Sundarajan, The sliding wear of Al–SiC particle composites. II. Microbehavior, Acta Mater. 44 (2) (1996) 451–460. [19] J. Zhang, A.T. Alpas, Transition between mild and severe wear in aluminium alloys, Acta Mater. 45 (2) (1997) 513–528. [20] C.S. Lee, Y.H. Kim, K.S. Han, T. Lim, Wear behaviour of aluminium matrix composite materials, J. Mater. Sci. 27 (1992) 793–801. [21] B. Venkataraman, G. Sundarajan, The sliding wear of Al-SiC particle composites. II. The charcterization of subsurface deformation and corelation with wear behavior, Acta Mater. 44 (2) (1996) 461–473. [22] K. Komvopoulos, N. Saka, N.P. Suh, Role of hard layers in lubricated and dry sliding, J. Tribol. 109 (2) (1987) 223–229. [23] S. Chung, B.H. Hwang, A micro structural study of the wear behavior of SiC/Al composites, Tribol. Int. 27 (1994) 307–314.
Dry sliding wear of garnet reinforced zinc/aluminium metal matrix composites G. Ranganath a,∗ , S.C. Sharma b , M. Krishna b a
b
Adhiyamaan College of Engineering, Hosur 635 109, Tamil Nadu, India Department of Mechanical Engineering, R.V. College of Engineering, Bangalore 560059, India
Abstract The unlubricated sliding wear behavior of ZA-27 alloy composites reinforced with garnet particles of size 30–50 m was evaluated. The content of garnet in the alloy was varied from 2 to 6% in steps of 2 wt.%. Liquid metallurgy technique was used to fabricate the composites. A pin-on-disc wear-testing machine was used to evaluate the wear rate, in which an EN24 steel disc was used as the counterface. Results indicated that the wear rates of the composites were lower than that of the matrix alloy and further decreased with the increase in garnet content. However, in both unreinforced alloy and reinforced composites, the wear rates increased with the increase in load and the sliding speed. Increase in the applied load increased the wear severity by changing the wear mechanism from abrasion to particle cracking induced delamination wear. It was found that with the increase in garnet content, the wear resistance increased monotonically. The observations have been explained using scanning electron microscopy (SEM) analysis of the worn surfaces and the subsurface of the composites. The debris size is of the order of millimeters at higher load while at the lower load, it is of the order of a few hundred micrometers. On the basis of the above experimental observation, the sequence of micromechnical events, which lead to the generation of wear debris, has been surmized. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Composite; ZA-27; Garnet; Sliding wear; Wear rate
1. Introduction Metal matrix composites (MMCs) offer designers benefits, they are particularly suited for applications requiring good strength at high temperature, good structural rigidity, dimensional stability, and light weight [1–5]. The trend is towards safe usage of the MMC parts in the automobile engine, which work particularly at high temperature and pressure environments [6,7]. Particle reinforced MMCs have been the most popular over the last two decades. The modern trend for potential applications, is to optimize the mechanical properties and heat treatment of MMCs. Zinc–base alloys are characterized by low initial cost, excellent foundry castability and fluidity, good mechanical properties, good machining properties, compared to many ferrous and nonferrous casting alloys [8]. In Europe and North America, the zinc–aluminum alloys have been applied for decades to bearing and many other parts of customarily high performance. However, limited attempts were made in the past to synthesize zinc matrix composites reinforced with particles by using the compocasting method [9–13]
∗
Corresponding author.
and also few studies were devoted to the heat treatment of ZA alloy MMCs [14,15]. In view of the above, an attempt has been made to evaluate the dry sliding wear behaviour of garnet particle reinforced MMCs over a range of loads and sliding speeds. The unreinforced ZA-27 was tested has a reference material. The operating wear mechanism causing material removal in all the cases has been examined.
2. Materials and experimental procedure In the present study, ZA-27 alloy having the chemical composition as per the ASTM B669-82 ingot specification given in Table 1 was used as the base matrix alloy. Garnet particles of size 30–50 m were used as reinforcement. The percentage of garnet was varied from 2–6% steps of 2wt.%. The liquid metallurgy technique was used to prepare the composite specimens, which is similar to the one used by Sharma et al. [10]. In this process, the matrix alloy (ZA-27) was first super-heated above its melting temperature and stirring was initiated to homogenize the temperature. The temperature was then lowered gradually until the alloy reached a semi-solid state. At this temperature (440◦ C), the preheated
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 7 8 1 - 5
G. Ranganath et al. / Wear 251 (2001) 1408–1413
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Table 1 Chemical composition of ZA-27 alloy (ASTM B669-82) Al (%)
Cu (%)
Mg (%)
Zn
25–28
1–2
0.01–0.02
Balance
garnet particles were introduce into the slurry. The temperature and stirring were continued until particle and matrix wetting occurred. The melt was then superheated above its liquidus temperature of 500◦ C and finally poured into the lower die-half of the press and the top die was brought down to solidify the composite by applying high pressure. Chemical analysis was carried out which showed that a significant part of the particles were retained in the melt. The wear specimen was cut from ZA-27/garnet particulate composite sample. The dimensions of the pin specimen measured 8 mm in diameter and 20 mm height. A pin-on-disc wear test configuration was used following the method of Sharma et al. [10], in which surface roughness of the specimen was R a = 2 m and was rubbed against a hardened steel disc (EN24 disc material, hardness H v = 0.5 kgf = 345 BHN), which had a better surface roughness of R a = 0.2 m. All the specimens followed a single-track 115 mm in diameter, with the tangential force (frictional force) during sliding being continuously monitored by a force transducer attached to the specimen holder. The ratio of the tangential to the normal force gave the coefficient of friction (µ). The test was conducted with a load range of 20–50 N at sliding velocities of 7.35–31.5 m s−1 and with sliding distance of 2 km; specific gravity measurements were conducted according to ASTM Standard C127-88. At each load, volume losses from the surface of specimens were determined as a function of sliding distance, sliding velocity, and applied load. The volume loss was calculated from the difference in weight of the specimen measured before and after the tests to the nearest 0.1 mg, using an electronic balance. All these tests were conducted at room temperature 27◦ C and relative humidity of 48%.
Fig. 1. Graph showing wear rate vs. applied load of the ZA-27 alloy composites at velocities of (a) 7.35; (b) 12.6; (c) 21; and (d) 31.5 m s−1 .
increase in the sliding velocity. Note that the composite specimens exhibited significantly lower wear rates than the base alloy specimens. The wear rate of each composite specimen reduced with the increase in garnet content (Fig. 3).
3. Results The incorporation of garnet particles to the ZA-27 alloy improves the sliding wear resistance in comparison with the unreinforced alloy. The effects of both applied load and speed were investigated as a function of garnet wt.%. The effect of the applied load on the wear rate at an different sliding distance is presented in Fig. 1(a)–(d) which indicate that the wear rate of the matrix alloy as well as the composite specimens increases with the applied load at sliding speeds of 7.35, 12.6, 21 and 31.5 m s−1 , respectively. Fig. 2(a)–(d) are the graphs representing the wear rates of the composites as well as the base alloy specimens as a function of the sliding velocity at applied load of 20, 30, 40 and 50 N, respectively. The wear rate of both the unreinforced alloy as well as the composite specimens increases with the
Fig. 2. Graph showing wear rate vs. sliding velocity of the ZA-27 alloy composites at applied load of (a) 20; (b) 30; (c) 40; and (d) 50 N.
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At 40 N (12.6 m s−1 ), the unreinforced alloy seizes, while the composites had good wear resistance. Similarly at high load of 50 N (31.5 m s−1 ), only the 6% garnet reinforced composite had good wear resistance, while the unreinforced as well as the 2 and 4% reinforced composites seize, thus, confirming the positive effect of the reinforcing garnet particles as well as its content in reducing wear rate of materials. A similar trend has been observed at speeds of 21 and 31.5 m s−1 .
tested. Similar micrographs were also generated for the composites of other compositions as well. For brevity and convenience, the micrographs of only 6% garnet composites at all the loads tested at 7.35 m s−1 have been presented. However, the explanation holds good even for the composites with 2 and 4% reinforcement as well. In all the specimens, rough wear surfaces were produced. The SEM examination of the worn surfaces shows areas from where material has been removed. As the load increases from low to higher values, the morphology of the worn surface gradually changes from fine scratches to distinct grooves and damaged spots in the form of craters can be seen. At low loads of 20 and 40 N, the composite specimens show a mixed abrasion-plastic deformation mechanism as is evident from Fig. 4(a) and (b). Abrasion is evident from the scoring grooves visible on the worn surfaces in the direction of sliding. It is possible that the scored grooves might have been formed due to the action of the wear-hardened deposits on the disk track. But at high loads of 40 and 50 N (Fig. 4(c) and (d)), the locally damaged and even fractured spots are observed. These are indications of severe deformation and fracture resulting in high wears rate. Surface fracture is clearly evident in SEM micrograph shown in Fig. 4(e). Fig. 4(c), which shows the morphology of the worn surfaces at an intermediate load of 30 N, is also provided for comparison. Under high loads, the protective layer of the reinforcing particles can no longer remain stable under the ploughing action, and the wear strips formed are more distinct. Material removal during the process is in the form of small pieces resulting in the formation of flake-type debris. The shear strains induced in the process are transmitted to the matrix alloy and the wear mechanism proceeds by a subsurface crack propagation caused the delamination wear. Das et al. [15] are of the opinion that in delamination wear, the subsurface cracks, which may either exist earlier or get nucleated due to the stresses, propagate during the course of wear. When such subsurface cracks join the wear surface, delamination is the dominant wear mechanism. Suh [16] has described the delamination wear of metal/materials and states that the large plastic strain in the deformed layers give rise to void nucleation and subsurface crack initiation and propagation. The surface material is removed and cracks get nearer to the surface and the shear strain is increased, thus causing the removal of the surface layers by delamination. These observations suggest that the main wear mechanism at high loads is delamination wear causing excessive fracture of the reinforcement and the matrix, resulting in deterioration of the wear resistance of the composite.
5. Morphology of the worn surfaces
6. Discussion
The SEM micrographs of a typical worn surfaces of the 6% garnet reinforced composite are presented in Fig. 4 which show the wear track morphology of the specimens
It is found from the all graphs that the wear rate of each ZA-27/garnet reinforced composite specimen reduces with the increase in garnet content in the specimens in dry sliding
Fig. 3. Graph showing wear rate vs. wt.% of garnet reinforced ZA-27 alloy composites at applied load of sliding velocity 7.35.
The experimental data suggest that there is a certain applied load, i.e. a transition phenomena at which there is a sudden increase in the wear rate of both reinforced as well as unreinforced materials. However, the transition loads for the composites were very much higher than that observed for the unreinforced alloy, and also the transition load increases with the increase in garnet particle content.
4. Effect of reinforcement on the wear resistance
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Fig. 4. (a) and (b) SEM showing the worn surface of the 6% garnet reinforced composites at low load of 20 and 40 N; (c) and (d) SEM showing the worn surface of the unreinforced matrix alloy at load of 30 and 40 N; (e) SEM showing the worn surface of the unreinforced matrix alloy at load of 50 N (delamination).
wear tests. The result obtained and the observations made are similar to that made by Alpas and Embury [17], who reported that the composite possesses better wear resistance than a matrix alloy under dry sliding conditions. The observed wear mechanisms involve abrasion wear in the ductile unreinforced alloys and delamination wear due to the subsurface
cracks in the less ductile composites. The increase in the reinforcing particle content decreased the abrasive wear, but enhanced delamination wear. Venkataraman and Sundarajan [18] reported in their research work, that the addition of 10% SiC to Al prevents the transition from mild to severe wear.
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At a speed of 21 m s−1 , the unreinforced matrix alloy showed a transition from mild to severe wear at a load of 30 N, while the 2 and 4% particle reinforced composites showed a transition at 40 N, and the 6% of garnet reinforced composites was showed no transition as shown in Fig. 1(a)–(c). This observation indicates that the presence of garnet reinforcement delays the transition from mild to severe wear, and increases the transition load of the 6% reinforced composite by almost two or more times with respect to the unreinforced alloy. This is clearly evident from the graph shown in Fig. 3. It follows from the results obtained that at lower loads, comparatively low wear rates exist indicating the regime of mild wear. In this regime of mild wear, the composites demonstrate significant wear resistance than the alloy counterpart. At higher loads, the materials exhibit rapid increase in wear rate. At loads greater than the transition load, severe wear occurs leading to seizure of the materials. The severe wear manifested itself by a rapid rate of material removal in the form of generation of coarse metallic debris, and also by massive surface deformation and material transfer to the counterface. Zhang and Alpas [19] pointed out that the transition from mild to severe wear was realized when the contact surface temperature exceeded a critical temperature which corresponds to about 0.4 times of the melting temperature of aluminium alloy. The material removal increased due to crack formation in the reinforcing particles as well as in the particle–matrix interface. The graphs shown in Fig. 2(a)–(d) reveal that the unreinforced alloy shows a transition at 30 N when tested at speeds of 21 m s−1 , while the same is observed at 40 N in case of 12.6 m s−1 test. Similarly, the composite with 2 and 4% garnet shows a transition at 30 N when tested at speeds of 21 m s−1 , while the same is observed at 50 N in case of 12.6 m s−1 test. The composite with 6% garnet shows no transition at all loads when tested at all speeds. The above observation clearly indicated that the sliding speeds employed have a significant effect on the wear rate transition of the materials. The transition in wear rate decreases with the increase in speed in all the materials. The results obtained are similar with those obtained by Lee et al. [20] who have reported that the wear mechanisms are strongly dependent upon the sliding speeds. At low speed, the wear rate is lower since the damage caused to the tribolayer formed between the mating surfaces is less, while under high speeds, there is a breakdown of the tribolayer leading to excessive wear. It is found that the mild wear of the alloy is oxidationdominated wear at low sliding speeds and loads. This kind of wear is maintained until higher loads are employed, at which the wear mechanism transforms from mild wear to severe wear. The morphologies of wear surfaces of 6% garnet reinforced composites at low loads of 20 and 40 N are shown in Fig. 4(a) and (b), respectively. The wear tracks show typical abrasive wear. The worn surfaces are smooth and the ploughing strips are seen on the surface, which are very shallow.
At higher loads, wear proceeds by a surface of the specimen (Fig. 4(e)) delamination process and because of the low ductility of the composite, the reinforcement does not provide a beneficial effect in improving the wear resistance of the composite at high loads. In addition to promoting surface cracking, wear debris also cause abrasion of the matrix. Under higher loads at which transition occurs from mild to severe wear, the wear rate quickly increases markedly. Due to the high loads employed, the friction and wear increased obviously, leading to severe wear. The morphology of the wear surface unreinforced matrix alloy at the intermediate load of 30 N is shown in Fig. 4(c), while the same at high loads of 40 and 50 N are shown in Fig. 4(d) and (e), respectively. Under high load condition, the wear surfaces are characterized by severe plastic deformation and extensive damage in the form of local cracks/cavities and delaminated surface layers. The presence of crack/cavities on the worn surface is clearly evident from the SEM micrograph shown in Fig. 4(e). Seizure may occur with the increase in the load applied. This improvement in wear resistance is attributed to the changes in the wear mechanism induced by the presence of the reinforcing particles. The mechanism of material removal during the wear process of the unreinforced alloy is by plastic deformation and gouging. In the composites, the operating mechanism in addition to these, involved fracture of the garnet particles leading to the formation of a thin layer at the interface, thereby providing protection to the matrix. The subsurface deformed layer beneath the worn surfaces of MMC pins is composed of a number of distinct layers like mechanically mixed surface (MMS) layers [21]. Moreover, the hard iron and aluminium oxide layer of matrix has very limited ductility and has the ability to withstand stress without plastic deformation or fracture under low load conditions. It is well established by investigators [22] that the wear rate and surface damage can be minimized if the plastic deformation of the material at the contact interface is prevented. The MMS layer withstands high stresses without plastic deformation or fracture and is very effective in reducing the wear rate in the case of composites. Hence, it can be concluded that the ability of the sheared reinforcement layers to adhere to the disc sliding surface decides the effectiveness of the garnet particles in reducing the wear rate of the composite materials. The layer formed at the surface of the specimen may have high hardness [23], but at the expense of low fracture toughness. Consequently, fracture and delamination becomes a dominant failure mechanism. At high loads, the garnet particle fractures and loses its ability to support the load, as a result the counterface consequently comes in direct contact with the matrix alloy in which high strains are developed causing removal of the surface layers by delamination. Thus, it follows from the above observation that the most significant feature of severe wear is that the garnet particles should remain intact during wear in order to support
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the applied load and act as effective abrasive elements. This observation is in consistence with the work done by Chung and Hwang [23]. In their work on effect of particle size and content on the dry sliding wear of SiC particles reinforced aluminium composites, they have reported that for a SiC size fraction, the wear resistance increased with increasing SiC content. They have also reported that for a constant SiC content, the composite containing coarser particles exhibited higher wears resistance. The coarser particles were found to provide greater resistance to the propagation of the subsurface cracks as compared to the finer SiC particles.
7. Conclusions 1. Garnet particle reinforced composites exhibited reduced wear rate than the unreinforced alloy specimens. The wear rate decreased with increasing garnet content. The wear rate of the composites as well as the matrix alloy increased with the increase in load applied. 2. The composite specimens exhibited abrasion wear at low loads, while at high loads delamination wear was dominant. SEM micrographs of the composite specimens tested at high loads revealed subsurface cracks which nucleate due to the stresses, propagate during the course of the wear, join the wear surface to cause delamination wear. 3. The wear rate increased abruptly above a critical load. The transition to high wear rate regime was induced by massive surface damage and material transfer to the counterface. The reinforcing garnet particles help delay the transition to the severe wear regime. References [1] S. Guldberg, H. Westengen, D.L. Albright, Properties of squeeze cast magnesium-based composites, SAE Trans. Section 5 (1991) 813–816. [2] K.R. Baldwin, D.J. Bray, G.D. Howard, R.W. Gardiner, Corrosion behaviour of some vapour deposited magnesium alloys, Mater. Sci. Techonol. 12 (1996) 937–943. [3] R. Oakle, R.F. Cochrane, R. Stevens, Recent development in magnesium matrix composites, Key Eng. Mater. 104-107 (1995) 387–416.
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