- Email: [email protected]
Influence of sliding speed on the dry sliding wear behaviour and the subsurface deformation on hybrid metal matrix composite
Wear 262 (2007) 1007–1012
Influence of sliding speed on the dry sliding wear behaviour and the subsurface deformation on hybrid metal matrix composite S. Basavarajappa a,∗ , G. Chandramohan a , Arjun Mahadevan a , Mukundan Thangavelu a , R. Subramanian b , P. Gopalakrishnan b a b
Department of Mechanical Engineering, PSG College of Technology, Coimbatore 641004, Tamil Nadu, India Department of Metallurgical Engineering, PSG College of Technology, Coimbatore 641004, Tamil Nadu, India Received 27 February 2006; received in revised form 30 August 2006; accepted 17 October 2006 Available online 28 November 2006
Abstract In recent years, more attention is being paid to the structure of both the surface and the subsurface of a material being subjected to wear. Surface and subsurface deformation can cause a considerable change in the microstructure of the material leading to a change in its properties. The present study investigates the influence of sliding speed on dry sliding wear behaviour and the extent of subsurface deformation in aluminium metal matrix composites, namely Al 2219/15SiCp and Al 2219/15SiCp-3graphite all fabricated by the liquid metallurgy route. Dry sliding wear tests were conducted using a pin-on-disc machine. The subsurface deformation was assessed as a measure of variation in microhardness along the depth normal to the cross-section of the worn surface. The results reveal that with increasing sliding speeds in the mild wear region the degree of subsurface deformation was also increasing. The graphitic composite exhibited less degree of subsurface deformation in comparison to the graphite free composite. © 2006 Elsevier B.V. All rights reserved. Keywords: Metal matrix composites; Wear rate; Subsurface deformation; Microhardness
1. Introduction Aluminium alloys are widely used in the automotive industry because of their high strength to weight ratio as well as high thermal conductivity. It is used particularly in automobile engines as cylinder liners as well as other rotating and reciprocating parts, such as the piston, drive shafts, brake rotors and in other applications in automotive and aerospace industries [1,2]. Aluminium and its alloys exhibit poor tribological properties leading even to seizure under adverse conditions. Hence, a strong drive to develop new materials with greater resistance to wear and better tribological properties led to the development of aluminum metal matrix composites [3,4]. The performance of metal matrix composite reinforced with ceramic particles has been reported to be superior to that of their unreinforced matrix alloy [5,6]. The addition of reinforcements significantly improves the wear resistance of aluminium alloy. Zou et al. [7] found that the wear resistance
∗
Corresponding author. Tel.: +91 422 2572177; fax: +91 422 2573833. E-mail address: [email protected] (S. Basavarajappa).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.016
of composites is proportional to the volume fraction and average diameter of SiC particles (SiCp). The wear resistance of 38 vol% SiCp particle (average size 57 m) reinforced composite is almost 10 times higher than that of the SiCp (5.5 m) reinforced composite with the same volume fraction. They also found that the depth of plastic deformation zone in the subsurface decreases with increasing volume fraction and average diameter of SiCp particles. Lim et al. [8] have reported that the dominance of particular wear mechanism for aluminium alloy based SiC particulate composite is primarily dependant on the sliding conditions. It was found that the addition of SiC particles to A356 grade aluminium alloy extended the mild wear regime to higher speeds and loads, thereby inhibiting severe wear. SiC particles assist in the retention of an oxide transfer layer on composite sliding surfaces which prevent metal-to-metal contact and keep wear behaviour within the mild wear regime [9]. Venkataraman and Sundararajan [10] have found that there exist a strong correlation between wear behaviour and the hardness, thickness and composition of the mechanically mixed layer (MML) that is formed in the sliding wear conditions. The best wear resistance was observed when a stable thin and hard MML layer is present.
1008
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
Reinforcement of SiCp with aluminum however gives rise to certain detrimental effects. The SiCp particles scuff the counterpart leading to greater wear loss [11]. The addition of solid lubricant particles such as graphite along with SiCp as hybrid reinforcements effectively improves the tribological properties of the whole system under sliding wear conditions [12]. These hybrid graphitic aluminium metal matrix composites provided greater seizure resistance even under dry sliding conditions. Wear becomes more stable as the amount of graphite addition increases [13]. The graphite smears on the sliding pin surface and forms a layer which reduces wear [14]. Riahi and Alpas [11] have found that SiCp–graphite reinforced composites displayed a transition from mild wear to severe wear at loads and sliding speed combinations which were higher than those of the unreinforced A356 aluminium alloy and graphite free A356 Al–20% SiCp composites. The uniform graphite microcrystalline layer on top of the worn surface helps to decrease the friction coefficient and alleviate the plastic deformation in the subsurface region avoiding severe adhesion wear [15]. In recent years, importance is being given to the structure of both the surface and the subsurface of the material being subjected to wear. Wear can cause considerable change in structure of the subsurface to a point where it becomes significantly different from that of the bulk material. These changes can lead to reduction in resistance to fatigue and creep [16]. In view of the account given above, a study has been made to compare the influence of sliding speeds on the extent of subsurface damage in Al 2219/15SiCp and Al 2219/15SiCp-3Gr composites. The subsurface damage is assessed as a measure of variation in microhardness along the cross-section of the worn surface. 2. Materials and experiment The matrix material selected for the present investigation was based on the Al–Cu–Mg alloy, designated by the American Aluminium Association as AA2219. This alloy was chosen as the matrix, since it provides excellent combination of strength and damage tolerance both at elevated and cryogenic temperatures [17]. The SiC particles, which were used to fabricate the composite, had an average particle size of 25 m and average density of 3.2 g/cm3 . Three different materials, namely Al 2219, Al 2219/15SiCp and Al 2219/15SiCp-3Gr were fabricated by the liquid metallurgy route [18]. Wear test specimen were machined from as-cast samples to form cylindrical pins having a diameter of 10 mm and a height of 30 mm. The specimen faces were then metallographically polished. The dry sliding wear tests were conducted using a pin-on-disc apparatus (Fig. 1) at room temperature as per the ASTM G9995 standards. The details of the wear test conditions are given in Table 1. The initial weight of the specimen was measured in a single pan electronic weighing machine with an accuracy of 0.0001 g. The wear of the composites were studied as a function of sliding velocity for the stated applied load and sliding distance. After running through the fixed sliding distance of 5000 m, the specimen was removed, cleaned with acetone and weighed to determine the weight loss. The difference in weight gives
Fig. 1. The schematic view of the pin-on-disc apparatus used in this study.
the wear of the specimen and then the wear rate was calculated [18]. Scanning electron microscope investigations were done to study the structure of the worn surfaces. Digital photographs of the worn surfaces were also taken for the study. The samples were then cut carefully along their diameter, perpendicular to the worn surface. The cut surface was then metallographically polished for measuring the microhardness along the depth using a Mitutoyo Vickers microhardness tester with a load of 50 g for a time of 10 s. The microhardness was measured at three locations and averaged to circumvent possible errors. Measurements of microhardness were carried out closest to 20 m from the worn surface and then intervals of 20 m were taken for subsequent indentations. The measurements were taken until the variation in hardness from the bulk hardness became negligible. 3. Results and discussion 3.1. Wear behaviour Wear rate versus sliding speed curves at an applied load of 40 N for Al 2219, Al 2219/15SiCp and Al 2219/15SiCp3Gr reinforced composite for a constant sliding distance of 5000 m are shown in Fig. 2. The wear rate for all the materials almost unchanged with an increase in sliding speed up to 3 m/s. Beyond 3 m/s the wear rate of the unreinforced alloy increased to higher magnitudes than those of the composites and seizure was observed at a sliding speed of 6.1 m/s. At 6.1 m/s, wear by delamination was observed to occur in the alloy with fragments from the pin being transferred to the disc and larger fragments Table 1 Details of the wear test conditions used in this study Pin material Disc material Pin dimensions Sliding speeds (m/s) Normal load (N) Sliding distance (m)
Al 2219, Al 2219/15SiCp, Al 2219/15SiCp-3Gr EN36 steel with a hardness of 65 HRc Cylinder with diameter 10 mm and height 30 mm 1.53, 3.0, 4.6, 6.1 40 5000
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
1009
Fig. 2. Variation of wear rate with the sliding speed for the three different materials subject to the dry sliding wear.
being thrown out. This increased wear rate finally leads to seizure of the material, which is evident from the micrograph shown in Fig. 3(a). This behaviour was also characterized by formation of large debris and burr on the leading edge of the pin (Fig. 3(b)). However, the wear rates of the composites almost unchanged with increase in sliding speed up to 4.6 m/s after which an increasing trend was observed. This unchanged wear rate until 4.6 m/s is due to the presence of SiCp particles. The introduction of reinforcement in the aluminium matrix reduces the wear rate and coefficient of friction [19]. The SiCp particles provide improved wear resistance to the composites than the unreinforced alloy by the formation of the mechanically mixed layer at the interface of the composite pin and the steel disc [10]. This layer was found to be composed of oxides of aluminium, oxides of iron which were scuffed off from the steel counter-face by SiCp particles and fractured SiCp particles [9,10]. In graphitic composites, the MML additionally contained graphite that acted as a solid lubricant further reduced the wear rate.
Fig. 3. Photographs of Al 2219 at an applied load of 40 N after running for a sliding distance of 5000 m at a sliding speed of 6.1 m/s: (a) material removed in severe wear and (b) formation of burr on the leading edge.
3.2. Surface and subsurface analysis Fig. 4 shows the variation of microhardness with distance from the worn surface for Al 2219/15SiCp composite at various sliding speeds. It can be observed that the pattern of variation in microhardness in the subsurface below the worn surface is different for different sliding speeds. At all sliding speeds the subsurface deformation observed in the present investigation has been divided into three zones, namely Zones 1–3. The microhardness just below the wear surface at a sliding speed of 3 m/s is about 139 VHN and stabilizes at a depth of 140 m from the worn surface. A large variation in microhardness up to a depth of 80 m (Zone 1) at a sliding speed of 3 m/s is due to the formation of MML on the surface of the pin, which was much harder than the bulk hardness of the tested material. Venkataraman and Sundararajan [10] have observed that the formation of MML on the pin surface is caused by turbulent plastic flow induced by the onset of shear instability at a critical depth below the wear surface. The hardness values between 80 and 140 m depth (Zone 2) showed a decreasing trend and stabilizes at 140 m (Zone 3).
Fig. 4. Variation of microhardness with the distance from the worn surface along the plane normal to the worn surface for Al 2219/15SiCp reinforced composite.
1010
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
Fig. 6. Variation of microhardness with the distance from the worn surface along the plane normal to the worn surface for Al 2219/15SiCp-3graphite reinforced composite.
Fig. 5. Worn surface of Al 2219/15SiCp composite at an applied load of load of 40 N after running for a distance of 5000 m at a sliding speed: (a) 3 m/s and (b) 6.1 m/s.
The increase in microhardness in the subsurface region (Zone 2) is because of several events such as shear deformation, void nucleation and growth as well as the onset of shear instability, taking place in the subsurface region [10]. Fig. 5(a) shows the presence of a large number of grooves over the entire surface of the pin. The abrasive wear mechanism is operative, as the MML is harder the grooves formed are shallow. This hard MML layer formed during the process reduces the amount of material removed from the surface of the pin. At a sliding speed of 4.6 m/s, the hardness of the subsurface is higher compared to the same material at 3 m/s up to an initial depth of 80 m after which it becomes more or less the same. The increase in the hardness in Zone 1 is due to the higher degree of compaction of the MML layer compared to those at lower speeds. The degree of deformation in Zone 2 extends to a further depth from the pin surface which is due to increased shear deformation in the subsurface regions with an increase in sliding speed. At a sliding speed of 6.1 m/s, the wear rate shows a rising trend which indicates more removal of material from the surface. Fig. 4 shows a decreased hardness in the vicinity of the top surface which may be due to the initiation of removal of the hard top surface layer. This also leads to an increase in the wear rate. The micrograph in Fig. 5(b) shows the removal of material
by delamination. Apart from this, cracks are generated along with particle pull out at the surface. The crack originating at various points propagates parallel to the surface of the pin and nucleates at one point which results in the removal of debris as sheets which are composed of MML [20]. With an increase in the depth from the surface the microhardness is seen to increase. This is due to the grain reorientation and refinement at the high sliding speed for which the reason has already been mentioned. Fig. 6 shows the variation of microhardness with distance from the worn surface for Al 2219/15SiCp/3Gr reinforced composites. The important microstructural features of the MML layers in case of graphite composite; in addition to the presence of SiCp particles is the presence of graphite films within the MML layer [11]. It was observed that at a sliding speed of 3 m/s, the surface damage increases up to a depth of 120 m from the worn surface. This is due to the fact that the graphite films squeezed out of the graphite particles elongated in the direction of shear over larger distances [11]. It has further been shown that the graphite films appear to have formed as a result of shearing of the graphite particles located immediately below the contact surfaces [11]. The microhardness reaches 131 VHN at the depth of 120 m from the worn surface. This high hardness in the subsurface could be attributed to the fact that the graphite layers were typically formed at depths of 20–100 m, where the magnitudes of subsurface strains were extremely high [11]. The microhardness increases just below surface at 20 m from the worn surface with increased sliding speeds of 4.6 and 6.1 m/s as observed in Fig. 6. But in subsurface regions beyond 40 m there is not much difference in the trend of microhardness variation between the sliding speeds of 4.6 and 6.1 m/s. The depth of subsurface deformations extends only up to 140 m below the worn surface for all the sliding speeds unlike the graphite free composite where the depth of subsurface deformation vary with the sliding speed. The degree of hardness and the depth of subsurface deformation in the graphite composite are distinctly less than the SiCp reinforced graphite free composite at all the
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
1011
Fig. 7. Worn surface of Al 2219/15SiCp-3graphite composite at an applied load of 40 N after running for a distance of 5000 m at a sliding speed: (a) 4.6 m/s and (b) 6.1 m/s.
sliding speeds studied. This can be attributed to the fact that the graphite layers serve to reduce the magnitude of shear stress transferred to the matrix underneath the MML [11]. It can also be observed from the SEM micrographs of the worn surface (Fig. 7(a and b)) that the formation of grooves is less distinct in the material at 4.6 m/s. As the speed increases to 6.1 m/s the grooves become deeper, since relatively more amount of material was removed. At a sliding speed of 6.1 m/s it can clearly be seen that there is no reduction in the hardness of the surface immediately below the worn surface so the transition to severe wear in the graphite composite has not occurred. In the graphite composite the transition to severe wear has been delayed due to the lubricating action of the graphite phase. Apart from delaying the transition to severe wear, the graphite composite enhances the resistance to scuffing damage imparted to the mating part by the MML layer. 4. Conclusions 1. The addition of SiCp reinforcement to Al 2219 alloy increases the wear resistance of the composites. The addition of graphite reinforcement in Al–SiCp composites further increases the wear resistance at all sliding speeds and effectively avoids the occurrence of severe wear. 2. The subsurface deformation from the contact surface to the region where the bulk hardness of the composite is reached can be divided into three zones. Zone 1 extends up to a depth of 60 m from the worn surface for both Al 2219/15SiCp and Al 2219/15SiCp-3Gr composite. Zone 2 corresponds to the subsurface that is 60–180 m from the worn surface in Al 2219/15SiCp and 60–140 m from the worn surface in Al 2219/15SiCp-3graphite composite. Zone 3 is the region that remains unaltered. 3. Variation of microhardness and the depth of deformation in the subsurface regions in Al 2219/15SiCp composite are different for different sliding speeds. The microhardness just below the wear surface is very high and as the depth increases the microhardness decreases. In the severe wear region the initial microhardness just below the worn surface is less, which can be attributed to the material removal in the form of sheets and particle pull out, exposing undeformed material.
4. A similar trend of microhardness variation in the subsurface regions is observed in Al 2219/15SiCp-3graphite composite at different sliding speeds. Further for all the sliding speeds, the depth of subsurface deformation in the graphite composite stabilizes at 140 m from the worn surface. The degree of subsurface deformation in graphite composite is less than that of the graphite free composite. References [1] A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Metal matrix composites—a review, J. Mater. Sci. 26 (1991) 1137–1157. [2] I. Sinclair, P.J. Gregson, Structural performance of discontinuous metal matrix composites, Mater. Sci. Technol. 3 (1997) 709–725. [3] A.P. Sannino, H.J. Rack, Dry sliding wear of discontinuously reinforced aluminium composites: review and discussion, Wear 189 (1995) 1– 19. [4] B.C. Pai, T.P.D. Rajan, R.M. Pillai, Aluminium matrix composite castings for automotive applications, Indian Foundry J. 50 (2004) 30–39. [5] K.G. Satyanarayana, R.M. Pillai, B.C. Pai, Recent developments and prospects in cast aluminium matrix composites, Trans. Indian Inst. Metals 55 (3) (2002) 115–130. [6] F. Gul, M. Acilar, Effect of the reinforcement volume fraction on the dry sliding wear behaviour of Al-10Si/SiCp composites produced by vacuum infiltration technique, Compos. Sci. Technol. l64 (2004) 1959–1970. [7] X.G. Zou, H. Miyahara, K. Yamamoto, K. Ogi, Sliding wear behaviour of Al–Si–Cu composites reinforced with SiC particles, Mater. Sci. Technol. 19 (2003) 1519–1526. [8] S.C. Lim, M. Gupta, L. Ren, J.K.M. Kwok, The tribological properties of Al–Cu/SiCp metal-matrix composites fabricated using the rheocasting technique, J. Mater. Process. Technol. 80–90 (1999) 591–596. [9] S. Wilson, A.T. Alphas, Wear mechanism maps for metal matrix composites, Wear 212 (1997) 41–49. [10] 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. [11] A.R. Riahi, A.T. Alpas, The role of tribolayers on sliding wear behaviour of graphitic aluminium matrix composites, Wear 251 (2001) 1396–1407. [12] M.L.T. Guo, C.Y.A. Tsao, Tribological behaviour of self lubricating aluminium/SiC/graphite hybrid composites synthesized by the semi-solid powder-densification method, Compos. Sci. Technol. 60 (2000) 65–74. [13] M.L.T. Guo, C.Y.A. Tsao, Tribological behaviour of aluminium/ SiCp/nickel-coated graphite hybrid composites, Mater. Sci. Eng. A333 (2002) 134–145. [14] S. Mohan, J.P. Pathak, R.C. Gupta, S. Srivastava, Wear behaviour of graphite aluminium composite sliding under dry conditions, Z. Metallkad. 93 (12) (2002) 1245–1251.
1012
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
[15] Y. Zhan, G. Zhang, The role of graphite particles in the high-temperature wear of copper hybrid composites against steel, Mater. Des. 27 (2006) 79–84. [16] J.A. Bailey, S. Jeelani, S.E. Becker, Surface integrity in machining AISI 4340 Steel, J. Eng. Ind. ASME 98 (3) (1976) 999–1000. [17] J.R. Davis, ASM Specialty Handbook, ASM, 1993.
[18] S. Basavarajappa, G. Chandramohan, Wear studies on metal matrix composites: a Taguchi approach, J. Mater. Sci. Technol. 21 (6) (2005) 348–350. [19] M.H. Korkut, Effect of particulate reinforcement on wear behaviour of aluminium matrix composites, Mater. Sci. Technol. 20 (2004) 73–81. [20] I.M. Hutchings, Tribology Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, pp. 77–122.
Influence of sliding speed on the dry sliding wear behaviour and the subsurface deformation on hybrid metal matrix composite S. Basavarajappa a,∗ , G. Chandramohan a , Arjun Mahadevan a , Mukundan Thangavelu a , R. Subramanian b , P. Gopalakrishnan b a b
Department of Mechanical Engineering, PSG College of Technology, Coimbatore 641004, Tamil Nadu, India Department of Metallurgical Engineering, PSG College of Technology, Coimbatore 641004, Tamil Nadu, India Received 27 February 2006; received in revised form 30 August 2006; accepted 17 October 2006 Available online 28 November 2006
Abstract In recent years, more attention is being paid to the structure of both the surface and the subsurface of a material being subjected to wear. Surface and subsurface deformation can cause a considerable change in the microstructure of the material leading to a change in its properties. The present study investigates the influence of sliding speed on dry sliding wear behaviour and the extent of subsurface deformation in aluminium metal matrix composites, namely Al 2219/15SiCp and Al 2219/15SiCp-3graphite all fabricated by the liquid metallurgy route. Dry sliding wear tests were conducted using a pin-on-disc machine. The subsurface deformation was assessed as a measure of variation in microhardness along the depth normal to the cross-section of the worn surface. The results reveal that with increasing sliding speeds in the mild wear region the degree of subsurface deformation was also increasing. The graphitic composite exhibited less degree of subsurface deformation in comparison to the graphite free composite. © 2006 Elsevier B.V. All rights reserved. Keywords: Metal matrix composites; Wear rate; Subsurface deformation; Microhardness
1. Introduction Aluminium alloys are widely used in the automotive industry because of their high strength to weight ratio as well as high thermal conductivity. It is used particularly in automobile engines as cylinder liners as well as other rotating and reciprocating parts, such as the piston, drive shafts, brake rotors and in other applications in automotive and aerospace industries [1,2]. Aluminium and its alloys exhibit poor tribological properties leading even to seizure under adverse conditions. Hence, a strong drive to develop new materials with greater resistance to wear and better tribological properties led to the development of aluminum metal matrix composites [3,4]. The performance of metal matrix composite reinforced with ceramic particles has been reported to be superior to that of their unreinforced matrix alloy [5,6]. The addition of reinforcements significantly improves the wear resistance of aluminium alloy. Zou et al. [7] found that the wear resistance
∗
Corresponding author. Tel.: +91 422 2572177; fax: +91 422 2573833. E-mail address: [email protected] (S. Basavarajappa).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.10.016
of composites is proportional to the volume fraction and average diameter of SiC particles (SiCp). The wear resistance of 38 vol% SiCp particle (average size 57 m) reinforced composite is almost 10 times higher than that of the SiCp (5.5 m) reinforced composite with the same volume fraction. They also found that the depth of plastic deformation zone in the subsurface decreases with increasing volume fraction and average diameter of SiCp particles. Lim et al. [8] have reported that the dominance of particular wear mechanism for aluminium alloy based SiC particulate composite is primarily dependant on the sliding conditions. It was found that the addition of SiC particles to A356 grade aluminium alloy extended the mild wear regime to higher speeds and loads, thereby inhibiting severe wear. SiC particles assist in the retention of an oxide transfer layer on composite sliding surfaces which prevent metal-to-metal contact and keep wear behaviour within the mild wear regime [9]. Venkataraman and Sundararajan [10] have found that there exist a strong correlation between wear behaviour and the hardness, thickness and composition of the mechanically mixed layer (MML) that is formed in the sliding wear conditions. The best wear resistance was observed when a stable thin and hard MML layer is present.
1008
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
Reinforcement of SiCp with aluminum however gives rise to certain detrimental effects. The SiCp particles scuff the counterpart leading to greater wear loss [11]. The addition of solid lubricant particles such as graphite along with SiCp as hybrid reinforcements effectively improves the tribological properties of the whole system under sliding wear conditions [12]. These hybrid graphitic aluminium metal matrix composites provided greater seizure resistance even under dry sliding conditions. Wear becomes more stable as the amount of graphite addition increases [13]. The graphite smears on the sliding pin surface and forms a layer which reduces wear [14]. Riahi and Alpas [11] have found that SiCp–graphite reinforced composites displayed a transition from mild wear to severe wear at loads and sliding speed combinations which were higher than those of the unreinforced A356 aluminium alloy and graphite free A356 Al–20% SiCp composites. The uniform graphite microcrystalline layer on top of the worn surface helps to decrease the friction coefficient and alleviate the plastic deformation in the subsurface region avoiding severe adhesion wear [15]. In recent years, importance is being given to the structure of both the surface and the subsurface of the material being subjected to wear. Wear can cause considerable change in structure of the subsurface to a point where it becomes significantly different from that of the bulk material. These changes can lead to reduction in resistance to fatigue and creep [16]. In view of the account given above, a study has been made to compare the influence of sliding speeds on the extent of subsurface damage in Al 2219/15SiCp and Al 2219/15SiCp-3Gr composites. The subsurface damage is assessed as a measure of variation in microhardness along the cross-section of the worn surface. 2. Materials and experiment The matrix material selected for the present investigation was based on the Al–Cu–Mg alloy, designated by the American Aluminium Association as AA2219. This alloy was chosen as the matrix, since it provides excellent combination of strength and damage tolerance both at elevated and cryogenic temperatures [17]. The SiC particles, which were used to fabricate the composite, had an average particle size of 25 m and average density of 3.2 g/cm3 . Three different materials, namely Al 2219, Al 2219/15SiCp and Al 2219/15SiCp-3Gr were fabricated by the liquid metallurgy route [18]. Wear test specimen were machined from as-cast samples to form cylindrical pins having a diameter of 10 mm and a height of 30 mm. The specimen faces were then metallographically polished. The dry sliding wear tests were conducted using a pin-on-disc apparatus (Fig. 1) at room temperature as per the ASTM G9995 standards. The details of the wear test conditions are given in Table 1. The initial weight of the specimen was measured in a single pan electronic weighing machine with an accuracy of 0.0001 g. The wear of the composites were studied as a function of sliding velocity for the stated applied load and sliding distance. After running through the fixed sliding distance of 5000 m, the specimen was removed, cleaned with acetone and weighed to determine the weight loss. The difference in weight gives
Fig. 1. The schematic view of the pin-on-disc apparatus used in this study.
the wear of the specimen and then the wear rate was calculated [18]. Scanning electron microscope investigations were done to study the structure of the worn surfaces. Digital photographs of the worn surfaces were also taken for the study. The samples were then cut carefully along their diameter, perpendicular to the worn surface. The cut surface was then metallographically polished for measuring the microhardness along the depth using a Mitutoyo Vickers microhardness tester with a load of 50 g for a time of 10 s. The microhardness was measured at three locations and averaged to circumvent possible errors. Measurements of microhardness were carried out closest to 20 m from the worn surface and then intervals of 20 m were taken for subsequent indentations. The measurements were taken until the variation in hardness from the bulk hardness became negligible. 3. Results and discussion 3.1. Wear behaviour Wear rate versus sliding speed curves at an applied load of 40 N for Al 2219, Al 2219/15SiCp and Al 2219/15SiCp3Gr reinforced composite for a constant sliding distance of 5000 m are shown in Fig. 2. The wear rate for all the materials almost unchanged with an increase in sliding speed up to 3 m/s. Beyond 3 m/s the wear rate of the unreinforced alloy increased to higher magnitudes than those of the composites and seizure was observed at a sliding speed of 6.1 m/s. At 6.1 m/s, wear by delamination was observed to occur in the alloy with fragments from the pin being transferred to the disc and larger fragments Table 1 Details of the wear test conditions used in this study Pin material Disc material Pin dimensions Sliding speeds (m/s) Normal load (N) Sliding distance (m)
Al 2219, Al 2219/15SiCp, Al 2219/15SiCp-3Gr EN36 steel with a hardness of 65 HRc Cylinder with diameter 10 mm and height 30 mm 1.53, 3.0, 4.6, 6.1 40 5000
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
1009
Fig. 2. Variation of wear rate with the sliding speed for the three different materials subject to the dry sliding wear.
being thrown out. This increased wear rate finally leads to seizure of the material, which is evident from the micrograph shown in Fig. 3(a). This behaviour was also characterized by formation of large debris and burr on the leading edge of the pin (Fig. 3(b)). However, the wear rates of the composites almost unchanged with increase in sliding speed up to 4.6 m/s after which an increasing trend was observed. This unchanged wear rate until 4.6 m/s is due to the presence of SiCp particles. The introduction of reinforcement in the aluminium matrix reduces the wear rate and coefficient of friction [19]. The SiCp particles provide improved wear resistance to the composites than the unreinforced alloy by the formation of the mechanically mixed layer at the interface of the composite pin and the steel disc [10]. This layer was found to be composed of oxides of aluminium, oxides of iron which were scuffed off from the steel counter-face by SiCp particles and fractured SiCp particles [9,10]. In graphitic composites, the MML additionally contained graphite that acted as a solid lubricant further reduced the wear rate.
Fig. 3. Photographs of Al 2219 at an applied load of 40 N after running for a sliding distance of 5000 m at a sliding speed of 6.1 m/s: (a) material removed in severe wear and (b) formation of burr on the leading edge.
3.2. Surface and subsurface analysis Fig. 4 shows the variation of microhardness with distance from the worn surface for Al 2219/15SiCp composite at various sliding speeds. It can be observed that the pattern of variation in microhardness in the subsurface below the worn surface is different for different sliding speeds. At all sliding speeds the subsurface deformation observed in the present investigation has been divided into three zones, namely Zones 1–3. The microhardness just below the wear surface at a sliding speed of 3 m/s is about 139 VHN and stabilizes at a depth of 140 m from the worn surface. A large variation in microhardness up to a depth of 80 m (Zone 1) at a sliding speed of 3 m/s is due to the formation of MML on the surface of the pin, which was much harder than the bulk hardness of the tested material. Venkataraman and Sundararajan [10] have observed that the formation of MML on the pin surface is caused by turbulent plastic flow induced by the onset of shear instability at a critical depth below the wear surface. The hardness values between 80 and 140 m depth (Zone 2) showed a decreasing trend and stabilizes at 140 m (Zone 3).
Fig. 4. Variation of microhardness with the distance from the worn surface along the plane normal to the worn surface for Al 2219/15SiCp reinforced composite.
1010
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
Fig. 6. Variation of microhardness with the distance from the worn surface along the plane normal to the worn surface for Al 2219/15SiCp-3graphite reinforced composite.
Fig. 5. Worn surface of Al 2219/15SiCp composite at an applied load of load of 40 N after running for a distance of 5000 m at a sliding speed: (a) 3 m/s and (b) 6.1 m/s.
The increase in microhardness in the subsurface region (Zone 2) is because of several events such as shear deformation, void nucleation and growth as well as the onset of shear instability, taking place in the subsurface region [10]. Fig. 5(a) shows the presence of a large number of grooves over the entire surface of the pin. The abrasive wear mechanism is operative, as the MML is harder the grooves formed are shallow. This hard MML layer formed during the process reduces the amount of material removed from the surface of the pin. At a sliding speed of 4.6 m/s, the hardness of the subsurface is higher compared to the same material at 3 m/s up to an initial depth of 80 m after which it becomes more or less the same. The increase in the hardness in Zone 1 is due to the higher degree of compaction of the MML layer compared to those at lower speeds. The degree of deformation in Zone 2 extends to a further depth from the pin surface which is due to increased shear deformation in the subsurface regions with an increase in sliding speed. At a sliding speed of 6.1 m/s, the wear rate shows a rising trend which indicates more removal of material from the surface. Fig. 4 shows a decreased hardness in the vicinity of the top surface which may be due to the initiation of removal of the hard top surface layer. This also leads to an increase in the wear rate. The micrograph in Fig. 5(b) shows the removal of material
by delamination. Apart from this, cracks are generated along with particle pull out at the surface. The crack originating at various points propagates parallel to the surface of the pin and nucleates at one point which results in the removal of debris as sheets which are composed of MML [20]. With an increase in the depth from the surface the microhardness is seen to increase. This is due to the grain reorientation and refinement at the high sliding speed for which the reason has already been mentioned. Fig. 6 shows the variation of microhardness with distance from the worn surface for Al 2219/15SiCp/3Gr reinforced composites. The important microstructural features of the MML layers in case of graphite composite; in addition to the presence of SiCp particles is the presence of graphite films within the MML layer [11]. It was observed that at a sliding speed of 3 m/s, the surface damage increases up to a depth of 120 m from the worn surface. This is due to the fact that the graphite films squeezed out of the graphite particles elongated in the direction of shear over larger distances [11]. It has further been shown that the graphite films appear to have formed as a result of shearing of the graphite particles located immediately below the contact surfaces [11]. The microhardness reaches 131 VHN at the depth of 120 m from the worn surface. This high hardness in the subsurface could be attributed to the fact that the graphite layers were typically formed at depths of 20–100 m, where the magnitudes of subsurface strains were extremely high [11]. The microhardness increases just below surface at 20 m from the worn surface with increased sliding speeds of 4.6 and 6.1 m/s as observed in Fig. 6. But in subsurface regions beyond 40 m there is not much difference in the trend of microhardness variation between the sliding speeds of 4.6 and 6.1 m/s. The depth of subsurface deformations extends only up to 140 m below the worn surface for all the sliding speeds unlike the graphite free composite where the depth of subsurface deformation vary with the sliding speed. The degree of hardness and the depth of subsurface deformation in the graphite composite are distinctly less than the SiCp reinforced graphite free composite at all the
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
1011
Fig. 7. Worn surface of Al 2219/15SiCp-3graphite composite at an applied load of 40 N after running for a distance of 5000 m at a sliding speed: (a) 4.6 m/s and (b) 6.1 m/s.
sliding speeds studied. This can be attributed to the fact that the graphite layers serve to reduce the magnitude of shear stress transferred to the matrix underneath the MML [11]. It can also be observed from the SEM micrographs of the worn surface (Fig. 7(a and b)) that the formation of grooves is less distinct in the material at 4.6 m/s. As the speed increases to 6.1 m/s the grooves become deeper, since relatively more amount of material was removed. At a sliding speed of 6.1 m/s it can clearly be seen that there is no reduction in the hardness of the surface immediately below the worn surface so the transition to severe wear in the graphite composite has not occurred. In the graphite composite the transition to severe wear has been delayed due to the lubricating action of the graphite phase. Apart from delaying the transition to severe wear, the graphite composite enhances the resistance to scuffing damage imparted to the mating part by the MML layer. 4. Conclusions 1. The addition of SiCp reinforcement to Al 2219 alloy increases the wear resistance of the composites. The addition of graphite reinforcement in Al–SiCp composites further increases the wear resistance at all sliding speeds and effectively avoids the occurrence of severe wear. 2. The subsurface deformation from the contact surface to the region where the bulk hardness of the composite is reached can be divided into three zones. Zone 1 extends up to a depth of 60 m from the worn surface for both Al 2219/15SiCp and Al 2219/15SiCp-3Gr composite. Zone 2 corresponds to the subsurface that is 60–180 m from the worn surface in Al 2219/15SiCp and 60–140 m from the worn surface in Al 2219/15SiCp-3graphite composite. Zone 3 is the region that remains unaltered. 3. Variation of microhardness and the depth of deformation in the subsurface regions in Al 2219/15SiCp composite are different for different sliding speeds. The microhardness just below the wear surface is very high and as the depth increases the microhardness decreases. In the severe wear region the initial microhardness just below the worn surface is less, which can be attributed to the material removal in the form of sheets and particle pull out, exposing undeformed material.
4. A similar trend of microhardness variation in the subsurface regions is observed in Al 2219/15SiCp-3graphite composite at different sliding speeds. Further for all the sliding speeds, the depth of subsurface deformation in the graphite composite stabilizes at 140 m from the worn surface. The degree of subsurface deformation in graphite composite is less than that of the graphite free composite. References [1] A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Metal matrix composites—a review, J. Mater. Sci. 26 (1991) 1137–1157. [2] I. Sinclair, P.J. Gregson, Structural performance of discontinuous metal matrix composites, Mater. Sci. Technol. 3 (1997) 709–725. [3] A.P. Sannino, H.J. Rack, Dry sliding wear of discontinuously reinforced aluminium composites: review and discussion, Wear 189 (1995) 1– 19. [4] B.C. Pai, T.P.D. Rajan, R.M. Pillai, Aluminium matrix composite castings for automotive applications, Indian Foundry J. 50 (2004) 30–39. [5] K.G. Satyanarayana, R.M. Pillai, B.C. Pai, Recent developments and prospects in cast aluminium matrix composites, Trans. Indian Inst. Metals 55 (3) (2002) 115–130. [6] F. Gul, M. Acilar, Effect of the reinforcement volume fraction on the dry sliding wear behaviour of Al-10Si/SiCp composites produced by vacuum infiltration technique, Compos. Sci. Technol. l64 (2004) 1959–1970. [7] X.G. Zou, H. Miyahara, K. Yamamoto, K. Ogi, Sliding wear behaviour of Al–Si–Cu composites reinforced with SiC particles, Mater. Sci. Technol. 19 (2003) 1519–1526. [8] S.C. Lim, M. Gupta, L. Ren, J.K.M. Kwok, The tribological properties of Al–Cu/SiCp metal-matrix composites fabricated using the rheocasting technique, J. Mater. Process. Technol. 80–90 (1999) 591–596. [9] S. Wilson, A.T. Alphas, Wear mechanism maps for metal matrix composites, Wear 212 (1997) 41–49. [10] 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. [11] A.R. Riahi, A.T. Alpas, The role of tribolayers on sliding wear behaviour of graphitic aluminium matrix composites, Wear 251 (2001) 1396–1407. [12] M.L.T. Guo, C.Y.A. Tsao, Tribological behaviour of self lubricating aluminium/SiC/graphite hybrid composites synthesized by the semi-solid powder-densification method, Compos. Sci. Technol. 60 (2000) 65–74. [13] M.L.T. Guo, C.Y.A. Tsao, Tribological behaviour of aluminium/ SiCp/nickel-coated graphite hybrid composites, Mater. Sci. Eng. A333 (2002) 134–145. [14] S. Mohan, J.P. Pathak, R.C. Gupta, S. Srivastava, Wear behaviour of graphite aluminium composite sliding under dry conditions, Z. Metallkad. 93 (12) (2002) 1245–1251.
1012
S. Basavarajappa et al. / Wear 262 (2007) 1007–1012
[15] Y. Zhan, G. Zhang, The role of graphite particles in the high-temperature wear of copper hybrid composites against steel, Mater. Des. 27 (2006) 79–84. [16] J.A. Bailey, S. Jeelani, S.E. Becker, Surface integrity in machining AISI 4340 Steel, J. Eng. Ind. ASME 98 (3) (1976) 999–1000. [17] J.R. Davis, ASM Specialty Handbook, ASM, 1993.
[18] S. Basavarajappa, G. Chandramohan, Wear studies on metal matrix composites: a Taguchi approach, J. Mater. Sci. Technol. 21 (6) (2005) 348–350. [19] M.H. Korkut, Effect of particulate reinforcement on wear behaviour of aluminium matrix composites, Mater. Sci. Technol. 20 (2004) 73–81. [20] I.M. Hutchings, Tribology Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, pp. 77–122.