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Tribological behaviour of aluminium alloy composites: A comparative study with a copper-based alloy
64
Wear, 162-164 (1993) 64-74
Tribological behaviour of aluminium alloy composites: study with a copper-based alloy
a comparative
S. Das and B. K. Prasad Regional Research Laboratory, CSIR, Hoshangabad Road, near Habibganj Naka, Bhopal 462026 (MP) (India)
Abstract This paper describes the results of dry sliding wear tests of LM13 alloy and graphite-particle-reinforced LM13 alloy composite in cast and heat-treated conditions, and their comparison with a conventionally used copperbased bearing alloy. Sliding wear tests were conducted on a pin-on-disc wear test apparatus against a rotating steel(EN25) counterface at a sliding velocity of 2.68 m s-‘. Results of the present investigation showed that the wear rate of the composite was considerably less than that of the copper-based alloy at an applied pressure of less than 4.0 MPa. Above 4.0 MPa the composite samples seized. However, the seizure pressure of the composite was less than that of the copper-based alloy. Detailed studies of wear surfaces, wear debris and subsurface deformation have been carried out. The overall results indicate that the heat-treated aluminium alloy-graphite composite could be considered as an excellent substitute for a conventional bearing alloy, such as Cu-Sn-Pb, at relatively low applied pressures (below 4.0 MPa), while the copper-based alloy can withstand higher pressures (up to 10 MPa).
1. Introduction Leaded-tin bronzes have been used in many tribological applications, including plain bearings [l], and, even now, they are among the universally accepted materials for such applications. However, in most studies, attention has been focused on evaluating the performance of their actual components (bearings) in service [l-7]. Few investigations devoted to the understanding of the wear mechanisms involved in such applications have been carried out, and are confined largely to those reported in refs. 8-14. Moreover, the high density of bronzes, associated with problems such as health hazards caused by the toxic properties of lead effective during melting and the use of the costly and scarce element tin, has led to the searching for alternative materials for possible applications [l]. In view of the above factors, considerable work has been carried out to explore the possibilities of using Al-Si alloys and, as a result, these have emerged as a potential material for a variety of tribological applications, including plain bearings [l, U-171. However, Al-Si alloys have been found to suffer from the problem of scuffing. The addition of graphite particles to Al-Si alloys has offered improved tribological properties under specific conditions [18, 191. The dispersoid phase acts as a solid lubricant under boundary lubrication conditions [20], in which case there always exists
0043-X48/93/$6.00
the possibility of a temporary scarcity of the lubricant. Smearing of graphite particles on the contacting surfaces reduces the metal-to-metal contact, thereby preventing seizure. Recent investigations indicate that the dispersion of graphite particles in a heat-treatable variety of Al-Si alloy and subjecting the composites to suitable heat treatments help to compensate for the deterioration in their mechanical properties; the deterioration occurs owing to the addition of weak graphite particles to the alloys [18, 191. Furthermore, the above measures also increase the efficiency of the smearing of graphite particles on the mating surfaces by way of morphological modifications of the matrix microstructure [18, 191. This ultimately leads to improved tribological properties of the graphitic Al-Si alloys. An attempt has been made to understand the dry sliding wear behaviour of a conventional leaded-tin bronze (SAE64) and a heat-treatable variety of Al-Si alloy (LM13). The LM13 alloy was also dispersed with graphite particles and heat treated to study the effect of graphite particles and the change in the silicon morphology on the wear properties of the alloy. 2. Experimental details 2.1. Material preparation The chemical compositions of the Al-Si alloy and the copper-based alloy are shown in Table 1.
0
1993 - Elsevier Sequoia. All rights reserved
S. Das, B. K Prasad / Tribological behaviour of Al alloy composites TABLE 1. Chemical composition copper-based alloy Element
Copper Magnesium Tin Manganese Lead Nickel Aluminium
10.0 1.0 1.0 0.8 1.5 Rest
persive X-ray spectroscopy (WDXS) capable of detecting C. The wear surfaces, wear debris and subsurface
deformation were also studied in the scanning electron microscope.
Composition (wt.%) LM13
Silicon
of the LM13 alloy and the
65
Copper-based alloy Rest 10.0 10.0 -
The copper-based alloy was prepared by the liquid metallurgy route, using a coke-fired furnace. The required quantities of the elements were added during melting. Low melting temperature elements, such as tin and lead, were added at the end of the melting operation to avoid their undesired loss by vaporization. The composite containing 3.0 wt.% graphite particles (size 63-120 pm) was synthesized by thevortex technique [21]. To induce wettability between the graphite particles and the molten aluminium alloy, 1.0 wt.% Mg metal was added. The copper-based alloy, Al-Si alloy and composite melts were solidified in a cast iron mould 18 mm in diameter.
3. Results
3.1. Microstructure The microstructure of the copper-based alloy mainly consists of Cu-Sn intermetallic phase G(Cu,,Sn,) distributed in the solid solution of tin in copper, and discrete particles of lead [22, 231, as shown in Fig. l(a). A higher magnification micrograph (Fig. l(b)) clearly reveals the intermetallic phase (marked ‘A’) and lead particles (marked ‘B’). The microstructure of the cast Al-Si (LM13) alloy depicts primary aluminium dendrites and eutectic silicon
2.2. Heat treatment The Al-Si alloy and its composite were heat treated in a tube furnace. The heat treatment cycle consisted of homogenization at 520 “C, quenching in warm water at 60 “C and ageing at 180 “C for 6 h. 2.3. Sliding wear tests Dry sliding wear tests were carried out using a Cameron-Plint pin-on-disc wear test apparatus. A detailed description of the test set-up and procedure is given elsewhere [18]. A sliding velocity of 2.68 m s-’ was maintained during the tests. The wear tests were carried out at various pressures. The wear rates were computed by weight loss measurements at sliding distance intervals of 500 m. The seizure pressures for all the specimens were determined. The onset of seizure was signalled by an abnormal vibration and noise from the pin-and-disc assembly. 2.4. Microscopy The samples for microstructural studies were polished metallographically, and etched with Keller’s reagent in the case of the Al-Si alloys and potassium dichromate in the case of the copper-based alloy. The specimens were observed in optical and scanning electron microscopes. The latter was equipped for wavelength-dis-
(4
(b) Fig. 1. (a) Microstructure of the copper-based alloy and (b) a higher magnificationview, showing the intermetallic phase Cu,,Sns (marked ‘A’) and lead particles (marked ‘B’).
66
S. Das, B. K. Prasad I Tribological
behaviour
of Al alloy composites
(4
(4 Fig. 2. Microstructure of (a) the cast LM13 alloy, (b) showing the distribution of graphite particles in the matrix, (c) a higher magnification view, showing graphite particles in the last freezing zone and (d) the alteration of the morphology of the eutectic silicon to a near-spherical morphology as the result of heat treatment.
in the interdendritic region and around the dendrites (Fig. 2(a)). The microstructure of the cast Al-Si alloy-graphite composite (Fig. 2(b)) reveals a reasonably uniform distribution of the graphite particles. A higher magnification micrograph (Fig. 2(c)) shows the pushing of graphite particles in the last freezing eutectic liquid. A typical microstructure of the heat-treated composite is shown in Fig. 2(d). The interesting feature to be observed is the alteration of the silicon morphology from a faceted to a near-spherical morphology (Fig. 2(d)). 3.2. Sliding wear Dry sliding wear rates of the LM13 alloy and composite, and of the copper-based alloy are shown in Fig. 3. It can be seen that the wear behaviour in all the
cases was non-linear and the wear rate increased with the applied pressure. The copper-based alloy showed
APPLIED
PRESSURE,
MPo
Fig. 3. Wear rate as a function of applied pressure. 0, LM13; A, LM13 (HT); n , LM13-3% graphite; A, Cu-Sn-Pb; 0, LM13-3% graphite (HT); *, seizure.
S. Das, B. K. Prasad / Tribologkal behaviour of Al alloy composites
0
2
4 APPLIED
6
0
PRESSURE,
Fig. 4. Maximum temperature function of applied pressure. Cu-Sn-Pb; *, seizure.
10
67
12
MPo
rise near the sliding surface as a 0, LM13-3% graphite (HP); A, Fig. 6. Wear surface of the copper-based alloy at 4.0 MPa, showing the sticking of a debris particle and cracks on the stuck particle.
(a)
Fig. 5. Wear surfaces of the copper-based alloy at 1.0 MPa pressure, showing (a) surface cracks (marked with an arrow) and (b) cracks along with the severely damaged region.
the highest wear rate of all the specimens, while the heat-treated composite showed the lowest. For example, at 3.0 MPa pressure the wear rate of the heat-treated composite was found to be around 1.0X lo-” m3 m-‘, whereas for the copper-based alloy it was approximately 30.0~ lo-l2 m3 m-l. However, the copper-based alloy seized at a considerably higher pressure than did the composite. For instance, the Al-Si alloy composite in the heat-treated condition seized at 4.0 MPa, while the copper-based alloy seized at 10.0 MPa. It can be seen that above 4.0 MPa, the slope of the wear rate curve of the copper-based alloy decreased. The seizure pressure of the LM13 alloy in the cast and heat-treated conditions, and the cast composite showed lower seizure pressures than that for the heat-treated composite (Fig. 3). Figure 4 shows the maximum temperature rise near the contact surface as a function of the applied pressure. The diagram clearly depicts that the maximum contactsurface temperature increased with an increase in the applied pressure. It also can be seen that, at and below 3.0 MPa, the temperature rise in the case of the heattreated composite was less than that of the copperbased alloy. However, above this pressure a reversal in the trend was observed. 3.3. SEA4 examination of wear surfaces The wear surface of the copper-based alloy at 1.0 MPa pressure is shown in Fig. 5. The wear surface reveals continuous grooves and severely damaged regions (Fig. 5(a)). Surface cracks (marked by an arrow) may also be seen in Fig. 5(a). A magnified view of Fig. 5(a) is shown in Fig. 5(b), which clearly reveals the cracks along with the severely damaged regions. Figure 6 shows the wear surface of the alloy at an applied pressure of 4.0 MPa. It may be observed that
S. Das, B. K. Prasad I Tribological behaviour of Al alloy composites
was less than that for the as-cast alloy (Fig. 8(b)). Figure 8(c) depicts the wear surface of the heat-treated composite. The presence of a graphite film on the wear surface was detected by the WDXS analysis (Fig. 8(d)), but no such graphite film formation could be resolved on the wear surface of the cast composite. 3.4. Debris analysis A wide range of sizes of wear debris was found in the case of the copper-based alloy. At 1.0 MPa the size of the debris was found to be large in general (approximately 200-400 pm) with a few fine particles (Fig. 9(a) and 9(b)). At a higher applied pressure, say 4.0 MPa, the debris size was found to be around 100-200 pm (Fig. 9(c)). Further increasing the applied pressure up to 10.0 MPa led to the generation of still finer wear debris, the majority being smaller than 100 pm (Fig. 9(d)). Most of the debris were found to be flaky in nature in all the cases (Fig. 9). Occasionally, machining chips of iron were also observed in the debris (Fig. 10(a)). The X-ray dot map of iron corresponding to Fig. 10(a) is shown in Fig. 10(b). Figure 11(a) shows typical equiaxed wear debris of the cast Al-Si alloy at an applied pressure of 1.0 MPa. When the applied pressure was increased to 1.5 MPa, the shape of the debris changed to flake type (Fig. 11(b)). In the case of the heat-treated composite, the shape of the debris was identical to that for the cast alloy, the only difference being the finer size in the case of the heat-treated composite (Fig. 11(c)). Occasionally, silicon particles were also observed in the debris (Fig. 11(d)). 0) Fig. 7. Wear surface of the copper-based alloy at the seizure pressure (10.0 MPa), showing (a) heavily damaged regions and (b) sticking of a debris particle on the wear surface.
the extent of the severely damaged regions in this case (Fig. 6) was less than that at 1.0 MPa (as shown in Fig. 5). Additionally, some debris particles were found to stick to the surface (Fig. 6). Cracks on the stuck particles also may be seen in Fig. 6. A typical wear surface at the seizure pressure (10.0 MPa) is shown in Fig. 7. Heavily damaged regions (Fig. 7(a)) and sticking of debris particles to the wear surface (Fig. 7(b)) were the two interesting features. Furthermore, it is worth mentioning that the presence of microcracks was found to be significantly less in this case than in the cases of the samples tested at relatively lower pressures (1.0 MPa). A typical wear surface of the cast Al-Si alloy tested at 1.5 MPa is shown in Fig. 8(a). It shows continuous grooves and damaged regions. In the case of the heattreated alloy, the tendency for surface cracking to occur
3.5. Subsurface studies Figure 12(a) shows a typical micrograph of the transverse section of the copper-based alloy at 1.0 MPa. The zone near the wear surface (top portion) may be seen in the figure. The deformed zone was found to extend to a depth of 10-20 pm. A large mass of material attached to the subsurface region and the presence of microcracks there are also shown (Fig. 12(a)). At higher applied pressures, say 4.0 and 10.0 MPa, a smaller mass was attached to the substrate as shown in Figs. 12(b) and 12(c) respectively. The interesting feature observed at 10.0 MPa was the flow of the intermetallic compound (Fig. 12(c)) and lead particles (Fig. 12(d)) along the sliding direction in the subsurface regions. The X-ray dot maps of copper and lead corresponding to Fig. 12(d) are shown in Figs. 12(e) and 12(f) respectively. The micrograph of the subsurface of the cast LM13 alloy is shown in Fig. 13(a). Breaking of eutectic silicon particles in the subsurface region (top) may be seen, while the unaffected bulk structure (away from the surface) showed the normal faceted silicon particles.
69
S. Das, B. K Prasad / Tribological behaviozu of Al alloy composites
(4
(b)
(c)
td)
Fig. 8. Wear surface of (a) the cast LM13 alloy, (b) the heat-treated and (d) the X-ray dot rn$ of C, corresponding to ic).
Subsurface cracks were also observed in this case. In the case of the heat-treated alloy, no such breaking of eutectic silicon particles and subsurface cracking could be observed, and the subsurface as well as the unaffected bulk revealed identical shapes of eutectic silicon (Fig. -
13(b)). 4. Discussion It may be noted that graphite has a hexagonal crystal structure with weak forces between the basal planes. This facilitates easy smearing of the graphite and the formation of a lubricating film [24]. The third-body graphite fihn minimizes the direct metal-to-metal contact and produces a lubricating effect. Similarly, lead also has an h.c.p. crystal structure and tends to flow during the sliding action on the contacting surface [25] and form a third-body film of lead, similar to graphite. This
LM13 alloy and (c) the heat-treated
composite
at 1.5 MPa
minimizes the direct contact between the contact surfaces and reduces the wear rate. The formation of the lead film on the mating surfaces will depend upon the degree of the shear stress produced during the sliding action and also on the extent of rise in temperature. The low wear rate of the heat-treated composite was mainly due to the formation of the graphite film on the contact surface of the composite (Fig. 8(c)), whereas the wear surface of the copper-based alloy showed surface cracks (Fig. 5) and the formation of large debris particles (Figs. 9(a) and 9(b)) at lower pressures (below 4.0 MPa). The above facts clearly indicate that the lead particles could not smear on the sliding surface of the copper-based alloy, rather they might have been engulfed in the large debris (Figs. 9(a) and 9(b)) as also evident from the subsurface analysis (Fig. 12(a)). The high seizure resistance of copper-based alloy was due to the high melting point of the matrix. Although
S. Das, B. K. Prasad / Tribological behaviour of AI alloy composites
(b)
(cl
(4
Fig. 9. Wear debris of the copper-based alloy at (a), (b) 1.0 MPa, showing large flaky particles, Reduction in the size of deb& with increasing p&s&k may be noted trom (a) to (d).
(a) Fig. 10. (a) A typical machining
(c) at 4.0 MPa and (d) 10.0 MPa.
(b) chip of iron and (b) the X-ray dot map of iron corresponding
to (a).
S. Das, B. K Prasad / Tribological behaviour of Al alloy composites
(4
(b)
(4 Fig. 11. (a) Equiaxed debris of the cast LM13 alloy at 1.0 MPa, (b) flake-type debris at 1.5 MPa (seizure), case of the heat-treated composite at 2.5 MPa and (d) a typical silicon particle in the debris.
the hardnesses of both the copper-based alloy and the heat-treated composite were comparable (about 90 HV), the ultimate tensile strength of the copper-based alloy (250 MPa) was higher than that of the heat-treated composite (180 MPa). This may be another reason for the higher seizure resistance of the copper-based alloy. The interesting feature observed in Fig. 3 is the existence of two distinct wear regimes in the case of the copper-based alloy. It may be noted from Fig. 3 that, above 4.0 MPa, the slope of the wear rate plot decreased with a further increase in the pressure. It appears that at higher pressures the lead particles smeared on the surface and offered a change in the trend (Fig. 3). Although the formation of the lead film could not be detected on the wear surface by WDXS, as also observed in an earlier study [9], the flow of lead was observed in the subsurface region (Fig. 12(d)).
(c) fine debris in the
The generation of finer debris particles (Fig. 9(d)) also indicates the possibility of the formation of the lead film on the wear surface. In addition, the wear debris formed in this pressure regime had a tendency to stick to the wear surface (Figs. 6 and 7); this could also be responsible for reducing the slope of the wear rate plot (Fig. 3). In the lower pressure range, adhesion of the copperbased alloy to the steel disc was practically nil, while adhesion was observed to a considerable extent at higher pressures. Similar observations have also been made in a recent study [ll]. In the low load regime, the results also showed the formation of large flaky debris (Fig. 9(b)). This may be due to the presence of softer lead particles, which could offer sites for the easy nucleation of cracks. When such cracks join each other, large debris particles would be generated (Fig. 9(b)). The presence of plenty of cracks on the wear surface
S. Das, B. K. Prasad I Tribological behaviour of AI alloy composites
(4
(e) Fig. 12. Transverse section of the copper-based alloy, showing (a) a large mass of material attached to the subsurface at 1.0 MPa, (b) at 4.0 MPa, (c) flow of the intermetallic at 10.0 MPa and (d) flow of lead particle at 10.0 MPa, and (e), (r) the X-ray dot maps of copper and lead, respectively, corresponding to (d).
S. Das, B. X Prasad / Tkbological behaviour of Al alloy composites
73
(4 Fig. 13. Subsurface of (a) the cast LM13 alloy, showing breaking of silicon particles heat-treated alloy, revealing practically unaffected subsurface microstructure.
(Fig. 5) and debris particles (Figs. 9(a) and 9(b)) in the case of the copper-based alloy tested at low pressures (below 4.0 MPa) also suggest its higher wear rate than that of the heat-treated composite (Fig. 3). The Al-Si alloy and its composite revealed the formation of relatively finer debris (equiaxed) (Fig. 11(a)) at lower pressures, which is consistent with their having a lower wear rate than at higher pressures (Fig. 3). The lowest wear rate of the heat-treated composite (Fig. 3) can be explained on the basis of the formation of a graphite film on the wear surface (Fig. 8(c)), the generation of finer debris (Fig. 11(c)) and there being significantly less surface damage (Fig. 8(c)). Alteration of the silicon morphology from faceted in the as-cast condition (Fig. 2(a)) to nearly spherical in the heat-treated alloy (Fig. 2(d)) helps in reducing the propensity of crack nucleation, thereby allowing the graphite particles to smear on the sliding surface. This ultimately improves the wear properties of the heattreated composite. The presence of subsurface cracks in the copperbased alloy, cast Al-Si alloy and the cast composite suggests that crack nucleation at the interface of the matrix and hard silicon/intermetallic or soft lead/graphite particles may be the rate-controlling mechanism of material removal. Recently, Alpas and Zhang [26] have suggested that the presence of subsurface cracks indicates that subsurface delamination is the wear mechanism. Deeper grooves on the wear surfaces (Fig. 5-8) could be due to three-body abrasion caused by the entrapment of oxide particles and machining chips generated from the disc (Fig. 10). Similar observations have also been made by earlier investigators [ll, 271.
in the subsurface
region (top), and (b) the
The above observations clearly indicate that the heattreated composite could be considered as an excellent substitute for conventional leaded-tin bronzes at low pressures from a wear point of view, while the bronze could withstand higher applied pressures.
5. Conclusions (1) The heat-treated Al-Si alloy-graphite composite showed the least wear rate among all the materials for operating pressures below 4.0 MPa. However, the copper-based, leaded bronze showed the maximum wear rate for operating pressures below 4.0 MPa; it seized at 10.0 MPa. (2) The temperature rise of the heat-treated composite was less than that of the bronze for operating pressures below 4.0 MPa, while a reverse trend was observed at higher pressures. (3) Observations of this investigation suggest that the Al-Si alloy-graphite composite in the heat-treated condition can be a cost- and energy-effective substitute for conventional leaded-tin bronzes in tribological applications subject to low pressure conditions.
Acknowledgment The authors are grateful to Prof. T. C. Rao, Director, RRL, Bhopal for his encouragement and for granting permission to publish this paper.
14
S. Das, B. K. Prasad / Triboiog’cal behaviour of Al alloy composites
References 1 G. C. Pratt, Materials for plain bearings, ht. Met. Rev., 18 (1973) l-27. 2 R. C. Erikson, Journal bearing applications of two zinc alloys, Proc. 2Ist Annual Conf Met. CIM, Toronto, Canada, 1982. 3 S. Murphy and T. Savaskan, Comparative wear behaviour of Zn-Al based alloys in an automotive engine application, Wear, 98 (1984) 151-161. 4 K. J. Altorfer, Zinc alloys compete with bronze bearings and bushings, Met. Prog. (November 1982) 29-31. 5 P. Delneuville, Tribological behaviour of Zn-Al alloys (ZA27) compared with bronze when used as a bearing material at high load and at very low speed, Wear, 105 (1985) 283-292. 6 T. S. Calayag, Zinc alloy replace bronze in mining equipment bushings and bearings, Min. Eng., 35 (1983) 727-728. 7 W. Mihaichuk, Zinc alloy bearings challenge the bronzes, Mach. Des., 53 (1981) 133-137. 8 P. P. Lee, T. Savaskan and E. Laufer, Wear resistance and microstructure of Zn-Al-Si and Zn-Al-Cu alloys, Wear, 117 (1987) 79-89. 9 W. A. Glaeser, Transfer of lead from leaded bronze during sliding contact, in K. C. Ludema (ed.), Proc. Int. Conf on Wear of Materials, Denver, CO, April 9-13, 1989, pp. 255-260. 10 V. E. Buchanan, P. A. Molian, T. S. Sudarshan and A. Akers, Frictional behaviour of non-equilibrium Cu-Pb alloys, Wear, 146 (1991) 241-256. 11 P. A. Molian, V. E. Buchanan, T. S. Sudarshan and A. Akers, Sliding wear characteristics of non-equilibrium Cu-Pb alloys, Wear, 146 (1991) 257-267. 12 W. A. Glaeser, Wear properties of heavy loaded Cu based bearing alloys, J. Met., 35(10)(1983) 50-55. 13 T. Okada and Y. Iwai, Resistance to the damage caused by the joint action of cavitation erosion and sliding wear of bearing alloys, Lubr. Eng., 44(1)(1988) 61-68. 14 J. Sikora, W. Majewski and A. Neyman, Wear of bearing alloys under incomplete lubrication, Tribologia, 19(l) (1988) 12-14, 21.
15 L. Bruni and P. Iguera, A new silicon-graphite-aluminium alloy for cylinder liners, Automot. Eng. (February/March 1978) 29-36. 16 B. P. Krishnan, N. Raman, K. Narayanaswamy and P. K. Rohatgi, Performance of aluminium alloy-graphite bearings in a diesel engine, Tribal. Int., 16 (1983) 239-244. 17 B. P. Krishnan and P. K. Rohatgi, Development of aluminium-graphite cast composites, Proc. 28th Annual Convention of the Institute of Indian Foundrymen, Bangalore, February 16-18 I979, paper no. 4.7. 18 S. Das, S. V. Prasad and T. R. Ramachandran, Microstructure and wear of cast (Al-Si alloy)-graphite composites, Wear, 133 (1989) 173-187. 19 S. Das, S. V. Prasad and T. R. Ramachandran, Tribology of Al-Si alloy-graphite composites: triboinduced graphite films and the role of silicon morphology, Mater. Sci. Eng. A, 138 (1991) 123-132. 20 P. K. Rohatgi and B. C. Pai, Seizure resistance of cast aluminium alloys containing dispersed graphite particles of different sizes, Trans. ASME, 101 (1979) 376-380. 21 B. P. Krishnan, M. K. Surappa and P. K. Rohatgi, The UPPAL process - a direct method to prepare cast aluminium alloy-graphite particle composites, J. Mater. Sci., 16 (1981) 1209-1216. 22 C. R. Brooks (ed.), Heat Treatment, Structure and Propenies of Non-ferrous Alloys, American Society for Metals, Metals Park, OH, 1982, pp. 275-327. 23 E. G. West (ed.), Copper and Its Alioys, Ellis Hotwood, New York, 1982, pp. 85-131. 24 E. R. Braithwaite and G. W. Rowe, Principles and applications of lubrication with solids, Sci. Lubr. (March 1963) 92-110. 25 Y. Tsuya and R. Takagi, Lubricating properties of lead films on copper, Wear, 7 (1964) 131-143. 26 A. T. Alpas and J. Zhang, Wear rate transitions in cast Al-Si alloys reinforced with SIC particles, Ser. Metall., 26 (1992) 505-509. 27 0. P. Modi, B. K. Prasad, A. H. Yegneswaran and M. L. Vaidya, Dry slidingwear behaviour of squeeze cast Al alloy-SiC composites, Mater. Sci. Eng. A, 151 (1992) 235-245.
Wear, 162-164 (1993) 64-74
Tribological behaviour of aluminium alloy composites: study with a copper-based alloy
a comparative
S. Das and B. K. Prasad Regional Research Laboratory, CSIR, Hoshangabad Road, near Habibganj Naka, Bhopal 462026 (MP) (India)
Abstract This paper describes the results of dry sliding wear tests of LM13 alloy and graphite-particle-reinforced LM13 alloy composite in cast and heat-treated conditions, and their comparison with a conventionally used copperbased bearing alloy. Sliding wear tests were conducted on a pin-on-disc wear test apparatus against a rotating steel(EN25) counterface at a sliding velocity of 2.68 m s-‘. Results of the present investigation showed that the wear rate of the composite was considerably less than that of the copper-based alloy at an applied pressure of less than 4.0 MPa. Above 4.0 MPa the composite samples seized. However, the seizure pressure of the composite was less than that of the copper-based alloy. Detailed studies of wear surfaces, wear debris and subsurface deformation have been carried out. The overall results indicate that the heat-treated aluminium alloy-graphite composite could be considered as an excellent substitute for a conventional bearing alloy, such as Cu-Sn-Pb, at relatively low applied pressures (below 4.0 MPa), while the copper-based alloy can withstand higher pressures (up to 10 MPa).
1. Introduction Leaded-tin bronzes have been used in many tribological applications, including plain bearings [l], and, even now, they are among the universally accepted materials for such applications. However, in most studies, attention has been focused on evaluating the performance of their actual components (bearings) in service [l-7]. Few investigations devoted to the understanding of the wear mechanisms involved in such applications have been carried out, and are confined largely to those reported in refs. 8-14. Moreover, the high density of bronzes, associated with problems such as health hazards caused by the toxic properties of lead effective during melting and the use of the costly and scarce element tin, has led to the searching for alternative materials for possible applications [l]. In view of the above factors, considerable work has been carried out to explore the possibilities of using Al-Si alloys and, as a result, these have emerged as a potential material for a variety of tribological applications, including plain bearings [l, U-171. However, Al-Si alloys have been found to suffer from the problem of scuffing. The addition of graphite particles to Al-Si alloys has offered improved tribological properties under specific conditions [18, 191. The dispersoid phase acts as a solid lubricant under boundary lubrication conditions [20], in which case there always exists
0043-X48/93/$6.00
the possibility of a temporary scarcity of the lubricant. Smearing of graphite particles on the contacting surfaces reduces the metal-to-metal contact, thereby preventing seizure. Recent investigations indicate that the dispersion of graphite particles in a heat-treatable variety of Al-Si alloy and subjecting the composites to suitable heat treatments help to compensate for the deterioration in their mechanical properties; the deterioration occurs owing to the addition of weak graphite particles to the alloys [18, 191. Furthermore, the above measures also increase the efficiency of the smearing of graphite particles on the mating surfaces by way of morphological modifications of the matrix microstructure [18, 191. This ultimately leads to improved tribological properties of the graphitic Al-Si alloys. An attempt has been made to understand the dry sliding wear behaviour of a conventional leaded-tin bronze (SAE64) and a heat-treatable variety of Al-Si alloy (LM13). The LM13 alloy was also dispersed with graphite particles and heat treated to study the effect of graphite particles and the change in the silicon morphology on the wear properties of the alloy. 2. Experimental details 2.1. Material preparation The chemical compositions of the Al-Si alloy and the copper-based alloy are shown in Table 1.
0
1993 - Elsevier Sequoia. All rights reserved
S. Das, B. K Prasad / Tribological behaviour of Al alloy composites TABLE 1. Chemical composition copper-based alloy Element
Copper Magnesium Tin Manganese Lead Nickel Aluminium
10.0 1.0 1.0 0.8 1.5 Rest
persive X-ray spectroscopy (WDXS) capable of detecting C. The wear surfaces, wear debris and subsurface
deformation were also studied in the scanning electron microscope.
Composition (wt.%) LM13
Silicon
of the LM13 alloy and the
65
Copper-based alloy Rest 10.0 10.0 -
The copper-based alloy was prepared by the liquid metallurgy route, using a coke-fired furnace. The required quantities of the elements were added during melting. Low melting temperature elements, such as tin and lead, were added at the end of the melting operation to avoid their undesired loss by vaporization. The composite containing 3.0 wt.% graphite particles (size 63-120 pm) was synthesized by thevortex technique [21]. To induce wettability between the graphite particles and the molten aluminium alloy, 1.0 wt.% Mg metal was added. The copper-based alloy, Al-Si alloy and composite melts were solidified in a cast iron mould 18 mm in diameter.
3. Results
3.1. Microstructure The microstructure of the copper-based alloy mainly consists of Cu-Sn intermetallic phase G(Cu,,Sn,) distributed in the solid solution of tin in copper, and discrete particles of lead [22, 231, as shown in Fig. l(a). A higher magnification micrograph (Fig. l(b)) clearly reveals the intermetallic phase (marked ‘A’) and lead particles (marked ‘B’). The microstructure of the cast Al-Si (LM13) alloy depicts primary aluminium dendrites and eutectic silicon
2.2. Heat treatment The Al-Si alloy and its composite were heat treated in a tube furnace. The heat treatment cycle consisted of homogenization at 520 “C, quenching in warm water at 60 “C and ageing at 180 “C for 6 h. 2.3. Sliding wear tests Dry sliding wear tests were carried out using a Cameron-Plint pin-on-disc wear test apparatus. A detailed description of the test set-up and procedure is given elsewhere [18]. A sliding velocity of 2.68 m s-’ was maintained during the tests. The wear tests were carried out at various pressures. The wear rates were computed by weight loss measurements at sliding distance intervals of 500 m. The seizure pressures for all the specimens were determined. The onset of seizure was signalled by an abnormal vibration and noise from the pin-and-disc assembly. 2.4. Microscopy The samples for microstructural studies were polished metallographically, and etched with Keller’s reagent in the case of the Al-Si alloys and potassium dichromate in the case of the copper-based alloy. The specimens were observed in optical and scanning electron microscopes. The latter was equipped for wavelength-dis-
(4
(b) Fig. 1. (a) Microstructure of the copper-based alloy and (b) a higher magnificationview, showing the intermetallic phase Cu,,Sns (marked ‘A’) and lead particles (marked ‘B’).
66
S. Das, B. K. Prasad I Tribological
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of Al alloy composites
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(4 Fig. 2. Microstructure of (a) the cast LM13 alloy, (b) showing the distribution of graphite particles in the matrix, (c) a higher magnification view, showing graphite particles in the last freezing zone and (d) the alteration of the morphology of the eutectic silicon to a near-spherical morphology as the result of heat treatment.
in the interdendritic region and around the dendrites (Fig. 2(a)). The microstructure of the cast Al-Si alloy-graphite composite (Fig. 2(b)) reveals a reasonably uniform distribution of the graphite particles. A higher magnification micrograph (Fig. 2(c)) shows the pushing of graphite particles in the last freezing eutectic liquid. A typical microstructure of the heat-treated composite is shown in Fig. 2(d). The interesting feature to be observed is the alteration of the silicon morphology from a faceted to a near-spherical morphology (Fig. 2(d)). 3.2. Sliding wear Dry sliding wear rates of the LM13 alloy and composite, and of the copper-based alloy are shown in Fig. 3. It can be seen that the wear behaviour in all the
cases was non-linear and the wear rate increased with the applied pressure. The copper-based alloy showed
APPLIED
PRESSURE,
MPo
Fig. 3. Wear rate as a function of applied pressure. 0, LM13; A, LM13 (HT); n , LM13-3% graphite; A, Cu-Sn-Pb; 0, LM13-3% graphite (HT); *, seizure.
S. Das, B. K. Prasad / Tribologkal behaviour of Al alloy composites
0
2
4 APPLIED
6
0
PRESSURE,
Fig. 4. Maximum temperature function of applied pressure. Cu-Sn-Pb; *, seizure.
10
67
12
MPo
rise near the sliding surface as a 0, LM13-3% graphite (HP); A, Fig. 6. Wear surface of the copper-based alloy at 4.0 MPa, showing the sticking of a debris particle and cracks on the stuck particle.
(a)
Fig. 5. Wear surfaces of the copper-based alloy at 1.0 MPa pressure, showing (a) surface cracks (marked with an arrow) and (b) cracks along with the severely damaged region.
the highest wear rate of all the specimens, while the heat-treated composite showed the lowest. For example, at 3.0 MPa pressure the wear rate of the heat-treated composite was found to be around 1.0X lo-” m3 m-‘, whereas for the copper-based alloy it was approximately 30.0~ lo-l2 m3 m-l. However, the copper-based alloy seized at a considerably higher pressure than did the composite. For instance, the Al-Si alloy composite in the heat-treated condition seized at 4.0 MPa, while the copper-based alloy seized at 10.0 MPa. It can be seen that above 4.0 MPa, the slope of the wear rate curve of the copper-based alloy decreased. The seizure pressure of the LM13 alloy in the cast and heat-treated conditions, and the cast composite showed lower seizure pressures than that for the heat-treated composite (Fig. 3). Figure 4 shows the maximum temperature rise near the contact surface as a function of the applied pressure. The diagram clearly depicts that the maximum contactsurface temperature increased with an increase in the applied pressure. It also can be seen that, at and below 3.0 MPa, the temperature rise in the case of the heattreated composite was less than that of the copperbased alloy. However, above this pressure a reversal in the trend was observed. 3.3. SEA4 examination of wear surfaces The wear surface of the copper-based alloy at 1.0 MPa pressure is shown in Fig. 5. The wear surface reveals continuous grooves and severely damaged regions (Fig. 5(a)). Surface cracks (marked by an arrow) may also be seen in Fig. 5(a). A magnified view of Fig. 5(a) is shown in Fig. 5(b), which clearly reveals the cracks along with the severely damaged regions. Figure 6 shows the wear surface of the alloy at an applied pressure of 4.0 MPa. It may be observed that
S. Das, B. K. Prasad I Tribological behaviour of Al alloy composites
was less than that for the as-cast alloy (Fig. 8(b)). Figure 8(c) depicts the wear surface of the heat-treated composite. The presence of a graphite film on the wear surface was detected by the WDXS analysis (Fig. 8(d)), but no such graphite film formation could be resolved on the wear surface of the cast composite. 3.4. Debris analysis A wide range of sizes of wear debris was found in the case of the copper-based alloy. At 1.0 MPa the size of the debris was found to be large in general (approximately 200-400 pm) with a few fine particles (Fig. 9(a) and 9(b)). At a higher applied pressure, say 4.0 MPa, the debris size was found to be around 100-200 pm (Fig. 9(c)). Further increasing the applied pressure up to 10.0 MPa led to the generation of still finer wear debris, the majority being smaller than 100 pm (Fig. 9(d)). Most of the debris were found to be flaky in nature in all the cases (Fig. 9). Occasionally, machining chips of iron were also observed in the debris (Fig. 10(a)). The X-ray dot map of iron corresponding to Fig. 10(a) is shown in Fig. 10(b). Figure 11(a) shows typical equiaxed wear debris of the cast Al-Si alloy at an applied pressure of 1.0 MPa. When the applied pressure was increased to 1.5 MPa, the shape of the debris changed to flake type (Fig. 11(b)). In the case of the heat-treated composite, the shape of the debris was identical to that for the cast alloy, the only difference being the finer size in the case of the heat-treated composite (Fig. 11(c)). Occasionally, silicon particles were also observed in the debris (Fig. 11(d)). 0) Fig. 7. Wear surface of the copper-based alloy at the seizure pressure (10.0 MPa), showing (a) heavily damaged regions and (b) sticking of a debris particle on the wear surface.
the extent of the severely damaged regions in this case (Fig. 6) was less than that at 1.0 MPa (as shown in Fig. 5). Additionally, some debris particles were found to stick to the surface (Fig. 6). Cracks on the stuck particles also may be seen in Fig. 6. A typical wear surface at the seizure pressure (10.0 MPa) is shown in Fig. 7. Heavily damaged regions (Fig. 7(a)) and sticking of debris particles to the wear surface (Fig. 7(b)) were the two interesting features. Furthermore, it is worth mentioning that the presence of microcracks was found to be significantly less in this case than in the cases of the samples tested at relatively lower pressures (1.0 MPa). A typical wear surface of the cast Al-Si alloy tested at 1.5 MPa is shown in Fig. 8(a). It shows continuous grooves and damaged regions. In the case of the heattreated alloy, the tendency for surface cracking to occur
3.5. Subsurface studies Figure 12(a) shows a typical micrograph of the transverse section of the copper-based alloy at 1.0 MPa. The zone near the wear surface (top portion) may be seen in the figure. The deformed zone was found to extend to a depth of 10-20 pm. A large mass of material attached to the subsurface region and the presence of microcracks there are also shown (Fig. 12(a)). At higher applied pressures, say 4.0 and 10.0 MPa, a smaller mass was attached to the substrate as shown in Figs. 12(b) and 12(c) respectively. The interesting feature observed at 10.0 MPa was the flow of the intermetallic compound (Fig. 12(c)) and lead particles (Fig. 12(d)) along the sliding direction in the subsurface regions. The X-ray dot maps of copper and lead corresponding to Fig. 12(d) are shown in Figs. 12(e) and 12(f) respectively. The micrograph of the subsurface of the cast LM13 alloy is shown in Fig. 13(a). Breaking of eutectic silicon particles in the subsurface region (top) may be seen, while the unaffected bulk structure (away from the surface) showed the normal faceted silicon particles.
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S. Das, B. K Prasad / Tribological behaviozu of Al alloy composites
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(b)
(c)
td)
Fig. 8. Wear surface of (a) the cast LM13 alloy, (b) the heat-treated and (d) the X-ray dot rn$ of C, corresponding to ic).
Subsurface cracks were also observed in this case. In the case of the heat-treated alloy, no such breaking of eutectic silicon particles and subsurface cracking could be observed, and the subsurface as well as the unaffected bulk revealed identical shapes of eutectic silicon (Fig. -
13(b)). 4. Discussion It may be noted that graphite has a hexagonal crystal structure with weak forces between the basal planes. This facilitates easy smearing of the graphite and the formation of a lubricating film [24]. The third-body graphite fihn minimizes the direct metal-to-metal contact and produces a lubricating effect. Similarly, lead also has an h.c.p. crystal structure and tends to flow during the sliding action on the contacting surface [25] and form a third-body film of lead, similar to graphite. This
LM13 alloy and (c) the heat-treated
composite
at 1.5 MPa
minimizes the direct contact between the contact surfaces and reduces the wear rate. The formation of the lead film on the mating surfaces will depend upon the degree of the shear stress produced during the sliding action and also on the extent of rise in temperature. The low wear rate of the heat-treated composite was mainly due to the formation of the graphite film on the contact surface of the composite (Fig. 8(c)), whereas the wear surface of the copper-based alloy showed surface cracks (Fig. 5) and the formation of large debris particles (Figs. 9(a) and 9(b)) at lower pressures (below 4.0 MPa). The above facts clearly indicate that the lead particles could not smear on the sliding surface of the copper-based alloy, rather they might have been engulfed in the large debris (Figs. 9(a) and 9(b)) as also evident from the subsurface analysis (Fig. 12(a)). The high seizure resistance of copper-based alloy was due to the high melting point of the matrix. Although
S. Das, B. K. Prasad / Tribological behaviour of AI alloy composites
(b)
(cl
(4
Fig. 9. Wear debris of the copper-based alloy at (a), (b) 1.0 MPa, showing large flaky particles, Reduction in the size of deb& with increasing p&s&k may be noted trom (a) to (d).
(a) Fig. 10. (a) A typical machining
(c) at 4.0 MPa and (d) 10.0 MPa.
(b) chip of iron and (b) the X-ray dot map of iron corresponding
to (a).
S. Das, B. K Prasad / Tribological behaviour of Al alloy composites
(4
(b)
(4 Fig. 11. (a) Equiaxed debris of the cast LM13 alloy at 1.0 MPa, (b) flake-type debris at 1.5 MPa (seizure), case of the heat-treated composite at 2.5 MPa and (d) a typical silicon particle in the debris.
the hardnesses of both the copper-based alloy and the heat-treated composite were comparable (about 90 HV), the ultimate tensile strength of the copper-based alloy (250 MPa) was higher than that of the heat-treated composite (180 MPa). This may be another reason for the higher seizure resistance of the copper-based alloy. The interesting feature observed in Fig. 3 is the existence of two distinct wear regimes in the case of the copper-based alloy. It may be noted from Fig. 3 that, above 4.0 MPa, the slope of the wear rate plot decreased with a further increase in the pressure. It appears that at higher pressures the lead particles smeared on the surface and offered a change in the trend (Fig. 3). Although the formation of the lead film could not be detected on the wear surface by WDXS, as also observed in an earlier study [9], the flow of lead was observed in the subsurface region (Fig. 12(d)).
(c) fine debris in the
The generation of finer debris particles (Fig. 9(d)) also indicates the possibility of the formation of the lead film on the wear surface. In addition, the wear debris formed in this pressure regime had a tendency to stick to the wear surface (Figs. 6 and 7); this could also be responsible for reducing the slope of the wear rate plot (Fig. 3). In the lower pressure range, adhesion of the copperbased alloy to the steel disc was practically nil, while adhesion was observed to a considerable extent at higher pressures. Similar observations have also been made in a recent study [ll]. In the low load regime, the results also showed the formation of large flaky debris (Fig. 9(b)). This may be due to the presence of softer lead particles, which could offer sites for the easy nucleation of cracks. When such cracks join each other, large debris particles would be generated (Fig. 9(b)). The presence of plenty of cracks on the wear surface
S. Das, B. K. Prasad I Tribological behaviour of AI alloy composites
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(e) Fig. 12. Transverse section of the copper-based alloy, showing (a) a large mass of material attached to the subsurface at 1.0 MPa, (b) at 4.0 MPa, (c) flow of the intermetallic at 10.0 MPa and (d) flow of lead particle at 10.0 MPa, and (e), (r) the X-ray dot maps of copper and lead, respectively, corresponding to (d).
S. Das, B. X Prasad / Tkbological behaviour of Al alloy composites
73
(4 Fig. 13. Subsurface of (a) the cast LM13 alloy, showing breaking of silicon particles heat-treated alloy, revealing practically unaffected subsurface microstructure.
(Fig. 5) and debris particles (Figs. 9(a) and 9(b)) in the case of the copper-based alloy tested at low pressures (below 4.0 MPa) also suggest its higher wear rate than that of the heat-treated composite (Fig. 3). The Al-Si alloy and its composite revealed the formation of relatively finer debris (equiaxed) (Fig. 11(a)) at lower pressures, which is consistent with their having a lower wear rate than at higher pressures (Fig. 3). The lowest wear rate of the heat-treated composite (Fig. 3) can be explained on the basis of the formation of a graphite film on the wear surface (Fig. 8(c)), the generation of finer debris (Fig. 11(c)) and there being significantly less surface damage (Fig. 8(c)). Alteration of the silicon morphology from faceted in the as-cast condition (Fig. 2(a)) to nearly spherical in the heat-treated alloy (Fig. 2(d)) helps in reducing the propensity of crack nucleation, thereby allowing the graphite particles to smear on the sliding surface. This ultimately improves the wear properties of the heattreated composite. The presence of subsurface cracks in the copperbased alloy, cast Al-Si alloy and the cast composite suggests that crack nucleation at the interface of the matrix and hard silicon/intermetallic or soft lead/graphite particles may be the rate-controlling mechanism of material removal. Recently, Alpas and Zhang [26] have suggested that the presence of subsurface cracks indicates that subsurface delamination is the wear mechanism. Deeper grooves on the wear surfaces (Fig. 5-8) could be due to three-body abrasion caused by the entrapment of oxide particles and machining chips generated from the disc (Fig. 10). Similar observations have also been made by earlier investigators [ll, 271.
in the subsurface
region (top), and (b) the
The above observations clearly indicate that the heattreated composite could be considered as an excellent substitute for conventional leaded-tin bronzes at low pressures from a wear point of view, while the bronze could withstand higher applied pressures.
5. Conclusions (1) The heat-treated Al-Si alloy-graphite composite showed the least wear rate among all the materials for operating pressures below 4.0 MPa. However, the copper-based, leaded bronze showed the maximum wear rate for operating pressures below 4.0 MPa; it seized at 10.0 MPa. (2) The temperature rise of the heat-treated composite was less than that of the bronze for operating pressures below 4.0 MPa, while a reverse trend was observed at higher pressures. (3) Observations of this investigation suggest that the Al-Si alloy-graphite composite in the heat-treated condition can be a cost- and energy-effective substitute for conventional leaded-tin bronzes in tribological applications subject to low pressure conditions.
Acknowledgment The authors are grateful to Prof. T. C. Rao, Director, RRL, Bhopal for his encouragement and for granting permission to publish this paper.
14
S. Das, B. K. Prasad / Triboiog’cal behaviour of Al alloy composites
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