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Studies on wear characteristics of Al–Fe–V–Si alloys
Wear 239 Ž2000. 211–218 www.elsevier.comrlocaterwear
Studies on wear characteristics of Al–Fe–V–Si alloys K.L. Sahoo a,) , C.S.S. Krishnan a , A.K. Chakrabarti b b
a National Metallurgical Laboratory, Jamshedpur 831007, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India
Received 23 September 1998; received in revised form 18 January 1999; accepted 15 December 1999
Abstract This paper deals with the wear behaviour of modified and unmodified cast Al–Fe–V–Si alloys studied through pin-on-disc wear test under three loads, namely 29.4, 49.0 and 68.6 N. Whilst all the alloys showed excellent wear resistance compared to the conventional eutectic Al–Si alloy, modification with Mg imparts further enhanced resistance to wear and reduced coefficient of friction. It was observed that during dry sliding wear extensive plastic deformation, work hardening and oxidation of a-Al matrix occurred. It has been shown that the primary mode of wear is delamination. q 2000 Elsevier Science S.A. All rights reserved. Keywords: a-Al matrix; Pin-on-disc wear test; Al–Fe–V–Si alloy
1. Introduction It is well known that Al–Fe–V–Si alloys are normally processed through the costly rapid solidification technology w1x, and that they derive strength due to the presence of finely dispersed Fe–V–Al–silicide intermetallic particles in the a-Al matrix. The conventional melting and casting route for these alloys, on the other hand, result in massive intermetallic precipitates and consequent casting defects causing deterioration of strength and toughness w2,3x. The present work has been taken up to examine the effects of changes in the microstructure of the precipitate phases in cast Al–Fe–V–Si alloys on their tribological behaviour. Modification of the microstructures in these experimental alloys was carried out by addition of pure Mg-and Mg-bearing master alloys. For the purpose of comparison, the modified Al–12.6% Si alloy was taken as a reference alloy to maintain identical experimental conditions.
2. Experimental procedure The compositions of the alloys and details of modification treatment are given in Table 1. The alloy was melted
)
Corresponding author.
in an electric resistance furnace in clay-bonded graphite crucible coated with alumina paint. The melt was cast in a 15-mm-diameter steel mould. The microstructure of the samples were examined under optical microscope. The sliding wear tests were carried out using a pin-on-disc machine. The pins of 8-mm diameter and 30-mm length were fabricated from 15-mm-diameter rods and made to slide against a low alloy steel disc Žmaterial: 103Cri-Eng31HRS60W61, equivalent to AISI 4340. of diameter 215 mm and hardness 62 Rc. The track radius and disc speed were maintained at 115 mm and 200 rpm, respectively, to maintain a constant sliding velocity of 1.19 mrs. The contact surfaces of the pins and disc were polished to a roughness of R a s 0.1 mm before wear testing. Three loads, namely 29.4, 49.0 and 68.6 N, were applied for each test in the present work. Tangential force and the coefficient of friction were measured continuously with an electronic sensor attached to the machine. Frictional force ŽN. and pin length reduction Žmm. were measured from the sensors output data as a function of time. The wear rates of the alloy specimens, defined as the cumulative wear loss per unit sliding distance per unit load, were calculated from the pin wear data. The wear test was carried out for a total sliding distance of about 2.142 km at a temperature of 208C and relative humidity of 50–55%. The surfaces of the worn samples, as well as the wear debris, were cleaned by acetone and examined under the scanning electron microscope ŽSEM.. The Vicker’s hardness of the wearing sur-
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 3 7 1 - 3
K.L. Sahoo et al.r Wear 239 (2000) 211–218
212 Table 1 Details of the alloys investigated Alloy designation
Alloy composition
Treatment, if any
Actual Mg content Žwt.%.
Density= 10 3 Žkgrm3 .
A1 A2 A3 A4 A5 A6 A7
Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–12.6% Si
Unmodified 1.0% pure Mg 1.5% Al–20% Mg 0.25% Ni–20% Mg 0.75% Ni–20% Mg 1.0% Ni–20% Mg Modified, 0.2% Na
– 0.18 0.22 0.02 0.15 0.19 –
2.95 2.94 2.95 2.96 3.02 3.03 –
face of each pin was measured and recorded under a 5-kg load after completion of each test.
3. Results The measured densities of the alloys are close to the theoretical values, which indicates that the samples were free from porosity. The microstructures of these alloys are shown in Fig. 1Ža–f.. It may be noted from these photomi-
crographs that the precipitate phases were uniformly distributed in the Al matrix in the Mg-treated alloys. On the contrary, the precipitates in the untreated alloy were the of-large, chunky type. Fig. 2Ža–b, c–d, e–f. represent the cumulative wear loss in microns Žmm. as a function of sliding distance in meters Žm., at different loads, namely, 29.4, 49.0 and 68.6 N, respectively. The wear test of the Al–12.6% Si alloy, however, could not be carried out at the highest load of 68.6 N due to seizure of the pins. After a transient period,
Fig. 1. Optical micrographs of the alloys subjected to wear testing Ža. alloy A 1 , Žb. alloy A 2 , Žc. alloy A 3 , Žd. alloy A 4 , Že. alloy A 5, Žf. alloy A 6 .
K.L. Sahoo et al.r Wear 239 (2000) 211–218
213
Fig. 2. The variation of cumulative wear loss vs. sliding distance Ža–b. 29.4 N, Žc–d. 49.0 N, Že–f. 68.6 N.
the wear loss was found to increase linearly with increasing sliding distance. The average volumetric wear rate ŽWR. counted from the beginning of the test to any particular stage is expressed as: WR s volumetric wear
loss Žmm3 .rwtotal distance traveled in meters Žm. = load applied in Newton ŽN.x. The average wear resistance ŽWRe. is defined as the inverse of the average wear rate and is expressed as: WRe s m Nrmm3. The values of average
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K.L. Sahoo et al.r Wear 239 (2000) 211–218
wear resistance and coefficient of friction Ž m . are presented in Figs. 3 and 4, respectively. Fig. 5Ža–c. are representative curves showing the variation of friction coefficient with sliding distance under the three different loads. The hardness ŽVPN. of the worn surfaces was determined and the data are reported in Table 2. This indicates that the hardness of the pin samples increased with increasing load. The wear resistance, however, increased with increasing load for only some of the alloys. In most cases, no steady state stage was attained during the period of wear test conducted. The sizes of the precipitate particles in unmodified and modified samples were measured by image analyser, and results are shown in Table 3. Low magnification SEM photographs of the worn surfaces of two pin samples Žtested under 49-N load. are presented in Fig. 6. It is evident from Fig. 6b that the worn surface of the Al–Si alloy has undergone plastic flow and cracking. The worn surfaces of the unmodified Al–Fe–V– Si alloy, on the other hand, as typified by Fig. 6a, show the presence of distinct grooves, suggesting ploughing of the pin surface. These worn surfaces are the areas from which the wear debris had been removed. Higher magnification SEM views of the worn surfaces are given in Figs. 7 and 8. Figs. 7Žb–c. and 8Ža. indicate plastic flow of the matrix, crack nucleation and propaga-
Fig. 4. Histogram showing the average coefficient of friction of different alloys Ža. 29.4 N load, Žb. 49.0 N load, and Žc. 68.6 N load.
tion and crevice formation in the Mg-treated alloys. The morphology of the wear debris collected from two samples are shown in Figs. 9 and 10. The unmodified alloy Žalloy A 1 . may undergo oxidation during the wear test. The existence of the probable oxide layer is shown in Figs. 8Žb. and 10.
4. Discussion From the results of the wear tests on the Al–Fe–V–Si alloys, the following aspects of wear can be assessed: Ža. overall wear resistance, Žb. effect of modifiers on the performance of individual alloys, and Žc. wear mechanism.
Fig. 3. Histogram showing the average wear resistance of different alloys Ža. 29.4 N load, Žb. 49.0 N load, and Žc. 68.6 N load.
The Al–Fe–V–Si alloy modified by 1% Ni–20% Mg Žalloy A 6 . offered the best wear resistance at the higher loads ŽFig. 3Žc... The wear resistance of the Na-modified Al–Si alloy ŽA 7 . was distinctly inferior to the experimental alloys. In fact, wear testing could not be conducted on the Al–Si alloy pin sample at 68.6 N Žhighest load in the
K.L. Sahoo et al.r Wear 239 (2000) 211–218
Fig. 5. The variation of friction coefficient Ž m . as a function of sliding distance Ža. 29.4 N load, Žb. 49.0 N load, and Žc. 68.6 N load.
present work. due to the occurrence of seizures. This can be explained by the mechanism proposed by Lim and Ashby w4x. When the pin surfaces are placed in contact with the disc, the real area of pin contact is usually very small. The large local pressure at the points of real contact Žthe asperity contacts. can forge soft metallic junctions between the surfaces even under static conditions. Under large-enough load, the real area of contact grows until it is equal to the nominal area of the pin surfaces and the surface seizes completely. The massive intermetallic particles in the unmodified Al–Fe–V–Si alloy are usually considered undesirable from toughness and ductility points of view. It is interesting that such non-uniform distribution of hard massive particles does not improve the wear resistance either. The wear resistance of the unmodified alloys decreased continuously with increasing wear load ŽFig. 3.. Better wear resistance should be achieved through even distribution of the intermetallic particles. In this context, the performance of different modifiers varied significantly. The extent of modification upon addition of 0.05% Mg as Ni–Mg alloy was insignificant Žactual Mg content in the pin sample is 0.02%.. The wear resistance of this alloy was, in fact,
215
lower than that of the unmodified Al–Fe–V–Si alloy. A marked increase in wear resistance, particularly at 68.6 N wear load, can be noted on increasing the dose of Mg as Ni–Mg wFig. 3Žc.; alloys A 5 and A 6 x. A good precipitated particle size distribution ŽTable 3. was achieved on modification with Al–Mg alloy ŽA 3 .. The corresponding sample recorded higher wear resistance at the highest load Žalloy A 3 .. It is, thus, obvious that well-distributed aluminide and silicide particles support the wear load better than chunky, non-uniformly distributed, particles. Crack nucleation generally occurs at some depth below the surface rather than very near the surface, owing to the very high hydrostatic compressive pressure acting near the asperity contact w5x. Thus, once a crack is nucleated, its propagation is slow and seizures do not occur, owing to the presence of well-distributed particles in the matrix w6x even at a high load of 68.6 N. In the Al–Si alloy, the presence of Si particles alone is not adequate for such high-wear load. With increasing test load, work hardening gradually takes place as is evident from the data on hardness of the worn specimen presented in Table 2. But the trends in cumulative wear curves ŽFig. 2. show that the steady state stage was not reached in any of the alloys at any load irrespective of the alloy composition or modification treatment. However, the overall wear resistance increased due to appreciable work hardening of the alloys modified with Ni–Mg or Al–Mg alloy ŽFig. 3.. On the contrary, the wear resistance of the unmodified alloy Žalloy A 1; Fig. 3. continued to decrease with increasing load. The extent of work hardening here was small. In this respect, the behaviour of this alloy was similar to that of the Al–Si alloy. In order to investigate the wear mechanism, the surfaces of the worn samples were examined under SEM. The low magnification SEM photographs show fine scoring marks. The worn surface of the Al–Si alloy at 49-N load was more heavily cracked than those of Al–Fe–V–Si alloys. The scoring may be due to abrasion by entrapped debris, work-hardened deposits on the counterface or hard asperities on the hardened steel counterface w7,8x. On examination of the worn surfaces at higher magnification, evidence of extensive plastic flow and cracking was observed. These are the two likely modes of crack initiation and propagation. Cracks may initiate in the highly
Table 2 Hardness ŽVPN. of as cast and worn samples Žload: 5 kg. Alloy designation
Hardness ŽVPN. As-cast
29.4 N load
49 N load
68.6 N load
A1 A2 A3 A4 A5 A6 A7
43 – 44 – 46 46 46.8
68 69 71 59 75 95 59.6
73 95 117 95 110 145 64.6
102 119 137 105 120 175 –
K.L. Sahoo et al.r Wear 239 (2000) 211–218
216 Table 3 Precipitate particle size distribution in various alloys Alloy designation
% Undersize Ž- 2 mm.
% Count Ž2–5.65 mm.
% Count Ž) 5.65–32 mm.
% Oversize Ž) 32 mm.
No. of particles per unit area Žmm2 .
A1 A2 A3 A6
6.20 13.0 22.04 37.42
60.90 54.80 46.85 43.29
30.80 29.60 30.66 18.81
2.10 2.60 0.45 0.48
93 216 323 453
work-hardened layer, particularly in the subsurface region. When cracks grow and get interconnected, a layer of metal is removed. This is delamination wear w5x. Figs. 7Žc. and 8Ža. suggest such a mechanism. The SEM photographs ŽFigs. 9 and 10. indicate that the particles in the wear debris were mainly sheet-like. This is an indication of delamination wear. It is also possible that the hard dispersoid particles or fractured pieces thereof are mechanically dislodged during wear. The pin-holes so formed act as potential sites for nucleation and growth of cracks, paving the way for delamination wear ŽFig. 7Žc... The dispersoid particles in the Al–Fe–V–Si alloys may act as asperities, which continue to support the load until they are fractured and leveled off. It appears from Table 2 that the extent of work hardening increases with increasing degree of modi-
Fig. 6. Low magnification SEM photographs of worn surfaces of Ža. alloy A 1 , Žb. alloy A 7 at 49.0 N load.
Fig. 7. Higher magnification SEM photographs of worn surfaces of Ža. alloy A 1 , Žb. alloy A 3 , Žc. alloy A 4 at 49.0 N load.
K.L. Sahoo et al.r Wear 239 (2000) 211–218
Fig. 8. Higher magnification SEM photographs of worn surfaces of Ža. alloy A 3 , Žb. alloy A 1 at 49.0 N load.
fication. This suggests that modified alloys with finer dispersoid particles undergo greater plastic deformation. The delaminated wear debris particles are already so severely work-hardened and fractured that they do not stick to the sliding surface any further and are removed. The high magnification SEM picture of the wear debris in Fig. 9Žb. confirms this fact. The worn surface and the wear debris of the unmodified Al–Fe–V–Si alloy appears to be oxidized, particularly at the highest load ŽFig. 10.. The coefficient of friction attained a value range Žallowing for the scatter in the data. after an initial transient period, as indicated in Fig. 5. The coefficient of friction increases uniformly with increasing load in most of the cases. The coefficient of friction in the case of the Al–Fe– V–Si alloys was lower at 29.4-N load but higher at 49-N load than the corresponding values for the Al–Si alloys ŽFig. 4.. The sudden increase in the friction coefficient at 68.6-N load indicates adhesion of the pin to sliding surface. Such adhesion was conspicuous in the case of the Al–Si alloy, which suffered seizure at 68.6-N load. At this load, the frictional heat generated is sufficient to cause localized heating, resulting in sticking of the pin to the disc. The rise in the coefficient of friction with the increase in wear load may be attributed to Ža. enhanced accumula-
217
Fig. 9. SEM views of wear debris from alloys A 3 , wear test load, 68.6 N load, sliding speed 1.19 mrs.
tion of the wear debris consisting of large volume fraction of hard aluminide and silicide particles pulled out of the matrix during wear ŽFig. 7Žb.. at the pin and disc interface and Žb. oxidation of the wearing surface. The deep furrows on the pin surfaces ŽFig. 7Žb.. may also be accounted for by the cutting action of the hard particles of aluminide and silicide having hardness of 600–800 VPN. Fortunately, a simultaneous work hardening of the matrix by plastic
Fig. 10. SEM views of wear debris from alloys A 1 , wear test load, 68.6 N load, sliding speed 1.19 mrs.
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K.L. Sahoo et al.r Wear 239 (2000) 211–218
deformation helped in reducing the extent of wear of the samples at high loads. During wear at high loads, the temperature increases appreciably, favouring the formation of aluminium oxide on the surface. The oxide layer at the interface is also responsible for a high coefficient of friction. The modified alloys A 3 and A 6 recorded the lowest coefficient of friction at 68.6-N load. The Mg oxide formed in Mg-modified alloy is flaky, peeling off in nature and could lead to reduce friction w7x. In case of the unmodified Al–Fe–V–Si alloy, the rapid oxidation of the matrix at high-wear load contributed to a steep rise in the coefficient of friction.
5. Conclusions Ž1. The Al–Fe–V–Si alloys investigated exhibit plastic deformation and work hardening during wear testing, the Mg-treated alloys work-hardened to a greater extent than the untreated alloys. Ž2. The wear resistance decreases with increasing load in case of unmodified alloys. In the modified alloys, it increases with increasing load due to extensive workhardening. Ž3. Wear occurs by plastic deformation and cracking of the matrix followed by delamination of flakes. In addition, detached hard dispersoid particles also contribute to wear.
Ž4. Coefficient of friction rises sharply at high-wear load. Ž5. Modification with Mg minimizes oxidation of the matrix. The best modified alloys recorded minimum coefficient of friction at high-wear load. Acknowledgements The authors wish to record their grateful thanks to Mr. Swapan Das for the SEM work, Prof. A. Basak of IIT, Kharagpur for providing facilities for wear test and discussion, and to the Director, National Metallurgical Laboratory for permission to publish this paper. References w1x D.J. Skinner, Dispersion strengthen Al-alloys, in: Kim Griffith ŽEd.., The Minerals, Metals and Materials Society,1988, p. 181, 420 Common Wealth Drive, Warrendale, Pennsylvania-15086, Ž412., 776– 9024. w2x L. Ananthanarayan, F.H. Samuel, J.E. Gruzleski, Metall. Mater. Trans. A 26 Ž1995. 2161–2174. w3x A. Couture, Int. Cast Met. J. 6 Ž1981. 9–12. w4x S.C. Lim, M.F. Ashby, Acta Metall. 35 Ž1. Ž1987. 1–24. w5x S. Jahanmir, N.P. Suh, Wear 44 Ž1977. 17–38. w6x C.C. Yang, W.-M. Hsu, E. Chang, Mater. Sci. Technol. 13 Ž1997. 687–694. w7x B.N. Pramila Bai, S.K. Biswas, Scr. Metall. 39 Ž5. Ž1991. 833–840. w8x B.N. Pramila Bai, S.K. Biswas, Wear 87 Ž1983. 237–249.
Studies on wear characteristics of Al–Fe–V–Si alloys K.L. Sahoo a,) , C.S.S. Krishnan a , A.K. Chakrabarti b b
a National Metallurgical Laboratory, Jamshedpur 831007, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India
Received 23 September 1998; received in revised form 18 January 1999; accepted 15 December 1999
Abstract This paper deals with the wear behaviour of modified and unmodified cast Al–Fe–V–Si alloys studied through pin-on-disc wear test under three loads, namely 29.4, 49.0 and 68.6 N. Whilst all the alloys showed excellent wear resistance compared to the conventional eutectic Al–Si alloy, modification with Mg imparts further enhanced resistance to wear and reduced coefficient of friction. It was observed that during dry sliding wear extensive plastic deformation, work hardening and oxidation of a-Al matrix occurred. It has been shown that the primary mode of wear is delamination. q 2000 Elsevier Science S.A. All rights reserved. Keywords: a-Al matrix; Pin-on-disc wear test; Al–Fe–V–Si alloy
1. Introduction It is well known that Al–Fe–V–Si alloys are normally processed through the costly rapid solidification technology w1x, and that they derive strength due to the presence of finely dispersed Fe–V–Al–silicide intermetallic particles in the a-Al matrix. The conventional melting and casting route for these alloys, on the other hand, result in massive intermetallic precipitates and consequent casting defects causing deterioration of strength and toughness w2,3x. The present work has been taken up to examine the effects of changes in the microstructure of the precipitate phases in cast Al–Fe–V–Si alloys on their tribological behaviour. Modification of the microstructures in these experimental alloys was carried out by addition of pure Mg-and Mg-bearing master alloys. For the purpose of comparison, the modified Al–12.6% Si alloy was taken as a reference alloy to maintain identical experimental conditions.
2. Experimental procedure The compositions of the alloys and details of modification treatment are given in Table 1. The alloy was melted
)
Corresponding author.
in an electric resistance furnace in clay-bonded graphite crucible coated with alumina paint. The melt was cast in a 15-mm-diameter steel mould. The microstructure of the samples were examined under optical microscope. The sliding wear tests were carried out using a pin-on-disc machine. The pins of 8-mm diameter and 30-mm length were fabricated from 15-mm-diameter rods and made to slide against a low alloy steel disc Žmaterial: 103Cri-Eng31HRS60W61, equivalent to AISI 4340. of diameter 215 mm and hardness 62 Rc. The track radius and disc speed were maintained at 115 mm and 200 rpm, respectively, to maintain a constant sliding velocity of 1.19 mrs. The contact surfaces of the pins and disc were polished to a roughness of R a s 0.1 mm before wear testing. Three loads, namely 29.4, 49.0 and 68.6 N, were applied for each test in the present work. Tangential force and the coefficient of friction were measured continuously with an electronic sensor attached to the machine. Frictional force ŽN. and pin length reduction Žmm. were measured from the sensors output data as a function of time. The wear rates of the alloy specimens, defined as the cumulative wear loss per unit sliding distance per unit load, were calculated from the pin wear data. The wear test was carried out for a total sliding distance of about 2.142 km at a temperature of 208C and relative humidity of 50–55%. The surfaces of the worn samples, as well as the wear debris, were cleaned by acetone and examined under the scanning electron microscope ŽSEM.. The Vicker’s hardness of the wearing sur-
0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 3 7 1 - 3
K.L. Sahoo et al.r Wear 239 (2000) 211–218
212 Table 1 Details of the alloys investigated Alloy designation
Alloy composition
Treatment, if any
Actual Mg content Žwt.%.
Density= 10 3 Žkgrm3 .
A1 A2 A3 A4 A5 A6 A7
Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–8.3Fe–0.8V–0.9Si Al–12.6% Si
Unmodified 1.0% pure Mg 1.5% Al–20% Mg 0.25% Ni–20% Mg 0.75% Ni–20% Mg 1.0% Ni–20% Mg Modified, 0.2% Na
– 0.18 0.22 0.02 0.15 0.19 –
2.95 2.94 2.95 2.96 3.02 3.03 –
face of each pin was measured and recorded under a 5-kg load after completion of each test.
3. Results The measured densities of the alloys are close to the theoretical values, which indicates that the samples were free from porosity. The microstructures of these alloys are shown in Fig. 1Ža–f.. It may be noted from these photomi-
crographs that the precipitate phases were uniformly distributed in the Al matrix in the Mg-treated alloys. On the contrary, the precipitates in the untreated alloy were the of-large, chunky type. Fig. 2Ža–b, c–d, e–f. represent the cumulative wear loss in microns Žmm. as a function of sliding distance in meters Žm., at different loads, namely, 29.4, 49.0 and 68.6 N, respectively. The wear test of the Al–12.6% Si alloy, however, could not be carried out at the highest load of 68.6 N due to seizure of the pins. After a transient period,
Fig. 1. Optical micrographs of the alloys subjected to wear testing Ža. alloy A 1 , Žb. alloy A 2 , Žc. alloy A 3 , Žd. alloy A 4 , Že. alloy A 5, Žf. alloy A 6 .
K.L. Sahoo et al.r Wear 239 (2000) 211–218
213
Fig. 2. The variation of cumulative wear loss vs. sliding distance Ža–b. 29.4 N, Žc–d. 49.0 N, Že–f. 68.6 N.
the wear loss was found to increase linearly with increasing sliding distance. The average volumetric wear rate ŽWR. counted from the beginning of the test to any particular stage is expressed as: WR s volumetric wear
loss Žmm3 .rwtotal distance traveled in meters Žm. = load applied in Newton ŽN.x. The average wear resistance ŽWRe. is defined as the inverse of the average wear rate and is expressed as: WRe s m Nrmm3. The values of average
214
K.L. Sahoo et al.r Wear 239 (2000) 211–218
wear resistance and coefficient of friction Ž m . are presented in Figs. 3 and 4, respectively. Fig. 5Ža–c. are representative curves showing the variation of friction coefficient with sliding distance under the three different loads. The hardness ŽVPN. of the worn surfaces was determined and the data are reported in Table 2. This indicates that the hardness of the pin samples increased with increasing load. The wear resistance, however, increased with increasing load for only some of the alloys. In most cases, no steady state stage was attained during the period of wear test conducted. The sizes of the precipitate particles in unmodified and modified samples were measured by image analyser, and results are shown in Table 3. Low magnification SEM photographs of the worn surfaces of two pin samples Žtested under 49-N load. are presented in Fig. 6. It is evident from Fig. 6b that the worn surface of the Al–Si alloy has undergone plastic flow and cracking. The worn surfaces of the unmodified Al–Fe–V– Si alloy, on the other hand, as typified by Fig. 6a, show the presence of distinct grooves, suggesting ploughing of the pin surface. These worn surfaces are the areas from which the wear debris had been removed. Higher magnification SEM views of the worn surfaces are given in Figs. 7 and 8. Figs. 7Žb–c. and 8Ža. indicate plastic flow of the matrix, crack nucleation and propaga-
Fig. 4. Histogram showing the average coefficient of friction of different alloys Ža. 29.4 N load, Žb. 49.0 N load, and Žc. 68.6 N load.
tion and crevice formation in the Mg-treated alloys. The morphology of the wear debris collected from two samples are shown in Figs. 9 and 10. The unmodified alloy Žalloy A 1 . may undergo oxidation during the wear test. The existence of the probable oxide layer is shown in Figs. 8Žb. and 10.
4. Discussion From the results of the wear tests on the Al–Fe–V–Si alloys, the following aspects of wear can be assessed: Ža. overall wear resistance, Žb. effect of modifiers on the performance of individual alloys, and Žc. wear mechanism.
Fig. 3. Histogram showing the average wear resistance of different alloys Ža. 29.4 N load, Žb. 49.0 N load, and Žc. 68.6 N load.
The Al–Fe–V–Si alloy modified by 1% Ni–20% Mg Žalloy A 6 . offered the best wear resistance at the higher loads ŽFig. 3Žc... The wear resistance of the Na-modified Al–Si alloy ŽA 7 . was distinctly inferior to the experimental alloys. In fact, wear testing could not be conducted on the Al–Si alloy pin sample at 68.6 N Žhighest load in the
K.L. Sahoo et al.r Wear 239 (2000) 211–218
Fig. 5. The variation of friction coefficient Ž m . as a function of sliding distance Ža. 29.4 N load, Žb. 49.0 N load, and Žc. 68.6 N load.
present work. due to the occurrence of seizures. This can be explained by the mechanism proposed by Lim and Ashby w4x. When the pin surfaces are placed in contact with the disc, the real area of pin contact is usually very small. The large local pressure at the points of real contact Žthe asperity contacts. can forge soft metallic junctions between the surfaces even under static conditions. Under large-enough load, the real area of contact grows until it is equal to the nominal area of the pin surfaces and the surface seizes completely. The massive intermetallic particles in the unmodified Al–Fe–V–Si alloy are usually considered undesirable from toughness and ductility points of view. It is interesting that such non-uniform distribution of hard massive particles does not improve the wear resistance either. The wear resistance of the unmodified alloys decreased continuously with increasing wear load ŽFig. 3.. Better wear resistance should be achieved through even distribution of the intermetallic particles. In this context, the performance of different modifiers varied significantly. The extent of modification upon addition of 0.05% Mg as Ni–Mg alloy was insignificant Žactual Mg content in the pin sample is 0.02%.. The wear resistance of this alloy was, in fact,
215
lower than that of the unmodified Al–Fe–V–Si alloy. A marked increase in wear resistance, particularly at 68.6 N wear load, can be noted on increasing the dose of Mg as Ni–Mg wFig. 3Žc.; alloys A 5 and A 6 x. A good precipitated particle size distribution ŽTable 3. was achieved on modification with Al–Mg alloy ŽA 3 .. The corresponding sample recorded higher wear resistance at the highest load Žalloy A 3 .. It is, thus, obvious that well-distributed aluminide and silicide particles support the wear load better than chunky, non-uniformly distributed, particles. Crack nucleation generally occurs at some depth below the surface rather than very near the surface, owing to the very high hydrostatic compressive pressure acting near the asperity contact w5x. Thus, once a crack is nucleated, its propagation is slow and seizures do not occur, owing to the presence of well-distributed particles in the matrix w6x even at a high load of 68.6 N. In the Al–Si alloy, the presence of Si particles alone is not adequate for such high-wear load. With increasing test load, work hardening gradually takes place as is evident from the data on hardness of the worn specimen presented in Table 2. But the trends in cumulative wear curves ŽFig. 2. show that the steady state stage was not reached in any of the alloys at any load irrespective of the alloy composition or modification treatment. However, the overall wear resistance increased due to appreciable work hardening of the alloys modified with Ni–Mg or Al–Mg alloy ŽFig. 3.. On the contrary, the wear resistance of the unmodified alloy Žalloy A 1; Fig. 3. continued to decrease with increasing load. The extent of work hardening here was small. In this respect, the behaviour of this alloy was similar to that of the Al–Si alloy. In order to investigate the wear mechanism, the surfaces of the worn samples were examined under SEM. The low magnification SEM photographs show fine scoring marks. The worn surface of the Al–Si alloy at 49-N load was more heavily cracked than those of Al–Fe–V–Si alloys. The scoring may be due to abrasion by entrapped debris, work-hardened deposits on the counterface or hard asperities on the hardened steel counterface w7,8x. On examination of the worn surfaces at higher magnification, evidence of extensive plastic flow and cracking was observed. These are the two likely modes of crack initiation and propagation. Cracks may initiate in the highly
Table 2 Hardness ŽVPN. of as cast and worn samples Žload: 5 kg. Alloy designation
Hardness ŽVPN. As-cast
29.4 N load
49 N load
68.6 N load
A1 A2 A3 A4 A5 A6 A7
43 – 44 – 46 46 46.8
68 69 71 59 75 95 59.6
73 95 117 95 110 145 64.6
102 119 137 105 120 175 –
K.L. Sahoo et al.r Wear 239 (2000) 211–218
216 Table 3 Precipitate particle size distribution in various alloys Alloy designation
% Undersize Ž- 2 mm.
% Count Ž2–5.65 mm.
% Count Ž) 5.65–32 mm.
% Oversize Ž) 32 mm.
No. of particles per unit area Žmm2 .
A1 A2 A3 A6
6.20 13.0 22.04 37.42
60.90 54.80 46.85 43.29
30.80 29.60 30.66 18.81
2.10 2.60 0.45 0.48
93 216 323 453
work-hardened layer, particularly in the subsurface region. When cracks grow and get interconnected, a layer of metal is removed. This is delamination wear w5x. Figs. 7Žc. and 8Ža. suggest such a mechanism. The SEM photographs ŽFigs. 9 and 10. indicate that the particles in the wear debris were mainly sheet-like. This is an indication of delamination wear. It is also possible that the hard dispersoid particles or fractured pieces thereof are mechanically dislodged during wear. The pin-holes so formed act as potential sites for nucleation and growth of cracks, paving the way for delamination wear ŽFig. 7Žc... The dispersoid particles in the Al–Fe–V–Si alloys may act as asperities, which continue to support the load until they are fractured and leveled off. It appears from Table 2 that the extent of work hardening increases with increasing degree of modi-
Fig. 6. Low magnification SEM photographs of worn surfaces of Ža. alloy A 1 , Žb. alloy A 7 at 49.0 N load.
Fig. 7. Higher magnification SEM photographs of worn surfaces of Ža. alloy A 1 , Žb. alloy A 3 , Žc. alloy A 4 at 49.0 N load.
K.L. Sahoo et al.r Wear 239 (2000) 211–218
Fig. 8. Higher magnification SEM photographs of worn surfaces of Ža. alloy A 3 , Žb. alloy A 1 at 49.0 N load.
fication. This suggests that modified alloys with finer dispersoid particles undergo greater plastic deformation. The delaminated wear debris particles are already so severely work-hardened and fractured that they do not stick to the sliding surface any further and are removed. The high magnification SEM picture of the wear debris in Fig. 9Žb. confirms this fact. The worn surface and the wear debris of the unmodified Al–Fe–V–Si alloy appears to be oxidized, particularly at the highest load ŽFig. 10.. The coefficient of friction attained a value range Žallowing for the scatter in the data. after an initial transient period, as indicated in Fig. 5. The coefficient of friction increases uniformly with increasing load in most of the cases. The coefficient of friction in the case of the Al–Fe– V–Si alloys was lower at 29.4-N load but higher at 49-N load than the corresponding values for the Al–Si alloys ŽFig. 4.. The sudden increase in the friction coefficient at 68.6-N load indicates adhesion of the pin to sliding surface. Such adhesion was conspicuous in the case of the Al–Si alloy, which suffered seizure at 68.6-N load. At this load, the frictional heat generated is sufficient to cause localized heating, resulting in sticking of the pin to the disc. The rise in the coefficient of friction with the increase in wear load may be attributed to Ža. enhanced accumula-
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Fig. 9. SEM views of wear debris from alloys A 3 , wear test load, 68.6 N load, sliding speed 1.19 mrs.
tion of the wear debris consisting of large volume fraction of hard aluminide and silicide particles pulled out of the matrix during wear ŽFig. 7Žb.. at the pin and disc interface and Žb. oxidation of the wearing surface. The deep furrows on the pin surfaces ŽFig. 7Žb.. may also be accounted for by the cutting action of the hard particles of aluminide and silicide having hardness of 600–800 VPN. Fortunately, a simultaneous work hardening of the matrix by plastic
Fig. 10. SEM views of wear debris from alloys A 1 , wear test load, 68.6 N load, sliding speed 1.19 mrs.
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K.L. Sahoo et al.r Wear 239 (2000) 211–218
deformation helped in reducing the extent of wear of the samples at high loads. During wear at high loads, the temperature increases appreciably, favouring the formation of aluminium oxide on the surface. The oxide layer at the interface is also responsible for a high coefficient of friction. The modified alloys A 3 and A 6 recorded the lowest coefficient of friction at 68.6-N load. The Mg oxide formed in Mg-modified alloy is flaky, peeling off in nature and could lead to reduce friction w7x. In case of the unmodified Al–Fe–V–Si alloy, the rapid oxidation of the matrix at high-wear load contributed to a steep rise in the coefficient of friction.
5. Conclusions Ž1. The Al–Fe–V–Si alloys investigated exhibit plastic deformation and work hardening during wear testing, the Mg-treated alloys work-hardened to a greater extent than the untreated alloys. Ž2. The wear resistance decreases with increasing load in case of unmodified alloys. In the modified alloys, it increases with increasing load due to extensive workhardening. Ž3. Wear occurs by plastic deformation and cracking of the matrix followed by delamination of flakes. In addition, detached hard dispersoid particles also contribute to wear.
Ž4. Coefficient of friction rises sharply at high-wear load. Ž5. Modification with Mg minimizes oxidation of the matrix. The best modified alloys recorded minimum coefficient of friction at high-wear load. Acknowledgements The authors wish to record their grateful thanks to Mr. Swapan Das for the SEM work, Prof. A. Basak of IIT, Kharagpur for providing facilities for wear test and discussion, and to the Director, National Metallurgical Laboratory for permission to publish this paper. References w1x D.J. Skinner, Dispersion strengthen Al-alloys, in: Kim Griffith ŽEd.., The Minerals, Metals and Materials Society,1988, p. 181, 420 Common Wealth Drive, Warrendale, Pennsylvania-15086, Ž412., 776– 9024. w2x L. Ananthanarayan, F.H. Samuel, J.E. Gruzleski, Metall. Mater. Trans. A 26 Ž1995. 2161–2174. w3x A. Couture, Int. Cast Met. J. 6 Ž1981. 9–12. w4x S.C. Lim, M.F. Ashby, Acta Metall. 35 Ž1. Ž1987. 1–24. w5x S. Jahanmir, N.P. Suh, Wear 44 Ž1977. 17–38. w6x C.C. Yang, W.-M. Hsu, E. Chang, Mater. Sci. Technol. 13 Ž1997. 687–694. w7x B.N. Pramila Bai, S.K. Biswas, Scr. Metall. 39 Ž5. Ž1991. 833–840. w8x B.N. Pramila Bai, S.K. Biswas, Wear 87 Ž1983. 237–249.