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Scanning electron microscopy study of worn Al-Si alloy surfaces
Wear, 87 (1983)
SCANNING ELECTRON ALLOY SURFACES B. N. PRAMILA Department (India)
237
237 - 249
MICROSCOPY
STUDY OF WORN Al-Si
BAI and S. K. BISWAS
of Mechanical
Engineering,
Indian Institute
of Science,
Bangalore 560012
N. N. KUMTEKAR Tata Consulting Engineers, Bangalore (India) (Received
May 15, 1982)
Summary A pin-on-disc machine was used to wear Al-Si alloy pins under dry conditions. Unmodified and modified binary alloys and commercial multicomponent alloys were tested. The surfaces of the worn alloys were examined by scanning electron microscopy to identify distinct topographical features to aid elucidation of the mechanisms of wear.
1. Introduction As-cast Al-S1 alloys are important wear-resistant materials. There is controversy over the optimum silicon content for maximum wear resistance. According to Clarke and Sarkar [l] the optimum silicon content is around the eutectic region but other workers suggest that it is in the hypereutectic region [ 2, 31. Clegg and Das [ 41 have shown that silicon has little effect on wear resistance. Rohatgi and Pai have reported [ 51 that the alloying elements improve the seizure resistance of Al-Si alloys. The wear of two-phase precipitation-hardened Cu-Cr alloys has been shown [6] to be of the delamination type. Biswas and Pramila Bai [7], in their work on the Al-Si alloy LM13 and its graphitic composites, observed that strength and ductility are important parameters for wear resistance. They also noted [8] that various modes such as delamination, abrasion and adhesion contributed to the wear process. Sarkar and Clarke [ 91 demonstrated the separation of work-hardened layers from the worn surface and showed [lo] that mutual transfer of material between pin and disc is important and that the features of worn surfaces can be explained by considering back transfer of material initially deposited on the disc, While different wear mechanisms such as delamination, adhesion and abrasion may be operating simultaneously the features of worn surfaces must be explained by these possible mechanisms. 0043-1648/83/0000-0000/$03.00
0 Elsevier
Sequoia/Printed
in The Netherlands
238
The present work is a continuation of earlier work on LM13 [ 7, 81. The effects of modification, alloying and silicon content on wear have already been studied. The effects of these parameters on features of the worn surfaces are now studied.
2. Experimental
details
2.1. Materials Wear and friction studies were carried out on three sets of materials: (a) Binary Al-55 alloys containing 4% - 21% Si were used (Table 1). The alloys were made by adding pure silicon to pure aluminium in different proportions and the silicon was not modified. (b) The binary alloys of (a) with modified silicon were also used. (c) Multicomponent alloys containing 0.2% - 24% Si were used. These alloys were made by adding commercially pure aluminium to LM29 in different proportions. The chemical compositions of these materials and the exact silicon contents of the alloys are given in Tables 2 and 3 respectively. All the alloys were melted in graphite crucibles and cast after degassing in permanent finger moulds of 12 mm diameter. The hypoeutectic alloys were modified with sodium and hypereutectic alloys with aluminium phosphide. The alloys were tested at room temperature under uniaxial tension. Figures l(a) and l(b) show the strength and percentage elongation characteristics of the alloys. TABLE Silicon
1 contents
of the binary
alloys tested
Serial number
Si (%)
1 2 3 4 5
3.91 8.47 13.97 18.61 20.88
TABLE
2
Chemical
composition
Material
Commercial LM29
of commercially Composition
Al
pure aluminium
and LM29
(%)
Si
Cu
Fe
Mg
Mn
Ti
Zn
Ni
Al
0.22 23.70
0.04 1.90
0.57 0.49
0.02 1.10
0.15 0.17
0.11 0.08
0.02 0.10
0.02 1.50
Balance Balance
239 TABLE 3 Silicon contents of the various modified multicomponent alloys used Serial number
Si (%)
1
0.22 4.48 9.36 12.09 15.12 17.16 23.70
2 3 4 5 6 7
ot
0
(a)
1
4
1
I
E; 12 16 % Stllcon
1
20
I
24
(b)
Fig. 1. Variation in (a) strength and (b) elongation with silicon content: 0, modified binary alloy; A, modified mufticomponent alloy; 0, unmodified binary alloy.
2.2. Equipment and methods A pin-on-disc-type wear rig described elsewhere [‘?I was used. The pin of the test material was held against a hardened (57 HRC) steel surface and the steel disc was rotated. The normal pressure applied on the pin was 0.038 kgf mme2. The average surface speed was 5.21 m s-l and the distance slid was 13 km. Experiments were conducted under dry conditions. Wear data were collected as the weight lost during wear. Worn surfaces and counterfaces and the debris were studied by scanning electron microscopy (SEM).
210
3. Results Under the load and speed conditions used sliding interaction caused severe wear. Figure 2 shows the dry wear characteristics of the three alloys as a function of the silicon content. The wear resistance of the binary alloy is not influenced significantly by silicon modification. The effect of alloying is to improve wear resistance slightly although there is a significant improvement when the alloy contains 17% Si. The wear data for both modified and unmodified binary alloys showed large scatter while those for the modified multicomponent system did not. Thus for the binary system at least ten experiments were carried out to obtain an average while for the multicomponent system only three experiments were sufficient to give consistent results. The points shown in Fig. 2 correspond to such average weight losses. The friction ch~acte~stics of these alloys hardly changed with variation in the modification, alloying or silicon content; the average value recorded was about 0.5. Worn pin surfaces exhibited several different features although there was no significant difference between the appearances of worn surfaces corresponding to modified and unmodified binary alloys. Therefore only those micrographs which correspond to unmodified binary alloys are presented here. The topographical features corresponding to the worn surfaces of binary alloys do not change significantly with silicon content up to 18%. Figure 3 shows diverse topographical features of a worn alloy containing about 8% Si. Figure 3(a) is a low magnification micrograph of a part of the worn surface. Most of the worn surface consists of smooth strips, the surfaces of which are characterized by fine scoring marks. These smooth strips were generally found to extend uninterrupted from one end of the specimen to the other. Cracks and strips of roughened surface are also seen in the same micrograph. Figure 3(b), a magnified view of the circled area in Fig. 3(a), shows cracks (marked A) of one type which propagate across the smooth
01
0
I
4
s
12 7.
16
20
24
SlllCW
Fig. 2. Wear us. silicon content: l, modified alloy; 0, unmodified binary alloy.
binary
alloy; A, modified
multicomponent
(9)
Fig. 3. SEM photographs of an unmodified binary alloy containing 8.47% Si (the sliding direction is from bottom to top): (a) low magnification view of the worn surface showing smooth strips, cracks and roughened strips; (b) magnified view of the circled area in (a); (c) magnified view of the rough flattened area circled in (b); (d) cracks on smooth strips; (e) a crater with a granular surface left behind after delamination; (f) another crater surface showing mud cracks; (g) a granular region being transformed to smooth strips.
242
strips in a direction inclined to the sliding direction. The cracks marked B propagate along the sliding direction, separating smooth strips. Figure 3(c) is a magnified view of the circled area in Fig. 3(b). The material, although rough, seems to have been flattened in a direction opposite to that of counterface sliding. A number of cracks inclined to the sliding direction are shown in Fig. 3(d). When these cracks intersect, flaky debris is detached leaving a crater. Figure 3(e) shows a crater which must have formed when cracks propagating across a smooth strip (the remnant can be seen on the right-hand side of the micrograph) intersected. The surface of separation has a granular appearance. The bottom of another crater shown in Fig, 3(f) is characterized by a set of shallow (possibly surface) cracks reminiscent of cracks in dried mud. In the same crater a patch of roughened oriented surface (similar to that of Fig. 3(c)) is seen on the left-hand side, Once the material is removed the surface of separation undergoes further sliding when the uneven crater surface as well as some of the debris left on the surface are flattened (left-hand side of Fig. 3(f)). It may be hypothesized that this flattened but rough surface gradually transforms into a smooth strip after considerable sliding. Such a process of transformation may be seen in Fig. 3(g), where the lower part of a penultimate surface layer can be seen to be in a more advanced stage of transformation than the corresponding top part. Figure 3(g) also shows that the edges of smooth strips are cracked and material is removed when the edge cracks intersect. Figure 4(a), which is a photograph of the worn surface of an alloy containing about 14% Si, shows smooth strips of finely scored material with cracked edges. Material flows in a direction normal to the sliding direction and heavily strained material at the edges cracks under longitudinal (i.e. in the direction of sliding) stress (Fig. 4(b)). A magnified view of the circled area in Fig. 4(b) shows the evidence of severe plastic deformation at the edge of a strip (Fig. 4(c)). Another feature of the worn surface is the presence of strips of overlapping flakes of material. The flakes are oriented and their surfaces are finely scored (Fig. 4(d)). Each flake is distinctly separated from the others and its leading edge is raised with respect to the adjacent flake (Fig. 4(e)). The worn surface of an alloy containing about 21% Si is shown in Fig. 5. The surface is disintegrated by the cracks. The appearances of worn surfaces of modified multicomponent alloys with up to 12% Si are similar to those of binary alloys containing up to 18% Si. Smooth strips with fine scoring marks are shown in Fig. 6(a). Edge cracking of the smooth strips is shown in Fig. 6(b). Overlapping flakes of material oriented in one direction are shown in Fig. 6(c). Oriented overlapping flakes were absent on the worn surfaces of hypereutectic alloys. Beth edge cracking and delaminated craters were seen only occasionally. The worn surfaces were dominated by smooth strips (Fig. 6(d)) which were disintegrated by a number of cracks when the silicon content was greater than 17% (Fig. 6(e)).
(c)
(e)
Cd) Fig. 4. SEM photographs of the worn surface of the binary alloy containing 13.97% Si (the sliding direction is from bottom to top): (a) a low magnification view of the surface showing finely scored smooth strips; (b) edge cracking of smooth strips; (c) plastic deformation of the edge of a strip (region circled in (b)); (d) oriented overlapping flakes; (e) magnified view of the circled area in (d) showing finer details of the flakes, which are distinctly separated from each other.
(a)
(b)
(d)
Fig. 6. SEM photographs of worn surfaces of modified multicomponent alloys containing (a) - (c) 12.09% Si, (d) 17.16% Si and (e) 23.7% Si (sliding directions:
(e)
(a) - (c) from top to bottom; (d), (e) from bottom to top): (a) finely scored smooth strips; (b) edge cracking seen on the worn surface shown in (a); (c) overlapping oriented flakes; (d) very smooth surface of the alloy containing 17.16% Si; (e) surface of the alloy containing 23.7% Si showing a number of cracks.
Most of the debris for binary alloys consisted of either laminates or string-type particles (Fig. 7(a)). Debris from multicomponent alloys was generally finer than that from binary alloys and the number of string-type particles was significantly less (Fig. 7(b)).
245
Seizure during sliding of binary alloys resulted in a “heavy” continuous smear on the counterface disc while that of the multicomponent alloys resulted in patchy discontinuous smear on the counterface disc, as shown in Figs. 8(a) and 8(b) respectively.
(a)
Fig. 7. Debris corresponding
to (a) an 8.47%
(b)
Si binary alloy and (b) a 9.36% Si multi-
(b)
Fig. 8. Counterfaces rubbed by (a) a binary alloy containing 18.61% component alloy containing 17.16% Si.
Si and (b) a multi-
4. Discussion 4.1. Wear characteristics This study corroborates the findings of other researchers [4] that modification has little effect on the wear characteristics of Al-% alloys. Rohatgi and Pai [ 51 reported that the wear resistance of an Al-Si alloy when rubbed against itself was improved by the addition of alloying elements. The present work shows that alloying elements do increase the wear resistance of Al-Si alloys when they are rubbed against steel. While the observed wear minimum for a multicomponent system agrees with the findings of Vandelli [ 21 and Okabayashi et al. [3], it disagrees with the findings of Clarke and Sarkar [ 11. It is believed that the operating conditions and the nature of the
246
test materials have a great bearing on the existence of such minima. The wear minima observed by other workers have usually not been significant when compared with the magnitude of wear. Even in the present work it gains some significance only when the multicomponent and binary alloy systems are compared. It is interesting that wear of the 17% Si alloy, which exhibits the wear minimum, gives rise to a uniformly smooth wear surface uncharacteristic of other test alloys. The difference in the extent of wear between binary and multicomponent systems can possibly be explained by the relative propensity of these alloys to seize when rubbed against a steel surface. Since seizure is a random phenomenon, scatter in the wear data could be related to seizure resistance. The higher the seizure the higher would be the scatter. This has been observed for binary alloys. This also possibly explains the relatively high wear of the binary system since the interaction between the pin and the disc will change to that between t,he pin and a smear; such an interaction being one between like metals will result in high wear. 4.2. Pin surface Considering the above-mentioned difference between the multicomponent and binary systems in their sliding interaction with steel we should expect to see it reflected on topographical features of the worn surfaces. The worn surfaces of binary and multicomponent systems, however, did not show any significant difference. Worn surfaces of all the alloys show a multitude of distinct topographical features that are common to most of the alloys studied. This indicates that each surface is simultaneously subjected to more than one mode of material removal. The greatest proportion of each worn surface was found to consist of smooth strips with fine scoring marks. The appearance of these strips is similar to that observed in grinding. In an in situ wear study conducted by SEM [lo] it was observed that scoring is a continuous process and the pattern of scoring marks changes with time. Scoring may be due to abrasion by entrapped debris, work-hardened deposits on the counterface or hard asperities on hardened steel. This mode, however, may not lead to a large wear volume since the amount of material removed from a fine groove formed during each scoring action is very small. In this sense, therefore, it does not constitute a major wear mechanism. The nature of the debris indicates that considerable amounts of material are removed as laminates. The cascade of cracks in the main body of a smooth strip such as that shown in Fig. 3(d) might have been initiated at the subsurface and, when the cracks interact, a particle is detached by a delamination process. The delaminated area has a rough granular appearance (Fig. 3(e)). Further sliding flattens the roughened surface and the delaminated area is completely scored, which results in the formation of a smooth strip (Figs. 3(f) and 3(g)). However, shear dimples seen on the fractured surface of ductile materials were not seen anywhere on the. worn surface. The absence of shear dimples has been observed by Doyle and Turley [ 111
247
for grinding and according to them shear dimples are not formed because of high compressive stresses which prevent the fo~ation and coalescence of voids. The cracks observed in the crater formed by delamination (Fig. 3(f)) are not easily explainable, These look like surface cracks which might have been formed by some secondary process. Another important mode of metal removal is edge cracking. Transverse cracks initiated at the edge of smooth strips propagating away from the edges intersect to form debris. Because the relative dimension of a strip in the transverse (normal to sliding) direction is small when compared with its length and because of the fact that a surface strip adheres to the penultimate surface layer high bearing pressures can be expected to produce plastic flow only at the edges of the strips and that also will be in the transverse direction. This leads to the formation of strip edges which are not anchored to the penultimate layer. The excessive plastic flow in the edge of the strip as seen in Fig. 4(c) most probably limits its ability to withstand further strain without fracture. Thus when the edge is subjected to high tensile stresses in the plane strain (lon~tudin~) direction cracks normal to the sliding direction are initiated. A similar feature was seen on ground surfaces as shown in
Fig. 9. A ground AI-B
alloy surface.
The oriented overlapping flaky feature shown in Fig. 4(d) has been _. observed by Glaeser [lo] in wear experiments. It was attributed to stick-slip motion during sliding. While this is possible it is interesting to note that similar features were also observed on fretted surfaces f 12,133. It is difficult to conceive of the existence of fretting in the severe wear regime under the present experimental conditions, This feature also resembles the “inclined shear plates” observed by Clarke and Sarkar 1143 for near-eutectic alloys. However, in the present work the feature was observed for silicon contents up to 18% for binary alloys and up to 12% for multicomponent alloys. Recently Clarke and Sarkar have reported [ 141 that mutual transfer of material between pin and disc is an important phenomenon by which most of the oriented features of the worn surfaces can be explained. The worn
surface is covered by a layer of material which was initially deposited on the disc and is then back transferred. They present an asperity model to explain orientation of the features on the worn surface, While it seems almost certain that some pin material transferred to the disc will be back transferred, for the following reasons the validity of Clarke and Sarkar’s model [ 141 as an explanation of the orientation of surface features is questioned. (1) As the pin surface and the subsurface [ 151 are subjected to extensive plastic flow the conical asperities existing on the original machined surface will be flattened to the extent of completely losing their conical shape. Thus the validity of a model based on two sharply projecting conical asperities seems doubtful. (2) Even if the material is back transferred in an oriented fashion as suggested by Clarke and Sarkar [14] the back-transferred material would be expected to coexist on the same surface with flattened pin asperities. Thus the topography should be a random mixture of opposing hyperbolae. No such feature was observed either by Clarke and Sarkar [ 141 or by the present authors. (3) The back-transferred material is repeatedly subjected to plastic flow while it remains on the original pin surface, on the disc surface and after it has been back transferred. The surface of such back-transferred material can be expected to have no special features related to orientation as shown in Fig. 10 which is the micro~aph of a surface with some back-tr~sfe~ed material.
Fig, 10. Surface of a 4% Si binary alloy showing an aggregate of back-transferred debris.
No definitive mechanism explaining the formation of oriented overlapping flakes could be suggested at this stage. Further investigation, especially into the possibility of a stick-slip type of mechanism on the wear surface, needs to be undertaken. Wear of high silicon alloys may be controlled by the brittleness and low strength imparted by the presence of large amounts of silicon. Figure 1 shows that, beyond a 20% Si content, both of these properties are very poor for all alloys studied here. It has been observed [7,15] that the subsurface of worn low strength brittle materials cannot sustain high plastic strains and
249
the energy input to the sliding process causes extensive surface cracking and disintegration. It is suggested that the same phenomenological reasons are responsible for the disintegration of the surfaces of high silicon alloys as seen in Figs. 5 and 6(e).
5. Conclusions (1) Modification has little effect on the wear resistance of binary Al-S1 alloys. The introduction of alloying elements slightly improves the wear resistance of the alloys. For the alloy containing 17% Si this improvement is significant. (2) Addition of alloying elements reduces the propensity of the alloy to seize against steel. (3) Except for the alloys containing more than 20% Si the worn surfaces of most alloys show similar topographical features. (4) While the major area of a worn surface consists of smooth strips, the surfaces of which are characterized by fine scoring marks, the major modes of metal removal are (a) by cracking of the edges of smooth strips and (b) by a process similar to delamination.
Acknowledgments The authors thank the Council for Scientific and Industrial Research (India) for providing the necessary grants for this work and Sargam Metals Private Ltd. (Madras, India) for donating the materials used in this investigation.
References 1 J. Clarke and A. D. Sarkar, Wear, 54 (1979) 7. 2 G. Vandelli, Aluminio, 37 (1968) 121. 3 K. Okabayashi, Y. Nakatani, H. Notani and M. Icawamoto,
14 (1964)
Keikinroku,
57. A. J. Clegg and A. A. Das, Wear, 43 (1977) 367. P. K. Rohatgi and B. C. Pai, Wear, 28 (1974) 353. N. Saka, J. J. Pamies-Teixeira and N. P. Suh, Wear, 44 (1977) 77. S. K. Biswas and B. N. PramiIa Bai, Wear, 68 (1981) 347. 8 B. N. PramiIa Bai, E. S. Dwarakadasa and S. K. Biswas, Wear, 76 (1982) 9 A. D. Sarkar and J. Clarke, Wear, 61 (1980) 157. 10 W. A. Glaeser, Wear, 73 (1981) 371. 11 E. D. Doyle and D. M. Turley, Wear, 51 (1978) 269. 12 J. A. AIic, A. L. Mawley and J. M. Urey, Wear, 56 (1979) 351. 13 G. L. Goss and D. W. Hoeppner, Wear, 24 (1973) 77. 14 J. Clarke and A. D. Sarkar, Wear, 69 (1981) 1. 15 B. N. PramiIa Bai and S. K. Biswas, Wear, 71 (1981) 381.
4 5 6 7
211.
SCANNING ELECTRON ALLOY SURFACES B. N. PRAMILA Department (India)
237
237 - 249
MICROSCOPY
STUDY OF WORN Al-Si
BAI and S. K. BISWAS
of Mechanical
Engineering,
Indian Institute
of Science,
Bangalore 560012
N. N. KUMTEKAR Tata Consulting Engineers, Bangalore (India) (Received
May 15, 1982)
Summary A pin-on-disc machine was used to wear Al-Si alloy pins under dry conditions. Unmodified and modified binary alloys and commercial multicomponent alloys were tested. The surfaces of the worn alloys were examined by scanning electron microscopy to identify distinct topographical features to aid elucidation of the mechanisms of wear.
1. Introduction As-cast Al-S1 alloys are important wear-resistant materials. There is controversy over the optimum silicon content for maximum wear resistance. According to Clarke and Sarkar [l] the optimum silicon content is around the eutectic region but other workers suggest that it is in the hypereutectic region [ 2, 31. Clegg and Das [ 41 have shown that silicon has little effect on wear resistance. Rohatgi and Pai have reported [ 51 that the alloying elements improve the seizure resistance of Al-Si alloys. The wear of two-phase precipitation-hardened Cu-Cr alloys has been shown [6] to be of the delamination type. Biswas and Pramila Bai [7], in their work on the Al-Si alloy LM13 and its graphitic composites, observed that strength and ductility are important parameters for wear resistance. They also noted [8] that various modes such as delamination, abrasion and adhesion contributed to the wear process. Sarkar and Clarke [ 91 demonstrated the separation of work-hardened layers from the worn surface and showed [lo] that mutual transfer of material between pin and disc is important and that the features of worn surfaces can be explained by considering back transfer of material initially deposited on the disc, While different wear mechanisms such as delamination, adhesion and abrasion may be operating simultaneously the features of worn surfaces must be explained by these possible mechanisms. 0043-1648/83/0000-0000/$03.00
0 Elsevier
Sequoia/Printed
in The Netherlands
238
The present work is a continuation of earlier work on LM13 [ 7, 81. The effects of modification, alloying and silicon content on wear have already been studied. The effects of these parameters on features of the worn surfaces are now studied.
2. Experimental
details
2.1. Materials Wear and friction studies were carried out on three sets of materials: (a) Binary Al-55 alloys containing 4% - 21% Si were used (Table 1). The alloys were made by adding pure silicon to pure aluminium in different proportions and the silicon was not modified. (b) The binary alloys of (a) with modified silicon were also used. (c) Multicomponent alloys containing 0.2% - 24% Si were used. These alloys were made by adding commercially pure aluminium to LM29 in different proportions. The chemical compositions of these materials and the exact silicon contents of the alloys are given in Tables 2 and 3 respectively. All the alloys were melted in graphite crucibles and cast after degassing in permanent finger moulds of 12 mm diameter. The hypoeutectic alloys were modified with sodium and hypereutectic alloys with aluminium phosphide. The alloys were tested at room temperature under uniaxial tension. Figures l(a) and l(b) show the strength and percentage elongation characteristics of the alloys. TABLE Silicon
1 contents
of the binary
alloys tested
Serial number
Si (%)
1 2 3 4 5
3.91 8.47 13.97 18.61 20.88
TABLE
2
Chemical
composition
Material
Commercial LM29
of commercially Composition
Al
pure aluminium
and LM29
(%)
Si
Cu
Fe
Mg
Mn
Ti
Zn
Ni
Al
0.22 23.70
0.04 1.90
0.57 0.49
0.02 1.10
0.15 0.17
0.11 0.08
0.02 0.10
0.02 1.50
Balance Balance
239 TABLE 3 Silicon contents of the various modified multicomponent alloys used Serial number
Si (%)
1
0.22 4.48 9.36 12.09 15.12 17.16 23.70
2 3 4 5 6 7
ot
0
(a)
1
4
1
I
E; 12 16 % Stllcon
1
20
I
24
(b)
Fig. 1. Variation in (a) strength and (b) elongation with silicon content: 0, modified binary alloy; A, modified mufticomponent alloy; 0, unmodified binary alloy.
2.2. Equipment and methods A pin-on-disc-type wear rig described elsewhere [‘?I was used. The pin of the test material was held against a hardened (57 HRC) steel surface and the steel disc was rotated. The normal pressure applied on the pin was 0.038 kgf mme2. The average surface speed was 5.21 m s-l and the distance slid was 13 km. Experiments were conducted under dry conditions. Wear data were collected as the weight lost during wear. Worn surfaces and counterfaces and the debris were studied by scanning electron microscopy (SEM).
210
3. Results Under the load and speed conditions used sliding interaction caused severe wear. Figure 2 shows the dry wear characteristics of the three alloys as a function of the silicon content. The wear resistance of the binary alloy is not influenced significantly by silicon modification. The effect of alloying is to improve wear resistance slightly although there is a significant improvement when the alloy contains 17% Si. The wear data for both modified and unmodified binary alloys showed large scatter while those for the modified multicomponent system did not. Thus for the binary system at least ten experiments were carried out to obtain an average while for the multicomponent system only three experiments were sufficient to give consistent results. The points shown in Fig. 2 correspond to such average weight losses. The friction ch~acte~stics of these alloys hardly changed with variation in the modification, alloying or silicon content; the average value recorded was about 0.5. Worn pin surfaces exhibited several different features although there was no significant difference between the appearances of worn surfaces corresponding to modified and unmodified binary alloys. Therefore only those micrographs which correspond to unmodified binary alloys are presented here. The topographical features corresponding to the worn surfaces of binary alloys do not change significantly with silicon content up to 18%. Figure 3 shows diverse topographical features of a worn alloy containing about 8% Si. Figure 3(a) is a low magnification micrograph of a part of the worn surface. Most of the worn surface consists of smooth strips, the surfaces of which are characterized by fine scoring marks. These smooth strips were generally found to extend uninterrupted from one end of the specimen to the other. Cracks and strips of roughened surface are also seen in the same micrograph. Figure 3(b), a magnified view of the circled area in Fig. 3(a), shows cracks (marked A) of one type which propagate across the smooth
01
0
I
4
s
12 7.
16
20
24
SlllCW
Fig. 2. Wear us. silicon content: l, modified alloy; 0, unmodified binary alloy.
binary
alloy; A, modified
multicomponent
(9)
Fig. 3. SEM photographs of an unmodified binary alloy containing 8.47% Si (the sliding direction is from bottom to top): (a) low magnification view of the worn surface showing smooth strips, cracks and roughened strips; (b) magnified view of the circled area in (a); (c) magnified view of the rough flattened area circled in (b); (d) cracks on smooth strips; (e) a crater with a granular surface left behind after delamination; (f) another crater surface showing mud cracks; (g) a granular region being transformed to smooth strips.
242
strips in a direction inclined to the sliding direction. The cracks marked B propagate along the sliding direction, separating smooth strips. Figure 3(c) is a magnified view of the circled area in Fig. 3(b). The material, although rough, seems to have been flattened in a direction opposite to that of counterface sliding. A number of cracks inclined to the sliding direction are shown in Fig. 3(d). When these cracks intersect, flaky debris is detached leaving a crater. Figure 3(e) shows a crater which must have formed when cracks propagating across a smooth strip (the remnant can be seen on the right-hand side of the micrograph) intersected. The surface of separation has a granular appearance. The bottom of another crater shown in Fig, 3(f) is characterized by a set of shallow (possibly surface) cracks reminiscent of cracks in dried mud. In the same crater a patch of roughened oriented surface (similar to that of Fig. 3(c)) is seen on the left-hand side, Once the material is removed the surface of separation undergoes further sliding when the uneven crater surface as well as some of the debris left on the surface are flattened (left-hand side of Fig. 3(f)). It may be hypothesized that this flattened but rough surface gradually transforms into a smooth strip after considerable sliding. Such a process of transformation may be seen in Fig. 3(g), where the lower part of a penultimate surface layer can be seen to be in a more advanced stage of transformation than the corresponding top part. Figure 3(g) also shows that the edges of smooth strips are cracked and material is removed when the edge cracks intersect. Figure 4(a), which is a photograph of the worn surface of an alloy containing about 14% Si, shows smooth strips of finely scored material with cracked edges. Material flows in a direction normal to the sliding direction and heavily strained material at the edges cracks under longitudinal (i.e. in the direction of sliding) stress (Fig. 4(b)). A magnified view of the circled area in Fig. 4(b) shows the evidence of severe plastic deformation at the edge of a strip (Fig. 4(c)). Another feature of the worn surface is the presence of strips of overlapping flakes of material. The flakes are oriented and their surfaces are finely scored (Fig. 4(d)). Each flake is distinctly separated from the others and its leading edge is raised with respect to the adjacent flake (Fig. 4(e)). The worn surface of an alloy containing about 21% Si is shown in Fig. 5. The surface is disintegrated by the cracks. The appearances of worn surfaces of modified multicomponent alloys with up to 12% Si are similar to those of binary alloys containing up to 18% Si. Smooth strips with fine scoring marks are shown in Fig. 6(a). Edge cracking of the smooth strips is shown in Fig. 6(b). Overlapping flakes of material oriented in one direction are shown in Fig. 6(c). Oriented overlapping flakes were absent on the worn surfaces of hypereutectic alloys. Beth edge cracking and delaminated craters were seen only occasionally. The worn surfaces were dominated by smooth strips (Fig. 6(d)) which were disintegrated by a number of cracks when the silicon content was greater than 17% (Fig. 6(e)).
(c)
(e)
Cd) Fig. 4. SEM photographs of the worn surface of the binary alloy containing 13.97% Si (the sliding direction is from bottom to top): (a) a low magnification view of the surface showing finely scored smooth strips; (b) edge cracking of smooth strips; (c) plastic deformation of the edge of a strip (region circled in (b)); (d) oriented overlapping flakes; (e) magnified view of the circled area in (d) showing finer details of the flakes, which are distinctly separated from each other.
(a)
(b)
(d)
Fig. 6. SEM photographs of worn surfaces of modified multicomponent alloys containing (a) - (c) 12.09% Si, (d) 17.16% Si and (e) 23.7% Si (sliding directions:
(e)
(a) - (c) from top to bottom; (d), (e) from bottom to top): (a) finely scored smooth strips; (b) edge cracking seen on the worn surface shown in (a); (c) overlapping oriented flakes; (d) very smooth surface of the alloy containing 17.16% Si; (e) surface of the alloy containing 23.7% Si showing a number of cracks.
Most of the debris for binary alloys consisted of either laminates or string-type particles (Fig. 7(a)). Debris from multicomponent alloys was generally finer than that from binary alloys and the number of string-type particles was significantly less (Fig. 7(b)).
245
Seizure during sliding of binary alloys resulted in a “heavy” continuous smear on the counterface disc while that of the multicomponent alloys resulted in patchy discontinuous smear on the counterface disc, as shown in Figs. 8(a) and 8(b) respectively.
(a)
Fig. 7. Debris corresponding
to (a) an 8.47%
(b)
Si binary alloy and (b) a 9.36% Si multi-
(b)
Fig. 8. Counterfaces rubbed by (a) a binary alloy containing 18.61% component alloy containing 17.16% Si.
Si and (b) a multi-
4. Discussion 4.1. Wear characteristics This study corroborates the findings of other researchers [4] that modification has little effect on the wear characteristics of Al-% alloys. Rohatgi and Pai [ 51 reported that the wear resistance of an Al-Si alloy when rubbed against itself was improved by the addition of alloying elements. The present work shows that alloying elements do increase the wear resistance of Al-Si alloys when they are rubbed against steel. While the observed wear minimum for a multicomponent system agrees with the findings of Vandelli [ 21 and Okabayashi et al. [3], it disagrees with the findings of Clarke and Sarkar [ 11. It is believed that the operating conditions and the nature of the
246
test materials have a great bearing on the existence of such minima. The wear minima observed by other workers have usually not been significant when compared with the magnitude of wear. Even in the present work it gains some significance only when the multicomponent and binary alloy systems are compared. It is interesting that wear of the 17% Si alloy, which exhibits the wear minimum, gives rise to a uniformly smooth wear surface uncharacteristic of other test alloys. The difference in the extent of wear between binary and multicomponent systems can possibly be explained by the relative propensity of these alloys to seize when rubbed against a steel surface. Since seizure is a random phenomenon, scatter in the wear data could be related to seizure resistance. The higher the seizure the higher would be the scatter. This has been observed for binary alloys. This also possibly explains the relatively high wear of the binary system since the interaction between the pin and the disc will change to that between t,he pin and a smear; such an interaction being one between like metals will result in high wear. 4.2. Pin surface Considering the above-mentioned difference between the multicomponent and binary systems in their sliding interaction with steel we should expect to see it reflected on topographical features of the worn surfaces. The worn surfaces of binary and multicomponent systems, however, did not show any significant difference. Worn surfaces of all the alloys show a multitude of distinct topographical features that are common to most of the alloys studied. This indicates that each surface is simultaneously subjected to more than one mode of material removal. The greatest proportion of each worn surface was found to consist of smooth strips with fine scoring marks. The appearance of these strips is similar to that observed in grinding. In an in situ wear study conducted by SEM [lo] it was observed that scoring is a continuous process and the pattern of scoring marks changes with time. Scoring may be due to abrasion by entrapped debris, work-hardened deposits on the counterface or hard asperities on hardened steel. This mode, however, may not lead to a large wear volume since the amount of material removed from a fine groove formed during each scoring action is very small. In this sense, therefore, it does not constitute a major wear mechanism. The nature of the debris indicates that considerable amounts of material are removed as laminates. The cascade of cracks in the main body of a smooth strip such as that shown in Fig. 3(d) might have been initiated at the subsurface and, when the cracks interact, a particle is detached by a delamination process. The delaminated area has a rough granular appearance (Fig. 3(e)). Further sliding flattens the roughened surface and the delaminated area is completely scored, which results in the formation of a smooth strip (Figs. 3(f) and 3(g)). However, shear dimples seen on the fractured surface of ductile materials were not seen anywhere on the. worn surface. The absence of shear dimples has been observed by Doyle and Turley [ 111
247
for grinding and according to them shear dimples are not formed because of high compressive stresses which prevent the fo~ation and coalescence of voids. The cracks observed in the crater formed by delamination (Fig. 3(f)) are not easily explainable, These look like surface cracks which might have been formed by some secondary process. Another important mode of metal removal is edge cracking. Transverse cracks initiated at the edge of smooth strips propagating away from the edges intersect to form debris. Because the relative dimension of a strip in the transverse (normal to sliding) direction is small when compared with its length and because of the fact that a surface strip adheres to the penultimate surface layer high bearing pressures can be expected to produce plastic flow only at the edges of the strips and that also will be in the transverse direction. This leads to the formation of strip edges which are not anchored to the penultimate layer. The excessive plastic flow in the edge of the strip as seen in Fig. 4(c) most probably limits its ability to withstand further strain without fracture. Thus when the edge is subjected to high tensile stresses in the plane strain (lon~tudin~) direction cracks normal to the sliding direction are initiated. A similar feature was seen on ground surfaces as shown in
Fig. 9. A ground AI-B
alloy surface.
The oriented overlapping flaky feature shown in Fig. 4(d) has been _. observed by Glaeser [lo] in wear experiments. It was attributed to stick-slip motion during sliding. While this is possible it is interesting to note that similar features were also observed on fretted surfaces f 12,133. It is difficult to conceive of the existence of fretting in the severe wear regime under the present experimental conditions, This feature also resembles the “inclined shear plates” observed by Clarke and Sarkar 1143 for near-eutectic alloys. However, in the present work the feature was observed for silicon contents up to 18% for binary alloys and up to 12% for multicomponent alloys. Recently Clarke and Sarkar have reported [ 141 that mutual transfer of material between pin and disc is an important phenomenon by which most of the oriented features of the worn surfaces can be explained. The worn
surface is covered by a layer of material which was initially deposited on the disc and is then back transferred. They present an asperity model to explain orientation of the features on the worn surface, While it seems almost certain that some pin material transferred to the disc will be back transferred, for the following reasons the validity of Clarke and Sarkar’s model [ 141 as an explanation of the orientation of surface features is questioned. (1) As the pin surface and the subsurface [ 151 are subjected to extensive plastic flow the conical asperities existing on the original machined surface will be flattened to the extent of completely losing their conical shape. Thus the validity of a model based on two sharply projecting conical asperities seems doubtful. (2) Even if the material is back transferred in an oriented fashion as suggested by Clarke and Sarkar [14] the back-transferred material would be expected to coexist on the same surface with flattened pin asperities. Thus the topography should be a random mixture of opposing hyperbolae. No such feature was observed either by Clarke and Sarkar [ 141 or by the present authors. (3) The back-transferred material is repeatedly subjected to plastic flow while it remains on the original pin surface, on the disc surface and after it has been back transferred. The surface of such back-transferred material can be expected to have no special features related to orientation as shown in Fig. 10 which is the micro~aph of a surface with some back-tr~sfe~ed material.
Fig, 10. Surface of a 4% Si binary alloy showing an aggregate of back-transferred debris.
No definitive mechanism explaining the formation of oriented overlapping flakes could be suggested at this stage. Further investigation, especially into the possibility of a stick-slip type of mechanism on the wear surface, needs to be undertaken. Wear of high silicon alloys may be controlled by the brittleness and low strength imparted by the presence of large amounts of silicon. Figure 1 shows that, beyond a 20% Si content, both of these properties are very poor for all alloys studied here. It has been observed [7,15] that the subsurface of worn low strength brittle materials cannot sustain high plastic strains and
249
the energy input to the sliding process causes extensive surface cracking and disintegration. It is suggested that the same phenomenological reasons are responsible for the disintegration of the surfaces of high silicon alloys as seen in Figs. 5 and 6(e).
5. Conclusions (1) Modification has little effect on the wear resistance of binary Al-S1 alloys. The introduction of alloying elements slightly improves the wear resistance of the alloys. For the alloy containing 17% Si this improvement is significant. (2) Addition of alloying elements reduces the propensity of the alloy to seize against steel. (3) Except for the alloys containing more than 20% Si the worn surfaces of most alloys show similar topographical features. (4) While the major area of a worn surface consists of smooth strips, the surfaces of which are characterized by fine scoring marks, the major modes of metal removal are (a) by cracking of the edges of smooth strips and (b) by a process similar to delamination.
Acknowledgments The authors thank the Council for Scientific and Industrial Research (India) for providing the necessary grants for this work and Sargam Metals Private Ltd. (Madras, India) for donating the materials used in this investigation.
References 1 J. Clarke and A. D. Sarkar, Wear, 54 (1979) 7. 2 G. Vandelli, Aluminio, 37 (1968) 121. 3 K. Okabayashi, Y. Nakatani, H. Notani and M. Icawamoto,
14 (1964)
Keikinroku,
57. A. J. Clegg and A. A. Das, Wear, 43 (1977) 367. P. K. Rohatgi and B. C. Pai, Wear, 28 (1974) 353. N. Saka, J. J. Pamies-Teixeira and N. P. Suh, Wear, 44 (1977) 77. S. K. Biswas and B. N. PramiIa Bai, Wear, 68 (1981) 347. 8 B. N. PramiIa Bai, E. S. Dwarakadasa and S. K. Biswas, Wear, 76 (1982) 9 A. D. Sarkar and J. Clarke, Wear, 61 (1980) 157. 10 W. A. Glaeser, Wear, 73 (1981) 371. 11 E. D. Doyle and D. M. Turley, Wear, 51 (1978) 269. 12 J. A. AIic, A. L. Mawley and J. M. Urey, Wear, 56 (1979) 351. 13 G. L. Goss and D. W. Hoeppner, Wear, 24 (1973) 77. 14 J. Clarke and A. D. Sarkar, Wear, 69 (1981) 1. 15 B. N. PramiIa Bai and S. K. Biswas, Wear, 71 (1981) 381.
4 5 6 7
211.