Wear lip formation during dry sliding

Wear lip formation during dry sliding

Wear, 126 f1988) 57 - 67 57 C. SUBRAMANIAN* Departmsnt of Metallurgy, Indian Institute of S&mea, Bangalore560 012 (h&n) (Recxived October 22,1937...

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Wear, 126 f1988)

57 - 67

57

C. SUBRAMANIAN* Departmsnt

of Metallurgy, Indian Institute of S&mea, Bangalore560 012 (h&n)

(Recxived October 22,1937;

accepted February 9,X9%3)

A pin-on-disc test con~~~ti~~ has been used to examine the formation of the sac-b~ened projection, or wear lips, especially at the traihng edge of the pin during dry sliding of aluminium alloys against steel discs, The mechanism of formation of such wear iips is studied with the aid of optical and electron microscopes. The plastic deformation of the pm, growth and eventual removal of the wear lip as wear debris are elucidated. The size and shape of the wear lips in pins of different shapes, i.e. square, rectangular, triangular and circufar cross-sections, are described,

Wear, ~co~d~g to one def~iti~~~ ‘“is a process of materisl remuvd from surfaces which are in relative movement under load” f l] a Such loss of material can be monitored in many ways, e.g, as changes in dimension, volume or mass of the specimen. If the conditions are favourable, in addition to loss of material, one of the sliding partners could develop projections at the wearing end, In the strict sense, the pm has worn uut because of the change in dimensiun although there may not be a phenomena change in the mass of the specimen. The study of these projections becomes important when pin specimens tested in a pin-on-disc wear unit fail to show any appreciable change in mass. These str~-h~de~~d projections, especially at the traihng edge of the pins, have recently been observed and termed wear lips [Z, 33 or mushrooms 143. The pin mater& in which wear lips were observed are ahxminium alloys 121 and ~~rn~iurn-~~hit~ particulate composites [3] during dry sliding, and aluminium alloys [5], copper alloys [4] and steel [6] during impact sliding wear conditions, However, the nature and the mechanisms of formation of wear lips are not clearly known. In the present study the formation mechanisms of the wear hp in pins of cummer-

*Rwsent address: ~p~~en~ of Mechanical tuxd ~anufactu~~~ University of Melbourne, Pmkville, Victoria 9052, Australia.

Engineering,

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cial aluminium alloys tested against hardened steel discs in a pin-on-disc machine under dry sliding situations in the ambient atmosphere, are described.

2. Experimental details Wear tests were performed in a pm-on-disc set up described elsewhere [7]. Briefly, it consists of a hardened AISI-SAE 52100 steel disc (Rockwe C hardness, 63 HRC) of 250 mm diameter and 10 mm thick rotated by a motor via a pulley system. The speed can be varied from 0.5 to 5.0 m s-i. The normal load is applied to the pin by placing known weights on it. The pins of different shapes were machined from extruded stock of the aluminium alloys listed in Table 1. One end of the pin was prepared by grinding it against a 600 mesh silicon carbide paper. The disc was ground to a surface finish of 0.3 pm centre-line average. Both the pin and the disc were cleaned in soapy water and degreased with acetone before the test. The wear tests were done in the normal laboratory atmosphere under unlubricated conditions with a load of 10.3 N at a speed of 2.6 m s-i for a sliding distance of 5000 m in all cases unless stated otherwise. Following the completion of a test, the worn pin was transferred to a scanning electron microscope (Cambridge Stereoscan 150 s) operated at 10 kV. For optical microscopy, the pin was sectioned pe~endicul~ to the worn surface and either parallel or perpendicular to the sliding direction. The sectioned sample was mounted, polished and etched with Keller’s reagent by the usual metallographic technique and the subsurface features were observed.

TABLE 1 Nominal composition of the alloys used Amount

Al cu Mg Mn Si Fe Ni Zn Zr + Ti

(wt.%) in the following days

ES L778

AA 2618b

Balance 3.9 - 5.0 0.2 - 0.8 0.4 - 1.2 0.5 - 0.9 0.5 0.2 0.2 0.2

Balance 1.8 - 2.5 1.2 - 1.8 0.2 0.25 0.9 - 1.4 0.8 - 1.4 -

=BS L77 alloy was used in the as-extruded condition. bAA 2618 alloy was solution treated at 800 K for 2 b, water quenched and aged at 460 K for 15 h.

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3. Results and discussion The results presented here are mostly confined to the cylindrical pins. The wear lip formation in the pins with other shapes will be discussed in Section 3.5. However, it should be noted that the nature of the wear lip in all the pins is the same. 3.1. Nature of the wear lip As a typical wear lip is a few millimetres in size, it could even be seen with the naked eye. Further, it could not be removed by pushing with a nail or a knife. This demonstrates the fact that it is attached firmly to the parent pin. After the wear test the mass of the pin was measured and the mass loss was determined (Table 2). Then the wear lip was removed by machining the projections on the pin, the mass of pin was redetermined and the mass loss was again estimated. The difference between these two mass loss measurements would be the mass of the wear lip. It is clear from Table 2 that there would be an error of 26% in the wear loss value if the wear lip is not taken into account. Figure 1 shows a typical surface of a wear lip with a layer-like morphology in an aluminium alloy 2618 pin. The layers of the material seem to have flowed from the wearing surface of the pin. TABLE 2 Mass of a wear lip Wear loss before wear lip removed Wear loss after wear lip removed Mass of the wear lip Wear-lip-to-wear-loss ratio

n b c-b-a c/b

0.06899 0.09279 0.02380 0.26

g g g

60

The lip is predom~antly present at the trailing edge of the pin. Figures Z(a) and 2(b) schematically show the wear lip at the trailing and the leading edges of the pm in the plane and the sectional elevation respectively. Figures 3(a) and 3(b) show the corresponding cross-sectional view of the aluminium alloy pin at the leading and trailing edges respectively. A close observation of the flow patterns in the lip region of Fig. 3(b) reveals that the flow lines are continuous from the pin to the lip region. This suggests that the material in the lip region is the result of the deformation of the trailing edge. The degree of work hardening at the edge of the lip is similar to that in the near-surface region of the pm whereas its centre part is close to that in the deformed subsurface of the pm. This point gets support from microhardness measurements taken at various locations in the lip and the subsurface regions of the pin (Table 3). The increase in the hardness of the wear lip and the flow pattern indicate that it has undergone heavy plastic deformation like the subsurface of the pin. It may be concluded that the ability to undergo plastic deformation is a necessary condition for a material to form a wear lip during dry sliding. Therefore, it appears that the ductility of the pin is important in

-I-

(al

Fig. 2. Schematic diagram of (a) the plane and (b) the sectional elevation of the leading and trailing edges of the pin (SD, sliding direction of the disc).

(a)

@f

Fig. 3. Optical micrographs of (a) leading and (b) trailing edges of aluminium alloy LW.

61 TABLE 3 Typical values of microhardness measurements surface section of an aluminium alloy 2618 pin

taken in various locations in the sub-

Location

Vickers hardnessa WV)

Undeformed pin Deformed subsurface Near the surface Centre of the wear lip Near the edge of the wear lip

150 200 240 190 230

aTaken under a 100 gf load.

deciding whether a material could form a wear lip during dry sliding. Nevertheless, the so-called brittle materials such as cast irons and aluminiumgraphite composites do form a wear lip, although its size is smaller than that formed in the ductile materials. This may be because the brittle materials also deform plastically under the influence of either a pure hydrostatic stress or a hydrostatic plus a superimposad tensile stress [ 8,9]. In a sliding situation such as this, the dominating stress between the asperities of the disc and that of the pin could be compressive in nature [lo] and therefore a significant amount of plastic deformation of the brittle pin at the wearing end could be observed. 3.2. Mechanism of wear lip formation The possible stages involved in the formation of a wear lip are schematically shown in Fig. 4 which is in accordance with the experimental

‘SD-.

SD-

Fig. 4. Schematic diagram illustrating the various stages involved in the process of wear lip formation (SD, sliding direction of the disc).

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evidence seen in Fig. 5. In the early stages of sliding, the asperities of the aluminium alloy pin are deformed and some degree of deformation could be present in the subsurface region of the pin without any significant amount of formation of the wear lip (Fig. 4(a)). Upon further sliding, the subsurface region extends and an appreciable wear lip is observed (Fig. 4(b)). Figures

(b)

(d)

(e)

ff)

Fig. 5. Optical micrographs of the cross-sections of aluminium alloy 2618 pins showing the formation of the weax lip at various stages after sliding for (a) 5 min, (b) 10 min, (c) 20 min, (d) 40 min, (e) 60 min and (f) 90 min.

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WEAR LIPCYCLE w--FORMATIONOF WEAR LIP REMDVAL$FWEAR+ g400-

gsoo-

g

, /'

s $I = 200-

0

0 0

100F 300. 0

0 2

4

6

/' . 8 10 12 14 lb SLIDINGDISTANCE,xlO'in

18

20

22

2I,

Fig. 6. The variation in the neck thickness of the wear lip with sliding distance for aluminium alloy L77 pin.

4(c) - 4(f) show the subsequent stages involved in the formation of a wear lip. Of particular interest is the feature depicting the bending and curling of the lip. When the end of the curled lip touches the side of the pin, further growth is constrained. At this moment of sliding, the “neck” (the region connecting the extended lip and the parent pin) reaches the maximum thickness and, upon further sliding, it starts to thin, eventually leading to the removal of the wear lip itself. Then, wear lip formation begins again, The time taken from the start of the lip formation to its eventual removal may be termed the “wear lip cycle”. The wear lip cycle could be followed by determining the thickness of the neck at various lengths of sliding. A typical plot is shown in Fig. 6. 3.3. Formation of debris particles from wear lip The wear mechanisms reported in the literature are concerned with the formation of wear debris particles from the wearing surfaces, e.g. the adhesion theory [ 111 and the delegation theory [ 121. Further, it appears that most of the studies on the wear me~h~isms do not discuss the formation of debris particles from the wear lip. However, one notable exception is that of Don et al. [ 131 who demonstrated the formation of lame&r particles from the “extrusions” at the exit side of a copper block in a block-on-ring machine. Nevertheless, it is worthwhile to investigate the formation of debris from the wear lip formed in a pin-on-disc machine. Figure 7(a) shows the features corresponding to an event which precedes the formation of a debris particle. The next possible event is demonstrated in Fig. 7(b) where the particle has been removed and the crater is seen. Figure 7(c) shows the sectional view of such removal of material from the wear lip. This type of debris formation mechanism is almost similar to that in the wearing surface as this part of the lip is just the extension of the wearing surface.

(a)

(b)

(cl Fig. 7. The process of debris formation from the wear lip: (a) the region labelled A may form a wear particle; (b) the crater labelled B may have been created by the removal of a debris particle; (c) a cross-sectional view of a pin showing a particle A about to detach and a particle B which has already been removed.

The second mechanism by which debris particles form from the wear lip is by the combination of radial cracking and the thinning-down process of the neck. As discussed earlier, with prolonged sliding, the thickness of the neck decreases (Fig. 6). Also, a few radial cracks start to appear in the lip region. The combination of radial cracking and neck thinning would result in the production of a few large particles. 3.4. Wear lip at the leading edge Figure 3 shows the perpendicular sections of the leading and trailing edges of the aluminium alloy L77 pin. It is observed that the wear lip in the trailing edge is larger than that in the leading edge. The flow pattern noticed in the trailing edge is absent in the leading edge. This suggests that a different mechanism is responsible for the formation of the lip in the leading edge. It is well known that aluminium is transferred to the steel counterface during dry sliding [ 14 - 181. The transfer patches encounter the leading edge of the pin after a revolution of the disc. When it meets the pin, the transfer

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elements forcibly enter the interface between the sliding components. While a considerable quantity of the transfer material could be thrown out of the wearing system, some could find entry into the wearing interface, resulting in the cold welding of the transfer layer to the pin at the leading edge. Upon further sliding, such back transfer of material leads to the formation of a small projection at the leading edge, which looks like a wear lip. This type of wear lip has a shorter life than those in the trailing edge and could become removed during the events following its temporary attachment.

3.5. Wear lip in pins of different shapes Figure 8 shows the schematic appearance of the wear lips and their side views along the circumference of the pins of various shapes. These profiles are drawn on the basis of the experimental observations. The size of the wear lip at a point in the periphery of the pin depends on its position relative to the sliding direction; it is a maximum at the trailing edge and a minimum at the leading edge. Another interesting factor to be noted is the absence of a flow pattern in the cross-sections taken perpendicular to the sliding direction in all the pins; this is illustrated in Fig. 9. Figure 9(a) shows a cross-section parallel to the sliding direction where the flow lines are seen. In contrast, flow lines are absent in the section perpendicular to both the wearing surface and the sliding direction.

Fig. 8. Wear lip formed on the surface of pins of various shapes (side view) (SD, sliding direction of the disc).

(a)

(b)

Fig. 9. Cross-sections (a) parallel and (b) perpendicular patterns are visible in the former but not in the latter.

to the sliding direction.

The flow

4 _ Conclusions When ~urn~i~ alloy pins were slid against hardened steel discs in a pin-on-disc testing machine, strain-hardened projections called wear lips were formed. On the basis of the present study on the nature and the formation process of such lips, the following conclusions can be drawn. (1) Considerable importance should be given to the flow of material and attendant lip formation in the pm to quantify the process of wear properly. (2) The size of the wear lip depends on (a) the pin shape, (b) its location around the pm and (c) the sliding conditions such as sliding distance. (3) Plastic deformation of the pin at the sliding contact aids the formation of the wear lip. (4) The lip region is similar in microst~cture and hardness to the subsurface region of the parent pin. (5) The removal of the wear lip results in the formation of a few large debris particles.

Acknowledgments The author would like to thank Professor S. Ranganathan, Chairman, and Professor Kishore, Department of Metallurgy, Indian Institute of Science, Bangalore, for providing the experimental facilities and helpful discussion. The kind assistance of Dr. T. S. P~ch~~e~ of Materials Research Laboratory in the scanning electron microscopy work is gratefully acknowl-

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edged. The author is also grateful to the University Grants Commission, Government of India, for the fellowship. The author also wishes to thank Dr. D. W. Borland of the Department of Mechanical and Industrial Engineering, University of Melbourne, for carefully reading the manuscript and for his critical comments.

References 1 N. P. Suh, Tri&ophysics, Prentice-Hall, Englewood Cliffs, NJ, 1986, p. 195. and Kiihore, 19th Annul Tech. Meet. ~nter~atiQ~1 Metul~o~aph~ 2 C. Subr~~i~

Society, Boston, MA, August 1986. Department of Metallurgy, Indian Institute of 3 C. Subramanian, IM.E. D~ertation, Science, Bangalore, 1983. 4 J, H. Groeger, S. F. Wayne, H. Nowotny and S. L. Rice, Mater. Sci. Eng., 49 (1981) 249. 5 S. L. Rice, Wear, 54 (1979) 291. 6 R. B. Griffin, Mechanical Engineering Department, Texas A & M University, Texas, personal communication, X987. 7 R. Ramesh, R. M. V. G. K. Rao and Kishore, Wear, 89 (1983) 131. 8 P. W. Bridgman, J. Appl. Phys., 18 (1947) 246. 9 P. W. Bridgman, Phys. Rev., 48 (1935) 825. 10 J. P. Hiith and D. A. Rigney, in P. Haasen, V. Gerotd and G. Kostorx (eds.), Proc. 5th Int. Conf. on the Strength of Metals and Alloys, Aachen, August 1979, Vol. 3, Pergamon, Oxford, 1980, p. 1500. 11 J. F. Archard, J. Appl. Phys., 24 (1953) 981. 12 N. P. Suh, The Deiumination Theory of Weor, Elsevier, Lausamre, 1977. 13 J. Don, T. C. Sun and D. A. Rigney, Wear, 91 (1983) 191. 14 S. Lingard, K. H. Fu and K. H. Cheung, Wear, 97 (1984) 75. 15 R. Antoniou and D. W. Borland, Mater. Sci. Eng., 93 (1987) 57. 16 A. D. Sarkar and J. Clarke, Wear, 75 (1982) 71. 17 T. S. Eyre, Microstruct. Sci., 8 (1980) 141. 18 L. B. Sargent, W. C. Milz and R. E. Atkinson, Lubr. Eng., 39 (1983) 706.