Wear of some F.C.C. metals during unlubricated sliding part II: effects of normal load, sliding velocity and atmospheric pressure on wear fragments

Wear, 35 (1975) 33I- 343 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

331

WEAR OF SOME F.C.C. METALS DURING UNLUBRICATED SLIDING PART II: EFFECTS OF NORMAL LOAD, SLIDING VELOCITY AND ATMOSPHERIC PRESSURE ON WEAR FRAGMENTS

N. SODA

The Institute of Physical and Chemical Research, Wake Saitama [Japan}

Y. KIMURA and A. TANAKA

Institute of Space and Aero~uti~~ Science, The university of Tokyo, Tokyo (Japan)

(Received March 26, 1975; in final form June 24, 1975)

Summary

The effects were determined of normal load, sliding velocity and atmospheric pressure on the volume and the formation rate of wear fragments obtained by experiments with Ni, Cu and Au under unlubricated conditions. The results show that the volume and the formation rate change in different ways. The change of wear with normal load or sliding velocity is predominantly due to a change in the volume of fragments. The change in wear with atmospheric pressure is due to a change of the formation rate. It is concluded that the volume of wear fragments is governed mainly by some mechanical factors and that the formation rate is dependent on chemical factors such as the chemisorption of oxygen.

1. Introduction

It is well known that the wear behaviour of metals is markedly influenced by various sliding conditions, such as normal load, sliding v&ocity and atmosphe~c pressure. In an earlier paper [l] , changes in the amount of wear with nickel, copper and gold during unlubricated sliding when these para- meters were varied were discussed.

Wear is the removal of material from sliding surfaces as a result of suc- cessive fracture. Since materials are in general removed in a number of small fragments, the total wear is given as the product of the volume and the number of the wear fragments. A change in the amount of wear must change either the volume or the number of wear fragments, or both, and what the change implies differs according to circumstances.

There are many investigations in which wear fragments are referred to. Most, however, are concerned with the qualitative observation of wear fragments [ 2 - 6 3 or with the exam~ation of the size or number of wear fragments without relation to the amount of wear [7 - 151. In the few experi-

332

Evocuotinq systsm

Fig. 1. Apparatus: A, pin specimen; B, disk specimen; C, arm; D, pivot joint; E, flexure plate; F, vertical shaft; G, loading spring; H, Wilson-type seal; I, vacuum chamber; J, watch glass.

ments in which wear fragments are quantitatively correlated with the rate of wear, only the abrasive nature of the wear fragments [ 161 or their compo- sition [ 171 is considered.

In this paper, the wear behaviour presented in the earlier paper [l] is interpreted in terms of the volume and number of wear fragments; the effects of the experimental variables are discussed.

2. Apparatus and procedure for wear experiments

A full description of the apparatus (Fig. 1) has been given earlier [ 11. Briefly, an inclined-flat-ended pin specimen 2 mm in diameter is allowed to rub on the lower surface of a horizontal disk mounted on the bottom end of a vertical shaft. The pin is mounted on a horizontal arm; one end of the arm is supported by a couple of ball bearings and the other end is suspended by a loading spring. These are enclosed in a vacuum chamber, the pressure in which can be varied between 5 X 10d6 and 760 Torr. The disk is driven by a variable speed electric motor placed outside the chamber. The ranges of velocity and load are 8 - 335 mm s-l and 130 - 1020 g, respectively. The materials investigated were Ni, Cu and Au, each of which slides on itself. The specimens were cleaned by washing in acetone and by electrolytic degreasing before each experiment. The sliding distance was 50 m unless otherwise stated.

(a)

Fig. 2. Examples of wear fragments: normal load 260 g; sliding distance 50 m. (a) Ni, sliding velocity 84 mm s-l, atmospheric pressure 760 Torr. (b) Ni, sliding velocity 168 mm s-l, atmospheric pressure 760 Torr. (c) Cu, slidin velocity 168 mm s-l, atmospheric pressure 760 Torr. (d) Cu, sliding velocity 168 mm s -B

(e) Au, sliding velocity 168 mm s-l, atmospheric pressure 5 x lop6 Torr.

atmospheric dressure 760 Torr.

3. Estimation of mean volume and number of wear fragments

The wear fragments were collected on a watch glass placed in the vacuum chamber under the specimens (Fig. If; all fragments were then photo- graphed through a microscope. Examples are shown in Fig. 2.

334

Unlike Finkin’s suggestion [ 121, the experimental conditions or materials did not make noticeable differences in the shape of wear fragments in this case; the extent of these differences was less than that observed between individual fragments under a single condition. The wear fragments all had an analogous shape, which can roughly be characterized as a flattened ellipsoid. The volume of a fragment is obtained reasonably accurately from the for- mula

w = (n/6)ubc

in which a, b and c are the length, width and thickness of the fragment, respectively. The values of a and b were directly read off the micrographs (Fig. 2) but c was approximated. Closer observation of some wear fragments obtained from different experiments and embedded in transparent synthetic resin showed that b was several times as large as c. Tentatively, c was assumed to be one-fifth of b. Thus, w was estimated from the relation

w = (n/30)ab2 (1)

3.2. Volume distribution of wear fragments The volume of individual wear fragments was determined under varying

experimental conditions (Fig. 3). Since sliding was between similar materials, it was not clear from which specimen of the pair a fragment originated. Therefore the fragments from the pin and those from the disk were both counted. As commonly observed, the volume of wear fragments is distrib- uted over a wide range.

It has been stated that the volume of wear fragments follows a log-normal distribution. However, the present results can be characterized by an exponential distribution with reasonable accuracy, as shown by the solid curve in Fig. 3. This was true for all distributions obtained.

3.3. A statistical method of estimation The mean volume of wear fragments is conveniently estimated by a

statistical method, developed by Kimura [ 181, which can eliminate the tedious work of measuring a number of minute fragments from each experiment. This method employs the fact that, if the initial distribution is known, a certain relation can be established between the mean of the several largest values and that of the population. As the distribution of the volume has been approximated by an exponential distribution, the mean volume and the total number of wear fragments can be estimated.

From wear fragments obtained in a run, a number i of the largest fragments (10 - 15) were selected and their volume was determined by eqn. (1). If the mean volume of these i fragments is ii+, it is related to the mean volume W of all the wear fragments by

ii. I= 1+

ii (2)

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0 0003 0 0006 00009

Volume of wear fragment (mm31

Fig. 3. Typical volume distribution of wear fragments of Ni: sliding velocity 84 mm s-l; normal load 260 g; atmospheric pressure 760 Torr; sliding distance 50 m.

where no is the total number of fragments. When no is sufficiently large, eqn. (2) may be approximated by

wi -=l+++lnn,- k k u,

k=l

(3)

where y = 0.577. . . is Euler’s constant. The amount of wear W is equal to the product of the mean volume and the number:

w=n,w (4)

When W and Wi are determined experimentally, iF and no can be obtained by solving eqns. (3) and (4) graphically.

Here, wear fragments from both the pin and the disk are treated as a whole. However, when the number of the fragments from either of the specimens is to be determined, the following relation is employed:

Number of fragments from pin (disk) Amount of wear of pin (disk) =

Total number of fragments Total amount of wear

where the mean volume of the wear fragments is assumed to be common. As to the number of wear fragments, those from the pin specimens are considered in this study.

4. Results

4.1. Mean volume of wear fmgmen ts Before the effects of other experimental variables on the mean volume

of wear fragments are investigated, changes, if any, in the volume of wear fragments with sliding distance are examined.

IO 20 30 40 50

Slldlng distance (r-n)

300 600 900 1200

Normal load ( 9 )

Fig. 4. Mean volume of wear fragments of Ni us. sliding distance: sliding velocity 168 mm se ‘, atmospheric pressure 5 X lop6 Torr; normal load 260 g.

Fig. 5. Dependence of the_rflean volume of wear fragments of Ni on the n_osrmal load: sliding velocity 168 mm s ; sliding distance 20 m; n 760 Torr, LJ 5 X 10 Torr.

100 200 300 10-5 10-3 IO-' IO’ IO3

Slldlng velocity (mm/s) Atmospheric pressure (Torr)

Fig. 6. Relation between the mean volume of wear fragments and the sliding velocity: normal load 260 g; sliding distance 50 m; m Ni, 760 Torr; 0 Ni, 5 X 1O--6 Torr; A Au, 760 Torr; i: Au, 5 x 10m6 Torr. (Although not shown, Cu behaves similarly to Ni.)

Fig. 7. Effects of atyospheric pressure on the mean volume of wear fragments: sliding velocity 168 mm s ; normal load 260 g; sliding distance 50 m; 0 Ni, 0 Cu, n Au.

Experiments were carried out for various sliding distances, and the mean volume of fragments was determined for each run. In Fig. 4 these mean values are plotted against the sliding distance in the high friction stage [l] . The result suggests that the mean volume of wear fragments is un- changed during an experiment and that the changes in the mean volume described below can be attributed solely to the changes in other experimental variables.

80

60

0 IO 20 30 40 50

Sliding distance Cm)

600

0 300 600 900 1200

Normal load (g)

Fig. 8. Dependenceyf the number of wear fragmenissof Ni on the sliding distance: sliding

velocity 168 mm s ; atmospheric pressure 5 X 10 Torr; normal load 260 g.

Fig. 9. Effects of the normal load on the number of wear fragments of Ni: sliding velocity

168 mm s-l, sliding distance 20 m; l 760 Torr; CJ 5 X lo-6 Torr.

The dependence of the mean volume of wear fragments on normal load is shown in Fig. 5. These results were obtained after sliding for 20 m only. The mean volume changes markedly with normal load: when the load is in- creased, the mean volume increases. Figure 6 shows the relation between the mean volume of wear fragments and the sliding velocity; in every case the mean volume is noticeably influenced: it increases greatly with increasing sliding velocity, regardless of the conditions or materials. The effects of atmospheric pressure are shown in Fig. 7. With Ni and Cu the mean volume of wear fragments changes, whereas with Au it is substantially constant. However, the change in the mean volume caused by varying the atmospheric pressure by a factor of lo8 is much less than that caused by only a tenfold change in normal load or sliding velocity.

4.2. Number of wear fmgments Figure 8 shows the dependence of the number of wear fragments on

sliding distance. The number of wear fragments is proportional to the sliding distance except when there is scatter in the very early stage, as expected from the dependence on sliding distance of the amount of wear and of the mean volume of the fragments.

The effects of normal load on the number of wear fragments are shown in Fig. 9. The number of wear fragments changes to a certain extent with normal load, irrespective of the experimental conditions. Figure 10 shows the relation between the number of wear fragments and the sliding velocity. The number changes little with sliding velocity over the greater part of the velocity range, but the change is considerable at very low velocities. When the atmospheric pressure is varied (Fig. 11) the number of wear fragments changes markedly for Ni and Cu, but not for Au. For Ni and Cu the number

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0 100 200 300

Sliding velocity (mm/s)

10-5 10-3 10.' IO' 103

Atmosphenc pressure (Torr)

Fig. 10. Relation between the number of wear fragments and the sliding velocity: normal

load 260 g; sliding distance 50 m; n Ni, 760 Torr; 0 Ni, 5 X 10e6 Torr;. Au, 760 Torr; nAu,5x lO@ Torr. (Results with Cu are not shown.)

Fig. 11. Changes in the number of wear fragments with atmospheric pressure: sliding velocity 168 mm s-l ; normal load 260 g; sliding distance 50 m; 0 Ni, 0 Cu, n Au.

greatly decreases on reducing the atmospheric pressure over an intermediate pressure range. The number continues to decrease in the high and low pressure ranges but only slightly.

4.3. Rate of wear fragment formation It was found during experiments that the pin specimen oscillated verti-

cally against the surface of the disk specimen after the abrupt increase of friction, a behaviour common to unlubricated sliding experiments [ 11, 191; it appears clearly in the present experiments owing to the fairly large moving mass attached to the pin. This suggests that, in such cases, a certain part of the running time is wasted without actual contact: the “real” sliding distance may be less than the “nominal” one (50 m in most cases). The real sliding distance was determined by changing the experimental variables (Figs. 12 - 14), as described in the Appendix. These results show that the real sliding distance differs considerably from the nominal sliding distance, the difference being dependent on the experimental variables.

Figures 9 - 11 are replotted as the relation between the experimental variables and the number of wear fragments formed in a unit real sliding distance, which is designated the rate of fragment formation (Figs. 15 - 17). The dependence of the rate of wear fragment formation on normal load is shown in Fig. 15. The rate is not influenced by the normal load, irrespective of the atmospheric pressure. Figure 16 shows the relationship between the rate of wear fragment formation and the sliding velocity. The rate of wear fragment formation does not change appreciably over a substantial part of the velocity range in all cases. This part corresponds to the velocity range

0 300 600 900 1200 0 100 200 300

Normal load (g ) Sildfng velcaty (mm/s)

Fig. 12. Dependence of the real sliding distance of Ni on the normal load: sliding velocity 168 mm s-r; atmospheric pressure 760 Torr; nominal sliding distance 20 m.

Fig. 13. Effects of the sliding velocity on the real sliding distance: normal load 260 g; atmospheric pressure 760 Torr; nominal sliding distance 50 m; u Ni, 0 Cu, n Au.

50

40

-F

8 6 30 g

P .- .s 20

0 $

IO

0

Atmospheric pressure (Torrl

IO'

^ 'E 8

.Z

z .G E IO’ k ?

B % b z B

I00 0 300 600 900 1200

Normal load ( g 1

Fig. 14. Change in the real sliding distance of Ni with atmospheric pressure: sliding velocity 168 mm s-l, normal load 260 g; nominal sliding distance 50 m.

Fig. 15. Dependence ofthe rate of wear fragment formation of Ni on!xmal load: slid. ing velocity 168 mm s ; sliding distance 20 m; n 760 Torr, 0 5 X 10 Torr.

where the amount of wear increases monotonically with increasing velocity. The rate appears erratic at lower velocities, e.g. below 100 mm s-l in the case of Ni in vacuum. Such low velocity ranges, however, are ignored in the following discussion, as wear behaviour in these ranges is dependent upon the apparatus [ 11. Figure 17 shows the effects of atmospheric pressure on the rate of wear fragment formation. Again, for Ni and c:u the rate of wear fragment formation is markedly influenced by the atmospheric pressure. Most of the reduction in the rate occurs over an intermediate pressure range.

340

102

T E

c 0

73 E IO’ 2

F k e 6 P

IO0

b 2

B

0' 0 100 200 300

Slidng velocity (mm/s)

,001 ’ IO.5 10-3 IO-' IO' IO3

Atmospheric pressure (Torr)

Fig. 16. Relation between the rate of wear fragment formation and the sliding velocity: normal load 260 g; sliding distance 50 m; m Ni, 760 Torr; 0 Ni, 5 x 10-e Tort-; A Au, 760 Torr;a Au, 5 x 10e6 Torr. (Dependence of Cu is similar to that of Ni.)

Fig. 17. Effects of atmospheric pressure on the rate of wear fragment formation: sliding velocity 168 mm s-l; normal load 260 g; sliding distance 50 m; 0 Ni, C) Cu, A Au.

Decreasing the pressure in the high range does not affect the rate of wear fragment formation, while in the low range, as far as examined, it continues to decrease slightly. However, the rate of wear fragment formation of Au is insensitive to atmospheric pressure.

5. Discussion

The unlubricated wear of metals is commonly characterized as adhesive wear. The adhesion theories predict hypothetically that, under a given normal load and sliding distance, wear of certain materials is governed solely by the probability of fragment formation. Other experimental variables, e.g. sliding velocity and environment, are regarded as having an influence on this probability and thus subsequently on wear. The validity of this reasoning is considered first.

As the probability of fragment formation is proportional to the number of wear fragments in a unit sliding distance, it will correspond to the rate of fragment formation in the present context. If the adhesion theories hold, changes in the amount of wear with sliding velocity or atmospheric pressure must be the result of a variation in the rate of fragment formation. As shown in Figs. 16 and 17, however, this is not the case. The change of rate is critical for wear behaviour when the atmospheric pressure is varied, but substantial change in wear is not accompanied by a change in the rate when the sliding velocity is varied. This shows that changes in the amount of wear cannot always be attributed to changes in the probability of fragment formation. Two classes of variables exist: one predominantly affects the probability; the other affects another factor, namely the fragment volume.

341

Instead of discussing wear as the total amount, therefore, both the formation rate and the mean volume of wear fragments are considered. With all metals examined, the mean volume of wear fragments shows simple behaviour when normal load, sliding velocity or atmospheric pressure is varied. As shown in Figs. 5 and 6, increasing the normal load or sliding velocity causes an extensive and monotonic increase in the mean volume, which almost coincides with a corresponding change in wear. The change of the mean volume with atmospheric pressure (Fig. 7) is less extensive and is contrary to the wear behaviour.

A simple relationship is also found between the number of wear fragments and the normal load, sliding velocity or atmospheric pressure. This is more clearly seen when the rate of fragment formation is considered. How- ever, this behaviour exhibits a striking contrast to that of the mean volume; except for Au, atmospheric pressure exerts more extensive effects than other variables. When the atmospheric pressure is reduced, the rate markedly decreases with Ni and Cu but remains constant with Au (Fig. 17). This dependence on atmospheric pressure of the rate agrees closely with that of the amount of wear. Neither normal load nor sliding velocity affects the rate, except for the erratic behaviour in the very low range (Figs. 15 and 16).

This characteristic behaviour of wear fragments has been referred to in the literature. The effects of normal load, sliding velocity or atmospheric pressure on the size of loose wear fragments or that of transferred fragments, which is believed to have some correlation with the former, were determined by several investigators [7, 8, 10, 11,151. The results almost agree in that increasing the normal load or the sliding velocity and reducing the atmospheric pressure cause the size to become larger. A few results on the number of loose or transferred fragments [8, lo] show that it decreases with increasing sliding velocity and increases with increasing atmospheric pressure. No com- parison has been carried out on the basis of the real sliding distance. This brief survey shows that the present results apparently confirm common characteristics of other experimental results.

When the earlier results on wear [ 11 are examined in the light of the discussion on the fragments, the following conclusions can be made. The change in wear with normal load or sliding velocity is predominantly due to a change in the volume of individual fragments. The change in wear with atmospheric pressure is due to a change in the rate of fragment formation. Some factors possibly of a mechanical nature were found to be responsible for the wear/sliding velocity behaviour; the chemisorbed oxygen film influenced the wear/atmospheric pressure behaviour [l] , as with some fatigue experiments. It seems likely that some “mechanical” factors affect the volume of wear fragments and “chemical” factors govern the rate of fragment formation. Further investigations are required to ascertain the validity of this reasoning.

342

6. Conclusion

The wear behaviour of Ni, Cu and Au, under varying normal load, sliding velocity and atmospheric pressure, is interpreted in terms of the volume and the formation rate of wear fragments. Not only the rate but also the volume was found to change; the changes take place in different ways depend- ing on the experimental variables. The change of wear with normal load or sliding velocity is predom~~~y due to a change in the volume of fragments, while atmospheric pressure affects wear through changing the rate of fragment formation. Mechanical factors affect wear by varying the volume of fragments. Some chemical factors govern wear by changing the formation rate. The effects on wear of factors of a mechanical and chemical nature must be treated separately.

References

1 N. Soda, Y. Kimura and A. Tanaka, Wear, 33 (1975) 1. 2 M. Antler, ASLE Trans., 5 (1962) 297. 3 R. Wibberley and T. S. Eyre, Wear, 13 (1969) 27. 4 J. R. Fehling and N. K. Sarkar, Wear, 14 (1969) 33. 5 M. Kawamoto and K. Okabayashi, Wear, 17 (1971) 123. 6 D. Scott, W. W. Seifert and V. C. Westcott, Sci. Am., 230 (1974) 88. 7 M. Kerridge and J. K. Lancaster, Proc. R. Sot. London, Ser. A, 236 (1956) 250. 8 W. Hirst and J. K. Lancaster, Proc. R. Sot. London, Ser. A, 259 (1961) 228. 9 A. W. J. De Gee and J. H. Zaat, Wear, 5 (1962) 257.

10 N. Soda and T. Sasada, Bull. Aeronautical Research Inst., Univ. of Tokyo, 4 (1964) 49.

11 E. Rabinowicz and R. G. Foster, Trans. ASME, D86 (1964) 306. 12 E. F. Finkin, ASLE Trans., 7 (1964) 377. 13 T. S. Eyre, R. F. IIes and D. W. Gasson, Wear, 13 (1969) 229. 14 Y. Mizutani and K. Nakajima, J. Jpn. Inst. Met., 33 (1969) 1202. 15 T. Sasada and H. Kando, J. Jpn. Sot. Lubric. Eng., 17 (1972) 96. 16 R. Takagi and Y. Tsuya, Wear, 5 (1962) 435. 17 B. J. Nield and 0. G. Griffin, Wear, 4 (1961) 111. 18 Y. Kimura, J. Jpn. Sot. Lubric. Eng., 17 (1972) 53. 19 Y. Miyakawa, K. Seki and M. Nishimura, J. Jpn. Sot. Lubric. Eng., 18 (1973) 323.

Appendix

~eter~~~~ tion of the real sliming distance Actual contact between the pin and the disk specimen during the exper-

iment was determined by electrical conductance measurements, using the circuit shown in Fig. Al. An example of the record obtained with an electromagnetic oscillograph is shown in Fig. AZ. From such traces the time frac- tion in which actual contact occurs can be estimated in each case. The real sliding distance shown in Figs. 12 - 14 is obtained by m~tiplying this frac- tion by the nominal sliding distance.

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Disk spec~me”

P1n spfmmen

Fig. Al. Circuit used to measure actual contact.

Fig. A2. Electromagnetic oscillograph trace of actual contact (redrawn). Natural frequen- cy of the instrument 400 Hz.

Documents

Wear of some F.C.C. metals during unlubricated sliding part II: effects of normal load, sliding velocity and atmospheric pressure on wear fragments