Wear, 90 (1983) 39 - 47 39

THE ROLE OF WEAR DEBRIS IN THE STUDY OF WEAR*

A. D. SARKAR

Department of Mechanical Engineering, University of Petroleum and Minerals, Dhahran (Saudi Arabia)

(Received October 22,1982)

Summary

The danger of using the steady state wear rates of metals to compare materials is indicated; the total wear must also be considered. Particle size analysis shows that, in sliding wear, the smallest particles are the most nu- merous. Surface topographies of steel and aluminium alloys as produced in laboratory machines were compared with the corresponding wear debris. It is suggested that a representative atlas of wear debris would be a valuable aid to the diagnosis of the state of industrial friction couples without disman- tling them. The beneficial role of ferrography in this regard is restated.

1. Introduction

Wear of sliding components results in reduced mechanical efficiency and an irretrievable loss of material in the form of wear debris. Since the world’s resources of material and energy are getting progressively denuded, by necessity there is a growing involvement in studies of wear on a global basis. It is hoped that an understanding of the mechanism of wear will result in better specifications from the viewpoint of composition, surface treatment etc. of friction couples. Although it is unlikely that wear occurs by a single mechanism, a comparative study of wear rates of different friction couples and a thorough examination of the worn surfaces and subsurfaces provide useful information regarding wear processes in materials.

2. Wear rates

Comparative studies of candidate materials can be made in the labo- ratory by using simple pin-on-disc type machines. When the weight or height loss of specimens is plotted against the sliding distance, two regimes of wear

*Paper presented at the First International Conference on Advances in Ferrography, University College, Swansea, Gt. Britain, September 22 - 24,1982.

0043-1648/83/$3.00 0 Elsevier Sequoia/Printed in The Netherlands

40

TABLE 1

Effect of the amount of phosphorus on the steady state wear rate of a grey cast iron

p (%I Wear rates (X10P6 kg km-l) at two loads

20 N 40N

0.26 4.0 8.5 0.36 3.2 8.0 0.56 4.8 8.2 0.76 4.0 1.5 1.13 4.0 8.5

are normally encountered, i.e. running-in and steady state wear. Normally, the slope of the steady state wear is taken as the wear rate and this becomes the parameter for comparison between materials. The steady state wear rate of a metal increases with increasing load although the relationship can be complex. The problem unfortunately is that a steady state wear rate cannot always be used to assess the role of alloy composition. For example, phos- phorus is known to affect the wear rate of grey cast irons. In a laboratory study [l] the effect of increasing the amount of phosphorus in a grey iron was examined using a pin-bush machine. As Table 1 shows, whereas increas- ing the load results in an increasing amount of wear, phosphorus has no effect on the steady state wear of this iron.

However, the total wear of an iron was high when the phosphorus con- tent was low. This is shown in Fig. 1 for grey iron bushes run against hard steel pins at a load of 20 N. The low phosphorus bush shows a very high amount of total wear when compared with the bush with a phosphorus con- tent of 1.13%. What happens in cases such as this is that, at some composi- tions, the running-in wear is high so that the parameter which must be chosen is the total amount of wear for a constant sliding distance. A true picture can then be obtained as shown in Fig. 2 where the total mass losses of grey iron pins sliding against hard steel bushes for a distance of 14 km each are plotted against their phosphorus content. The total wear decreases as the amount of phosphorus increases.

3. Practical systems

A laboratory rig such as a pin-bush machine rejects most of the debris from the interface, particularly when it is run dry. Under those situations it is easy to observe a profusion of wear debris during the running-in stage such as for the iron in Fig. 2 with 0.26% P. For an enclosed system such as a bear- ing, the wear products are washed away from the interface by the lubricant.

Attempts have therefore been made to establish the severity of wear by withdrawing an oil sample periodically and examining the solids. In one

41

m

‘3 o.3 ,”

5 0.2 - 3

-3 0.1 -

0.2 0.4 0.6 0.0 1.0 1.2

‘1. Phosphorus

Fig. 1. Grey iron bushes with 0.26% and 1.13% P. The lower phosphorus iron shows heavy wear. Both bushes were slid for a total distance of 5 km against a hard steel pin 6.25 mm in diameter; the load was 20 N and the speed was 118 m min-l. (Magnification, 1.875x.)

Fig. 2. Total mass loss of grey iron pins sliding against hard steel bushes as a function of the phosphorus content (speed, 118 m min- ‘; total distance slid, 14 km): 0, load of 20 N; 0, load of 40 N.

method, a double-beam spectrophotometer analyses the used oil. For aircraft bearings, the most usual elements recorded are iron, chromium, silver and copper. From analyses of the unused oil and on the basis of experience, a guideline for the maximum tolerable amount of the element, iron say, is drawn up. Once this is exceeded, some distress of the bearing is anticipated.

Spectroscopic oil analysis programmes have at least one limitation: the state of the sliding surfaces cannot be ascertained because dismantling a machine for examination is not acceptable.

The morphology of the wear product is that of the sliding surfaces and it has been demonstrated that the interfacial topography of a friction couple depends, inter alia, on the applied load. If surfaces and the corresponding wear debris under controlled conditions were microscopically examined, an atlas could be constructed. The topography of wear debris from practical situations could then be compared and postulations regarding the state of the sliding interface made.

4. Ferrography

This approach has been taken by various workers using ferrography and findings have been documented [2 - 161. Ferrography provides a severity of

42

wear index which has been successful in the condition monitoring of various types of industrial machines including aircraft. A Ferrogram can be exam- ined under high magnification and important features of debris can be re- corded.

5. The role of particles

Interest in wear particles has led to a school of thought [ 17, 181 that uses the energy criterion to arrive at an order-of-magnitude size of a wear product. The idea is that an asperity held in a matrix receives a quantum of energy owing to its interaction with the counterface. As this opposing sur- face passes, an amount of residual stress is left in the junction until, even- tually, after repeated encounters, the cumulative energy stored exceeds the work of adhesion of the asperity to the substrate. The radius r of a wear par- ticle is shown [ 191 to be related to the applied load W and the hardness H of the matrix as follows:

It is suggested that hard metals have a low W/H ratio so that small wear par- ticles are expected. For steel sliding on steel [ 191, the average size r of a wear fragment is about 12 X 10m2 mm. Also, the mass M of a wear fragment is related to the applied load W:

M=CWCV (2)

where C and cy are empirical constants; (11.c 0.3 for copper sliding on steel. On an asperity scale, the load dependence of M in eqn. (2) can be explained. At the first contact, the summits flatten and are removed by shearing stresses. The compliance of the interface improves and contact occurs at numerous points instead of at a few favourably disposed asperities. If the load is in- creased, the true area of contact increases so that the mass of a wear frag- ment becomes larger.

It is erroneous to assign an average particle size to wear debris. As a machine starts, a few large particles will detach but, as the number of con- tact spots increases, the propensity for smaller particles to form should increase.

To emphasize the fact that wear debris should be considered statis- tically from the point of view of size, particles from a pin-bush machine were examined methodically and some of the results reported [ 201. The fric- tion couple comprised Al-Si alloy pins and martensitic steel bushes. The composition of the wear product was first evaluated qualitatively by X-ray diffraction analysis which showed that, apart from aluminium and y-Al,O,, the debris contained steel particles worn from the hard bush. A particle size analysis was carried out simultaneously with the aid of a Quantimet. The results were expressed as a range of mean particle sizes in terms of the

43

--- 1%

d, rnrn~lO-~

Fig. 3. Mean particle size distribution of wear debris from an Al-Si alloy pin (diameter, 6.25 mm) sliding against hard steel bushes: the data satisfy the relationship N = A exp(-kd) where N is the number of particles, d is the mean particle size and A and k are empirical constants.

Fig. 4. Longitudinal section of a steel pin running on a steel bush [ 211 (load, 20 N; speed, 120 m min-‘; the arrow in this and other diagrams shows the direction of counterface motion).

amount of collected debris. If the number N of particles is plotted against the mean diameter d of the particles in the respective ranges of size, a typical relationship is obtained as shown in Fig. 3. The smallest particles are the most numerous, which suggests that the large particles may be produced at the infant stage of the sliding couple. Empirically,

N = A exp(--hd)

where A and k are constants which may depend on the applied load.

(3)

6. Debris morphology

It is reasonable to suppose that wear products originate from asperity interactions. However, the prior surfaces of friction couples are completely altered because of the sliding under a load and it is important to study their morphology.

A longitudinal section of a worn sample [21] shows a heavily deformed layer (Fig. 4) which is not necessarily part of the parent metal but probably consists of layers of metal deposited during sliding. The layers form by con- tinued transfer and back transfer of material across the sliding interface. The first transfer is initiated by the softer material but the harder surface appears to soften in isolated microcontacts and takes part in the transfer process. The phenomenon has been fully argued elsewhere [ 221 and it has been sug- gested that material migration across the interface of a friction couple occurs throughout a wear run. These are clearly microevents but the result is evi- dent on a macroscale owing to the cumulative deposition of one material onto another.

44

Fig. 5. The surface of the pin in Fig. 4 at a higher load of 40 N [ 211.

Fig. 6. A wear particle from the surface shown in Fig. 5.

Fig. 7. The underside of an Al-lS%Si alloy debris particle obtained after sliding on steel. (Magnification, 475x.)

Fig. 8. Wear particles from a steel pin sliding on steel (load, 40 N;speed, 120 m min-‘).

Figure 4 shows a gap between the heavily deformed layer (HDL) and a relatively less deformed metal below. The latter is probably the parent metal but the former is in all probability a layered deposit. Cracks inside this composite could be formed by surface forces or because the interlayer zones opened up during sliding under the load.

The surface of the same pin at a higher load is shown in Fig. 5. The load is high enough for surface damage but well below that which tends to induce seizure. The notable feature of this surface is the inclined shear plates which can also be observed on a wear particle (Fig. 6) from the experiment which produced the surface in Fig. 5. It is important to compare the topography of the debris with that of the sliding surface. Otherwise, a misleading conclu- sion is likely as shown in Fig. 7 for the underside of an Al-15%Si alloy par- ticle. The fluted appearance could be confused with inclined shear plates or some other morphology indicating heavy deformation.

High surface stresses inevitably give rise to sheet-like particles as shown in Fig. 8 for steel sliding on steel. There are some small particles but the predominant ones are large.

45

Fig. 9. Fine debris ready to be detached from the bush surface. The debris is formed from the deposit of the counterface which was an Al-Bl%Si pin (pin diameter, 6.26 mm; load, 10 N). (Magnification, 1560x.)

Fig. 10. A steel pin surface produced at a minuted. (Magnification, 154x.)

load of 20 N. Large debris particles are com-

Fig. 11. Adherent micromachined chips in a steel pin slid at 10 N. The surface was exam- ined once steady state wear was well established. (Magnification, 154x.)

Generally, small particles are produced from the counterface by dis- integration of the layered deposit on it. An example is shown in Fig. 9 which is a hard steel counterface running against an Al-2l%Si alloy pin that is 6.25 mm in diameter under a load of 10 N.

Fine particles are also produced by a process of comminution as ob- served on a steel pin surface under a load of 20 N (Fig. 10). The deposited layers seem to break up in a brittle manner and instead of escaping some remain attached to the surface to become comminuted as sliding continues.

A feature not uncommon with worn steel surfaces under light loads is adherent micromachined chips (Fig. 11). These are not generally noticed in the expelled wear debris, possibly because of comminution.

46

7. Discussion

Even under light loads, material from the low yield point member of a friction couple transfers onto the counterface. The same material transfers back to the parent and the process of migration and repatriation continues throughout the life of the sliding members. The hard surface also takes part in this transfer process and certainly wears with time.

The effect of combined normal and tangential loads is to deform this transferred layer and a thin subsurface of the parent metal. Deformation alters the prior surface topography, the exact nature of which can be re- corded by microscopy.

A wear particle detaches either because transmitted forces nucleate cracks or simply by further weakening of the inter-layer zones, the deposits being held by inefficient microwelding. The topography of the detached wear particles reproduces that of the sliding surface. Therefore surface examination of wear particles is clearly a reliable technique for predicting the physical nature of sliding surfaces. If an atlas of wear debris is con- structed for a particular friction couple under varying conditions of load and speed, this could be used to diagnose the state of an industrial system using the same material combinations. Ferrography, apart from giving a severity- of-wear index, isolates wear particles whose topography can be recorded photomicrographically.

Examination of wear particles can also aid the elucidation of wear mechanisms. The early theories of wear assume asperity interactions which may be true provided that it is recognized that the prior surfaces change radically according to the applied stresses to assume a new and dynamic topography. It is evident that wear products are much larger than a single asperity so that it is essential to consider the wear process in terms of crack propagation over large distances.

The results presented show that small particles are probably formed by comminution so that the size of the first particle may be considered to be governed by such variables as load and speed. Detachment of this particle from the surface is time dependent so that there is ample scope for further fragmentation as sliding continues.

8. Concluding remarks

The topo~aphy of sliding surfaces depends on such variables as load and speed. Wear particles are much larger than the size of asperities and exhibit the topography of the sliding surfaces. Periodic inspection of wear products from industrial machines, as in ferrography, will therefore yield valuable information regarding the state of the sliding interfaces.

Acknowledgment

I am grateful to the University of Petroleum and Minerals for providing funds to attend the conference.

47

References

1 A. D. Sarkar, unpublished work. 2 D. Scott, W. W. Seifert and V. C. Westcott, Sci. Am., 230 (5) (1974) 88. 3 D. Scott, Wear, 34 (1975) 15. 4 E. R. Bowen, D. Scott, W. W. Seifert and V. C. Westcqtt, Tribal. ht., 9 (1976) 109. 5 D. Scott and V. C. Westcott, Proc. Tribology Conv., Swansea, 1977, Institution of

Mechanical Engineers, London, 1977. 6 D. Scott and G. H. Mills, Wear, 24 (1973) 235. 7 W. W. Seifert and V. C. Westcott, Wear, 21 (1972) 27. 8 J. L. Middleton, V. C. Westcott and R. W. Wright, Wear, 30 (1974) 275. 9 D. Scott and G. H. Mills, Nature (London), 241 (1973) 115.

10 R. A. Cummins, E. D. Doyle and B. Rebecchi, Wear, 27 (1974) 115. 11 G. Pocock, Wear, 38 (1976) 189. 12 F. T. Barwell, E. R. Bowen, V. P. Bowen and V. C. Westcott, Wear, 44 (1977) 164. 13 M. H. Jones, Wear, 56 (1979) 93. 14 A. A. Reda, R. Bowen and V. C. Westcott, Wear, 34 (1975) 261. 15 W. Hoffmann, Wear, 65 (1981) 307. 16 L. J. Hampson, Chart. Eng., 28 (1981) 60. 17 R. Davies, Friction and Wear, Elsevier, Amsterdam, 1959. 18 E. Rabinowicz, J. Appl. Phys., Suppl. 1, 82 (1951). 19 E. Rabinowicz, Proc. R. Sot. London, Ser. A, 66 (1953) 929. 20 J. Clarke and A. D. Sarkar, Wear, 54 (1979) 7. 21 B. Campillo and A. D. Sarkar, unpublished work. 22 J. Clarke and A. D. Sarkar, Wear, 69 (1981) 1.