Wear, 106 (1985) 177 - 201 177

THE UNLUBRICATED FRETTING WEAR OF MILD STEEL IN AIR*

D. ALDHAM and J. WARBURTON

Central Electricity Generating Board Berkeley Nuclear Laboratories, Berkeley, Glos. CL13 9FB (Ct. Britain)

R. E. PENDLEBURY

Central Electricity Generating Board South Western Region, Bridgwater Rd., Bristol BS13 8AN (Gt. Britain)

Fretting experiments were carried out on mild steel in air to long sliding distances over a range of loads and stroke amplitudes. For peak-to-peak fretting amplitudes greater than 5 pm, three fretting stages were identified: an initial adhesive transfer stage, an in~rrne~a~ stage where both metallic and oxide debris were produced and a terminal stage characterized by extensive oxide debris beds and continuing wear rates some 50 times smaller than in the previous stage. The nature of the contact and the ability of the oxide beds to bear load are discussed. Some dependence of wear rate on ~plitude was found, but this was smaller than that found by other workers from short-term tests.

1. Introduction

Several studies of the unlubricated fretting wear of mild steel surfaces in air at room temperatures have been reported [ 1 - 61. Early studies were limited in that the course of wear was usually described in terms of one measured parameter, e.g. weight change, or wear scar dimensions or speci- men separation. Different investigators used different specimen contact macrogeometries, and in some cases only one worn specimen of an asym- metric fretting pair was subsequently examined. The resulting picture was confused and data from various early sources were often non-comparable. However, two points of general agreement have emerged, i.e. that wear rates fall with distance and that wear rates can increase dramatically above a critical threshold of fretting amplitude.

The wider use of profilometry equipment to measure the wear volume of each specimen of a contacting pair has removed some of the earlier con- fusion. Ohmae and Tsukizoe’s [ 31 studies appear to confirm Halliday and Hirst’s [4 3 earlier observations that the amount of wear and the wear mecha- nism itself are ~plitude dependent, with the th~~old ~plitude around

*Paper presented at the Institution of Mechanical Engineers’ Fretting Wear Seminar, Nottingham, Gt. Britain, April 2 - 3,1985.

~0~3-~6~61351~3.3~ @ Elsevier Sequoia/Printed in The Netherlands

178

70 pm peak to peak, above which values of the wear rate are similar to those seen in reciprocating-sliding wear. The reported behaviour is summarized in Fig. 1.

At lower amplitudes with flat-on-flat contact geometry it has been widely observed that, after long fretting times, the specimen surfaces become increasingly covered by thin beds of compacted oxide debris. The load- bearing capacity of these has been the subject of recent experiment and speculation [ 71. However, the dehberate introduction of oxide particles be- tween the fretting pair cannot provide an unambiguous demonstration of the normal fretting mechanism when the introduced particle size is greater than that normally seen. Of recent investigators, Toth [8] alone considers that metallic wear particle formation and comminution continue in steady state fretting at low amplitudes.

The experimental investigation described in this paper was therefore designed to examine the fretting wear behaviour of mild steel in air, with specific reference to the effects of amplitude and sliding distance and the role of oxide debris on the wear mech~ism. Experiments were conducted at

SLIP AWPLITUOE $m)

Fig. 1. Fretting wear data as a function of slip amplitude for mild steel: X, 443 kgf, 9 Hz and 67 800 cycles [2]; *, 15 kgf, 10 Hz and 1 - lo6 cycles [3]; a,19 kgf, 50 Hz and lo6 cycles [4]; fi, 19 kgf, 50 Hz and IO6 cycles [5]; v, 8 kgf, 25 Hz and (1 - 5) x lo5 cycles 161.

179

peak-to-peak amplitudes of between 5 and 80 pm under normal loads of 50, 100, 200 and 1000 N with a flat-on-flat contact geometry.

Two rigs were used; Figs. 2 and 3 show their essential features. The rig employed at lower loads and intermediate amplitudes has dead-weight normal loading and the specimens are driven by an eccentric on a motor via a lever arm. Changes in specimen separation are measurable on this rig. The apparatus used at the highest loads and smallest amplitudes has servo- hydraulic control of both normal load and fretting amplitude. Both rigs have provision for the display of frictional forces and specimen displacements, measured using capacitance probes mounted on the specimens.

At the highest loads, wear was sufficiently large to be followed gravi- metrically. At lower loads, each specimen of the pair was examined by sur- face profilometry before and after chemical removal of the oxide and, in many longer tests, any debris ejected was collected and weighed. All tests were performed in air; neither temperature nor humidity was controlled, but each was monitored. The spread of values within and between tests is in the

Fig. 2. Fretting apparatus (lower loads): 1, eccentric bearing; 2, rocker-arm; 3, rocker-arm pivot; 4, carrier pivot; 5, carrier shaft; 6, carrier shaft diaphragm levels; 7, stationary specimen arm; 8, stationary specimen arm pivot; 9, loading arm.

180

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range where wear rates have been shown [l] to be relatively insensitive. The contribution of these environmental effects to the scatter in wear rates is felt to be insignificant.

2. Measurements of relative specimen separation

In some preliminary tests at 100 N load and 38 pm stroke, measure- ments were made of the relative specimen separation within a stroke of the fretting cycle. An initial contact profile of the specimen pair confirmed that the mounting of the moving specimen accurately places the wearing face co- planar with the fretting motion (Fig. 4).

In one test, after 3000 cycles, the form of the friction trace was noted, the apparatus was stopped and the contact profile was remeasured. This

181

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showed that specimen separation increased towards each stroke end, indicat- ing the formation of shallow ramps (about 29 on the specimen. After about 3 X lo6 cycles the inclinations of the ramps had increased to about 3” at one stroke end and had decreased to near zero at the other. Similar profiles were seen in other tests after long running times. The rapid establishment of these ramps suggests that the early stage of wear involves the creation, by adhesive transfer, of engaging metallic “peak” and voidage “trough” features above and below the original contact surfaces. Their persistence after long fretting times might also reflect the hardness of the highly worked material.

3. Friction force measurements

The contact profile measurements suggest that interpretation of the form of the recorded friction traces should take account of the inclined loci of contact known to be present. This is considered further in Appendix A.

With such high frequency test rigs it is difficult to measure the peak friction force in the first few hundred fretting cycles, but typically the peak value of the coefficient of friction /.L, is about 1 after 500 cycles and falls to about 0.6 over the first 300 000 cycles. The “shoulder” coefficient ps shows less variation, soon settling to about 0.5.

The analysis given in Appendix A shows that, in addition to the contact being inclined to the nominal fretting plane at short running times, some additional process, perhaps the ploughing displacement of metal, occurs towards the stroke end. The existence of a constant p. value for all long- and short-time tests suggests that the nature of contact, perhaps metal to metal, remains the same. This is discussed later in connection with the load-bearing ability of compacted oxide debris beds.

182

4. Appearance of wear scars after tests

Typical photographs of worn specimens are shown in Fig. 5. After short tests (Fig. 5(a)) the small worn area consists of disturbed metal. After long fretting times the early disturbances can often be identified by their eleva- tion and reflectivity, but they are surrounded by much larger areas of red- brown compacted Fe203 debris, with outer fringes of paler loose oxide debris (Fig. 5(b)). These areas were measured and found to increase slowly with time. The beds are generally elliptical with the major axis in t,he fretting direction. This may reflect the higher debris migration velocity with, rather than across, the fretting direction.

The exceptions to this occur at very low fretting amplitudes, e.g. see Fig. 5(d), where wear debris tends to follow the lay of the original grinding

(a) Zmm

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Fig. 5. Fretted specimens: (a) 3 x lo3 cycles, 66 I.trn peak-to-peak amplitude, 100 N; (b) lOa cycles, 66 pm peak-to-peak amplitude, 100 N; (c) lo8 cycles, 80 pm peak-to-peak amplitude, 1000 N; (d) lo9 cycles, 5 pm peak-to-peak amplitude, 1000 N. The arrows show the direction of motion.

183

direction of one of the specimens: in the case shown it is orthogonal to the fretting motion and the lay of the specimen.

5. Ejected debris

With the first rig it was possible to collect all the debris which escaped from the fretting interface. Debris escaping within the first about 30 000 cycles contained some metallic particles, but thereafter it was exclusively oxide. The debris was collected, weighed and assigned a compact volume which, when added to the other measured oxide volumes, gave the total amount of metal converted to oxide during fretting. No correlation was found between the amount of ejected debris and load or ~plitude condi- tions at a given fretting time. Debris ejection seems most likely to be governed by the proximity of the initial contact area to the specimen edge.

After fretting for up to about 6 X 10s cycles, net volume losses of the specimen pair are often seen. In view of the ratio of the volumes of oxide and metal from which it is formed (2.14:1 for Fe,03:Fe), more than half of the oxide volume created by fretting wear can be ejected as debris before the oxide beds are well established.

6. Wear meas~ements

6.1. 1000 N tests 22 tests were conducted at 1000 N at peak-to-peak amplitudes of be-

tween 5 and 80 pm at a frequency of 200 Hz. Specimen weight losses, after the mechanical removal of oxide debris, were used to assess wear. Owing to the relatively large amounts of transferred material on some specimens, com- bined weight losses for each specimen pair are presented. A statistical analy- sis of the weighings gives a 95% confidence limit for this method of rt0.2 mg per specimen pair. The average difference in weight loss between specimen pairs in the same experiment is less than 20%.

Figure 6 shows the results for all ~plitudes at up to 400 m sliding distance. The line shown corresponds to a wear rate per specimen pair of 3.5 X lOPi m3 N-’ m- i. Figure 7 shows the longer term wear test data with the results grouped for 22 - 80 pm and 10 - 15 E.crn amplitude ranges. For the higher amplitude range the longer term wear rate is 1 X IO-l6 m3 N-’ m-i and for the lower amplitudes it is 2 X lo-i7 m3 N-i m-i.

6.2. Lower loads 24 tests were conducted at loads of between 50 and 200 N with peak-

to-peak amplitudes in the range 38 - 70 pm at 50 Hz. After fretting the sur- face features of each specimen were investigated using an automated Talysurf profilometer. The equipment used provides an isometric plot of the specimen surface and calculates above-surface material and voidage volumes to an accuracy of about 5 X 10e4 mm3.

0 50 100 150 200 250 300 350 400

SLIDING DISTANCE (ml

Fig. 6. Weight loss us. sliding distance (1000 N tests).

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Fig. 7. Weight loss us. sliding distance (1000 N tests, short-term data not shown).

The above- and below-datum volumes measured for different specimen pairs after various fretting times under one set of conditions are shown in Fig. 8 as joint volumes for the specimen pair. The volumetric course of. fretting wear and rates derived from these and other similar sets of measure- ments are discussed later.

Table 1 shows the highest peak and deepest trough features measured from the profilometry plots. This shows that the average maximum peak

185

Fig. 8. Fretting wear us. sliding distance (100 N load, 44 pm peak-to-peak amplitude, 50 Hz).

height seen after short fretting times is nearly proportional to the load. At a given load, smaller peak heights are generated at the higher stroke. The average maximum peak height and trough depth are closely similar, since these features form as contacting engaged pairs on the two specimens.

After long fretting times, the average maximum peak height is only a little smaller than that at short times, although this reduction is generally greater at higher load. This observation suggests that the early-formed peaks are very long lived. The trough depths seen after long fretting times are again similar to the peak heights with which they engage, suggesting that the one is determined by the other. Oxide stripping confirms this.

After oxide stripping, the maximum peak heights show little reduction suggesting that the peak tops persist as bare metal features. Troughs, how- ever, become deeper by 10 - 15 pm after oxide stripping. It seems possible that their as-worn depths are determined by the motion of the engaged metal peak sweeping aside the oxide debris of various degrees of compaction which partially backfills a deeper earlier-formed trough. Trough depths after oxide stripping often lie within the observed range of values of early-formed peaks and reflect the earlier heights of peaks with which they once engaged.

These observations suggest that, throughout the course of fretting wear, specimen separation is determined by contact at, or on slopes close to, peak tops and trough bottoms. If this is so, the beds of compacted oxide debris seen may simply fill available volume between the specimens without neces- sarily bearing load.

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7. Oxide stripping

The stripping of compacted oxide debris beds involves reductive dis- solution of the iron(II1) ions to complexed iron(I1) ions using a low oxida- tion state transition metal ion (LOMI reagent) [ 91. Microscopic detail of the substrate metal surface is preserved. Examination of worn areas reveals no preferential attack of work-hardened metal by the reagent.

Figure 9 shows the appearance of a specimen from a long duration fretting test before and after oxide stripping. The oxide debris beds are completely removed and deepening of troughs is seen, but the elevation of the highest peak feature is essentially unchanged, as discussed earlier.

It seems possible that during the course of fretting wear some metallic particles might become embedded within compacted oxide debris to form a breccia-like structure. Stripping should remove both oxide and embedded metal. The density of stripped material confirms this. Many measurements of weight and volume losses after stripping provide an average density of 5.82 X lo3 kg mW3 for the material removed, equivalent to 20 vol.% Fe in Fez03. This value was used to correct the oxide volumes on which some wear rates were based.

Fig. 9. Fretted specimen before and after oxide stripping.

188

8. Suggested course of fretting wear

8.1. The early stage The early stage is defined as up to 3 X lo4 cycles fretting or about 5 m

sliding distance. Figure 10 shows profilometer plots of a specimen fretted for 3 X lo3

cycles at 200 N load, 38 pm and 50 Hz. The specimen’s appearance was similar to that shown in Fig. 5(a), i.e. metallic surfaces with virtually no red- brown oxide. The fretting action has formed nesting pairs of peak and trough features. The near-equality of joint above- and belowdatum volumes seen in all similar tests at various loads shows that this first stage of fretting involves only displacement and transfer of metal by adhesive contact (and possibly ploughing), without significant metal loss or oxide formation (see Fig. 8). The measured volumes and feature elevations are essentially un- changed after stripping. The base lengths of the features seen in the fretting direction exceed the amplitude used by an order of magnitude or more.

Etched sections AB and CD through the specimens in the plane of the fretting motion are shown in Fig. 11. The relative positioning of the two photographs corresponds closely to the actual specimen positions at the null position of the fretting motion and shows the engagement of peak and trough features. The rolling direction of the rod from which specimens were machined is clearly seen in the substrate microstructure, as are the

Fig. 10. Profilometer plots of a fretted specimen pair.

189

C

Y limits of fretting motion

Fig. 11. Wear scar sections.

orthogonal displacements of various microfeatures as a result of the fretting action. In many cases these displacement strains greatly exceed the extent of the fretting motion, showing that they must have been accumulated over many fretting cycles.

The volumes of the metallic peak and voidage trough features formed in fretting times of up to 3 X lo4 cycles were measured in several tests. The joint volumes V, for the specimen pair are shown in Fig. 12 for various load and stroke combinations. It can be seen that V, is proportional to load and insensitive to amplitude. The sections seen in Fig. 11 suggest that the volume of metal which has been plastically deformed is much larger than the mea- sured value of Vo. This larger volume is not readily measurable, but it might be expected to be proportional to load. Once some metal displacement has occurred, the work hardening involved should make further displacement more difficult. The measured V, values might therefore reflect not the initial metal hardness, but its hardenability. The measured volumes imply that, at the lower loads, this initial metal displacement stage could be essentially complete after about 3 X lo3 cycles and involves metal volume displacement rates of about 5 X lo-l3 m3 N-i m-i for the specimen pair.

8.2. The intermediate stage The intermediate stage is defined as up to 3 X lo5 cycles fretting or

about 40 m sliding distance.

Fig. 12. Joint volumes V. of specimen pair (above or below datum) us. the load.

It is of interest to compare the relative rates of wear, as measured by the amount of metal lost or oxide formed, over successive time periods. Since the friction behaviour in all tests had steadied by about 3 X lo5 cycles (see Fig. A2), this point is convenient for making a comparison.

The amount of oxide formed is small at all load and stroke combina- tions and is difficult to measure accurately. Calculations of wear rates at lower loads were therefore based on a simpler measurement, i.e. the net volume gain of the as-worn specimen pair including any loose debris. The calculated values fall mostly in the range (2 - 5) X 10P15 m3 N-’ m-l metal volume per specimen pair converted to oxide. Similar wear rates were ob- served in the 1000 N tests from weight loss measurements only (see Section 6.1). The early-stage metal displacement rates (Section 8.1) are about 100 times larger.

Since early-formed peaks persist to long fretting times, wear appears to proceed predominantly by increasing trough volumes which should therefore roughly double between 3 X lo3 and 3 X 10s cycles. Examination of speci- mens confirms this.

8.3. The tertiary stage The tertiary stage is defined as more than 3 X lo6 cycles or more than

4 km sliding distance. Figure 7 shows that the 1000 N data at long sliding distances can be

separated into two distinct groups on the basis of the slip amplitude. When including the low load results, it is appropriate to make a division at 20 pm. Four pairs of results obtained at 5 pm peak-to-peak amplitude (not shown

191

in Fig. 7) were thought to form a third distinct group. Let us consider first the fretting behaviour at amplitudes greater than 20 pm.

8.3.1. Peak-to-peak amplitudes greater than 20 pm Figure 13 shows two views of a specimen fretted for 6 X 10’ cycles at

200 N, 38 pm and 50 Hz. An extensive oxide debris plateau surrounds an inner area of highest peak features which engage with corresponding troughs on the counterspecimen. The outer rim of the plateau carries some uncom- pacted oxide debris which has been swept aside by the successive traverses of the profilometer stylus. A section of this specimen is shown in Fig. 14. The crease below the substantial above-datum feature suggests that its formation involved folding as well as metal displacement. To the left of this are a shallow trough and the compacted debris plateau. The higher magnification photographs show the trough to be substantially backfilled by a stratified mixture of metal and oxide; the lower strata contain larger metallic frag- ments. Some metal appears to be present at all levels within the oxide, sug- gesting that metallic wear particles are still being produced. The inner debris plateau, about 10 pm thick, has an irregular lower surface, perhaps cor- responding to an earlier contact location. Here the oxide contains no free metal but has within it a line of discontinuity which suggests that deposition of oxide debris at this location may have been interrupted at some time. The outer debris plateau is thinner, about 3 pm, is wholly oxide and overlies un- worn metal. It is difficult to estimate the likely integrated metal content of strippable material from polished sections, but values of 10 - 20 vol.% do not seem unreasonable.

Figure 8 shows the volumetric course of fretting wear for one load- stroke combination. After long fretting times (about 3 X 10’ cycles) the ejected debris represents a significant fraction of the total wear volume.

Fig. 13. Fretted specimen (long-duration test).

192

(a)

Fig. 14. Sections of the fretted specimen shown in Fig. 13. (Magnifications: (a) 98x; (b) 245x.) Regions I, II and III in (a) refer to those in (b).

Oxide stripping of worn specimens shows that at long times wear is domi- nated by increases in trough volume by growth in, and transverse to, the fretting direction rather than by deepening. Indeed, the troughs may be no deeper than they were at very early times.

In Fig. 8 the broken curves show how the joint below-datum volume would increase with time if the early-created volume V, grew at the steady rate k. Similar curves indicate that for all load and stroke combinations studied, the overall wear rates suggested by the longest tests are of the order of lo-l6 m3 N-’ m-’ loss for the specimen pair, except when the amplitude falls below about 20 E.trn (see Fig. 7).

Where debris collection and oxide stripping were done, the rate in each test could be calculated in three ways: (i) from the net metal volume loss revealed by profilometry after stripping; (ii) from the total oxide volume revealed by the profilometry-strip-profilometry sequence (corrected for embedded metal) and the debris collected; (iii) from the net volume gain of the as-worn specimen pair plus collected debris.

In general, the rates calculated by each method are in good agreement (at about 5 X 10-l’ m3 N-’ m-l) and similar to the gravimetric results at 1000 N, i.e. lo-l6 m3 N-’ m-l. This suggests the same proportionality between wear damage and frictional work done at large sliding distance.

8.3.2. Peak-to-peak amplitudes of 10 - 20 pm Figure 7 shows that the tertiary fretting wear rate remains essentially

constant for amplitudes between 80 and 22 pm but is a factor of about 5 smaller for lower amplitudes.

193

8.3.3. Peak-to-peak amplitudes of 5 pm Two fretting experiments (giving four pairs of results) were conducted

at 1000 N and 5 pm peak-to-peak amplitude. The results at about 150 m (Fig. 6) suggest that the early and intermediate fretting stages may be short lived. This view is supported by the general appearance of the fretted sur- faces, which do not show the type of significant disruption seen at larger amplitudes (see Figs. 5(c) and 5(d)). However, the tertiary 5 pm wear rate appears to be similar to that at 10 - 15 pm.

9. Discussion

Fretting wear has been described as a progression through three stages, defined from the experimental results. The early stage is marked by surface roughening resulting from the displacement and transfer of metal, mainly by adhesive contact, with little loss of material or oxide formation. At most amplitudes studied, this stage appears to be significant in establishing a sur- face morphology which is long lasting and therefore strongly influences the future course of wear. However, at 5 pm slip amplitude this initial stage seems to be much less pronounced, at least in terms of persistent significant surface disruption.

It is possible to explain qualitatively the early-stage behaviour in terms of the probability of asperity collisions between the contacting surfaces. At the larger fretting amplitudes studied (greater than 20 pm) an asperity has a high probability of interacting with many adjacent opposing asperities. Adhesive transfer of material between asperities and the formation of peaks and their engaging troughs then results, giving the surface roughening ob- served. Strain hardening of the disrupted surface eventually halts this stage. When fretting amplitudes are less than the mean distance between asperities the probability of asperity interactions falls and therefore the amount of adhesive transfer is less. Surface roughening would be expected to reduce accordingly. For still lower fretting amplitudes, which are less than the interasperity contact size, the probability of interactions, and hence the amount of surface roughening, becomes very small.

The intermediate fretting stage is marked by an end to the early dis- ruption stage and the start of metallic and oxide debris production. This appears to proceed by extension, rather than by deepening, of troughs by the persisting peaks. This behaviour continues into the tertiary stage where conversion of metal to oxide continues, albeit at a lower rate.

The formation of oxide, at what is an enhanced rate compared with static oxidation, might involve the formation and comminution of metallic wear particles to submicron size. Their collective surface area might then be greatly enhanced with respect to the plan area over which they are active, explaining the enhanced oxidation rate.

Eventually, comminution to oxide debris of the persistent peak features formed in the early stage may take place, since we do not normally observe

194

these features on plant. The decline in wear rate with amplitude observed in the tertiary stage may therefore reflect the lower initial surface disruption, i.e. smaller peaks and trough volumes. Since these small features are less persistent, this would also explain the earlier onset of the tertiary stage for the smaller amplitudes studies.

Removal of compacted debris beds from long-test specimens often reveals many light scratches on those areas of the surface likely to have been covered by stable oxide beds after about lo6 cycles. The scratch orientations are in the fretting direction and not the ground finish direction of the counterspecimen. Their lengths often greatly exceed the fretting stroke, sug- gesting that their formation involved many cycles in which repeated adhesion and transfer of metallic wear particles occurred. Their depths are often less than 1 E.trn (Fig. 15).

The covering of these scratched areas by compacted oxide debris beds suggests that the area over which metallic particles form and migrate may pass through a maximum with time. By the time oxide debris has covered these areas, it seems likely that wear particle generation may be restricted to a smaller area lying within the engaged peak and trough features. Lateral migration and comminution of wear particles may be hampered by a higher probability of capture within the oxide in the non-load-bearing areas of troughs. This could explain the observed lowering of the rate at which metal is converted to oxide.

In sliding wear, a bed or plateau of compacted oxide debris is often seen by the time wear rate and frictional behaviour have steadied [lo]. Even without any measure of the conversion of metal to oxide in the steady state, many investigators have linked the above observations mechanistically, con- cluding that the load-bearing ability of the oxide bed is responsible for the lower steady state wear rate [ 71. For fretting in air, Toth [ 81 alone considers that adhesion, transfer and comminution of metallic particles continue in the steady state.

The etch sensitivity of “metallic” strata within compacted wear debris shows that they are pearlitic rather than ferritic. In undisturbed oxidation, no significant difference is seen between the oxidation rates of ferrite and pearlite. Here, ferrite appears to have been selectively converted to oxide. This would not be expected if the conversion process involved a repeated scraping off of thin oxide layers from the substrate as envisaged by some, e.g. Uhlig [ 111. If the enhanced oxidation rate seen were due to an increased surface area in the form of a large number of small wear particles, the harder pearlitic particles would be more resistant to comminution and conversion to oxide than the softer ferrite, as observed.

In this work, even at the longest times, wear still appears to be continu- ing at a measurable rate. If oxide contacts are present, they must act in a way which permits rather than prevents further conversion of metal to oxide. In the present studies, the constant underlying coefficient of friction of about 0.5 implies an unchanging nature of contact, unless the coefficients of friction of metal on metal, metal on oxide and oxide on oxide are,

195

/

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fretting motion

Fig. 15. Surface scratches revealed by oxide stripping.

fortuitously, similar. The friction behaviour thus supports Toth’s [ 81 view of some continuing metal-to-metal contacts.

The separation of the specimen pair indicated by the measured engage- ment of peaks and troughs is closely similar to that of the double thickness of the compacted oxide debris beds also seen. Either could govern specimen separation. After long fretting times, the joint above-datum volume mea- sured for the specimen pair is largely that of the compacted oxide debris beds. This, divided by their common area, gives the oxide double-bed average thickness. In Fig. 16, beds of increasing double thickness are indicated by

196

lines of increasing slope. The volume-area combinations measured from various long-duration test specimens are shown as points, one per specimen pair. The double-bed thicknesses seen range from about 2 - 7 pm with a mean of about 4 pm. The scatter of points suggests some trend towards a larger double-bed average thickness at higher load. Mechanical strength con- siderations make it seem unlikely that the steady state thickness of a com- pacted oxide debris bed should increase with the load it is bearing. The ob- served trend would be expected if, instead of being parallel, the specimen faces in different tests contacted in various low angle V-configurations. The larger oxide volumes produced in higher load tests might then fill the divergent available volume over a larger area and so give a larger double-bed average thickness. This, it is suggested, is the more likely explanation of the observations, and it is concluded that in these tests the oxide debris beds do not bear load but simply infill the available volume. Similar investigation of the 1000 N load specimens has not yet been completed. Compacted oxide debris might be more able to bear load in conditions which provide lateral

WEAR SCAR 8 DEBRIS AREA ( cm2 )

Fig. 16. Oxide double-bed thickness for various loads and peak-to-peak amplitudes: 0, 50 N, 48 pm; Q, 100 N, 44 pm; 0, 200 N, 38 pm; 0, 50 N, 70 pm; 0, 100 N, 68 pm; +,

200 N, 66 pm.

197

Metal

Oxide

Oxide

Metal

t-4 Fretting stroke

Fig. 17. Long-term contact geometry

restraint for the bed, e.g. at low amplitudes where the oxide volume just fills the initial surface roughness. Figure 5(d) shows what may be an example of this form of behaviour. It is under such conditions that the lowest overall wear rates have been seen.

The envisaged tertiary contact geometry for the larger fretting ampli- tudes studied is shown in Fig. 17 with contact at, or on slopes close to, peak top and trough bottom. Within this configuration lies an unswept volume at the trough bottom. Here, oxides and embedded metallic debris might lie un- disturbed. Loose metal here will be removed by stripping and hence included in the wear rate as determined by profilometry. Conversion of the metal to oxide may involve two stages in series, i.e. particle formation and particle comminution, which increase the surface area. For the envisaged geometry, an observed fall in incremental wear rate with time must involve a reduced probability of wear particle formation.

It is instructive to compare the present wear rate data with those of other workers. The data shown in Fig. 1 refer to relatively short-duration tests (less than 40 m sliding distance). In this study, although frictional behaviour has steadied by about 3 X lo5 cycles, incremental wear rates con- tinue to fall for a much longer time. It is not clear that a steady state wear rate has been reached even in the longest duration tests (about 25 km sliding distance), but at this sliding distance the observed terminal wear rates de- crease by a factor of about 5 over the ~plitude range 22 - 10 pm. This illustrates the importance of comparing wear rate data at similar sliding distances. Calculated lives of wear components on the basis of observed short-term wear rates might be grossly pessimistic.

10. Conclusions

Two series of tests, carried out on separate rigs, have produced com- parable and reproducible data, in spite of differences in the specimen size, loads and frequencies of fretting. Some test durations exceeded 20 km sliding distance.

(1) Profilometry, oxide stripping and gravimetric methods have revealed that, for amplitudes greater than 5 pm, fretting appears to proceed through three stages:

198

(a) An initial stage, lasting about 3000 cycles, involves plastic deforma- tion of metal, adhesive contact and transfer but no significant metal loss or oxide formation. A measure of the displaced metal volume indi- cates that this is proportional to load, as expected. Metal displacement rates during this stage are about 5 X lo-l3 m3 N-’ m-’ for the specimen pair. (b) An intermediate stage, typified by running times of up to 3 X 10’ cycles (when friction behaviour steadies), involves conversion of metal to oxide at a rate of about 3 X lo-” m3 N-’ m-l per specimen pair. (c) A terminal stage is characterized by the presence of extensive beds of compacted oxide debris. Wear continues at a much reduced rate, i.e. about (5 - 10) X 10-l’ m3 N-’ m-l for the specimen pairs above 20 ,um amplitude. At lower amplitudes, terminal wear rates appear to be established at short sliding distances and are about a factor of 3 - 5 lower, i.e. about 2 X 10-l’ m3 N-’ m-’ for a specimen pair. (2) At 5 pm amplitude, the early and intermediate stages appear to be

minor and relatively short lived. (3) Frictional behaviour steadies after about 300000 cycles and is

consistent with the continued presence of small inclination slopes. Continu- ing production of oxide via wear particle formation and comminution would seem to require, at the very least, intermittent metal-to-metal contact at all fretting times.

(4) No unambiguous evidence was found for the oxide beds bearing the load. The fall in wear rates generally observed with increasing sliding distance may reflect a lower particle formation rate once the debris beds are estab- lished.

(5) Where the long-term wear behaviour is of primary concern, tests of duration at least as great as those reported here would seem more appro- priate in quantifying the effects of amplitude, load etc.

Acknowledgment

The authors wish to thank the Central Electricity Generating Board by whose permission this work is published.

References

1 K. H. R. Wright, hoc., Inst. Me&. Eng., London, (Part 1B) (1952 - 1953) 556. 2 I. Ming-Feng and H. H. Uhlig, J. Appl. Me&., 21 (1954) 395. 3 N. Ohmae and T. Tsukizoe, Wear, 27 (1974) 281. 4 J. S. Halliday and W. Hirst, Proc. R. Sot. London, Ser. A, 236 (1956) 411. 5 J. S. Halliday, hoc. Cont. on Lubrication and Wear, Institution of Mechanical

Engineers, London, 1957, p. 640. 6 M. J. Lewis and P. B. Didsbury, cited in R. B. Waterhouse, Treatise Mater. Sci.

Technol., 13 (1979) 267.

199

7 C. Colombie, Y. Berthier, A. Floquet, L. Vincent and M. Godet, Trans. ASME, 106 (1984) 194.

8 L. Toth, Wear, 20 (1972) 277. 9 M. Segal and R. M. Sellers, J. Chem. Sot., Faraday Tmns. 1, 78 (1981) 1149.

10 T. F. Quinn, Tribol. Znt., 16 (1983) 257, 305. 11 H. H. Uhlig, J. Appl. Me&., 21 (1954) 401.

Appendix A

A.1. Variations in friction force Figure Al shows traces typical of the force-time shape seen at short

and long times. Also shown is the sinusoidal specimen displacement output. Its minimum corresponds to the top of the moving specimen’s motion, when it is closest to the force transducer. In this position also the maximum positive (compressive) friction force F, at point 1 in Fig. Al is always seen.

Following the reversal of motion at point 1, IF21 is always smaller than F,, suggesting that the nature of the process responsible for F, is both posi- tion and direction dependent within the stroke. Along the return stroke, I F41, is always smaller than F3, suggesting that similar processes operate at both stroke ends at early running times.

(a) 0)

Fig. Al. Friction force-time traces (observed) at (a) 750 cycles and (b) 6 X 10’ cycles. (c) The sinusoidal specimen displacement output.

200

1.2 -

1.0 ki w c-l t; 0.8 -

Y

g 0.6. 5 ;I I

: 0.4 E

0.2 -

0- 1 10 102 103 104

SLIDING DISTANCE ( m 1

Fig. A2. Friction coefficients vs. time (200 N, 38 pm peak-to-peak amplitude, 50 Hz).

The way in which the form of the friction-time trace changed with time in this work may be illustrated by defining (i) a peak friction coeffi- cient

F, -Fs Elp= -

2L

and (ii) a shoulder friction coefficient

F,---2 Ps= -

2L

where L is the horizontal load applied to the stationary specimen. The varia- tion in p, and cc, with time is shown in Fig. A2. Several calculated curves from different uninterrupted fretting tests at 200 N load and 38 pm peak-to- peak amplitude are shown.

Similar sets of curves were obtained under the other load and amplitude conditions tested.

The peak friction force I.c,, recorded after 500 cycles running falls from a value of about unity to about 0.6 after 6 X 10’ cycles. The shoulder coef- ficient ps shows less variation, soon settling to a value of about 0.5.

A, 1.1. Friction forces on inclined planes Let us consider contact between two surfaces inclined at an angle 8 to

the horizontal, with a friction coefficient p0 and with the upper surface attached to a horizontal force transducer. If the lower surface moves so as to increase apparently the surface separation the observed friction coefficient is given by

j,+) cos 8 + sin 0 (Al)

201

- Contact inctination

63 + Zero

e -

0 - Zero

0 +

(a) (bf (Cl (df (e)

Fig. A3. Friction force-time traces (idealized): (a} zero inclination; (b) constant inclina- tion at each stroke end; (c) increasingly inclined over whole stroke; (d) symmetrically inclined over each half stroke; (e) zero inclination with unidirectional ploughing.

which is greater than p, and equal to FJL. For decreasing surface separation, the observed friction coefficient is given by

~,cos6-sin0

cos e + p, sin 0 (A2)

which is less than ,u, and equal to F2/L. These considerations suggest that contacts of various inclinations

should give force-time traces of the types shown in Fig. A3. The traces seen in the present work after short running times closely resemble Figs. A3(c) and A3(d), and suggest that the point of greatest inclination to the plane of the fretting motion lies at the stroke end, in agreement with the specimen separation measurement described in Section 2.

The measured friction coefficients and contact profiles are consistent with eqns. (Al) and (A2) for a constant cl0 value of 0.5, at long running times and at short running times when specimen separation is decreasing. For motion towards the stroke end (separation increasing) at short running times, the known inclination angles predicted a smaller pP value than that actually observed. A ploughing friction coefficient of about 0.5, augmenting clo, satisfactorily explains the observed pP values at short running times.