The unlubricated wear of sintered iron

Wear, 113 (1986) 371 - 382 371

THE UNLUBRICATED WEAR OF SINTERED IRON

S. C. LIM* and J. H. BRUNTON

Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ (U.K.)

(Received February 13,1986; accepted April 22,1986)

Summary

The unlubricated wear of sintered pure iron has been investigated under different sliding conditions using a dynamic wear rig operating in the scanning electron microscope and a simple pin-ondisk apparatus operating in air. Delamination and ploughing mechanisms were found to be responsible for wear during sliding under the different conditions. Using normalized variables, the measured wear rates agreed well with the predictions of Archard’s law and correlated with wear data from other sources. The effect of porosity in the iron has also been studied. Open pores on sliding surfaces were found to be important in generating and trapping wear debris. When the trapping mechanism was bypassed by cleaning, or rendered ineffective by pore closure, the wear behaviour of high porosity sintered iron approached that of the lower porosity specimens.

1. Introduction

In comparison with the extensive studies on wear of non-sintered metals, there has been relatively little work on sintered materials. In the case of sintered iron and iron-based alloys, important contributions include the work of Amsallem et al. [ 11. In this investigation, on the unlubricated slow sliding behaviour of 12.0% porosity sintered iron in air and nitrogen, they found that, in general, it behaved similarly to non-sintered iron but showed higher friction and lower wear. Eyre [2] and Eyre and Walker [3] reported that sintered iron exhibited a transition from mild to severe wear similar to its non-sintered counterpart. They found that the wear resistance of sintered iron could be improved with increasing the pressing pressure and sintering temperature, presumably giving a reduced porosity, as well as with the addition of phosphorus which formed a hard phosphide eutectic.

Gopinath et al. [4] investigated the influence of sliding velocity u (0.5 - 8 m s-i) and contact pressure P (lo5 - lo6 N m-*) on the unlubricated

*Present address: Department of Mechanical and Production Engineering, National University of Singapore, Kent Ridge, Singapore 0511, Republic of Singapore.

0043-1648/86/$3.50 @ Elsevier Sequoia/Printed in The Netherlands

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friction and wear behaviour of 26.0% porosity sintered iron. They found that the wear behaviour underwent two transitions; (1) a high wear at low PZJ values to a low wear at medium values of Pu, and (2) a transition to high wear at high values of Pu. Gopinath [5] extended this work to study the influence of u (0.5 - 16 m s-l) on the unlubricated wear of sintered iron- based alloys. He found that the constant load wear rate decreased with increasing sliding velocities up to a critical value and then increased. He showed that this transition was the result of an increase in surface temperature; frictional heating at the higher velocities caused thermal softening of the metal and greater wear.

The wear resistance of sintered iron-based alloys has been reported by James et at. [6] who found that the wear resistance of sintered Fe-Ni-Mo- Cu alloys could be increased by cold work. This was related to the defor- mation-induced transformation of metastable austenite to martensite and a consequential large increase in hardness. In the case of resistance to abrasive wear, Razavizadeh and Davies [7] found that the wear properties of sintered iron and Fe-Cu alloys were improved by the addition of copper in amounts up to 8 wt.% and by subjecting the sintered metals to steam oxidation at 525 “C.

This brief review shows that, although some work has been done on the wear behaviour of sintered iron, there have been no reports on the wear mechanisms that operate under different sliding conditions. Similarly, there has been no mention of the role of porosity in wear. The present experimental investigation has been carried out on sintered iron to study the wear mechanisms that operate in unlubricated sliding and to examine the role of porosity in wear.

2. Dynamic wear tests in the scanning electron microscope

2.1. Experimental de tails Two series of wear tests were carried out using a dynamic wear rig

operating inside the scanning electron microscope (SEM) [S]. In one series, hard steel pins were run on soft commercially pure sintered iron disks and in the other series the situation was reversed.

The first set of tests involved the sliding of case-hardened mild steel pins on soft sintered iron disks. The tips of the pins were prepared to provide a wide range of nominal contact areas from about 10m9 to 10e5 m2. The pins were hardened to an average hardness of 426 HV. The resulting contact pressures at the interface ranged from about lo4 to lo* N m-* under an average normal load of 0.50 N. As will be shown below, this range of contact pressures was sufficient to allow two different wear mechanisms to operate, The sliding velocities used ranged from 0.28 to 0.38 mm s-i.

In order to examine the behaviour of pores in the first series of tests iron specimens with porosities of 22.3% were selected from a range of sintered materials (supplied by GKN Technology Ltd.). These were cut into

373

disks which were either mechanically polished to a mirror finish using diam- mond paste or were prepared by electrolytic polishing. Electrolytic polishing produced surfaces with many more open pores than was the case with the mechanically prepared surfaces. The average hardness of the disks was 50 HV.

In the second series of tests soft sintered pins were run on mirror smooth case-hardened mild steel disks of average hardness 480 HV. The pins with porosities of 22.3% were machined from plate specimens and annealed to yield an average hardness of 40 HV.

2.2. Wear mechanisms When a hard pin ran under a high contact pressure (sharp pin) on a

soft sintered iron disk it was observed that a ploughing mechanism operated (Fig. l(a)). The hard pin ploughed a groove on the surface and the wear

(a)

-,c

iotffl

(b)

- \I Thin wear sheets

(cl (d) Fig. 1 (a) The ploughing mechanism operates when the contact pressure is high. A case- hardened mild steel pin is shown here sliding on a mechanically polished soft sintered iron disk where the wear fragments are removed by a mechanism similar to a cutting process. (b) The schematic representation of (a) showing the sharp pin ploughing the soft disk. (c) The delamination mechanism operates when the contact pressure is reduced by using a pin with a larger nominal contact area. Thin wear sheets or flakes are seen at the interface between the hard pin and a mechanically polished soft disk. (d) The schematic representation of (c) showing the blunt pin sliding on the soft disk. The arrow in each photomicrograph indicates the direction of rotation of the disk.

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fragments were produced by a mechanism analogous to a cutting process. This is similar to the observations of Bates et ~2, f9] on the sliding of a di~ond rider on steel and cast iron disks. They described this type of wear as penetrative wear.

When the pressure was decreased by increasing the nominal contact area (blunt pin) the mechanism was observed to change to delamination wear [lo]. Figure l(c) shows the thin wear sheets or flakes produced at the pin- disk interface. It was a general observation in these tests, as Fig. 1 clearly illustrates, that the dominant wear mechanism was sensitive to changes in the contact pressure. All these tests were carried out on disks which had been mechanically polished using diamond paste.

(al

Debris in

(cl

(b)

(d) Fig. 2. Features on the electrolytically polished soft sintered iron disks after blunt case - hardened mild steel pins have run on them for an average distance of 1150 mm. (a) The number of open pores on the wear track has been reduced by the flow of material due to sliding. The numerous open pores are clearly visible outside the wear track. (b) The flow of material over the edge of an open pore produces thin extended tongues overhanging the pore cavity. (c) Small debris collected inside a pore. The flow of material over the edge of the pore can also be seen. (d) A large wear fragment trapped by an open pore. The arrow in each photomicrograph indicates the sliding direction of the pin.

375

Tests using electrolytically polished disks and blunt pins produced wear tracks where the surface material flowed along the track in the direction of sliding, as illustrated in Fig. 2(a). This figure also shows fewer open pores on the wear track than can be found outside it. The flow of material at the edge of a pore produces thin extended tongues overhanging the pore cavity (Fig. 2(b)). After several traverses the tongues break away to form debris which accumulates in the pores as shown in Fig. 2(c). This mechanism is schemat- ically illustrated in Fig. 3 and is consistent with the observation that far less wear debris was observed to leave the pin-disk interface during sliding in these tests than in the tests using mechanically polished disks. This differ- ence is accounted for by the open pores trapping much of the free debris produced under the pin, thereby leaving little to emerge from the interface.

An

asperity

9 Wear debris ’

Wear debris /

(a)

A crack

Wear debris

Fig. 3. A schematic representation of a process whereby debris is formed from the edge of an open pore. (a) An asperity sliding over an idealized pore containing trapped debris. (b) A tongue is formed at the edge of the pore. (c) Repeated traverses of the asperity result in further extension of the tongue. A crack is nucleated which eventually causes the tongue to separate from the edge of the pore.

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Occasionally, larger fragments of debris from the surface of the disk were seen entering the pores. An example of this is shown in Fig. 2(d).

Turning now to the second series of tests using soft sintered iron pins sliding on case-hardened mild steel disks. In these tests the open pores in the surface of the pins had been covered during the machining process and the wear behaviour of these pins was found to be identical with the behaviour of non-sintered iron pins [ 111. The covering and possibly filling of the open pores prior to these tests effectively formed a surface similar to a pore-free material. Figure 4 shows the same thin iron flakes produced when

pins were run on mirror-smooth disks.

fb) Fig. 4, (a) The accumulation of thin wear sheets at the leading edge of a sintered iron pin after it has run on a mirrorsmooth casehardened mild steel disk, The total sliding distance is 970 mm, (b) The same accumulation of thin wear sheets at the leading edge is observed when a nonsintered high purity iron pin is used instead. The total sliding distance is 1110 mm. The arrow in each photomicrograpb indicates the direction of rotation of the disk.

__ _.

(a) (b) Fig. 5. {a) The plastically extruded tail showing a layered appearance at the trailing edge of a sintered iron pin after it has run on a casehardened mild steel disk having a centre- line average surface roughness of 0.42 pm. The total sliding distance is 1080 mm. (b) The same layered appearance of the plastic tail at the trailing edge of a nonsintered high purity iron pin after it has run on a disk with the same surface roughness. The total sliding distance is 1190 mm.

311

Furthermore, examination of the worn pins after sliding on roughened disks showed that the structure of the extruded tails was the same in both cases and both had a similar layered appearance. Figures 5(a) and 5(b) show respectively the layered surface of a sintered pin end and the same feature for a non-sintered iron pin. This layered appearance is indicative of flaking by a delamination process.

These observations on the wear processes indicate that in sintered iron the open pores initially play an important role in generating and also in removing wear debris. Mechanical working or polishing which acts to cover the open pores results in wear behaviour very similar to that found in non- sintered materials.

3. Wear tests in air

3.1. Experimental de tails The tests in air were carried out on sintered iron specimens with

porosities of 12.0% and 22.3% using a simple pin-ondisk test rig described elsewhere [ 121. Vacuum annealing of all the pins together gave a uniform hardness of 40 HV. The flat-ended pins used were 5 mm in diameter. A constant linear sliding velocity of 0.44 m s-i was selected for all the tests and loads ranging from about 3 N to just over 30 N were employed. In order to arrive at a reliable value for the wear rate at each load at least 40 runs were made at that load. The duration of each run was kept to 30 s to reduce the effect produced by the accumulation of debris. The pins were removed, cleaned in alcohol and dried in warm air before being weighed to determine their weight loss and hence the wear rate.

3.2. Wear rates and mechanisms It has been shown [ll] that wear rates can be satisfactorily presented

using the following normalized variables: (F/A,H,), the normalized pressure; (W/A,), the normalized wear rate; where F is the applied load, W is the wear rate (defined as the volume removed per unit sliding distance), A, is the nominal contact area and Ho is the initial room temperature hardness of the softer sliding member.

The results of the present wear tests in air are now presented using the normalized variables. For sintered iron A, takes into account porosity. Since the area fraction of pores is equal to their volume fraction A, will be approximately 77% and 88% of the cross-sectional areas of the sintered pins with porosities of 22.3% and 12.0% respectively.

Figure 6 gives the wear rate data for the two sintered irons (12.0% and 22.3% porosity). For the low porosity specimens and using the normalized variables there is little scatter and a good agreement with Archard’s law [ 131. Figure 7 is the same as Fig. 6 but with additional data from the works of Amsallem et al. [ 11, Gopinath et al. [ 41 and Eyre and Walker [ 31. These researchers used sintered iron with a porosity range similar to that used in

I& / ~~ ~____ -. ._I____-_. .__ / 10-q 10-B 10-7 l(

NORMALISED WEAR RATE (W/A,)

Fig. 6. A plot of normalized pressure against the normalized wear rate of sintered iron having two different porosities. The high porosity specimens generally have a lower normalized wear rate than the lower porosity specimens.

I ”

I

wzi ,o-9 ~-

/

to-8 to-7 10-s

N@~ALISED WEAR RATE (W/A, 1

Fig. 7. Comparison of the present results with the data from Amsallem et al. [ 1], Gopinath et al. [4 ] and Eyre and Walker [3 1. Results from the present work are represented by the shaded region and deviations from the present results are only observed at higher normalized pressures.

the present tests. When the normalized variables are used the data points from Gopinath et al. agree well with the present results. Some of the results of Amsallem et al. [l] show significant deviations at higher normalized pressure. This deviation could be related to differences in test geometry since they used a flat inning against a hardened ring. Eyre and Walker’s results

319

10-C ’ 10-e 10-7 1o-6

NORMALISED WEAR RATE (W/A,)

Fig. 8. The effect of a different cleaning interval on the wear rate of the 22.3% porosity sintered iron. The normalized wear rate is seen to increase when the pins are cleaned six times during each run rather than only at the end of each run. This intermediate cleaning has rendered the open pores less effective in accumulating the free wear debris, thus increasing the measured wear rate.

also deviate at high normalized pressures. The reason for this may relate to the higher sliding velocities used in their tests. In spite of these deviations Fig. 7 demonstrates the usefulness of the normalized variables as a way of comparing wear data obtained from different sources as well as bringing out the agreement between the wear data and Archard’s law.

Measurements of wear rates for the 22.3% porosity specimens are shown in Fig. 8. Two sets of results are plotted for these specimens. The first set refers to runs of 30 s duration followed by cleaning and weighing and the second set refers to runs of 30 s duration but with intermediate cleaning of the pins every 5 s to remove debris. It can be seen from Fig. 8 that the specimens with the intermediate cleaning showed a higher normalized wear rate than those cleaned only at the end of a run. The two sets of data points are well separated on the figure. The explanation proposed for this behaviour is that, as seen in the SEM work (Section 2.2 and Fig. 2(c)), debris is fre- quently trapped by open pores and therefore does not contribute to the weight loss. The measured wear rates are therefore lower in those specimens cleaned once per run than in the specimens cleaned six times during a run. For these last specimens cleaning removes much of the loose debris before it becomes trapped in the pores. This increases the measured weight loss and the observed wear rates.

Re-examining the data in Fig. 6, an explanation can now be offered for the observed reduction in wear rate with increasing porosity. The less porous specimens, having fewer open pores, are less effective in trapping debris than the more porous specimens, hence the higher wear rate. It is interesting to

380

note in Figs. 6 and 8 that for the high porosity specimens there is a sharp transition in normalized wear rate with increasing normalized pressure when the normalized pressure reaches a value of about 2 X 10-3. Above this transition the normalized wear rates coincide with the rates found in the low porosity specimens and these rates are in close agreement with Archard% law. No such transition has been observed in low porosity and non-sin&red specimens [ 111, It is considered that this sharp increase in normalized wear rate with pressure is due to surface flow covering over most of the pores {by a process similar to that illustrated in Figs. Z(a) and (b)), so that the surface behaves similarly to a material of lower porosity. In Fig. 8, for example, the data points for the specimens cleaned at the two different intervals (5 and 30 s) coincide above the transition pressure indi~at~g an equivalence in surface behaviour.

The wear debris, the noise level during sliding and the appearance af the worn pin ends all indicated that wear took place primarily in the mild wear regime. SEM examinations of the worn sintered pin ends revealed surface features similar to those found in the case of non-sintered iron pins run under similar conditions [ 111, indicating that delamination is the dominant wear mechanism for the tests in air. A typical example of the worn surface is shown in Fig. 9.

It is interesting to note that the crater near the open pore in Fig. 9 appears to be the result of a flake having detached itself from the sliding surface. This suggests that the severe material flow generated during the tests in air (at a higher sliding velocity) quickly covers the open pores which are

Cracks perpendicular Co

the sliding direction

A flat crater

left by a detached flake

A wear fl&x?

Fig. 9. A typical example of the surface features on a worn sintered iron pin end after sliding on a case-hardened miid steel disk in air. The arrow indicates the sliding direction of the disk.

381

also continuously re-opened by a flaking, or delamination, process. These two processes compete with each other and the dominant process determines the number of open pores on the surface. The normalized wear rates given in Figs. 6 and 8 indicate that at lower normalized pressures the flaking mechanism dominates and it produces a sufficient number of open pores on the sliding surface to trap wear debris giving rise to lower “apparent” wear rates. At higher sliding pressures pore closure due to the increase in surface flow leaves few open pores remaining on the sliding surface. This results in surface behaviour similar to that found in lower porosity specimens and in non- sintered specimens. These effects are thought to be responsible for the coin- cidence of wear rates at higher pressures shown in Figs. 6 and 8.

4. Conclusions

For case-hardened mild steel pins sliding on soft sintered iron disks the following conclusions can be made.

(1) The pores play an important role in the wear behaviour of sintered iron. Under dry sliding conditions the open pores on the sliding surfaces act as sites for the generation and collection of wear debris.

(2) When most of the open pores are covered by material flow on the surface owing to sliding or mechanical polishing, the wear behaviour of sintered iron approaches that of its non-sintered counterpart.

(3) Studies in the SEM showed that for the sliding conditions investigated, and using mechanically polished disks, the dominant wear mechanisms were ploughing at high normalized pressures and delamination at lower pressures.

For sintered pure iron pins sliding on case-hardened mild steel disks the following conclusions can be made.

(1) SEM studies showed that the mechanism of wear for soft sintered iron pins is one of delamination.

(2) In the tests in air mild delamination wear took place and the wear rates agreed well with Archard’s law when presented using the normalized variables.

(3) The wear rate data for tests in air have been explained in terms of mechanisms suggested by the observations using the SEM. These showed the open pores acting as generators and collectors of wear particles. The data also indicated that as the collecting mechanism is interfered with by cleaning or by pore closure the wear of the high porosity specimens approaches that found in specimens of lower porosity and in specimens of non-sintered material.

Acknowledgments

One of the authors (SCL) would like to thank the National University of Singapore for the award of a scholarship as well as granting him study

382

leave to undertake this piece of work. We wish to thank GKN Technology Ltd. for supplying sintered specimens for this investigation and Miss S. Glassford for her assistance with some of the experimental work.

References

1 C. Amsallem, A. Gaucher and G. Guilhot, The unlubricated frictional behaviour of sintered iron, Wear, 23 (1973) 97 - 112.

2 T. S.’ Eyre, Wear resistance of metals, Treatise Mater. Sci. Technol., 13 (1979) 363 - 442.

3 T. S. Eyre and R. K. Walker, Wear of sintered metals, Powder Metall., 19 (1) (1976) 22 - 30.

4 K. Gopinath, G. V. N. Rayudu and R. G. Narayanamurthi, Friction and wear of sintered iron, Wear, 42 (i977) 245 - 250.

5 K. Gopinath, The influence of speed on the wear of sintered iron-based materials, Wear, 71 (1981) 161 - 178.

6 B. A. James, J. E. Garner and G. T. Brown, Work hardening sintered materials -a metallographic study, Powder Metall., 20 (4) (1977) 221 - 226.

7 K. Razavizadeh and B. L. Davies, The effects of steam treatment on the wear resistance of sintered iron and Fe-Cu alloys, Wear, 69 (1981) 355 - 367.

8 S. C. Lim and J. H. Brunton, A dynamic wear rig for the scanning electron microscope, Wear, 101 (1985) 81 - 91.

9 T. R. Bates, Jr., K. C. Ludema and W. A. Brainard, A rheological mechanism of penetrative wear, Wear, 30 (1974) 365 - 375.

10 N. P. Suh, The delamination theory of wear, Wear, 25 (1973) 111 - 124. 11 S. C. Lim and J. H. Brunton, The unlubricated wear of iron, Wear, 113 (1986) 383

393. 12 S. C. Lim, Wear mechanisms with particular reference to the wear of sintered mate-

rials, Ph.D. Thesis, Cambridge University, 1986. 13 J. F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys., 24 (8) (1953)

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Documents

The unlubricated wear of sintered iron