Wear, 28 (1974) 89-94 0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

89

EFFECT OF WEAR PARTICLES ON THE CONTACT OF UNLUBRICATED SLIDING METALS

J. M. GEORGES, B. LAMY and L. DELASSAUSSE

Laboratoire de Technologie des Surfaces, Ecole Centrale de Lyon, Ecully (France)

(Received June 20, 1973; in final form November 13. 1973)

SUMMARY

The behaviour of tin bronze when rubbed at high speed against a case hardened steel, was studied. Removal of the particles of tin bronze which adhere to the steel affects the process of transfer, wear rate, mean surface temperature, coefficient of friction and surface topography.

NOTATION

P dynamic friction coefficient T mean surface temperature AP weight loss t time K wear rate K = d(AP)/dt

;

mean standard deviation of the asperity heights mean radius of curvature of the tips of the asperities

c/fi geometrical criterion of surface topography

INTRODUCTION

The engineer with a dry friction problem often needs to know the inter- facial temperature. The generally accepted theories of Jaeger’ and Archard’ state that the surface temperature depends upon the two materials in contact, the type of contact, the load and the speed. However, experiments have shown3 that measurements are unreliable due to the influence of wear debris on the contact.

The formation and the rupture of the junctions between two contacting surfaces, is an important cause of wear. Kerridge4 showed that wear occurs in different stages: transfer of one material to another, formation of a relatively thick transferred layer, mechanical or physical-chemical transformations and the permanent regeneration of this layer leading to the formation of wear debris. Such material transfer is observed when a brass pin rubs against a steel cylinder; the cylinder is soon covered with bronze particles.

According to Buckley5, metals such as aluminium, copper and silver of lower cohesive energy than iron, were transferred onto iron in a static, adhesive contact

90 J. M. GEORGE& B. LAMY, L. DELASSAUSSE

when tests were made in a vacuum and in the absence of oxides. When oxide was present on the metal surface, fracture of the adhesive junction took place between the metal and oxide, where the metal to oxygen bond was weaker than the cohesive bonds in the.metal. Rabinowicz6 explains the release of wear fragments by interracial energy. Archard’ and Kragelsky’ consider that the wear fragment is detached by a fatigue mechanism. When the material is not homogeneous, each phase behaves differently. Thus Courtel et al.’ showed that in wear of a two phase tin bronze, the transferred material consisted mainly of a phase, while the wear fragments were 6 phase. Thus, after a certain length of time, in a pin and cylinder wear test, the tin bronze pin no longer rubs against the steel cylinder but against a tin bronze layer adhering to the steel resulting in increased wear of the pin’ O.

It has been shown that by removing transferred particles after their formation that the wear of the pin decreases” and the contact parameters improve (co- efficient of friction, mean temperature of the surface, rate of wear and surface topography).

APPARATUS AND MATERIALS

The wear apparatus used, a pin and cylinder machine similar to that used by Ling’ ’ consists of a stationary cylindrical pin (6 mm diam.) loaded (50 N) against a rotating cylinder (120 mm diam.) at its outer surface. The tangential sliding speed is constant (3 m/s). The apparent pressure is 1.8 N/mm’. All tests were performed under atmospheric conditions at room-temperature (21°C). The pin is supported by a frame (Fig. 1); the driving couple is measured by strain- gauges attached to a flexible blade. Estimates of the mean surface temperature were obtained by inserting chromel-alumel thermocouples into the brass pins at four points, approximately 0.4, 0.8, 1.2 and 1.6 cm from the rubbing surfaces. The temperature gradient between these points was measured after reaching thermal equilibrium, and, by linearity, the surface temperatures were then calculated. Calibration of the thermocouples was carried out by a hot oil bath. From the weight of specimens before and after tests of different duration, the weight-loss AP of specimens in relation to time was determined and a wear rate K, the slope of the curve AP=f( t) calculated. After each test a statistical analysis of two

B: Flexible blade

F : Frame

Fig. 1. Schematic diagram of the wear machine.

CONTACT OF UNLUBRICATED SLIDING METALS 91

perpendicular profiles of the surfaces by the method of Greenwood and Williamson1 3, gave the asperity tip distribution, (mean standard deviation) as well as the mean radius of asperity curvature. The criterion a/p was used for the study of changes of surface topography.

Specimens The tin bronze pins contain 12% tin and less than 0.1% lead. A soft c1

phase encloses globules of 6 phase (diameter: 5-10 p). Before test, the end of the pin was run-in to the shape of the cylinder surface for two minutes under test conditions designated “without intervention”. The surface hardness of the pins after this shaping was 280 to 290 HVlOO. The steel cylinder of nominal compo- sition of 0.16% C 1.5%% Ni 0.9% Cr was case hardened to 900 HVlOO. Before use the cylinder had a surface roughness corresponding to a//I = 0.006. All specimens were ultrasonically cleaned in alcohol; the shaping of pins by running-in completed their cleaning.

RESULTS AND DISCUSSION

To study the role of particles during friction, three experiments were carried out. In a first series of tests, the shaped pin was rubbed against a cylinder which initially had a clean surface. Tin bronze is transferred to the cylinder and this test is called “without intervention” (W I). In a second series, a razor blade was pressed against the cylinder throughout the test to remove some of the transferred tin bronze particles (tests R B). Finally in a third series a Joseph paper wiper was pressed against the cylinder (tests J P), to remove most of the particles. Coefficient of friction, wear rate, mean temperature of the surface and surface topography, in relation to time, are shown in Figs. 2, 3, 4 and 5 respectively. The values of these different parameters after seven minutes of testing are shown in Table I.

In all ““W I” tests, tracks resulting from friction are first covered with small, spherical particles (about 50 p). After one minute, tin bronze fragments removed by transfer become platelets of thickness small compared with the other two dimensions (length about 0.5 mm and width 0.2 mm). The results agree

1 2 3 4 5 6 7 Tih4E (min)

0 1 2 3 4 5 6 f

TIME (min)

Fig. 2. Variation of the coefficient of friction with time. Fig. 3. Variation of wear rate with time.

92 J. M. GEORGE& B. LAMY, L. DELASSAUSSE

TABLE 1

TESTS WITHOUT LUBRICATION

Particles

prrsen t Particles p

Tci

K fvF removed (v/s)

WI little large 0.57 150 0.65 0.05

RB little large large 0.57 150 0.32 0.03

JP little little 0.37 80 0.002 0.002

0’ / / I I I

1 2 3 4 5 6 TIME

Fig. 4. Variation of temperature with time.

I .- -. ip ,-._ -. .-. .-_ 7 0 1 2 3 4 5 6 7

(min) TIME (min)

Fig. 5. Variation of surface topography with time.

with those of Courtel’ and Lancaster14. Some of these fragments are transferred back to the pin, producing an irregular surface of rough surface topography (this process corresponds to part A of curve W I, Fig. 5). After live minutes, both large and small fragments are present, but there is less transfer back to the pin, and the surface topography becomes more regular (part B of curve W I, Fig. 5). In test “R B” the razor blade removes large fragments attached to the cylinder without preventing their further formation; it does not collect small particles. The paper wiper collects some small particles (without preventing their further forma- tion) but also prevents the formation of large fragments.

The formation of large fragments seems to result from the presence of a thick layer of small fragments already attached to the surface. For example:

(a) tests W I: large fragments only appear when the layer of small fragments adhering to the cylinder is of sufficient thickness (after about one minute).

(b) tests J P: many small fragments are removed after formation and large particles no longer form.

From the measured parameters the large fragments have an important effect on the interface. When they are present (tests W I and R B), the coefficient of friction is high and fluctuating (0.55 <p ~0.65) as is the temperature (146°C <p < 165°C). The surface topography of the pins is bad and the wear is severe.

In the absence of large fragments (tests J P and the beginning of tests W I) the friction coefficient is lower and more stable, and the same in both cases (0.35 <p ~0.37). The mean temperature of contact is lower (79°C). The surface

CONTACT OF UNLUBRICATED SLIDING METALS 93

topography improves and wear is insignificant. Two aspects of the problem require elucidation. First how are large particles

formed from the transferred tin bronze layer and secondly is the action of paper a mechanical or physical-chemical phenomenon?

It appears that there are several superimposed phenomena: the adhesion of tin bronze on steel and tin bronze on transferred tin bronze and the work- hardening of the surface layers of the tin bronze pin?4. Further research is needed to explain these phenomena.

Small fragments could be picked up by the paper fibres (mechanical), or the paper could prevent the adhesion of fragments by adsorption as a lubricant on the cylinder surface (physical-chemical). However, since the friction coefficient is the same for both the J P and initial W I tests the lubricating effect is negligible. To confirm this point, two experiments were conducted using two poor lubricants; ethyl alcohol and acetone. The results were similar.

Two kinds of tests were conducted. In one the alcohol dripped (10 cm’jmm) onto the friction track (tests D B D). The other was similar to the paper tests described earlier, only the Joseph paper was soaked in alcohol (tests S P). The results are given in Table II.

If the function of the paper was mainly the physical-chemical effect of a poor lubricant (~=0.37 tests J P) with the addition of a poor lubricant on the surface, there should be no difference between tests with paper (S P) and with- out paper (D B D). However this is not the case so the main function of the paper is not a physical-chemical phenomenon. As there is wear to the pins, the use of a lubricant does not provide fluid lubrication. In the experiment the mechanical nature of the paper seems to be effective. As in the unlubricated tests, use of paper reduces the friction coefficient, the mean surface temperature, the rate of wear and improves the surface topography.

TABLE II

TESTS WITH ETHYL ALCOHOL

Particles present

Particles IJ 4B removed

DBD little 0.4 50 0.05 0.012

SP little little 0.2 30 0.015 0.003

CONCLUSION

The wear of tin bronze on case-hardened steel (with a mean apparent pressure of 1.8 N/mm2 and a sliding speed of 3 m/s) is modified by removal of the particles transferred by adhesion during friction. Wear, coefficient of friction and the mean surface temperature are reduced and the surface topography of the tin bronze pin is improved.

Adsorption of lubricants lowers the energy of adhesion and the rate of transfer”. By mechanically lifting off some of the adherent particles, the process of transfer and the various parameters of the contact are affected.

94 J. M. GEORGES, B. LAMY, L. DELASSAUSSE

ACKNOWLEDGEMENTS

Thanks are due to M. Chappuis, Daronnat, Magnien, Philippe for their advice.

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