Plate-like wear particle formation in a lubricated ball-on-plate friction pair

Plate-like wear particle formation in a lubricated ball-on-plate friction pair I. Iliuc* The mechanism of formation of plate-like wear particles in a ball-on-plate lubricated friction pair has been examined for wear constants of K < 10 -1° (mm 3 mm -1 N -~ ). The plate Vicker's hardness was 2 . 8 0 - 3 . 0 0 kN/mm 2, the sliding speed 1.74 m s -1 and the load 50 N. The following mechanism is suggested: scratching of the surface and formation of ridges at the scratch border, lateral deformation of ridges and formation of thin sheets, and cracking and separation of plate-like particles from these sheets. Keywords: wear, lubricated friction pair, wear particles Much work has been published over the years on the sliding wear of metal contacts. Efforts have been made both to establish quantitative relationships between wear and the parameters involved in the process and to elucidate the wear mechanism and the formation of wear particles. These earlier works covered adhesive wear, oxidative wear, delamination wear and others. Only a small number of them dealt with the wear mechanism under lubricated conditions. 1 Two main difficulties confront workers in this field: the complexity of the process as a consequence of the presence o f lubricant and additive; and the low rate of wear, particularly when it is desired to simulate conditions similar to those existing in real machines. To offset the problem of low wear rate, either very sensitive wear measurement techniques can be used, which are rather cumbersome for more comprehensive investigations, or resort can be made to accelerated wear tests. This leads to rapid alteration of the wear body shapes and to modified operating conditions. The present paper uses a sensitive and convenient wear measurement technique combined with observation of the wear track. Arguments are given in favour of a wear particle formation mechanism that implies only scratching of the surface and plastic flow of the ploughed-up material.

Experimental A ball-on-plate friction pair was used for the tests, with a ball radius of 11.5 mm. Since the bush axle was stationary the relative motion between ball and plate was pure sliding. The friction pair was submerged in the lubricant, whose temperature was kept constant at 50°C. During operation a wear track formed in the shape of a spherical cap. By measuring the cap diameter and depth with a profilometer, volumes of worn-out metal o f the order of 10 -s mm 3 could be determined. 2 The friction parts were made of carbon steel containing about 0.38%C and l%Cr. The ball, which in fact was a spherical bush, had a Vickers' hardness of 4 . 8 0 - 5 . 0 0 kN mm -2 and the plate a hardness of 2 . 8 0 - 3 . 0 0 kN mm -2 . The roughness of the plate was taken to a minimum (R a = 0.02 ~m) and that of the bush

*Institute o f Physics and Technology o f Materials, Str. Constantin Mille 15, Bucharest, Romania

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to Ra = 0.045/am, by polishing with metalographic paper and diamond paste. In one test the bush had roughness R a = 0.075/am. The basic lubricant was a white oil with viscosity 185 mm 2 s -~ at 25°C and 10.5 mm 2 s -~ at 90°C. The load applied was 50 N and the sliding speed was 1.74 m s -~ . Examination of the wear track and the surrounding region was made under optical and scanning electron microscopes.

Results and discussion As a consequence of the conformability of the wear surfaces the wear rate of the pairs with concentrated contacts decreases with time. For the conditions described above and a ball roughness of 0.045 tam the mean wear constant decreases f r o m K = 1.7 x 10 -1° (ram 3 mm -1 N -1) for an operating time of 5 s to _K = 1.15 x 1 0 - n for 300 s and = 1 x 10-12 for 3600 s. The instantaneous wear constant K decreases from K ~ 1 x 10-12 for an operating time of 300 s to K - 0 for 3600 s. The mean values of wear constant (,~) are taken over the whole running time: K = V/SL. The instantaneous values (K) are obtained from K = A V / A S . L ; where V is the wear, S is the distance travelled and L is the load. The values AV and A S are calculated from the wearrunning time diagram for t = 300 s and t = 3600 s. Note that the wear constant assumes low values even from the first moments of operation. The wear track surface exhibited characteristic aspects of sliding (adhesive) wear after 5 s operation and after 1800 s;

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Fig 1 Wear tracks after 5 s running time

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of these sheets, parallel to the scratches, can be observed in all tracks, not only in those shown here (denoted by a solid arrow in Figs 2 and 3). It is not in all cases that the ridges are deformed into sheets according to the mechanism described above. Parts of these ridges can be pressed into the neighbouring scratches and cover them. The border line of the deformed material can also be observed in the wear track (see the dotted arrows in Fig 3(b)). The initial stage of the formation of lips can be seen in Fig 5, where an accidentally formed large scratch may be noted, the track boundary being marked by a dashed line. That the deformed material forms a detachable lip can also be observed at the scratch tip where a small portion of it was removed (see the arrow in Fig 5). Two characteristic images, where the laminated sheets formed at the scratch border are separated from the basic material are shown in Figs 6 and 7. The wear tracks in these figures were produced by bushes with R a = 0.075/am. The increased roughness led to severe wear with a wear constant of K" ~ 10-8. Note that the sheet free edge is parallel to the

Fig 2 Surface of the wear track after 5 s running time. Two magnifications are given. The solid arrows mark the free edge of the deformed material that is, scratches of various depths, parallel to the bush motion (Figs 1,2 and 3). The wear track boundary is seen to be toothed due to the intersection of scratches of various depths with the plate surface. This configuration is most apparent during the first instants of formation of the wear track (Fig 1), when the wear is at its most intensive. There is, however, an obvious difference between the track surface obtained after 5 s running time and that resulting after 1800 s; that is, the number of scratches after 1800 s is lower and the surface is smoother than after 5 s. The scratches in the wear track are almost certainly produced by the asperities of the bush, by the particles transferred from one surface to another and by solid contaminants in the lubricant. Although these mechanisms could not be observed directly, measurements of electrical contact resistance and material transfer seem to give strong support to the theory. Any factor that reduces the surface interaction, such as increased lubricant film thickness or reduction of the surface roughness, diminishes the wear, the particle transfer and the number of electrical contacts. Surface scratching itself does not cause wear. The material displaced from the scratch is not removed as chips but is mounded up, the scratches being produced in a manner similar to ploughing. Long or filament wear particles were not observed in the present tests. When wear becomes mote severe larger wear particles may appear following their expulsion from the scratches. Formation of the scratch by ploughing produces ridges either side of it. The pressure acting in the loaded zone produces lateral plastic flow of the ridges and the subsequent formation of sheets or lips (Fig 4). The free edges

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Fig 3 Surface o f the wear track after 1800 s running time. Two magnifications are given. The solid arrows mark the free edge o f the deformed material and the dotted arrows a crack parallel to the direction of motion formed by the covering o f a scratch with deformed material

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Fig 4 Formation o f thin sheets o f material by the deformation o f marginal ridges of a scratch: (a) formation of a scratch and ( b ) deformation of marginal ridges

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being much smaller. They appear to be smooth, with irregular borders. If the running time is prolonged the formation of metal wear particles, by the scratching-plastic deformation process described, gradually ceases as a consequence o f the surfaces conforming. The surfaces are, at this stage, separated by a thin, continuous lubricant film. It was found that under these circumstances, use of an oil without additives leads to the wear track being covered by a thin oxide layer. 4 In the absence of accidental scratching during this operation mode slow wear of the oxide layer occurs yielding very fine oxide particles much smaller than 1 /am. The rate of wear is very small, being below the sensitivity

Fig 5 Deformation o f ridges observed in an accidental large scratch. The dotted line shows the wear track boundary and the arrow the place o f separation o f a particle o f the sheet formed by deformation

Fig 7 Separation o f the plate-like particle. The wear track was produced under severe wear

Fig 6 Thin sheets formed by deformation are separated from the surface. The wear track was produced under severe wear scratches, that is, parallel to the sliding direction. In Fig 6 some cracks can be observed, marked by arrows, produced by the strong lateral deformation of the sheets. The sheets appear to be smooth, their thickness being much less than 1/am. Fig 7 shows the separation of a metal sheet from the base surface, that probably occurred when the plate was removed from the installation. When the sheets are sufficiently thin, parts o f them separate and form free plate-like wear particles. To observe the tree wear debris, use was made of friction polymer. It is known that some of the wear products generated in the loaded zone are deposited around the wear track, forming a characteristic pattern. If the reaction products contain a viscous product, such as a friction polymer, this will largely retain the particles expelled from the loaded zone. In this way they can be easily observed immediately after the run. 3 Before examination, the plate must be carefully washed without wiping. Fig 8 shows the boundary of a wear track in the lubricant outlet region, a more detailed image being shown in Fig 9. The operation time was 1800 s and the basic oil had 5% oleic acid added to it. The oleic acid yields a high viscosity adherent fluid, ie a friction polymer that accumulates around the wear track. From Fig 8 it can be seen that a large number of wear particles is retained by the polymer. Fig 9 shows the wear particles to be in the shape of small plates, their major dimension being 1 - 2 / a m , the thickness

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Fig 8 Plate-like wear debris in the lubricant outlet region. The base lubricant had 5% oleic acid added and the running time was 1800 s

Fig 9 Plate-like wear particles retained on the plate surface. Detail o f Fig 8

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of the measurement technique used. The wear of the oxide layer was observed only due to the colouration of the polymer layer formed around the wear track by the oxide particles separated from the loaded region, The mechanism of formation of plate-like particles suggested in this paper has also been mentioned under conditions of boundary and dry lubrication in a vacuum, s'6 It can be seen, however, from the present work that this mechanism appears independent of the initial presence of machining marks or the existence of strong deformations characteristic of dry friction.

• The thinning of these sheets finally produces cracking and separation of the plate-like wear particles. When the wear rate increases the material is probably expelled during scratching and other processes occur that generate larger wear particles. If a wear particle is not expelled at the very time of its formation it will be gradually worn down to a very small thickness. It should be noted that the lubricant film thickness in a friction pair such as that used in the runs described; is smaller than 1/am. When the particles are finally expelled, they are in the shape of small plates.

Conclusions The main sliding wear mechanism of lubricated friction surfaces at very low wear rate could be described as follows: • Scratching of the plate surface by the asperities of the other wear body, transferred particles or particles contained in the lubricant. Scratches are formed by ploughing without free particles being expelled. The material from scratches forms mounds along the scratch border. This process probably prevails in the case of fine scratches characteristic of low values of wear constant. • The deformation of marginal ridges leads to the formation of sheets, the free edges of which are parallel to the scratch.

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References 1. Childs T.H.C. The sliding wear mechanisms of metals, mainly steels. Tribology Int., 1980, 13(6), 285 2. lliuc 1. Verschleiss bei elastohydrodynamischer Teilschmierung. Schmiertechnik Tribologie, 1978, 25 hr. 5 164. 3. Iliuc I. Reaction products in and around the wear track in the lubricated mild wear regime. Wear, 1984, 93, 271 4. lliuc 1. Untersuchungen an Triborektionschichten. Schmiertechnik Tribologie , 1984, 31(1), 21

5. 3ahanmir S. Wear mechanism of boundary lubricated surfaces. Wear, 1981, 73, 169 6. GlaeserW.A. Wear experiments in the scanning electron microscope. Wear, 1981, 73, 371

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