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Effect of reciprocating and unidirectional sliding motion on the friction and wear of copper on steel
Wear 249 (2001) 582–591
Effect of reciprocating and unidirectional sliding motion on the friction and wear of copper on steel Etsuo Marui∗ , Hiroki Endo Department of Mechanical and Systems Engineering, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu-shi 501-1193, Japan Received 10 May 2000; received in revised form 6 March 2001; accepted 8 May 2001
Abstract From the practical necessity to obtain the friction and wear characteristics of materials used in machinery, various types of wear-testing machines have been developed and used. To obtain useful data for practical application, it is desirable that the investigation is carried out by a full-scale wear-testing apparatus having approximately similar contact conditions. Generally speaking, the obtained wear characteristics are different for every wear-testing apparatus used. When the type of wear-testing machine is not suitable, one cannot obtain the required wear characteristics. There are various parameters in wear-testing machines, such as configuration of contact surface and form of the relative motion between the test specimens. Thus, in this report, the effect of relative motion between contacting pair and friction process on obtained test results is investigated, for the combination of a copper pin specimen and a flat steel specimen. The microscopic structure of the pin specimen may be varied due to the friction and wear process. An optical microscope is used for the observations. The pin specimen is made from pure copper, which can be deformed easily. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Wear; Pin-on-disc wear-testing machine; Pin-on-flat wear-testing machine; Optical microscopic observation; Unidirectional friction; Reciprocating friction
1. Introduction Since olden times, practical methods to reduce wear have been explored. Burwell [1] might have been the first researcher to explain scientifically wear phenomenon with complex properties. He studied wear from the standpoint of material science and tribology. It is important to estimate wear characteristics appropriately. However, it is rare for friction or wear characteristics of materials or machine elements to be estimated from a full-scale test. These characteristics are obtained in many cases from a simplified model test on a small scale. The friction or contact modes in such model apparatuses for estimation are as follows [2]: (A) pin (or ball)/disk (unidirectional friction type); (B) pin (or ball)/flat (reciprocating friction type); (C) cylinder/cylinder (contact with end surface); (D) pin/cylinder; (E) four-ball; and (F) two-parallel cylinder type. Test by type (E) machine is used for judging the lubricant ability, and type (F) machine is for machine elements of rolling contact. The decision as to which contact mode is superior to obtain the wear and friction ∗ Corresponding author. E-mail address: [email protected] (E. Marui).
characteristics of materials and machine elements, must be made from equivalence between a full-scale machine and reduced-scale model machine. There are three modes in practical contact surface, point contact, line contact and surface contact. The case in which pins with curved end surface or balls are used in (A) or (B), and the case (E) belong to point contact. Cases (D) and (F) are of line contact. The cases that a pin with flat end is used in (A) and (B) and case (C) are for surface contact. As mentioned above, there are many structure-types of wear-testing machine and contact mode. So in this research, the wear test is carried out using two different types of wear-testing machine in our laboratory. Contact modes are the same in both machines, however, the friction or relative motion modes are different. One is (a) unidirectional friction or continuous sliding type and the other (b) a reciprocating friction type [3]. The effect of relative motion on the friction and wear characteristics and the features of each machine are investigated from the experiment of flat/flat contact of a pin having a flat end (surface contact mode). Experiments are carried out on the friction pair of copper and steel for machine structure use. The effect of reciprocating friction in type (b) machine on wear characteristics of the copper pin is a particular focus of attention.
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2. Structure of friction pair (test specimens) Both tests of unidirectional friction and reciprocating friction are carried out using the same friction pairs of test specimens. The pin specimen is made from pure copper [4] (purity: 99.9%), which is frequently used in electric circuits. The mechanical strength of pure copper is low, and may deform easily, so the features of the wear-testing apparatus may be sensitively detected and used in our experiment. Vickers hardness number of pure copper is about 110. Its Young’s modulus is 113 GPa. The surface roughness (mean peak-to-valley height [5]) of the pin specimen at the contact surface is 5 m Rz before the wear test. The thermal conductivity of pure copper is large (393 W/m K). Plane specimen is made from carbon steel for machine structure use, whose carbon content is 0.47%. Its Vickers hardness number and Young’s modulus are 210 and 210 GPa, respectively. The contact surface of the flat specimen is finished by grinding, and its surface roughness before the wear test is about 4 m Rz . It will be desirable that the wear map [6] of pure copper sliding on the steel surface is constructed for the entire clarification of the friction and wear characteristics of pure copper. However, the main purpose of this paper is to discuss how the sliding mode of different wear-testing apparatus influences the friction and wear characteristics of pure
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copper sliding on the steel surface. Although experiments are carried out for wide experimental conditions that are possible in each wear-testing apparatus, the wear map is not constructed here. Experiment under the same experimental conditions for both wear-testing apparatus is carried out. In the following, characteristic experimental results obtained in both wear-testing apparatus are explained briefly.
3. Unidirectional friction wear test by pin-on-disk wear-testing machine 3.1. Structure of pin-on-disk wear-testing machine The structure of the pin-on-disk wear-testing machine in this experiment is shown in Fig. 1. In the figure, A is a flat specimen and B a pin specimen. A calibrated vertical load is exerted between them through a pneumatic pressure mechanism. This apparatus is the same as the one used in the previous report [7], so the details of the measurements are not repeated here. The static bending rigidity of the main spindle–pin specimen system is 800 kN/m, when a pure copper pin specimen 5 mm in diameter is set on the main spindle with a 10 mm overhang. The capable range of experimental conditions in this wear-testing machine is as follows. Maximum vertical load:
Fig. 1. Pin-on-disk wear-testing machine with unidirectional friction.
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Fig. 2. Variation of pin specimen length due to sliding distance (vertical load: 29.4 N).
98 N; continuously variable rotational speed of flat specimen: 25–1000 rpm. The effective contact radius between pin and flat specimen is 80 mm, and the sliding velocity between both specimens can be varied within about 0.1 and 4.2 m/s. In our wear-testing machine, the magnitude of wear can be obtained by monitoring continuously the pin specimen length and by weighing the mass of the pin specimen before and after the wear test. 3.2. Experimental procedure Wear measurement is carried out up to a sliding distance of 500 m, varying the vertical load and the sliding velocity under the room temperature condition.
The surface of the pin and the flat specimens are washed by acetone thoroughly, and wax adhering to the surfaces is removed. The worn mass of the pin specimen is measured by a precise electric-balance, and its minimum increment is 1 mg. 3.3. Experimental result and discussion Variation of the pin specimen length corresponding to the sliding distance is shown in Fig. 2. The sliding velocity is given in the figure. In the case of the small sliding velocity, the transition from the severe wear regime to the mild wear regime is recognized. The transition from the severe to the mild wear regime does not take place for a larger sliding velocity. The pin specimen shortens approximately in
Fig. 3. Relation between worn mass and sliding velocity (vertical load: 29.4 N).
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Fig. 4. Relation between surface roughness and sliding velocity.
proportion to the sliding distance. Corresponding to such rapid decrease of the pin specimen length, a large lip is formed behind the pin specimen. Details on this phenomenon will be discussed later. The correct worn mass of the pin specimen accompanied by a lip formation cannot be obtained from the change in pin specimen length. Then, the worn mass is weighed by means of a precise electric-balance, and the above-mentioned change in the pin specimen length is converted into the change in mass by multiplying the density of the copper, simultaneously. In brief, the former mass is called here a measured mass and the latter a converted mass. The relation between both masses and the sliding velocity is given in Fig. 3. The symbol (䊊) indicates a converted mass, and the symbol (䊉) a measured mass. For a larger sliding velocity, the difference between both masses becomes remarkable. The difference in masses corresponds to the mass of the lip. The measured mass decreases with the increase in sliding velocity, in the lip formation condition. The surface roughness change of the flat specimen corresponding to the sliding velocity is shown in Fig. 4, when sliding distance reaches 500 m. No characteristic dependence of the friction coefficient on the vertical load or the sliding velocity can be observed. The magnitude of the friction coefficient is within the range from 0.7 to 0.8.
4. Reciprocating friction wear test by pin-on-flat wear-testing machine 4.1. Structure of pin-on-flat wear-testing machine The structure of pin-on-flat wear-testing machine with reciprocating friction is shown in Fig. 5. Both flat and pin specimens are fixed to the reciprocating stage or to the pin
specimen holder by setting screws. The diameter of the pin specimen is 4 mm. Vertical load is applied by a weight, as seen in the figure. Applied vertical load is determined so that the mean contact pressure is equal to the value in the pin-on-disk wear-testing machine. Maximum sliding velocity obtainable in this machine is 11.06 mm/s, which is approximately equal to the minimum sliding velocity in pin-on-disk wear-testing machine. Pin specimen and flat specimen materials in this section are identical to the former pin-on-disk wear-testing machine. The static bending rigidity of the main spindle–pin specimen system (pure copper specimen diameter: 4 mm and its overhang length: 10 mm) with leftward loading is 28 kN/m, and that for rightward loading is 21 kN/m. Small difference is recognized according to the loading direction. 4.2. Experimental results and discussion The magnitudes of wear (worn mass) are shown in Fig. 6 corresponding to the sliding distance. The transition from severe wear regime to mild wear regime is not recognized here. The experimental condition of sliding velocity 11.06 mm/s is common to both testing machines. From the comparison of the wear rate calculated from worn mass of the pin specimen, it is clarified that the magnitude for the pin-on-flat wear-testing machine is slightly larger than for the pin-on-disk wear-testing machine. However, the difference is quite small and the feature of the wear-testing machines cannot be investigated. The surface roughness of the flat specimen becomes worse from 4 m Rz (before experiment) to 12 m Rz (after experiment). The deterioration in the surface roughness of the pin specimen is more remarkable as in the case of the pin-on-disk wear-testing machine, and the surface roughness of the pin specimen is not measured here.
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Fig. 5. Pin-on-flat wear-testing machine with reciprocating friction.
An example of the magnitude of friction coefficient monitored during the wear test by the pin-on-flat wear-testing machine is given in Fig. 7. This example is obtained in the most severe experimental condition of the vertical load 18.6 N and the sliding velocity 11.06 mm/s. In the figure, the maximum and minimum values of the friction coefficient corresponding to the right- and leftward motion of the stage carrying the flat specimen are shown. The friction coefficient in the rightward motion is slightly larger than in the leftward
motion. As seen in the measurement of the static rigidity of the main spindle–pin specimen system, the rigidity of the system is different corresponding to the loading direction, and the deflection or inclination of the pin specimen is also different from the sliding direction of the flat specimen. This is reflected in the measured friction coefficient. Until we saw the experimental result, we thought the formation of the lip on the pin specimen could not be observed, since the sliding direction is always reciprocating in the
Fig. 6. Increase of worn mass due to sliding distance (vertical load: 18.6 N).
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5. Observation of pin specimen microscopic structure by optical microscope 5.1. Observation method
Fig. 7. Variation of friction coefficient due to sliding distance (vertical load: 18.6 N).
pin-on-flat wear-testing machine with reciprocating friction. Seeing the worn pin specimen, we noticed that a small lip is formed on the end of the pin specimen in the pin-on-flat wear-testing machine with reciprocating friction, too. This may be related to the fact that the friction force may tilt the pin specimen during sliding, and a unidirectional sliding or friction condition is realized at the end of the pin specimen as in the experiment by pin-on-disk wear-testing machine. To indicate the lip formation quantitatively, the worn mass obtained from the pin specimen weighing is compared with the worn mass converted from the decrease of the pin specimen length as in the section above. The result is given in Fig. 8. In this figure, the results for the condition of the vertical load 15.7 N are compared. The worn mass obtained from the decrease in the pin specimen length is somewhat large, and a small lip formation is recognized.
Fig. 8. Relation between worn mass and sliding velocity (vertical load: 15.7 N).
In the previous experiments, the lip formation parallel to the sliding direction is recognized in some experimental conditions. To clarify the formation mechanism of the lip, and to examine the effect of the difference in sliding modes on the experimental results, the microscopic structure of the pin specimen is observed by optical microscopy. The pin specimen made of pure copper is cut in a determined direction by a fine cutter. The surface is then buffed to a mirror finish. After that, etching treatment is performed on the surface, and the microscopic structure of the pin specimen is observed by means of an optical microscope at an adequate magnification. The etching agent is a mixture of 30% liquid ammonia, 3% oxygenated water and distilled water. The mixing ratio is 1:1:1. Etching time is 2 min. 5.2. Observation in pin-on-disk wear-testing machine An example of the small lip growth (sliding velocity 0.1 m/s) and an example of large lip growth (sliding velocity 0.7 m/s) are treated here. In both examples, the vertical load is set exceptionally large at 24.5 N. The sliding distance is 500 m. The magnification of the photographs is given in the following figures. Fig. 9 is a photograph showing the microscopic structure of the cross-section in parallel to the disk rotation at the pin specimen center, when the lip growth is small. Some plastic deformation zone stretched in the direction of the disk rotation is recognized on the whole contact surface of the pin specimen. On the opposite end, no lip formation is seen. Fig. 10 shows the microscopic structure of the same cross-section, when the sliding velocity is large enough to
Fig. 9. Optical micrograph of pin specimen when cut by a plane parallel to the disk specimen rotation (pin-on-disk wear-testing machine, sliding velocity: 0.1 m/s).
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Fig. 11. Optical micrograph of pin specimen when cut by a plane perpendicular to the disk specimen rotation (pin-on-disk wear-testing machine, sliding velocity: 0.7 m/s). Fig. 10. Optical micrograph of pin specimen when cut by a plane parallel to the disk specimen rotation (pin-on-disk wear-testing machine, sliding velocity: 0.7 m/s).
sliding, the growth of the lip continues, and the lip is layered up as shown in Fig. 10. 5.3. Observation in pin-on-flat wear-testing machine
form a large lip. The lip develops approximately in the radial direction from the joint. In this case, the depth of the plastic deformation zone at the contact surface is about 0.05 mm. Fig. 11 is a photograph showing the cross-section perpendicular to the disk rotation. Corresponding to Fig. 10, it is seen that the lip grows in stratified construction. From the above observations, the growing process of the lip in pin-on-disk wear-testing machine is considered to be as follows: when sliding is started, a small burr is formed around the pin specimen. Burr in the disk rotation is little large. For longer sliding distance, softening of the pin specimen may occur due to the friction heating and the burr is stretched into the direction of disk rotation by some ratchetting mechanism. At last, a lip is formed. By successive
The same observation of the pin specimen is carried out in the case of pin-on-flat wear-testing machine with reciprocating friction. Fig. 12 is a photograph at the sliding velocity of 11.06 mm/s and vertical load of 18.6 N. The sliding distance is 200 m. Figure (a) is at the left end of the flat specimen stroke and (b) at the right end of the flat specimen stroke. In both cases, a small lip is growing at the left and the right ends. Differing from the pin-on-disk wear-testing machine, the layered microscopic structure of the lip is discontinuous from the microscopic structure of the original pin specimen, as seen in these photographs. In some cases, the lip is separated from the pin specimen. From these, there is a possibility that the lip is formed by re-adhering
Fig. 12. Optical micrograph of pin specimen when cut by a plane parallel to the flat specimen movement (pin-on-flat wear-testing machine with reciprocating friction, vertical load: 18.6 N): (a) left end; (b) right end.
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Fig. 13. Optical micrograph of pin specimen when cut by a plane perpendicular to the flat specimen movement (pin-on-flat wear-testing machine with reciprocating friction, vertical load: 18.6 N): (a) left side; (b) right side.
of the separated wear debris to the pin specimen, due to the reciprocation of the flat specimen motion. Fig. 13 shows the microscopic structure of the pin specimen cut by a plane perpendicular to the direction of the flat specimen reciprocation. On both the right and left sides of the pin specimen, the lip formation with continuous layered structure to the original pin specimen is recognized. From these observations, the lip in the pin-on-flat wear-testing machine with reciprocating friction may be formed due to the following process. Small lips are formed at the front and rear ends of the pin specimen, corresponding to the reciprocating flat specimen motion when the sliding started. After that, the lip spreads over the whole circumference of the pin specimen. However, the motion of the flat specimen is always reciprocating, so the lip growth in the sliding direction is suppressed. Passing through such processes, the lip grows perpendicular to the flat specimen movement (Fig. 13). It may be considered that the difference in the wear mechanism of the pin-on-disk wear-testing machine from the pin-on-flat wear-testing machine with reciprocating friction exists in such a point. Above-mentioned unique deformation of the pin specimen may be influenced by the contact temperature rise due to friction heating and the plastic ratchetting. A consideration from these aspects is given in the following chapter. 6. Considerations 6.1. Temperature rise at contact surface due to friction heating Temperature at the contact surface is a main factor influencing the result of wear test. So, the temperature at the contact surface between the pin specimen and disk specimen or between the pin specimen and flat specimen is estimated here. There are many researches [8,9] that estimated the temperature rise due to friction heating at the contact surface. A common fundamental idea for the temperature rise
estimation is that the work done against the friction force will be liberated as heat and this heat will flow into the pin specimen and flat specimen in some ratios. As a result, the temperature rise of the specimens is induced. Friction heat flowing into the pin specimen is carried away from the contact surface by conduction and is emitted into surrounding air. Bowden and Tabor [10] presented an equation to calculate temperature at the contact surface between an infinitely long pin specimen and a flat specimen, where Newton’s law of cooling is introduced. Estimation result of the temperature at the contact surface may differ owing to the equation for temperature estimation. However, a qualitative feature of the temperature estimated is not seriously influenced by the equation used. Here, the equation by Bowden and Tabor is used in our estimation of temperature rise at the contact surface. Physical meaning of their equation is clear and the temperature is easily calculated using small number of characteristic values, although the effect of surface roughness and the mass of pin specimen holder are not considered in their equation. This equation is written as follows: 1 αµNv T = T0 + π 2σ kr where T is the temperature at contact surface, T0 the room temperature, N the normal load, v the sliding velocity, r the radius of the pin specimen, µ the kinematic friction coefficient, k the thermal conductivity, σ the cooling coefficient of Newton’s law of cooling and α is the certain fraction of friction heat going into the pin specimen. Then, the temperature rise due to friction heating is T − T0 . Here, the temperature rise of the pin specimen is estimated by the above equation, within the experimental conditions treated in this paper. Following parameter values are used in the estimation: the fraction α is set at 0.5 by referring the account by Bowden and Tabor thermal conductivity k and cooling coefficient σ of copper are k = 393 W/m K, σ = 40 W/m2 K, respectively. The magnitude of the cooling coefficient is determined
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by referring to the value of constantan (which is an alloy of copper and nickel) cited by Bowden and Tabor [10]. Radii of the pin specimen in the pin-on-disk wear-testing machine and the pin-on-flat wear-testing machine are 2.5 and 2 mm, respectively. The magnitude of the kinematic friction coefficient µ is determined referring to the experimental results. The magnitudes of vertical load and sliding velocity are settled as the experimental conditions. The maximum and minimum temperature rises in the pin-on-disk wear-testing machine are estimated as 155 and 10◦ C, respectively. Those in the pin-on-flat wear-testing machine are 15 and 5◦ C. Such large difference in estimated temperature rise corresponding to the types of wear-testing machine is induced by the difference of attainable sliding velocity. It is quite natural that the estimated temperature rise is equivalent regardless of the type of wear-testing machine, when the vertical load and the sliding velocity are equivalent each other. A higher estimation of temperature rise at contact surface is obtained when a large magnitude of the fraction α is used in the estimation. From the above consideration on the temperature rise at contact surface, the influence of friction heating on the wear of copper is negligible, except the case of large vertical load and large sliding velocity in the pin-on-disk wear-testing machine. 6.2. Lip formation by plastic ratchetting Kapoor and co-workers [11,12] stated that material deformation proceeds by a plastic ratchetting mechanism, when cyclic stress beyond the plastic shakedown limit is applied. Let us consider the case that a thin surface layer is subjected to a steady compressive stress σ zz together with an alternating orthogonal shear stress ±τ xz . These stresses satisfy the von-Mises yield criterion twice a stress cycle on σ –τ plane. The plastic strain increments arising from the cyclic shear are equal and opposite, so that no accumulation of shear strain occurs here. However, the increments of plastic compression are always in the same direction, then the material accumulates unidirectional increments of compression within each stress cycle. This phenomenon is called a plastic ratchetting. As a result of plastic ratchetting, a filmy layered deformation orthogonal to the sliding direction (frictional traction) arises in the material and a lip is built up. When this lip is dropped, that phenomenon is a wear by plastic ratchetting. Yang and Torrance [13] considered the wear of metal by plastic ratchetting, too. When an orthogonal siding to the asperity ridge is applied, the extrusion in the sliding direction is prevented, but the lateral extrusion by plastic ratchetting is possible. From the above-mentioned point of view, the lip formation in the pin-on-disk and pin-on-flat wear-testing machine can be discussed in detail.
6.3. Consideration on wear test of copper sliding on steel flat In the first place, results by the pin-on-disk wear-testing machine are discussed. The disk specimen rotates at a steady rotational speed. A shearing stress in the disk rotation direction acts on the pin specimen. The pin specimen yields under the combined action of this shearing stress and contact pressure. Small shear deformation in the direction of disk rotation arises in the pin specimen. New stress field acts on the pin specimen. In this new stress condition, an additional shear deformation of the pin specimen arises in the direction of disk rotation and is accumulated on the first deformation. Such a process continues during the wear test and the lip is grown up in the rear end of the pin specimen. So, the lip has a layered structure. This is ascertained in a photograph of cross-section of the pin specimen, which is perpendicular to the disk rotation or sliding velocity. A lip to the orthogonal direction to disk rotation is an accumulation of the deformation by contact pressure acting on the pin specimen. This lip is smaller than the lip at the rear end of the pin specimen. In the next place, results by the pin-on-flat wear-testing machine are discussed. Above-mentioned lip formation by the accumulation of shear deformation cannot happen, in ideal case that only the direction of shear force is reversed accompanying by the sliding direction reciprocating. Lip into the orthogonal direction to the sliding may be formed. However, in the experiment, a small lip formation parallel to the sliding is recognized. The static bending rigidity of the cantilever type main spindle–pin specimen system is quite small as mentioned above. So, the pin specimen holder deflects. The direction of this deflection is reversed according to the sliding velocity. Following this, contact position between the pin specimen and the flat specimen always deviates to the rear ends of the pin specimen. Both ends of the pin specimen contact the mating flat specimen in only one way of rightwards or leftwards during each reciprocating stroke. Relative movement between the pin specimen and the flat specimen is reversed, however, direction of friction force acting on each end does not change. As a result, the contact state at the end of the pin specimen is equivalent to that of the pin specimen in the pin-on-disk wear-testing machine. Lip recognized in the pin-on-flat wear-testing machine is formed by the similar mechanism at the pin-on-disk wear-testing machine. Contact pressure is settled so as to be equivalent in both types of the wear-testing machine. Owing to the reason in machine structure, sliding velocity attainable in the pin-on-flat wear-testing machine is not so high and the influence of friction heating may be small compared with the pin-on-disk wear-testing machine. The size of formed lip is also small. If the main spindle–pin specimen system is fully rigid, the shearing force is reversed corresponding to the reversal of sliding velocity. The accumulation of deformation by sliding friction disappears and the lip formation
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parallel to the sliding velocity cannot be recognized. Only a small lip may be formed in the orthogonal direction to the sliding velocity.
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the pin-on-flat wear-testing machine with reciprocating friction. References
7. Concluding remarks The influence of the friction mode on the wear regime is examined by a wear experiment using two types of wear-testing machines, a pin-on-disk wear-testing machine with unidirectional friction and a pin-on-flat wear-testing machine with reciprocation motion. As a result, a small difference is recognized in the wear rate. However, no fundamental effects on the wear characteristics of friction modes, such as unidirectional sliding or reciprocating sliding, are recognized. As the pin specimen is made from pure copper, which can be deformed easily, the lip is formed at the rear end of the pin specimen. The microscopic structure of this lip is observed by means of an optical microscope. Then, following points are clarified: formation of the lip is remarkable in the pin-on-disk wear-testing machine with unidirectional friction, and there are some differences in the lip formation mechanism by two types of wear-testing machines. Detailed considerations on the lip formation are carried out from the point of the friction heating and the plastic ratchetting mechanism. There is a weak point in the machine structure that sufficiently large sliding velocity is difficult to realize in
[1] J.T. Burwell, Survey of possible wear mechanisms, Wear 1 (1957–1958) 119–141. [2] I.M. Huchings, Tribology, Friction and Wear of Engineering Materials, Edward Arnold, London, 1992, p. 79. [3] C.-R. Yang, Y.-C. Chiou, R.-T. Lee, Tribological behavior of reciprocating friction drive system under lubricated contact, Tribol. Int. 32 (1999) 443–453. [4] I.I. Garbar, The effect of load on the structure and wear of friction pair materials: example of low-carbon steel and copper, Wear 205 (1997) 240–245. [5] T.R. Thomas, Rough Surfaces, Longman, London, 1982, p. 85. [6] S.C. Lin, M.F. Ashby, Wear-mechanism map, Acta Metallogr. 35 (1987) 1–24. [7] E. Marui, N. Hasegawa, H. Endo, K. Tanaka, T. Hattori, Research on the wear characteristics of hypereutectoid steel, Wear 205 (1997) 384–392. [8] J.F. Archard, The temperature of rubbing surface, Wear 2 (1958–1959) 438–455. [9] A. Yevtushenko, O. Ukhanska, R. Chapovska, Friction heat distribution between a stationary pin and a rotating disc, Wear 196 (1996) 219–225. [10] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1986, pp. 33–35. [11] A. Kapoor, K.L. Johnson, Plastic ratchetting as a mechanism of metallic wear, Proc. Roy. Soc. London A 445 (1994) 367–381. [12] A. Kapoor, Wear by plastic ratchetting, Wear 212 (1997) 119–130. [13] Y. Yang, A.A. Torrance, Wear by plastic ratchetting: an experimental evaluation, Wear 196 (1996) 147–155.
Effect of reciprocating and unidirectional sliding motion on the friction and wear of copper on steel Etsuo Marui∗ , Hiroki Endo Department of Mechanical and Systems Engineering, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu-shi 501-1193, Japan Received 10 May 2000; received in revised form 6 March 2001; accepted 8 May 2001
Abstract From the practical necessity to obtain the friction and wear characteristics of materials used in machinery, various types of wear-testing machines have been developed and used. To obtain useful data for practical application, it is desirable that the investigation is carried out by a full-scale wear-testing apparatus having approximately similar contact conditions. Generally speaking, the obtained wear characteristics are different for every wear-testing apparatus used. When the type of wear-testing machine is not suitable, one cannot obtain the required wear characteristics. There are various parameters in wear-testing machines, such as configuration of contact surface and form of the relative motion between the test specimens. Thus, in this report, the effect of relative motion between contacting pair and friction process on obtained test results is investigated, for the combination of a copper pin specimen and a flat steel specimen. The microscopic structure of the pin specimen may be varied due to the friction and wear process. An optical microscope is used for the observations. The pin specimen is made from pure copper, which can be deformed easily. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Wear; Pin-on-disc wear-testing machine; Pin-on-flat wear-testing machine; Optical microscopic observation; Unidirectional friction; Reciprocating friction
1. Introduction Since olden times, practical methods to reduce wear have been explored. Burwell [1] might have been the first researcher to explain scientifically wear phenomenon with complex properties. He studied wear from the standpoint of material science and tribology. It is important to estimate wear characteristics appropriately. However, it is rare for friction or wear characteristics of materials or machine elements to be estimated from a full-scale test. These characteristics are obtained in many cases from a simplified model test on a small scale. The friction or contact modes in such model apparatuses for estimation are as follows [2]: (A) pin (or ball)/disk (unidirectional friction type); (B) pin (or ball)/flat (reciprocating friction type); (C) cylinder/cylinder (contact with end surface); (D) pin/cylinder; (E) four-ball; and (F) two-parallel cylinder type. Test by type (E) machine is used for judging the lubricant ability, and type (F) machine is for machine elements of rolling contact. The decision as to which contact mode is superior to obtain the wear and friction ∗ Corresponding author. E-mail address: [email protected] (E. Marui).
characteristics of materials and machine elements, must be made from equivalence between a full-scale machine and reduced-scale model machine. There are three modes in practical contact surface, point contact, line contact and surface contact. The case in which pins with curved end surface or balls are used in (A) or (B), and the case (E) belong to point contact. Cases (D) and (F) are of line contact. The cases that a pin with flat end is used in (A) and (B) and case (C) are for surface contact. As mentioned above, there are many structure-types of wear-testing machine and contact mode. So in this research, the wear test is carried out using two different types of wear-testing machine in our laboratory. Contact modes are the same in both machines, however, the friction or relative motion modes are different. One is (a) unidirectional friction or continuous sliding type and the other (b) a reciprocating friction type [3]. The effect of relative motion on the friction and wear characteristics and the features of each machine are investigated from the experiment of flat/flat contact of a pin having a flat end (surface contact mode). Experiments are carried out on the friction pair of copper and steel for machine structure use. The effect of reciprocating friction in type (b) machine on wear characteristics of the copper pin is a particular focus of attention.
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2. Structure of friction pair (test specimens) Both tests of unidirectional friction and reciprocating friction are carried out using the same friction pairs of test specimens. The pin specimen is made from pure copper [4] (purity: 99.9%), which is frequently used in electric circuits. The mechanical strength of pure copper is low, and may deform easily, so the features of the wear-testing apparatus may be sensitively detected and used in our experiment. Vickers hardness number of pure copper is about 110. Its Young’s modulus is 113 GPa. The surface roughness (mean peak-to-valley height [5]) of the pin specimen at the contact surface is 5 m Rz before the wear test. The thermal conductivity of pure copper is large (393 W/m K). Plane specimen is made from carbon steel for machine structure use, whose carbon content is 0.47%. Its Vickers hardness number and Young’s modulus are 210 and 210 GPa, respectively. The contact surface of the flat specimen is finished by grinding, and its surface roughness before the wear test is about 4 m Rz . It will be desirable that the wear map [6] of pure copper sliding on the steel surface is constructed for the entire clarification of the friction and wear characteristics of pure copper. However, the main purpose of this paper is to discuss how the sliding mode of different wear-testing apparatus influences the friction and wear characteristics of pure
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copper sliding on the steel surface. Although experiments are carried out for wide experimental conditions that are possible in each wear-testing apparatus, the wear map is not constructed here. Experiment under the same experimental conditions for both wear-testing apparatus is carried out. In the following, characteristic experimental results obtained in both wear-testing apparatus are explained briefly.
3. Unidirectional friction wear test by pin-on-disk wear-testing machine 3.1. Structure of pin-on-disk wear-testing machine The structure of the pin-on-disk wear-testing machine in this experiment is shown in Fig. 1. In the figure, A is a flat specimen and B a pin specimen. A calibrated vertical load is exerted between them through a pneumatic pressure mechanism. This apparatus is the same as the one used in the previous report [7], so the details of the measurements are not repeated here. The static bending rigidity of the main spindle–pin specimen system is 800 kN/m, when a pure copper pin specimen 5 mm in diameter is set on the main spindle with a 10 mm overhang. The capable range of experimental conditions in this wear-testing machine is as follows. Maximum vertical load:
Fig. 1. Pin-on-disk wear-testing machine with unidirectional friction.
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Fig. 2. Variation of pin specimen length due to sliding distance (vertical load: 29.4 N).
98 N; continuously variable rotational speed of flat specimen: 25–1000 rpm. The effective contact radius between pin and flat specimen is 80 mm, and the sliding velocity between both specimens can be varied within about 0.1 and 4.2 m/s. In our wear-testing machine, the magnitude of wear can be obtained by monitoring continuously the pin specimen length and by weighing the mass of the pin specimen before and after the wear test. 3.2. Experimental procedure Wear measurement is carried out up to a sliding distance of 500 m, varying the vertical load and the sliding velocity under the room temperature condition.
The surface of the pin and the flat specimens are washed by acetone thoroughly, and wax adhering to the surfaces is removed. The worn mass of the pin specimen is measured by a precise electric-balance, and its minimum increment is 1 mg. 3.3. Experimental result and discussion Variation of the pin specimen length corresponding to the sliding distance is shown in Fig. 2. The sliding velocity is given in the figure. In the case of the small sliding velocity, the transition from the severe wear regime to the mild wear regime is recognized. The transition from the severe to the mild wear regime does not take place for a larger sliding velocity. The pin specimen shortens approximately in
Fig. 3. Relation between worn mass and sliding velocity (vertical load: 29.4 N).
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Fig. 4. Relation between surface roughness and sliding velocity.
proportion to the sliding distance. Corresponding to such rapid decrease of the pin specimen length, a large lip is formed behind the pin specimen. Details on this phenomenon will be discussed later. The correct worn mass of the pin specimen accompanied by a lip formation cannot be obtained from the change in pin specimen length. Then, the worn mass is weighed by means of a precise electric-balance, and the above-mentioned change in the pin specimen length is converted into the change in mass by multiplying the density of the copper, simultaneously. In brief, the former mass is called here a measured mass and the latter a converted mass. The relation between both masses and the sliding velocity is given in Fig. 3. The symbol (䊊) indicates a converted mass, and the symbol (䊉) a measured mass. For a larger sliding velocity, the difference between both masses becomes remarkable. The difference in masses corresponds to the mass of the lip. The measured mass decreases with the increase in sliding velocity, in the lip formation condition. The surface roughness change of the flat specimen corresponding to the sliding velocity is shown in Fig. 4, when sliding distance reaches 500 m. No characteristic dependence of the friction coefficient on the vertical load or the sliding velocity can be observed. The magnitude of the friction coefficient is within the range from 0.7 to 0.8.
4. Reciprocating friction wear test by pin-on-flat wear-testing machine 4.1. Structure of pin-on-flat wear-testing machine The structure of pin-on-flat wear-testing machine with reciprocating friction is shown in Fig. 5. Both flat and pin specimens are fixed to the reciprocating stage or to the pin
specimen holder by setting screws. The diameter of the pin specimen is 4 mm. Vertical load is applied by a weight, as seen in the figure. Applied vertical load is determined so that the mean contact pressure is equal to the value in the pin-on-disk wear-testing machine. Maximum sliding velocity obtainable in this machine is 11.06 mm/s, which is approximately equal to the minimum sliding velocity in pin-on-disk wear-testing machine. Pin specimen and flat specimen materials in this section are identical to the former pin-on-disk wear-testing machine. The static bending rigidity of the main spindle–pin specimen system (pure copper specimen diameter: 4 mm and its overhang length: 10 mm) with leftward loading is 28 kN/m, and that for rightward loading is 21 kN/m. Small difference is recognized according to the loading direction. 4.2. Experimental results and discussion The magnitudes of wear (worn mass) are shown in Fig. 6 corresponding to the sliding distance. The transition from severe wear regime to mild wear regime is not recognized here. The experimental condition of sliding velocity 11.06 mm/s is common to both testing machines. From the comparison of the wear rate calculated from worn mass of the pin specimen, it is clarified that the magnitude for the pin-on-flat wear-testing machine is slightly larger than for the pin-on-disk wear-testing machine. However, the difference is quite small and the feature of the wear-testing machines cannot be investigated. The surface roughness of the flat specimen becomes worse from 4 m Rz (before experiment) to 12 m Rz (after experiment). The deterioration in the surface roughness of the pin specimen is more remarkable as in the case of the pin-on-disk wear-testing machine, and the surface roughness of the pin specimen is not measured here.
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Fig. 5. Pin-on-flat wear-testing machine with reciprocating friction.
An example of the magnitude of friction coefficient monitored during the wear test by the pin-on-flat wear-testing machine is given in Fig. 7. This example is obtained in the most severe experimental condition of the vertical load 18.6 N and the sliding velocity 11.06 mm/s. In the figure, the maximum and minimum values of the friction coefficient corresponding to the right- and leftward motion of the stage carrying the flat specimen are shown. The friction coefficient in the rightward motion is slightly larger than in the leftward
motion. As seen in the measurement of the static rigidity of the main spindle–pin specimen system, the rigidity of the system is different corresponding to the loading direction, and the deflection or inclination of the pin specimen is also different from the sliding direction of the flat specimen. This is reflected in the measured friction coefficient. Until we saw the experimental result, we thought the formation of the lip on the pin specimen could not be observed, since the sliding direction is always reciprocating in the
Fig. 6. Increase of worn mass due to sliding distance (vertical load: 18.6 N).
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5. Observation of pin specimen microscopic structure by optical microscope 5.1. Observation method
Fig. 7. Variation of friction coefficient due to sliding distance (vertical load: 18.6 N).
pin-on-flat wear-testing machine with reciprocating friction. Seeing the worn pin specimen, we noticed that a small lip is formed on the end of the pin specimen in the pin-on-flat wear-testing machine with reciprocating friction, too. This may be related to the fact that the friction force may tilt the pin specimen during sliding, and a unidirectional sliding or friction condition is realized at the end of the pin specimen as in the experiment by pin-on-disk wear-testing machine. To indicate the lip formation quantitatively, the worn mass obtained from the pin specimen weighing is compared with the worn mass converted from the decrease of the pin specimen length as in the section above. The result is given in Fig. 8. In this figure, the results for the condition of the vertical load 15.7 N are compared. The worn mass obtained from the decrease in the pin specimen length is somewhat large, and a small lip formation is recognized.
Fig. 8. Relation between worn mass and sliding velocity (vertical load: 15.7 N).
In the previous experiments, the lip formation parallel to the sliding direction is recognized in some experimental conditions. To clarify the formation mechanism of the lip, and to examine the effect of the difference in sliding modes on the experimental results, the microscopic structure of the pin specimen is observed by optical microscopy. The pin specimen made of pure copper is cut in a determined direction by a fine cutter. The surface is then buffed to a mirror finish. After that, etching treatment is performed on the surface, and the microscopic structure of the pin specimen is observed by means of an optical microscope at an adequate magnification. The etching agent is a mixture of 30% liquid ammonia, 3% oxygenated water and distilled water. The mixing ratio is 1:1:1. Etching time is 2 min. 5.2. Observation in pin-on-disk wear-testing machine An example of the small lip growth (sliding velocity 0.1 m/s) and an example of large lip growth (sliding velocity 0.7 m/s) are treated here. In both examples, the vertical load is set exceptionally large at 24.5 N. The sliding distance is 500 m. The magnification of the photographs is given in the following figures. Fig. 9 is a photograph showing the microscopic structure of the cross-section in parallel to the disk rotation at the pin specimen center, when the lip growth is small. Some plastic deformation zone stretched in the direction of the disk rotation is recognized on the whole contact surface of the pin specimen. On the opposite end, no lip formation is seen. Fig. 10 shows the microscopic structure of the same cross-section, when the sliding velocity is large enough to
Fig. 9. Optical micrograph of pin specimen when cut by a plane parallel to the disk specimen rotation (pin-on-disk wear-testing machine, sliding velocity: 0.1 m/s).
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Fig. 11. Optical micrograph of pin specimen when cut by a plane perpendicular to the disk specimen rotation (pin-on-disk wear-testing machine, sliding velocity: 0.7 m/s). Fig. 10. Optical micrograph of pin specimen when cut by a plane parallel to the disk specimen rotation (pin-on-disk wear-testing machine, sliding velocity: 0.7 m/s).
sliding, the growth of the lip continues, and the lip is layered up as shown in Fig. 10. 5.3. Observation in pin-on-flat wear-testing machine
form a large lip. The lip develops approximately in the radial direction from the joint. In this case, the depth of the plastic deformation zone at the contact surface is about 0.05 mm. Fig. 11 is a photograph showing the cross-section perpendicular to the disk rotation. Corresponding to Fig. 10, it is seen that the lip grows in stratified construction. From the above observations, the growing process of the lip in pin-on-disk wear-testing machine is considered to be as follows: when sliding is started, a small burr is formed around the pin specimen. Burr in the disk rotation is little large. For longer sliding distance, softening of the pin specimen may occur due to the friction heating and the burr is stretched into the direction of disk rotation by some ratchetting mechanism. At last, a lip is formed. By successive
The same observation of the pin specimen is carried out in the case of pin-on-flat wear-testing machine with reciprocating friction. Fig. 12 is a photograph at the sliding velocity of 11.06 mm/s and vertical load of 18.6 N. The sliding distance is 200 m. Figure (a) is at the left end of the flat specimen stroke and (b) at the right end of the flat specimen stroke. In both cases, a small lip is growing at the left and the right ends. Differing from the pin-on-disk wear-testing machine, the layered microscopic structure of the lip is discontinuous from the microscopic structure of the original pin specimen, as seen in these photographs. In some cases, the lip is separated from the pin specimen. From these, there is a possibility that the lip is formed by re-adhering
Fig. 12. Optical micrograph of pin specimen when cut by a plane parallel to the flat specimen movement (pin-on-flat wear-testing machine with reciprocating friction, vertical load: 18.6 N): (a) left end; (b) right end.
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Fig. 13. Optical micrograph of pin specimen when cut by a plane perpendicular to the flat specimen movement (pin-on-flat wear-testing machine with reciprocating friction, vertical load: 18.6 N): (a) left side; (b) right side.
of the separated wear debris to the pin specimen, due to the reciprocation of the flat specimen motion. Fig. 13 shows the microscopic structure of the pin specimen cut by a plane perpendicular to the direction of the flat specimen reciprocation. On both the right and left sides of the pin specimen, the lip formation with continuous layered structure to the original pin specimen is recognized. From these observations, the lip in the pin-on-flat wear-testing machine with reciprocating friction may be formed due to the following process. Small lips are formed at the front and rear ends of the pin specimen, corresponding to the reciprocating flat specimen motion when the sliding started. After that, the lip spreads over the whole circumference of the pin specimen. However, the motion of the flat specimen is always reciprocating, so the lip growth in the sliding direction is suppressed. Passing through such processes, the lip grows perpendicular to the flat specimen movement (Fig. 13). It may be considered that the difference in the wear mechanism of the pin-on-disk wear-testing machine from the pin-on-flat wear-testing machine with reciprocating friction exists in such a point. Above-mentioned unique deformation of the pin specimen may be influenced by the contact temperature rise due to friction heating and the plastic ratchetting. A consideration from these aspects is given in the following chapter. 6. Considerations 6.1. Temperature rise at contact surface due to friction heating Temperature at the contact surface is a main factor influencing the result of wear test. So, the temperature at the contact surface between the pin specimen and disk specimen or between the pin specimen and flat specimen is estimated here. There are many researches [8,9] that estimated the temperature rise due to friction heating at the contact surface. A common fundamental idea for the temperature rise
estimation is that the work done against the friction force will be liberated as heat and this heat will flow into the pin specimen and flat specimen in some ratios. As a result, the temperature rise of the specimens is induced. Friction heat flowing into the pin specimen is carried away from the contact surface by conduction and is emitted into surrounding air. Bowden and Tabor [10] presented an equation to calculate temperature at the contact surface between an infinitely long pin specimen and a flat specimen, where Newton’s law of cooling is introduced. Estimation result of the temperature at the contact surface may differ owing to the equation for temperature estimation. However, a qualitative feature of the temperature estimated is not seriously influenced by the equation used. Here, the equation by Bowden and Tabor is used in our estimation of temperature rise at the contact surface. Physical meaning of their equation is clear and the temperature is easily calculated using small number of characteristic values, although the effect of surface roughness and the mass of pin specimen holder are not considered in their equation. This equation is written as follows: 1 αµNv T = T0 + π 2σ kr where T is the temperature at contact surface, T0 the room temperature, N the normal load, v the sliding velocity, r the radius of the pin specimen, µ the kinematic friction coefficient, k the thermal conductivity, σ the cooling coefficient of Newton’s law of cooling and α is the certain fraction of friction heat going into the pin specimen. Then, the temperature rise due to friction heating is T − T0 . Here, the temperature rise of the pin specimen is estimated by the above equation, within the experimental conditions treated in this paper. Following parameter values are used in the estimation: the fraction α is set at 0.5 by referring the account by Bowden and Tabor thermal conductivity k and cooling coefficient σ of copper are k = 393 W/m K, σ = 40 W/m2 K, respectively. The magnitude of the cooling coefficient is determined
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by referring to the value of constantan (which is an alloy of copper and nickel) cited by Bowden and Tabor [10]. Radii of the pin specimen in the pin-on-disk wear-testing machine and the pin-on-flat wear-testing machine are 2.5 and 2 mm, respectively. The magnitude of the kinematic friction coefficient µ is determined referring to the experimental results. The magnitudes of vertical load and sliding velocity are settled as the experimental conditions. The maximum and minimum temperature rises in the pin-on-disk wear-testing machine are estimated as 155 and 10◦ C, respectively. Those in the pin-on-flat wear-testing machine are 15 and 5◦ C. Such large difference in estimated temperature rise corresponding to the types of wear-testing machine is induced by the difference of attainable sliding velocity. It is quite natural that the estimated temperature rise is equivalent regardless of the type of wear-testing machine, when the vertical load and the sliding velocity are equivalent each other. A higher estimation of temperature rise at contact surface is obtained when a large magnitude of the fraction α is used in the estimation. From the above consideration on the temperature rise at contact surface, the influence of friction heating on the wear of copper is negligible, except the case of large vertical load and large sliding velocity in the pin-on-disk wear-testing machine. 6.2. Lip formation by plastic ratchetting Kapoor and co-workers [11,12] stated that material deformation proceeds by a plastic ratchetting mechanism, when cyclic stress beyond the plastic shakedown limit is applied. Let us consider the case that a thin surface layer is subjected to a steady compressive stress σ zz together with an alternating orthogonal shear stress ±τ xz . These stresses satisfy the von-Mises yield criterion twice a stress cycle on σ –τ plane. The plastic strain increments arising from the cyclic shear are equal and opposite, so that no accumulation of shear strain occurs here. However, the increments of plastic compression are always in the same direction, then the material accumulates unidirectional increments of compression within each stress cycle. This phenomenon is called a plastic ratchetting. As a result of plastic ratchetting, a filmy layered deformation orthogonal to the sliding direction (frictional traction) arises in the material and a lip is built up. When this lip is dropped, that phenomenon is a wear by plastic ratchetting. Yang and Torrance [13] considered the wear of metal by plastic ratchetting, too. When an orthogonal siding to the asperity ridge is applied, the extrusion in the sliding direction is prevented, but the lateral extrusion by plastic ratchetting is possible. From the above-mentioned point of view, the lip formation in the pin-on-disk and pin-on-flat wear-testing machine can be discussed in detail.
6.3. Consideration on wear test of copper sliding on steel flat In the first place, results by the pin-on-disk wear-testing machine are discussed. The disk specimen rotates at a steady rotational speed. A shearing stress in the disk rotation direction acts on the pin specimen. The pin specimen yields under the combined action of this shearing stress and contact pressure. Small shear deformation in the direction of disk rotation arises in the pin specimen. New stress field acts on the pin specimen. In this new stress condition, an additional shear deformation of the pin specimen arises in the direction of disk rotation and is accumulated on the first deformation. Such a process continues during the wear test and the lip is grown up in the rear end of the pin specimen. So, the lip has a layered structure. This is ascertained in a photograph of cross-section of the pin specimen, which is perpendicular to the disk rotation or sliding velocity. A lip to the orthogonal direction to disk rotation is an accumulation of the deformation by contact pressure acting on the pin specimen. This lip is smaller than the lip at the rear end of the pin specimen. In the next place, results by the pin-on-flat wear-testing machine are discussed. Above-mentioned lip formation by the accumulation of shear deformation cannot happen, in ideal case that only the direction of shear force is reversed accompanying by the sliding direction reciprocating. Lip into the orthogonal direction to the sliding may be formed. However, in the experiment, a small lip formation parallel to the sliding is recognized. The static bending rigidity of the cantilever type main spindle–pin specimen system is quite small as mentioned above. So, the pin specimen holder deflects. The direction of this deflection is reversed according to the sliding velocity. Following this, contact position between the pin specimen and the flat specimen always deviates to the rear ends of the pin specimen. Both ends of the pin specimen contact the mating flat specimen in only one way of rightwards or leftwards during each reciprocating stroke. Relative movement between the pin specimen and the flat specimen is reversed, however, direction of friction force acting on each end does not change. As a result, the contact state at the end of the pin specimen is equivalent to that of the pin specimen in the pin-on-disk wear-testing machine. Lip recognized in the pin-on-flat wear-testing machine is formed by the similar mechanism at the pin-on-disk wear-testing machine. Contact pressure is settled so as to be equivalent in both types of the wear-testing machine. Owing to the reason in machine structure, sliding velocity attainable in the pin-on-flat wear-testing machine is not so high and the influence of friction heating may be small compared with the pin-on-disk wear-testing machine. The size of formed lip is also small. If the main spindle–pin specimen system is fully rigid, the shearing force is reversed corresponding to the reversal of sliding velocity. The accumulation of deformation by sliding friction disappears and the lip formation
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parallel to the sliding velocity cannot be recognized. Only a small lip may be formed in the orthogonal direction to the sliding velocity.
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the pin-on-flat wear-testing machine with reciprocating friction. References
7. Concluding remarks The influence of the friction mode on the wear regime is examined by a wear experiment using two types of wear-testing machines, a pin-on-disk wear-testing machine with unidirectional friction and a pin-on-flat wear-testing machine with reciprocation motion. As a result, a small difference is recognized in the wear rate. However, no fundamental effects on the wear characteristics of friction modes, such as unidirectional sliding or reciprocating sliding, are recognized. As the pin specimen is made from pure copper, which can be deformed easily, the lip is formed at the rear end of the pin specimen. The microscopic structure of this lip is observed by means of an optical microscope. Then, following points are clarified: formation of the lip is remarkable in the pin-on-disk wear-testing machine with unidirectional friction, and there are some differences in the lip formation mechanism by two types of wear-testing machines. Detailed considerations on the lip formation are carried out from the point of the friction heating and the plastic ratchetting mechanism. There is a weak point in the machine structure that sufficiently large sliding velocity is difficult to realize in
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