Different aspects of the role of wear debris in fretting wear

Transcription

1 Wear 252 (2002) Different aspects of the role of wear debris in fretting wear M. Varenberg, G. Halperin, I. Etsion Department of Mechanical Engineering, Technion, Haifa 32000, Israel Received 17 August 2001; received in revised form 11 February 2002; accepted 12 March 2002 Abstract Two different aspects of the role of oxide wear debris in fretting wear are studied by allowing them to escape from the interface during sliding. This is accomplished by laser surface texturing that forms regular micro-pores topography on the friction surfaces which enables this escape. It is found that the role of oxide wear debris depends on the dominant fretting wear mechanism. Their presence in the interface protects the friction surfaces when the dominant wear mechanism is adhesive and harms the friction surfaces when this mechanism is abrasive. The escape of oxide wear debris into the micro-pores results in up to 84% reduction in the electrical contact resistance of the textured fretting surfaces Published by Elsevier Science B.V. Keywords: Fretting wear; Wear debris; Wear mechanism; Laser surface texturing 1. Introduction Fretting is a relative cyclic motion with small amplitude which occurs between two oscillating surfaces [1]. Depending on loading conditions, material properties and environment, fretting can cause fretting wear, which combines all four basic wear mechanisms (oxidative, adhesive, surface fatigue and abrasive) [2], or fretting fatigue, which combines surface and bulk fatigue. These two forms of damage can arise in any assembly of engineering components if a source of vibration is present [3]. They have been identified as two of the plagues of modern machinery since they significantly reduce the life of machine elements. They are encountered in all quasi-static loaded assemblies such as keys, cables, conduits, lugs, bearing races and shafts, orthopedic implants, cranes, turbine blade roots, electrical contacts, nuclear reactor components, power plant machinery, etc. [4]. Fretting wear has the characteristic of forming (by any one of the basic wear mechanisms) considerable amounts of very fine oxide debris. Their behavior is a significant factor in governing fretting wear [5]. There is a strong relation between wear particles presence in the contact zone and surface roughness, i.e. on a rough surface debris can escape from areas of contact into adjacent hollows on the surface [1]. Depending on the ease of wear debris escape from the contact regions, fretting can either increase the separation or lead to seizure of the fretting surfaces [6]. Corresponding author. Tel.: ; fax: address: etsion@tx.technion.ac.il (I. Etsion). The role played by wear particles presence at the contact zone has been discussed (in direct or indirect form) in several papers [4,5,7 13] and two opposite effects, of either reducing or increasing the wear, were identified. It was shown that keeping oxide debris in the interface by better surface finish or by directly supplying them into the contact may have either a beneficial effect [4,7 9] or a harmful one [1,10,11]. The two opposite effects were also observed depending on materials selection [12], loading conditions [5] and blowout (with different frequencies) of wear debris from the contact zone [13]. The purpose of the present work, in light of the previous uncertainty, is to study various aspects of the role of oxide wear debris presence during fretting. This will be accomplished by on-line monitoring of the contact response and friction parameters behavior while allowing the escape of wear particles into the micro-pores of a laser-textured surface. 2. Test details 2.1. Test apparatus Two types of test devices are usually used in the study of fretting: (1) devices that form single-point or line contact (ball-on-flat, cylinder-on-flat); (2) devices designed to produce vibration in the contact of plane surfaces. Both types have a number of merits and shortcomings [1,14]. Therefore, in order to use the merits of both schemes of contact, /02/$ see front matter 2002 Published by Elsevier Science B.V. PII: S (02)

2 M. Varenberg et al. / Wear 252 (2002) Fig. 1. Schematic diagram of the experimental apparatus. a new experimental device that allows investigation of the fretting phenomenon in both flat-on-flat and ball-on-flat contacts was designed and built. A schematic diagram of the experimental apparatus constituting the flat-on-flat scheme is illustrated in Fig. 1. A crank drive (1), containing a motor, eccentric with variable eccentricity and connecting rod, transforms the rotary motion of the motor shaft into a linear cyclic motion of a moving table. Two rotary degrees of freedom self-aligning joint (2) carrying a moving specimen (3) is located on the moving table. The conditions required for fretting in the contact zone consist of loading a quasi-static specimen (4) against the moving one by the weight (6). The self-aligning joint axes are coplanar with the friction plane of the two specimens. Hence, the friction forces acting in this plane do not produce moments about the joint axes and full contact is always retained between the friction surfaces. All the system s joints are specially designed to eliminate unwanted clearances. The amplitude of vibration is defined by the variable eccentricity, but is also affected by the normal load (through the friction force) and by the system s stiffness. The tangential (friction) force between the specimens is measured by a force transducer (7). This force transducer that carries the quasi-static specimen is mounted on a hinged arm, which allows lifting and lowering of the quasi-static specimen. The loading is transmitted to the quasi-static specimen via a self-aligning ball bearing (8), thus, preventing any tangential load component. The loading arm is balanced by a counterweight (9). An eddy current proximity probe mounted on the quasi-static specimen holder is used to measure the relative displacement of the moving specimen. The electrical resistance of the contact, R, which is affected by the presence of oxide wear debris, is recorded on-line. This is accomplished by transmitting a constant current, I, through the specimens, and measuring the voltage drop, V, between them at two points near the contact zone. The tested specimens consist of two rings having 38 mm outer diameter, 26 mm inner diameter and 10 mm height. The moving specimen ring (see Fig. 2) has three protruded legs milled from the ring body at an angle of 120 apart. Each leg has a square flat top surface destined for the laser surface treatment and the total area of all three legs is 100 mm 2.For Fig. 2. Schematic diagram of the surface treatment on the lower (moving) specimen. Typical pore diameter is m.

3 904 M. Varenberg et al. / Wear 252 (2002) every new test, a new moving specimen mounted always in the same position is used, while the quasi-static specimen is turned around its axis by 30 in order to fret a new surface. Converting of the experimental apparatus into a ball-on-flat scheme is achieved by replacing the quasi-static ring holder with a ball holder, taking the self-aligning joint out and fastening the moving specimen holder directly on the moving table. In this case, a ring without the legs is used as the lower specimen. Both the ball and the ring specimen can be used for a number of tests by turning them in their holders. Adjustment for the different mass of the hinged arm is made by the counterweight (5) Measurements and data processing The present experimental device permits on-line monitoring and measuring of the relative contact displacement, the tangential (friction) force and the electrical contact resistance as functions of time, using a special program, which was developed in the LabVIEW software package. A 12-bit analog-to-digital converter of multifunctional I/O board Lab-PC-1200, used for data acquisition and control, samples the output voltage signals from the respective probes. The sampling is performed with a multiplex technique at a rate much higher than the fretting frequency, resulting in synchronous acquisition of the measured signals. The function used for the data sampling makes continuous, time-sampled measurements of four channels, stores the data in a circular buffer, and returns a specified number of scan measurements on each call of this function. The digitized data received from each channel is calibrated and low pass filtered to eliminate high frequency noise. A force displacement hysteresis loop, which characterizes operating fretting regime [15], is obtained by plotting the tangential force versus the contact relative displacement during a complete fretting cycle. The area enclosed by this loop represents the friction energy that was dissipated in the contact during this cycle [16,17]. Averaging and analyzing of scan measurements returned on each call of the sampling function give the evolution of the contact response during the test. A data chart shows time changes in the relative displacement amplitude, responding tangential force, electrical contact resistance, dissipated energy per fretting cycle, cumulated dissipated energy and fretting distance. A three-dimensional friction log, in which the force displacement hysteresis loop is recorded as a function of the number of cycles, is also presented Experimental conditions and procedure Two different tribo-pairs were used for the flat-on-flat scheme. The upper (quasi-static) specimen in the first tribopair was made of phosphate bronze C51100 with hardness of HRB. The lower (moving) one was made of 4140 steel hardened to HRC. In the second tribo-pair both the lower and upper specimens were of the same 4140 steel. The mating surfaces of the specimens were super-polished to a roughness average of m. Several lower specimens were laser-textured and then super-polished again to remove the bulges which are formed on the micro-pores rims. After this process, the pores diameter was mm, the pores depth was m, and the pores area density was 42 47%. Schematic diagram of this surface treatment is illustrated in Fig. 2. The experiments were carried out under imposed displacement amplitude, A, of145 m, normal load, N, of5,10 and 25 N and a frequency, f, of 3 Hz. Each test consisted of 20,000 cycles of which the first 300 were required to reach steady state conditions. Only the steady state portion of the test was analyzed. The temperature and relative humidity in the laboratory were 21 ± 2 C and 45 ± 5%, respectively. The data were sampled at a rate of 600 Hz resulting in 200 data points over a full fretting cycle. The 1200 data points (six cycles) were returned from the circular buffer and then analyzed for each call of the sampling function. The cut-off frequency of the low pass filter was 30 Hz. At least, two tests were performed for each combination of experimental parameters. Before each test both specimens were ultrasonically cleaned with ethyl alcohol 95%. After being mounted on the holders, the specimens were cleaned again with glass cleaning paper, brought precisely into the contact and loaded. After each test, the specimens were carefully disassembled, examined by optical microscopy, and ultrasonically cleaned to remove loose wear particles. The fretted surfaces were again examined by optical microscopy to identify the dominant wear mechanism and observe the surface damage. The damaged surfaces were also analyzed by two-dimensional profilometry Damage assessment methodology Recently published research [17] confirmed that wear tends to be linearly related to the cumulated dissipated energy. Hence, in the present work, a comparison of the dissipated energy values was used for evaluating the effect of the surface micro-topography on the wear volume. In order to verify the use of this method for the present test conditions, a special experiment was performed with the ball-on-flat contact scheme. This allows fast and simple wear volume estimation by two-dimensional profilometry using the model of semi-ellipsoid wear scar volume [17]. The same tribo-pairs materials as for the flat-on-flat scheme were used for verifying the wear estimation method. A standard 5 mm diameter bearing steel ball was fretted against bronze and steel test rings. The tests were carried out under the same conditions of the imposed displacement amplitude, normal loads and frequency. The test duration was 10,000 and 20,000 cycles. The mating surfaces of the specimens were not laser-textured and the roughness average was m.

4 M. Varenberg et al. / Wear 252 (2002) Fig. 3. Wear volume as a function of cumulated dissipated energy in a ball-on-flat scheme. 3. Results and discussion 3.1. Ball-on-flat contact scheme Wear volume and dissipated energy The results presented in Fig. 3 validate the linear relation between the wear volume and the cumulated dissipated energy. These results were obtained with the two ball-on-flat tribo-pairs under test conditions identical to these intended for the flat-on-flat scheme. Hence, similar linear relation between wear and cumulated dissipated energy can be considered in the flat-on-flat contact tests under the same wear process. As can be seen the measurement error for the steel-onbronze contact is larger than that for the steel-on-steel. This can be attributed to the different hardness of steel and bronze. The harder steel is less sensitive to crushing, which occurs as soon as the steel ball is loaded against the flat even before fretting starts. The crushing affect adds erroneous volume to the actual fretting wear volume, and thus, results in a larger measured volume error in the softer bronze. Note that crushing effect is not feasible in the flat-on flat scheme due to larger contact area and, hence, the linear relation between measured wear volume and cumulated dissipated energy is expected to be more accurate Contact response and surface damage Fig. 4 presents the time variations of the tangential force and the electrical contact resistance. The fluctuations in these two variables are due to wear debris formation and escape from the contact zone. It can be clearly seen from the chart that for the steel-on-steel tribo-pair with 23 N load these two variables change out of phase, i.e. an electrical resistance reduction is accompanied by a tangential force increase and vice versa. This phenomenon is indicative of a dominant adhesive mechanism of fretting wear during the recorded period. Low electrical resistance is the result of wear debris escaping from the contact zone. This allows for smaller separation between the friction surfaces and leads to increased adhesion interaction and higher friction. High Fig. 4. Dominant adhesive wear mechanism. Steel-on-steel contact, ball-on-flat scheme, untextured surfaces, N = 23 N.

5 906 M. Varenberg et al. / Wear 252 (2002) Fig. 5. Dominant abrasive wear mechanism. Steel-on-bronze contact, ball-on-flat scheme, untextured surfaces, N = 10 N. electrical resistance accompanied by tangential force reduction is the result of an increase of the wear debris amount in the contact. The wear debris separate the friction surfaces and, if the dominant wear mechanism is adhesive, the friction is reduced. Consequently, an electrical resistance reduction accompanied by a tangential force increase and vice versa proves the adhesive mechanism dominance. Fig. 5 presents the time variation of the tangential force and electrical contact resistance for the steel-on-bronze tribo-pair. For this material combination at 10 N load, the two variables change in phase, i.e. an electrical resistance reduction is accompanied by a tangential force reduction and vice versa. This phenomenon points out that abrasive mechanism of fretting wear is dominant. In this case, the escape of wear debris from the contact zone, which reduces electrical resistance, also reduces the tangential force whereas an increasing amount of wear debris participating in the friction process leads to increasing tangential (friction) force. Hence, abrasive action of wear debris is more dominant than potential protection against adhesion that could be afforded by their presence in the contact. Fig. 6 shows a portion of the wear scar region in the steel-on-steel contact under 25 N load. The bright zones in the upper right side of the picture (corresponding to regions that are elevated above the original surface plane) show marks of material transfer. The surface hollow shown as the dark zone in the lower left side of the picture marks material detachment resulting from seizure. This surface damage appearance is typical when the adhesive mechanism of fretting wear is dominant. Fig. 7 shows a portion of the wear scar region in the bronze-on-steel contact under 25 N. The marks on the upper side of the picture (randomly crossed grooves) characterize undamaged original surface. The damaged zone on the lower side of the picture is characterized by clearly oriented grooves resulting from an abrasive dominant fretting wear mechanism. The dark spots mark the oxide debris conglomerates. It can be concluded, from the discussion earlier, that different wear mechanisms may play the dominant role in Fig. 6. An optical microscope photograph showing a portion of the wear scar region. Steel-on-steel contact, ball-on-flat scheme, N = 25 N. Bright zones correspond to material transfer and dark spot to material detachment due to seizure. Fig. 7. An optical microscope photograph showing a portion of the wear scar region. Bronze-on-steel contact, ball-on-flat, N = 25 N. Lower part of picture presents oriented grooves resulting from abrasive mechanism on the flat bronze.

6 M. Varenberg et al. / Wear 252 (2002) Fig. 8. Contact electrical resistance in steel-on-steel contact without (a) and with (b) pores, N = 10 N. fretting wear depending on material combination and operating conditions. The dominant mechanism can be determined from on-line observation of time variation of the tangential (friction) force and the electrical contact resistance as well as from posttest examination of damaged surfaces Flat-on-flat contact scheme Electrical contact resistance and pores filling In the steel-on-steel tribo-pair, changing the load in the range from 5 to 25 N caused only about 3% difference in the mean contact resistance for the untextured case. The insensitivity of the mean electrical resistance to the normal load is probably due to formation of a thick enough insulating layer of oxide wear debris. This insulating layer remains intact having about the same thickness for the entire load range and resulting in a constant mean electrical resistance regardless of the normal load. The differences in the mean electrical resistance for the textured case were about 32% with a reduction in the resistance as the load increases. In this case, the insulating layer is either much thinner or does not exist. Large variation in the contact resistance with respect to its mean value was observed in both the textured and untextured cases as shown in Fig. 8. This is probably due to the stochastic nature of wear debris formation and escape from the contact zone. From a t-test [18], it was found that the dependence of the contact resistance on the load is statistically insignificant. Similar behavior with somewhat different percentage changes was observed in the bronze-on-steel tribo-pair. The representing averages of the mean electrical contact resistance for the three loads are shown in Fig. 9, which presents the effect of the surface texturing over the test duration for the two tribo-pairs. It can be clearly seen that the surface treatment has a dramatic effect of reducing the mean electrical contact resistance by 69% in the steel-on-steel case and by 84% in the bronze-on-steel case. Similar effect was reported in [19], where escape of wear debris from the contact zone caused significant reduction in the electrical contact resistance. During fretting, a layer of oxide wear debris, with high electrical resistance, builds up and separates the mating surfaces. This process increases the electrical contact resistance. When wear particles can escape from the contact zone into adjacent pores, the insulating layer is much thinner or does not form at all. The interface of the friction surfaces remains free of contamination and the electrical resistance is low. The differences in the electrical contact resistance between the two tribo-pairs result from different specific resistance, Fig. 9. The effect of a surface regular micro-topography on mean contact electrical resistance.

7 908 M. Varenberg et al. / Wear 252 (2002) Fig. 10. Schematic description of pore filling mechanism. Filling starts from top, stage (1) toward center and bottom, stages (2) to (4). which in the case of bronze is two to seven times smaller than that of steel. The mechanism by which wear debris fills the pores is shown schematically in Fig. 10, where the process is developed from stages (1) to (4). The pore filling occurs from the topside of the pore surface toward its center and bottom. New wear debris from the outside are dragged into the pore space and, because of the external normal load, push the previously arrived wear debris inside. This mechanism of pores filling may be explained by the tendency of wear particles to gather into compacted conglomerates. Stage (4) was confirmed by the fact that after the end of each test, when pores appeared to be completely full, it was possible to push the pore contents downwards. Stages (2) and (3) were observed when tests were stopped early. In these cases, most of the pores appeared like the pore shown in Fig. 11. As can be seen, all the debris is attached to the pore surface close to its circumference while the pore bottom (the black area in the middle of the pore) is still exposed Dimensionless cumulated dissipated energy (coefficient of friction) The frictional work or dissipated energy, E d,isgiven by Eq. (1), where F t is the momentary tangential (friction) force, and L the overall sliding distance. This energy can be normalized according to Eq. (2) to yield the parameter µ which is the cumulated dissipated energy of friction per Fig. 11. Optical microscope photograph of a partially full pore. unit sliding distance and per unit normal load F n. From Eqs. (1) and (2), it appears that µ actually represents an average fretting coefficient of friction: E d = F t dl (1) L µ = E d LF n (2) Figs. 12 and 13 present the effect of the surface texturing, or wear debris presence in the contact zone, on the dimensionless cumulated dissipated energy, µ (the average coefficient of friction). In the steel-on-steel contact (Fig. 12), two opposite effects can be seen over the entire load range. The escape of wear debris from the contact zone into the micro-pores at normal load of 5 N reduces the dissipated energy, and hence, the wear (according to the relation shown in Fig. 3) compared to the untextured case. This result confirms the observation, made in [1,11,12,14], that the presence of wear debris in a contact accelerates wear. In such cases, the abrasive action of wear particles is the dominant mechanism, causing the main damage to the fretted surfaces. In contrast, the escaping of wear debris from the contact zone under the normal loads of 10 and 25 N increases the dissipated energy (see Fig. 12) and, hence, the wear. This confirms the other observation, in [4,7 9,13], that protection afforded by wear Fig. 12. The effect of surface regular micro-topography on the dimensionless cumulated dissipated energy in the steel-on-steel contact.

8 M. Varenberg et al. / Wear 252 (2002) Fig. 13. The effect of surface regular micro-topography on the dimensionless cumulated dissipated energy in the bronze-on-steel contact. debris against adhesion is greater than the damage caused by their presence. In these cases, adhesion is the expected dominant fretting wear mechanism. It seems, therefore, that in the steel-on-steel case, the dominant fretting wear mechanism depends on the load level. Fig. 13, for the bronze-on-steel contact, shows that escape of wear debris from the contact zone into the micro-pores reduced the dissipated energy over the entire load range. This result supports the hypothesis that because of weak adhesive interaction between bronze and steel the dominant mechanism in this case is abrasive. Hence, removing abrasive wear debris from the contact zone reduces the damage. As can be seen from Figs. 12 and 13, the dimensionless cumulated dissipated energy decreases with increasing normal load in the two tested tribo-pairs for all test conditions. Because of the finite stiffness of the test rig a reduction of the fretting amplitude is associated with increasing friction due to increasing load. Hence, the rate of increase in E d (see Eq. (1)), in the present tests, is smaller than the rate of load increase. For this reason, the dimensionless cumulated dissipated energy, µ, decreases with an increasing load. As shown in Figs. 12 and 13, the dimensionless cumulated dissipated energy for a given normal load is always higher in the bronze-on-steel than in the steel-on-steel case. Steel oxide debris is approximately 30 times harder than bronze [3]. Hence, harder steel oxide particles can be embedded in the softer bronze surface and cause more damage, by plowing the mating steel surface, compared to the damage caused by loose particles in the steel-on-steel case. The statistical meaning of the reported results was examined by variance analysis of the large amount of data received from the two groups of textured and untextured specimens. Using the t-test [18], and assuming the two groups of results to be approximately normal with unknown but equal variances we concluded at the 0.95 confidence level that these groups do not belong to the same population. Thus, the effect of the surface regular micro-topography on the dimensionless cumulated dissipated energy that varies between 3% and 7% is statistically significant. 4. Conclusion Different aspects of the role of oxide wear debris in fretting wear were experimentally investigated. This was accomplished by providing for wear debris removal from the contact zone into laser-textured micro-pores. The main findings of the investigation are given as follows. 1. The role of oxide wear debris depends on the dominant wear mechanism of fretting wear. 2. With dominant adhesive wear mechanism the oxide wear particles act like a solid lubricant to reduce the damage caused by fretting. 3. When the abrasive mechanism is dominant the oxide wear particles facilitate the wear rather than protect against it. 4. Surface regular micro-topography enables the removal of oxide wear debris from the contact zone and, therefore, affects the damage caused by fretting according to the dominant wear mechanism. 5. Surface regular micro-topography significantly improves the electrical conductivity under fretting conditions. Acknowledgements This research was partially supported by the German Israeli Project Cooperation (DIP) and by the Center for Absorption in Science, Ministry of Immigrants Absorption the State of Israel. Help by Surface Technologies Ltd., in preparation of laser-textured specimens and by Dr. G. Ryk in useful discussions is gratefully acknowledged. References [1] R.B. Waterhouse, Fretting Corrosion, Pergamon Press, Oxford, [2] H. Czichos, Tribology A System Approach to the Science and Technology of Friction, Lubrication and Wear, Elsevier, Amsterdam, [3] P.L. Hurricks, The mechanism of fretting a review, Wear 15 (1970)

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