An Experimental Study on the Effect of Machining-Induced White ...

Tribology Transactions, 53: 127-136, 2010 C Society of Tribologists and Lubrication Engineers Copyright ISSN: 1040-2004 print / 1547-397X online DOI: 10.1080/10402000903283250

An Experimental Study on the Effect of Machining-Induced White Layer on Frictional and Wear Performance at Dry and Lubricated Sliding Contact

Downloaded By: [Guo, Y. B.] At: 18:03 31 December 2009

Y.B. GUO and R.A. WAIKAR Department of Mechanical Engineering University of Alabama, Tuscaloosa, Alabama 35487, USA

The frictional and wear performance of a machined component depends strongly on the surface properties. A white layer on the machining surface is often produced at abusive machining conditions. However, the effect of white layer on frictional and wear performance has received little attention. Dry and lubricated sliding contact tests for white layer surfaces by turning and grinding were carried out at different load levels on a ball-on-disk tribometer with real-time monitoring of the wear process using an acoustic emission sensor. The experimental results show that the existence of a turned white layer slightly decreases the coefficient of friction (COF), while a ground white layer significantly increases COF at dry conditions. At lubricated conditions, the turned white layer only slightly increases COF while the ground white layer slightly reduces it. The trends of acoustic emission (AE) amplitude are consistent with those of COF at dry conditions for the turned or ground surfaces. At lubricated conditions, a turned white layer increases the AE amplitude with the increase of normal load when compared with a turned fresh surface. The ground white layer makes AE amplitude much smaller than that of the ground fresh surface at high normal load. The third body wear debris may act as solid lubricants leading to reduced friction. A turned white layer shows better wear resistance than the turned fresh surface. However, the ground white layer has poor wear resistance than the ground fresh one at large load.

INTRODUCTION

material by a single point tool, thereby generating a surface that is anisotropic in nature from the surface topography point of view. In contrast, grinding removes material by an abrasive action. Multiple abrasives remove small amounts of material in a statistical manner and produce a surface that is much more isotropic compared to a turned surface (Waikar and Guo (1)). Although turning and grinding are characterized by severe plastic deformation, high strain rates, and high temperatures, especially with a worn cutting tool or grinding wheel, the depth of a strain-hardened layer and temperature penetration in the subsurface are very different due to the distinct tool/workpiece contact nature (Warren and Guo (2); Guo and Warren (3)). High surface temperatures may occur in both processes when using a worn cutting tool or dull grinding wheel without sufficient coolant. The rapid quenching by the surrounding air or coolant often induces a phasetransformed layer of material on component surface, commonly referred to as white layer because of its white appearance under an optical microscope (Guo and Sahni (4)). The metallurgical transformation of the white layer makes its mechanical properties very different from the bulk material in terms of microstructure, hardness, and residual stresses (Guo and Janowski (5)). Although the white layer has been shown to be very detrimental to rolling contact fatigue life by Schwach and Guo (6), the role of a white layer on tribological performance such as friction and wear is not without controversy. There is little experimental or theoretical work to clarify the white layer effect on tribological performance of the machined surfaces in dry or lubricated conditions. Therefore, the objectives of this study are threefold: (i) First, to produce turned and ground surfaces with and free of white layers; (ii) second, conduct sliding experiments to determine the effect of different surface types with or without a white layer by turning or grinding on the coefficient of friction in dry and lubricated cases; (iii) third, establish a relationship between the surface type, the wear rate, and the samples signals at different loading and speeds.

Hard turning and grinding are competitive finishing processes that produce distinct surface integrity due to the inherent differences in the material removal mechanisms. Hard turning removes

BACKGROUND ON THE WHITE LAYER EFFECT ON SLIDING WEAR

KEY WORDS Friction; Sliding Contact; White Layer; Hard Turning; Grinding

Surface Topography Effect Manuscript received December 12, 2008 Manuscript accepted August 7, 2009 Review led by Fred Higgs

Several types of testing facilities have been used to assess the effect of surface topography on the wear behavior of machined

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surfaces. The experimental setups include cylinder-on-cylinder (Jhahanmir and Suh (7)), block-on-cylinder (Bartha, et al. (8)), pin-on-disk, and ball-on-disk. Early studies by Abrahamson, et al. (9) and Jhahanmir and Suh (7) have shown that the surface roughness influences only the initial wear behavior and not the steady-state wear rate in dry sliding conditions. Under low normal loads the initial wear rate of a smoother surface is higher than that of a rougher surface, provided that the surfaces are free from machining-induced surface damage. The opposite is found under high normal loads since original asperities are removed immediately during the run-in stage. Similar conclusions were also reported by other researchers (Gahr and Heinz (10)). The machined surface finish did not affect the sliding distance to reach the steady-state wear or the final surface finish of the samples after testing (Bartha, et al. (8)).


Surface Integrity Effect Subsurface damage (in the form of subsurface deformation, voids, and cracks) induced by a chipped tool was found to increase the initial wear rate of the machined surfaces at dry conditions (Jhahanmir and Suh (7)). As for the effect of strain hardening on the sample surface, Welsh (11) noted that at the T1 transition, softening of a strain-hardened layer occurred with a consequent increase in wear rate. Welsh also found that hard layers only formed above the T2 transition load and mild wear occurred when this layer was covered with oxide. At the T3 transition a further reduction in wear rate occurred when a sufficient hard layer formed, which enabled mild wear to predominate even in the absence of an oxide film. In the presence of a machining-induced white layer, several researchers have investigated the function of a white layer on wear resistance. Eyre and Baxter (12) expected that the high hardness of a white layer might improve wear resistance. Griffiths and Furze (13) studied the effect of white layers produced by turning EN24 steel on wear performance using a block-on-ring setup at dry conditions. White layers of 2 and 7 µm were produced and compared with the wear performance of samples free of a white layer under normal loads from 0.6 to 3 kg. For all the load levels they found that the white layer samples had better wear performance than the ones without the white layer. Also, the thicker the white layer, the higher its wear resistance was. The gradual transition of the white layer in the bulk gives superior adhesive properties than any coatings that may have issues of adhesion to the surface. Tomlinson, et al. (14) studied the effects of grinding white layers on the run-in performance of EN24 steel in sliding dry conditions. White layers of 0.29 and 3.5 µm thick were produced by centerless grinding. The test results revealed that the thicker white layer had a narrower wear track than the thinner white layer. Also, the depth of the wear track was shallower for the specimen with a thicker white layer. Both these features clearly showed that the thicker white layer increased the wear resistance of the machined surface. Bartha, et al. (8) showed that the white layer induced by turning AISI 52100 steel had a lower wear rate than the bulk material (tempered martensite). In addition, the white layer and overtempered martensite have an equivalent wear resistance. However,

the effect of machining-induced residual stress on wear performance was not clear. Even though some researchers (Bartha, et al. (8); Griffiths and Furze (13); Tomlison, et al. (14); Griffiths and Furze (15)) showed that a white layer has significant tribological advantage, some others (Bai (16); Yang, et al. (17)) found a white layer by shot peening to be detrimental to wear resistance. In addition, Xu, et al. (18) reported that white layers by milling had negligible effects on wear. Therefore, the function of a white layer in sliding contact still has some ambiguity.

SAMPLE PREPARATION AND SURFACE INTEGRITY CHARACTERIZATION Typical bearing steel AISI 52100 steel discs of 76.2 mm diameter and 19.05 mm thickness were heat treated via austenizing, quenching, and tempering to the hardness of 62 (±1) HRc. To study the effects of surface integrity including the white layer on tribological performance, four types of test samples were prepared by face turning or grinding at different machining conditions (Waikar and Guo (1)).

r r r r

Hard-turned fresh surface (without a white layer) (HTF); Hard-turned white layer surface (HTWL); Ground fresh surface (GF); Ground white layer surface (GWL).

The 3D surface topography of the machined surfaces was measured using a Taylor Hobson Talysurf CLI 2000 3D surface profiling system. Due to the small geometrical features of the precision machined surfaces, the measurement was carried out using the inductive gauge with a resolution of 10 nm and a measurement range of 2.5 mm. The stylus was used to scan across a set area of the workpiece. Table 1 lists the measured 3D surface parameters. The optical images of the cross sections of the machined samples are shown in Fig. 1. White layers of the turned and ground surfaces can be clearly seen. A comprehensive characterization of the mechanical properties of the machined surfaces including the white layers can be found in the authors’ previous work (Warren and Guo (2); Guo and Warren (3); Guo and Sahni (4); Guo and Janowski (5)) and are not presented in this article for brevity. To calculate the wear rate of each surface type, the cross section of a wear track was measured using the 3D surface profilometer. 2D line scans across wear tracks were obtained. The cross section area of each track was calculated using the associated data

TABLE 1—3D SURFACE TOPOGRAPHY PARAMETERS Amplitude Parameters

HTF

HTWL

Sa (µm) Sq (µm) Sp (µm) Sv (µm) St (µm) Sz (µm) Ssk Sku

0.229 0.28 0.98 0.653 1.63 1.35 0.53 2.88

0.175 0.219 0.773 0.715 1.49 1.3 0.228 2.89

GF 0.158 0.196 0.612 0.679 1.29 1.25 −0.127 2.79

GWL 0.202 0.256 1.11 1.01 2.11 1.8 0.427 3.19


Frictional and Wear Performance of a Machined Component

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Fig. 1—Optical images of the cross sections of test samples: (a) HTF (hard-turned fresh surface), (b) GF (ground fresh surface), (c) HTWL (hard-turned white layer surface, (d) GWL (ground white layer surface).

analysis software. This area multiplied by the circumference of the track yielded the wear volume for the track.

WEAR TEST SETUP AND CONDITIONS The sliding tests were carried out on a universal microtribometer shown in Fig. 2. The microtribometer basically consists

Fig. 2—Tribometer setup of sliding contact.

of a lower rotary drive on which a sample is mounted, a vertical positioning system to apply the normal load, and a lateral positioning system to adjust the wear track diameter. The load cell is mounted on the vertical positioning system and it simultaneously measures the normal and frictional forces in a sliding test. The applied load was controlled by an active feedback system with an

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TABLE 2—SLIDING WEAR TESTING CONDITIONS Surface Type


HTF HWL GF GWL

Sliding Speed (mm/s) 30 30 30 30

Normal Load (N)

Test Time (h)

# of Dry Tests

# of Lubricated Tests (SAE 85W-140)

10, 30, 50 10, 30, 50 10, 30, 50 10, 30, 50

2 2 2 2

3 3 3 3

3 3 3 3

accuracy of ±1 N. The sapphire ball was placed against the flat machined surface. The ball was placed in a ball holder that was fixed to the load cell. The ball holder prevents any rotary movement of the ball. An acoustic emission (AE) sensor was mounted on the ball holder with vacuum grease, which served as the coupling media between the sensor and the ball holder. The forces and the acoustic emission signals were real-time collected during the test for post analysis. In situ monitoring of sliding wear was accomplished using an AE signal acquisition and processing package AEWin (PAC (19)). The AE sensor has a 125-kHz resonant frequency and connects to an 18-bit PCI-2 data acquisition board that was incorporated into a PC. Before the data reached the PC it was passed through a preamplifier that was set at a 40 dB gain. A threshold of 25 dB was used. The sampled AE signal parameters in this study include amplitude, counts, and RMS (root mean square). Table 2 shows that wear test conditions for each surface type were conducted at both dry and lubricated conditions with SAE 85W-140. The sliding tests were carried out at a constant speed at the three load levels. Three identical tests were carried out at each load level for experimental repeatability. The rotational speed of the lower rotary drive and the wear track radius were adjusted to ensure the constant sliding speed. Before the test, the

wear samples were ultrasonically cleaned in acetone. After a sliding test was finished the sapphire ball was rotated to a new position so that a fresh ball surface can be used for each test. The test time was set to 2 h.

RESULTS AND DISCUSSIONS Coefficient of Friction Comparison at Dry Conditions Figures 3-5 show the effect of surface type on coefficient of friction (COF) at loads of 10, 30, and 50 N. In the run-in stage, COF rises rapidly as soon as the sample begins to rotate and reaches a local peak level, then decreases by a small amount and then increases continuously before reaching a steady state. This phenomenon was observed for all of the dry sliding tests. The formation of this “hump” in a frictional coefficient graph in the run-in stage is due to the wornout asperities on the surface and the existence of a third body (solid film of lubricant). In the steady state at 10 N normal load, the COF in the decreasing order is GWL (1.1) > HTF (0.85) > HTWL (0.75) > GF (0.5). Obviously, the ground white layer increases COF, while the turned white layer behaves oppositely. This may be reconciled by observing the respective SEM wear tracks as shown in Figs. 35. The GF surface shows clearly a layer of solid lubricant film (wear debris) on the wear track, whereas the GWL surface shows a much wider and deeper wear track free of wear debris. Whatever small amounts of debris are present have been pushed to the side of the wear track for the GWL surface. The HTWL surface also shows more debris than the HTF surface. It can hence be observed that the surface with more debris has a lower coefficient of friction, which leads to the conclusion that wear debris acts as solid lubricants and reduces the coefficient of friction. However, the COF difference (0.6 vs. 0.1) between GWL and GF is much larger than that of HTF and HTWL.

Fig. 3—Effect of surface type on coefficient of friction (COF) at dry conditions (10 N).



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When the normal load increases to 30 N, the COF in the decreasing order is GWL (0.85) ∼ HTF (0.83) > HTWL (0.75) > GF (0.65). It seems that the load increase does not change the basic order of COF. However, the COF difference between GWL and GF reduces from 0.6 to 0.2, while that of the HTF and HTWL virtually has no change. Figure 5 shows the effect of the surface type on COF at the higher normal load of 50 N. Only the HTWL surface went through to the stipulated testing time of 2 h. All other surfaces did not reach the stipulated testing time of 2 h. The HTF surface started off smoothly but soon failed by scuffing failure (extremely high pitch sound). The GF and HWL sur-

faces suffers from substantial vibrations due to the ball sliding over the alternating parallel and perpendicular feed marks. However, all the surfaces successfully passed through the runin stage. The COF order in this stage is GF > HTF > GWL > HWL. Figure 6 shows the effect of load on COF for the turned HTWL surfaces. The formation of the run-in hump occurs almost at the same time. The hump magnitude is not proportional to the normal load. However, the larger the normal load, the lower the COF when the steady state is well established. Figure 7 shows the effect of load on COF for the GWL surfaces. The larger the normal load, the lower the run-in hump



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Fig. 6—Effect of load on coefficient of friction (COF) of hard-turned white layer surfaces (HTWL) at dry conditions.

for the GWL surface. The load increase reduces the steady-state COF.

COF Comparison at Lubricated Conditions The effect of the surface type on COF at lubricated conditions for different load levels is shown in Figs. 8-11. Contrasted to the COF graphs at dry conditions, there were no sharp and well-defined run-in humps. Generally, the COF started out high as the test began and then quickly became stable. The rates of COF decrease are different for different surface types. However, the HTWL surface took relatively the longest time to reach the steady state.

Fig. 7—Effect of load on coefficient of friction (COF) of ground fresh surfaces (GWL) at dry conditions.

Fig. 8—Effect of load on coefficient of friction (COF) of hard-turned fresh surfaces (HTF) at lubricated conditions.

Compared with the COF at dry conditions, the distinctive COF feature in lubricated conditions is that COF dramatically reduces due to the presence of lubricants. For example, the COF of the GWL surface reduces from 1.1 to 0.118. The steady-state COF in the decreasing order at 10, 30, and 50 N, respectively, is HTWL (0.14) ∼ HTF (0.14) > GF (0.13) > GWL (0.12), GF (0.13) > HTWL (0.13) > GWL (0.12) ∼ HTF (0.12), GF (0.14) ∼ HTWL (0.14) > GWL (0.125) ∼ HTF (0.125). In general, the COF values for all the surface types are less than 0.14 at each of the load levels. The low COF values can be correlated to the barely noticeable wear tracks on the lubricated surfaces. In

Fig. 9—Effect of load on coefficient of friction (COF) of hard-turned white layer surfaces (HTWL) at lubricated conditions.



Fig. 10—Effect of load on coefficient of friction (COF) of ground fresh surfaces (GF) at lubricated conditions.

addition, there was also no observed wear debris on the contact surfaces. It is clear that the turned white layer increases COF while the ground white layer reduces it at each of the load levels. The difference seems to be consistent with 3D surface roughness, which is characterized by 3D surface arithmetic mean deviation Sa or root mean square deviation Sq in Table 1. It can be seen that the HTWL surface has a lower surface roughness than the HTF surface. The GF surface roughness is less than that of the GWL surface. Hence, it indicates that the smoother turned or ground surface may have higher COF than that of a relatively rougher surface.

Fig. 11—Effect of load on coefficient of friction (COF) of ground white layer surfaces (GWL) at lubricated conditions.

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Fig. 12—Effect of load on acoustic emission (AE) amplitude of hardturned white layer surfaces (HTWL) at dry conditions.

Figures 8-11 also show the effect of normal load on the COF for the individual machined surfaces. For the HTF and HTWL surfaces the highest COF occurs at the lowest load level at the end of each test. For the GF surface, the COF increases as the load increases, while the GWL surface behaves oppositely, but did not show any clear trend.

The White Layer Effect on the AE Signal Dry conditions: AE refers to the generation of transient elastic waves caused by the rapid energy release of a localized deformation within a material during sliding contact. Figures 12 and 13 show the white layer effect on AE amplitude in dry conditions.

Fig. 13—Effect of load on acoustic emission (AE) amplitude of ground white layer surfaces (GWL) at dry conditions.


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Fig. 14—Effect of load on acoustic emission (AE) amplitude of hardturned white layer surfaces (HTWL) at lubricated conditions.

Fig. 15—Effect of load on acoustic emission (AE) amplitude of ground white layer surfaces (GWL) at lubricated conditions.

Amplitude is the maximum (positive or negative) AE signal excursion during an AE hit. The amplitude is expressed in db using the relationship:

Wear Track Characterization

db = 20 log (Vmax /1µ − volt) − (Preamplifier Gain)

Figures 3-5 show the SEM images of the wear tracks of the four surface types at different loads. Table 3 lists the measured width and depth of the wear tracks. Generally speaking, wear track width and depth increase with the applied load for each surface type. A turned white layer surface has a narrower and shallower wear track than the fresh counterpart, while a ground white layer has a wider wear track at 10 N but a narrower track at load 30 N than the fresh ones. The wear tracks have varying amounts and types of third body wear debris. The third body debris ranges from scattered fragments to a continuous solid film depending on the surface type and applied load. The third body film may act as a solid lubricant to reduce friction. The wear tracks of the HTF surfaces at the three different loads show that wear debris in the form of solid fragments is parallel to the feed marks, which have been worn away to a considerable extent for the HTF surfaces. The solid wear debris is smeared into the valleys of the feed marks. As the load increases from 10 to 30 N, the increased random solid wear debris was formed in the central part of the track. Solid wear debris on the wear track can also be seen at the load of 50 N. However, further analysis of the wear track at 50 N is difficult due to the early scuffing failure. The wear tracks of the HTWL surfaces show that the width and depth of the wear tracks for the HTWL surface are much narrower than the HTF surface at the same load. But, the wear track seems to have more wear debris than those of the HTF

[1]

The AE amplitude graphs show the typical run-in hump in which the amplitude initially increases and then decreases in the early state of the tests. The AE amplitude of the GWL surface shows a spike during run-in at the load of 10 N. The corresponding COF is also highest among all the tests. It has been shown that the steady-state AE amplitudes for the white layer surfaces, HTWL and GWL, are higher than their HTF and GF counterparts at all load levels. The higher hardness of the white layer (800 HV for turned surface and 1100 HV for ground surface; Guo and Sahni (4)) than the bulk material may contribute to the higher AE amplitudes of the white layer surfaces. Figures 12 and 13 show the effect of load on AE amplitude for the HTWL and GWL surfaces. The AE amplitude increases with the normal load for the HTWL surface. The GWL surface has the highest amplitude at the lowest load in the run-in stage. Lubricated conditions: At lubricated conditions, Figs. 14 and 15 show that the AE amplitudes are much smaller for the HTWL and the GWL surfaces when compared with the same surfaces free of a white layer at dry conditions. The AE amplitude increases when the load is increased for the HTWL surface. But the amplitude drops slightly for the GWL surface. TABLE 3—WIDTH AND DEPTH OF WEAR TRACKS Surface Type Load (N) Width (µm) Maximum depth (µm)

HTF 10 300 0.4

30 400 0.9

HTWL 50 NA NA

10 200 0.6

30 250 0.6

GF 50 400 1.5

10 200 1.3

30 550 1

GWL 50 NA NA

10 300 3

30 300 1

50 NA NA


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Fig. 16—Effect of surface type on wear rate at low load (10 N).

surfaces, which explains why the HTWL surface has a lower COF than the turned ones. The wear tracks of the GF surfaces in Figs. 3-5 show that the wear debris is in the form of a film, which prevented direct ballsurface contact at 10 N load. The ball rides on this film, which acts as a solid lubricant. A strip of wear debris formed on the track at the load of 30 N. Due to the vibration-induced early failure of the GF surface at 50 N, the wear track at this load level is very narrow. However, a periodic wear debris band forms perpendicular to the sliding direction. This observation may explain the severe vibration the ball experienced during the test. The wear tracks of the GWL surfaces show that the GWL surfaces have much less solid films than the GF surfaces, which contribute to the smaller COF for the GF surfaces.

Wear Rate Comparison Figures 16 and 17 show the effect of surface type on the wear rate coefficient. The wear rate coefficient was calculated by k = V LS [2] where k is the wear rate coefficient in mm3 /Nm, V is the volume of material lost in mm3 on a wear track, L is the load applied in Newton, and S is the sliding distance in meter. Figure 16 shows the wear rates for the four types of surfaces at the load of 10 N. It can be seen that the HTWL surface has

Fig. 17—Effect of surface type on wear rate at low load (30 N).

a lower wear rate coefficient than the HTF surface, which indicates a better wear resistance capability. The GWL surface has less wear resistance capability than the GF surface. As shown in Fig. 17 at 30 N, both the HTWL and GWL surfaces have a better wear resistance than the fresh surfaces. Figures 16 and 17 also show that for the HTF and GF surfaces the increased load increase wear resistance, while the HTWL and GWL surfaces reduce the wear rate when the load is increased. Due to the scuffing failure of the HTF surface and severe vibration failure of and GF and GWL surfaces at 50 N, the wear rates at the steady state cannot be compared at this load level.

EXPERIMENTAL ERROR ANALYSIS There are various sources of experimental errors while conducting the sliding tests. The errors may be classified into two categories; i.e., errors due to deviation in test parameters such as load and contact frequency and errors due to variation in surface topography. The normal load was applied by the upper positioning system of the microtribometer. As the sample was rotated, the minute difference in the height variation of the surfaces caused slight fluctuations in the applied load. The load fluctuations were counteracted by a feedback system in the upper positioning system, which made minute adjustments in the ball position in order to


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control the load within the designed level. Even so, the load variation was about ± 1 N. This variation in load could cause error in the output graphs. For each surface at each load, three sliding experiments were carried out in order to get output data that is repeatable in statistical nature. Since the sliding tests were unidirectional and rotational, the circular wear tracks of different diameters were chosen, ensuring that a constant linear speed was maintained. Hence, each track had a different rotational speed. Owing to the complex nature of contact between the ball and the surface, even a slight variation in surface topography and contact frequency may affect the output friction and AE signals. As a result, small variations were found in the COF magnitudes. In such a case, an average of three signals was used for comparison. Another contributing error source is the difference in surface topography of the turned and ground surfaces. For the turned surfaces the sliding direction was always along the feed marks. The sliding direction changed alternatively from parallel to perpendicular for the ground surfaces. This was due to the unidirectional grinding feed marks. The influence of the surface texture alone will be a future research subject.

CONCLUSIONS In this study a set of four types of surface properties of AISI 52100 bearing steel (62 HRc) were prepared by gentle and abusive hard turning or grinding conditions. The gentle machining conditions were chosen to produce surfaces free of a white layer, while the abusive conditions were chosen to produce white layer surfaces. Dry and lubricated wear tests were carried out at different load levels on a ball-on-disk tribometer with real-time monitoring of the wear process using an acoustic emission sensor. The main conclusions are summarized as follows.

r

r

At dry conditions, the existence of a turned white layer slightly decreases the coefficient of friction, while a ground white layer significantly increases the coefficient of friction. The coefficient of friction in the decreasing order is GWL, HTF, HTWL, and GF. At the steady state, the coefficient of friction for the HTF surface is not sensitive to the normal loads, while it increases with the load for the GF surfaces. The increased load reduces the coefficient of friction for both turned and ground white layer surfaces. At lubricated conditions, the turned white layer only slightly increases the coefficient of friction while the ground white layer slightly reduces it. The difference seems to be consistent with 3D surface arithmetic mean deviation Sa or root mean square deviation Sq. It also indicates that a smoother turned or ground surface may not necessarily have a higher coefficient of friction than that of a relatively rougher surface as long as the surface roughness is within a certain range. The highest friction occurs at the lowest load level for the turned surfaces regardless of the presence of a white layer. However, friction increases as the load increases for the ground fresh surfaces, while the ground white layer surface behaves oppositely, but did not show a clear trend.

r

r

The trends of AE amplitude are consistent with those of the coefficient of friction at dry conditions for the turned or ground surfaces. In lubricated conditions, a turned white layer increases the AE amplitude with the increase of normal load when compared with a turned fresh surface, while a ground white layer makes the AE amplitude much smaller than that of the ground fresh surfaces at high normal load. The third body wear debris may act as solid lubricants leading to reduced friction. A turned white layer shows better wear resistance than the turned fresh surface. However, the ground white layer has poorer wear resistance than the ground fresh surface at a large load.

ACKNOWLEDGMENT This research is based upon work supported by the National Science Foundation under Grant No. CMMI-0447452.

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