- Email: [email protected]
Experimental studies of dynamic sliding wear for PEHL contacts
Tribology for Energy Conservation / D. Dowson et al. (Editors) 1998 Elsevier Science B.V.
275
Experimental studies of dynamic sliding wear for PEHL contacts W. Wang and P.L. Wong Department of Manufacturing Engineering and Engineering Management City University of Hong Kong Hong Kong A two-disk wear test rig which is incorporated with an optical attachment for monitoring the surface roughness changes during the wear tests was developed. The optical system adopts a newly developed technique which is able to capture the roughness of moving surfaces. The new wear rig was validated to be sensitive enough to detect the roughness changes with two running-in tests for PEHL contacts. Keywords: wear, surface roughness, PEHL, optical method
l.lntroduction In modem machinery, there are many friction pairs actually running under partial elastohydrodynamic lubrication (PEHL)conditions where the film thickness is of the same order of magnitude of surface roughness. The studies of the variation of surface roughness during operation are important to understand the wear mechanism of both the runningin and the mild wear under PEHL regime. However, none of the existing common wear testers such as the pin-on-disk or two-disk machine provides means to capture the changes of surface roughness during wear tests. The surface roughness measurements can only be taken by stopping the machine at different stages. Interrupting the continuous wear test in order to check the roughness change is certainly not able to obtain the exact data. A sliding wear model for PEHL contacts which is based on the thermal desorption and the oxidative wear mechanisms was proposed by Cheng [1]. A test-rig was set up to evaluate the model and for the determination of the coefficients of the model. The wear model has a shortcoming of not being a dynamic system. Hence, it is not suitable for modeling the dynamic wear or' running-in processes. The test rig was a two-disk type wear tester which can run under different sliding/rolling ratios for the simulation of gear operation such that the effects of different sliding/rolling ratios can also be studied. A similar design of wear test rig was chosen for the present work.
A dynamic system model which is entirely based on theoretical analyses of lubricated sliding wear and running-in was established by Hu et al [2]. They recognized wear as a dynamic process which is mostly distinct during the running-in stage. Surface topography can be altered due to wear and in turn the wear rate is affected by the surface roughness. The simple dynamic wear model for running-in can be written as:
W = P * ( a o + a I / c r + a 2 ~or ~ ) •
•
(1)
dcr / dt = (-boer + b, cre+ bz P ) . ( W - Win) Meaning of the symbols of Eq. 1 can be referred to [I]. Eq. 1 shows that the wear rate W , surface roughness cr and the rate of change of roughness d cr/dt are all interrelated. The model is reasonably constructed with theoretical backgrounds and all the parameters can be obtained with purely theoretical analyses upon some wear models. Nevertheless, its validity has to be confh'med with experimental results. For wear tests running under actual conditions, the roughness of PEHL contact surfaces actually keeps changing. Obviously, one foreseeable problem is to measure the surface roughness changes during a wear test at a real-time mode. The traditional method of surface roughness measurement is the stylus probe type. It is difficult to be developed for measuring roughness of moving surfaces. There are non-contact methods which are mainly optical in nature, such as holographic interferometry [3], Nomarski-type scanning [4],
276
focus-multiplexing [5], scattered light [6] and speckle correlation [7]. None of these methods are suitable for roughness measurements on moving surfaces because of the vibration nature of moving surfaces, limited measuring range or low resolution. The scattering light method can have large measuring range, but the resolution is low. Contrarily, the speckle correlation has high resolution but limited measuring range(a quarter of wavelength). A new optical technique which is based on the these methods such that large measuring range with high resolution can be provided was developed by the authors [8]. To be able to measure the change of surface roughness during wear tests without intermediate stops is thus significant to the research on the wear model for PEHL contacts. Therefore, a wear test rig which is incorporated with a real time surface roughness measuring attachment was designed and constructed. 2. Two disk wear test-rig and optical attachment
circuit was designed such that the changes of the contact area can be observed through contact potentials. The lubricating condition can be monitored by the contact potential across the two disks since the amount of asperity contacts is related to the potential values. The potential qualitatively indicates the lubrication film thickness and can be applied to determine the lubrication condition.
Fig.2 Pattern of R~=0.2~tm Surface
~,~ 1
Loa
=
~
_
!=~,°°, *,-°~,I
I
Fig. 1 Schematic setup of the test rig The set up of the test-rig is schematically shown in Fig. 1. The two disks are driven by two servo-motors independently. The speed can be continually varied from zero to 3000 rpm. Thus different sliding/rolling ratios can be set up. A non-contact type torque sensor which is used to measure the friction torque is mounted between the supporting shaft of the large disk and the motor. A simple but effective electronic
The optical attachment which is utilized to measure the roughness of the rotating disk surface are composed of a CCD camera, an amplifier, a HeN¢ laser source, a microscope and a personal computer with an image-card. The principle of the optical method is based on the phenomenon that when a rough surface is projected with a laser beam, the surface roughness can be inferred from by the overall intensity of the reflected pattern which is formed by the combination effect of speckles and scattering of the light. Fig.2 and 3 are the reflected patterns of two surfaces with Ra=0.2~m and 0.4~m
277
respectively. It can be seen that surfaces with different roughness have different sizes in dark or bright area on their pattern. It is found that the ratio of dark or bright area to the total area of the image pattern correlates well with the roughness of specimen surface. Fig.4 shows that the two ratios have monotonous relations with the surface average roughness 1~. Hence, by knowing these two relations, R, can be inferred from either one of the two ratios. Dark ratio and bright ratio can be used at different measuring roughness range to maximize the sensitive of the technique. 0.7
0.6
In order to measure the roughness of moving surfaces, the main problem that needs to be tackled is the harmful effect of vibration which, in another words, renders the surface to deviate from the preset focal plane. The effect of deviation from the focal plane on the dark ratio, D, is shown in Fig.5. It shows that even the amplitude of the vibration is 0.3 mm, the value of dark ratio is only changed marginally. For ordinary engineering designs, the amplitude of the radial movement of a rotating disk surface is easily controlled below 0.03 mm. The new optical technique is thus proved to be suitable for roughness measurement of moving surfaces. More details of the technique can be referred to [8].
3. Experiment results and Discussion
.2 0.5 .,
0.4
~
0~3
.g
Preliminarily running-in wear tests using two steel disks with structure as shown in Fig.6 were performed with the test rig.
L..
~ o.2 0.1 • ~-
0
0.2
-0-
.,__
0.4
0.6
0.8 Gdark
1 1.2 e bright
1.4
1,6
1.8
Surface Roughness, ~ (gm)
Fig.4 Relation of dark ratio and bright ratio with surface roughness Ra 0.6
.2
0,4
0.3
0.2
-O3
-02
o t
Deviation
0
0!
02
from focal plane (ram)
Fig.5 Effect of vibration of surface
0.3
Fig. 6 Specimens As for preliminary tests, the outer rim of the large steel disk is not crowned as shown in Fig. I. The two disks are thus in line contact. The outside diameter of the large and small disks are 86 mm and 34 mm respectively. The length of the contact line which is equal to the width of the large disk is !2 mm. The small disk is with higher hardness, HRD=70 and the large one is with HRD=48. Two pairs of specimens were used. The first set is with higher roughness R~ of 1.723~tm for the big disk and 0.66~tm for the small one respectively. The second set is with R~ of 0.8728~tm and 0.4071am. A He-Ne laser was projected onto the wear track of the large disk during the test. The reflected pattern was captured at different time intervals and analyzed. Friction torque and contact potential were also measured during the wear test. Having completed the test, the dark ratio of each reflected pattern recorded at different time intervals was calculated.
278
The variation of dark ratio illustrates the changes of surface roughness. The surface roughness of specimens were also measured before and aRer the test by Talysulf stylus roughness measuring equipment. Fig.7 and 8 are experimental results of specimens with higher initial roughness. The working conditions and the surface roughness of two disks before and after the test are listed in Table. 1. The subscript b and s mean big and small disks respectively.
,
,,,
,.
.
.
.
.
.
.
,
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Nm
0
;O 0.6
14
o
-4 o
A • A & &
0.55
•
&
•
&&
•
&
•
12"~c tl)
&•
A
I
•
e •
e
e
l
10 z seconds
0
1
2~ ..........
- 3 .......... 4
5
!
2
3
5
4
STime 7
Fig.8 Changes of contact potential under the conditions listed in table, l
13D, ,,.. 0 ::3
o
E 40
10 3 seconds
"11
13t 0.65
A
>
10
.
1E-3
0,7
50
c 20 o O
Tab!e_ 1 testing parameters nb=n~=263rpm, sliding/rolling ratio =0.87, lubricant = Shell vitrea 32 oil, Initial surface roughness ( R ~ = 1.723Iam and (RO,=0.66I~m Roughness after test (R,)~= 1.521 lam and (R~)~=0.606pm Loading=2. lkg ,
in and acquires a stable value which is about 10% less than the initial value after the run-in. The total running-in period for the test is about 70 minutes.
6 Time 7
Fig.8 depicts the variation of potential difference during the test. The starting potential difference is 55 mV which represents a complete separation of the two surfaces with lubricant. The rig was loaded right after the start of the test. The potential dropped largely to a small value of 15mV, which indicates that the two surfaces are under PEHL contact because the potential is zero if they are under full contact. The potential difference then increased gradually and settled at a constant value of 20mV after running-in. That means the asperity contact area decreased and the lubrication effect was enhanced.
l
Fig.7 Dark ratio and friction torque changes under the conditions listed in table. I
tE-3
0.5
Nm
9
17i0.45 a)
o •
•
0.4 0
"!"i
9, m.
•
•
•
•
The test lasted for near 2 hours. Fig.7 shows the variation of dark ratio and friction torque during the experiment. Both curves drop exponentially against with time, which is typical for usual running-in wear tests. The dark ratio D decreases with time and approaches to a steady value. This means that the roughness of specimen surface decreases drastically in the beginning and then the rate of change gradually decreases. The specimen surface becomes smoother after the completion of the running-in stage. The friction torque also drops during running-
10
e
•
• •
•
e
•
80
• •
&
e
--t O
&A
&&
•
•
•
•
•
A
&
&
&
7.=3
0.35 103 s e c o n d s 0.3 . . . . . . . . . . . . . . . . . . . . . . . . . . ~. . . . . . . . . . . . . . 5 0 1 ........... 2 3 4_ 5 6Time 7 l.:':Dark Ratio ~ Frictiont~qu~ 1
Fig.9 Dark ratio and friction torque changes under the conditions listed in table.2
279
Fig.9 and 10 illustrate experimental results of another pair of specimens which have smoother surfaces than the first set of specimens. The surface roughness of the specimens measured before and after the test were recorded and listed in Table 2. The test conditions are the same as the first set, as listed in Table I. Table.2 R, of specimens . Roughness at test beginning (Ra)b=0.8728pm and (l~)s=0.407pm Roughness after test (R~)b=0.806~tm and (Ra)s=0.384pm The changes of dark ratio and friction torque with time are shown in Fig.9. It is clearly indicated from the figure that the running-in of the test was completed at about 45 minutes. Comparing with Fig.7, the duration required for running-in is shorter for specimens with smoother surface while all other running conditions are kept the same. Owning to the smoother surfaces, the initial dark ratio is smaller than that of the first test. Furthermore, the reduction of dark ratio value after running-in is also less which agrees well with the smaller change of surface roughness before and atier the test. The settling surface roughness is about 0.4pm while that of first test is about 0.61am. It seems that the settling roughness value is affected by the initial roughness of the components. The friction torque is smaller for the smooth pair of disks than that of rough disks pair as shown in Fig.7.
Fig. 10 shows the contact potential changes during the test. It is about the same as that of the fh'st test with rougher specimens as shown in Fig.8. It means that the lubrication is also under PEHL conditions.
4. Conclusion A two-disk wear test rig which is incorporated with an optical attachment for monitoring the change of surface roughness of the specimen during running-in test was developed. The optical measuring attachment which adopts a newly developed technique for roughness measurements on moving surfaces was proved sensitive enough to detect the roughness changes with two running-in tests for PEHL contacts. Results illustrate that the roughness of specimens decreases significantly in the start of running-in process and the rate of decrease diminishes gradually. The surface roughness settles with a constant value after running-in. The higher the initial roughness of the surface is, the longer the duration required for the running-in process and the higher the settling surface roughness will be.
Acknowledgment The authors would like to express their appreciation to the Research Grants Council of Hong Kong and the City University of Hong Kong for financial support to the project.
60 ,50
References
A
> E,IO t~
>
30
"6
t~ •" 20 0 o
10 10 3 seconds O,
1
2
3
4
5
6 Time 7
Fig. I0 Changes of contact potential under the conditions listed in table.2
[1] S. Wu, H.S. Cheng, "A Sliding Wear Model for Partial-EHL Contacts", J. of Tribology, pp.134-14 l January 199 !. [2] Y.Z Hu, N. Li, K. Tonder, "A Dynamic System Model for Lubricated Sliding Wear and Runningin", J. of Tribology, July 1991, Vol. 113, pp.499505, July 1991. [3] W.B. Ribbens,"Surface roughness measurements by holographic interferometry." Appl. Opt. l l., pp.807-810, 1972. [4] T.C. Bristow and K. Arakellian, "Surface rough-
280 ness measurements using a Nomarsky type scanning instruments." Proc. SPIE 749, pp.114-118, 1987. [5] G. Molessini, P.Poggi and F. Quercioli, "Focusmultiplexed optical profilometef' in ICO 13. Conference digest, pp 188-189, 1984. [6] D. H. Hensler, "Light scattering from surface polycrystalline aluminum oxide surfaces." Appl. Opt. 11. 2522, 1972. [7] U. Persson, "Measurement of surface roughness on rough machined surfaces spectral correlation and image analysis." Wear, 160, pp.221-225, 1993. [8] W. Wang, P.L. Wong, J.B. Luo and Z. Zhang, "A New Optical Technique for Rouglmess Measurement on Moving Surface.", submitted, 1997.
275
Experimental studies of dynamic sliding wear for PEHL contacts W. Wang and P.L. Wong Department of Manufacturing Engineering and Engineering Management City University of Hong Kong Hong Kong A two-disk wear test rig which is incorporated with an optical attachment for monitoring the surface roughness changes during the wear tests was developed. The optical system adopts a newly developed technique which is able to capture the roughness of moving surfaces. The new wear rig was validated to be sensitive enough to detect the roughness changes with two running-in tests for PEHL contacts. Keywords: wear, surface roughness, PEHL, optical method
l.lntroduction In modem machinery, there are many friction pairs actually running under partial elastohydrodynamic lubrication (PEHL)conditions where the film thickness is of the same order of magnitude of surface roughness. The studies of the variation of surface roughness during operation are important to understand the wear mechanism of both the runningin and the mild wear under PEHL regime. However, none of the existing common wear testers such as the pin-on-disk or two-disk machine provides means to capture the changes of surface roughness during wear tests. The surface roughness measurements can only be taken by stopping the machine at different stages. Interrupting the continuous wear test in order to check the roughness change is certainly not able to obtain the exact data. A sliding wear model for PEHL contacts which is based on the thermal desorption and the oxidative wear mechanisms was proposed by Cheng [1]. A test-rig was set up to evaluate the model and for the determination of the coefficients of the model. The wear model has a shortcoming of not being a dynamic system. Hence, it is not suitable for modeling the dynamic wear or' running-in processes. The test rig was a two-disk type wear tester which can run under different sliding/rolling ratios for the simulation of gear operation such that the effects of different sliding/rolling ratios can also be studied. A similar design of wear test rig was chosen for the present work.
A dynamic system model which is entirely based on theoretical analyses of lubricated sliding wear and running-in was established by Hu et al [2]. They recognized wear as a dynamic process which is mostly distinct during the running-in stage. Surface topography can be altered due to wear and in turn the wear rate is affected by the surface roughness. The simple dynamic wear model for running-in can be written as:
W = P * ( a o + a I / c r + a 2 ~or ~ ) •
•
(1)
dcr / dt = (-boer + b, cre+ bz P ) . ( W - Win) Meaning of the symbols of Eq. 1 can be referred to [I]. Eq. 1 shows that the wear rate W , surface roughness cr and the rate of change of roughness d cr/dt are all interrelated. The model is reasonably constructed with theoretical backgrounds and all the parameters can be obtained with purely theoretical analyses upon some wear models. Nevertheless, its validity has to be confh'med with experimental results. For wear tests running under actual conditions, the roughness of PEHL contact surfaces actually keeps changing. Obviously, one foreseeable problem is to measure the surface roughness changes during a wear test at a real-time mode. The traditional method of surface roughness measurement is the stylus probe type. It is difficult to be developed for measuring roughness of moving surfaces. There are non-contact methods which are mainly optical in nature, such as holographic interferometry [3], Nomarski-type scanning [4],
276
focus-multiplexing [5], scattered light [6] and speckle correlation [7]. None of these methods are suitable for roughness measurements on moving surfaces because of the vibration nature of moving surfaces, limited measuring range or low resolution. The scattering light method can have large measuring range, but the resolution is low. Contrarily, the speckle correlation has high resolution but limited measuring range(a quarter of wavelength). A new optical technique which is based on the these methods such that large measuring range with high resolution can be provided was developed by the authors [8]. To be able to measure the change of surface roughness during wear tests without intermediate stops is thus significant to the research on the wear model for PEHL contacts. Therefore, a wear test rig which is incorporated with a real time surface roughness measuring attachment was designed and constructed. 2. Two disk wear test-rig and optical attachment
circuit was designed such that the changes of the contact area can be observed through contact potentials. The lubricating condition can be monitored by the contact potential across the two disks since the amount of asperity contacts is related to the potential values. The potential qualitatively indicates the lubrication film thickness and can be applied to determine the lubrication condition.
Fig.2 Pattern of R~=0.2~tm Surface
~,~ 1
Loa
=
~
_
!=~,°°, *,-°~,I
I
Fig. 1 Schematic setup of the test rig The set up of the test-rig is schematically shown in Fig. 1. The two disks are driven by two servo-motors independently. The speed can be continually varied from zero to 3000 rpm. Thus different sliding/rolling ratios can be set up. A non-contact type torque sensor which is used to measure the friction torque is mounted between the supporting shaft of the large disk and the motor. A simple but effective electronic
The optical attachment which is utilized to measure the roughness of the rotating disk surface are composed of a CCD camera, an amplifier, a HeN¢ laser source, a microscope and a personal computer with an image-card. The principle of the optical method is based on the phenomenon that when a rough surface is projected with a laser beam, the surface roughness can be inferred from by the overall intensity of the reflected pattern which is formed by the combination effect of speckles and scattering of the light. Fig.2 and 3 are the reflected patterns of two surfaces with Ra=0.2~m and 0.4~m
277
respectively. It can be seen that surfaces with different roughness have different sizes in dark or bright area on their pattern. It is found that the ratio of dark or bright area to the total area of the image pattern correlates well with the roughness of specimen surface. Fig.4 shows that the two ratios have monotonous relations with the surface average roughness 1~. Hence, by knowing these two relations, R, can be inferred from either one of the two ratios. Dark ratio and bright ratio can be used at different measuring roughness range to maximize the sensitive of the technique. 0.7
0.6
In order to measure the roughness of moving surfaces, the main problem that needs to be tackled is the harmful effect of vibration which, in another words, renders the surface to deviate from the preset focal plane. The effect of deviation from the focal plane on the dark ratio, D, is shown in Fig.5. It shows that even the amplitude of the vibration is 0.3 mm, the value of dark ratio is only changed marginally. For ordinary engineering designs, the amplitude of the radial movement of a rotating disk surface is easily controlled below 0.03 mm. The new optical technique is thus proved to be suitable for roughness measurement of moving surfaces. More details of the technique can be referred to [8].
3. Experiment results and Discussion
.2 0.5 .,
0.4
~
0~3
.g
Preliminarily running-in wear tests using two steel disks with structure as shown in Fig.6 were performed with the test rig.
L..
~ o.2 0.1 • ~-
0
0.2
-0-
.,__
0.4
0.6
0.8 Gdark
1 1.2 e bright
1.4
1,6
1.8
Surface Roughness, ~ (gm)
Fig.4 Relation of dark ratio and bright ratio with surface roughness Ra 0.6
.2
0,4
0.3
0.2
-O3
-02
o t
Deviation
0
0!
02
from focal plane (ram)
Fig.5 Effect of vibration of surface
0.3
Fig. 6 Specimens As for preliminary tests, the outer rim of the large steel disk is not crowned as shown in Fig. I. The two disks are thus in line contact. The outside diameter of the large and small disks are 86 mm and 34 mm respectively. The length of the contact line which is equal to the width of the large disk is !2 mm. The small disk is with higher hardness, HRD=70 and the large one is with HRD=48. Two pairs of specimens were used. The first set is with higher roughness R~ of 1.723~tm for the big disk and 0.66~tm for the small one respectively. The second set is with R~ of 0.8728~tm and 0.4071am. A He-Ne laser was projected onto the wear track of the large disk during the test. The reflected pattern was captured at different time intervals and analyzed. Friction torque and contact potential were also measured during the wear test. Having completed the test, the dark ratio of each reflected pattern recorded at different time intervals was calculated.
278
The variation of dark ratio illustrates the changes of surface roughness. The surface roughness of specimens were also measured before and aRer the test by Talysulf stylus roughness measuring equipment. Fig.7 and 8 are experimental results of specimens with higher initial roughness. The working conditions and the surface roughness of two disks before and after the test are listed in Table. 1. The subscript b and s mean big and small disks respectively.
,
,,,
,.
.
.
.
.
.
.
,
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Nm
0
;O 0.6
14
o
-4 o
A • A & &
0.55
•
&
•
&&
•
&
•
12"~c tl)
&•
A
I
•
e •
e
e
l
10 z seconds
0
1
2~ ..........
- 3 .......... 4
5
!
2
3
5
4
STime 7
Fig.8 Changes of contact potential under the conditions listed in table, l
13D, ,,.. 0 ::3
o
E 40
10 3 seconds
"11
13t 0.65
A
>
10
.
1E-3
0,7
50
c 20 o O
Tab!e_ 1 testing parameters nb=n~=263rpm, sliding/rolling ratio =0.87, lubricant = Shell vitrea 32 oil, Initial surface roughness ( R ~ = 1.723Iam and (RO,=0.66I~m Roughness after test (R,)~= 1.521 lam and (R~)~=0.606pm Loading=2. lkg ,
in and acquires a stable value which is about 10% less than the initial value after the run-in. The total running-in period for the test is about 70 minutes.
6 Time 7
Fig.8 depicts the variation of potential difference during the test. The starting potential difference is 55 mV which represents a complete separation of the two surfaces with lubricant. The rig was loaded right after the start of the test. The potential dropped largely to a small value of 15mV, which indicates that the two surfaces are under PEHL contact because the potential is zero if they are under full contact. The potential difference then increased gradually and settled at a constant value of 20mV after running-in. That means the asperity contact area decreased and the lubrication effect was enhanced.
l
Fig.7 Dark ratio and friction torque changes under the conditions listed in table. I
tE-3
0.5
Nm
9
17i0.45 a)
o •
•
0.4 0
"!"i
9, m.
•
•
•
•
The test lasted for near 2 hours. Fig.7 shows the variation of dark ratio and friction torque during the experiment. Both curves drop exponentially against with time, which is typical for usual running-in wear tests. The dark ratio D decreases with time and approaches to a steady value. This means that the roughness of specimen surface decreases drastically in the beginning and then the rate of change gradually decreases. The specimen surface becomes smoother after the completion of the running-in stage. The friction torque also drops during running-
10
e
•
• •
•
e
•
80
• •
&
e
--t O
&A
&&
•
•
•
•
•
A
&
&
&
7.=3
0.35 103 s e c o n d s 0.3 . . . . . . . . . . . . . . . . . . . . . . . . . . ~. . . . . . . . . . . . . . 5 0 1 ........... 2 3 4_ 5 6Time 7 l.:':Dark Ratio ~ Frictiont~qu~ 1
Fig.9 Dark ratio and friction torque changes under the conditions listed in table.2
279
Fig.9 and 10 illustrate experimental results of another pair of specimens which have smoother surfaces than the first set of specimens. The surface roughness of the specimens measured before and after the test were recorded and listed in Table 2. The test conditions are the same as the first set, as listed in Table I. Table.2 R, of specimens . Roughness at test beginning (Ra)b=0.8728pm and (l~)s=0.407pm Roughness after test (R~)b=0.806~tm and (Ra)s=0.384pm The changes of dark ratio and friction torque with time are shown in Fig.9. It is clearly indicated from the figure that the running-in of the test was completed at about 45 minutes. Comparing with Fig.7, the duration required for running-in is shorter for specimens with smoother surface while all other running conditions are kept the same. Owning to the smoother surfaces, the initial dark ratio is smaller than that of the first test. Furthermore, the reduction of dark ratio value after running-in is also less which agrees well with the smaller change of surface roughness before and atier the test. The settling surface roughness is about 0.4pm while that of first test is about 0.61am. It seems that the settling roughness value is affected by the initial roughness of the components. The friction torque is smaller for the smooth pair of disks than that of rough disks pair as shown in Fig.7.
Fig. 10 shows the contact potential changes during the test. It is about the same as that of the fh'st test with rougher specimens as shown in Fig.8. It means that the lubrication is also under PEHL conditions.
4. Conclusion A two-disk wear test rig which is incorporated with an optical attachment for monitoring the change of surface roughness of the specimen during running-in test was developed. The optical measuring attachment which adopts a newly developed technique for roughness measurements on moving surfaces was proved sensitive enough to detect the roughness changes with two running-in tests for PEHL contacts. Results illustrate that the roughness of specimens decreases significantly in the start of running-in process and the rate of decrease diminishes gradually. The surface roughness settles with a constant value after running-in. The higher the initial roughness of the surface is, the longer the duration required for the running-in process and the higher the settling surface roughness will be.
Acknowledgment The authors would like to express their appreciation to the Research Grants Council of Hong Kong and the City University of Hong Kong for financial support to the project.
60 ,50
References
A
> E,IO t~
>
30
"6
t~ •" 20 0 o
10 10 3 seconds O,
1
2
3
4
5
6 Time 7
Fig. I0 Changes of contact potential under the conditions listed in table.2
[1] S. Wu, H.S. Cheng, "A Sliding Wear Model for Partial-EHL Contacts", J. of Tribology, pp.134-14 l January 199 !. [2] Y.Z Hu, N. Li, K. Tonder, "A Dynamic System Model for Lubricated Sliding Wear and Runningin", J. of Tribology, July 1991, Vol. 113, pp.499505, July 1991. [3] W.B. Ribbens,"Surface roughness measurements by holographic interferometry." Appl. Opt. l l., pp.807-810, 1972. [4] T.C. Bristow and K. Arakellian, "Surface rough-
280 ness measurements using a Nomarsky type scanning instruments." Proc. SPIE 749, pp.114-118, 1987. [5] G. Molessini, P.Poggi and F. Quercioli, "Focusmultiplexed optical profilometef' in ICO 13. Conference digest, pp 188-189, 1984. [6] D. H. Hensler, "Light scattering from surface polycrystalline aluminum oxide surfaces." Appl. Opt. 11. 2522, 1972. [7] U. Persson, "Measurement of surface roughness on rough machined surfaces spectral correlation and image analysis." Wear, 160, pp.221-225, 1993. [8] W. Wang, P.L. Wong, J.B. Luo and Z. Zhang, "A New Optical Technique for Rouglmess Measurement on Moving Surface.", submitted, 1997.