Fretting Friction

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Wear 249 (2001) 379388

Effects of ambient pressure on fretting friction and wearbehavior between SUS 304 steels

Rong Chen, Akira Iwabuchi, Tomoharu ShimizuDepartment of Mechanical Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan

Received 27 March 2000; received in revised form 11 September 2000; accepted 22 February 2001

Abstract

Effects of ambient pressure on fretting friction and wear behavior between SUS 304 steels were investigated. The ambient pressure

varied from 105 to 104 Pa. The experiments were conducted at normal loads of 0.6, 1.0 and 1.8 N, slip amplitudes of 50 and 100m,

frequency of 10 Hz and 105 cycles. It was found that there was a critical pressure below which coefficient of friction was increased. Thepeak wear volume occurred at pressure of 103 Pa, not at atmospheric pressure (105 Pa), it was revealed that the wear volume depends on the

formation of metallic particle or oxide particle and the effects of these particles on the contact surface. In order to examine the formation

and removal of the oxide debris particles on the wear scar, another experiments were carried out in which ambient pressure was changed

alternately from 105 to 103 Pa and from 103 to 105 Pa for every 103 cycles. Oxidized particle volumes were measured by subtracting

the disk wear volumes before and after 3% HCl solution treatment which was used to remove oxidized particles. Oxide particle volume

was proved to be larger after fretting wear at 105 Pa. These particle remains in the roughs of wear scar were difficult to be removed during

the consequent fretting wear in 103 Pa even up to 106 cycles which results in coefficient of friction not increasing at 103 Pa as expected.

2001 Elsevier Science B.V. All rights reserved.

Keywords: Fretting; Friction and wear; Coefficient of friction; Oxide layer; Oxide particles; Ambient pressure

1. Introduction

Fretting wear is caused by external vibration with small

amplitude. The important factor of the mechanism for fret-

ting wear is oxidation process and subsequent behavior of

oxide debris particles between surfaces. As a result, fretting

wear is sometimes referred to as fretting corrosion [1].

Fretting wear is also complicated by the contact problem

[2,3].

One of the authors had examined the oxidation process in

fretting. It was found that the high temperature fretting ac-

celerated the oxidation rate on the fretting surface, and com-

pacted oxide layer formed on the wear surface could reduce

fretting damage and the coefficient of friction [4]. Frettingin vacuum up to 103 Pa was also investigated for under-

standing the effect of oxidation and oxide particles on fret-

ting wear, where the oxidation process was forbidden [57].

It was found that the change in the coefficient of friction

with fretting cycles was affected by the formation of oxide.

In the first few 100 cycles, coefficient of friction increased,

then it dropped to certain value depending on the pressure.

Corresponding author. Tel.: +81-19-621-6417; fax: +81-19-621-6417.

E-mail address: [email protected] (R. Chen).

Oxide plays an important role in fretting. The wear con-trol factor is generally considered as the debris separated

form the bulk surface for usual sliding wear. Trapped wear

particles at the interface are not taken into account seriously.

However, it is the removal rate of wear particles from the in-

terface, which controls the fretting wear [8]. The oxide par-

ticles are compacted and act as the solid lubricant in fretting.

The separation and following recovery of this compacted

oxide layer determines the wear rate. However, oxide par-

ticles abrade the surface before the compaction as a loose

particle [9]. In order to explore the oxidation or adsorption

of oxygen on metal surface affecting the growth process of

the transferred particles, the ambient pressure was alternated

between air and vacuum [10]. This experimental method isalso useful to clarify the formation and separation process of

compacted oxide layer in fretting. However, until now, it is

still not clear if the trapped oxide particle could be removed

form the fretting surface in lower pressure up to long cycles

at the condition that the oxide particles are already formed

during wear in higher pressure.

In this paper, fretting wear experiments at constant am-

bient condition and varying ambient condition were con-

ducted. In first part, the coefficient of friction at lower

pressure below 103 Pa was examined. We are interested

0043-1648/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 5 4 7 - 6


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in how the coefficient of friction is affected in much lower

pressure, whether it increases or saturates at a certain value

with decreasing pressure. Lower pressure can be obtained

by using a turbo-molecular pump.

In second part, the formation and removal of oxide debris

and wear behavior at 105 and 103 Pa were investigated. The

experiments were done by alternating the pressure from 105

to 103 Pa and from 103 to 105 Pa for every 103 cycles.

The oxide particle volume was measured by subtracting the

disk wear volumes before and after 3 vol.% HCl solution

treatment which was used to remove the oxide debris. From

the change of the coefficient of friction, we try to understand

at how long cycles, the oxide particle could be removed form

the wear scars. The adhesive wear behavior of fretting wear

in lower pressure was also investigated.

2. Experiment

2.1. Materials

The specimen material was type-304 (JIS SUS 304) stain-

less steel for both specimens. A moving upper specimen

was a ball with a 8 mm diameter. The lower disk specimen

was turned to a diameter of 12 and 4 mm thick. Surface was

buff-polished. The peak-to-galley surface roughness Ry was

0.60m for a ball and 0.26 m for a disk. Hardness of the

specimen Hv was 2087 MPa for a ball and 1852MPa for

a disk. Specimens were cleaned ultrasonically with acetone

for 10 min before setting-up.

2.2. Fretting wear tests

A schematic diagram of the experimental apparatus used

in this work is shown in Fig. 1 [11]. Piezoelectric actuator

provided the oscillating motion. A moving ball specimen

was supported with a horizontal beam consisting of two elas-

tic hinges, and the relative slip amplitude between specimens

could be amplified by about eight times of the vibration of

the actuator. A lower disk-like specimen was pressed onto a

Fig. 1. Schematic diagram of an experimental apparatus.

ball specimen using an elastic beam, and a screw thread and a

piezoelectric actuator gave the normal load. Frictional force

and normal force were measured with strain gages attached

to the elastic beam supporting the lower specimen. Relative

slip amplitude was measured with a leaf spring type dis-

placement transducer. Frictional force, normal load and slip

amplitude were recorded in PC with a sampling time of 2 ms.

Normal load and slip amplitude were controlled with PC.

The apparatus was put in a vacuum chamber, which was

evacuated with a rotary pump and a turbo-molecular pump

to 104 Pa. The ambient pressure was measured with a mer-

cury manometer, a Schlutz gage and a BA gage depending

on the pressure. The pressure was controlled to a certain

value manually. The residual gas pressure below 103 Pa

was measured with a mass-spectroscopy. At 103 Pa resid-

ual gas includes 62% of H2O, 28% of N2, 7% of O2 and

other gases.

Two series of experiment were carried out, as noted above.

The experimental condition for the first one was as follows:

a normal force of 1.0 N, slip amplitudes of 50 and 100m, afrequency of 10 Hz, and 105 fretting cycles at room temper-

ature. Ambient pressure was changed form 105 to 104 Pa

with 10 steps. Coefficient of friction was measured contin-

uously.

The second experiment was under the following con-

dition: the ambient pressure was alternated from 105 to

103 Pa and from 103 to 105 Pa for every 103 cycles. Nor-

mal load was given 1.0 N, slip amplitudes were 50 and

100m and frequency was 10 Hz. At first situation, the test

was first carried out for 103 cycles at 105 Pa. Then, the fret-

ting test was stopped to evacuate the chamber to 103 Pa.

After evacuation, the fretting test of 103 cycles was restarted.

At second situation, the test was first carried out for 103

cycles at 103 Pa. Then, after air was introduced into the

chamber, the fretting test was restarted. These tests were re-

peated up to 6000 cycles. In order to evaluate the possibility

if the oxide particles could be removed form the wear scar,

a test was carried out first at 105 Pa for 103 cycles and then

put in vacuum up to 106 cycles. The tests were carried out

more than three times at the same condition.

2.3. Fretting scar assessment

Wear scars were observed by SEM, EDX was used for

chemical element analysis of wear debris. Wear volumes

were evaluated by a 3D profilometer with a trace pitch of20m.

3. Results

3.1. Constant ambient conditions

3.1.1. Friction

Fig. 2 shows the change in coefficient of friction with

fretting cycles at different pressures from 104 to 105 Pa. It

was very steady after 100 cycles for higher pressures above


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Fig. 2. Coefficient of friction against the number of cycles at 1.0N, 50,

100m, and various ambient pressures.

10 Pa. However, it increased with cycles and decreased for

the pressures below 101 Pa. The peak coefficient of friction

with the cycles shows that the peak depend on the ambient

pressure. Apparently, the peak cycle is delayed by the lower

pressure.

Fig. 3 shows the coefficient of friction against the ambient

pressure at slip amplitudes of 50 and 100 m, where it was

obtained as the mean value after 104 cycles. The coefficient

of friction was kept at 0.81.0 independent of pressure and

slip amplitude when the pressure was above 1.0 Pa. It started

Fig. 3. Dependence of mean coefficient of friction on ambient pressure

at 105 cycles.

to increase below 1.0 or 0.1 Pa depending on slip amplitude

It reveals the effect of slip amplitude on the coefficient of

friction.

3.1.2. Wear

Wear volume loss of the upper specimens ball and lower

specimens disk was measured for 50 and 100 m slip ampli-

tude, as shown in Fig. 4. The negative wear volume below

the critical pressure means the apparent volume increase due

to surface roughening with plastic deformation and transfer

from the opposite surface. It reveals that the extensive wear

volume was obtained above the critical pressure. The wear

of an upper specimen was greater than that of a lower sta-

tionary specimen.

Besides, Fig. 4 shows that the maximum wear volume

was not at the atmospheric pressure (105 Pa), but at 103 Pa.

Wear rate is dependent on the dimension of wear debris

and the number of debris particles. In the previous tests, the

wear peak appeared at around 10 Pa for pin-on-disk type

unidirectional sliding wear test [13,14]. Such a wear peakmay be resulted form the formation of large metallic wear

particles, because of lack of oxygen to oxidize the particles.

Fig. 4 also shows the decrease in wear at 102 Pa at 50m

Fig. 4. Dependence of wear volume of upper (ball) and lower (disk) on

ambient pressures.


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382 R. Chen et al./ Wear 249 (2001) 379388

or 10 Pa at 100m. Below these pressures, wear increases

again, and decreases.

3.1.3. Scar observations

Apparently, patterns of wear scar and wear debris was

different at varied pressure form SEM observation. Fig. 5

represents the typical wear scars from 10

5

to 10

3

Pa at thecondition of 1 N and 50 m. The chemical elements of the

wear debris were analyzed by EDX. The results in Fig. 6(a)

reveals that the debris at 105 Pa after 105 cycles was oxide

particles, for the shape of O element distribution was the

same as that of particle. However, the debris at 103 Pa after

105 cycles was metallic particles, for no O element could

be detected at the position of the two particles, as shown in

Fig. 5. SEM photographs of wear scars after 105 fretting cycles at 1 N and 50m at (a) 105 Pa; (b) 100 Pa; (c) 103 Pa.

Fig. 6(b). The morphology of wear scar in Fig. 5 reveals that

the degree of oxidization was changed by ambient pressure.

Many small oxide particles occurred in high ambient pres-

sure. However, larger metallic particles were easy to form in

very low ambient pressure. Comparative to the oxide parti-

cles occurred in high pressure, the number of metallic debris

particles in lower pressure was less.

3.2. Varying ambient conditions

3.2.1. Friction

Fig. 7 shows the typical coefficient of friction at alternat-

ing pressure between 105 and 103 Pa at 1 N and 50m. The

coefficient of friction was not increased from 105 to 103 Pa


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Fig. 6. Element analysis of wear debris by EDX after 10 5 fretting cycles at 50m and 1N (a) at 105 Pa; (b) 103 Pa.

Fig. 7. Typical example of the change in coefficient of friction with an

alternation of ambient pressure between 105 and 103 Pa.

as expected. It may be due to the oxide particle remained in

the wear scars.

In order to determine if the oxide particles could be ex-

cluded from wear scars after long cycles, we did the experi-

ment at the process first at 105 Pa for 1000 cycles, and then

at 103 Pa up to 106 cycles. The result in Fig. 8 shows that

the coefficient of friction was not increased even up to 10 6

cycles.If the fretting wear was firstly conducted at 103 Pa from

0 to 1000 cycles, the coefficient of friction was high, about

2.2, as shown in Fig. 9. It decreased to 0.5 immediately

at 105 Pa at the first cycle in the consequent fretting wear,

which may be due to compacted oxide layer formed soon

as the pressure was changed. When wear at 103 Pa again

form 2000 to 3000 cycles, the coefficient of friction was

kept at 0.5 and could not go back to the original high value.

The same result was shown form 4000 to 5000 cycles at

103 Pa.


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Fig. 8. Coefficient of friction of SUS 304 steel first under fretting wear

for 103 cycles at 105 Pa and then for 106 cycles at 103 Pa.

3.2.2. Wear

The wear volumes at the alternation pressure from 105 to

103 Pa at slip amplitudes of 50 and 100m were shown inFig. 10. Here, only wear volumes of disk were measured.

The white and dark points were the wear volumes before and

after HCl solution treatment, upper and lower were sign

as the heap and loss volumes which were above and below

the original unworn specimen surface. It shows that at 105 Pa

by 1000 cycles, the heap and loss volumes were small. They

were increased at 103 Pa after 2000 cycles, especially for

the heap volume. It may be due to the fact that adhesive

wear easily occurred in low pressure. The difference of the

volume before and after HCl treatment was connected with

the oxide debris. It was discussed in the next chapter.

The wear volumes form 103 to 105 Pa at50m were also

measured, the results were shown as in Fig. 11. It also proves

the heap volume was larger at 103 Pa by 1000 cycles. It

may be due to the plastic deformation for there was less

oxide particle formation at 103 Pa. After 2000 cycles at

105 Pa, the oxide particle volume was increased.

Fig. 9. Typical example of the changes in coefficient of friction with an

alternation of ambient pressure between 103 and 105 Pa.

Fig. 10. Wear volume of disk specimen before and after 3 vol.% HCl

treatment at an alternation of ambient pressure between 105 and 103 Pa

3.2.3. Scar observationsWear scars during varying ambient pressure at 105 Pa for

1000 cycles and 103 for 2000 cycles were investigated by

SEM and EDX. Fig. 12 shows the wear scar surface after

1000 cycles at 105 Pa, there were many small oxide particles

Fig. 11. Wear volume of disk specimen before and after 3 vol.% HCl

treatment at an alternation of ambient pressure between 103 and 105 Pa


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Fig. 12. SEM photographs of wear scars after 1000 cycles at (a) 50 mand 1.0 N; (b) magnitude.

on the wear scar, the feature of oxidization of wear scar was

very obvious. The chemical element analysis of the wear

scar by EDX shows that the distribution of O element was

similar with the shape of wear scars, as shown in Fig. 13(a),

it means the wear scar surface was oxidized after 1000 cycles

at 105 Pa. For comparison, the wear scars after consequent

wear at 103 Pa up to 2000 cycles was measured, as shown

in Fig. 13(b), O element cannot be detected at the area of

wear scar. The wear scar surface was not obviously oxidizedafter 2000 cycles at 103 Pa. The compacted oxide layer

formed after 1000 cycles at 103 Pa could be removed by the

consequent fretting wear. However, parts of oxide particles

were still remained in the scar.

Fig. 14(a) shows oxide particles remained in the roughs

of wear scars at 103 Pa and 2000 cycles. These particles

could be removed by 3 vol.% HCl treatment, as shown in

Fig. 14(b). Fig. 15 shows the oxide particles were clustered

at the center of the wear scar after fretting wear at 103 Pa

by 6000 cycles. They were difficult to be removed.

4. Discussion

4.1. The effect of amplitude on fretting behavior at

constant ambient conditions

The critical pressure is defined as the pressure at which

the coefficient of friction begins to increase. It seems that thecritical pressure was higher for small slip amplitude, and the

coefficient of friction at small slip amplitude was higher than

that at large slip amplitude at the same ambient pressure,

as shown in Fig. 3. This behavior can be explained as the

great attack of residual oxygen to the contacting surfaces

with larger slip amplitude. The mutual overlap coefficient

(MOC) was defined as

MOC =Ac

Aw(1)

where Ac was the apparent contact area and Aw the wear

scar area [12]. For pin-on-disk unidirectional sliding wear

test, Ac was the worn area of a pin and Aw the worn area ofa disk. In this case, Ac was the wear scar of a ball and Awthat of disk. Compared with pin-on-disk, MOC for fretting

becomes high and approaches unity depending on slip am-

plitude because the sliding distance was rather smaller than

a dimension of contact area. MOC determines the area that

oxygen can attack or the difficulty of movement of trapped

wear particles to be removed form the interface. As MOC

was smaller with larger slip amplitude, the exposed area to

oxygen attack becomes large and oxide formation was cer-

tainly facilitated. Therefore, coefficient of friction becomes

low with increasing slip amplitude at a certain pressure be-

low the critical pressure. At the pressure above the critical

pressure oxygen attack was not important, because sufficient

oxide was formed to maintain the low coefficient of friction.

In other words, the rate of controlling factor for the forma-

tion of oxide was not the oxygen attack to the surface in this

pressure region.

The increase in coefficient of friction with decreasing

pressure also results from the decrease in oxygen attack rate

to the interface even at a constant slip amplitude, i.e. MOC.

When the pressure was decreased form 103 to 104 Pa,

the increase of coefficient of friction was very obvious. The

oxygen attack in super lower pressure was decreased obvi-

ously at smaller slip amplitude.

4.2. The oxide particle volume at varying ambient

conditions

HCl solution was used to remove the oxidized particles

at wear scar surface, the time should be controlled to avoid

removing the metallic surface. The oxide particle volumes

could be obtained by subtracting the disk wear volume be-

fore and after HCl treatment. As shown in Fig. 10, at first

1000 cycles at 105 Pa and 50m before HCl solution treat-

ment, the heap volume was about 10 106 mm3, after


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386 R. Chen et al./ Wear 249 (2001) 379388

Fig. 13. Chemical element analysis of the fretting wear scar by EDX after (a) 1000 cycles at 10 5 Pa; (b) 2000 cycles in 103 Pa at 50m and 1.0N.

Fig. 14. SEM photographs of wear scars after 2000 cycles at 50m and 1.0 N (a) before; (b) after 3 vol.% HCl treatment.


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Fig. 15. SEM photographs of wear scars after 6000 fretting cycles at analternation ambient pressure at 50m and 1N from 105 to 103 Pa (a)

wear scar; (b) magnitude.

HCl treatment, it was near to 0. This heap volume was re-

moved by HCl solution treatment. The loss volume before

HCl treatment was near 0, after treatment, it was about to

10 106 mm3. The removal volume was equal to the loss

volume, it was same at 50 and 100m. It was concluded

that an oxide particle layer as heap volume was formed in

the wear scar after 1000 cycles. This layer could be removed

by consequent fretting wear after 2000 cycles at 103 Pa.

Because after 2000 cycles, both the heap and loss volumesbefore and after HCl treatment were the same, there was

less oxide particle volume. It coincides with the Figs. 12

and 13. When wear at 105 Pa up to 3000 cycles, the oxide

particle volume was increased again. It was increased to the

largest after 5000 cycles at 105 Pa. It may be due to the re-

mained oxide particles and new oxide particles were more

easy to be produced, for the existence of particles may re-

sult in abrasive wear. The oxide particle volumes at 105 Pa

at cycles of 1000, 3000 and 5000 were larger than that of

at 103 Pa at cycles of 2000, 4000 and 6000, and increased

with the cycles even at 103 Pa. It proves that the oxide par-

ticles were easily produced at high pressure, and difficult to

be removed form the wear scars after consequent fretting

cycles at 103 Pa.

The coefficient of friction was not obviously increased af-

ter 1000 cycles in 103 Pa, as shown in Fig. 7. It reveals that

even the compacted oxide layer could be removed at 103 Pa

by 2000 cycles, the oxide particles could not be totally es-

caped from the wear scar. Parts of them were dropped into

the wear roughs, as shown in Figs. 14 and 15. The trapped

oxide particles may be due to decrease in the coefficient of

friction.

From Fig. 9, it reveals that the coefficient of friction

at 103 Pa was high only in the condition of very clean

surface. If the contacted surface remained with oxide

particles due to fretting wear at 105 Pa, the coefficient

of friction was smaller, for it was difficult to remove the

oxide particles out of the wear scars at the small fretting

amplitude.

5. Conclusions

From two series of fretting experiment of type-304 steel

at various ambient pressures, the following conclusions are

drawn:

1. the critical pressure existed, above which the frictional

coefficient was constant and below which the coefficient

of friction increased, and it became lower pressure with

increasing slip amplitude;

2. the maximum wear volume was not at the atmospheric

pressure (105 Pa), but at 103 Pa. It depends on the possi-

bility of formation of metallic particle or oxide particle

and the effects of these particles on the contact surface;

3. the oxide particle volume was proved to be larger after

fretting wear at 105 Pa, these particles remained in the

roughs of wear scar and were difficult to be removed

during the consequent fretting wear at 103 Pa even up

to 106 cycles, which results in the coefficient of friction

not being changed at the alternation pressure form 105

to 103 Pa.

Acknowledgements

Authors thank Mr. N. Fukuda, undergraduate student, for

his serious help in the experiment. Authors thanks Mr. K.

Matusmoto, technician at our department, for preparation of

specimens and an experiment rig.

References

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