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Effect of MoS2 additive on electrical pitting mechanism of lubricatedsurface for Babbitt alloy/bearing steel pair under ac electric field

Chung-Ming Lin a, Yuang-Cherng Chiou b,, Rong-Tsong Lee b

a Department of Mechanical Engineering, Cheng-Shiu University, Kaohsiung 833, Taiwanb Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

Received 1 December 2003; accepted 21 May 2004

Abstract

In this study, a static electrical pitting tester, SEM, and EDS are employed to investigate the effects of supply voltage, supply current,

MoS2 concentration, and oil film thickness on the mechanism of electrical pitting for the lubricated surface of Babbitt alloy/bearing steel

pair under ac electric field. The pitting regimes, consisting of pitting and no-pitting regimes, have been established in terms of the supply

voltage, the oil film thickness, and MoS2 concentration at different supply currents. The area of pitting regime increases with increasing

MoS2 concentration and supply current. It is seen from the pitted surfaces of the Babbitt alloy block that there exists an obvious concave

crater with a few plateaus. According to the force measurement, the formation process of the metal column is proposed. The cross-sectional

area of the plateau significantly increases with increasing oil film thickness and MoS2 concentration. Moreover, the ratio of pitting area

to interface power (Ap/P) increases with increasing MoS2 concentration and oil film thickness at the oil film thickness less than 6 m.

When the oil film thickness increases from 6 to 10m, the value ofAp/P quickly increases to about 10 times, because MoS2 powders can

sufficiently suspend on the oil across the gap.

2004 Elsevier B.V. All rights reserved.

Keywords: Molybdenum disulfide; Electrical pitting mechanism; Lubricated surface

1. Introduction

Electric current passed through the bearings has been rec-

ognized as one of the sources of bearing failure, and the

sources of bearing currents have been previously described

[13]. Generally, the damage of the bearing due to the shaft

current can corrugate the surface and can considerably ac-

celerate the mechanical wear [4]. Its effect on the deterio-

ration of lubricant has also been studied [5,6]. Costello [7]

indicates that there are four distinct types of bearing damage

due to shaft current: (a) frosting, (b) spark track, (c) pitting,

and (d) welding.

The addition of solid lubricants to mineral oils is used

to improve the friction and wear properties of oils [811].

It was indicated by Stock [8] that the lubricated wear de-

creased with increasing amount of MoS2 at high load case,

but somewhere between 2 and 4% should be the optimum

concentration at light loads. Bartz [1214] thought that the

solid-lubricant additive raises the load-carrying capacity of

liquid lubricant in most cases, but in some cases, an enlarge-

Corresponding author. Tel.:+886 7 525 2000; fax: +886 7 525 4299.

E-mail address: ycchiou@mail.nsysu.edu.tw (Y.-C. Chiou).

ment of the wear scar causes this improvement. Generally,

the lubricating effectiveness was improved with finer parti-

cles because finer particles more easily formed a complete

and continuous film than coarser particles. However, very

few works investigate the effect of MoS2 concentration on

the electrical pitting under ac electric field. El Beqqali et al.

[15] studied electrical properties of MoS2. Furthermore, the

breakdown of hydrodynamic lubrication is correlated with

the dielectric strength of the lubricant, and this dielectric

strength depends on the oil film thickness and the additive

concentration. Hence, it is worth investigating.

In this study, the mechanism of electrical pitting on the

lubricated surface of Babbitt alloy/steel pair is investigated

by using the SEM micrograph and EDS analysis, and the

pitting formation diagram for the Babbitt alloy/steel pair is

also established.

2. Experimental apparatus and procedures

The experimental apparatus used here shown in Fig. 1

has been described in a previous experiment [16]. Using the

plastic oil tank insulates the block specimen, and a small

0043-1648/$ see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.wear.2004.05.002

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834 C.-M. Lin et al./ Wear 257 (2004) 833842

Fig. 1. Schematic diagram of the static electrical pitting tester.

ball inserted in the pin specimen is held in an insulating

holder. They are immersed in the oil tank, so that they can

constitute an electric circuit. The oil film thickness between

the ball and the block can be adjusted from 0.2 to 25m

by 2m heads of non-rotating spindle type. Moreover, an

analog mu-checker with a graduation of 0.1 m is employed

to calibrate this oil film thickness. The sequence of operation

for this device has been described in the earlier work [17].

To avoid the effect of MoS2 particle on the accuracy of the

oil film thickness, the oil with the MoS2 powders is filled in

the oil tank after the gap distance is set to a certain value.

To select a certain size of powder, they are sequentially

filtered by the ashless filter papers of pore size 10, 8, and

7m after the MoS2 powders are uniformly mixed with

acetone. The SEM micrograph shown in Fig. 2 indicatesthat the particle shape is flake-like and the average particle

size is about 8m. Then the MoS2 powders are uniformly

mixed to the mineral paraffin base oil (26.59 cSt at 40 C,

4.76 cSt at 100 C) so that their concentrations range from

0.1 to 5 wt.% and their resistivity of the oil from 1.061014

to 0.756 1014 cm measured by using the digital super

megohmmeter (maximum range: 2 1017 cm). The oil

Fig. 2. SEM micrograph of MoS2 powder.

temperature was maintained at 25 3 C. The specimens

of ball and block are made of commercial bearing steel

(SUJ2) and Babbitt alloy (85.84% Sn, 8.76% Sb, 4.88% Cu,

0.34% Pb), respectively. The test specimens are the same

one used in the previous paper [17]. The arithmetic average

roughness for the ball and the block surfaces are about 0.01

and 0.02m, respectively.When the oil film thickness has been adjusted to a certain

value, the r.m.s. supply voltage is preset from 1 to 100 V

through an ac power supply, and the r.m.s. supply current

between ball and block specimens can be adjusted from 1

to 8 A through a variable resistor. The time of applied elec-

trical field between the specimens is 30 s. The strain-gage

and piezoelectric load cells simultaneously measure the

interface force between the specimens. The variations of in-

terface voltage (Va), current (Ia) and force can be measured

on the data acquisition system and fed to a personal com-

puter for data analysis. Moreover, a digital oscilloscope is

also employed to observe the variations of interface voltage,

current, and force. The interface impedance, Ra, is calcu-lated from Ohms law (Ra = Va/Ia). The interface force

is used to judge the molten and the adherence states. The

optical microscope or the scanning electronic microscope

(SEM) with the energy dispersion spectrometer (EDS) can

be used to observe the electrical pitting mechanism.

3. Experimental results and discussion

3.1. Variations of interface voltage and impedance

According to the experimental procedure mentioned

above, the typical results for the interface voltage and the in-

terface impedance versus the oil film thickness for different

supply voltages, supply currents, and MoS2 concentrations

are shown in Figs. 3 and 4. Fig. 3 shows the effects of MoS2concentration and oil film thickness on interface voltage and

impedance at supply voltage of 100 V and supply current

of 1 A. It is seen from Fig. 3 that the interface voltage and

impedance increases with increasing oil film thickness (h),

reaches to maximum at h = 3m, and finally decreases to

a certain value at h = 8m. When the oil film thickness is

larger than 10m, the interface voltage abruptly increases

to the magnitude of the supply voltage, and the interface

impedance increases to infinity, meaning that the insulation

condition has been achieved and the pitting disappears. On

the other hand, at the pitting condition, with decreasing the

MoS2 concentration, the interface voltage and impedance

increase, but the effect of MoS2 concentration on the min-

imum oil film thickness for the non-pitting region is not

significant.

When the supply current is increased to 8 A, the effects

of MoS2 concentration and oil film thickness on interface

voltage and impedance are shown in Fig. 4. By comparing

the results of Figs. 3 and 4, with increasing the supply

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Fig. 3. The effects of MoS2 concentration and oil film thickness on

interface voltage and interface impedance at supply voltage of 100 V, and

supply current of 1 A.

current, there exists the same tendency for the effect of oil

film thickness on interface voltage and impedance, mean-

ing that the interface voltage and impedance reaches to

maximum at h = 3m, and decreases to a certain value at

h = 8m. However, the minimum oil film thickness for the

Fig. 4. The effects of MoS2 concentration and oil film thickness on

interface voltage and interface impedance at supply voltage of 100 V, and

supply current of 8 A.

insulation condition significantly increases with increasing

supply current and MoS2 concentration. On the other hand,

at the pitting condition, the interface voltage and impedance

are significantly larger for 0.1 wt.% MoS2 than for the other

MoS2 concentrations.

3.2. Threshold condition of pitting formation

It is seen from Figs. 3 and 4 that with increasing oil film

thickness, the interface condition varies from the arc dis-

charge to the insulation. Under the arc discharge state, the

pitting occurs on the specimen surfaces. Hence, there ex-

ists a critical oil film thickness to judge the pitting status

at a certain of supply voltage and MoS2 concentration. On

the other hand, at a certain of oil film thickness and MoS 2concentration, when the supply voltage is larger than a crit-

ical value, the arc discharge occurs on the interface. This

critical supply voltage is known as the threshold voltage.

Hence, at a certain of supply current and MoS2 concentra-tion, the boundary between no-pitting and pitting regimes

can be plotted in terms of the supply voltage and the oil film

thickness, as shown in Figs. 5 and 6.

Fig. 5 shows the diagram of pitting regimes at supply cur-

rent of 1 A for different MoS2 concentrations. It is seen from

this figure that the pitting region is significant influenced by

adding MoS2 particles when the supply voltage is larger than

30 V and the oil film thickness is larger than 1.5m. At the

supply voltage of 100 V, the critical oil film thickness can

be increased to more than twice by adding MoS2 particles.

Moreover, the effect of MoS2 concentration on the critical

Fig. 5. The diagram of electrical pitting formation at supply current of

1 A.

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Fig. 6. The diagram of electrical pitting formation at supply current of

8 A.

oil film thickness is just a little. Fig. 6 shows the diagram of

pitting regimes at supply current of 8 A for different MoS2concentrations. It is seen from this figure that at the supply

voltage of 100 V, the critical oil film thickness can be in-

creased to more than five times by adding MoS2 particles,

and the effect of MoS2 concentration on the critical oil filmthickness becomes significant.

It is seen from Figs. 5 and 6 that the pitting region be-

comes wider with the addition of MoS2 powder on the oil.

However, the supply current almost does not influence the

pitting region for the pure oil. The reason for this phe-

nomenon is that the addition of MoS2 powder on the oil

exhibits the electro-rheological effect when it is subjected

to a strong electric field over 4000 V mm1 [1820]. The

electro-rheological effect is based on the tendency of polar

particles to be influenced by the presence of an electric

field. In the strong electric field, the particles orient them-

selves along the electric field lines, and the attractive force

between particles is increased due to the proximity of op-

posite poles. This attractive force causes particles to form

chains along the electric field lines, and these chains then

slowly aggregate to form clusters. These clusters cause the

decrease of interface impedance, and the current flowing

through the cluster becomes easier. Moreover, when the

supply current and the MoS2 concentration increase, the

distribution of powders in the clusters becomes close. Con-

sequently, the threshold voltage decreases and the critical

oil film thickness for the arcing discharge increases. This

phenomenon is similar to the electrical-discharge machining

(EDM) by adding the powders with different conductivity

on the dielectric fluid [2123]. On the other hand, at the oil

film thickness less than 0.6m, the supply current and the

MoS2 concentration do not influence the pitting region. This

results from the powder size always larger than 0.6m,

and the MoS2 powders cannot suspend on the oil across the

gap.

3.3. Observation of pitting surface

As mentioned above, the effect of MoS2 concentration on

the pitting region is notable under high supply voltage and

current. Hence, the important role of MoS2 concentration

on the pitting formation can be revealed by using SEM to

observe the feature of pitting and using the EDS to inves-

tigate the mass transfer between two specimens under the

severe conditions, such as high supply voltage and current

with large oil film thickness. The typical SEM micrograph

and the EDS analysis of the pitted surfaces are shown in

Figs. 79.

After the discharge time of 30 s, Fig. 7 shows the backscat-tered electron images (BEI) of pitted surfaces on the block

and the ball for different MoS2 concentrations under the

supply voltage of 100 V, the supply current of 8 A, and the

oil film thickness of 10m. It is seen from the pitted sur-

faces of the block that there exists an obvious concave crater

with a few plateaus. A few concave pits on the plateaus

within the crater can be observed. Moreover, the solidified

metal strip, which flows from the crater during the arcing,

can be observed outside the concave crater. The crater size

and the area of the plateaus on the block surface increase

with increasing MoS2 concentration. Because the plateau is

subjected to the squeeze action of the ball, it is quite flat,and its height is larger than the thickness of strip outside the

crater.

Fig. 8(a) and (b) shows the SEM micrograph of pitted

surfaces on the block and the ball for the same electricity and

MoS2 concentration conditions as Fig. 7(c) and (d) except

for the oil film thickness of 20m. By comparing the pitted

surfaces ofFigs. 7(c) and (d) to 8(a) and (b), the crater size,

and the area and the height of the plateaus on the block

surface increase a little with increasing oil film thickness.

However, only one plateau remains. Fig. 9(a) and (b) shows

the EDS analysis of the pitted surfaces for Fig. 7(a) and

(d), respectively. It is seen from Fig. 9(a) and (b) that a

large amount of tin element, which comes from the transfer

layer of Babbitt alloy block, is left over the pitted surface

of steel ball. On the other hand, a little amount of ferrous

element, which comes from the transfer layer steel ball, is

left over the pitted surface of block. It should be noted that

the molybdenum element could not be found on the pitted

surface of the block.

3.4. Formation mechanism of plateau

To understand the formation mechanism of plateau, the

interface force between the specimens are measured by using

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Fig. 7. SEM micrograghs of pitted surfaces under different MoS2 concentrations at supply voltage of 100 V, supply current of 8 A, and oil film thickness

of 10m.

the strain-gage and piezoelectric load cells simultaneously

during the test. The typical results are shown in Fig. 10 for

the supply voltage of 100 V, the supply current of 8 A, MoS2concentration of 3 wt.%, and the oil film thickness of 10m.

To easily explain the relationship between the interface force

and the plateau, the electricity process is divided into four

time stages: (I) 2.5 s, (II) 2.530 s, (III) 3050 s (the power

has been switched off), and (IV) 5060 s (pull the specimens

apart slowly).

It is seen from Fig. 10 that the arcing struck surface

produces the repulsive force, which can be measured

by the piezoelectric load cell within 1 ms. However, the

attractive force can be measured after 1 ms due to the

electro-rheological effect under the action of an electric field

between the opposite poles. In the stage (I), the attractive

force dominates the discharge process, but the interface force

switches from the attractive force to the repulsive force with

increasing time. Generally, the load cell of strain-gage type

cannot measure the instantaneous variation of the interface

force. It is inferred from the variation of interface force in

the first time stage that the molten metal column grows and

is in contact with the ball surface within a few milliseconds,

and then it pushes and squeezes the ball surface in the stage

(I). In the stage (II), the interface force measured from the

piezoelectric load cell almost keeps a certain of value, but

the interface force measured from the strain-gage load cell

increases with increasing time. This indicates that the molten

metal column grows slowly, and it pushes the ball surface

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Fig. 8. SEM micrograghs of pitted surfaces under different MoS2 concentrations at supply voltage of 100 V, supply current of 8 A, and oil film thickness

of 20m.

further. However, the piezoelectric load cell cannot mea-

sure the repulsive force between the specimens. In the stage

(III), when the power is switched off, the interface force

decreases with time. This indicates that the molten metal

column starts to solidify and cool. Because of the shrink-

age of solidified metal, the repulsive force decreases. In the

stage (IV), when the specimens are pulled apart slowly, thepull force increases quickly with time, and its peak value is

larger than the other stages. This situation indicates that the

surfaces weld together during the stage (III), thus, it need a

larger force to part them. Moreover, the variation of inter-

face impedance indicates that a high momentary current is

passed through surfaces during the stages (I) and (II) or at

the discharge duration. This result implies that the molten

metal column forms within few milliseconds, and it grows

with time during the stages (I) and (II). The weld strength is

defined as the pull force to break welds during the last stage

or the ratio of the peak value of repulsive force to the area of

plateaus. In this case, the weld strength is about 26 N/mm2.

This value is about one-third of tensile strength for Babbitt

alloy. This result indicates that the weld between the plateau

and the ball surface is not completely coherent, as shown in

Fig. 7.

Based on the results of Figs. 710, the pitted surfaces

formed at a certain oil film thickness during the discharge

process can be considered as follows. When the arcing

strikes across the gap between surfaces, the Babbitt alloy

block with lower melting point first melts, and then flows

radially outward. Under the action of the strong electrical

field, the molten metal orients itself along the electric field

line. The attractive force between the opposite poles causes

the molten metal to form columns within a few millisec-

onds, and the interface impedance decreases to the order of

0.1. These columns grow and are in contact with the ball

surface. When the power is switched off, the columns start

to solidify and cool. Finally, the surfaces weld together.

When the specimens are pulled apart, a flat plateau can be

found. Consequently, the formation process of the columnis proposed, as shown in Fig. 11.

To further understand the effects of MoS2 concentra-

tion and oil film thickness on the cohesive condition, the

pull force to break welds is plotted, as shown in Figs. 12

and 13. Fig. 12 shows the pull force for the supply voltage

of 100 V, the supply current of 8 A, and MoS2 concentration

of 3 wt.% under different oil film thickness. Results show

that the pull force is very small at the oil film thickness less

than 6m. When the oil film thickness is larger than 8 m,

the pull force is parabolically increased with increasing oil

film thickness. This result indicates that when the oil film

thickness is larger than 8m, and the MoS2 powders can

effectively suspend on the oil across the gap, and they cause

the significant increase for the cross-sectional area of the

plateau. Hence, the pull force significantly increases with oil

film thickness. These results agree with the SEM observa-

tions shown in Figs. 7(c) and 8(a). Fig. 13 shows the effect of

MoS2 concentration on the pull force under the same elec-

tricity condition as Fig. 12. Results show that the pull force

is parabolically increased with increasing MoS2 concentra-

tion. These results indicate that the cross-sectional area of

the plateau increases with increasing MoS2 concentration,

and they agree with the SEM observations shown in Figs. 7

and 8.

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Fig. 9. EDS analysis of pitted surfaces under different MoS2 concentrations at supply voltage of 100V, supply current of 8 A, and oil film thickness of

10m.

3.5. Formation mechanism of electrical pitting

It is known from a general interface discharge that the for-

mation of electrical pitting is the action of the Joule heat and

the arc strike, which directly relates to the interface power.

Under the action of the discharge energy, the interface mate-

rial first melts and then splashes. Finally, a crater is formed.

This discharge energy mainly dissipates on the medium and

the energy to cause melting and erosion of the surfaces. In

this study, the addition of MoS2 particles on the oil slightly

lowers the resistivity of the oil. The block surface appears

a crater with the protrudent plateau, as shown in Figs. 79.

Moreover, when the oil film thickness is larger than 8 m,

the cross-sectional area of the plateau increases with increas-

ing MoS2 concentration and oil film thickness. Hence, not

only the interface power but also MoS2 concentration and

oil film thickness influence the formation of electrical pit-ting. To investigate the formation mechanism of electrical

pitting, the ratio of pitting area to interface power for differ-

ent MoS2 concentrations and oil film thickness is arranged

in Fig. 14 under the same electricity condition.

Fig. 14 shows the ratio of pitting area to interface power

(Ap/P) versus the oil film thickness under different MoS2concentrations at the supply voltage of 100 V, the supply

current of 8 A. It is seen from this figure that when the oil

film thickness is less than 6m, since MoS2 powders cannot

sufficiently suspend on the oil across the gap, Ap/P increases

with increasing MoS2 concentration and oil film thickness.

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Fig. 10. The variations of interface force and impedance at supply voltage of 100 V, supply current of 8 A, oil film thickness of 10m, and MoS2

concentration of 3 wt.%.

Fig. 11. The formation process of the metal column: (a) the crater forms and the molten metal column initiates; (b) the metal column grows; (c) the

metal column grows and pushes the ball; (d) the column solidifies and shrinkages.

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Fig. 12. The effect of oil film thickness on the pull force to break welds at

supply voltage of 100V, supply current of 8 A, and MoS2 concentration

of 3 wt.%.

It is noted from this figure that the value of Ap/P can be

increased to more than twice by adding MoS2 of any con-

centration to the pure oil. On the other hand, when the oil

film thickness increases from 6 to 10 m, the value of Ap/P

Fig. 13. The effect of MoS2 concentration on the pull force to break

welds at supply voltage of 100 V, supply current of 8 A, and oil film

thickness of 10m.

Fig. 14. The ratio of pitting area to interface power vs. oil film thickness

under different MoS2 concentrations at the supply voltage of 100V and

the supply current of 8 A.

quickly increases to about 10 times, because MoS2 powders

can sufficiently suspend on the oil across the gap to form

a cluster structure. This leads the molten metal to form a

column. Hence, the column connects the specimens and the

interface power mainly dissipates on the Joule heat. This

situation becomes more obvious with increasing MoS2 con-

centration, and it results in the increase of the cross-sectionalarea of the plateau. Hence, the force to break weld also in-

creases, as shown in Fig. 13.

As mentioned above, the cross-sectional area of the

plateau and the pitting area increase by adding MoS2 parti-

cles to the pure oil. Hence, when the bearing surfaces are in

relative motion, they must shear the column connected the

surfaces. Consequently, the friction force and the damage

area of the electrical pitting increase.

4. Conclusions

In this study, a static electrical pitting tester, SEM, andEDS are employed to investigate the effects of supply volt-

age, supply current, MoS2 concentration, and oil film thick-

ness on the mechanism of electrical pitting for the lubricated

surface of Babbitt alloy/bearing steel pair under ac electric

field. The main results are as follows:

1. The pitting regimes, consisting of pitting and no-pitting

regimes, have been established in terms of the supply

voltage, the oil film thickness, and MoS2 concentration

at different supply currents. The area of pitting regime

increases with increasing MoS2 concentration and supply

current.

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2. It is seen from the pitted surfaces of the Babbitt alloy

block that there exists an obvious concave crater with a

few plateaus. According to the force measurements, re-

sults show that the attractive force between the specimens

causes the molten metal to form columns within a few

milliseconds, and the columns grow and are in contact

with the ball surface. When the power is switched off, thecolumns start to solidify and cool. Finally, the surfaces

weld together, and the plateau is formed.

3. The cross-sectional area of the plateau significantly in-

creases with increasing oil film thickness and MoS2concentration. The pull force to break welds is very

small at the oil film thickness less than 6m, but it

parabolically increases with increasing oil film thickness

and MoS2 concentration at the oil film thickness larger

than 8m.

4. The ratio of pitting area to interface power (Ap/P) in-

creases with increasing MoS2 concentration and oil film

thickness at the oil thickness less than 6m. When the

oil film thickness increases from 6 to 10m, the value ofAp/P quickly increases to about 10 times, because MoS2powders can sufficiently suspend on the oil across the gap.

Acknowledgements

The authors would like to express their appreciation to

the National Science Council (NSC-92-2212-E-110-023) in

Taiwan, ROC, for the financial support.

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