Kazuhisa Miyoshi

Glenn Research Center, Cleveland, Ohio

Masanori Iwaki, Kenichi Gotoh, Shingo Obara, and Kichiro Imagawa

Tsukuba Space Center, Tsukuba, Ibaraki, Japan

Friction and Wear Properties of Selected

Solid Lubricating Films

Part 1: Bonded and Magnetron-Sputtered Molybdenum

Disulfied and Ion-Plated Silver Films

NASA/TM—1999-209088/PART1

October 1999
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NASA/TM—1999-209088/PART1

October 1999

National Aeronautics and

Space Administration

Glenn Research Center

Kazuhisa Miyoshi

Glenn Research Center, Cleveland, Ohio

Masanori Iwaki, Kenichi Gotoh, Shingo Obara, and Kichiro Imagawa

Tsukuba Space Center, Tsukuba, Ibaraki, Japan

Friction and Wear Properties of Selected

Solid Lubricating Films

Part 1: Bonded and Magnetron-Sputtered Molybdenum

Disulfied and Ion-Plated Silver Films
Acknowledgments

This research activity was basic research conducted under the Reimbursable Space Act Agreement between NASA

Glenn Research Center and K Systems Corporation for Measurements and Analysis of Aerospace Tribology

for Various Materials.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03
NASA/TM—1999-209088/PART1 1

Friction and Wear Properties of Selected Solid Lubricating Films

Part 1: Bonded and Magnetron-Sputtered Molybdenum Disulfied and Ion-Plated Silver Films

Kazuhisa Miyoshi

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Masanori Iwaki, Kenichi Gotoh, Shingo Obara, and Kichiro Imagawa

National Space Development Agency of Japan

Tsukuba Space Center

Tsukuba, Ibaraki 305–8505 Japan

Summary

To evaluate commercially developed solid film lubricants for aerospace bearing applications, an investigation

was conducted to examine the friction and wear behavior of bonded molybdenum disulfide (MoS2), magnetronsputtered MoS2, and ion-plated silver films in sliding contact with 6-mm-diameter American Iron and Steel Institute

(AISI) 440C stainless steel balls. Unidirectional sliding friction experiments were conducted with a load of 5.9 N

(600 g), a mean Hertzian contact pressure of 0.79 GPa (maximum Hertzian contact pressure, 1.19 GPa), and a sliding velocity of 0.2 m/s. The experiments were conducted at room temperature in three environments: ultrahigh

vacuum (vacuum pressure, 7×10–7 Pa), humid air (relative humidity, ~20 percent), and dry nitrogen (relative humidity, <1 percent). The resultant films were characterized by scanning electron microscopy, energy-dispersive x-ray

spectroscopy, and surface profilometry.

Marked differences in the friction and wear of the bonded MoS2, magnetron-sputtered MoS2, and ion-plated

silver films investigated herein resulted from the environmental conditions and the solid lubricating film materials.

The main criteria for judging the performance of the solid lubricating films were coefficient of friction and wear

rate, which had to be less than 0.3 and on the order of 10–6 mm3/N·m or less, respectively. The bonded MoS2 and

magnetron-sputtered MoS2 films met the criteria in all three environments. Also, the wear rates of the counterpart

AISI 440C stainless steel balls met that criterion in all three environments. The ion-plated silver films met the criteria only in ultrahigh vacuum but failed in humid air and in dry nitrogen. In humid air and dry nitrogen both the silver

film wear rate and the ball wear rate were higher than the criteria and on the order of 10–5 mm3/N·m.

In ultrahigh vacuum the coefficient of friction and endurance (wear) life of the bonded MoS2 films were superior to those of the magnetron-sputtered MoS2 and ion-plated silver films. In humid air, an oxidative environment,

the bonded MoS2 films had higher coefficient of friction (0.14) and shorter wear life (113 570 passes) than those

(0.10 and 277 377 passes, respectively) of the magnetron-sputtered MoS2 films. The ion-plated silver films had a

high coefficient of friction (0.43) in humid air but relatively low coefficients of friction in the nonoxidative environments (0.20 in ultrahigh vacuum and 0.23 in dry nitrogen).

Adhesion and plastic deformation played important roles in the friction and wear of the solid lubricating films

in sliding contact with 440C stainless steel balls in the three environments. All sliding involved adhesive transfer of

materials: transfer of solid lubricant wear debris to the counterpart 440C stainless steel and transfer of 440C stainless

steel wear debris to the counterpart solid lubricant.

Introduction

Tribology, the multidisciplinary science of lubrication, friction, and wear, is application oriented, with a rich

methodology. Once the initial shortcomings relating to lubrication in design and application had been dealt with, it

became increasingly clear that materials science and technology ranked equal with design in reducing the friction

and wear of machinery and mechanical components (ref. 1). This conclusion applied particularly in the field of aerospace mechanisms.

In modern technology coefficient of friction and wear are regarded to be widely variable, depending on operational

variables, lubricants, substrate properties, and surface films. Therefore, testing is central, and particularly round-robin

testing at different laboratories is of great importance to tribologists, lubrication specialists, designers, and engineers.
NASA/TM—1999-209088/PART1 2

This present investigation was conducted at NASA Glenn Research Center to examine the friction and wear

properties of three types of solid lubricating film in ultrahigh vacuum, in humid air at a relative humidity of

~20 percent, and in dry nitrogen at a relative humidity of less than 1 percent. The three types of solid lubricating

film selected were bonded molybdenum disulfide (MoS2), magnetron-sputtered MoS2, and ion-plated silver, which

were furnished by the National Space Development Agency of Japan (NASDA) through K Systems Corporation

(Dayton, Ohio) under a Space Act Agreement. Unidirectional pin-on-disk sliding friction experiments were conducted with 440C stainless steel balls in sliding contact with the solid lubricating films at room temperature in ultrahigh vacuum, in humid air, and in dry nitrogen. The resultant solid lubricating films and their wear surfaces were

characterized by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), and surface

profilometry. SEM and EDX were used to determine the morphology and elemental composition of wear surfaces

and wear debris. The sampling depth of EDX for elemental information ranged between 0.5 to 1 μm in this investigation. Surface profilometry was used to determine the surface morphology, roughness, and wear of the films.

Selected Materials

Three specimens of each type of film (bonded MoS2, magnetron-sputtered MoS2, and ion-plated silver) produced on 440C stainless steel disk substrates were used in this investigation (table I). The bonded MoS2 films were

relatively rough, having a centerline-average roughness Ra, measured using a cutoff of 1 mm, of 1.2 μm with a standard deviation of 0.24 μm. The magnetron-sputtered MoS2 films were relatively smooth, having an Ra, measured

using a cutoff of 1 mm, of 32 nm with a standard deviation of 4.0 nm. The ion-plated silver films were also relatively smooth, having an Ra, measured using a cutoff of 1 mm, of 30 nm with a standard deviation of 3.2 nm. The

6-mm-diameter 440C stainless steel balls (grade number, 10) used were also smooth, having Ra of 0.025 μm with a

standard deviation of 0.02 μm or less.

Experiment

The pin-on-disk tribometer used in the investigation was mounted in a vacuum chamber (fig. 1). Unidirectional

pin-on-disk sliding friction experiments were conducted at room temperature in ultrahigh vacuum (7×10–7 Pa), in

humid air (relative humidity, ~20 percent), and in dry nitrogen (relative humidity, <1 percent). All experiments were

conducted with 6-mm-diameter 440C stainless steel balls in sliding contact with the solid lubricating films deposited

on 440C stainless steel substrate disks. All experiments were conducted with a load of 5.9 N (600 g) at the sliding

velocity of 0.2 m/s. The mean Hertzian contact pressure of the 440C stainless steel substrates in contact with the

440C stainless steel balls was ~0.79 GPa (maximum Hertzian contact pressure, 1.19 GPa). The pin-on-disk tribometer can measure friction in vacuum, in humid air, and in dry nitrogen during sliding. The friction force was continuously monitored during the friction experiments.

The sliding wear life for the coatings (film wear life, or film endurance life) in this investigation was determined to

be the number of passes at which the coefficient of friction rose to 0.3 in a given environment. Wear can be quantified by

measuring the wear scars and wear tracks on the specimens after the wear experiments. Film wear volumes were

obtained by measuring the average cross-sectional area, determined from stylus tracings, across the wear tracks at a

minimum of eight locations in each wear track. Then, the average cross-sectional area of the wear track was multiplied

by the wear track length. The wear rate, known as the dimensional wear coefficient, is defined as the volume of material

removed at a unit load and in a unit sliding distance expressed as cubic millimeters per newton·meter.

Results and Discussion

Friction Behavior

Figures 2 to 4 present typical friction traces obtained in ultrahigh vacuum, in humid air, and in dry nitrogen for

the bonded MoS2, magnetron-sputtered MoS2, and ion-plated silver films in sliding contact with 440C stainless steel

balls as a function of the number of passes.
NASA/TM—1999-209088/PART1 3

Ultra-high-vacuum environment.—The friction traces for the bonded MoS2 and magnetron-sputtered MoS2

films obtained in vacuum are smooth, but the friction trace for the ion-plated silver films fluctuates (fig. 2). The

coefficient of friction for the bonded MoS2 films gradually increased to ~0.05 at 1 million passes. The coefficient of

friction for the magnetron-sputtered MoS2 films suddenly rose from 0.06 to 0.3 at 274 130 passes. The coefficient of

friction for the ion-plated silver films gradually increased to 0.3 at 364 793 passes from 0.2 at the initial stage of the

experiment.

Figure 2 reveals that in ultrahigh vacuum the coefficient of friction and endurance life of the bonded MoS2

films were superior to those of the magnetron-sputtered MoS2 and ion-plated silver films.

Humid-air environment.—The coefficient of friction for the bonded MoS2 films began relatively high and

gradually increased to ~0.16 at ~80 000 passes (fig. 3). It then became erratic and rose to 0.3. The coefficient of

friction for the magnetron-sputtered MoS2 films was relatively high and fluctuating. The coefficient of friction for

the ion-plated silver films rapidly rose to 0.3 in a small number of passes, say 10 passes.

Figure 3 reveals that in humid air the coefficient of friction and endurance life of the bonded MoS2 and

magnetron-sputtered MoS2 films were superior to those of the ion-plated silver films. Furthermore, the tribological

performance of the three types of solid lubricating film was much better in ultrahigh vacuum than in humid air.

Dry-nitrogen environment.—The friction traces for the bonded MoS2 and magnetron-sputtered MoS2 films

obtained to 1 million passes in dry nitrogen are smooth, but the friction trace for the silver films obtained to

1040 passes fluctuates (fig. 4). The coefficient of friction for ion-plated silver gradually increased to 0.3 at

1040 passes.

Figure 4 reveals that in dry nitrogen the coefficient of friction and endurance life of the bonded MoS2 and

magnetron-sputtered MoS2 films were superior to those of the ion-plated silver films.

Wear Behavior

Figures 5 to 7 present SEM photomicrographs of wear scars on the 440C stainless steel balls and wear tracks on

the solid lubricating films deposited on 440C stainless steel disks after sliding contact in ultrahigh vacuum, in humid

air, and in dry nitrogen, respectively. The SEM observations were made either at the end of the wear life of the solid

lubricating films or at 1 million passes.

The wear scars on the 440C stainless steel balls were generally smooth, regardless of either the type of solid

lubricating film or the environment. Thin, smeared wear patches and particles of the solid lubricating films generally

covered the smooth wear scars. Smeared tongues of thin, layered, agglomerated wear debris were also present. Most

of the loose and smeared wear debris accumulated outside the wear scars.

The wear tracks on the solid lubricating films revealed that the sliding action usually generated a smooth wear

surface, fine wear debris particles, and agglomerated, pasty wear debris, regardless of the environment. The wear

tracks on the ion-plated silver films in ultrahigh vacuum and in dry nitrogen revealed delaminated, flat plate-like

wear debris.

Wear Life (Endurance Life)

The sliding wear (endurance) life for the solid lubricating films deposited on 440C stainless steel disks was

determined to be the number of passes at which the coefficient of friction rose to 0.3. The sliding wear lives are presented in figure 8 and table II.

The sliding wear life varied with the environment and the type of solid lubricating film. The wear lives of the

bonded MoS2 films were over 1 million passes in ultrahigh vacuum and in dry nitrogen but only 113 570 passes in

humid air. The wear lives of the magnetron-sputtered MoS2 films in dry nitrogen were over 1 million passes, much

greater than in ultrahigh vacuum and in humid air. The wear lives of the ion-plated silver films were relatively

greater in ultrahigh vacuum than in dry nitrogen and in humid air.

In ultrahigh vacuum the bonded MoS2 films had longer wear lives than the magnetron-sputtered MoS2 and

ion-plated silver films. In humid air the wear lives of all three solid lubricating films were relatively short. In dry

nitrogen the bonded MoS2 and magnetron-sputtered MoS2 films had longer wear lives than the ion-plated silver

films.
NASA/TM—1999-209088/PART1 4

Comparison of Steady-State Coefficients of Friction and Wear Rates

Table II and figure 9 present the steady-state coefficients of friction, the wear rates for the solid lubricating

films, and the wear rates for the 440C stainless steel balls after sliding contact in all three environments. The data

presented in the table reveal the marked differences in friction and wear resulting from the environmental conditions

and the solid lubricating film materials.

Both the bonded MoS2 and magnetron-sputtered MoS2 films had low coefficients of friction and low wear rates

in ultrahigh vacuum and in dry nitrogen but relatively high coefficients of friction and high wear rates in humid air.

Also, the 440C stainless steel balls that slid against the bonded MoS2 and magnetron-sputtered MoS2 films had low

wear rates in ultrahigh vacuum and in dry nitrogen but moderate wear rates in humid air.

In contrast to the bonded MoS2 and magnetron-sputtered MoS2 films, the ion-plated silver films had a higher

coefficient of friction and greater wear rates in all three environments. However, the lowest values for the silver

films were obtained in the ultra-high-vacuum environment.

Ultra-high-vacuum environment.—In sliding contact with 440C stainless steel balls in ultrahigh vacuum the

bonded MoS2 films had the lowest coefficient of friction, lowest film wear rate, and lowest ball wear rate. The

magnetron-sputtered MoS2 films also had low coefficient of friction, low film wear rate, and low ball wear rate. The

ion-plated silver films had relatively low film and ball wear rates but a high coefficient of friction.

Humid-air environment.—In sliding contact with 440C stainless steel balls in humid air the magnetronsputtered MoS2 films had the lowest coefficient of friction. Both bonded MoS2 and sputtered MoS2 films generally

had low coefficients of friction, low film wear rates, and low ball wear rates in humid air. However, the ion-plated

silver films had high friction and high wear.

Dry-nitrogen environment.—In sliding contact with 440C stainless steel balls in dry nitrogen the magnetronsputtered MoS2 films had the lowest coefficient of friction. Both the bonded MoS2 and sputtered MoS2 films had

low coefficients of friction, low film wear rates, and low ball wear rates. However, the ion-plated silver films had

high friction and high wear.

Sliding Wear, Wear Debris, and Transferred Wear Fragments

Adhesion and plastic deformation played important roles in the friction and sliding wear of the solid lubricating

films in sliding contact with the 440C stainless steel balls in all three environments. The worn surfaces of both the

films and the balls contained wear debris particles. Examining the morphology and composition of the worn surfaces

by SEM and EDX provided detailed information about plastic deformation of the solid lubricating films, wear

debris, and transferred wear fragments produced during sliding (figs. 10 to 20). Marked plastic deformation occurred

in the three solid lubricating films. Smeared, agglomerated wear debris accumulated around the contact border,

particularly on the ends of wear scars. All sliding involved adhesive transfer of materials.

Ultra-high-vacuum environment.—Figure 10(a) presents a typical wear track on a bonded MoS2 film over

which a 440C stainless steel ball has passed in ultrahigh vacuum leaving transferred steel wear fragments. The

asperities (or texture) of the bonded MoS2 were deformed plastically and the tips of the asperities were flattened

under a load during sliding. The wear scar on the counterpart 440C stainless steel ball (fig. 10(b)) contained transferred wear fragments of MoS2. Wear fragments of MoS2 and 440C stainless steel usually adhered to the other surface or came off in loose form. Another form of adhesive transfer of MoS2 is found in sliding wear. SEM and EDX

showed that a thin MoS2 layer (or sheet) was generated over the entire wear scar of a 440C stainless steel ball.

Figure 11(a) presents a typical wear track on a magnetron-sputtered MoS2 film over which a 440C stainless

steel ball has passed in ultrahigh vacuum leaving transferred steel wear fragments. The fine asperities of the sputtered MoS2 film were flattened and elongated in the direction of sliding by plastic deformation, revealing a burnished appearance. The wear scar on the counterpart 440C stainless steel ball (fig. 11(b)) contained transferred wear

fragments of MoS2. However, according to the elemental concentrations, much less transfer occurred between the

440C stainless steel balls and the films and vice versa with the magnetron-sputtered MoS2 (figs. 11(a) and (b)) than

with the bonded MoS2. With the magnetron-sputtered MoS2 a thin MoS2 layer was also generated over the entire

wear scar of the 440C stainless steel ball.

Figure 12(a) presents a typical wear track on an ion-plated silver film over which a 440C stainless steel ball has

passed in ultrahigh vacuum leaving a small amount of transferred steel wear fragments. The fine asperities of the

ion-plated silver film were flattened and elongated in the direction of sliding by plastic deformation, revealing a
NASA/TM—1999-209088/PART1 5

burnished appearance. The entire wear scar on the counterpart 440C stainless steel ball (fig. 12(b)) contained thick

transferred layers (or sheets) of silver. Plate-like silver debris particles were found at the edges of the wear track on

the ion-plated silver film, as shown, for example, in figure 13. Severe plastic deformation and shearing occurred in

the silver film during sliding.

Humid-air environment.—Figure 14(a) presents a typical wear track on a bonded MoS2 film over which a

440C stainless steel ball has passed in humid air leaving a small amount of transferred steel wear fragments. The

asperities of the bonded MoS2 were deformed plastically and the tips of the asperities were flattened under a load

during sliding. The wear scar on the counterpart 440C stainless steel ball (fig. 14(b)) contained a large amount of

transferred MoS2 wear fragments. Wear debris particles usually adhered to the other surface or came off in loose

form. Also, a thin MoS2 layer (or sheet) was generated over the entire wear scar on the 440C stainless steel ball.

Figure 15(a) presents a typical wear track on a magnetron-sputtered MoS2 film over which a 440C stainless

steel ball has passed in humid air leaving transferred steel wear fragments. The fine asperities of the sputtered MoS2

film were flattened and elongated in the direction of sliding by plastic deformation, revealing a burnished appearance. The wear scar on the counterpart 440C stainless steel ball (fig. 15(b)) contained transferred MoS2 wear fragments. The elemental concentrations of 440C stainless steel transferred to the films in humid air were greater with

the sputtered MoS2 film than with the bonded MoS2 film. On the other hand, the elemental concentrations of MoS2

transferred to the 440C stainless steel were much less with the sputtered MoS2 than with the bonded MoS2. A thin

MoS2 layer was also generated over the entire wear scar on the 440C stainless steel ball.

Figure 16(a) presents a typical wear track on an ion-plated silver film over which a 440C stainless steel ball has

passed in humid air leaving a small amount of transferred steel wear fragments. The fine asperities of the ion-plated

silver film were flattened and elongated in the direction of sliding by plastic deformation, revealing a burnished

appearance. Significantly, the wear scar on the counterpart 440C stainless steel ball (fig. 16(b)) contained an

extremely small amount of transferred silver debris particles. This result suggests that oxidation of silver during

sliding in humid air may prevent large silver transfer. In addition to the small wear debris particles, plate-like silver

debris was found at the edges of the wear track on the ion-plated silver film, as shown, for example, in figure 17.

Severe plastic deformation and shearing occurred in the silver film during sliding.

Dry-nitrogen environment.—Figure 18(a) presents a typical wear track on a bonded MoS2 film over which a

440C stainless steel ball has passed in dry nitrogen leaving a small amount of transferred steel wear fragments. The

asperities of the bonded MoS2 were deformed plastically, and the tips of the asperities were flattened under a load

during sliding. The wear scar on the counterpart 440C stainless steel ball (fig. 18(b)) contained a large amount of

transferred MoS2 wear fragments. The wear debris particles usually adhered to the other surface or came off in loose

form. Also, a thin MoS2 layer (or sheet) was generated over the entire wear scar on the 440C stainless steel ball.

Figure 19(a) presents a typical wear track on a magnetron-sputtered MoS2 film over which a 440C stainless

steel ball has passed in dry nitrogen leaving transferred steel wear fragments. The fine asperities of the sputtered

MoS2 film were flattened and elongated in the direction of sliding by plastic deformation, revealing a burnished

wear track. The wear scar on the counterpart 440C stainless steel ball (fig. 19(b)) contained transferred MoS2 wear

fragments. The elemental concentrations of 440C stainless steel transferred to the films in dry nitrogen were greater

with the sputtered MoS2 film than with the bonded MoS2 film. On the other hand, the elemental concentrations of

MoS2 transferred to the 440C stainless steel were much less with the sputtered MoS2 than with the bonded MoS2. A

thin MoS2 layer was also generated over the entire wear scar on the 440C stainless steel ball.

Figure 20(a) presents a typical wear track on an ion-plated silver film over which a 440C stainless steel ball

has passed in dry nitrogen leaving a small amount of transferred steel wear fragments. The fine asperities of the

ion-plated silver film were flattened and elongated in the direction of sliding by plastic deformation, revealing a

burnished appearance. The wear scar on the counterpart 440C stainless steel ball (fig. 20(b)) contained transferred

silver debris and particles. In addition to the small wear debris particles, plate-like silver debris was found at the

edges of the wear track on the ion-plated silver film. Severe plastic deformation and shearing occurred in the silver

film during sliding.

Conclusions

To evaluate commercially developed solid film lubricants for aerospace bearing applications, unidirectional

sliding friction experiments were conducted with bonded molybdenum disulfide (MoS2), magnetron-sputtered

MoS2, and ion-plated silver films in contact with AISI 440C stainless steel balls in ultrahigh vacuum, in humid air,
NASA/TM—1999-209088/PART1 6

and in dry nitrogen. The main criteria for judging the performance of the solid lubricating films were coefficient of

friction and wear rate, which had to be less than 0.3 and on the order of 10–6 mm3/N·m or less, respectively. The

following conclusions were drawn:

1. The bonded MoS2 and magnetron-sputtered MoS2 films met the criteria in all three environments.

2. The wear rates of the counterpart 440C stainless steel balls met the criteria in all three environments.

3. The ion-plated silver films met the criteria only in ultrahigh vacuum, failing in humid air and in dry nitrogen

where both the film and ball wear rates were on the order of 10–5 mm3/N·m.

4. In ultrahigh vacuum the coefficient of friction and endurance (wear) life of the bonded MoS2 films were

superior to those of the magnetron-sputtered MoS2 and ion-plated silver films.

5. With the bonded MoS2 and magnetron-sputtered MoS2 films lower coefficients of friction, film wear rates,

and ball wear rates were obtained in dry nitrogen than in ultrahigh vacuum.

6. In humid air, an oxidative environment, the bonded MoS2 films had higher coefficient of friction (0.14)

and shorter wear life (113 570 passes) than the magnetron-sputtered MoS2 films (0.10 and 277 377 passes,

respectively).

7. The ion-plated silver films had a high coefficient of friction (0.43) in humid air but relatively low coefficients of friction in nonoxidative environments (0.20 in ultrahigh vacuum and 0.23 in dry nitrogen).

8. Adhesion and plastic deformation played important roles in sliding friction and wear of the solid lubricating

films in contact with 440C stainless steel balls in all three environments. All sliding involved adhesive transfer of materials: transfer of solid lubricant wear debris to the counterpart 440C stainless steel and transfer of

440C stainless steel wear debris to the counterpart solid lubricant.

Reference

1. Jost, H.P.: Tribology—Origin and Future, Wear, vol. 136, 1990, pp. 1–17.

TABLE I.—CHARACTERISTICS OF SELECTED SOLID LUBRICATING FILMS

[Substrate material, 440C stainless steel.]

Film Film material Film

thickness,

μm

Surface roughness of films,

Ra,

nm

Mean Standard

deviation

Bonded MoS2 MoS2 , polyamide-imide,

others (proprietary blend)

10 ± 4 1.2×103 2.4×102

Magnetronsputtered MoS2MoS2 1.0 ± 0.2 32 4.0

Ion-plated

silver

Silver 0.5 ± 0.2 30 3.2
NASA/TM—1999-209088/PART1 7

Figure 1.—Pin-on-disk tribometer in vacuum chamber and conditions of unidirectional, pin-on-disk, sliding

 friction experiments.

Load, N (g)

Mean Hertzian contact pressure, GPa

Maximum Hertzian contact pressure, GPa

Ball specimen

Sliding velocity, m/s

Environments

5.9 (600)

0.79

1.19

6-mm-diameter 440C stainless steel ball

0.2

R J C

Ball or pin

specimen

Friction

force

sensor

Detail

Disk specimen

Load

Vacuum, 710–7 Pa

Humid air (relative humidity, ~20%)

Dry nitrogen (relative humidity, <1%)

TABLE II.—STEADY-STATE COEFFICIENT OF FRICTION, WEAR RATES, AND

WEAR LIFE FOR SELECTED SOLID LUBRICATING FILMS IN SLIDING

CONTACT WITH 440C STAINLESS STEEL BALLS

Film Film

material

Steady-state

coefficient of

friction

Film wear

(endurance)

lifea

Film wear

rate,

mm3/N·m

Ball wear

rate,

mm3/N·m

Bonded MoS2 Vacuum 0.045 >1 million 6.0×10–8 1.3×10–9

Air .14 113 570 2.4×10–6 8.1×10–8

Nitrogen .04 >1 million 4.4×10–8 6.9×10–10

Magnetron- Vacuum 0.070 274 130 9.0×10–8 2.5×10–9

sputtered MoS2 Air .10 277 377 2.4×10–7 1.5×10–7

Nitrogen .015 >1 million 1.6×10–8 9.9×10–10

Ion-plated silver Vacuum 0.20 364 793 8.8×10–8 2.4×10–8

Air .43 8 5.5×10–5 1.2×10–5

Nitrogen .23 1040 1.6×10–5 1.6×10–5

a

Film wear life is determined to be the number of passes at which the coefficient of

 friction rises to 0.3.
NASA/TM—1999-209088/PART1 8

0.1

0

0.4

0.3

0.2

0.1

0

0.3

0.2

0.1

0

0 2 4 6 8 10x105

Number of passes

0 2 4 6 8 10x105

Number of passes

0 2 4 6 8 10x105

Number of passes

Coefficient of friction

(a)

(b)

(c)

Figure 2.—Friction traces for films of (a) bonded

 MoS2, (b) magnetron-sputtered MoS2, and (c) 

 ion-plated silver in sliding contact with 440C 

 stainless steel balls in ultrahigh vacuum.
NASA/TM—1999-209088/PART1 9

Figure 3.—Friction traces for films of (a) bonded 

 MoS2, (b) magnetron-sputtered MoS2, and (c) 

 ion-plated silver in sliding contact with 440C 

 stainless steel balls in humid air.

0.3

0.2

0.1

0

0.4

0.3

0.2

0.1

0

0 6 12 18 24 30x104

Number of passes

0 6 12 18 24 30x104

Number of passes

0 6 12 18 24 30x104

Number of passes

Coefficient of friction

0.4

0.3

0.2

0.1

0

0.4

0.3

0.2

0.1

0 0 10 20

(c)

(b)

(a)
NASA/TM—1999-209088/PART1 10

Figure 4.—Friction traces for films of (a) bonded 

 MoS2, (b) magnetron-sputtered MoS2, and (c) 

 ion-plated silver in sliding contact with 440C 

 stainless steel balls in dry nitrogen.

(c)

(b)

(a) 0.1

0

0.1

0

0 2 4 6 8 10x105

Number of passes

0 2 4 6 8 10x105

Number of passes

0 2 4 6 8 10x105

Number of passes

Coefficient of friction 0.4

0.3

0.2

0.1

0

0.3

0.2

0.1

0

0 600 1200
NASA/TM—1999-209088/PART1 11

Sliding direction

Film wear track Ball wear scar

Figure 5.—Wear tracks produced on solid lubricating films and wear scars produced on 440C stainless steel balls

 in ultrahigh vacuum. (a) Materials pair of bonded MoS2 film and 440C ball at 1 million passes. (b) Materials pair 

 of magnetron-sputtered MoS2 film and 440C ball at 274 130 passes. (c) Materials pair of ion-plated silver film 

 and 440C ball at 364 793 passes.

100 μm

100 μm

100 μm

100 μm

100 μm

100 μm

(a)

(b)

(c)
NASA/TM—1999-209088/PART1 12

Sliding direction

Film wear track Ball wear scar

Figure 6.—Wear tracks produced on solid lubricating films and wear scars produced on 440C stainless steel 

 balls in humid air. (a) Materials pair of bonded MoS2 film and 440C ball at 113 570 passes. (b) Materials pair

 of magnetron-sputtered MoS2 film and 440C ball at 277 377 passes. (c) Materials pair of ion-plated silver film 

 and 440C ball at 1000 passes.

100 μm

100 μm

100 μm

100 μm

100 μm

100 μm

(a)

(b)

(c)
NASA/TM—1999-209088/PART1 13

Sliding direction

Film wear track Ball wear scar

Figure 7.—Wear tracks produced on solid lubricating films and wear scars produced on 440C stainless steel 

 balls in dry nitrogen. (a) Materials pair of bonded MoS2 film and 440C ball at 1 million passes. (b) Materials 

 pair of magnetron-sputtered MoS2 film and 440C ball at 1 million passes. (c) Materials pair of ion-plated silver

 film and 440C ball at 1040 passes.

(a)

(b)

(c)

100 μm 100 μm

100 μm

100 μm 100 μm

100 μm
NASA/TM—1999-209088/PART1 14

1.0x106

0.6

0.4

0.2

0

Environment

Number of passes

0.8

Figure 8.—Sliding wear life of bonded MoS2, 

 magnetron-sputtered MoS2, and ion-plated silver 

 films in sliding contact with 440C stainless steel 

 balls in ultrahigh vacuum, in humid air, and in dry 

 nitrogen.

Vacuum Air Nitrogen

Bonded MoS2, 

Magnetron-sputtered MoS2

Ion-plated silver 
NASA/TM—1999-209088/PART1 15

10–4

10–5

10–6

10–7

10–8

10–4

10–5

10–6

10–7

10–8

10–9

10–10

10–6

10–7

10–8

10–9

10–10

0 0.1 0.2 0.3 0.4 0.5

Coefficient of friction

Wear rate, mm3/N·m

(a)

(b)

(c)

Figure 9.—Steady-state (equilibrium) coefficients of 

 friction and wear rates (dimensional wear coeffic-

 ients) for solid lubricating films and 440C stainless

 steel balls (a) in ultrahigh vacuum, (b) in humid air,

 and (c) in dry nitrogen.

Bonded MoS2

Sputtered MoS2

Ion-plated silver

440C ball against bonded MoS2

440C ball against sputtered MoS2

440C ball against ion-plated silver

0 0.1 0.2 0.3 0.4 0.5

Coefficient of friction

Wear rate, mm3/N·m

0 0.1 0.2 0.3 0.4 0.5

Coefficient of friction

Wear rate, mm3/N·m
NASA/TM—1999-209088/PART1 16

Figure 10.—Morphology and elemental composition (a) of wear track produced on bonded MoS2 film and (b) of

 wear scar produced on 440C stainless steel ball in ultrahigh vacuum.

SEM

(b)

10 m

10 m

(a)

EDX

x105

1.0

1.5

0.5

 0

Ck Alk Sbl Crk

Fek

Sk

Mol

Ck

Sbl

Crk

Fek

Sk

Mol

Counts

1.5 x105

0.5

1.0

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
NASA/TM—1999-209088/PART1 17

Figure 11.—Morphology and elemental composition (a) of wear track produced on magnetron-sputtered MoS2

 film and (b) of wear scar produced on 440C stainless steel ball in ultrahigh vacuum.

SEM

(b)

10 m

10 m

(a)

EDX

x105

1.0

2.0

1.5

0.5

 0

Ck Crk Fek

Sk

Mol

Ck

Crk

Fek

Sik

Mol

Counts

1.5 x105

0.5

1.0

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV

Sk
NASA/TM—1999-209088/PART1 18

Figure 12.—Morphology and elemental composition (a) of wear track produced on ion-plated silver film and (b)

 of wear scar produced on 440C stainless steel ball in ultrahigh vacuum.

SEM

(b)

10 m

10 m

(a)

EDX

x105

1.0

1.5

0.5

 0

Ck

Fek

Ck

Crk Sik Fek

Agl

Agl

Counts

1.5 x105

0.5

1.0

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV

Figure 13.—Plate-like silver wear debris produced

 on ion-plated silver film in ultrahigh vacuum.

10 m
NASA/TM—1999-209088/PART1 19

Figure 14.—Morphology and elemental composition (a) of wear track produced on bonded MoS2 film and (b) of

 wear scar produced on 440C stainless steel ball in humid air.

SEM

(b)

10 m

10 m

(a)

EDX

x105

1.0

2.0

1.5

0.5

 0

Ck

Ok

Ok

Crk

Sbl

Sbl

Fek

Sk

Mol

Ck

Alk

Crk

Fek

Sk

Mol

Counts

7x104

4

3

2

6

1

5

0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
NASA/TM—1999-209088/PART1 20

Figure 15.—Morphology and elemental composition (a) of wear track produced on magnetron-sputtered MoS2

 film and (b) of wear scar produced on 440C stainless steel ball in humid air.

SEM

(b)

10 m

10 m

(a)

EDX x105

1.0

1.5

0.5

 0

Ck

Crk

Fek

Sk

Sk

Mol

Ck

Ok

Crk

Fek

Sik

Mol

Counts

1.5 x105

0.5

1.0

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
NASA/TM—1999-209088/PART1 21

Figure 16.—Morphology and elemental composition (a) of wear track produced on ion-plated silver film and (b) 

 of wear scar produced on 440C stainless steel ball in humid air.

SEM

(b)

10 m

10 m

(a)

EDX x105

1.0

1.5

0.5

 0

Ck

Crk

Fek

Ck Crk

Fek

Sik

Sik

Agl

Agl

Counts

1.5 x105

0.5

1.0

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
NASA/TM—1999-209088/PART1 22

Figure 17.—Plate-like silver wear debris produced on

 ion-plated silver film in humid air.

10 m

Figure 18.—Morphology and elemental composition (a) of wear track produced on bonded MoS2 film and (b)

 of wear scar produced on 440C stainless steel ball in dry nitrogen.

SEM

(b)

10 m

10 m

(a)

EDX x105

1.0

2.0

1.5

0.5

 0

Ck Sbl

Sbl

Fek

Sk

Mol

Ck

Crk

Fek

Sk

Mol

Counts

1.5 x105

0.5

1.0

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
NASA/TM—1999-209088/PART1 23

Figure 19.—Morphology and elemental composition (a) of wear track produced on magnetron-sputtered MoS2

 film and (b) of wear scar produced on 440C stainless steel ball in dry nitrogen.

SEM

(b)

10 m

10 m

(a)

EDX x105

1.0

2.0

1.5

0.5

 0

Ck Crk

Fek

Mol

Ck

Crk

Fek

Sk

Sk

Mol

Counts

1.0 x105

0.5

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
NASA/TM—1999-209088/PART1 24

Figure 20.—Morphology and elemental composition (a) of wear track produced on ion-plated silver film and (b) of

 wear scar produced on 440C stainless steel ball in dry nitrogen.

SEM

(b)

10 m

10 m

(a)

EDX x105

1.0

1.5

0.5

 0

Ck

Crk Fek

Agl

Agl

Ck Crk

Fek

Sik

Counts

1.0 x105

0.5

 0

Counts

0 2 4 6 8 10

Energy, keV

0 2 4 6 8 10

Energy, keV
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Unclassified Unclassified

Technical Memorandum

Unclassified

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

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9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

October 1999

NASA TM—1999-209088-PART1

E–11673

WU–523–22–13–00

30

A03

Lubricants

Unclassified -Unlimited

Subject Category: 27 Distribution: Nonstandard

Kazuhisa Miyoshi, NASA Glenn Research Center, Cleveland, Ohio; Masanori Iwaki, Kenichi Gotoh, Shingo Obara, and

Kichiro Imagawa, National Space Development Agency of Japan, Tsukuba Space Center, Tsukuba, Ibaraki 305–8505,

Japan. Responsible person, Kazuhisa Miyoshi, organization code 5140, (216) 433–6078.

Friction and Wear Properties of Selected Solid Lubricating Films

Part 1: Bonded and Magnetron-Sputtered Molybdenum Disulfied and Ion-Plated Silver Films

Kazuhisa Miyoshi, Masanori Iwaki, Kenichi Gotoh, Shingo Obara,

and Kichiro Imagawa

To evaluate commercially developed solid film lubricants for aerospace bearing applications, we investigated the friction and wear

behavior of bonded molybdenum disulfide (MoS2), magnetron-sputtered MoS2, and ion-plated silver films in sliding contact with

6-mm-diameter AISI 440C stainless steel balls. Unidirectional sliding friction experiments were conducted with a load of 5.9 N

(600 g), a mean Hertzian contact pressure of 0.79 GPa (maximum, 1.19 GPa), and a sliding velocity of 0.2 m/s at room temperature in

three environments: ultrahigh vacuum (7×10–7 Pa), humid air (~20 percent humidity), and dry nitrogen (<1 percent humidity). The

resultant films were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and surface profilometry.

Marked differences in friction and wear resulted from the environmental conditions and the film materials. The main criteria for judging

the performance were coefficient of friction and wear rate, which had to be less than 0.3 and on the order of 10–6 mm3/N·m or less,

respectively. The bonded MoS2 and magnetron-sputtered MoS2 films met the criteria in all three environments. Also, the wear rates of

the counterpart AISI 440C stainless steel balls met that criterion in all three environments. The ion-plated silver films met the criteria

only in ultrahigh vacuum. In ultrahigh vacuum the bonded MoS2 films were superior. In humid air the bonded MoS2 films had higher

coefficient of friction and shorter wear life than did the magnetron-sputtered MoS2 films. The ion-plated silver films had a high coefficient of friction in humid air but relatively low coefficients of friction in the nonoxidative environments. Adhesion and plastic deformation played important roles in all three environments. All sliding involved adhesive transfer of materials.