Angular-contact, 440C stainless steel, ball bearings with sputtered MoS2 films 0.0000006 x 10-7m (6000 A) thick were evaluated in a vacuum bearing chamber (1750 rpm, 137.9-N- (31-lbf-) thrust load) for endurance. Two types of sputtered films were evaluated:  (1) MOS2 sputtered directly onto bearing components, and (2) a thin 0.0000001 x 10-7m (1000 A) underlayer of Cr3Si2 subsequently sputtered with MoS2. Bearing test evaluations in vacuum showed that endurance lives of more than 1000 hours (105,000,000 cycles) were obtained with bearings (cage, races, and balls) directly sputtered with MoS2. The same endurance lives were also obtained when only the races and cage were sputtered with an underlayer of Cr3Si2 and subsequently with MoS2

Spalvins, T.

English

NASA Technical Reports Server

NASA TM X-3]93 NASA TECHNICAL 
MEMORANDUM 
cv, 
>< 
I-
1 
BEARING ENDURANCE TESTS IN VACUUM FOR 
SPUTTERED MOLYBDENUM DISULFIDE FILMS 
Talivaldis Spa/vms 
Lewis Research Center 
Cleveland, Ohio 44135
, 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION • WASHINGTON, D. C. • JANUARY 1975
https://ntrs.nasa.gov/search.jsp?R=19750006982 2020-03-23T01:32:22+00:00Z
1. Report No. 2. Government Accession No. 3.	 Recipient's Catalog No. 
NASA TM X-3193 
4. Title and Subtitle 5. Report Date 
BEARING ENDURANCE TESTS IN VACUUM FOR SPUTTERED January	 1975 
6. Performing Organization Code MOLYBDENUM DISULFIDE FILMS 
7. Author(s) 8. Performing Organization Report No. 
Talivaldis Spalvins E-8166 
10. Work Unit No. 
- 
506 16 
________________________________________________________________________________ 
9. Performing Organization Name and Address 
Lewis Research Center 11. Contract or Grant No. 
National Aeronautics and Space Administration 
Cleveland, Ohio 44135 13. Type of Report and Period Covered 
Technical Memorandum 12. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 14. Sponsoring Agency Code 
Washington, D. C.	 20546 
15. Supplementary Notes 
16. Abstract 
Angular-contact, .440C stainless-steel, ball bearings with sputtered M0S 2 films 6x10 7 m 
(6000 A) thick were evaluated in a vacuum bearing chamber (1750 rpm, 137. 9-N- (31-lbf-) thrust 
load) for endurance. 	 Two types of sputtered films were evaluated: 	 (1) MoS2 sputtered directly 
onto bearing components, and (2) a thin (1x10 7 m (1000 A)) underlayer of Cr 3Si2 subsequently 
sputtered with MoS2 .	 Bearing test evaluations in vacuum showed that endurance lives of more 
than 1000 hours (10. 5X10 7 cycles) were obtained with bearings (cage, races, and balls) directly 
sputtered with MoS2 .	 The same endurance lives were also obtained when only the races and 
cage (not the balls) were sputtered with an underlayer of Cr 3Si2 and subsequently with MoS2. 
17. Key Words (Suggested by Author(s)) 18. Distribution Statement 
Bearings Unclassified - unlimited 
L&ibricant STAR Category 37 (rev.) 
Friction 
19.	 Security Classif. (of this report) 20.	 Security Classif. (of this page) 21. No. of Pages 22.	 Price 
Unclassified Unclassified 10 $3. 00
For sale by the National Technical Information Service, Springfield, Virginia 22151 
BEARING ENDURANCE TESTS IN VACUUM FOR SPUTtERED 
MOLYBDENUM DISULFIDE FILMS
bylalivaldis Spalvins
Lewis Research Center 
SUMMARY 
Angular-contact, 440C stainless-steel, ball bearings with sputtered molybdenum 
disulfide (MoS 2) films 6x10 7 meter (6000 A) thick were evaluated in a vacuum bearing 
chamber (1750 rpm, 137. 9-N- (31-lbf-) thrust load) for endurance. Two types of sput-
tered MoS films were tested: (1) MoS 2 sputtered directly onto bearing components, 
and (2) a thin (1x10 m (1000 A)) underlayer of Cr 35i 2 subsequently sputtering with 
MoS . Bearing test evaluations in vacuum showed that endurance lives of more than 
1000 hours (10. 5x10 cycles) were obtained with bearings (cage, races, and balls) 
directly sputtered with MoS 2 . The same endurance lives were also obtained when only 
the races and cage (not the balls) were sputtered with an underlayer of Cr3Si 2 and sub-
sequently with MoS2.
INTRODUCTION 
One of the most recent methods for applying molybdenum disulfide (MoS 2) as a dry-
film lubricant for sliding or rotating machinery components is the sputter-deposition 
technique. The major advantage of sputtering MoS 2 lies in the strong adherence to the 
surface and strong coherence between the sputtered particles of the film. As a conse-
quence of these characteristics, only thin sputtered MoS2 films are needed for lubrication 
In practical lubrication, sputter deposition has several advantages, because these films 
can be accurately controlled and reproduced in terms of thickness, uniformity, density, 
etc. It is an attractive technique for coating curved and indirectly accessible surfaces 
such as bearing balls and the ball pockets of cages. The frictional behavior and endur-
ance lives of sputtered MoS 2 films about 2x10 7 meter (2000 A) thick have been studied in 
sliding-type experiments (pin and disk) in vacuum (ref S. 1 and 2). 
No studies establishing the endurance lives of bearings lubricated with sputtered
MoS 2
 films have been reported. Many aerospace mechanisms contain bearings which 
operate at low speeds and light loads. Rotational speeds of a few revolutions per minute 
are quite common. Even in some of the mechanisms involving higher speed operation 
(e. g., radiometer bearings), bearings do not exceed speeds of 3000 rpm (ref. 3). The 
objective of this investigation was to demonstrate bearing endurance for sputtered Mo52 
film lubrication in vacuum. A speed of 1750 rpm and a thrust load of 137. 9 newtons 
(31 lbf) were applied to a 204-size, 440C stainless-steel, ball bearing. For many aero-
space mechanisms, such parameters constitute accelerated testing. 
APPARATUS AND PROCEDURE

Sputtering Apparatus 
The sputtering apparatus used in this study was a radiofrequency planar diode con-
figuration with a superimposed direct-current bias for sputter etching or biasing pur-
poses, as shown schematically and photographically in figure 1. This apparatus is de-
scribed in detail in references 1 and 2. The sputtering targets which were utilized in 
this study were 12. 7-centimeter-diameter MoS 2 and Cr3Si 2
 disks. The sputtering con-
ditions were kept constant during this investigation. The radiofrequency power density 
was 3. 5 W/cm 2
 at a frequency of 7 megahertz and at argon pressure of 2X10 2
 torr. The 
distance from bearing to target was maintained at 2. 5 centimeters. The bearings to be 
sputter coated were cleaned by sputter etching at -2000 volts dc for about 10 minutes 
immediately prior to coating. The balls to be coated were placed in a screen pan (insert 
to fig. 1(b)) and were kept in constant motion during the sputtering process in order to 
maintain a uniform film coverage. The thickness of the MoS 2
 films was maintained at 
6x10 7
 meter (6000 A).
Bearing Endurance Apparatus 
The bearing endurance test apparatus is shown schematically in figure 2. The 
bearing test assembly was mounted horizontally in the vacuum chamber. It consisted of 
two test bearings mounted on a rotatable shaft; the load was exerted by a precalibrated 
spring. The test bearings were equally loaded by a 137. 9-newton (31-lbf) spring. The 
bearings were rotated through a magnetic drive assembly capable of varying the speeds 
up to 3600 rpm. The running conditions (smoothness) of the bearing were monitored by 
the motor current, the endurance life was registered by an automatic timer, and the 
vacuum during the tests was maintained at io 8 torr. 
2
The ball bearings which were sputter coated and subsequently tested for endurance 
were of the following type: 20-millimeter-bore 440C stainless steel, 11 balls of 0. 714-
centimeter diameter, radial clearance of 0. 0058 centimeter, ball pocket diametral 
clearance of 0. 018 centimeter, and retainer diametral clearance of 0. 028 centimeter. 
Procedure 
Two series of sputtered coating films were prepared, and their endurance was de-
termined. In the first series, MoS 2
 films were directly sputtered on the 440C stainless-
steel bearing components. In the second series, thin underlayers ofCr 3Si2 were sput-
tered on the races and cages, which were subsequently coated with a sputtered film' of 
MoS . The bearings to be sputtered were cleaned by sputter etching at -2000 volts dc 
in 2x10 torr of argon for about 10 minutes, immediately prior to coating. Where the 
sputtered underlayer of Cr3Si 2
 was used, the thickness was maintained at 1x10 7
 meter 
(1000 A). The sputtered MoS 2
 lubricant films in all instances were about 6x10 7
 meter 
(6000 A) thick. 
The bearing running conditions during the tests were monitored by the motor current. 
At the beginning of the test the motor current was somewhat high, 1.3 to 1. 4 amperes. 
As the test progressed the current generally dropped to 1. 2 amperes, and the running 
conditions were very smooth. As the run became rougher and more noisy the current 
increased. An automatic cutoff was set at 1. 8 amperes, and the test was discontinued 
when the operating current reached that level. An endurance time limit of 1000 hours 
(10. 5x10 7
 cycles) was set for evaluation purposes. Therefore, if the motor current had 
not exceeded 1.8 amperes at 1000 hours, the test was terminated. 
DISCUSSION AND RESULTS 
In this investigation the endurance of sputtered MoS 2
 films on bearings was evaluated 
Two methods of application were used: (1) MoS 2
 films were directly sputtered on the 
bearing components, and (2) first an underlayer of Cr3Si 2
 was sputtered on the bearing 
components which were subsequently sputter coated with a MoS 2 film. 
Bearing Tests with Directly Sputtered Mo5 2
 Films 
In the first series of tests, sputtered MoS 2
 films about 6<10 meter (6000 A) thick 
were deposited directly on the races and cage, but not on the balls. When tested for en-
3
ciurance in vacuum at io8 torr at a speed of 1750 rpm and a thrust load of 137. 9 newtons 
(31 lbf), the average endurance life of these bearings was about 184 hours. When all the 
components (races, cage, and balls) were sputtered with MoS 2
 and tested under these 
conditions, the endurance life was extended to over 1000 hours. Since 1000 hours was 
selected as the maximum endurance limit, the tests were discontinued once 1000 hours 
was reached. Several endurance tests were conducted where Only the balls were sput-
tered with MoS 2 . The endurance life for this combination was about 19 hours before 
failure. A bearing tested without a lubricant under identical experimental conditions 
failed (very noisily and with high motor current) in about 35 minutes. These results are 
graphically shown in figure 3. 
In several instances when the bearing endurance tests were progressing very 
smoothly (motor current, 1. 2 A) at 1750 rpm, the speed was increased to 2250 rpm. 
After the speed was increased, the bearing had a satisfactory performance for only about 
an additional 95 hours. In several other tests, while the bearing was performing satis-
factorily (motor current, 1. 2 A), air was introduced into the vacuum test chamber to 
reach the standard atmospheric conditions. Immediately upon the introduction of air, 
the bearing began to operate at a high motor current (-'1. 4 to 1. 6 A). In 1 hour the per-
formance was very rough and noisy, and the test was discontinued. 
Bearing Tests with Sputtered Underlayer of Cr 3Si 2
 with Sputtered MoS 2
 Film 
In the second series of the bearing endurance tests the races and cages were sputter 
coated with a thin underlayer of Cr 3Si 2
 and subsequently covered with a sputtered MoS2 
film. The balls were left uncoated. This combination showed a significant improvement 
in endurance when tested under the same experimental conditions as the previously di-
rectly MoS2
 sputtered cages and races. The endurance life with the Cr 3 Si 2
 underlayer 
was over 1000 hours, compared to 187 hours for the directly sputtered MoS 2
 films. A 
comparison of these endurance lives is shown in figure 4. 
The application of the Cr 3Si 2
 underlayer was based on the decision to coat the bear-
ing surfaces with a very hard, thin, uniform, glassy surface finish. This finish would 
serve to contour the surface asperities and would function as a barrier layer during as-
perity deformation. Direct metal-to-metal contact, which leads to seizure, would be 
hindered. The Cr 3Si2
 film also has a tendency to act as an inert diffusion barrier layer. 
Coefficient of Friction against Atmospheric Pressure for Sputtered MoS 2 Films 
It is well documented that the coefficient of friction for MoS 2
 increases with the par-
tial pressure of water vapor in the air (ref. 4). The effect of atmospheric pressure on 
4
the coefficient of friction for sputtered MoS 2 films was also evaluated. The changes in 
the coefficient of friction with changes in pressure from atmospheric pressure (760 torr) 
and a relative humidity of 60 percent to 10 torr are shown in figure 5. This study was 
conducted in an ultrahigh-vacuum friction apparatus (ref. 5) with a 0. 47-centimeter-
radius hemispherical rider sliding on the sputtered MoS 2
 flat surface of a rotating 6. 35-
centimeter-diameter disk. The curve in figure 5 was constructed by starting the friction 
test at atmospheric conditions and gradually evacuating the chamber and monitoring the 
change in friction. The reverse procedure was also performed, wherein the friction test 
was started at 10 torr and the pressure was gradually increased until atmospheric con-
ditions were reached. The resulting friction curves had a very close agreement in their 
profile, whether the curve was constructed by increasing the pressure from 1O 9 torr or 
by decreasing the pressure from 760 torr. The reversibility in the procedure (evacu-
ating or pressurizing) has no effect on the coefficient of friction. As seen in figure 5, a 
continuous change occurred in the coefficient of friction between 100 and 400 torr. Above 
400 torr the coefficient of friction was steady at '-'0. 15, and below 100 torr it stabilized 
at 0. 04. Friction tests of sputtered MoS 2 films were also conducted in dry air, nitrogen, 
and argon. The coefficient of friction under these conditions was the same, 0. 04. 
SUMMARY OF RESULTS 
The following results were obtained from the bearing endurance tests when 204-size, 
440C stainless-steel, ball bearings were sputtered with a 6X10 7 -meter- (6000-A-) thick 
film. Two types of molybdenum disulfide (M0S 2) sputter depositions were performed: 
(1) direct sputtering with MoS 2 , and (2) sputtering with an underlayer of Cr 3Si 2
 and sub-
sequently with MoS2. 
1. Endurance lives of more than 1000 hours were obtained with bearings (cage, 
races, and balls) directly sputtered with MoS 2 . The same endurance lives (>1000 hr) 
were also obtained when only the races and cage were sputtered with an underlayer of 
Cr 3Si 2 and subsequently with MoS 2 . In comparison, an endurance life of less than 
200 hours was obtained with the races and cage coated only with MoS2. 
2. When air was admitted into the vacuum bearing chamber during the test, the bear-
ing failed in about an hour. Higher bearing speeds (2250 rpm) appreciably reduced the 
life.
3. The effect of pressure on the coefficient of friction for sputtered MoS 2 films was 
determined during sliding (disk and pin) experiments. Below 100 torr the coefficient of 
friction remained constant at 0. 04, above 400 torr the coefficient of friction was about
5
0., 15, and between 400 torr and 100 torr the coefficient of friction gradually changed with 
pressure. 
Lewis Research Center, 
National Aeronautics and Space Administration, 
Cleveland, Ohio, December 9, 1974, 
506-160
REFERENCES 
1. Spalvins, T.: Lubrication with Sputtered Mo5 2
 Films0 ASLE Trans., vol0 14, no0 4, 
Oct0 1971,	 267-274., 
2. Spalvins, T.: Structure of Sputtered MoS2
 Films at Various Substrate Temperatures0 
ASLE Trans0, vol0 17, no0 1, Jan. 1974, pp 17o 
3 Bisson, E. E.; and Anderson, W. J.: Advanced Bearing Technology. NASA SP-38, 
1964, pp. 279-283 
4. Winer, W. 0.: MoS2
 as Lubricant: A Review of the Fundamental Knowledge. Wear, 
vol. 10, 1967, pp. 422-452. 
5. Spalvins, T.; and Buckley, D. H.: Vapor Deposited Thin Gold Films as Lubricants 
in Vacuum (10_li mm Hg). J. Vac. Sci0 Tech0, vol0 3, no. 3, May-June 1966, 
pp. 107-113. 
6
High-voltage dc	 rf power 
power supply	 Matching netrk	 supply. 
(5kV)	 1kW
— Top plate 
tr j 
Specimen	 / I -	 _ Rotatable shutter 
/	 I (cathode	 /	 I 
or ground)	 '—Metal ring
	
Pyrex cylinder 
(anode)
CS-W020 Base plate 
(a) Schematic diaqram. 
(bi View of apparatus during sputter coating.
Figure 1. - Radiofrequency diode sputtering apparatus with direct-current bias.
7
a) 
a) 
I-
Test bearings -..
N
N 
Spring-loadl 
coils 
Figure 2. - Schematic of vacuum bearing endurance assembly. 
Without	 Sputtered M0S 2	 Sputtered MoS2 Sputtered M0S 2 on 
lubricant	 on balls only	 on races	 races, cage, 
and cage	 and balls 
Figure 3. - Endurance lives of 440C stainless-steel ball bearings with and 
without sputtered M0S 2 films.
F 
0) 
E 
0 I-
Directly	 Cr3Si2 under-
sputtered	 layer with 
with M0S 2	 sputtered MoS2 
Figure 4. - Endurance lives of 440C 
stainless-steel ball bearings with 
sputtered M0S 2
 films on races 
and cage - with and without a 
Cr3Si2
 underlayer. 
18 
.14 
0 
U 
•	 .10 
C 
0) 
U 
0) 0 
L)
02111	 I	 I	 I	 I	 I	 I	 I 
800 700 600 5(X) 400 3(10 200 100 10 10-1 io 2 io- io io ia-6 io 
Pressure, torr 
Figure 5. - Effect of pressure on coefficient of sliding friction for sputtered M0S2. 
Load, 250 grams; speed, 40 rpm; substrate/rider, Ni/Ni; room temperature. 
NASA-Langley, 1975' i-8166
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Bearing endurance tests in vacuum for sputtered molybdenum disulfide films

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