NASA/TM--2001-210941 ARL-TR-2467

1

v

U.S. ARMY

RESEARCH LABORATORY

The Effect of Journal Roughness and Foil

Coatings on the Performance of Heavily

Loaded Foil Air Bearings

Kevin C. Radil

U.S. Army Research Laborator_ Glenn Research Center, Cleveland, Ohio

Christopher DeIIaCorte

Glenn Research Center, Cleveland, Ohio

June 2001

https://ntrs.nasa.gov/search.jsp?R=20010069210 2018-05-14T14:47:40+00:00Z
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NASA/TM--2001-210941

U.S. ARMY

ARL-TR-2467

RESEARCH LABORATORY

The Effect of Journal Roughness and Foil

Coatings on the Performance of Heavily

Loaded Foil Air Bearings

Kevin C. Radil

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Christopher DellaCorte

Glenn Research Center, Cleveland, Ohio

Prepared for the

56th Annual Meeting

sponsored by the Society of Tribologists and Lubrication Engineers

Orlando, Florida, May 20-24, 2001

National Aeronautics and

Space Administration

Glenn Research Center

June 2001
NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http: //gltrs.grc.nasa.gov/GLTRS
The Effect of Journal Roughness and Foil Coatings on the

Performance of Heavily Loaded Foil Air Bearings

Kevin C. Radil

U.S. Army Research Laboratory

Glenn Research Center

Cleveland, Ohio 44135

Christopher DellaCorte

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Foil air bearing load capacity tests were conducted to

investigate if a solid lubricant coating applied to the

surface of the bearing's top foil can function as a break-in

coating. Two foil coating materials, a conventional soft

polymer film (polyimide) and a hard ceramic (alumina), were

independently evaluated against as-ground and worn (run-in)

journals coated with NASA PS304,

a high-temperature solid lubricant composite coating. The

foil coatings were evaluated at journal rotational speeds of

30000 rpm and at 25°C. Tests were also performed on a foil

bearing with a bare (uncoated) nickel-based superalloy top

foil to establish a baseline for comparison.

The test results indicate that the presence of a top

foil solid lubricant coating is effective at increasing the

load capacity performance of the foil bearing. Compared

to the uncoated baseline, the addition of the soft polymer

coating on the top foil increased the bearing load

coefficient by 120% when operating against an as-ground

journal surface and 85% against a run-in journal surface.

NASA/TM--2001-210941 1
The alumina coating increased the load coefficient by 40%

against the as-ground journal but did not have any affect

when the bearing was operated with the run-in journal. The

results suggest that the addition of solid lubricant films

provide added lubrication when the air film is marginal

indicating that as the load capacity is approached foil air

bearings transition from hydrodynamic to mixed and boundary

lubrication.

Introduction

Compliant foil air bearings operate by generating a

self-acting hydrodynamic air film between the stationary

inner top foil and the surface of the rotating shaft.

Because ambient air functions as the lubricant, foil

bearings do not have the same speed and temperature

limitations imposed on oil lubricated rolling element

bearings and, therefore, can be used at extreme temperatures

(to 650°C) and speeds well above 3 million DN (ref. I). To

capitalize on these advantages, research is underway to

incorporate compliant foil air bearings into high-speed

rotating machinery such as turbochargers, auxiliary power

units (APU's), and gas turbine engines (ref.2).

The rotor-bearing system proposed for several high

temperature, oil-free applications consists of state-of-theart foil bearings coupled with rotors coated with a wear

resistant solid lubricant composite coating, NASA PS304.

The foil bearings belong to the Generation III class of foil

NASA/TM--2001 - 210941 2
air bearings and have demonstrated in load capacity tests an

empirical load coefficient, _, equal to 1.0 based on the

Rule Of Thumb (ROT) model proposed in reference 3. The

PS304 solid lubricant coating's purpose is to reduce the

sliding wear and friction that occurs on the top foil and

rotor during start-up and shutdown in the absence of an air

film (refs 4 and 5). Endurance test results from start-stop

cycles indicate that the coating provides bearing wear lives

well in excess of requirements for turbomachinery

applications (ref 6).

Recent room temperature load capacity tests performed

in the authors' lab on Generation III bearings and PS304

coated journals have exhibited load coefficients of 0.3

instead of 1.0 as previously measured. An ensuing

investigation determined that the discrepancy resided with

the journals, specifically, the surface finish of the PS304

coating.

In both series of load capacity tests that resulted in

= 1.0 and _ = 0.3 the journals were prepared identically

by first applying the PS304 material using plasma spraying

followed by grinding the as-sprayed surface with a 500 grit

diamond wheel to a final average surface roughness of 0.8_m.

However, the earlier tests that exhibited high load capacity

(7 = 1.0) were conducted with the same PS304 coated

journals that were previously subjected to high temperature

NASA/TM--2001-210941 3
endurance stop-start cycles. This is important for three

reasons.

First, repeated high temperature rubbing associated

with each start-stop cycle polishes the surface of the asground coating and generates a thin oxide layer that is

smooth, dark, glossy and functions as a solid lubricant.

Measurements on this run-in surface (i.e., oxide layer)

with a profilometer confirm that the rubbing wear cycles

decreased the initial average roughness of the as-ground

coating surface from 0.8_m to 0.05-0.2_m, very close to the

0.05-0.1_m range considered for optimum bearing performance.

Second, it is believed that the localized start-stop rubbing

may also generate better conforming surfaces between the

bearing top foil and the coating surface. Third, the high

temperature sliding causes a transfer of solid lubricants

from the PS304 shaft coating to the top foil thereby

producing a solid lubricant film that leads to good bearing

performance (ref. 7).

An investigation into the cause of the 0.8_m surface

finish of the as-ground PS304 coating identified voids, i.e.

porosity, created in the coating during the plasma spray

process. The presence of open pores typically leads to an

unfavorable surface finish when standard grinding techniques

are used. For example, as mentioned previously, machine

grinding the as-sprayed coating with a 500 grit diamond

wheel results in an average surface finish of approximately

NASA/TM--2001-210941 4
0.8_m. The average surface finish can be improved to

.l-.2_m by following the initial grinding with a more

elaborate process of using 600 grit sandpaper, 9-15_m

diamond paste and 0.008_m alumina. However, these

additional steps are time consuming and costly.

The concern for the surface finish of the PS304 coating

is warranted due to its connection with a phenomenon known

as galling. Referred to as scuffing or scoring in the

internal combustion engine community galling is a failure

mechanism characterized by the formation of asperity

microwelds between surfaces in sliding contact. It is

believed that galling of the top foil and the nickel

constituent in the coating is playing a role in limiting

load capacity in the same way severe galling between piston

rings and liners leads to engine seizure.

The susceptibility of a foil bearing to galling stems

from its reliance on a very thin film of air, nominally 5um

under normal operating conditions, that separates the top

foil and shaft surface. If the average surface finish of

the rotor coatlng is on the same order of magnitude of the

air film a load less than the bearing's designed load

capacity will instigate local asperity contact with the top

foil, resulting in hot spots, friction welding, and galling.

The high asperities can also prevent full separation of the

top foil from the rotor surface resulting in a bearing

NASA/TM--2001-210941 5
operating in a mixed lubrication regime. In either case

galling will occur once contact is initiated.

Ruscitto et al. proposed another explanation for the

relationship between surface roughness and foil bearing load

capacity over twenty years ago (ref. 8). In their report

they speculated that rough surfaces contain relatively large

microscopic voids that provide alternate paths of escape for

the air molecules that otherwise would be used to generate

and sustain the hydrodynamic air film. This explanation is

plausible but is not relevant to this situation because the

pores in the PS304 are not interconnected and probably do

not assist in any additional or parasitic leakage of air.

Even though galling can transpire at any time, given

the right conditions, its most pronounced affect on bearing

load capacity will be while the coating is in the as-ground

condition. Therefore, until the coating and bearing

experience the required number of high temperature startstop cycles to produce the thin oxide layer, generate

conforming surfaces, and build-up a lubricious transfer film

on the top foil galling must be alleviated in order to

insure maximum performance of the foil bearing.

The purpose of this paper is to report on a possible

solution to sustaining the designed load capacity of a foil

bearing while using PS304 in an as-ground condition. The

concept to be evaluated is to place a coating directly onto

the top foil to inhibit galling and thus allowing the foil

bearing to support its designed maximum load. Two coatings,

NASA/TM--2001-210941 6
a soft polymer (polyimide) and a hard ceramic alumina, were

chosen for the study. Their effectiveness was evaluated by

conducting a series of room temperature load capacity tests

on foil bearings with coated top foils against ground and

run-in PS304 coated journals. Room temperature was selected

as the best test temperature because it presents the most

severe galling condition and does not prompt oxide formation

on the surface of the coating. To establish a baseline,

uncoated top foils were also tested under the same

conditions.

Experimental Apparatus/Procedure

Foil Bearings

The Generation III class foil bearings used in the

tests are shown in figure 1 and are nominally 35 mm in

diameter, 25 mm long (or wide) and made up of several layers

of nickel-based superalloy foils. The top foil supports the

hydrodynamic gas film while the underlying foils, known as

bump foils, provide the elastic and compliant support

structure for the top foil. The bump foils also provide

Coulomb damping to the bearing through frictional microsliding with the top foil and adjacent surfaces. There are

other foil bearing designs in commercial use that utilize

overlapping leaves and perforated spring foils. The results

presented in this paper should also be relevant to these

different designs.

NASA/TM--2001-210941 7
Test Journals

The test journals shown in figure 2 are nominally 35 mm

in diameter, 84 mm long and made from a nickel-based

superalloy. The journal is an improvement over the previous

two-piece design that used a heat dam and test journal

secured with a tie bolt (ref. 9). The new journal

eliminates the need for a tie bolt by combining the heat dam

and test journal into one part. This provides a 40% weight

savings and a more rotordynamically stable test rig. A

0.25 mm deep undercut is machined on the journal to

accommodate the deposition of the PS304 solid lubricant

coating. The journals have twelve equally-spaced threaded

holes for in-place, high-speed, dynamic balancing.

For the tests with run-in journals, i000 start-stop

endurance cycles under a 10kPa load at 538°C were completed

using an identical Generation III bearing that was not

associated with the tests. This separate bearing was used

to avoid the development of a lubricious transfer film on

the three test top foils or to generate conforming surfaces.

Therefore, any differences in load capacity results reflect

changes in the PS304 coating surface finish and perhaps

chemical composition and/or the presence of the top foil

coating

Top Foil Coatings

The polymer (polyimide) and ceramic (alumina) coatings

were applied to the bearing top foils prior to installation

and assembly of the bearing. The polymer material is a

NASA/TM--2001-210941 8
commercially available polyimide based dry film lubricant

that provides a low coefficient of friction, excellent

adhesion and resiliency and long wear life. The top foil

was coated by spraying the polymer with an atomizer gun to

a final thickness of 25.4 _m and cured according to the

manufacturer's directions.

The alumina was applied to the top foil using a

sputtered deposition to produce a coating thickness of

25.4 _m according to the procedures described in reference

i0. This material was chosen because it demonstrated

promising lubricating properties during previous partial-arc

bearing endurance tests (ref. ii) and also high temperature

rigid gas bearing tests (ref. 12).

Test Apparatus/Procedure

The high-speed test rig used to conduct the tests is

shown in figure 3 and is described fully in reference 9.

The rig consists of a drive shaft that is supported by two

hybrid ceramic ball bearings that are lubricated by oil jets

and cooled by temperature controlled water. An impulse

turbine attached to the drive shaft is capable of driving

the shaft to rotational speeds up to 70000 rpm. Referring

to figure 4, radial loading of the bearing is accomplished

via a vertical cable system with one end attached to the

bearing and the other to a pneumatic load cylinder mounted

below the test rig. A rod attached to the bearing loader

is used to relay the bearing torque to a load cell.

NASA/TM--2001-210941 9
Testing commenced by first placing the bearing on the

journal and then accelerating the rig to a rotational speed

of 30000 rpm. The speed was held constant for a few minutes

in order to thermally stabilize the test rig. A chart

recorder continuously monitored and displayed the bearing

torque signal and the shaft rotational speed. The rig was

then accelerated by a few thousand rpm and the pneumatic

load cylinder was used to apply a starting load equal to

half of the expected load capacity according to the ROT

model. When the desired load was reached the rig was

decelerated to the final speed of 30000rpm and held for

5 minutes unless there were indications of bearing failure.

In this paper foil bearing load capacity is defined

as the maximum load supported by the bearing while still

operating at constant speed and steady-state conditions.

The chart recorder output shown in figures 5 and 6 better

elucidate this definition of load capacity. The two traces

in figure 5a represent speed and torque output from a

bearing supporting a moderate load and operating in a stable

condition. The spikes in the torque data are caused by

sporadic noise from the data acquisition system. Conversely,

the speed and torque traces shown in figure 6 reflect a

bearing operating close to its maximum load capacity. The

erratic behavior exhibited by these traces is indicative of

intermittent surface asperity contact between the top foil

and the coating. While operating at this load condition

frequent speed corrections, in response to the fluctuating

NAS A/TM--2001-210941 10
bearing torque, were needed in order to maintain the test

speed of 30000 rpm.

If failure didn't occur the speed of the test rig was

again increased by a few thousand rpm and an additional 22 N

(5.0 Ib) was applied. The rig speed was reduced to 30000 rpm

and the load sustained for another 5 min. This methodology

of loading the bearing in 22 N (5.0 Ib) increments with

5 min. hold periods was continued until speed and torque

readings indicated that the hydrodynamic film was beginning

to rupture. When this occurred the load was returned to the

previous stable test condition and a smaller load increment

of 4.4 N (i ib) was used.

Test Results and Discussion

The test results for the various top foil coating

conditions are summarized in Table i. It is clearly evident

from the data that the top foil coatings and the PS304

surface finish, either separately or in combination, have a

direct effect on the load coefficient of a foil bearing.

The as-ground PS304 coating, vulnerable to galling because

of its high surface roughness and exposed nickel

constituent, consistently produced the lowest load

coefficients. The baseline bearing's load coefficient of

7 = 0.33 falls significantly short of the '7 = 1 design

point for the Generation III class foil bearings thus

initiating the present research (on foil break-in coatings).

The two candidate break-in coatings tested in this

study, alumina and the soft polymer, exhibited anti-galling

NAS A/TM--2001 - 210941 11
characteristics when paired with the as-ground coating but

at varying degrees of effectiveness. Alumina's presence on

the top foil increased the load coefficient 40% from 0.33 to

0.47. The polymer coating's effect on the load coefficient

was more pronounced than the alumina as it expanded the

margin over the baseline by 120%, from 0.33 to 0.73.

The polymer's superior performance may classify it

as a better anti-galling agent but it is believed that a

significant portion of the coefficient's increase is due to

the polymer's solid lubricant properties. When operating

the bearing close to its load capacity the air film is

marginal indicating contact between surface asperities.

During sliding contact minute wear particles are generated

and introduced into the air film causing a transition from

a purely hydrodynamic bearing to one that may also be in a

state of quasi-hydrodynamic powder lubrication as explained

by Heshmat(13). Based on this assumption, the small

increase in the load coefficient caused by the alumina

coating is probably an indication that, for this

application, it is a less effective solid lubricant.

The tests also demonstrated that PS304 in the run-in

condition plays a major role in defining the load

coefficient of a foil bearing. When the baseline foil

bearing was tested against the run-in PS304 coating the load

coefficient improved 70% over the as-ground result from

= 0.33 to _ = 0.56. The run-in shaft coating had the

same effect on the bearing with the alumina coated top foil

NASA/TM--2001-210941 12
as it increased the coefficient 21%, from _ = 0.47 to

=0.56 but, when compared to the baseline, the alumina

coating was inconsequential as both exhibited coefficients

of 0.56. Continuing the trend when paired with the run-in

shaft coating the foil bearing with the polymer coating

realized a 43% load coefficient increase from 7 = 0.73 to

= 1.05. If run-in results for the polymer and baseline

are compared there is an 85% increase, from _ = 0.56 to

= 1.05, in the load coefficient caused by the coating's

ability to perform as a solid lubricant.

The success of run-in PS304 at improving the foil

bearing's load coefficient can be attributed to the presence

of the thin complex oxide/lubricant surface layer formed

during the high temperature sliding cycles of the stop-start

endurance tests. The oxide layer alleviates the galling

failure mechanism by separating the nickel constituent from

the top foil and its smooth surface finish allows for a much

thinner air film to be tolerated. Furthermore, as with the

polymer coating, the oxide layer produces minute wear

particles that act as a solid lubricant. However, based

upon the baseline and alumina coating results, achieving a

coating surface as smooth as the oxide layer does not

automatically guarantee an adequate bearing load

coefficient. Other additional factors such as the formation

of a lubricious transfer film on the top foil and generating

NASA/TM--2001-210941 13
conforming surfaces through high temperature start-stop

cycles appear to be essential for optimum bearing

performance.

The test results of the polymer and alumina coatings

against the as-ground surface suggests that top foil

coatings can be used to increase bearing performance by

inhibiting the onset of galling. However, it is evident

from the polymer coating results that if high temperature

foil break-in coatings are to be used they must possess

anti-galling and solid lubricant properties. Also, since

the PS304 coating surface finish plays a major role in the

performance of a foil bearing, concurrent work is needed on

improving application and/or surface preparation methods.

Conclusions

i. The current practice of using plasma spraying to

apply the PS304 composite results in a porous coating

that limits the load coefficient of foil bearings.

Further research is needed to identify alternate

deposition techniques that produce higher density

coatings.

2. The test results for the polymer and alumina coatings

indicate that break-in foil coatings that inhibit

galling can, at varying degrees, increase the bearing

load coefficient. Coatings that possess solid

lubricant properties further increase the load

coefficient by providing a solid lubrication

NASA/TM--2001-210941 14
component when the bearing is operating under

boundary and mixed lubricating conditions.

3. To maximize performance, foil bearings operating

against PS304 coated shafts together must undergo

high temperature start-stops in order to produce the

smooth oxide layer, conforming surfaces, and the

lubricious transfer film on the beating's top foil.

References

i. Heshmat, H.: "Advancements in the Performance of

Aerodynamic Foil Journal Bearings: High Speed and Load

Capability," ASME Jour. Of Trib., Vol. 116, pp. 287-

295, April, 1994.

2. Heshmat, H., Walton, J.F., DellaCorte, C., and Valco,

M.: "Oil-Free Turbocharger Demonstration Paves Way to

Gas Turbine Engine Applications," Presented at the ASME

International Gas Turbine and Aeroengine Congress and

Exhibition, Munich, Germany, May 8-11, 2000, Paper No.

2000-GT-620.

3. DellaCorte, C., and Valco, M.J.: "Load Capacity

Estimation of Foil Air Journal Bearings for Oil-Free

Turbomachinery Applications," Tribology Transactions,

Vol. 43, 2000, pp. 795-801.

4. DellaCorte, C., and Edmonds, B.J.: "Preliminary

Evaluation of PS300: A New Self-Lubricating High

Temperature Composite Coating for Use to 800°C, " NASA

TM-I07056, 1995.

NASA/TM--2001-210941 15
5. DellaCorte, C., and Fellenstein, J.A.: "The Effects of

Compositional Tailoring on the Thermal Expansion and

Tribological Properties of PS300: A Solid Lubricant

Composite Coating," NASA TM-I07332, 1996.

6. DellaCorte, C., Valco, M.J., Radil, K.C. and Heshmat,

H.: "Performance and Durability of High Temperature

Foil Air Bearings for Oil-Free Turbomachinery," NASA

TM-2000-209187, 1999.

7. DellaCorte, C.: "The Evaluation of a Modified Chrome

Oxide Based High Temperature Solid Lubricant Coating

for Foil Gas Bearings." NASA TM-1998-208660, 1998.

8. Ruscitto, D., McCormick, J. and Gray, S.: _Hydrodynamic

Air-Lubricated Compliant Surface Bearing for an

Automotive Gas Turbine Engine. I - Journal Bearing

Performance." NASA CR-135368, April, 1978.

9. DellaCorte, C.: _High Speed Test Rig for Use to 700°C

and 70,000 rpm." NASA TM-I07405, 1997.

i0. Lei, J.F.: "Advances in Thin Film Sensor Technologies

for Engine Applications", ASME Paper No. 97-GT-458,

1997.

ii. DellaCorte, C., Fellenstein, J.A. and Benoy, P.A.:

_Evaluation of Advanced Solid Lubricant Coatings for

Foil Air Bearings at 25 and 500°C. " NASA TM-1998-

206619.

12. Murray, S.F.: "Research and Development of High

Temperature Gas Bearings. Volume II: Selection and

NAS A/TM--2001-210941 16
Evaluation of Gas Bearing Materials in Argon at 900°F

13.

and 1400°F ", NASA Contract No. NAS3-9433, 1969.

Heshmat, H.: "The Rheology and Hydrodynamics of Dry

Powder Lubrication." Tribology Transactions, Vol. 34,

1991, pp. 433-439.

Table i. Bearing load coefficients for coated foils running

against PS304 in the ground and run-in condition.

Foil Coating

Soft Polymer

Foil Coating

Roughness,

sum Ra

Load Coefficient,

As-ground,

0.90

PS304

0.73

Run-in,

PS304

1.05

Ceramic(Alumina) 0.15 0.47 0.56

Bare (Baseline) 0.I0 0.33 0.56

Beadng

sleeve _ _

Figure 1 .-Schematic of Generation III foil air bearing.

NASA/TM--2001-210941 17
250-Fm thick PS304 coating applied--

29

35

diam

\

'---1 of 12 set Screw balancing holes

8

CD-00-80986

Figure 2.-Schematic of the modified test journal for the high-temperature, high-speed foil

air bearing test rig. Measurements in mm.

......... =_ _. g ::( ;;

NASA/TM--2001-210941 18
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NASA/TM--2001-210941 19
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LVDT load cell

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Figure 4.--Test rig partial front view showing torque and load measurement

system.

NASA/TM--2001-210941 20
40

O

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T =,

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10 0,,

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0 1O0 200 300 400 500 600

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Figure 5.--Graphs illustrating a foil air bearing operating in a stable condition at 30 000 rpm

and supporting half of its load capacity. (a) Speed. (b) Torque.

NASA/TM_2001-210941 21
A4° o 30 Io

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Figure 6.--Graphs illustrating the erratic behavior of a foil air bearing operating at 30 000 rpm

and supporting close to its load capacity. (a) Speed. (b) Torque.

NASA/TM--2001-210941 22

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4. TITLE AND SUB_i/LE

The Effect of Journal Roughness and Foil Coatings on the

Performance of Heavily Loaded Foil Air Beatings

6. AUTHOR(S)

Kevin C. Radii and Christopher DellaCorte

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center

Cleveland, Ohio 44135-319I

and

U.S. Army Research Laboratory

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, DC 205464)001

and

U.S. Amay Research Laboratory

Adelphi, Maryland 20783-1145

5. FUNDING NUMBERS

WU-708-18-13--00

ILl61102AH45

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12794

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210941

ARL-TR-2467

11. SUPPLEMENTARY NOTES

Prepared for the 56th Annual Meeting sponsored by the Society of Tribologists and Lubrication Engineers, Orlando,

Florida, May 20-24, 2001. Kevin C. Radii, U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio,

and Christopher DellaCorte, NASA Glenn Research Center. Responsible person, Kevin C. Radil, organization code 5960,

216--433-5047.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b, DISTRIBUTION CODE

Unclassified - Unlimited

Subject Category: 23 Distribution: Nonstandard

Available electronically at htto://dtrs.m-c.nasa._ov/GLTRS

This publication is available from the NASA Center for AeroSpace Information, 301-621_)390.

13. ABSTRACT (Maximum 200 words)

Foil air beating load capacity tests were conducted to investigate if a sofid lubricant coating applied to the surface of the

hearing's top foil can function as a break-in coating. Two foil coating materials, a conventional soft polymer film

(polyimide) and a hard ceramic (alumina), were independently evaluated against as-ground and worn (run-in) journals

coated with NASA PS304, a high-temperature solid lubricant composite coating. The foil coatings were evaluated at

journal rotational speeds of 30 000 rpm and at 25 °C. Tests were also performed on a foil bearing with a bare (uncoated)

nickel-based superalloy top foil to establish a baseline for comparison. The test results indicate that the presence of a top

foil solid lubricant coating is effective at increasing the load capacity performance of the foil bearing. Compared to the

uncoated baseline, the addition of the soft polymer coating on the top foil increased the beating load coefficient by

120 percent when operating against an as-ground journal surface and 85 percent against a run-in journal surface. The

alumina coating increased the load coefficient by 40 percent against the as-ground journal but did not have any affect

when the bearing was operated with the run-in journal. The results suggest that the addition of solid lubricant films

provide added lubrication when the air film is marginal indicating that as the load capacity is approached foil air bearings

transition from hydrodynamic to mixed and boundary lubrication.

14. SUBJECT TERMS

Foil gas beatings; Coating; Tribology; Solid lubricant

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

NSN 7540-01-280-5500

19. SECURITY CLASSIRCATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

28

16. PRICE CODE

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribedby ANSI Std. Z39-18

298-102