NASA/TM--2001-210806

Preventing Spacecraft Failures Due to

Tribological Problems

Robert L. Fusaro

Glenn Research Center, Cleveland, Ohio

Prepared for the

2001 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

April 2001
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, ME) 21076

Price Code: A03

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03

Available electronically at http: //gltrs.grc.nasa.gov/GLTRS
Preventing Spacecraft Failures Due to Tribological Problems

Robert L. Fusaro

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

ABSTRACT

Many mechanical failures that occur on spacecraft are caused by tribological problems. This

publication presents a study that was conducted by the author on various preventatives, analyses,

controls and tests (PACTs) that could be used to prevent spacecraft mechanical system failure. A matrix

is presented that plots tribology failure modes versus various PACTs that should be preformed before a

spacecraft is launched in order to insure success. A strawman matrix was constructed by the author and

then was sent out to industry and government spacecraft designers, scientists and builders of spacecraft

for their input. The final matrix is the result of their input. In addition to the matrix, this publication

describes the various PACTs that can be performed and some fundamental knowledge on the correct

usage of lubricants for spacecraft applications. Even though the work was done specifically to prevent

spacecraft failures the basic methodology can be applied to other mechanical system areas.

INTRODUCTION

A wide variety of tribological components is required to operate in a space environment for long

durations and at very low torque. Satellites and space vehicles have bushings and rolling contact

bearings in components like hatch doors, manipulators, solar array drives, control moment gyros, antenna

and camera pointing mechanisms, infrared horizon scanners, gear boxes etc. Sliding contacts are found

in brakes, clutches, hinges, deployment devices, traction drives, etc. Mechanisms for extending or

retracting devices often use wire cables requiring internal lubrication of the cable. Telemetry and

electrical connections to moving parts require slip or roll rings whose contact resistance often must be

low and unvarying. All these components involve sliding or rolling contacts and because of limited power

available, they must operate with minimum friction. In addition, many of these tribological devices must

operate both in air and in the vacuum of space.

Satellites and space vehicles are lubricated either by using liquid lubricants or solid lubricants.

How well a lubricant functions is extremely system dependent and thus the choice of a lubricant (for any

particular application) must be made consistent with the operating conditions of the system. Liquids and

solids can lubricate in more than one lubrication regime and very different mechanisms can take place in

each regime. Therefore the methods of evaluating and testing a lubricant depend upon which lubricating

regime is operating.

This article will present different preventatives, analyses, controls and tests (PACTs) for selecting

and employing lubricants for spacecraft tribological applications. Unfortunately one cannot use any one

of these techniques with absolute assuredness to predict the friction coefficient, the wear, or the

NASA/TM--2001-210806 1
endurance life of a lubricating system. The only certain way to do that is to evaluate the lubricant or

lubricant system in the end use device under the actual operating conditions. It is almost impossible to

do that on the ground. Two excellent books (Conley 1998, Sarafin 1995) discuss many of the problems

and methodologies that can be used to prevent failures and are recommended reading.

FUNDAMENTALS OF FRICTION AND WEAR

Friction

Friction is the resistance to movement when one object moves relative to another while in

contact. There are three basic laws of friction:

(1)

(2)

(3)

The friction force (f) is proportional to the normal force (F), which leads to the

relationship:

f = I_F, where I_ is defined as the coefficient of friction,

The friction force is independent of the apparent area of contact, and

The friction force is independent of the sliding velocity.

There are some exceptions to these laws, especially in vacuum, but for most situations they are

applicable.

There are basically two reasons why friction occurs. The first is due to adhesion that occurs

between the molecules of the two surfaces. The second is due to the fact that surfaces are not absolutely

flat. When a hard surface slides across a softer surface, small asperities on that hard surface "plow"

through the soft surface. This is known as abrasion.

For surfaces with contaminant films on them, friction is not a constant. This is because films wear

off on repeated sliding which usually causes friction to increase. Also if the yield stress of one of the

materials is exceeded, friction will increase due to plastic deformation of that surface.

Most metals have oxide films on them. When they are slid in air, these oxide films can be

reformed as they are worn off. If the same surfaces were slid in vacuum (where there is no oxygen)

they would not be reformed and friction would increase significantly.

Friction Measurement

Friction should be measured continuously during a test. Usually a test commences with high

friction. This is a time where either the lubricant or the materials in contact are plastically rearranged to

accommodate the sliding forces. It is called the "running-in" phase. After "running-in", the next phase is a

period of time when the friction coefficient is fairly constant and reduced from the initial level. Generally,

this is the value reported as the friction coefficient for this lubrication system. The final phase is a period

of constantly increasing friction. Increasing friction is an indication that the lubricant is being depleted.

NASA/TM--2001-210806 2
Depending on the type of lubricant, the type of materials, surface finish, etc. the final phase may take

place over a long time or a relatively short time.

Another friction characteristic that should be noted and recorded is the "roughness" of the friction

produced during a test. This "roughness" or variation of friction over a very short time span is due to the

fact that at least one of the surfaces is constantly moving over a new surface on the other specimen and

that the friction between the two different surfaces may not be exactly the same. The ability to pick up

this difference is dependent on the time reponse of the device that is used to record the friction and the

sliding speed of the rotating specimens. Often the width of a friction trace ("roughness" or variation of the

trace) on a chart recorder can be quite broad. For example, the width of the trace might range from a

friction value of 0.06 to 0.08. The width of this trace should always be noted. In general, the less the

"roughness" (less variation of the trace), the better is the lubrication system.

Wear

Similar to friction, wear is caused by both adhesion and abrasion. In addition, corrosion, fatigue,

erosion, and cavitation can cause wear. Also there is a type of wear called fretting which occurs in joints

that are tightly held down and subject to vibrations. When fretting occurs the amplitude of the vibration is

very small. More information can be found about these wear mechanisms in an article by Zaretsky 1997.

Friction and wear are not necessarily related. When there is low friction there is not necessarily

low wear. Corrosion for example may provide a film that provides low friction, but corrosive wear can be

very high. Every material combination has both friction and wear characteristics that are dependent on

the conditions or environment under which they are being slid. Thus it is important to evaluated materials

and lubricants in accelerated tests under conditions that they will experience in their end use application.

Some of these conditions are: temperature, atmosphere (including moisture content), speed, load,

geometry of parts, type of motion, etc. For more details on various aspects of vacuum tribological

fundamentals see the publication by Buckley 1971.

Wear is loss of material from two surfaces in sliding contact. Archard 1980 has stated for

adhesive wear that:

(1)

(2)

(3)

Wear is proportional to the load (L).

Wear is proportional to the distance-slid (x).

Wear is inversely proportional to the hardness (p) of the surface being worn.

Thus Archard 1980 formulated an equation for the volume of material (W) worn away to be:

W = kLx/p, where k is the wear coefficient.

NASA/TM--2001-210806 3
Since most wear is adhesive, this is the most common equation used to determine wear. In some

instances, hardness is included as part of the wear coefficient and in this case the wear coefficient is

expressed as:

k = W/Lx.

Since both load and sliding distance are known, the wear coefficient can be determined by measuring the

wear volume. The most common methods of doing this are: (1) determining a weight change, (2) optical

measurement of the scars and then calculating the wear volume, and (3) surface profiles of the change in

shape of the surfaces and then calculating the volume of material removed.

To obtain the most accurate value for wear, several measurements of wear volume should be

taken at various sliding distances. Since most surfaces experience "running-in," this will help separate

running-in from steady state wear. The sections on solid lubrication and liquid lubrication testing will

discuss this in more detail.

Wear Measurements

There is inherent scatter of wear test results. Therefore, multiple tests must be conducted under

standard test conditions to provide a measure of the scatter. It is impractical and expensive to run large

numbers of wear tests, so statistical methods sometimes must be used to analyze the data.

For example, a test that the American Society for the Testing of Materials (ASTM) recommends

is called Standard G83-96, "Standard Test Method for Wear Testing with a Crossed-Cylinder Apparatus."

The standard details statistical methods and provides tables for determining the minimum number of

tests. See Table 1.

Table 1.mFactors for estimating standard deviation based of sample size.

Sample Size (n) d2 1/d2

2 1.128 0.8865

3 1.693 0.5907

4 2.059 0.4857

5 2.326 0.4299

6 2.534 0.3946

7 2.704 0.3698

8 2.847 0.3512

9 2.970 0.3367

10 3.078 0.3249

The minimum sample size for a series of wear tests can be determined from a preliminary round of tests

from which the coefficient of variability can be obtained. The following parameters can be calculated from

the wear results.

NASA/TM--2001-210806 4
S I

R=

d2 =

V =

S =

X =

e =

n =

Standard deviation (for sample size less than 10 =R/d2)

Difference between highest and lowest test value

Deviation factor (Varies with sample size)

Coefficient of variation, percent = (s/x) - 100

Standard deviation

Arithmetic average wear for n tests

Allowable sampling error expressed in %.

Sample size

(95 percent confidence level) = (1.96v/e) 2

If the allowable sampling error is a given, then the minimum number of tests for 95 percent

confidence level can be determined. This method has been used in round-robin tests for given material

combinations run at selected conditions. The method was found useful for comparing the results from a

number of different laboratories. Coefficient of variation among the labs that did this testing was reported

in ASTM G83-90 as 30 percent.

PRETESTING ANALYSES

Controlling Specifications

For spacecraft design, if a proper set of specification documents are developed and then

followed, many tribological problems can be eliminated. MIL-STD-961establishes a standard format

for specification content. Specifications should include statements of scope and mission requirements.

References to applicable documents and requirements such as mechanism function, operating

environments, tribological interfaces, endurance life, materials compatibility, torque limits, etc. are also

necessary. In addition, project-engineering documents with sufficient requirements specified to enable a

detailed design of the mechanism and a process to be used to fabricate and assemble the mechanisms

should be given. The materials to be used and a process for fabricating the mechanism should also be

specified.

Material Certification/Review

Tribological materials are very system dependent. Extreme care should be taken in selecting

the materials for a particular application. That not only includes the lubricants but also the materials from

which the tribological components are made. Certify that these materials have functioned properly in

other similar applications and review the literature for possible failures. To help in selecting the materials

to use in space, NASA has written a Space Materials Handbook (Rittenhouse and Singletary, 1969).

NASA/TM--2001-210806 5
Lubricant Evaporation Analysis

An analysis should be made on the liquid lubricant evaporative losses for each particular bearing

system that is used on a spacecraft. The rate of loss will depend on the vapor pressure of the lubricant

used, the vapor pressure of the additives used in the lubricant and the type of seal that is used to seal

the system. In addition, if an active replenishment system is used, the rate of dispersal should also be

figured into the calculation.

Lubricant Life Analysis

It is almost impossible to calculate an exact life for liquid lubricants because so many variables

are involved. The most important criterion for life is to have enough liquid lubricant available so that the

bearing does not run in a "starved" condition. If starvation occurs, wear to the bearing surfaces can take

place that will cause mechanical noise (vibrations), higher torque and shortened life of the bearing. If

adequate lubricant is supplied to the bearing and it is not over-loaded or run at excessive speeds, life

should be dependent on the quantity of lubricant available for lubrication. In this case, the liquid lubricant

evaporation analysis coupled with other loss factors (such as creep from the contact areas) should be

able to give a ballpark prediction of the life of the lubricant.

Solid lubricants have a finite life. A solid lubricant film has an average life of so many passes

or cycles slid over it or total distance slid. Solid lubricant composites tend to wear until the clearances

between the surfaces become too large to operate effectively. In general, solid lubricant films or composites tend to wear at a constant rate if conditions remain constant. Reasonable life predictions can

be made by running accelerated tests on solid film lubricants or composites under similar operating

conditions to those that they will experience in the end-use application. Since life is very system

dependent, it must not be assumed that wear rates given by a manufacturer are applicable for an

application. The only way to obtain valid data for a particular application is to evaluate the solid lubricant

under the exact conditions of an application.

Review Tolerance Stack-up

Dimensions are controlled in a spacecraft at the part or assembly level. The usual approach is

to sum up the dimensions of the parts. This sum willthen be considered the dimension of the assembly.

This methodology is adequate as long as there are no thermal gradients. But thermal gradients do arise

and must be taken into account. Heating can occur from outside thermal sources or from tribological

sliding.

A literature review can provide expected friction coefficients needed to calculate possible thermal

expansion. Thermal expansion calculations from these above mentioned sources will ensure that binding

of a mechanism will not occur if there is thermal growth due to friction. Opening of clearance can also

occur due to thermal cooling and this should also be considered. Sarafin 1995 discusses tolerance

problems in more detail.

NASA/TM--2001-210806 6
PHYSICAL PROPERTIES TESTING OF MATERIALS/LUBRICANTS

Quality control of lubricants can be accomplished using the physical property tests that are

relative to your application. The following are some standard tests than can be conducted on liquid

and solid lubricants as well as on space mechanism structural materials.

ASTM B195

ASTM B347

ASTM D91

ASTM D92

ASTM D97

ASTM D 149

ASTM D217

ASTM D256

ASTM D455

ASTM D621

ASTM D790

ASTM D942

ASTM D972

ASTM 1310

ASTM D1478

ASTM D 1743

ASTM D2265

ASTM D2270

ASTM D2510

ASTM D2511

ASTM D2649

ASTM D5949

ASTM El0

ASTM E18

ASTM E228

ASTM E595

ASTM 2595

ASTM F312

MIL-L-6085

Electrical resistively of metallic materials

Hardness of self-lubricating composite materials

Precipitation number of liquid lubricants.

Flash and fire points of liquid lubricants.

Pour point of liquid lubricants

Dielectric breakdown and strength of solid lubricant coatings

Penetration of greases

Impact resistance of tribological materials

Kinematic Viscosity of liquid lubricants.

Deformation under load of solid film lubricants

Flexural properties of materials.

Oxidation stability of greases.

Evaporation loss of liquid lubricants.

Flash point of solid lubricant coatings prior to application.

Low temperature torque of greases

Corrosion prevention of greases.

Dropping point of greases.

Viscosity Index of liquid lubricants.

Adhesion of solid lubricant coatings.

Thermal shock of solid lubricant coatings.

Corrosion characteristics of solid lubricant coatings.

Pour point of a liquid lubricant

Standard test method for Brinell hardness of metallic materials.

Standard test for Rockwell hardness and superficial hardness of metallic

materials.

Linear thermal expansion of solids.

Material outgassing in vacuum.

Evaporation loss of greases.

Particulate contamination of liquid lubricants.

Low Temperature stability of liquid lubricants

NASAfI'M--2001-210806 7
MIL-STD-453C

MIL-STD-6866.

MSCF-SPEC-527

Radiographic Testing.

Liquid Penetrant Testing.

Materials Selection List for Space Hardware Systems.

TRIBOLOGICAL ACCELERATED BENCH TESTING

Since friction, wear and lubrication are very dependent on atmospheric conditions, it is very

important to control the atmosphere when simulating conditions for the end use requirement. For the

space environment, obviously the experiments should be conducted in a vacuum. However, for some

initial bench screening tests, it may be possible to achieve a degree of simulation by evaluating the

specimens in a very dry argon or nitrogen atmosphere. This will negate the effect that oxygen and water

vapors have on the lubrication process. This section will describe a number of accelerated bench testing

devices assuming that they are enclosed in a vacuum chamber or a chamber that would facilitate testing

in dry argon or dry nitrogen. ASTM standards for testing would apply, but they should be adjusted

appropriately for a vacuum condition.

Block-on-Ring Tribometer

A schematic of the block-on-ring test elements is shown in figure 1. The device consists of a

rectangular block pressed against the periphery of a ring. The block can be flat (line contact) or it can be

conforming (area contact). For area contact, the same radius of curvature is given to the contacting block

face as to the ring so as to produce a large area of contact. ASTM has issued a standard testing

designation for this device specified as ASTM G77. Commercial machines are builtto this specification.

The block is stationary and loaded with a dead weight against the ring. The ring is attached to

a rotating shaft that can rotate in one direction or oscillate. A probe attached to the block holder contacts

a load transducer and measures frictional force between the block and the rotating ring. A thermocouple

is imbedded near the contact area of the block to measure temperature. Also when testing a liquid

lubricant, a thermocouple should be immersed in the lubricant supply cup to measure the lubricant

temperature.

If liquid lubricants are tested, a lubricant reservoir (cup) should be filled up so that the ring dips

into it. If a solid lubricant film is tested, the film is applied to the contact area around the diameter of

the ring. The block can also be made from a composite material for testing. In all cases, the surface

roughness of the ring is very important and can influence the results. Generally speaking, the smoother

the ring, the lower the block wear rate. To most closely reproduce the end-use application, the roughness

should closely match that of the end-use application. ASTM standard tests for this machine are:

D-2714-94 for calibration and operation (with liquid lubricants) and D2981-94 for wear life of bonded

solid lubricants in an oscillating motion.

NASA/TM--2001-210806 8
Pin-on-Disk Tribometer

Schematics of two different pin-on-disk (ball-on-flat) testing devices are shown in figures 2 and 3.

A hemispherically tipped pin (or a ball clamped in a holder) is pressed against a flat disk and the disk is

rotated relative to the pin. The load is applied by a dead weight on a lever arm system. Wear volume of

the pin is determined by measuring the weight loss or by measuring the change in diameter of a circular

wear scar on the pin and then calculating the volume of material removed. Wear volume of the disk is

determined by measuring the weight change or by measuring the wear track cross-sectional area using

a surface profilometer and then calculating the volume removed. Wear should always be reported as a

volume change (for a disk, the cross-sectional-area of the scar multiplied by the disk track length or for

a ball or pin, the volume of the material removed from the tip) because as discussed previously adhesive

wear has been established to be a volume phenomenon (Archard 1980). It is recommended that one

relies more on optical and surface profilometry measurements rather than weight-loss measurements,

since material transfer from one surface to another can provide erroneous weight change measurements.

An advantage of pin-on-disk testing as compared to some other testers is that flat wear surfaces are

produced which are very easy to analyze using surface analytical techniques.

For solid lubrication testing, a film or coating is applied to the disk or the disk is made from a

composite material and then a metallic or ceramic pin is slid against the disk. Films are usually not

applied to the pins in pin-on-disk testing, but sometimes (to approximate certain end use geometries)

composite pins are slid against uncoated metallic or ceramic disks. It is important that the uncoated

metal or ceramic surface be as smooth as possible since rough surfaces can abrade solid lubricants.

A discussion on how to evaluate solid lubricants in a pin-on-disk tester is given in the next section and

by Fusaro 1986.

For liquid lubricant testing, the disk must be dipped into a lubricant reservoir in the vertical

position (figure 2) or immersed in an oil reservoir in the horizontal position (figure 3). Both pin and disk

should be finished to the smoothest surface condition possible to reduce abrasive wear. A discussion of

how to evaluate liquid lubricants on a pin-on-disk tester is given in a paper by Loomis and Jones 1980.

This tester is well adapted to operation in a vacuum chamber. A system designed for ultra-high

vacuum friction and wear testing and surface analysis is shown in figure 4 (Miyoshi and Pepper 1992).

In this configuration, the beam on which the pin is mounted is attached to a vacuum flange through a

gimbal. It is sealed with a metal bellows as shown. The beam contains two flats on which strain gages

are mounted. These gages measure the applied load and the frictional force between the pin and the

disk. The pin load is applied by moving the beam against the disk. A 6-mm steel ball slides against the

flat disk. The entire apparatus is mounted on a 15 cm (6-inch) flange, which attaches to the chamber of

an XPS spectrometer. Ion sputtering can clean the flat surface and then a solid lubricant can be

deposited by ion deposition without removing the specimens from the vacuum chamber.

NASA/TM--2001-210806 9
A second type of vacuum pin-on-disk tester is shown in figure 5 (Miyoshi and Pepper 1992). This

tester uses very small disks (2.0 cm diameter). The advantage of using these small disks is that the disks

can easily be inserted into a scanning electron microscope for observation of the wear surfaces or they

can be inserted into other surface analytical instruments for elemental and compound analysis. Large

test specimens cannot be easily inserted into these instruments.

Four-Ball Tribometer

This tester is used primarily for determining the wear resistance of rubbing steel surfaces in

lubricating oils. The ASTM Standard for the four-ball wear tester is ASTM D4172-94. This test should

not be confused with the four-ball EP tester, which is used to determine the galling point of heavily loaded

lubricated balls.

The four-ball tester uses a single ball in sliding contact with three stationary balls in a closepacked array. The three stationary balls are mounted in an assembly, which contains a lubricant

reservoir and a lever arm for measuring the friction force between the rotating ball and the stationary

balls. The rotating ball is held in a collet on a rotating shaft. The balls are standard size 12.7 mm

(1/2-inch) bearing balls. Loads of 147 or 392 N (15 or 40 kg) are applied to the rotating shaft. The nested

balls are submerged in the test lubricant. The relative size of the scars developed on the three stationary

balls after a test gives a measure of the lubricant's ability to inhibit wear. The high loads and pure sliding

condition result in a considerable generation of heat. This accelerates chemical reactions between the

lubricant and the steel ball surfaces. Therefore, it can be considered as a very good accelerated test.

A schematic of a vacuum version of the four-ball tester is shown in figure 6 (Jones et al. 1994).

The specimen configuration is the same as the conventional four-ball tester, except that 9.5-mm (3/8-

inch) diameter precision balls (grade 10) are used. The apparatus is mounted in a vacuum chamber that

uses a mechanical pump and then a turbo molecular pump to achieve a vacuum of approximately 104 to

10 -6 Pa. The chamber is equipped with a hot filament ionization gage to measure chamber pressure and

a mass spectrometer (residual gas analyzer) to measure vaporization products.

Spiral Orbit Tribometer (SOT)

This apparatus was designed to simulate the rolling contact in angular contact ball bearings using

a planar geometry (Pepper et al. 1996). The device consists of one to three balls rolling between a

stationary bottom plate and a rotating top plate. A schematic view of the SOT components is shown in

figure 7 and a photograph of the SOT is shown in figure 8. To simulate a space environment, the tester

is contained in a cubical vacuum chamber maintained at a pressure of about 10 _ Pa by a turbo-molecular

pump. An external motor drives the top plate though a ferro-fluidic feed through. Load is applied upward

on the bottom plate with a deadweight through a lever system that is located below the apparatus. Grade

10 precision bearing ball bearings are used.

NASA/TM--2001-210806 l0
The balls are placed between the plates. If more than one ball is used, a positioning device is

used to equally space them and position them at the same radial distance from the center of the plates.

After positioning, the positioning device is removed and the load is applied through the bottom plate.

Movement is obtained by rotating the top plate. The balls tend to gradually spiral out to the plate

periphery during rotation. Their path is eventually stopped by a bumper, which nudges the balls back

to their original track orbit. This causes repositioning scrub marks on the bottom plate. The bumper

assembly contains a transducer to determine the force with which each ball makes as it contacts the

bumper. The length of the scrub and the bumper force indicate the degree of boundary lubrication.

To determine chemical products released during the rolling and bumping interaction, a quadrupole mass

spectrometer and a cold cathode ionization pressure gauge are used. Electrical resistance is used to

determine the separation between the ball and the plates caused by insulating lubricant films.

Lubricants are applied by dipping the balls into a dilute solution of the desired lubricant. After

removal from the lubricant solution, the solvent evaporates and leaves a thin residue of lubricant on the

balls. No other lubricant is applied to the plates or balls; thus only a minimum amount of lubricant is used

in this test. More details about this test device can be found in the papers by Pepper, et al. 1996, and

Jones, et al. 2000.

Pin and V-Block Tribometer

Another accelerated test machine that is used to evaluate lubricants is the Pin and V-Block

tribometer. The test configuration consists of a pin rotating between two V-blocks (Figure 9). Generally,

these tests are conducted in air or in a controlled atmosphere. The device is used to determine the life

and the load carrying capacity of solid lubricant coatings. The solid lubricant coatings are applied to the

pin and/or the V-blocks. The methodology for conducting these tests is specified in ASTM standard

D2625-94.

This tribometer can also be used to determine the wear resistance of materials sliding in a fluid

lubricant or the extreme pressure properties of fluid lubricants. The ASTM standards for conducting these

tests are D2670-95 and D-3233-93, respectively.

Instrument Bearing Tester

A device has been designed and built to test angular contact instrument bearings in a simulated

space environment (Jones et al. 1994). The device is shown in figure 10. The tester is contained in a

cubical vacuum chamber and is driven by an external motor through a ferro-fluidic feed through. The

motor is a micro-stepper that is computer controlled to provide either continuous rotation or a precise

dither motion. A precision screw mechanism is used to load the bearings. Bearing torque is measured

with a flex pivot assembly instrumented with micro-strain gages. The angular contact bearings to be

tested (size 1219) have an O.D. of 30.16-mm, a bore of 19.05-mm, and eighteen 3.275-mm balls.

NASA/TM--2001-210806 11
The test bearing can be electrically isolated so that the contact resistance can be measured.

This helps determine if the bearing is in a starved condition at any time during a test. A mass spectrometer is also attached to the vacuum chamber to measure any out-gassing products from the lubricants

or retainer materials. Test temperature range is from room temperature to 50°C. Loads can be varied

from 25 to 200N. Speeds can be from 1 to 1200 rpm in a linear mode or 1Hz in a dither (oscillating)

mode.

Roller Contact Tribometer

A device has been designed and built to evaluate coatings under rolling and sliding contact in

a vacuum environment. A schematic of the device is shown in figure 11 and a detailed view of the test

rollers is shown in figure 12. A motor drives a crowned roller that is loaded against cylindrical roller.

Various solid lubricants can be applied to the cylindrical roller or to both rollers if desired. Slip between

the crowned roller and the cylindrical roller is controlled by a brake applied to the cylindrical roller.

Friction coefficients can be measured and traction coefficients can be calculated. Wear to the coatings

can be determined either by stopping the tests at preset intervals and measuring the wear or at the end

of a test. The device has been used to evaluate coatings for use in traction drives and bearings for space

applications; however, to date no technical papers have been published in the literature on data from this

tribometer.

Brake Tester

Brake pads are needed for some applications in space such as the remote manipulator system

that will be used on the space station. In vacuum, the friction of some materials can decrease due to

dehydration of the material or due to accumulation of wear debris on the conforming interface where the

brake pad slides. To evaluate various materials for use in applications such as these, a tester has been

constructed in Canada to test brake materials in high vacuum (Hawthorne 1990). A schematic of the

tester is shown in figure 13.

In this system, the pads are driven by an electric motor through a ferro-fluidic rotating seal and

slide against a metallic counterface. Loads up to 70-N can be applied to the pads by a deadweight

system through a bellows seal in the vacuum chamber. Speeds up to 100 rpm can be attained in a

vacuum level of 1.3 x 10 4 Pa (10 .6 Torr).

NASA/TM--2001-210806 12
CORRECT USE OF SOLID LUBRICANTS

Cleaning of Test Specimens

Contamination can adversely affect the performance of a solid lubricant coating. Thus, it is

extremely important that the test specimens be thoroughly cleaned before coating. It is also important

that after cleaning they be kept in a clean environment and that the specimens are not touched after

cleaning. The hand contains oil that could affect the results of the tests.

Before coating, metallic specimens should be first cleaned with a good solvent like ethanol to

remove organic deposits from the surfaces. Frequently, the solvent will leave a deposit behind when it

evaporates from the surface. Evidence for this is when water is put on the surface, the water will "beadup" and not fully wet the surface. Fusaro 1986, recommends that the surfaces be scrubbed with a water

paste of levigated alumina as a final cleaning method. The very fine alumina particles will remove oxides

on the surfaces and any minute traces of solvent particles. If water is applied to these surfaces (after

adequately cleaning with levigated alumina), it will wet the surfaces uniformly. To remove the alumina

particles, the surfaces should be scrubbed with a clean nylon bristle brush under running water and then

given a final rinse with distilled water. Because some surfaces will oxidize if not dried quickly, it is

advisable to use clean, dry, compressed air to blow-dry the surfaces.

After the coatings are applied, care should be taken to keep them clean. They should not be

cleaned again. Solvents can absorb through the surfaces into the bulk of the coating and degrade the

coating's lubricating ability. Similarly, water vapor in the atmosphere can absorb and degrade the

coatings. Once coated, the specimens should be stored in a clean dry environment to prevent

contamination.

There are not many good techniques to clean polymers or polymer composite materials. Many

polymers (polyimides, polyamides, epoxies, etc.) have free hydrogen bonds in their molecular structures

that will attract water vapor molecules. Water vapor will absorb into the surface and then hydrogen bond

to the molecules and affect the tribological properties. Thus, it is recommend not to clean polymers or

polymer composites with an aqueous solution. A satisfactory method for cleaning polymers (if it is

deemed necessary) is to use a fast drying solvent to remove organic deposits on the surface.

Substrate Preparation for the Application of Coatings

An important consideration for the application of solid lubricant coatings is that they must be well

bonded to the substrate. The first step for accomplishing this is to ensure that the surfaces are clean; that

is, that they are free from oil, grease and oxide films. The surfaces should be cleaned as described in the

previous section.

Cleaning is very important for achieving a good bond of the solid lubricant coating to the surface.

But improved bonding can be achieved by either mechanically or chemically treating the surface to be

coated. Mechanical treating means roughening the surface by such techniques as sanding, glass

peening, sand blasting, etc. Chemical treating is done by reacting the surface with a chemical to promote

NAS A/TM--2001-210806 13
the formation of a thin layer of a chemical compound on the surface. For a comparison of some different

surface pretreating methods see Fusaro 1984.

Mechanical treating increases the surface area and provides a reservoir for the solid-lubricant

material. When this technique is used, it is important to remove any high or sharp asperities that are

produced. If not removed, they can abrade the counterface of the material sliding against the coating.

Lightly polishing the surfaces after mechanical treating should remove any high spots or sharp asperities

created by the mechanical treating process.

Chemically treated surfaces can improve the lubricating ability of coatings in three ways. (1)

They can act as a rough surface to improve bonding and to serve as a reservoir for the lubricant (similar

to a mechanically treated surface. (2) They can form a conversion surface layer that can combine with

the solid-lubricant coating forming a mixture that will improved bonding as well as improved lubrication.

(3) The coating can improve the chemical bonding of the solid lubricant to the surface (some solid

lubricants do not bond well to certain materials). For more information on surface pretreatment see

Fusaro 1984 and Fusaro 1986..

Application of Solid Lubricant Coatings

There are many methods of applying solid-lubricant coatings. The simplest method is to use a

polishing cloth and manually burnish (rub) a solid-lubricant powder onto a substrate surface. A better way

than burnishing by hand is to devise a mechanical method that will more vigorously rub the solid-lubricant

onto the surface. For example, a brush rotating on a drill can be used. To get the best bonding of the

solid lubricant to the surface with this method, the substrate must have some roughness to it. It is

recommended to sandblast or glass peen the surface to a roughness of 0.90 to 1.25 micrometers (35 to

50 p.m) RMS.

Another way to apply solid-lubricants is to impel them at high velocities at the substrate surface.

This method will physically imbed the powders into the surface and provide a good bond. There are

commercial vendors that will apply films by this method.

The most common way of applying solid-lubricant coatings for long life is to incorporate them into

a binder system. The binder functions much like a paint, holding the solid-lubricant particles (and any

other desired additives) and attaching them to the substrate surface. The binder can function merely as a

material that attaches the particles to the surface; or, if the binder is a good lubricating material itself (like

the polymer polyimide), it can mix with the solid lubricant additives to produce an even better lubricating

coating. Methods for applying bonded solid lubricant coatings are dipping, painting with a brush, or

spraying. Any method used to apply paint could be used to apply a bonded solid-lubricant coating. A

caveat should be given for applying bonded coatings. The coating should not be applied too thick and it

should be applied very uniformly to the surface. If it is not, it can be worn away very rapidly during the

"running-in" process (i.e., dispersed from the contact area) and an adequate amount may not be left to

provide good lubrication.

NASA/TM--2001-210806 14
Another technique for applying coatings is called plasma spraying (Sliney 1987, Sliney 1991).

In this method, a carrier gas such as argon is passed through a very high electric potential and ionized to

create a plasma stream. Solid-lubricant powder is injected into the stream before it exits the plasma gun

nozzle and these particles when striking the substrate surface become fused to it. A disadvantage of this

method is that very high temperatures are produced in the plasma and only materials which have high

thermal stability can be applied by this method. In addition, these coatings can only be applied to

materials with high thermal stability. Another disadvantage is that the coatings applied by this method

are often very thick and "rough." Thus, after application they must be machined (or finished by some

other method) to reduce their thickness and to smooth the surfaces.

Ion plating, sputtering or chemical vapor deposition (CVD) processes can also be used to apply

coatings. A vacuum system is needed for ion plating or sputtering. The advantage of these methods is

that highly adherent, very dense, thin coatings of solid-lubricant materials can be applied to irregular

surfaces. The disadvantage is that the methods are expensive and it is hard to coat large parts. For

more information about these techniques see Spalvins 1969a, Spalvins 1969b, or Bunshah et a11982.

Factors Which Affect Solid-Lubricant Performance

How well a particular solid lubricant performs is system dependent, Thus the solid lubricant

should be evaluated as closely as possible to the same conditions and in a methodology that

approximates the end-use application. Some factors that affect solid lubricant coating performance

are given in Table 2. The parameters listed in the table must be considered when planning a testing

sequence to evaluate solid lubricants for a particular end-use application.

Table 2.mFactors which effect solid lubricant coating performance.

• Type of materials in sliding contact.

• Surface to which a solid lubricant film is applied.

• Geometry of sliding materials.

• Contact stress or pressure.

• Substrate hardness.

• Substrate surface topography or roughness.

• Temperature.

• Sliding speed.

• Environment.

• Atmosphere.

• Fluids.

• Cleanliness or contaminants.

NASA/TM--2001-210806 15
The material to which a solid lubricant coating is to be applied is the first factor to consider. Some

metals are intrinsically hard to lubricate, such as AISI 300 Series stainless steel, aluminum and titanium.

A solid lubricant applied to these materials may fail immediately. This failure is not due to the solid

lubricant, but to the material that the solid lubricant was applied to. The solid lubricant and the metal it

lubricates are a system. Ideally, the type of material to be lubricated should be chosen just as carefully

as the solid lubricant. Unfortunately, in many instances, the type of material for a mechanical component

is chosen long before the solid lubricant. When this occurs, it will limit the technologist in finding the best

low friction, low wear, long life system. However, in either case, a thorough literature search should be

conducted to determine which lubricant worked well with a particular metal. Literature studies, though,

will only give an indication of which solid lubricants might work. You will need to test the lubricant and

metal combination under your particular conditions to determine whether it will be appropriate for your

application.

Applying the solid lubricant to the correct surface is also important. It is not advisable to apply a

solid lubricant coating to both sliding surfaces. Generally, the coating should be applied to the surface

that has the largest surface area. For example, in a pin-on-disk test, one would want to apply the coating

to the disk and not the pin.

In general, hard materials can be lubricated to produce lower friction, lower wear and longer life

than soft materials. A very high-contact stress applied to a coating could cause the substrate to either

elastically or plastically deform. If the coating does not follow this deformation, it could either brittlely

fracture or plastically deform, permitting metal-to-metal contact to occur. Thus, the hardness of the

substrate relative to the applied Hertzian contact stress is a very important consideration.

In addition to the substrate hardness, the roughness of the substrate is also very important. As

a general rule, most burnished or bonded solid lubricant films will not adhere adequately to very smooth

surfaces, so to ensure a good bond; the disk substrate surfaces should be roughened to a value of 0.90

to 1.25 micrometers (35 to 50 _in) RMS. The opposite is needed for the counterface sliding against a

coating. This counterface surface must be extremely smooth or it will abrasively wear the coating.

Temperature and speed are related, the higher the speed, the higher the temperature of the

sliding contact. Sometimes, higher temperatures are beneficial to a solid-lubricants performance; but, in

most cases, higher temperatures decrease the endurance life of a solid-lubricant coating. One general

statement that can be made is that friction, wear and endurance life are highly dependent on temperature.

Thus, if possible, this factor should be controlled in an accelerated test. In addition to affecting the

temperature of a coating, speed can also affect the rheological properties of a polymer coating. Flow

properties of these coatings can be time dependent. Thus, if the speed of a counterface sliding over the

coatings is too fast, instead of the coating flowing plastically (shearing) it will fracture brittlely.

The atmospheric environment in which a solid lubricant coating is evaluated can have a marked

effect on the tribological properties. For example, the relative humidity of laboratory air (if not controlled)

can vary from 80 % in the summer to 20 % in the winter. A graphite or MoS2 film would give totally

NASA/TM--2001-210806 16
different results evaluated under these two conditions. The graphite film would be better than the MoS2

film in summer (humid air), while the MoS2 film would prove to be better than the graphite film in winter

(dry air). In order to determine how well a coating well perform in a certain application, the coating must

be tested in whatever atmospheres the end use will experience.

Most solid lubricant bonded coatings do not function well in a liquid environment, whether it be

water or oil. The oil or water will absorb into the structure and cause it to degrade. As mentioned

previously, even a fingerprint can dramatically affect the tribological properties. Cleanliness in terms

of dust or dirt is also important. Small hard particles can imbed in a film, a polymer or a composite and

severely abrade the counterface (Fusaro 1985).

Macroscopic Mechanisms of Wear and Lubrication

To properly evaluate solid lubricant coatings, one must be aware of the macroscopic mechanisms

by which solid lubricant coatings wear. There are two major mechanisms that can take place. The

strength of the coating and the applied contact stress are the two main factors that determine which

mechanism will occur (Fusaro 1981, Fusaro 1982). In the first mechanism, the coating has enough

structural strength to support the stresses induced by the load and the sliding interface. In addition, for

low wear, a very thin layer of the coating material must become "ordered" on the coating surface and the

lubrication process then becomes the shear of this layer between the coating and the counterface (the

surface sliding against the coating). In general, the best solid lubricants produce very thin layers. They

also produce very thin transfer layers to the counterface. Thick transfer layers tend to produce stress

risers and higher adhesive wear. Endurance life is determined by how long it takes the counterface to

wear through the coating. Often life can be extended beyond this point because this lubricating process

can transition into the second lubricating mechanism.

The second lubricating process occurs when the coating does not have enough structural

strength to support the stresses due to the load and the sliding interface. In this case, the coating is

rapidly worn or "spalled" away. Even though this occurs, a very thin "secondary film" can be formed at

the surface of the metallic substrate and this film can provide lubrication. How well a solid lubricant will

function when this mechanism occurs is determined by several factors. One is the thickness of the

original coating. If the coating is too thick, most of it will be worn or "spalled" away very quickly (ejected

from the contact area) and little will be left behind to form the secondary film. A thinner coating will tend

to be compressed rather than to be "spalled" away and more material will tend to stay in the contact zone

to form a better "secondary film."

Substrate surface roughness is also important. A properly tailored topography on the substrate

will provide a reservoir for the solid lubricant and a "dump" for metallic wear particles. If this second

mechanism is to function, the coating materials must have the ability to form this "secondary film." Sliding

speed, load, temperature, and relative humidity will affect its formation. The formation of this film is a

NASA/TM--2001-210806 17
"running-in" process. Often a better secondary film can be formed if the applied solid lubricant coating is

"run-in" under less severe conditions rather than under actual operating conditions.

The type of wear occurring to the counterface surface is also dependent on which mechanism is

operating. In the first mechanism, very little wear to the counterface occurs since only the coating

material comes in contact with the metallic counterface. In the second mechanism, wear to the

counterface can occur since the "secondary film" is very thin and can allow metallic asperities to protrude

through the film to abrade or cause higher adhesive wear to the counterface. For more discussion on

these mechanisms see Fusaro 1981, and Fusaro 1982.

Experimental Procedures for Testing Solid Lubricants

The specimens should be inserted into the apparatus and the chamber sealed. If it is a vacuum

test, the chamber should be pumped down until the desired pressure is obtained and let stand until there

is no measurable out-gassing. For controlled atmosphere testing, the chamber should be purged with

the desired atmosphere before starting the test and continuously monitored throughout the test. The test

should not be started until the atmosphere stabilizes. The time for this will depend on the size of the

chamber and the flow rate of the purging gas atmosphere. Once the atmosphere is stable, the

temperature should be adjusted to the correct value, if the test is not being conducted under ambient

conditions. Once the temperature has stabilized, the specimens can be set in motion and the load should

be gradually applied.

Two types of friction and wear testing procedures can be followed: (1) the "continuous testing

method" or (2) the "interval testing method." In the continuous testing method, the test is run continuously

until some maximum, predetermined friction coefficient occurs. The test is then stopped and the wear

scars measured. The time to reach this value of friction coefficient is defined as the endurance life of the

coating. In the "interval testing method," the specimens are removed from the test chamber at

predetermined intervals of sliding and the wear scars are measured and a visual microscopic inspection

of the surfaces takes place. The specimens are then put back into the chamber and the previous test

procedure is repeated. Sliding continues until the predetermined friction coefficient is obtained or when

enough data is obtained for wear analysis. The advantage of this method is that wear as a function of

sliding distance can be determined. In continuous testing, only wear at the end of a test can be

determined and run-in wear cannot be separated from steady-state wear. One caveat on the "interval

testing method" is that care must be taken to replace the specimens with the same orientation and

alignment that they had before they were removed. This is not a trivial task, but can be done.

Evaluation of Solid Lubricants Results

The accelerated friction and wear test will enable the evaluation of friction coefficients, coating

wear, counterface wear and endurance life of the coatings. In addition, if the interval testing method is

used, the wear mechanism can be determined. Coatings can be evaluated under different sliding

NASA/TM--2001-210806 18
conditions to determine how load, speed, temperature, etc. influence the results. This is important

because these are accelerated tests and the results will not necessarily directly apply to the end use

application. It is most likely that friction and wear are not constant throughout the test and that there will

be a running-in period with higher friction and wear, a steady-state period with constant friction and wear

and a period where friction and wear increase gradually with time. However, every coating is different

and these general rules may not apply. What these tests will provide is a comparison of the friction and

wear characteristics of various coatings for an application. The results are only a relative indication of

how well the coating will perform. The coatings may not perform as well in the end use application or it is

possible they may perform better, but hopefully the relative performance of the lubricants in the final

application will stay the same as that found in the accelerated test.

LIQUID LUBRICANT BENCH TESTING

Regimes of Liquid Lubrication

There are four defined regimes of liquid lubrication: (1) hydrodynamic, (2) elastohydrodynamic,

(3) boundary, and (4) mixed (a combination of elastohydrodynamic and boundary) (Booser 1984, Jones

1982, and Cameron 1966). When specifying or testing a liquid lubricant, it very important to determine

under which lubrication regime or combination of regimes the mechanism will be operating. The viscosity

characteristics of the oil, the load, contact geometry and the speed are the main factors that determine

which regime will be operating. During hydrodynamic and elastohydrodynamic lubrication, no contact

takes place between the surfaces because the viscosity of the oil is high enough, the load is low enough

and/or the speed is high enough to separate the moving surfaces. Thus, in the hydrodynamic and

elastohydrodynamic regimes, friction is very low and no wear of the surfaces takes place. Ideally, one

would always like to design a mechanism so that either of the above two lubricating mechanisms takes

place, but this is not always possible.

When the other two regimes of lubrication occur (mixed and boundary lubrication), rubbing

contact between the surfaces takes place. Rubbing contact causes wear. In order to provide lubrication

in these two regimes, a thin layer of a boundary lubricant type material must form between the interface of

the two sliding surfaces. This boundary lubricant material must adhere to at least one of the surfaces and

must shear under sliding contact to mitigate the wear and provide low friction. A good boundary lubricant

oil may be capable of providing for the formation of these adhered lubricant type films by itself. However,

to optimize the process, chemical additives are often dissolved or suspended in the oils. In effect, the oil

serves as a carrier for supplying the surfaces with material that will react with the surface to form a type of

solid lubricant material.

One must be careful in formulating the oils, however. Problems have occurred in boundary

lubrication situations when a boundary lubricant film which is too thick has been deposited on bearing

raceways. The deposited film can cause mechanical noise (vibrations) (Kannel and Dufrane 1986,

NASA/TM--2001-210806 19
Zaretsky 1990 and Kingsbury 1992) during unidirectional sliding or torque bumps (Zaretsky 1990, Todd

1981) during oscillating bearing motion.

Specimen Preparation

In general for liquid lubrication, the smoother the test surfaces the better. Specimens should be

ground, lapped and polished to surface finishes of better than 0.1 pm (4 pin). After polishing, they should

be cleaned before testing. The cleaning procedure is the same as was stated in the solid lubrication

section.

Liquid lubricants for space use should also be degassed before using or testing them. Loomis

and Jones 1980 used a degassing procedure of 150°C (302 °F) for one hour under a pressure of

270 N/m 2to degas lubricants. Measurements made after degassing showed that this procedure reduced

the dissolved water content in the fluid to less than 20 ppm.

Experimental Accelerated Testing Procedures

Since there is no wear and very low friction when operating in the hydrodynamic and

elastohydrodynamic lubrication regimes, there is no need to do accelerated experimental testing. As

long as adequate oil remains in the system and the operating conditions do not change, almost unlimited

life is possible. In this case, testing needs to be conducted in the final end use device to determine if

enough lubricant will be present to ensure that starvation will not take place over the projected life of the

spacecraft.

For boundary lubrication however, accelerated testing is often useful. Boundary lubrication is

very similar to solid lubrication and the statements made in the previous section on solid lubrication also

apply here. The advantage of boundary lubrication over solid film lubrication is that the boundary

lubricant film can be constantly replaced by reactions of the surfaces with the oil or additives in the oil.

With dry solid lubricant films, once the films are depeted they cannot be replaced except by taking the

specimens apart, refinishing the surfaces and reapplying the films.

When conducting tests to evaluate boundary lubrication, make sure that conditions are indeed in

the boundary lubrication regime. As mentioned previously, oil viscosity, sliding speed, contact geometry

and contact stress are the factors that determine whether or not boundary lubrication will take place. In

addition, if there is not enough oil present, a "starved" condition can take place that could cause a nonlubricated condition. Care must be taken to determine what the testing conditions will be for each

situation. For example, if the temperature and the oil viscosity are constant, loads and speeds should

be adjusted to insure a boundary lubrication condition exists. Oil viscosity can change with temperature,

thus it is important to monitor temperature to insure it remains constant.

There is a caveat, which could occur when testing in the boundary lubrication regime. After long

sliding duration, sufficient wear of the specimens could take place to reduce the contact stress to the

NASA/TM--2001-210806 20
point that mixed or even hydrodynamic lubrication could occur. If the experimenter is not aware of this,

the wear results could be misinterpreted.

The same type of friction and wear testing methods as described for solid lubricant films can be

used for oils. Usually one does not run life tests since life could be indefinite. A test is usually conducted

for a predetermined time and then stopped. The specimens are removed and the wear measured and

the boundary lubricant films on the surfaces evaluated. To separate running-in from steady-state wear,

it is a advisable to conduct interval type tests.

Liquid lubricants are as a rule not as dependent on the type of atmosphere as are solid lubricants.

Often a good evaluation of liquids can take place by evaluating the lubricants in a dry inert atmosphere.

This involves less time and is cheaper than evaluating lubricants in a vacuum environment.

Evaluation of Results

Essentially, when applying accelerated testing methods to evaluate liquid lubricants, one lubricant

formulation is being compared to another. The target is to find the lubricant formulation that provides the

least amount of wear and gives the lowest friction coefficient for the conditions under which the lubricant

operates. Besides friction and wear, there are other characteristics that can demonstrate a good

lubricant formulation. For example, the "smoothness" or consistency of the friction force, the consistency

of the wear rate, and the types of boundary lubricant films found on the wear surfaces are important

characteristics. Very thin, continuous, smooth boundary lubricant films are indicative of a good lubrication

condition. In addition, the chemical composition of these films can indicate the "quality" of the lubricants.

Various surface analysis techniques that can be used to study the wear surfaces will be discussed in the

next section.

CHEMICAL ANALYSIS AND CHARACTERIZATION TECHNIQUES OF SURFACES

This section gives a short overview of some of the surface analytical techniques available for the

investigation of wear mechanisms taking place on the sliding surfaces of accelerated testing devices.

The same techniques can be used for the analysis of surfaces of actual space mechanical components.

The difficulty with actual space components is that they are often too large to fit into the analysis

chamber. Therefore, they must be sectioned for analysis. For more detailed discussion of these surface

analysis techniques see the papers by Ferrante, 1982, Ferrante 1989, Kane and Larrabee1974, and

Hilton and Didziulis1993.

Visual Light Microscope

A good optical microscope can be used to measure the size of the wear scars, to determine what

type of deposits are found on the wear surfaces and to observe the type of wear taking place. Low

magnification (up to IOOX) is used to measure the wear scar while high magnification (up to 1500X) is

NASA/TM--2001-210806 21
used to examine the deposits and wear processes. Photography can be used to record the wear scars

and the nature of wear or deposits on the surfaces. A disadvantage of this technique (especially for high

magnifications) is that it is hard to focus on non-flat surfaces. Thus, an advantage of using pin-on-disk

testing devices is that they produce flat wear surfaces that are much easier to keep in focus over a larger

area under an optical microscope.

An advantage of the visual light microscope for looking at surfaces is that very small differences

in surface irregularities can be observed. Being able to observe the surfaces in color is also an

advantage. Interference fringes can sometimes be observed which can be used to calculate film

thickness. Film thickness can also be determined by using the focusing micrometer to focus on the

top and then the bottom of a feature and then subtracting the micrometer readings obtained. Onemicrometer differences in thickness can be discerned by this method.

Surface Profilometry

A surface profilometer can be used to measure the cross sectional area of wear on the sliding surfaces.

The simplest instruments use a stylus that generally is pyramidal or conical in shape. Typically a stylus

cone is 60 degrees with a radius of 12.5 pro. The stylus is made from diamond and the tip can be either

flat or rounded. Traversing the surfaces can cause some damage to the surfaces, but the loads are so

light it is usually insignificant. If this is a concern, there are some units available that use lasers or

interference methods to scan the surface, but these units are more expensive.

A surface profilometer is used to make recordable traces of the surface topography or wear

surface area. By taking passes over the surface, that cover both the unworn edges of the specimen as

well as the wear track, the depth of the wear scar and thereby the wear volume can be calculated. Wear

volume measured over several different sliding distances can be used to determine a value of the wear

rate. Very high vertical magnifications can be obtained using this instrument.

Scanning Electron Microscopy

Wear specimens can also be examined by inserting them into a scanning electron microscope

(SEM). One disadvantage of this instrument is that oils must be removed from the specimens before

insertion. Another disadvantage is that the instrument will not generally accommodate large specimens,

although there are some that can handle specimens up to 200 mm diameter. A third disadvantage is that

many well-lubricated wear surfaces are very smooth and featureless and this instrument does not give

good pictures under this condition. Charging of specimens can also be a problem if the specimens are

not conducting. This can be mitigated by shadowing the sample with a thin layer of conductive material

such as carbon or gold. An advantage is that SEM's have better spatial resolution than optical

microscopes. Another advantage is that wear features show up very well and the surfaces remain

in focus to very high magnifications (10,000x or more).

NASA/TM--2001-210806 22
Energy Dispersive Spectroscopy

An additional advantage of the SEM is that while the surfaces are being observed visually, an

elemental analysis of the deposits on the surfaces can be made using electron dispersive spectroscopy

(EDS). An overall analysis of the whole surface can be taken or small particles or small areas can be

focused upon and their spectra taken. One disadvantage of this technique is that the analysis goes rather

deep into the surface (- 1 _m) and the substrate material under a particle may be included in the spectra.

Also the technique is not good for the detection of light elements with energies less than 170 eV. In

addition there is some problem in discriminating between elements whose x-ray energies are too close

together, for example, Mo and S. However, more expensive modern equipment using ultra-thin windows

can resolve x-rays down to carbon and can separate Mo and S. There are ways to improve detection by

using wavelength diffractometers (WD) and electron probe micro-analyzers as accessories in the SEM.

Mass Spectroscopy

A residual gas analyzer (RGA) can be attached to a vacuum chamber in which a friction and wear

test is being conducted. The RGA can detect out-gassing of materials that occurs when a material is

exposed to vacuum or during the friction and wear experiment. By knowing the masses of atoms, the

technique can be used to determine what the out-guessing products are. The analyzer can also be used

to insure that there is a "clean" vacuum before starting a test.

Auger Electron Spectroscopy

Auger electron spectroscopy (AES) or scanning Auger microscopy (SAM) is used to determine

the elemental composition of very thin surface layers on the wear surface (-10 nm). The technique

works by bombarding the surface with electrons to generate Auger electrons with various energies. The

energies can then be analyzed to determine the elements present. The chemical information obtained

can be semi-quantitative. The electron beam can also be scanned over the surface to give an elemental

composition map. This is called scanning auger microscopy (SAM).

The chemical composition of very thin films or deposits (-2 rim) on a wear scar can be

determined by using x-ray photoelectron spectroscopy (XPS). In this technique the surface is

bombarded with x-rays that cause photo-emitted core and valence electrons to be emitted. Semiquantitative analysis of the surface composition can also be made with XPS data.

Secondary Ion Mass Spectroscopy

Another technique to determine elemental information on wear surfaces is secondary ion mass

spectroscopy (SIMS). In this technique, a beam of ions is directed at the surface that sputters away

charged particles, the mass of which can be analyzed. The technique is surface sensitive and both

elemental and compound analysis can be conducted. The advantage of this technique is that the

NASA/TM--2001-210806 23
sensitivity is better than AES, but the spatial resolution is not as good. The technique is also destructive

because it involves the sputtering away of the surface during the analysis.

Fourier-Transform Infrared Microscopy

This technique combines optical microscopy with infrared spectroscopy to provide structural

information of films or deposits on wear surfaces. The advantage of this technique over conventional

infrared spectroscopy techniques is that a very small area on the contact area can be focused upon and

isolated from the rest of the surface to obtain structural information.

FINAL CHECKS AND TESTS

Assemble in Class 10,000 Clean Room

Tribological surfaces can be markedly affected by contamination such as dirt, dust, water vapor,

etc. As mentioned previously, even touching a surface with a "clean" hand can affect the tribological

performance. Once cleaned, tribological surfaces should never be touched! When applying lubricants to

a clean surface they should be applied under very clean conditions. The specimens should then be

stored under very clean conditions and when ready to be assembled into the final configuration, the

components should be assembled in a class 10,000 clean room. A clean room is rated by the number of

pollutant particles it allows per unit volume of air. For example, a class 10,000 clean room would have to

maintain a level of no more than 10,000 particles larger than 0.5 microns in diameter per cubic foot

(353,000 particles per cubic meter). For more information see Sarafin 1995.

QA Assembly Inspection

A quality assurance assembly inspection is necessary to make sure that all processes that went

into designing, manufacturing, testing and assembling the mechanism will deliver a product that will meet

the operating objectives of the mechanism. One should verify that the mechanism meets the

specifications of the engineering drawings, that the tests match the test plan, and those critical elements

such as materials, parts, processes and configuration are controlled and are as was specified.

Run-in Testing

Many mechanisms need to be "run-in" before they are actually used. Liquid lubricated devices

need to have "rough areas" worn off at lower loads and speeds to insure that proper elastohydrodynamic

films will form and starvation of the bearing surfaces will not occur. Solid lubricated devices need to also

be "run-in" at lower speeds and loads to insure that a smooth dense film is developed. Solid lubricants,

which are not "run-in", can spall and not produce low friction, long life films.

NASA/TM--2001-210806 24
Cycle or Life Testing (with Thermal Simulation)

To insure that the desired life of a spacecraft will occur, life testing with simulated thermal cycling

should be conducted on either the entire spacecraft or the particular unit mechanism. This will verify

whether or not binding, friction or cold welding, wear, corrosion, high torque, blocking, contamination by

the lubricant, mechanical noise, or electrical noise will occur. In addition, one will be able to assess if the

right type of lubricant has been applied, and if it has the right viscosity characteristics, whether there is

any material, lubricant or environmental incompatibility, and whether there has been excessive lubricant

depletion. MIL-STD-150B states that the unit should be tested for a time period that exceeds twice the

total expected operating cycles that would be expected for both the service life and for the acceptance

testing. Life tests should be done in a hard vacuum of less than 1.3 x 10 .4Pa (10 e Torr) and at the

temperature extremes expected for the mission.

Vibroacoustic Testing

Acoustic response can occur during launch or when a pyrotechnic device is activated.

Fluctuating pressure waves are produced which can cause fretting of contacting surfaces and lead to

friction welding. They can also cause liquid lubricant depletion when there is only a small amount of

lubricant available for lubrication. Both of these actions can lead to a decreased life of the spacecraft.

Vibroacoustic testing of tribological components coupled with post inspection can help mitigate problems

caused by random acoustic responses.

Thermal Vacuum Testing

Thermal vacuum tests are conducted to verify the performance of the fully integrated spacecraft

under vacuum conditions for the full spectrum of thermal conditions that the spacecraft will encounter.

This testing can assess tribological problems such as binding, cold welding, liquid lubricant depletion,

temperature-related lubricant viscosity changes, environmental compatibility, electrical noise and

mechanical noise. Outgassing of the lubricant can also be ascertained. Methods for conducting these

tests are discussed in Sarafin 1995.

Post Test Functional Testing

Post-test functional testing is conducted after an environmental test to verify that the mechanism

is still operating as it did before the environmental testing and to determine if it still meets the mission

requirements. The results should be compared to the results that were obtained on the mechanism

before the environmental test was conducted. If the results are not similar, one should not proceed until

it is understood why there is a deviation in the two sets of data. If it is determined that the deviations do

not violate the specified requirements, the mechanism will be considered verified. Such tribological

problems as binding, cold welding, friction welding, wear, insufficient liquid lubricant, liquid lubricant

depletion, contamination of the lubricant, high torque, insufficient life, mechanical noise, electrical noise,

NASA/TM--2001-210806 25
and incomplete application can be determined. These types of tests are discussed in more detail in the

book by Serafin 1995.

Active Replenishment System

To insure and adequate supply of liquid lubricant for the life of the spacecraft, an active

replenishment system should be incorporated into the bearing system. There are several different types

that are available. One should be chosen that best fits the particular bearing application. Review Conley

1998 and/or Zaretsky 1997 for the best type suitable for the application.

Post Test Inspection

After completing any individual test. The mechanisms should be examined closely for

mechanical flaws. Sliding surfaces should be examined both visually and with an optical microscope to

make sure that no excessive damage has occurred that will affect the mission. In some cases, a

scanning electron microscopy or other surface analytical technique may be used to insure no damage

has occurred.

FINAL REMARKS

It is important to note that to date there have been no accelerated tribological tests or bench tests

developed that can absolutely predict how a lubricant will perform in an end-use application. Accelerated

testing can be used to screen lubricants, to obtain an idea of what the friction and wear properties of a

particular lubricant are, and to determine which one might function better; but accelerated testing will not

absolutely predict how well a lubricant will perform in a particular end use application. The only certain

way to do that is to test the solid lubricant or liquid lubricant formulation in the end use application under

the conditions it will experience.

Also, someone skilled in the field of tribology should make the evaluation and selection of the

type of lubricant to use for a particular application. Without adequate training, it is not possible to make

good decisions on which lubricant to choose for a particular application. Table 3 gives a matrix of

tribological failure modes versus preventions, analyses, controls and tests (PACTs) that can be used

to help prevent spacecraft tribology problems.

NASA/TM--2001-210806 26
REFEREENCES

Archard, J.F., "Wear Theory and Mechanisms," Wear Control Handbook, ASME, p.65 (1980).

ASTM D2625-94 (1998), Standard Test Method for Endurance (Wear) Life and Load-Carrying Capacity of

Solid Film Lubricants (Falex Pin and Vee Method), American Society for Testing and Materials, West

Conshohocken, PA.

ASTM D2670-95 (1999) Standard Test Method for Measuring Wear Properties of Fluid Lubricants (Falex

Pin and Vee Block Method), American Society for Testing and Materials, West Conshohocken, PA.

ASTM D2714-94 (1998), Standard Test Method for Calibration and Operation of the Falex Block-on-Ring

Friction and Wear Testing Machine, American Society for Testing and Materials, West Conshohocken,

PA.

ASTM D2981-94 (1998), Standard Test Method for Wear Life of Sofid Film

Lubricants in Oscillating Motion, American Society for Testing and Materials, West Conshohocken, PA.

ASTM D3233-93 (1998), Standard Test Methods for Measurement of Extreme Pressure Properties of

Fluid Lubricants (Falex Pin and Vee Block Methods), American Society for Testing and Materials, West

Conshohocken, PA.

ASTM D4172-94 (1999), Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid

(Four-Ball Method), American Spociety for Testing and Materials, West Conshohocken, PA.

ASTM G77, Standard Test Method for Ranking Resistance of Materials to Sliding

Wear Using Block-on-Ring Wear Test, American Spociety for Testing and Materials, West

Conshohocken, PA.

ASTM G83-93, Wear Testing with a Crossed Cylinder Apparatus, American Society for Testing and

Materials, West Conshohocken, PA

Booser, E.R., Ed., Handbook of Lubrication, 2, "Theory and Design," CRC Press, Inc., Boca Raton, FL,

(1984).

Buckley, D. H., "Friction, Wear and Lubrication in Vacuum," NASA SP 277 (1971).

NASA/TM--2001-210806 27
Bunshah, R.F., et al., Deposition Technologies for Films and Coatings, Noyes Publications (1982).

Cameron, A., Principles of Lubrication, Longmans, London (1966).

Conley, P. L., ed., Space Vehicle Mechanisms, John Wiley & sons, Inc., NY (1998).

Ferrante, J., "Practical Applications of Surface Analytic Tools in Tribology," Lubr. Eng., 38, 4, pp. 223-236

(1982).

Ferrante, J., "Applications of Surface Analysis and Surface Theory in Tribology," Surface and Interface

Analysis, 14, pp.809-822 (1989).

Fusaro, R.L., "Mechanisms of Lubrication and Wear of a Bonded Solid Lubricant Film," ASLE Trans. 24,

2, pp.191-204 (1981).

Fusaro, R. L., "Effect of Load, Area of Contact and Contact Stress on the Wear Mechanisms of a Bonded

Solid Lubricant Film, "Wear, 75, pp.403-422 (1982).

Fusaro, R.L., "Effect of Substrate Chemical Pretreatment on the Tribological Properties of Graphite

Films," NASA TM 83574, (1984).

Fusaro, R.L.: "Friction, Wear, Transfer, and Wear Surface Morphology of Ultrahigh-Molecular-Weight

Polyethylene". ASLE Trans., 28, 1, pp. 1-10(1985).

Fusaro, R.L., "How to Evaluate Solid Lubricant Films Using a Pin-on-Disk Tribometer," Lubr. Eng., 43, 5,

pp.330-338, (1986).

Hawthorne, H.M., "Tribomaterial Factors in Space Mechanisms Brake Performance," Proceedings 24th

Aerospace Mechanisms Symposium, NASA CP 3062, pp.231-242 (1990).

Hilton, M.R. and Didziulis, S.V., "Surface Analysis of Bearings," Aerospace Report No. TR-93 (3935)-1

(1993).

Jones, W.R., Jr., "Boundary Lubrication Revisited," NASA TM-82858 (1982).

NASA/TM_2001-210806 28
Jones, et el., 'q'he Preliminary Evaluation of Liquid Lubricants for Space Applications by Vacuum

Tribometry," Proceedings of the 28th Aerospace Mechanisms Symposium, NASA CP 3260 pp.265-275

(1994).

Jones, et el., "A New Apparatus to Evaluate Lubricants for Space Applications--The Spiral Orbit

Tribometer (SOT), NASA TM-2000-209935 (2000).

Kane, P.F. and Larrabee, G.B., Eds. Characterization of Solid Surfaces, Plenum Press, NY (1974).

Kannel, J. and Dufrane, K. "Rolling Element Bearings in Space," 20th Aerospace Mechanism

Symposium, NASA CP 2423, pp. 121-132 (1986).

Kingsbury, E., "Influences on Polymer Formation Rate in Instrument Ball Bearings," STLE Trans., 35,

pp.184-188 (1992).

Loomis, W. R. and Jones, W.R., Jr., "Steady-State Wear and Friction in Boundary Lubrication Studies,"

NASA TP 1658 (1980).

Marirrodriga, C., and Cunningham, J., "Assessment of Data Repeatability and Reproducibility in Friction

and Wear Tests," Preparing for the Future, (ISSN 1018-8657), Vol.6, No.2, 1996.

MIL-HDBK-340, Application Guidelines for MIL-STD-1540B, Washington, DC, Dept. of Defense, 1985.

MIL-STD-961, Department of Defense Standard Practice for Defense Specifications, Washington, DC,

Dept. of Defense, 1995.

MIL-STD-1540B, Test Requirements for Space Vehicles, Washington, DC, Dept. of Defense, 1982.

MIL-A-83577B, General Specifications for Assemblies, Moving Mechanical for Space and Launch

Vehicles, Los Angeles, CA, SSAF Space Div, SD/ALM, 1988.

Miyoshi, K., and Pepper, S.V., "Properties Data for Opening the Galileo's Partially Unfurled Main

Antenna," NASA TM 105355 (1992).

Pepper, S.V., Kingsbury, E. and Ebihara, B.T., "A Rolling Element Tribometer for the Study of Liquid

Lubricants in Vacuum," NASA TP 3629 (1996).

NASA/TM--2001-210806 29
Rittenhouse, J. B. and Singletary, J.B., Space Materials Handbook, 3rd Ed., NASA SP-3051 (1969).

Sarafin, T. P, ed., Spacecraft Structures and Mechanisms -- From Concepts to Launch, Microcosm, Inc.,

Torrance, CA (1995).

Sliney, H.E., "Coatings for High Temperature Bearings and Seals," NASA TM-100249,

DOE/NASA/50112-71 (1987).

Sliney, H.E, "PM200/PS200: Self-Lubricating Bearing and Seal Materials for Applications to 900°C, ''

NASA TM-103776 (1991 ).

Spalvins, T., "Deposition of MoS2 Films by Physical Sputtering and Their Lubrication Properties in

Vacuum," ASLE Trans., 12, p.36-43 (1969a).

Spalvins, T., "Energetics in Vacuum Deposition Methods for Depositing Solid Film Lubricants," NASA TM

X-52549 (1969b).

Todd, M.J., "Investigation of Torque Anomaly in Oscillating PDM Bearings," European Space Agency

(ESTL) Report No. 49, May 1981.

Zaretsky, E. "Liquid Lubrication in Space," Tribology International, Vol. 23, April 1990, p. 75-93.

Zaretsky, E.V., Tribology for Aerospace Applications, STLE, Park Ridge, IL (1997).

NASA/TM--2001-210806 30
Table 3.--Tribology Problems Versus Preventions, Analyses, Controls and Tests (PACT's).

Preventions, Analyses, Controls & Tests (PACTs)

Tribology Problems

Binding

Cold welding

Friction welding

Surface wear

Insufficient Liquid lubricant

Liquid lubricant depletion

Liquid lubricant decompositiondegradation

Contamination of lubricant

Contamination by lubricant

Temp-related viscosity change

Incorrect lubricant

Lubricant incompatibitity

Material incompatibility

Environmental Compatibility

Corrosion

High torque

Insufficient life

Zero gravity effects

Electrical noise

Mechanical noise

B_ocking

Solvent incompatibility

Incorrect application

Coating Adhesion

Fretting

Galling

High Hertzian contact stress

Incorrect Loads

Coeffiicient of Friction Sensitivity

E

oo

N

_ _= . oo _, _, _ _

_ _ -- .._, -_ .-

.... _ o ,_, _, _ _ ,., o E

E _ .¢2_ _ o .,,:., .- o o

E

,-- = O .--

X X x X

X X X

X X X X

X X X X

X

x

X X X X

x X X

X

X X

X X x

X X X

X X X

X X X

X X X

X X

X X

X X X

x x X

X X X

X X X

X x X

X X X X

X X

X X X X

X

x X X

X

X

X

X

x X

X X

x

X X

X

X x x X

X X x X x

X X X X x

X x X X

X x X x

X x x X X X x X

X

x x

X

X X X

X

X

X

X x

X

x X

X

X X

X x

X

x X

X X X x X X

X X X X x

X X _( X

X X X X

X X X X X

X X X x

NASA/TM--2001-210806 31
Gage

Contact block

(shoe) flat or

conformal --_

To load

cell

I

Thermocouple --_1 __i

,<'],:, .

Optional '_

cover plate //-

for lubd.cant /./"

reservoir J

-I, couple _! F- Contact

block

Contact .-

f block

holder

assembly

ing

---" Rotation

, Detail •

..... Load calibration arm

\_- Test ring

n

Figure 1._Schematic view of the block-on-ring tribometer test elements.

.- Rider

/ (0.476-cm

/ rad)

Applied Friction . _ Disk

load force

CD-12360-I 5

Figure 2.--Schematic view of pin-on-disk tribometer (disk in vertical position).

NASA/TM--2000-209642 32
L Preload

Strain gauge

Gimbal

assembly

Applied load

F -- Counter

f

Motor

Environmental

chamber

rranslation _ Ball holder

plate Disc

Figure 3.--Schematic view of the liquid pin-on-disk tribometer.

Motorized

Y-axis

micrometer _,

Z axis Bellows _ "

'\

Contact

indicator

/

Flexbe __. I

p|vot _ igage ",,\

"_ _ \\ "-Electrical

,_ ' \_ feedthrough

\\

Sample _ _-Bearing ball _- 6-ino flange

Figure 4.--Schematic view of the ultra-high-vacuum pin-on-disk tribometer.

X-axis

micrometer

NASA/TM--2000-209642 33
Wall-mounted

gas manifold

system _

Vacuum

chamber _\

Turbomolecular

vacuum

pump -_

Data acquisition

system ---_

_Vacuum

/ gage

\

\

\

\

• \

i I \

/

Quadrupole

,, gas analyzer

!: ,//

//_ "_- Rotary

./ feedthrough

Speed

pickup \- Drive motor

Speed /"

control -1/ _- Pneumatic isolation table

Figure 5.---Schematic view of the small specimen vacuum pin-on-disk tribometer.

Ddve motor

seal

Rotating ball

__ To vacuum pump

, balls

Pneumatic loading system

Figure 6.--Schematic view of vacuum four-ball tribometer.

NASA/TM--2000-209642 34
/_ Ball slides on

"Scrub" --_ / top plate

Force I/ /

transducer _ Guide /

\ plate --_ /

\ X II /

\ l I I Orbit _

Spiral track

Bottom plate _ / 11

Figure 7._Schematic view of the Spiral Orbit tribometer (SOT) components.

Figure 8.--Schematic view of the ultra-high-vacuum pin-on-disk tdbometer.

NASA/TM--2000-209642 35
Brass

locking pin _\ _L _-'=''" J

___ourna, revo,ves

_ ,' at_,O,_m

\

\

\

\

\

\

(

\ i

_-V blocks J

Figure 9.--Schematic view of the V-block tribometer components.

_ ,,--Drive system

Ferrofluidic

_ ,_" feq_:lthrough

S'

•"-- Test bearing

• fixture

,._::_ .- .--Torque

, .- sensor

o

'_ Load cell

t i I I "'-- Load system

Figure 10.--Schematic view of the angular contact instrument-bearing tester.

NASA/TM--2000-209642 36
/- Vacuum chamber (10 .6 torr)

Torq ue/thrust

transducer -, " .- Traction/normal load gimbal

\ t

_ Drive motor

t

II t

I

I

'

I Test rollers (external ,' '_

I or intemal contact) -J

Misalignment

Figure 11 .-----Schematic view of vacuum roller contact tribometer.

Load

.s"

Driving roller

(fiat or crown_l_

_Driven roller

To brake

Figure 12._chematic view of vacuum roller contact tribometer test specimens.

NASA/TM--2000-209642 37
Cooling

coil -_

\

Viton O-ring -_\ \

Bellows -_\

F Flexure \

Pivot -_

Counter- ¢

balance -/

Thermocouple -/

I

F Heater

\ coil /- Brake disks

_- Flexible

\ / _ coupling

\ // ,/_ _-Tachometer

=_,/ // / /-Torque

!___ ucer

WJ _ LL171 \_Gearbox

B _'_ I i F_rrofluZ_ I

___T -_ Mass

_ sl:_trometer

Ion [

pump

Figure 13.--Schematic view of view of vacuum brake tester.

NASA/TM--2000-209642 38

REPORT DOCUMENTATION PAGE Form Approved

OMBNO.0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

April 2001

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Preventing Spacecraft Failures Due to Tribological Problems

6. AUTHOR(S)

Robert L. Fusaro

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

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, DC 20546-0001

3. REPORT TYPE AND DATES COVERED

Technical Memorandum

WU-274-00-00-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12713

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210806

11. SUPPLEMENTARY NOTES

Prepared for the 2001 Annual Meeting sponsored by the Society of Tribologists and Lubrication Engineers, Orlando,

Florida, May 20-24, 2001. Responsible person, Robert L. Fusaro, organization code 5950, 216--433-6080.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 27 Distribution: Nonstandard

Available electronically at htto://gltrs._rc.nasa._ov/GLTRS

This publication is available from the NASA Center for AeroSpace Information. 301-621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 2OOwords)

Many mechanical failures that occur on spacecraft are caused by tribological problems. This publication presents a study

that was conducted by the author on various preventatives, analyses, controls and tests (PACTs) that could be used to

prevent spacecraft mechanical system failure. A matrix is presented in the paper that plots tribology failure modes versus

various PACTs that should be performed before a spacecraft is launched in order to insure success. A strawman matrix

was constructed by the author and then was sent out to industry and government spacecraft designers, scientists and

builders of spacecraft for their input. The final matrix is the result of their input, In addition to the matrix, this publication

describes the various PACTs that can be performed and some fundamental knowledge on the correct usage of lubricants

for spacecraft applications. Even though the work was done specifically to prevent spacecraft failures the basic methodology can be applied to other mechanical system areas.

14. SUBJECT TERMS

Aerospace tribology; Lubricants; Space systems; Friction; Wear;

Surface analytical techniques; Wear testing devices

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44

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A03

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