effects of vacuum

7/23/2019 Effects of Vacuum

1/190

NASA SP-277

FRICTION, WEAR, AND

LUBRICATION

IN V A C U U M

b y

DONALD H. BUCKLEY

P r e p a r e d a t N A S A L e w i s R e s e a r c h C e n t e r

5 A N D

L

191

1 1 4

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION


2/190

L i b r a r y

of

C o n g r e s s C a ta l o g C a r d N u m b e r

72-

174581

For sale by the National Technical Inf or mat ion Service, Springf ield, Vi rg in ia 22151 - Price 3.00


3/190

PREFACE

This publication is intended

as

a review of s tudies and ob-

serva tions on the friction, wear, and lubrication behavior of ma-

teria ls in a vacuum environment. The specific subject of adhe-

sion

was

not included. Th er e

is

a genera l discussion of the

subject, however, with refe rence to friction and adhesive wear.

The intent in this document was to satisfy two inte res ts in the

field of tribology: that of the basi c researcher and that of the

engineer confronted with lubrication design problems. Vacuum

provides the basic re sear ch er with a tool that enables him to

eliminate normal environmental effects and the ir influence on

friction, wear, and lubrication. It offers a means of examining

the bas ic properties of ma te rial s that influence tribological

characteris tics. Fo r the engineer, it is one more environment

w i t h

which he must concern himself in the design of lubrication

systems. It is hoped that this document w i l l have something to

offer both.

.

i i i


4/190

CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . .

1

Adhesion Theory of Frict ion

. . . . . . . . . . . . . . .

1

Surface Films in

a

Normal Environment . . . . . . . . .

4

Metal oxide films

. . . . . . . . . . . . . . . . . . . .

5

Other films . . . . . . . . . . . . . . . . . . . . . . .

6

Solid solubil ity concept . . . . . . . . . . . . . . . . .

8

The Rebinder ef fect

. . . . . . . . . . . . . . . . . . .

11

15

Static friction . . . . . . . . . . . . . . . . . . . . . .

15

Dynamic friction

. . . . . . . . . . . . . . . . . . . .

17

Factor s that influence dynamic friction

. . . . . . . .

17

Relation of Adhesion to Static and Dynamic Fric tion .

.

WEAR AND VARIOUS TYPES OF WEAR . . . . . . . . . .

20

Abrasive Wea r . . . . . . . . . . . . . . . . . . . . . .

20

Corrosive Wear

. . . . . . . . . . . . . . . . . . . . . . 21

Adhesive Wear

. . . . . . . . . . . . . . . . . . . . . . . 22

Interatomic bonds in adhesion . . . . . . . . . . . . .

23

Relation between cohesion and elasticity

. . . . . . . .

25

The adhesive wear partic le

. . . . . . . . . . . . . . . 29

Part ic le generation by cleavage . . . . . . . . . . . .

31

Effect of inclusions . . . . . . . . . . . . . . . . . . .

32

Ductility in metals . . . . . . . . . . . . . . . . . . .

36

Lattice mismatch . . . . . . . . . . . . . . . . . . . .

37

Fatigue

Wear . . . . . . . . . . . . . . . . . . . . . . .

40

Fatigue in britt le materials

. . . . . . . . . . . . . .

42

Therma l fatigue

. . . . . . . . . . . . . . . . . . . . . 43

Fatigue in ductile ma ter ial s

. . . . . . . . . . . . . . 42

V


5/190

F RICT ION. WEAR. A ND L UB RICA T ION I N VACUUM

Page

INFLUENCE OF REDUCING AMBIENT PRESSURES

ON FRICTION

. . . . . . . . . . . . . . . . . . . . . . . .

44

Effect on Metal Oxides

. . . . . . . . . . . . . . . . . .

44

Effect on Bearing Steel

. . . . . . . . . . . . . . . . . .

47

Effect on Carbon

. . . . . . . . . . . . . . . . . . . . .

50

Effect on Covalent and Ionic Solids . . . . . . . . . . . . 55

INFLUENCE OF ULTRAHIGH VACUUM ON

FRICTION OF CLEAN METALS

. . . . . . . . . . . . . . .

56

Matched Single Crystals . . . . . . . . . . . . . . . . . . 56

Dissimilar Atomic Planes in Contact

Dissimila r Metal Crysta ls in Contact . . . . . . . . . . .

Influence of Crystal Structure . . . . . . . . . . . . . . .

. . . . . . . . . . .

60

62

67

Cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Thallium

. . . . . . . . . . . . . . . . . . . . . . . . .

71

Ra re ear th elements

. . . . . . . . . . . . . . . . . . .

71

Lattice ratio

. . . . . . . . . . . . . . . . . . . . . . .

76

Magnesium . . . . . . . . . . . . . . . . . . . . . . . . 79

Recrysta llization and Texturing

. . . . . . . . . . . . . .

83

Anisotropy

. . . . . . . . . . . . . . . . . . . . . . . . .

86

Hexagonal metals

. . . . . . . . . . . . . . . . . . . .

86

Body- centered- cubic metal s . . . . . . . . . . . . . . 89

Ionic solids

. . . . . . . . . . . . . . . . . . . . . . .

94

Metal Alloys . . . . . . . . . . . . . . . . . . . . . . . . 95

Equilibrium Segregation

. . . . . . . . . . . . . . . . . .

96

OTHER METAL PROPERTIES RELATED TO

FRICTION AND VACUUM

. . . . . . . . . . . . . . . . . .

101

Elastic Propertie s . . . . . . . . . . . . . . . . . . . . . 101

Ductile to Brittle Transitions

. . . . . . . . . . . . . . .

102

Order-Disorder Reactions

. . . . . . . . . . . . . . . .

105

FRICTION BEHAVIOR OF NONMETALS

IN

A

VACUUM ENVIRONMENT . . . . . . . . . . . . . . . . . . 108

Aluminum Oxide

. . . . . . . . . . . . . . . . . . . . . .

109

Carbons

. . . . . . . . . . . . . . . . . . . . . . . . . . .

117

v i


6/190

CONTENTS

Page

ADSORBED FILMS AND THEIR EFFECT ON

METALLIC FRICTION

. . . . . . . . . . . . . . . . . . . .

117

Differences in the Types of Adsorption

. . . . . . . . . . 118

Effect of Adsorbed Fil ms on Frict ion . . . . . . . . . . . 122

Kinetics of Adsorption . . . . . . . . . . . . . . . . . . . 130

Effect of Hydrocarbon Bond Saturation

. . . . . . . . . . 132

Effect

of

Hydrocarbon Chain Length

. . . . . . . . . . . .

134

FRICTION BEHAVIOR OF POLYMERS

IN A VACUUM . . . . . . . . . . . . . . . . . . . . . . . . 135

Polyimide

. . . . . . . . . . . . . . . . . . . . . . . . . 143

Advantages

of

Solid Polymer Bodies . . . . . . . . . . . 149

Polytetrafluoroethylene (PTFE) . . . . . . . . . . . . . 137

SOME ADDITIONAL FACTORS RELATING TO

WEAR

I N

VACUUM

. . . . . . . . . . . . . . . . . . . . .

150

CONVENTIONAL LUBRICATION

. . . . . . . . . . . . . .

154

SOLID

FILM LUBRICANTS

. . . . . . . . . . . . . . . . .

158

Layered Lat tice Compounds

. . . . . . . . . . . . . . . 159

Methods of Application . . . . . . . . . . . . . . . . . . 166

Soft Metal Fi lms

. . . . . . . . . . . . . . . . . . . . . .

170

REFERENCES

. . . . . . . . . . . . . . . . . . . . . . . . 175

v i i


7/190

INTRODUCTION

ADHESIONTHEORYOF FRICTION

In or der t o gain a fundamental understanding of the frict ion

and wear behavior of mat er ia ls in

a

vacuum environment, it i s

important t o consider those fac tors that determine and influence

fric tion and wear in general. The topographical, physical, me-

chanical, and chemical nature of the surface influence the friction

and wear behavior of solid bodies in contact.

properties such as deformation characteris tics, fra ctu re behav-

ior, and structure

w i l l

exer t an effect on fric tion and wear.

The fi rs t step to gaining insight into the friction and wear

behavior of solid sur face s in contact is to examine the surface

profile or topography.

neering applications a r e not atomically flat.

A

surface, when

examined microscopica lly or with a profilometer, even though

highly polished, has an irr egu lar nature. The surface consis ts

of high and low spots

as

shown schematically in figure l(a). The

nature of the irr egu lar itie s

will

be influenced to some degree by

the method of surface preparation: Mechanically prepared sur -

Further, bulk

Most sol id metal su rfaces used in engi-

a) Asperities.

(b )

Cleavage

steps.

(c l Contacting asperities. d ) Plowing.

Figure 1. - Nature

of

surfaces and surface interactions.

1


8/190

2 FRI CTIO N, WEAR, AND LU BR IC AT IO N I N VACUUM

faces tend to be the most rough, and electropolished sur faces tend

to be the least. The surface high spots or protuberances a r e

called asperities.

solid-state contact.

Although most surfaces have aspe ri ti es of varying heights and

degrees, atomically flat surfaces can be and have been generated.

When diamond, sapphire, lithium fluoride, or sodium chloride

is

fractured along

a

cleavage plane, an atomically flat sur face can

resu lt. Even metals such

as

zinc can, a t cryogenic tempera -

tures, b e cleaved along its basal plane producing atomically

smooth flat surfaces. In the cleavage process, however, steps

result on the su rface as shown in figure l (b) . These steps de-

velop in the cleavage process because in cleaving large

areas,

frac ture cracks develop between adjacent paralle l cleavage

planes and leave steps in the surface . Between such steps the

surfaces may be atomically flat. Thus, even if atomically flat

step s a r e developed

as

shown schemat ical ly in figure l(b), when

two such surfaces

are

placed into contact with one another, the

presence of the steps

will

prevent complete contact over the ap-

parent contact a r ea unless bifurcation was done in such

a

man-

ner

as

to permit the two surfaces to remate in the same manner

in which separation occurred.

When two solid surfaces whose surface profiles are like that

shown in figure l(a ) a r e brought into contact, the interfacial con-

tact

is

as shown in figure l(c).

As shown, the real o r actual

contact is only a small port ion of the apparent contact area. The

points o r high spots that make solid contact a r e the asperiti es.

If a

very light load

L is

applied to the surfaces in contact, these

surface asperities w i l l at fi rst deform elastically . If the load

is

sufficiently light, these local regions will always deform elas -

tically. However, because the amount of re al contact a r ea is

generally small, not only elast ic but plastic deformation of these

surface irregularities occurs as well. In addition, elas tic de-

formation occurs in the bulk of the asperi ty while plastic flow is

occurring in its surfic ial layers .

The deformation process of the surface aspe ri tie s w i l l con-

tinue until the load

is

supported, that is, until the real contact

Surface irr eg ular it ie s determine the nature of


9/190

I NTRODUCT

I

ON

3

a r e a is sufficiently large such that the applied force

is

below the

elastic limit or yield pre ss ur e in compression . When the two

surfaces in contact a r e of the same material, the deformation

w i l l occur to both sur faces. Where the sur faces a r e of differ-

ent mat eri als , the deformation w i l l generally take place in the

weaker of the two materials or in the material with the lower

elast ic limit.

over the other, there

is

a

res istance to t his tangential motion,

which

is

simply the frictional res ist ance of the two bodies to

such motion. The fr iction of

a

materi al pai r in contact is usually

expressed as the coefficient of friction.

may b e written simply as

When one of the su rf ac es in figure l( c) is pulled tangentially

The fr iction coefficient

where 1 1 is the f ric tion coefficient, F i s the friction force, and

L

is the no rm al load. The gre ate r the normal load, the grea ter

the fr iction force.

mined by the true interfacial contact a r ea A and the shear

strength S of the weakest ma te rial in the surf icial region.

The friction force in this relation is deter-

F = AS

The contact area

is

proportional to the applied normal load

L

and the deformation or strain containing both an elastic and

plastic component D.

When the applied normal load is removed, elastic recovery

occurs.

The tr ue contact area, then, can be represented by the

contact ar ea determined by plastic deformation. From these re-

lations, it can be readily seen that the coefficient of friction may

be expressed

as


10/190

4 FRICTION,

WE A R,

A N D

L UB RICA T ION IN VACUUM

S

p = -

D

This relation

is

applicable when the re

is

negligible plastic

flow of the mate rials in contact.

If,

however, one surface will

flow into another, such

as

a st ee l ball would do on

a

sheet of lead

(see fig. l(d )), the friction in addition to

a

shearing component

w i l l

have a plowing component. The plowing component

w i l l

sim-

ply be the force required to deform the mate rial ahead of the ball

i n

figure l(d).

friction w a s formulated by Leonard0 da Vinci in the lat e fifteenth

century (ref. l ) , by the French engineer Amontons at the end of

the seventeenth century (ref.

2),

and by Coulomb in the eight-

eenth century (ref.

3) .

Coulomb originally believed that adhe-

sion contributed to the friction of solid bodies in cbntact, but he

la te r discounted thi s theory in favor of the concept that fric tion

was the physical r esi stance to the pulling of the mechanically in-

terlocked aspe ri tie s over one another. Bowden and Tabor la ter

established that Coulomb's original concept with respect to fric-

tion being related to adhesion

at

the asperit ies w a s correct

(ref.

4).

According to the adhesion theory of friction

as

proposed by

Bowden and Tabor, adhesion occurs

at

the contacting asperities,

and the force required to shear the adhered junctions S

w i l l

be

the shear strength in the surficial region.

Much of our present understanding of the basi c concepts of

SURFACE FILMSN A NORMAL NVIRONMENT

The surfaces of most solids in

air at

atmospheric press ure

contain films. Most metal surfaces, for example, a r e repre-

sented by figure

2.

The bulk metal or alloy has a near surface

region

of

highly worked metal , which develops in the process of

forming the surface . Depending on the nature of the sur face pro-

cess ing, the str uc tura l makeup of thi s layer

w i l l

vary. For

meta ls that a r e prone to develop surface textures, such as the

hexagonal metals , these textures may be presen t on the surface.


11/190

I NTRODUCTI

ON

5

Figure

2. -

Schematic representation

of rubbing metal

surface.

The nature of reactivity of the meta l surface depends largely

on

the physical and chemical nature of surface layer.

Metal oxide films. - With single-crystal metal su rfaces,

the surface activity to fo rm oxides

(as

shown in fig.

2) w i l l de-

pend on orientation. The da ta of table

I

indicate the zones on

single-crystal sph ere s where pre fer red oxidation in air occurs

TABLE I . - OXIDATION O F ME TA L SINGLE CRYSTALS

IN A IR (R E F .

5)

Metal

C r

F e

Cd

Zn

C r y s t a l s t r u c t u r e

Face-cen te red cub ic

Face-cen te red cubic


Face- cen te red cubic


Face- centered cubic

Fody-cen te red cubic

Body-centered cubic

Close-pack ed hexagonal

Close-pack ed hexagonal

Act ive regions

(100)

(100) (210)

N o change to 55

(100) (311)

(100) (110)

(100)

(100) (111)

No pre fe rence

s ix a r e a s 45' to

(0001)

Hexagonal ring cen-

t e r e d a t

(0001)

Inactive regio ns

(111)

(311) (111)

l o c

(111) (210)

(111)

(111)

None

Region between

(100) and (110)

(0001)

(0001)


12/190

6

FR I CT1 ON, WEAR, AND LUBR ICA T1 ON IN VACUUM

(ref . 5). In the table these a r e called active regions.

With polycrystalline metal surfaces where different orien-

tations a r e present on the sur face , the nature of the oxide film

will vary from grain to gra in because of the differences in su r-

face energies as demonstrated by the da ta of table I.

energy, more densely packed

{

111} planes in the face-centered-

cubic ( F C C ) system a r e le ss r eact ive with oxygen than the le ss

densely packed { 100) and

{

l l O } planes. The gra in boundaries

themselves on the polycrystalline metal su rface tend to interact

with oxygen because of the high energy in these regions.

Alloying elements can markedly alt er the nature of the su r-

face oxide. Auger emission spectrometer analysi s of stainless

steels

h a s

shown chromium to be present on the surface and,

on

oxidation, to form chromium oxide. With alloys, it is possible

to have an oxide present on the surface that is principally the re -

action product of oxygen w i t h the solvent, with thk parent metal,

with

a

solute element, which may be presen t in minor amounts,

or with a mixture of oxides of the metallic elements present in

t h e alloy.

Thus, the oxide region

of

figure 2 may be complex,

and it

is

well established

as

being present on

all

metal surfaces

except gold.

and adsorbed water vapor by the metal oxides. Many of the

gases present

in a

normal a i r environment w i l l chemisorb rap-

idly to the common oxides found on friction and wear surfaces .

Table I1 presents some of these ga se s and metal oxides and the

heats of chemisorption together with the speed of adsorption

(ref. 6).

Thus, one can see that the sur faces of me tal s a r e

thoroughly c overed with film

s.

The presence of environmental constituents a r e not only

present on meta l sur faces but on nonmetal surface s as well .

For example, the presence of water vapor on graphite is essen-

tial to it s lubricating prope rties. Carbon, in general, contains

chemisorbed oxygen that frequently can only be removed by heat-

ing it to temperatures in excess of 1500' C (1793 K), whereupon

the oxygen evolves

as

carbon monoxide.

The lower

Other films. - The top layer in figure 2 represents gases

With ionic crystal s, the presence of adsorbed surface spe-


13/190

~

INTRODUCTION

-

G a s

TABLE

II.

- CHEMISORPTIONS ON OXIDES AT ROOM

TEMPERATURE (REF.

6)

Oxide

~

NiO

Cr203

Fe203

MnO

ZnO .

C r p O

NiO

c 00

ZnO

c u z o

MnO

Cr203

ZnO

.CrZO

NiO

c

0 0

NiO

c0 0

c0 0

ZnO

ZnO .

C r 2 0

c u z o

c u 2 0

nit ial heat of

adsorption,

kcal /mole

26

20

9 to 13

20

6 4 . 4

28

14 . 9

28

23

--

13

13

2 5 . 2

11

20

37

Velocity

of

adsorption

Initially rapid adsorption, then slow

Same as NiO

S a m e a s NiO

Smal l , very rapid adsorpt ion

Extrem ely rapid

Rapid adsorpt ion; cove rage .


14/190

8

F R C T I ON, WEAR,

A N D

LUBR I CAT1 ON I N V A C U U M

ci es not only influences the surface prope rties of the crys ta ls but

al so affects the mechanical behavior of the solids. The effect of

environmental constitutents on the behavior of ionic sol ids

is

discus sed in detail in the l ite rature

(refs. 7

to

11) .

When two metal su rface s a r e brought into contact, the as-

perities w i l l be covered by the oxides and adsorbed films.

If

the

load is sufficiently high, such that the sur faces deform plast i-

cally

at

the interface, metal to metal adhesion can occur through

the sur face fi lms present . The imposition of tangential motion

on the surfaces in contact

w i l l

resu lt in the shear of the weakest

region in the surfici al layer. The shea r force required coupled

with the rea l ar e a in contact,

as

discussed earlier, w i l l deter-

mine the friction force.

Metal oxides, in general, .have higher

shear

strengths than

the corresponding parent meta l (see table III; refs.

1 2

and 13).

The only metal oxides in table III that have lower shear st rengths

than the parent metal a r e molybdenum and silver. Thus, in

most instances, for two solid metal surfa ces in contact, tangen-

tial motion

w i l l

result in the shear of the weakest bonds formed

a t the interface which, in general,

w i l l b e the metallic bonds

ra th er than the oxide. The oxide, however, reduces the amount

of meta l-to-metal bonding that can occur and thereby reduces,

also, the true metal-to-metal contact a re a and the cros s-

sectional interfacial a r ea of metal that must b e sheared. The

effect of the oxides and other surface contaminating films on

metal surfaces

is

very important to the understanding of t h e dif-

fer ences in the friction behavior of metallic s urf aces in air and

vacuum.

Solid solubility concept. - Based on the fact that

shear

w i l l

occur in metallic junctions, Erns t and Merchant (ref. 14) calcu-

lated the friction coefficients for va rious metal p ai rs using the

relation p

=

S/H

w h e r e

S

is

the shear strength of metal

at

the

interface and H the hardn ess of the metal. (Hardness, instead

of deformation or str ain, was used in the previous relation for

fric tion coefficient.

)

The friction coefficients, which they deter-

mined from the relation S/H and experimentally, a r e presented


15/190

INTRODUCTION

9

TABLE

111.

- RELATIVE SHEAR STRENGTHS

MEASURED FO R VARIOUS M ET AL S AND

METAL OXIDES

IN

COMPRESSION

TWISTING (REFS. 12 AND 13)

-

detal

A1

c u

N i

Fe

C r

Mo

W

g

Pb

c o

Z n

T i

Z r

-

Shear strengths

for metal at

high pressures ,

ss,

k / m m

3 1 .0

49.0

87.0

1 0 0 .0

122.0

121.0

128.0

47.0

6.8

63 .0

1 8 .4

130.0

45.

0

Metal mide

A1203

N i O

Fe203

Cr203

Moo3

CU20

wo3

Ag2O

P b 3 0 4

P b0

PbOZ

ZnO

T i 0 2

ZrOZ

Shear strengths for

metal

oxides at

high pressures ,

ss, 2

kg/mm

94

103

119

167

134

111

140

3 5

24

99

81

117

126

1 4 5

121

in table IV. In table N( a) the metal pai rs in contact were capa-

ble of forming solid solutions. Examination of the predicted and

observed friction coefficients indica tes good agreement. With

the su rface films present, the friction coefficient fo r meta ls o r

alloys in contact ra re ly exceed

a

value

of

1.5.

In table N(b ) a comparison

is

made between predicted and

observed friction coefficients fo r pa i rs of metals that exhibit

almost mutual insolubility. Here the predicted and observed

friction values va ry considerably. Erns t and Merchant were in-

dicating with table IV the effect of mutual solubility on friction.

Although, in general , the friction proper ties of insoluble pairs

a r e lower than for mutually soluble metal pair s, ca re must be

taken in using bulk properties, such as solid solubility, for pre-


16/190

10 FR IC T

I

ON, WEAR, AN 0 LUBR CA T

I

ON I N VACUUM

T AB L E

IV.

- COMPARISON O F

PREDICTED~

AND

OBSERVED

FRICTION C OEFFICIENT S

( R E F .

14)

(a)

Pairs

forming sol id solu-

t ions a t r oom t em per a t u r e

Pair

Al- Fe

Al- Zn

C o - F e

c o - c u

Co-A1

Cu-Cd

Cu-Zn

Zn-Fe

Zn-Sb

Fr ict ion coeff ic ient

P r ed i c t ed

1.05

. a 5

_ _ - -

.90

1.

0 5

. a 3

. a 5

. 8 5

. a 5

Observec

1 . 0 5

. 8 2

. 5 4

. a 9

1.01

. 8 5

.86

. 8 5

. a 5

( b) P a i r s a l m os t mu t ua l ly in -

s o lu b le a t r o o m t e m p e r a t u r e

Cd- Fe

. 6 2

1.05

aBased on p =

S/H

and

S

=

0 . 4 2 7 L/3p log, T,/T.

dicting sur face behavior. A s w i l l be shown later, adhesion and

high friction can occur for insoluble

as

well

as

soluble metal

pairs.

tallic adhesion w i l l almost always occur.

a r y

lubricants

is

to reduce this adhesion

as

much

as

possible.

The presence of lubricating fi lms on metal surfaces w i l l reduce

the friction coefficients for metals in contact to values consider-

When two sol id surfaces are in contact, some degree of me-

The function of bound-


17/190

INTRODUCTION

1 1

ably below those presented in table

IV.

With effect ive boundary

lubricants, such

as

fatty acids, friction coefficients for me tals

in contact w i l l generally be 0.

1

or

less.

With poorer fluid lubri-

cants present on meta l su rfaces, friction coefficients may be

as

high

as 0.2.

These organic fi lms fur the r reduce the amount of

metallic contact occurring acr os s the interface. They act with

the metal oxides present in reducing metal lic contact, the total

adhesion, and consequently

the

friction force.

The presence of adsorbed surface films greatly influences

the fric tion behavior of nonmetals, also. The degree to which

these fil ms reduce friction

w i l l

be discussed in reference to vac-

uum resul ts. The presence of surface active agents on non-

metal s, par ticularly on ionic solids, can influence the mechan-

ica l behavior of these solid sur faces . If surface active species

can influence deformation, they can al so influence friction. Sur-

face active fi lms can influence the deformation behavior of ionic

solids by various mechanisms. These include

(1)

strengthening

by dissolution of the solid sur face or the Joffe effect (ref. 15);

(2 )

surface hardening

o r

Roscoe effect (refs.

16

and

17);

and

(3) surface

softening

or

Rebinder effect

(refs. 9 and

11).

The Rebinder effect.

-

The Rebinder effect

has

been studied

most and, because of i t s significance to the friction behavior of

solids, w i l l

be discussed in some detail here. \The fi rs t disclo-

sure of

th is

effect

w a s

reported in 1928

b y

Russian resea rch ers

(refs. 9 and 11). Although considerable information

w a s

gener-

ated in this a r ea by the Russian laborator ies, particularly by

the founder of the concept (Rebinder), li ttle support

w a s

given to

it outside Russia until recently.

The Rebinder effect

has

been shown in the deformation of

ionic solids such

as

lithium fluoride (ref. 18) and magnesium

oxide (ref. 19). This inc rease in ductility or ability to deform

plastically in the presence of adsorbed surface species has also

been observed in metals (refs.

11, 17,

and

20).

It has been ob-

served in the covalent ma teria l germanium and in metal carb ides

(ref.

21).

the ionic sol id lithium fluoride to de termine the influence of sur-

Some sliding friction experiments have been conducted with


18/190

1 2

FR ICT I ON, WEAR, AND LU BR ICAT1 ON

I

N VACUUM

(a)

D r y

air.

(b) Water.

(c) Water and 5. 0 ~ 1 0 - ~ myr ist ic

acid.

Figure

3. -

Cross section of wear tracks on l it hi um fl uori de in sli din g fri cti on experiments. Load,

200 grams (2.0 N); 1.6-millimeter-diameter sapphire ball; temperature, 20' C (293

K);

sl iding

velocity,

0.005

mil lim ete r per second. Ball made a single pass across surface covered wit h th re e

different medi a air, water, and water wi th myr is ti c acid.


19/190

INTRODUCTION

13

face fi lms on friction and deformation. A sapphire

ball

was slid

across

a

freshly cleaved lithium fluoride (100) surface. The

lithium fluoride specimen

w a s

then cleaved normal to the sliding

track and etch pitted. The subsurface deformation and the de-

velopment of cleavage cra ck s is shown in figu re 3(a). Examina-

tion of fi gure 3(a) reveals that s lip has taken place along the

O l l }

and

{

l O l } s e t s of planes. Since the se are the slip planes,

plastic deformation might be expected to occur in such

a

fashion.

In addition to the slip bands, cleavage crac ks developed along

the { 011) sli p bands and have their origin a t the surface.

Cra cks can form in lithium fluoride a t

the

inter section of { 110)

slip planes according to the equation

It is important to note f rom the etch pitted sl ip bands in fig-

u r e 3(a) that a britt le m aterial such as lithium fluoride w i l l de-

form plastical ly in sliding.

In order to show the marked influence that atmospheric

constituents can have on the mechanical behavior of ionic c ry s-

ta ls in sliding friction studies, equivalent experiments were

conducted with lithium fluoride in water. Rather than simply

comparing behavior in moist air with that in dry

air,

water w a s

used. The lithium fluoride cr ys ta ls were cleaved in water and

friction exper iments were conducted

w i t h

water present on the

cry sta l surface. The cry sta ls were then cleaved normal to the

wear tra ck and etched. The tra ck subsurface deformation is

shown in figure 3(b). Note that, although sl ip bands are evident

from the dislocation etch pi ts along the (110) plane, a subsurface

crac k has formed in the crystal. This crac k lies in a (001)

plane. In dry

air

(fig. 3(a)) the c rac k formed at the surface

along (110) planes rat he r than in the subsurface. With the plas-

tic deformation of lithium fluoride, cr ac ks can develop along a

(100) plane with the intersect ion of { 110

}

slip bands in accor-

dance with the equation


20/190

14

FRI CT I ON, WEAR, AND LUBR IC AT IO N I N VACUUM

The crack developed in figure 3(b) w a s the re su lt of both com-

pressive for ces acting on the crys ta l surface in the form of the

normal load and tangential f or ce s associated with sliding.

Figure 3(c) is a sliding friction tra ck in c ro ss section after

a sliding friction experiment w a s conducted in a

5.

OX10-6 normal

solution

of

myrist ic acid. In the presence of the acid, t her e

w a s

no evidence of either surface or subsurface crack formation

as

seen in figu res 3(a) and (b).

which the (011) slip bands extend

is

appreciably grea ter than that

observed in the other two environments.

Thus,

a

greater de-

gre e of plasticity appears to exis t in the presence of the my ri s-

ti c acid. The energy associa ted with the sliding friction process

ap pear s to have been absorbed completely in plasti; behavior.

The influence of environment on the behavior of ionic sol ids

is

further shown in some sliding fric tion experiments conducted

on the (111) cleavage face of calcium fluoride.

Figure 4 pre-

sen ts deformation data as a function of the molar concentrat ion

of dimethylsulfoxide in water . The data indicate that with de-

creasing concentrations of dimethylsulfoxide o r increasing con-

centrations of water , the width of the wear track inc reases .

In figure 3(c) the subsurface depth to

,-Pure dimethylsulfoxide Pu r e water-,

\


21/190

INTRODUCTION 15

This increase in the width of the wear tr ack may be attributed to

an inc rease in the plasticity of the surface.

The foregoing discussion on the influence of surface fi lm s

on the deformation and fra cture of lithium fluoride and calcium

fluoride indicates that the presence of surface films on ionic

solids influences not only surface behavior but a ls o subsurface

behavior.

Not only does the abi lity of su rface fi lms to influence

deformation behavio r influence friction because it de termines

true contact

area,

but a ls o it influences the wear of solid sur-

face s in contact. The presence of surface o r subsurface cracks

can, with repeated tra ver sal s over the same surface, give r i s e

to the formation of wear particles.

This h a s been demonstrated

with the ionic solids lithium and calcium fluorides. Deformation

re su lt s with calcium fluoride indicate the ext reme sensitivity of

ionic solids to sma ll changes

in

environmental constituents.

RELATION

OF ADHESION TO STATIC

AND

DYNAMICRICTION

When two solids are placed in contact, the contact occurs

ac ro s s the interface between surface asperities.

Plas ti c flow

of

these metal asper itie s w i l l then occur, and

a

certain amount of

metal-to-metal contact

w i l l

take place through any surface films

present.

The force required to initiate motion between the two

solid surfaces w i l l repres ent the st atic friction force.

Once one

of the solid sur faces is in motion, the friction force measured i s

the dynamic friction.

In general, the s tat ic friction coefficient fo r

sol ids in contact is higher than the dynamic fric tion coefficient.

There are a number of fac to rs that account fo r the difference .

types of friction is the tim es that a spe rit ies o r microjunctions

a r e in contact. When two solid bodies are placed into contact

under a

load, f i r s t elastic and then plastic deformation w i l l

occur. The solids generally

w i l l

undergo fu rth er deformation

while standing in contact. This time-dependent additional defor -

mation of the so lids in contact under an applied s t r e s s

is

re-

Static friction. - The first major difference between the two


22/190

16

FR 1 C T I ON, WEAR, AND LUBR 1 CAT 1 ON I N VACUUM

ferred to as creep.

where the c reep st rain may be completely (or nearly

so)

re-

covered on the removal of the s t r e s s , or in the plastic range,

where the creep deformation is permanent.

tion in the plastic range an elastic component al so exists, but it

may be t rivial by comparison with the plastic (ref. 22) . Metals

a r e particularly prone to creep.

Adhesion at the interface w i l l increase with increasing in-

terfacial

contact

area as creep

in the microjunctions continues.

With increasing adhesion there should be

a

corresponding in-

cr ease in the static friction. This in fact

has

been observed, as

is

shown by the data of figure 5 (from

ref.

23), where the stat ic-

friction coefficient is plotted as a function of contact time.

The data of figure

5

do not show

the

sta tic friction becoming

less dependent on contact time with prolonged contact times.

The curve should resemble cre ep curves. Static'-friction curves

obtained by Ishlinski and Kraghelsky (ref. 24) show that it does.

After some period of time the static-friction coefficient becomes

less dependent on contact tim e, and the slope of the curve

changes. Adhesion and sta tic- fric tion coefficient measurements

are

therefore ex tremely dependent on the time the solid surf aces

Creep may be exhibited in the elastic range,

For creep deforma-

.6

c

c

U

.-l .5

s

c . 4

.-

-

c

U

L

U

.-

.-

.-

2

. 3

. 2

. (

0

.1

1

10 100

loo0

Time, T, sec

Figure 5.

-

Static-friction coefficient as func-

t ion o f t ime for unlubricated steel sl iding on

steel (ref. 23).


23/190

NTRODUCTI ON

17

a r e in contact.

With metal s more prone to creep than nonmetals,

it

might

be

anticipated that great er differences would exist between the

stat ic and dynamic-f ric tions of me ta ls than of nonmetals. This

is

in fact what

is

generally observed.

The static-friction coefficient of ma te ri al s not only is im-

portant in the init iation of relative motion between surfaces but

al so manifests itself during rela tive motion between solid s u r

-

faces in the so-called s tick-s lip phenomenon. This phenomenon

is

par ticula rly prevalent with metal surfaces in contact. If one

surface

is

slowly moved across another and

i f

the friction force

is

recorded during movement, the friction for ce

will rise

to

some high value and then drop ve ry suddenly. This process

w i l l

continuously repeat itself. With relative motion strong bonds of

adhesion will form ac ro ss the interface, motion

w i l l

momen-

tarily stop, and the friction force, or force required to over-

come the bonding of the adhered junctions,

w i l l

continue to in-

c rea se until the bond forces of adhesion

are

exceeded. When

th i s

occurs the bonds

w i l l

break, and motion

w i l l

occur with a n ac -

companying sudden decrease in friction.

T h e

process continu-

uously repeat s itself. The stick portion of the process is in

reality

a

measu re of the force necessary to overcome static fr ic-

tion.

hes ive junctions between solid surfaces a r e continuously being

made and broken very rapidly, and the dynamic-friction coeffi-

cient repr esent s an averaging of the making and breaking of ad-

hesive contacts.

Dynamic friction.

-

In dynamic-friction measuremen ts, ad-

Factors that influence dynamic friction. - When two surfaces

are

in dry sliding contact, very high interfacial temperatures are

momentarily generated a t the contacting microjunctions or

as-

perities. These temperatures may reach

1000

C

(1293 K)

(ref.

4).

Because they occur with junction contact and

are

of

short duration, they a r e called flash temperatures. In addition,

the energy put into the surface

as a

resul t of the friction process

w i l l

generally incr ease the temperature in the surficial layers.


24/190

18 FR IC T I ON, WEAR, AND LUBR I CAT1 ON I N VACUUM

These temperatures will be influenced by the normal load, slid -

ing velocity, the nature of the sol ids , and the proper ties of sur-

face films. With increasing sliding velocity, the surface tem-

pe ra tu re s increase until the melting point of the lower melting

material

is

reached. When thi s occurs ,

a

marked decrea se in

the friction forc e w i l l occur because the liquid metal

w i l l

have

much lower shea r strength than the same metal in the solid

state.

be markedly influenced al so by the normal load applied to the

su rfaces in contact.

tion for copper sliding on copper as a function of applied load

(ref. 25). At very light loads the coefficient of fr ic tion is ap-

proximately

0 . 4 .

This lower friction re pres ents basically the

fric tion proper tie s of the copper oxide fi lms presen t on the two

sur faces. As the load

is

increased, metallic contact begins to

occur through the oxide films, and the fric tion coefficient begins

to incr ease and continues to do

s o

with increased loading as more

and more metal to metal contact occurs.

sult in an inc rease in friction, even the oxides themselves a r e

The coefficient of f ric tion for meta l surfaces in contact can

Figure 6

is a

plot of the coefficient of fr ic-

Although oxides can be penetrated on metal surfaces to r e -

2.0

.- 1 . 5

t;

c

1.0

L

c

0

cl

U

al

.-

.-

c

s

. 5


25/190

I

NTRODUCTI ON

c

u

.-

c

L

-

-

c

a,

u

iE

a,

u

-

.-

c

0

.4-

/ d

2 -

f i gu re

7. -

Coefficient of fri ct io n as fun cti on of load for

sapphire sl idin g on sapphire

in

ai r (760 torr). Slidinq

velocity,

0.013

cent imeter pe r second; ambient tempe;-

ature. 25

C

(298 ) .

sensitive to load. Figure

7

for single-crystal aluminum oxide

(sapphire) sliding on itself in

air

shows the effect of loading on

fric tion ccefficient. At the lighter loads the fric tion coefficient

w a s 0.15. With increased loading and penetration of adsorbed

water vapor and oxygen, the fric tion coefficient ro se to

a

value

of

0.25.

chemisorbed surface films in vacuum will resu lt in a fourfold in-

c rease in friction coefficient of aluminum oxide.

As already mentioned, sliding speed, may al so influence the

coefficient of friction between two sur faces ( se e fig.

8).

At very

As wil l be shown

later,

complete removal of these

Slidi ng velocity

-

coefficient as function of sliding

velocity (ref.

26).

f i gu re

8. -

Typical plot of kinetic


26/190

2 0

FR I C T l ON,

WEAR, AND

LUBR

I

CAT1ON I N VACUUM

high sliding veloci ties , i f localized melt ing on the surface of the

ma terial s in contact does not occur

so

as

to result in

a

marked

decrea se in friction coefficient, other changes can take place in

the materials. Sufficient interfacial heating, which w i l l increase

with increasing sliding speed, can produce these localized

changes. One such change would be

a

metallurgical transforma-

tion. Another can be diffusion of alloy consti tuents to the s u r -

fic ial region.

WEAR AND

VARIOUS

TYPES

OF

WEAR

The wear of solid su rfaces in contact can be caused by one

or a combination of wear mechanisms. The most common types

of wear

are

abras ive, adhesive, co rrosiv e, erosive, and fatigue.

ABRASIVE

WEAR

Abrasive wear occurs when two solid su rfaces a r e in contact

and one of the two solids

is

considerably harder than the other.

The harder surface asperities w i l l pr es s into the softer surface

with plastic flow of the so fter surface occurring around the as-

per iti es from the harder surface. When a tangential motion is

imposed, the harder surface w i l l move, shearing and removing

the softer material. Abrasive wear is the mechanism involved

in the finishing of many surfaces . Filing, sanding, and grinding

of surfaces all involve the proces s of abrasive wear. The mech-

anism of wear in the se proc esse s may not, however, b e exclu-

sively ab rasive wear. Chemical interaction of sur face oxides

frequently a r e involved, which can give r ise to an element of

cor ros ive wear. The fatigue mechanism may also be involved.

Kruschov and Babichev (ref. 27) found that the res is tance of

metals t o abrasive wear w a s related to their relative hardness.

Figure 9 is a plot of what they te rm abrasiv e wear resistance as

a function of meta l hardness. The relation is readily apparent.

In general, the abr asive wear behavior of m ate ria ls is propor-

tional to the load applied to the sur faces in contact, proportional

to the dis tance of sliding, and inversely proport ional to the hard-

ness.


27/190

WEAR A N D VARIOUS TYPES

OF

WEAR 21

Figure 9. - Wear resistance

of

vario us metals and steel (ref. 27).

The abrasion of metal sur faces can resul t in the development

of

prefe rred surface orientations or texturing (ref. 28). These

pre fer red surface orientations can, as w i l l be discussed with

reference to the

vacuum

results, markedly influence friction and

wear behavior.

CORROSIVE

WEAR

Cor ros ive wear occurs when the environment interac ts with

solid surfaces in contact to contribute to the attrition of the sur -

faces.

If

two surf aces react actively w i t h the environment, the

rubbing of su rf aces together in such an environment can re su lt

in the continuous formation and removal of reac tion products.

Since the ma te ri al of the surfaces in contact a r e contained in the

reaction product, material

is

being removed from the sur face.

With rubbing, fresh surface

is

continuously being exposed for

further reaction. If the reaction products a r e solids, they a r e

genera lly moved out of the contact zone

as

solid wear particles.

When the reaction products are gaseous,

corros ive wear may

occur very rapidly.

An

example of this situation o ccu rs when

solid carbon

is

used in air above 680 C ( 9 5 3 K).


28/190

2 2

FR CTI ON, WEAR, AN D LUBRICATION N VAC UUM

ADHESIVE

WEAR

Adhesive and fatigue wear

are

important to the behavior of

materials in contact in vacuum

as

they a r e the most frequent

types of wear encountered in

a

vacuum environment. A detailed

discussion of these types of wear a t th is point w i l l eliminate the

need for the same in discussing vacuum data.

Adhesive wear

is

the most detrimental and

is

frequently en-

countered. It can very rapidly destroy such mechanical compo-

nents

as

bearings, gear s, and seal s. Adhesive wear involves

the adhesion of solid sur faces ac ro ss an inter face with subse-

quant subsurface frac ture in one or both mate rial s. Material

may be transfe rred from one surface to another, from each sur-

face to the other, or back and forth from one su rface to another.

The transfer process of course is a materi al removal process.

Figure

10

shows schematically how the process can occur. In

order for adhesive

wear

to take place, fractur e must occur in

the subsurface of one or both mater ials. If fra cture occurred

at the adhesive junction, that i s , a t the interface between the two

surfaces, no adhesive wear would occur. This, of course,

means that when adhesive wear t akes place, the adhesive junc-

tion between the two solid sur faces

is

stronger than some region

subsurface where fract ure has taken place.

step necessi tates that adhesion take place between the two solids

in contact. Thi s can occur in an ordinary environment by the

Considering adhesive wear in

a

stepwise manner, the fi rs t

Motion-

Figure 10. - Adhesive wear.


29/190

WEAR AND VARIOUS T Y P E S OF WEAR

23

penetrat ion of surface fi lms with deformat ion of the sol ids in

contact. It can occur in vacuum with the simple approach of

two clean surfaces.

Intera tomic bonds in adhesion. - The plot of figure

11

shows

the interatomic forces acting ac ro ss the interface where nascent

surfaces come into contact.

An

atom from each of the two sur-

faces in contact will form a bond which

w i l l

be in equilibrium at

some distance

rl .

The equilibrium distance of separation

occurs a t the minimum in the potential energy, the distance at

which the attract ive fo rce s just balance the repulsive forces .

If

the atoms

are

moved together

under

applied force in the form

of load, a strong repulsive force ari ses . When the atoms a r e

pulled apart, the force required for separation, which

w i l l

in-

fluence cohesive or fric tion fo rces , w i l l fi rs t go through

a

maximum omax and then fall off asymptotically to ze ro (see

fig. 11). The value of umax corresponds to the ultimate

strength or theoretical strength at a distance of r1 on a strain

of (r - ro)/ro.

A

line may be drawn tangent to the resultant force curve at

r

=

ro.

stress

to strain. Thus, in figure 11, the modulus is given by

E

The modulus of elasticity i s defined a s the ratio of

a 0

i

1

Resultant force

Figure 11. - Interatomic forces between two atoms

as function of distance of separation.


30/190

24

FR IC TI ON , WEAR, AND LU BR IC ATI ON I N VACUUM

where

or the slope of the curve

at r

= ro

multiplied by

ro.

The inter-

cept of t h i s line of tangency

on

the

force

axis

is also

E

by the

rule of sim ila r triangles (ref.

29).

The elastic -str ess range in

real metals, for example,

is

only about 1/100th to 1/1000th the

value umax in figure

11.

The modulus of elastici ty

is,

how-

ever , the only commonly measured mechanical property which

directly reflec ts these interatomic forces.

If

two solid sur faces in contact were perfect solids and

i f

the bonds formed ac ro ss

the

interface in the micrdjunctions were

perfect, the force necessa ry to separate o r move

the

surfaces

apa rt could be determined using the theoret ical strength of the

interatomic junctions from

amax

(fig.

11)

and knowledge of the

true contact

area:

w h e r e E is

the elas tic modulus,

a is

the la ttice parameter of

the crys talline solids, and S

is

the sur face energy.

If

values ar e

used in thi s equation, the theoretical strength is found to be ap-

proximately equal to E/10.

6

would b e 2x10 kilograms per square centimeter. The strength

of real solids such

as

iron, a r e considerably below th is value.

The deviation of real solids from ideal

w i l l

result in

a

greater

true contact a r ea when sur faces a r e pressed together under load

because plast ic flow

w i l l

occur at much lower applied st re ss . At

the same time, however,

the shear strength

is

less . Thus, one

might expect that the frict ion behavior of wiskers (crystalline

sol ids with

a

minimum of defects) might not be too different from

le ss perfect solids of the sam e material.

Fo r iron, by way of example, it


31/190

WEAR AND VARIOUS TYPES OF WEAR

25

Relation between cohesion and elasticity. - Based on the

foregoing considerations,

a

rela tion might be anticipated between

the mechanical property of elas tic modulus and the more basic

property of cohesive energy. Figure

1 2

is

a

plot of

Young's

modulus of elasticity

as a

function of cohesive energy for va rious

face-centered-cubic metals. Where bonding occurs ac ro ss an

interface for like metals in contact,

i f

the contact ar ea were the

same in each case, the force to frac tur e the cohesive bonds of

lead would be considerably

less

than the force to fractu re iridium

cohesive bonds

(ref.

30). Since, however, the elastic modulus

for lead is considerably le ss than that for iridium, the contact

ar ea under

a

given load

w i l l

also

be

larger, and this increases

the for ce necessary to frac ture cohesive junctions where the ap-

plied load is the same for the two metals.

It

w a s

mentioned ea rl ie r that, for adhesive wear to occur,

interfac ial bonding between the two sur faces in contact must be

st ronger than cohesive bonding in one of the two solids.

LEED

600

1

-

120

c

/

L O

2

3

4

5

6

0

Young's modulus of elasticity, kg/cm2

I

I I

I I I

0

10 20 u)

40

50

60

Young's

modu l u s

of elast icit y, N / C J

Figure

12. -

Relation of Young's modulus

to

cohesive energy

for

var ious face-centered cubic metals.


32/190

26

FRI CTI ON, WEAR, AND LU BR IC AT IO N

I N

VACUUM

M etal couple

(low energy electron diffraction) studies with various face-

centered-cubic metal crys ta ls in contact show that, for dissim-

ilar

metals in contact, the adhesive junction a t the interface is

st ronger than the cohesive junctions in the cohesively weaker of

the two solids in contact. The data of table V present results

considering the effects of orientation, the effect of alloying, and

the effect of di ss imila r metal pairs. In each cas e of adhesive

contact, the cohesively weaker of the two face-centered-cubic

meta ls transferred to the

cohesively

stronger.

LEED pat terns a r e presented in figure 13 showing the

changes in the diffraction patte rn of the nickel (111) surface as

a

resu lt of adhesive contact. Figure 13(a) shows a clean nickel

(111) sur face before adhesive contact. In figure 13(b) that same

surface

is

shown

after

being contacted by copper. Copper ad-

herred to the cohesively stronger nickel. The copper accounts

Meta l wh ich t ra ns -

f e r r e d t o t h e o t h e r

s u r f a c e

Au

Au

Au

I

I

Orien ta t ion e f fec t s

(100) Au to (100) C u

(100) Au to (110) C u

(100)Au to

(111)

C u

Eff ects of a l loy con st i tuents

(111)

Au to

(111)

Cu-A1 al loysa

Au

O t h e r d i s s i m i l a r m e t a l p a i r s

(111)

A u t o

(111)

Ni

(111) Ag to (111) Ni

(111) C u to (111) Ni

(111) AI to (111) Ni

(111) Au to (111) Ag

(111) A u t o (111) A1

(111) P t t o (111) A1

ao. I

to

10 at . s

AI.


33/190


27

la ) Before contact.

bl Contacted

by

copper

c )

Contacted by lead.

Id )

Contacted by platinum.

Figure 13.

- LEED

photographs

of

nickel

(111)

surface before and after adhesive contact w it h var ious

metals. Contact load, 20 dynes ( ~ O X ~ O - ~

)

at 20 C 2 9 3 K I ; contac t time,

10

seconds at

lo-''

tor:.

for the additional diffraction spots in figure 13(b) not seen in

figure 13(a). The regular arrangement of the spots indicates

that the copper is present on the nickel in an ordered fashion.

hesive contact with lead. Note the large number of additional

diffraction spots due to the presence of the lead on the nickel.

Again, the cohesively weaker metal, lead in this instance, trans-

fe rr ed to the cohesively stronger nickel.

the

LEED

pattern of the nickel (111) surface after adhesive con-

tact with a platinum (111) surface. There a r e no new diffraction

Figure 13(c) repr esen ts the surface of figure 13(a) after ad-

The diffraction pattern of figure 13(d) shows the change in


34/190

28 FR ICT ION , WEAR, AND LU BR ICA TIO N I N VACUUM

spots, and the nickel spots have become elongated (compare

(a)

and (d) of fig. 13).

On tens ile fr ac tu re of the adhered pair,

nickel transfer red to the cohesively s tronger platinum. The

elongation of the diff raction spots

is

due to lattice st rain pro-

duced by the tensile fr ac tu re in the nickel.

occurs acr os s the interface, the theoretical strengths

of,

for

example, metals cannot b e used because the real strength of the

junction wil l be

so

much less.

There are

three

principal

reasons

w h y the theoretical strength and the re al strength

of

the junctions

will be different:

Fi rs t, with r ea l metals , on the application of s t r e s s in the

form of load to the surfa ces in contact and the removal of load,

there

w i l l

not be complete elasti c recovery, even where the

s t re sses applied a r e very low in relation to the yield strength.

tropy. With iron, for example, the modulus of elastic ity normal

to the (111) plane is 3 .0X lO kilograms per square centimeter,

and that normal to the

(100)

plane is 1 .3X10 kilograms per

square centimeter, or l es s than half the (111) plane.

6

used value for friction and wear su rfaces of 2 .0X lO kilograms

per square centimeter actually is an average of these two ex-

tre mes and

all

other intermediate orientations. Thus, for two

identical microjunctions topographically under the sam e load, the

final a r e a in tr ue adhesive contact

w i l l

differ simply if the orien-

tation

of

the grai ns vary.

junction differ because of the presence of defects. This

is

one of

the most important reasons in considering both adhesive and fa-

tigue wear.

The defects can be point defects, such

as

vacancies

and interstitials, or line defects, such as disloca tions and other

surface defects produced a t the inte rface between the two contact-

ing sol ids. These defects may be very much like those encoun-

tered at

a

grain boundary.

In the loading process used when two solid sur faces a r e

placed into contact,

energy

is

stored in the bodies. If the load

is

sufficiently light and only elas tic deformation occurs , the

When two solid su rfaces a r e placed into contact and adhesion

Second, re a l crysta lline solids exhibit substad tial aniso-

6

6

The usually

Third, the theoretical strength and the re a l strength of the


35/190

WEAR AND VARIOUS TYPES OF WEAR 29

energy input is equal to that stored up and the process

is

reversi-

ble. Where the energy exceeds

a

cr iti ca l point, instability r e -

sult s and the bodies can relax to

a

more stable s tate by plastic

flow or f racture or by flow followed by frac ture. For flow fol-

lowed by fracture, that portion

of

stored energy which is dissi-

pated by relaxation is an irreversible process.

tact in the weakest zones of the surficial regions. The fracture

process

is

a

progress ive separation of bonds star ting a t some

site, sur face or subsurface, where the dissipation of the input

energy associated with sliding o r rolling contact under load can-

not be dissipated. When

a

sufficient amount of energy, which

cannot be dissipated

as

heat, has been accumulated a t

a

particu-

lar site,

fracture

w i l l

occur. This may be

at

the adhesive junc-

tion, in which case no adhesive wear w i l l occur; or it may occur

in

the subsurface regions, in which case

a

particle may be r e -

moved from

a

surface. Fracture

is

most likely to be initiated at

a r e weaker

at

these sit es than in the normal structure.

sid er those phenomena that occur in mate rials that give r i s e to

the formation of an adhesive wear particle. A s already dis-

cussed,

the f i rs t thing that must happen is adhesion. What gives

rise

to

the

removal of ma te rial from one surface afte r adhesion

has taken place? The answer to this question is dictated by the

mechanisms of f racture and those fact ors that influence fr ac ture

mechanisms. Since nearly

all

materia ls a r e the subject of ad-

hesive

wear,

the fr ac tu re mechanisms that can give ri se t o the

generation of

a

wear part icle in the various classes of mater ial s

w i l l

vary.

relate to wear

by

fatigue

as

w e l l

as

adhesion.

With metal s,

if

single-crystal surfaces a r e in sliding con-

tact, deformation

w i l l

occur by slip. This involves the gliding

of one sl ip plane over another. With tangential motion of two

sur faces in contact, slip sometimes

w i l l

continue until complete

separation

has

occurred. The termination of the sl ip process

occurs when the two pa rt s

are

formed from

a

single one of the

Fracture

w i l l

occur in one or both of the solid bodies in con-

some imperfection in the su rfic ial lay er s because bond energies

The

adhesive

wear

particle.

-

At th is point it

is wel l

to con-

Many of the fr act ur e concepts to be discussed w i l l


36/190

30 FR ICT

ION, WEAR,

AND

LUBR

I CAT1

O N

I N VACUUM

two cry sta ls in contact.

ing fracture . It can occur, fo r example, where

a

hexagonal

metal is involved with bas al planes parall el to the surface.

Thus, sl ip in an asperi ty of a hexagonal metal can, when carried

to completion by the sliding process, give r i se to an adhesive

wear particl e due to shearing fractu re along the preferred (0001)

slip plane. This would be adhesive wear in it s simples t form.

In most engineering applications, metals

or

alloys used a r e

in

a

polycrystalline form whose pr efe rre d slip planes a r e not

parallel to the surface but rather a r e

at

a variety of orientations.

Sliding or rolling on these s urf aces

as

encountered in friction and

wear devices can produce pre ferred orientations on the surface by

the proc es s of texturing. This is shown by the sketch in figure 14

for a hexagonal metal such

as

beryllium. Once such surface tex-

turing has occurred, the polycrystalline surface may behave with

respect to shearing fractu re like the single-crystal surface.

In light of the foregoing discussion, it might be anticipated

that the fric tion force between hexagonal metals such

as

beryl-

lium, where texturing has occurred as shown in figure 14 and

where sl ip occurs only along the basa l planes,

w i l l

b e nearly the

same for single crys ta ls and polycrystals. This, in fact, has

been observed where the basa l plane in the single crystal is par-

allel to the surface (ref.

31).

This

is

commonly referred to as shear-

-

brasion direc tion

Normal

[Ooll

f iber orientat ion

Oblique

[Ooll

f iber orientation

[OOll

Figure

14. -

Diagrammatic form of fragmented surface region of abraded b er yl li um

cryst al (ref.

28).


37/190

WEAR AND VAR I O U S TYPES OF WEAR

3 1

With me tal s having sli p syst ems other than the basa l sli p

mechanism, the shearing process

is

complicated

b y

sl ip plane and

sl ip plane dislocation interaction.

centered-cubic metal s Lomer-Cottrell locks, due to the insection

of {

111}

slip plane dislocation, w i l l give r i s e to work hardening

and an increase in the force to shear.

Pa rt ic le generation by cleavage. - Frac ture by cleavage can

als o occur in the sur ficial reg ions of metals in contact giving

rise

to wear partic les. This usually occurs

at

low temperatures with

separation taking place along

w e l l

defined crysta llographic planes

producing the type of sur face ref er red to in reference to fig-

u r e l(b). No face-centered-cubic metal is known to fail by this

mechanism. Furthe r, in sliding friction exper iments with iron-

silicon crystals

(a

body-centered-cubic alloy), no evidence for

cleavage has been observed

at -195

C (78

K)

during the proce ss

of sliding (ref. 32).

Fracture for a number of mater ia ls , whether by initiation of

cleavage cra cks or in

a

plastic manner along slip planes, occurs

along

w e l l

defined planes. The planes involved fo r some typical

materials are shown in table VI (ref.

33).

There

is

a marked difference in micromechanism of f rac tur e

that occurs in surfic ial laye rs of metals, inorganic crystalline

sol ids indicated in table VI, and br it tle amorphous materials . In

amorphous materials, the stra in ra te is proportional to the ap-

plied s tr es s, and strain may be somewhat uniformly distributed.

The viscosity of br itt le ma ter ial s shows a notable temperature

dependency. Glass, for example , on changing from 80' to 60 C

(353 to

333

K) w i l l undergo

a

10 000-fold increa se in i ts viscosity

coefficient. At room temperature, flow in glass

is

practically

nil and ther e is no possibility of relieving st r e s s e s by flow. Met-

als and inorganic crysta lline solids , however, a r e relatively in-

sensitive to tempera ture with respect to re sis tan ce to plastic

flow. It

is

believed that, in metals and inorganic crystal line

solids, the origin of microcrack s li es in the hetergeneous nature

of the plastic flow proce ss of these ma te ri al s under applied

stresses.

For example, with face-


38/190

3 2

T e

Rock salt

Rock sa l t a t low tem -

pe r a t u r e

SrC12 in rock sa lt

SrC12 in rock sa lt

a t low tem pera tu re

Solid solution

of

Solid solu tion of

FR I C T I ON, WEAR, AND LUBR 1 CAT1 ON

I

N VACUUM

( i o i o )

(001)

(001)(011)

(001)

(001)

TABLE VI.

-

MODE O F FRACTURE O F METAL CRYSTALS

A N D

MINERALS (REF. 33)

Crys ta l s t ruc ture

Face- cen te red

cubic

Body-centered

cubic

Close-packed

hexagonal

Body- cen tere d

rhombohedral

Hexagonal

Cubic

M ater ia l C leavage o r f r ac ture

plane

c o

A1

Al-Zn solid solution

F e

Fe-Si alloy

Fe-Si alloy containing

ove r 4 percen t Si

or

a t low tem peratu re

Mo

C r

Mg

Zn

Zn containing 0. 13

percent Cd

Zn containing

0. 53 Cd

Not observed

Not observed

(111)

(oooi)(ioii)(ioi2)(ioio)

(0001)-

(0001)(1010)

(0001)

( l l1 )

Bi

Slip plane

(011)(123)(112)

(011)(123)(112)

(011)

(011)(123)(112)

?

(0001)

(000 1)

(0001)

(0001)

(0001)

There a r e mechanisms other than simple slip or cleavage

that can give r i se to the formation of

a

wear particle in crystal-

line solids. These more complex modes w i l l occur with mate-

rials commonly encountered in friction and wear surfaces.

Effect of inclusions. - The presence of obstacles in metals

can give r i se to dislocation coalescence and the initiation of sub-

surface cr ac k nuclei. The development of such voids

is

shown


39/190


33

in figure

15

(ref.

34).

This mechanism of cr ack initiation can

be sta rted by such obstacles

as

oxide inclusions in metal s such

as copper (ref. 35) and in iron by nitrogen and carbon (ref.

36) .

readily fo r sur faces in sliding o r rol ling contact. Oxide inclu-

sions a r e very prevalent in the nea r surface layer s of relatively

soft meta ls because these oxides can be buried by the sliding

process. Fur ther , with bearing and gear stee ls, carbide in-

clusions a r e always present. Thus, where adhesion occurs with

This type of subsurface void development can occur ve ry

0 . .

. . .

0 . .

0 . .

. . .

0 . .

0 . .

. . .

. . . .

0 0 . 0

0 0

i i i i

. .

. . .

0 0 . 0 0 . .

0 0 . 0 0 . .

0 0 . 0 0 . .

. . . . . . .

0 0 0 . 0

. . . . . .

. . . . . .

0 0 . 0

* .

(a) Extra row of atoms pu t (b) Collapse of atoms to

in lattice

t o

form edge mor e stable positions

dislocation. leaving crack nuc le us

below extra row of

atoms.

Obstacle Obstacle

u u u v w 1

-

(c) Dislocation pil eup against obstacles (w it h enough applied

force, en d pa ir s can coalesce).

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . .

. . . . . . .

0 . .

. .. .

*v;;

0. 77:..

0 0 . .

0 .

0 .

. . . . . .

0 .

* .

0 . . 0 . .

. . . .

0 . . 0 . . 0 . 0 .

Id ) Coalescence of two

( e )

Coalescence of three

dislocations. dislocations.

Figure

15.

- Formation of crack nuclei as result of dislocation

coalescence (ref. 34).


40/190

34

FR ICT ION , WEAR, AND LUB RIC AT ION I N VACUUM

such subsurface voids presen t, the weakest region may not be

the adhesive interfacial a r e a but rat her the cohesive subsurface

region about the void. On application of a force, the fracture

progr esse s from the subsurface void to the sur face by frac ture

of cohesive bonds more readily than by fracture of the adhesive

interfacial bonds. This re su lt s in the generation of an adhesive

wear

particle. Surfaces that a r e subjected to repeated loading

can develop wear par tic les readily because, with repeated rolling

or sliding, the ma teri al zone between the void and surface may

undergo an exhaustion in ductility. This w i l l result in the forma-

tion of disordered lay er s and the propagation of the subsurface

Figure 16. - Generation of

wear particle.


41/190

WEAR AND VARIOUS

T Y P E S

OF WEAR 35

void to the su rface as shown in figure 16. The ultimate result as

shown in figure 16

is

the development of the adhesive or fatigue

wear

particle.

cr ac ks and the s tart of the formation of an adhesive or fatigue

wear

particle. These mechanisms a r e presented schematically

in figure 17. The mechanisms were summarized by Cottrell

(ref.

37)

n refe rence to bulk ma terial behavior but they can be

applied here equally

w e l l

to near sur face phenomena. The

sub-

sur face cra ck developed from sliding on a lithium fluoride sur-

face (fig. 3(b)) resulted from the coalescence of (107) sl ip plane

dislocations to fo rm

a

crack along a (001) cleavage plane

as

shown schematically in figure 17(b). All the possib le fo rm s of

crack initiation shown in figure 17 can occur fo r solids in sliding

Ther e a r e other mechanisms that can initiate subsurface

(a) Crack formed by pileup

of dislocations

in

sli p band

against grai n boundary.

( b ) Coalescence of two s l i p dislocations

to f orm crack di slocatio n on cleavage

plane.

(c l Coalescence of two s lip bands to form

cleavage crack.

(d) Crack r esu lti ng from

shear on

two

bands.

(e ) Crack formed at tilt

boundary.

Figure

17.

-Mechanisms of crack init iat ion (ref. 37).


42/190

36 FR C T I ON, WEAR, AND LUBR CA T1 ON

I

N VACUUM

or rolling contact.

ized, there

is

plas tic instability. This plast ic instability can con-

cent rate shear on certain planes.

these bands of concentrated flow.

been conducted for br it tl e sol ids and not until recently has much

attention been paid to ductile frac tur e behavior. It is the ductile

frac tur e behavior that

is

most frequently encountered with su r-

fa ce s where friction and wear

is

involved. With respect to rea lly

br itt le solids, the Griffith theory (ref. 38)

is

applicable to frac-

tu re , and the mechanics of f racture by thi s mechanism have been

extensively examined for such materials by Irwin and associates

(refs. 39 to 41).

Ductility in metals. - With most crystalline solids in sliding

or rolling contact, plastic deformation

is

observed.

relatively bri ttle solids, such

as

aluminum oxide and lithium

fluoride, plas tic flow e xe rt s an influence on observed sliding be-

havior. Metals, which a r e generally considered ductile, vary in

th ei r ductility, and these varia tions can influence the mode of

frac ture . In general, meta ls that deform by slip

o r

le ss than

five independent systems cannot undergo significant plastic de-

formation before fracture .

criterion.

hexagonal metals such

as

zinc a t low temperatures. Basa l slip

provides only three slip systems.

other systems may be activated, these metal s exhibit excellent

ductility.

als

most commonly encountered in fric tion and wear sur faces,

the Von Mis cr ite rion

is

met a t all temperatures.

less,

with most body-centered-cubic metals,

a

ductile-brittle

transition

is

observed. This, of course,

w i l l

influence the na-

tu re of the frac tu re observed.

Alloying can influence the ductile-britt le transition behavior

of me ta ls such

as

iron.

Stoloff (ref.

42) has

shown that, for iron

base solid solutions, the alloying elements, cobalt, silicon, va-

nadium, and aluminum, suppress dislocation tangle formation

In surface microjunctions where plast ic flow i s very local-

Fracture can intiate along

Most fracture initiation and fracture mechanics studies have

Even with

This

is

the

so

called von

M i s 6 s

This accounts for the relatively brittl e behavior of

At higher tempera tures where

In

body-centered-cubic

(BCC)

metals , the type of met-

Neverthe-


43/190

WEAR AND VARIOUS TYPES

O F

WEAR 3 7

and markedly increase both yield s t r e s s and the ductile-brittle

transition tempera ture.

Nickel, in contrast, has lit tle influence

on dislocation subs tructu re and

is

not

as

effective in this r e -

spect.

that can give r i s e to the generation of adhesive wear par tic les

have been discussed as if they were two independent processes.

The adhesion pr ocess itself may,

however, give r is e to the de-

velopment of subsurface defects.

Lattice mismatch.

-

When clean copper and clean gold are

placed in contact in a vacuum environment

s o as

to maintain

clean surfaces, adhesion occurs ac ro ss the interface.

If

the

sam e atomic planes in copper and gold a r e used and the crystal-

lographic directions a r e matched acro ss the interface, LEED

studies have shown that the gold w i l l transfer epitaxially to the

copper (ref. 30). The trans fe r of the gold to the copper might

be anticipated from the ear li er discussion on cohesive energy

(see fig.

12).

Because the gold is bonded epitaxially to the cop-

pe r and the gold has lattice pa ra me te rs which differ from cop-

per, the gold atoms must undergo strain to enable them to take

on the lat tice charact er is tic s of the copper. The manner in

which th i s occurs is shown in figure

18.

copper-gold alloy and the arrangement of gold on the copper

surface in an epitaxial manner.

cohesive bonds occur as indicated.

contact

is

relatively large as it is for gold on copper, the mis-

match cannot be ent irely accommodated simply by st rain .

When the atomic di sreg is try becomes sufficiently large, misfit

dislocations like those shown in figure

19

w i l l

develop.

In refe rence to figure

19,

if an arb it ra ry point on the su r-

face

is

selected where a toms match ac ro ss the interface and

the re i s general lattice mismatch, as the matched pa ir of atoms

is moved

away ,

each successive row w i l l enounter an increa se in

the degree of mismatch. If the lattice pa ra me te rs do not differ

grea tly and there fore the mismatch is not large, as is the case

T h i s far, adhesion and the generation of subsurface defect s

Figure

18

indicates the atomic arrangement in an ordered

Lattice st ra in and frac ture of

Where the lattice mismatch between the two sur faces in


44/190

38

FRI CTIO N, WEAR, AND LU BRI CA TI ON

IN

VACUUM

Copper-gold alloy Adhesion

of

gold t o

copper epitaxially

o o o 3 o

Copper-gold- nterfacebonding

Fracture

of

gold

@cohesive bonds

Lattice str ain in gold on adhesion t o copper

Figure 18. - Atomic arra ngement a nd lat tice bonding.

with gold and si lver , the mismatch may be accommodated by

strain . Where the mismatch is large, as

is

the ca se for gold and

copper, misfit dislocations w i l l develop near the interface ( see

fig. 19) because st ra in alone cannot satisfy the atomic disreg-

is t ry .

The presence

of

such defects has been discussed in the

li tera ture in reference to the deposition of one mate ria l on an-

Interface

Figure

19.

- Accommodation of lattice mismatch i n copper-gold contact w it h mi sf it dislocation.


45/190

WEAR A N D VARIOUS

TYPES

OF WEAR

Metal Lattice

parameter ,

A

( o r I m )

Au 4.078

Ai3 4.086

A1

4 . 0 4 9

c u 3 .6 15

39

Percent m is - Fo rce

of

adh esion of

fit w ith (111) Au (111) to (111)

Au me tal surfa ce, a

dyne (or 10- N)

0 >400

* 19 >400

. 7 1 >400

11. 1 80

other (refs.

43

to 46).

u re

19

would reduce the number of gold bonds across the inter -

face. The average strength of the adhesive bond would be gre ate r

than the cohesive strength of the weaker of the meta ls (namely,

gold) and fracture would occur in the gold.

materia ls adhering one to another may in and

of

itself introduce

defects which w i l l reduce the strength of one of the ma te ri al s sub-

sur face and give r i s e on tensile fr ac ture to the formation of ad-

hesive wear.

ticipated that the gr ea te r the degree of lattice mismatch, the

grea te r should be

the

concentration of subsurface defects (i. e.,

misf it dislocations) and this should affect the fo rce necessary to

frac ture two surfaces in adhesive contact. Table

VII

presen ts the

resu lts of adhesion measurements for various face-centered-

cubic metal s to

a

gold

111)

surface. Adhesive contact

w a s

made

to the gold with the (111) su rface s of gold, si lver , aluminum, and

copper. Where the lattice misfit

w a s

le ss than

1

percent, the ad-

hesi ve force s exceeded 400 dynes

(400~10-~

)

with an applied

force of only 20 dynes

(20X10-5N).

With copper in contact with

The presence of the in terfacial mis fit dislocations in fig-

Thus, the process of

If

what has been said thus

far is

correct, then it might b e an-

TABLE

VII.

- EFFECT OF LATTICE MISFIT ON

ADHESION OF GOLD TO VARIOUS FACE


46/190

40 FR

I

CTI ON, WEAR, AND LUBR

I

CAT1 O N

I

N VACUUM

the gold and where the la ttice misf it

w a s

in excess of 11percent,

the adhesive force measured

w a s

only

80

dynes

80X10-5 N).

Even with

a

consideration of differences in cohesive energies and

deformation characteristics

as

were presented in figure

12 ,

the

degree of lattice misfi t and, therefore, the presence of surf icial

defects appear to have an influence on adhesive behavior.

FATIGUEE R

The concept of fatigue wear

is

normally associated

w i t h

fric-

tion,

wear,

and lubrication with repeated cycling in bearing or

gear components

(ref.

47). Fo r example, one of the major con-

cer ns of bearing des igners in the fatigue life of bearings . The

bearings operate in a normal environment and a r e well lubri-

cated. After

a

repeated number of s t r e s s cycles of the bea rings

during operation, the bearings

w i l l

fail by fatigue, Material

will have become dislodged, in the case of

a

ball bearing either

from the ball o r ra ce , destroying the usefulness of the bearing.

Wear by fatigue can al so occur during

dry

solid-state con-

tact . It can occur in vacuum for mater ia ls that do not adhere

strongly because of the presence of surface films or where there

is

a

lubricant film. When two surfaces

are

in rubbing contact,

the surface microcontac ts a r e subjected to both compress ive and

tensile forces. This fact has been determined experimentally by

Radchik an

effects of vacuum

Documents