MITC 2013 　　 2013. 11. 9

Boundary Lubrication as Surface Chemistry

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Shigeyuki Mori

Tribology and Surface Chemistry

Iwate University

１． Surface chemistry２． Chemical Analyses of BL３． Tribochemical Reaction４． Conclusions

Mt. IwateMorioka

１． Surface chemistry２． Chemical Analyses of BL３． Tribochemical Reaction４． Conclusions

Space station

Tribology and Surface Chemistry

shear

Oil moleculeBall

Disk

Complicated phenomena

Lubricating contact

Simplification

Components

Contact conditions Materials

Atmosphere

1

2

M

S

C R

F

I

YO

X

Q

M

S

C R

F

I

YO

X

Q

Friction of clean metals and influence of adsorbed gases

F. P. Bowden and T. P. Hughes, Proc. Roy. Soc., A172 (1939) 263.

Hg diffusion pump

There is no simple method of determining the nature or the amount of any impurity which may remain on a metal surface.

Glass chamber

metal slider metal wire

Study on boundary lubrication under vacuum

Prof. Gerhard Ertl

Chemical Processes of Solid Surfaces

PEEM

N2 + 3H2 　　→　 2NH3

Haber-Bosch method(1913)

Fe

2007 Nobel prize　

Chemical composition

CO

O2

0 s 10 s 20 s 30 s Time

CO + 1/2O2 → CO2

Pt

2D100 μm

3

１． Surface chemistry２． Chemical Analyses of BL３． Tribochemical Reaction４． Conclusions

Time

FTIR

AES

XPS

EPMAelement

bonding

Sizeμm nm

ChemistryTOF-SIMS

TEM-EELS

mm

composition

AFM

Raman

XANES

Spatial resolution

Ch

emic

al r

eso

luti

on

Surface analytical tools

S

FeS2

FeSO4

FeSR

RSSR

S

Sulfur compounds

Mechanical properties

Chemical structure

Where ? What? When?

4

Principle of Time of Flight Secondary Ion Mass Spectroscopy

TOF-SIMS

Advantages of TOF-SIMS1.High sensitivity less than monolayer2.High mass resolution chemical characterization3.High 2D resolution 1 μm

Ion gun

Sample

Detector

m = (2eU/L2) t2

m/e is estimated by time of flight of secondary ions

5

Chemical analysis of boundary films formed from PTFE in PEEK

PTFE

PEEK

Boundary film

Bearing composite

Steel ring

6

PEEK + PTFE0.5 to 6 wt%

Contact pressure 2 to 12 MPaSliding speed 2 m/sLubricant oil 40 ℃

7

Mass spectrum

Chem. structure

Chemical image

Distribution of tribo-film

2D 1μm

Thickness less than 1 nm

Chemical analysis of tribo-films by TOF-SIMS

what ？

where ？

Chemical images of boundary layer from PTFE in PEEK-PTFE composite

Massspectrum

8

TOF-SIMS mass spectrum of material surface

300 1000Mass number

CF2 = 50

PTFE = (CF2CF2)n

9

Chemical images of CF+ from PTFE on composite material after tribo-tests at different contact pressures

Contact pressure

10

Effect of contact pressure on surface coverage of PTFE

0

20

40

60

80

100

120

0 5 10 15 20 25 30

　 MPa面圧 （ ）

　Ｐ

ＴＦ

Ｅ被

覆率

（％）

Seizure

Seizure was occurred at 20% of the coverage of PTFE-tribofilm.

PTFE Coverage ＝ constant×

Contact pressure (MPa)

PT

FE

Cov

erag

e (%

)

PTFE intensity

Fe intensity

11

PTFE

（ CF+)

Steel

（ Fe+)

③6→18→6MPa②18MPa①6MPa

Removal and recovering of tribofilm

Chemical images of PTFE and steel components on bearing material

12

Formation model of tribofilm from PTFE-PEEK composite

Removal of tribofilm

High contact pressure

Recovery of tribofilm

Low contact pressureLow contact pressure

Formation of tribofilm

steel

PEEK + PTFE

Summary

Contact Pressure

Time

13

Observation of tribochemical degradation of nano-lubricant film on hard disks

Magnetic layer

DLC (3 nm)

Lubricant oil (1 nm)Hard disk

Head

HOCH2CHCH2OCH2-CF2O(CF2CF2O)m-(CF2O)nCF2-CH2OCHCH2OH

OHHO

Perfluoropolyether (PFPE) oil

detected by TOF-SIMS

14

Tribo-tester　installed　in　TOF-SIMS　

Rotating assembly

15

Friction tester of a hard disk in the main chamber of TOF-SIMS

1.8 inch magnetic disk

slider

load ： 0.2 ～ 0.8 mNvelocity ： 8 rpm (0.01m/s)

Conditions

Analyzing position

coating ： Al2O3 ， TiN ， DLC ， c-BN

0.5μ ｍ

50μ ｍ

Si tip

10 mm

Fomblin Z-dol

16

Mass (amu)

0 50 100 150 200

Inte

nsi

ty，

10

6

counts

0

0.5

1.0

2.0

1.5

2.5

C+

CF+

CF2+

CF3+CFO+

C2F4+

C2F5+

Mass spectrum of hard disk

What? and Where?

20 µm

Chemical image of friction track

Friction track

Original surface

CF+

Al2O3

17

Subtracted mass spectrum of TOF-SIMS

0 50 100 150 200

CF2+ CF3

+

CFO+

C2F4+

C2F5+

C+

CF+

27

0

20000

40000

-20000

-40000

Component in friction track

In original surfaceInte

nsity

， c

ount

s

Mass number (amu)

Al2O3 slider ， 0.8mN ， friction repeated 100

Chemical image of friction track

?

18

　 Al species 　⇒　where ？

102.6 102.8 103.0 103.2 103.40

50

100

150

200

250

mass

AlF4-

(c)　track（ negative　ion)

Al+C2H3

+

26.6 26.8

27.0 27.2 27.40

200

400

600

26.6 26.8 27.0

27.2 27.40

200

400

600

800

C2H3+

（ a ）　 track

（ b ）　 original　surface

Ion

inte

nsity

, cp

s

Chemical　wearAl2O3 + (CF2O)

AlF3 + CO2

Material　transfer　of　Al　on　disk　surface

Sapphire(Al2O3)

Disk

20　µm

Al+ 　

sapphire(Al2O3)

Disk

TOF-SIMS analysis

19

(a) 　 CF+ chemical image

CF+

0 20 40 60 80 100 120

Position ， μ ｍ

0

5

10

15

20

CF

＋in

tensi

ty，

Counts

(b) Profile of ion intensity

　　　 lubricant loss

20

0

20

40

60

Al2O3 DLC ｃ‐ BNTiN

Slider materials

Lubr

ican

t lo

ss

，C

ount

s×μ

m

　　 Effect of slider material on lubricant loss

lubricant ： FOMBLIN ZDOL

load ： 0.8 mN, speed ： 8 rpm, friction repeated ： 400

Fragment ion ：C2F4

+

Lewis acid

21

Long life of head-disk interface(HDI)

3D analysis of chemical structure at friction truck

lubricant(1 nm)DLC

Al2O3 ・TiChard

Chemically stableDLC

TOF-SIMS analysis

Long lige

To develop a new tribo-system for HDI

A better combination of lubricant and material

22

Organic additives Organic contaminants

C, H, O

2H(D), 13C, 18O

Stable isotope as a tracer

Tracer method using stable isotopes

Analysis with TOF-SIMS

23

Lubrication of nitrides with ethyl alcohol

Y. Enomoto, S. Mori, et al.,(2010)

Surface product analyzed by TOF-SIMS

Wear track Outside

CrN

CD3CD2OHTiN or CrN?

CH3CH2OH

TiN

CrNFric

tion

coef

ficie

nt

Time, s

24

１． Surface chemistry２． Chemical Analyses of BL３． Tribochemical Reaction４． Conclusions

・ ・ ・・ ・・ ・ ・・ ・

Causes of tribochemical reaction

Ball

Space station

Tribology and Surface Chemistry

DiskBase oil additive

Reaction Conditions Surface Activities

High temp. + high pressure nascent surfacedefectradical site・・・・・・

Mechanical Energy

Boundary films are formed through tribochemical reactions.

25

Metal oxide

Organic contaminant

Nascent surface

adsorption desorption

Monitoring the chemical process on nascent surface

Surface defects

Monitor with Q-mass

26

Sliding speed 2, 3, 4, 5, 6 cm/s

Load 2, 4, 8, 12, 16 N

Temp. R.T.

Vacuum less than 2×10-4 Pa

Table 1 Conditions

To TM pump

Q-mass

Fig.6 Experimental apparatus with Q-mass

Magnetic rotating assembly

Lub. oil disk

ballload

Ion gauge

Ball Disk

Material 52100

Diameter

6.25mm 20mm

Table 2 Test piece

Friction force

Variable leak valve

27

Time, s

m/e = 84

m/e = 78

Inte

nsi

ty, 1

0-

9 A

m/e = 82

20 40 60 80 1000

10

8

6

4

0.6

0.40.2

12Austopstart

Gold becomes to be active chemically by scratching

Catalytic dehydrogenation and hydrogenation of olefin by nascent surface of Au

+ H2

Au

adsorption

desorption

28

Metal oxide

Organic contaminant

Nascent surface

adsorption desorption

Monitoring the chemical process on nascent surface

Surface defects

Monitor with Q-mass

(RO)3P=OPolar compd.

RSSRNon-polar compd.

Additives are effective under

severe condition Mild condition

EP additives

29

Load

Metal oxide

ball

friction

Organic contaminant

Steel disk

Active sites on nascent surface

Catalytic effect

Hydrocarbon oilH2, CH4

・・・

Frictional heat

Tribochemical decomposition of hydrocarbon oil

30

Gas evolution during lubrication (MAC, 2cm/s, 8N, 4km)

Rd=C2(P2-P)/kT=C2ΔP/kT sliding

Rate of gas formation

MAC: multi-alkylated cyclohexane

Decomposition of hydrocarbon oil

31

Conclusions

1. Chemical analyses of boundary layers TOF-SIMS is a powerful tool to analyze boundary films and

tribochemical reactions. TOF-SIMS analyses revealed that very thin layer of PTFE was

removed at higher contact pressure and was recovered under the sliding condition of low contact pressure.

It was found that thin layer of PFPE oil was decomposed catalytically by Al2O3 and TiN as a slider material, but the decomposition was deactivated by DLC and c-BN coating. Chemically inert coatings such as DLC make a longer life of HDI.

Stable isotopes such as D(2H), 13C and 18O can be used as a tracer to detect boundary film without the effect of organic contamination.

Boundary lubrication from the viewpoint of surface chemistry

2. Nascent surfaces of metals as a active source of tribochemical reactions are characterized by our method.

Even gold becomes to be active chemically and benzene decompsed on nascent gold surface.

Sulfide is more active on nascent steel surface than phosphate.EP additives should be selected depending on lubricating conditions;Sulfides and phosphates are effective under severe and mild conditions, respectively. It was found that hydrocarbon oil was decomposed by the effect of nascent surface and temperature rise at mechanical contact. Phosphate is more effective to reduce the decomposition than sulfide.

Thank you for your attention

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