MITC 2013
2013. 11. 9
Morioka
Mt. Iwate
Boundary Lubrication as Surface Chemistry
１．Surface chemistry
２．Chemical Analyses of BL
３．Tribochemical Reaction
４．Conclusions
Iwate University
Shigeyuki Mori
Tribology and Surface Chemistry
Photo by Obara
１．Surface chemistry
２．Chemical Analyses of BL
３．Tribochemical Reaction
４．Conclusions
Simplification
Oil molecule
Ball
Complicated phenomena
shear
Atmosphere
Disk
Materials
Contact conditions
Components
Space station
Tribology and Surface Chemistry
Lubricating contact
1
Friction of clean metals and influence of adsorbed gases
F. P. Bowden and T. P. Hughes, Proc. Roy. Soc., A172 (1939) 263.
S
Glass chamber
M
F
Q
X
C
metal slider
I
Y
O
Hg diffusion pump
R
metal wire
There is no simple method of determining the nature or the amount
of any impurity which may remain on a metal surface.
Study on boundary lubrication under vacuum
2
Chemical Processes of Solid Surfaces
0s
10 s
20 s
30 s
Time
2007 Nobel prize
100 μm
2D
Pt
CO + 1/2O2 → CO2
Prof. Gerhard Ertl
N2 + 3H2
CO Chemical
O2 composition
Fe
→ 2NH3
Haber-Bosch method(1913)
PEEM
3
１．Surface chemistry
２．Chemical Analyses of BL
３．Tribochemical Reaction
４．Conclusions
Surface analytical tools
Where ? What? When?
Chemistry
Chemical resolution
TOF-SIMS
bonding
S
TEM-EELS
Raman
FTIR
XPS
XANES
Sulfur compounds
composition
S
FeS2
AES
Chemical structure
element
FeSO4
FeSR
EPMA
RSSR
AFM
mm
Time
μm
Spatial resolution
nm
Size
Mechanical
properties
4
Ion gun
Detector
m/e is estimated by time of
flight of secondary ions
Sample
m = (2eU/L2) t2
Advantages of TOF-SIMS
1. High sensitivity less than monolayer
2. High mass resolution chemical characterization
3. High 2D resolution 1 μm
Principle of Time of Flight Secondary Ion Mass Spectroscopy
TOF-SIMS
5
Chemical analysis of boundary films
formed from PTFE in PEEK
Bearing composite
PTFE
PEEK
Boundary film
Steel ring
6
Contact pressure 2 to 12 MPa
Sliding speed
2 m/s
Lubricant oil
40 ℃
PEEK + PTFE
0.5 to 6 wt%
7
Chemical analysis of tribo-films by TOF-SIMS
Chemical image
what？
Mass spectrum
Distribution of tribo-film
2D 1μm
Chem. structure
where？
Thickness less than 1 nm
Mass
spectrum
Chemical images of boundary layer from PTFE
in PEEK-PTFE composite
8
PTFE = (CF2CF2)n
CF2 = 50
300
Mass number
1000
TOF-SIMS mass spectrum of material surface
9
Contact pressure
Chemical images of CF+ from PTFE on composite
material after tribo-tests at different contact pressures
10
Seizure was occurred at 20% of the coverage
of PTFE-tribofilm.
ＰＴＦＥ被覆率　（％）
Coverage (%)
PTFE
120
100
80
Seizure
60
40
20
0
0
5
10
15
20
25
30
Contact
pressure (MPa)
面圧　（MPa）
Effect of contact pressure on surface coverage of PTFE
PTFE intensity
PTFE Coverage ＝constant×
Fe intensity
11
Chemical images of PTFE and steel components on bearing material
①6MPa
②18MPa
③6→18→6MPa
PTFE
（CF+)
Steel
（Fe+)
Removal and recovering of tribofilm
12
Summary
Removal of tribofilm
Contact Pressure
High contact pressure
Formation of tribofilm
Recovery of tribofilm
PEEK + PTFE
steel
Low contact pressure
Low contact pressure
Time
Formation model of tribofilm from PTFE-PEEK composite
13
Observation of tribochemical degradation of
nano-lubricant film on hard disks
Lubricant oil (1 nm)
Hard disk
DLC (3 nm)
Magnetic layer
Head
detected by TOF-SIMS
HOCH2CHCH2OCH2-CF2O(CF2CF2O)m-(CF2O)nCF2-CH2OCHCH2OH
OH
HO
Perfluoropolyether (PFPE) oil
14
Rotating assembly
Tribo-tester installed in TOF-SIMS
15
50μｍ
coating：Al2O3 ，TiN ，DLC，c-BN
0.5μｍ
Si tip
10 mm
slider
Analyzing position
1.8 inch magnetic disk
Fomblin Z-dol
Conditions
load ：
0.2～0.8 mN
velocity ： 8 rpm (0.01m/s)
Friction tester of a hard disk in the main chamber of TOF-SIMS
16
Intensity， 106 counts
2.5
Mass spectrum of hard disk
2.0
C+
1.5
CF+
1.0
C2F5+
CFO+ CF3+
CF2+
0.5
C 2 F4 +
0
0
50
100
150
200
Mass (amu)
Chemical image of friction track
What? and Where?
CF+
Friction track
20 µm
Al2O3
Original surface
17
Intensity， counts
Chemical image of friction track
?
40000
20000
C+
Component in friction track
27
0
-20000
-40000
0
CF+
CFO+
CF2+ CF3+
In original surface
C2F4+
50
C2F5+
100
150
200
Mass number (amu)
Subtracted mass spectrum of TOF-SIMS
Al2O3 slider，0.8mN，friction repeated 100
18
600
（a） track
C2H3+
400
Ion intensity, cps
Al+
sapphire(Al
O3)
20 2µm
Al+
TOF-SIMS
analysis
200
Disk
8000
600
26.6
26.8
（b） original surface
27.0
27.2
27.4
Sapphire(Al2O3)
C2H3+
400
200
Disk
0
26.6
250
200
150
26.8
27.0
27.2
27.4
Material transfer of Al
on disk surface
Al species ⇒
AlF4-where？
(c) track
（negative ion)
Chemical wear
100
Al2O3 + (CF2O)
50
0
102.6
102.8
103.0
mass
103.2
103.4
AlF3 + CO2
19
CF+
(a) CF+ chemical image
(b) Profile of ion intensity
lubricant loss
CF＋intensity，Counts
20
15
10
5
0
0
20
40
60
80
100
120
Position，μｍ
20
Lubricant loss， Counts×μm
Fragment ion： C2F4+
60
40
Lewis acid
20
0
Al2O3
TiN
DLC
ｃ‐BN
Slider materials
Effect of slider material on lubricant loss
load ： 0.8 mN, speed ： 8 rpm,
friction repeated ： 400
lubricant ： FOMBLIN ZDOL
21
Long life of head-disk interface(HDI)
Al2O3・TiC
DLC
hard
Chemically stable DLC
lubricant(1 nm)
Long lige
TOF-SIMS analysis
3D analysis of chemical structure at friction truck
To develop a new tribo-system for HDI
A better combination of lubricant and material
22
Tracer method using stable isotopes
Organic contaminants
Organic additives
C, H, O
Stable isotope as a tracer
2H(D), 13C, 18O
Analysis with TOF-SIMS
23
Lubrication of nitrides with ethyl alcohol
CH3CH2OH
CD3CD2OH
Friction coefficient
TiN or CrN?
TiN
CrN
CrN
Time, s
Wear track
Outside
Surface product analyzed by TOF-SIMS
24
１．Surface chemistry
２．Chemical Analyses of BL
３．Tribochemical Reaction
４．Conclusions
Boundary films are formed through tribochemical reactions.
Ball
Causes of tribochemical reaction
Mechanical Energy
・ ・ ・・ ・
・ ・ ・・ ・
Base oil
Disk
additive
Reaction Conditions
High temp. + high pressure
Space station
Tribology and Surface Chemistry
Surface Activities
nascent surface
defect
radical site
・・・・・・
25
Monitor with Q-mass
adsorption
desorption
Organic contaminant
Surface defects
Metal oxide
Nascent surface
Monitoring the chemical process on nascent surface
26
Variable leak valve
Ion gauge
Friction force
To TM pump
ball
load
Lub. oil
disk
Q-mass
Magnetic rotating assembly
Fig.6 Experimental apparatus with Q-mass
Table 1 Conditions
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 2 Test piece
Ball
Material
Diameter
Disk
52100
6.25mm
20mm
27
12
Intensity, 10-9 A
10
Au
stop
start
adsorption
m/e = 82
8
6
m/e = 78
4
0.6
0.4
0.2
m/e = 84
desorption
0
20
40
80
60
100
Time, s
Gold becomes to be active chemically by scratching
+ H2
Au
Catalytic dehydrogenation and hydrogenation of olefin by nascent surface of Au
28
Additives are effective under
Monitor with Q-mass
severe condition
Non-polar compd.
RSSR
adsorption
Mild condition
Polar compd.
(RO)3P=O
EP additives
desorption
Organic contaminant
Surface defects
Metal oxide
Nascent surface
Monitoring the chemical process on nascent surface
29
Hydrocarbon oil
Organic contaminant
Metal oxide
Load
H2, CH4 ・・・
Active sites on nascent surface
ball
Steel disk
Frictional heat
Catalytic effect
friction
Tribochemical decomposition of hydrocarbon oil
30
Rate of gas formation
Rd=C2(P2-P)/kT=C2ΔP/kT
sliding
Decomposition of hydrocarbon oil
Gas evolution during lubrication (MAC, 2cm/s, 8N, 4km)
MAC: multi-alkylated cyclohexane
31
Conclusions
Boundary lubrication from the viewpoint of surface chemistry
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.
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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