"All roads lead to Rome"
– On all the different routes
leading to triboformation
and self-alignment of
low-friction WS2 films
Fredrik Gustavsson
Ångström Tribomaterials Group
Uppsala University
[email protected]
?
Tungsten disulphide (WS2)
– How does it lubricate?
- Origin from mineral tungstenite, but is synthetically produced
- May form a lamellar structure with easily sheared planes
- Intrinsic lubricant - µ 0.002 in vacuum
- Sensitive to humidity -> higher friction (µ 0.2 -0.3) and rapid wear
- Similarities with graphite?
Anisotropic structure
Covalent bonds within the planes
Repulsion between planes
W, (Mo)
S, (Se)
Examples of ways of formation
Dry contact
PVD coatings of WS2
Pure
Alloyed
Metastable TiC coating containing S
sliding against a W countersurface
Lubricated contact
WS2 nanoparticles solved into oil
W-doped DLC coating + S from oil
additives in engine oils
W-doped DLC coating + S from fuel
Electrolytical coatings with WS2
nanoparticles
WS2 nanoparticles mixed with silver
for electrical sliding contact
Dry sliding
contact
PVD sputtered WS2 coatings (pure)
-
Low hardness: 1.5 - 3 GPa, E < 50 GPa)
Porous, columnar structure, low load carrying capacity
(Mechanical properties and structure depends on stoichiometry)
Very sensitive to humidity
µ 0.01 in dry air, 0.1 - 0.3 in humid air
Wear is negligible in dry air
but severe in humid air
100 μm
Crystallized and
aligned tribofilm
Mechanism: WS2 crystallizes to molecular
layers/planes in the contact, transfers to the
counter surface and align themselves parallel to
the surface => Low shear stress
Unaffected coating
F. Gustavsson, S. Jacobson, et al
100 μm
Dry sliding
contact
PVD (co-) sputtered WS2 coatings
Idea: To increase mechanical properties but maintaining the lowfriction properties, forming low shear strength tribofilms supported
by a hard coating.
Tabor-Bowden model: µ = Shear stress/Hardness
And to acchieve better resitance towards humid air
Many approaches have been tried:
Example 1: WS2 Alloyed with metals, e.g. Ti, Cr:
- Higher hardness: 5 – 13 GPa, E ~100-170 GPa
- Amorphous dense structure
- Improved wear resistance
- No larger improvement of friction properties in humid air
W-S-Cr: T. Polcar, F. Gustavsson, S. Jacobson et al
Dry sliding
contact
PVD (co-) sputtered WS2 coatings
Alloyed with light elements, e.g. C, N:
-
WS2 + C
Higher hardness: 5 – 10 GPa
Nanocomposite (WS2 + C)
or amorphous structure (WS2 + N)
Improvement in mechanical properties
Large improvements in sensitivity
towards humidity
WS2 + N
WS2 + C in air
RH ~30%
W-S-N: F. Gustavsson, T. Polcar, S. Jacobson, et al
W-S-C: T. Polcar, A. Cavaleiro, et al
50 nm
5 nm
Dry sliding
contact
WS2 + 30% N
µ~0.003 in dry air
Counter surface
WS2 is formed on the top surface
but with a large amount of WO3
present. N is removed completely
from the contact.
Coating
AES
Wear rate after 2 million passages
at ~1.2 GPa mean contact pressure:
2.1 x 10-11 mm3/Nm
~1 atomic layer per 250 passages.
F. Gustavsson, T. Polcar, S. Jacobson, et al
100 µm
Dry sliding
contact
PVD (co-) sputtered WS2 coatings
Alloyed with metals and carbon, e.g. Ti, Cr + C:
-
Higher hardness: 8 – 18 GPa
Nanocomposite or amorphous dense structure
depending on metal content
Much better mechanical properties
No improvement of friction properties
Increased metal content
ot to the sensitivity to humidity
More amorphous
Dry sliding
contact
WS2 + 40% C + 7% Cr
Aligned WS2
Amorphous
oxide layer with
streaks of WS2
“unaffected” coating
The transfer film has poor
adhesion, contains a lot of
oxides and is not fully
covering the contact
Only at the top surface of
the transfer film aligned
WS2 layers are seen
EELS
T. Polcar, F. Gustavsson, S. Jacobson, et
al
Dry sliding
contact
WS2 + ~70% C, 0 % Ti WS2 + 50% C, 30 % Ti WS2 + 30% C, 30 % Ti
WS2 + Ti + C
Pt/C from FIB
6 GPa
Tribofilm
”Unaffected” coating
TiO2
12 GPa
18 GPa
Different structures were seen
depending on the amount of Ti and C added.
Friction levels varying, with best results ~0.02
in a non-humid environment
Mechanism by adding metal and C:
The metal forms oxides that stays in
the contact and increase the friction, while
carbon is removed from the contact.
WS2 is formed, but is primarily present
in the interface between the surface where
it can help reduce µ.
Transferfilms are unstable with poor
adhesion to the steel ball surface
H. Nyberg, J. Sundberg , F. Gustavsson, S. Jacobson, et al
Dry sliding
contact
TiC coatings deposited by reactive sputtering with H2S gas
Nanocomposite TiC grains in a disordered C matrix.
S goes into the TiC structure
metastable TiCxSy phase (slightly larger unit cell than TiC)
Mechanism: Sulphur released from the
coating during sliding reacts with W on
the ball and forms WS2 clearly decreasing
friction
Sliding in
dry air RH < 1 %
J. Sundberg, H. Nyberg, S. Jacobson, et al
Dry sliding
contact
Electrolytical coatings with
WS2 Nanoparticles
µ 0.008 in non-humid
environment despite
very inhomogenous and
rough coating
100 nm
In humid air the whole layer of nanoparticles
is removed rapidly
Mechanism: Nanoparticles are crushed
and smeared out in the contact, forming a soft
WS2 film on the surfaces with the already
crystalline sheets/shards
F. Gustavsson, S. Jacobson, et al
Dry sliding
contact
WS2 nanoparticles mixed with silver
for use in electrical sliding contacts
Pure silver => Very low contact resistance,
but high friction and wear
Silver + WS2 => Still low contact resistance,
but low friction and better wear properties
WS2 nanoparticles were burnished
into the soft silver material
Mechanism: nanoparticles breaks and
forms WS2 in the interface with low friction,
and as a semiconductor WS2
also has good electrical properties
B. André, U. Wiklund et al
Lubricated
contact
WS2 nanoparticles solved in PAO base oil
PAO+ 1% IF WS2 (+dispergent)
Steel vs steel 100Cr6 (~8 GPa ), 1 Hz
Load: 10N ~ 0.7 GPa mean contact pressure
Friction boundary lubrication
20-30 % reduction in friction
Wear ~ 5-7 times higher for pure PAO
0.12
0.12
0.10
0.10
0.08
0.08
PAO+IF
PAO
0.06
0.04
0.04
0.02
0.02
0.00
0.00
0
5000
10000
15000
20000
25000
Revolutions
Rough surface: Ra 100 nm, Δq: 3
F. Svahn, et al
30000
PAO+IF
PAO
0.06
0
5000
10000
15000
20000
25000
30000
Revolutions
Fine polished surface: Ra: 5 nm, Δq: 0.1
Lubricated
contact
WS2 nanoparticles solved in PAO base oil
Mechanism in oil
3.
Adhesion of WS2 layers 4.
2.
Rupture of particles to the surfaces
Shearing of basal planes
1.
Particles enters
the contact
Boundary lubrication
Lubricated
contact
W containing amorphous carbon coating in oil
Steel ball
W and C containing coating ~2 µm
Lubricated with PAO base oil + EP and AW additives
Boundary lubrication
Mechanism:
S from the EP additive reacts
with W from the coating and
forms WS2 on the surfaces
Dramatic reduction of friction
Effect enhanced by higher
contact pressures and T
but reduced if the amount
of AW additives is too high
B. Podgornik, N. Stavlid, S. Jacobcon, et al
Courtesy: B. Podgornik
Lubricated
contact
W-containing amorphous carbon coating in fuels
Counter surface wear was reduced
dramatically in Diesel and FAME
compared to a non alloyed
amorphous carbon coating
Friction was reduced by up
to 40% in some of the tests
The coatings showed negligible wear
DLC with W top layer
DLC with C top layer
Mechanism:
S from the fuel (ppm levels)
reacts with W from the coating
And forms WS2 in the contact
Reducing friction and wear of the ball
F. Gustavsson, P. Forsberg, S. Jaobson et al
Surface analysis
proved the formation
of WS2 in the contact
correlating with low µ
and low wear of the
counter surface
Steel ball
W and C containing
DLC coating ~2 µm
Boundary lubrication
Conclusions
- Alloying WS2 coatings increases the mechanical properties,
but also changes the friction properties. The added elements
contributes differently and influence the conditions of the
contact.
- All elements present in the sliding interface influence the WS2
lubrication properties negatively. Best results seen for N and
C, which are removed from the contact
- Added metals form oxides that generally has a large negative
effect on the friction and wear. However, formation of WO3
appears to not be as bad as TiO2, Cr2O3 and Fe2O3
- WS2 has the possibility to regenerate and is continuously
formed as long as the coating sustains
Conclusions
- When W and S are present in a tribological system
(even for ppm levels of S) the formation of WS2 is highly
probable, often with a large reduction of friction (and wear)
- The formation of smooth and fully covering tribofilms should
take place on both surfaces for the best effect
- Oxides and other reactive compounds (AW-additives) inhibits
the formation of the tribofilms
- The high probability of forming WS2 has been capitalized in
several applications with very promising results
Thank you for listening!