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Environmental Dependence of Tribological Behavior of DLC Films
Se-Jun Park and Kwang-Ryeol Lee
Future Technology Research DivisionKorea Institute of Science and Technology
AEPSE 2003, Jeju, 2003. 10. 1.
Properties of Solid Carbon
Property Diamond DLC Graphite
Density (g/cm3) 3.51 1.8 – 3.6 2.26
Atomic Number Density (Mole/cm3)
0.3 0.2 – 0.3 0.2
Hardness (Kgf/mm2) 7000 - 10000 2000 - 8000 <500
Friction Coeff. 0.05 0.03 – 0.2
Refractive Index 2.42 1.8 – 2.6 2.15 – 1.8
Transparency UV-VIS-IR VIS-IR Opaque
Resistivity (cm) >1016 1010 - 1013 0.2 – 0.4
Tribological Properties of Hard Coatings
DLC
WC
TiN
CrN
TiCN
Wear Rate Friction Coefficient
2.0 1.6 1.2 0.8 0.4 0.2 0.4 0.6 0.8 1.0Relative value
Applications of DLC Film
Problems of DLC Films
• Thermal Instability– Degradation at High Temperature (400 – 600oC)
• High Residual Compressive Stress– Max. 10 GPa
• Poor Adhesion– Stable Chemical Bonds– Especially on Ferrous Materials
• Environmental Dependence of the Tribological Properties
Humidity Dependence of the Tribological Behavior
R. Gilmore et al Surf. Coat. Technol. 133-134, (2000), 437
Tribochemical Reaction
• The environmental dependence strongly implies that the tribochemical reaction between the test environment, the film and the counterface materials are significant.
DLC
Purposes of The Present Work
• To investigate systematically the friction behavior of DLC film in various test environments including relative humidities.
• To find the reason for the humidity dependence of the tribological behavior of DLC film in the point of tribochemical reaction.
• What happens in Si incorporated DLC films?
Film Deposition Condition
• RF PACVD(13.56 MHz)
• Precursor Gas : C6H6, C6H6 + SiH4,
• Deposition Pressure : 1.33 Pa
• Bias Voltage : - 400 Vb
Substrate : P-type (100) Si-wafer
• Film thickness : 1 ㎛
• Si concentration : 2 at.%
Friction Test
•Ball : AISI 52100 steel ball
•Normal Load : 4 N
•Sliding Speed : 20 cm/s
•Temperature : Room temperature
•Environmental Gas :
Ambient atmosphere
(relative humidity : 0 – 90 %)
High pure oxygen
Humidity Dependence of Friction
Pure DLC Si-DLC
Scar Surface with Humidity Variation
RH 0 % RH 50 % RH 90 %
250 ㎛
100 ㎛ 100 ㎛ 100 ㎛
Raman Spectra of the Transfer Layer
Chemical Composition of Debris
Wear Rate of Track and Ball
Track Ball
(e)
250㎛
(a)
250㎛
(c)
2 ㎛
(b)
2 ㎛
(d)
2 ㎛
(f)
250㎛
(e)
RH=0% RH=50% RH=90%
a-C:H
a-C:H
a-C:H
FeFe-O
FeFe-O
FeFe-O
Debris Composition & Friction
M. G. Kim et al., Surf. Coat. Tech. 112, 204 (1999).
In Dry Oxygen Environment
(a)
2 ㎛
(b)
2 ㎛
in Dry Oxygen Environment
a-C:H
Al2O3
a-C:H
Fe
Fe-O
a-C:H
The Environmental Dependence• The increased friction coefficient in humid
environment is closely related with the increased Fe concentration in the debris due to the enhanced surface oxidation of the steel ball.
• The humidity dependence is not an inherent property of the DLC films.
1. The Fe rich debris itself degrades the lubricating property of the DLC film.
2. The Fe rich debris enhance the agglomeration of small debris into larger one that requires larger energy dissipation to be deformed during sliding
1. The Fe rich debris itself degrades the lubricating property of the DLC film.
2. The Fe rich debris enhance the agglomeration of small debris into larger one that requires larger energy dissipation to be deformed during sliding
Chemical Composition of Debris with Humidity Change
Si-DLC DLC Wear Rate of the Ball
0 % 50 % 90 %
3 ㎛
3 ㎛ 3 ㎛
3 ㎛ 3 ㎛
3 ㎛
Si-DLC
DLC
Raman Spectra of the Transfer Layer
Si-DLC DLC
Scar Surface with Humidity Variation
100 ㎛ 100 ㎛
RH 0 % RH 50 % RH 90 %
250 ㎛ 250 ㎛ 250 ㎛
Friction in Dry Oxygen Environment
3 ㎛
DLC
Si-DLC
3 ㎛
Wear Rate of Ball and Track in O2 Env.
Raman Spectra of the Transfer Layer
The Environmental Dependence
• Fe rich debris formed by oxidation of the steel ball increased the friction coefficient in humid environment.
• Less dependent on the test environment– Bond structure of the debris varied with relative
humidity. Diamond-like structure in humid environment suppress the agglomeration of the debris.
Conclusions
• Humidity dependence of the friction behavior of DLC film is not an inherent property of the DLC film.
• Humidity dependence should be understood in terms of the tribochemical reaction of the tribo-system.
• Two major factors were suggested.– Fe concentration in the debris : Conterface Materials
– Debris agglomeration : Chemical bond of the debris
Acknowledgement
• Discussion with Dr. H-S. Kong, Dr. E-S. Yoon & Dr. J-K. Kim.
• Financial Support – Center for Nanostructured Materials Technology
– Center of Advanced Plasma Surface Engineering
– J&L Tech. Co., Ltd.