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.