Humidity Dependence of Humidity Dependence of Tribological Behavior of DLC Tribological Behavior of DLC FilmFilm

Se Jun Park*#, Kwang-Ryeol Lee*, Seung-Cheol Lee* and Dae-Hong Ko#

*Korea Institute Science and Technology#Yonsei University

Tribological Properties Tribological Properties of Hard Coating Materialsof Hard Coating Materials

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

Dependence of Tribological Behavior Dependence of Tribological Behavior of DLC Films on Humidityof DLC Films on Humidity

It is important to understand the tribological behavior of DLC films in humid environment

R. Gilmore et al Surf. Coat. Technol. 133-134, (2000), 437

Previous WorksPrevious Works

Surface smoothness of DLC film by chemisorbed oxygen

The increase of real contact area

The prevention of the growth of the carbonaceous transfer film by the water vapor

• C. Donnet et al., Tribo. Letters, 4 (1998) 259

• B. Marchon et al, IEEE Trans. Magn. 26 (1990) 2670

Tribology Process of DLC FilmTribology Process of DLC Film

DLC

Counterface

Wear

Reaction with environment

Reactionwith environment

Materialtransfer Transfer layer

FF

FN

Friction Coefficient

F

N

FF

=

Tribo-chemical reaction

Purposes of Present WorkPurposes of Present Work

To characterize the tribological behavior of DLC film with the humidity change in various tribo-system.

• DLC against steel ball• DLC against DLC coated ball

To find the reason for the humidity dependence of frictional behavior of DLC film in view of tribo-chemical reaction.

Deposition ConditionDeposition Condition

•RF PACVD(13.56 MHz)

•Precursor Gas : C6H6

•Deposition Pressure : 1.33 Pa

•Bias Voltage : - 400 Vb

•Substrate : P-type (100) Si-wafer

•Film Thickness : 1 ㎛

•Residual Stress : 0.9 ± 0.02 Gpa

•Hardness : 10.5 ± 0.5 GPa

Tribo-test ConditionTribo-test Condition

• Ball-on-disc type tribometer isolated by chamber

• Ball : AISI 52100 steel ball

DLC coated steel ball

• Normal Load : 4 N

• Rotation Speed : 220 rpm

• Temperature : Room temperature

• Environmental Gas :

Ambient atmosphere

(Relative humidity : 0 – 90 %)

Film

Hygrometer

NormalLoad

Loadcell

Humidity controller

Rotary Pump

Friction Coefficient with Humidity Friction Coefficient with Humidity Change Using Steel ballChange Using Steel ball

Images of Transfer Layer on ScarImages of Transfer Layer on Scar

RH 0 %

100 ㎛

RH 50 %

100 ㎛

RH 90 %

100 ㎛

Morphology of Debris on TrackMorphology of Debris on Track

2 ㎛ 2 ㎛ 2 ㎛

RH 0 % RH 50 % RH 90 %

Relativehumidity

Size ofdebris

Frictioncoefficient

Chemical Composition of DebrisChemical Composition of Debris

Chemical Composition of DebrisChemical Composition of Debris

• Kim et al, Surf. Coat. Techno. 112 (1999) 204

Chemical Composition of DebrisChemical Composition of Debris

Wear of steel ballFriction

coefficientFe concentration

in debris

Friction Behavior with HumidityFriction Behavior with Humidity

RH : 50 % RH : 0 % RH : 90 %

Steel

DLC

Steel

DLC

C-O-Fe C-O

Steel

DLC

Fe-Olow oxidation of ball high oxidation of ball

Frictioncoefficient

RelativeHumidity

Formation of Fe-rich debrisIncrease of the debris size

MotivationsMotivations

Steel

DLC

DLC

If the formation of Fe-rich debris was suppressed ?

Friction Coefficient with Humidity Friction Coefficient with Humidity ChangeChange

Chemical Composition of DebrisChemical Composition of Debris

Micrographs of Debris on Track Micrographs of Debris on Track with Humidity Changewith Humidity Change

0 % 50 % 90 %

3 ㎛ 3 ㎛ 3 ㎛

Friction Coefficient with Humidity Friction Coefficient with Humidity ChangeChange

Raman Spectra of Transfer LayerRaman Spectra of Transfer Layer

Steel ballDLC coated ball

Schematic Diagram of Friction Schematic Diagram of Friction Behavior in high humidityBehavior in high humidity

Increase of debris size

Formation of Fe-rich Debris Formation of graphitic layer highly

sensitive to humidity Degradation of lubricant properties of

DLC film

C-O-Fe

Formation of C-O Debris The diamond-like bonding

structure of the transfer layer

Steel

DLC

Steel

DLC

DLC

The Role of Fe in DebrisThe Role of Fe in Debris

Steel

Fe-rich debris

Steel

Fe-rich debris

RH : 90 % RH : 0 %

Formation of graphitic layer highly sensitive to humidity Degradation of lubricant properties of DLC film

Abrupt change of humidity

If the Fe in debris degraded the lubricant properties of DLC film

The friction coefficient slowly dropped though the humidity abruptly drop

Friction Coefficient with HumidityFriction Coefficient with Humidity

The effect of Fe in debris on the friction behavior The formation of the graphitic transfer layer highly sensitive to humidity`

The immediate drop of friction coefficient

RH : 90 % RH : 0 %

ConclusionsConclusions

Humidity dependence of the friction behavior of DLC film is not an inherent property of the DLC film

The graphitic layer itself was very sensitive to humidity in case of using steel ball Fe element in debris enhanced the graphitization of the transfer

layer

Tribological behavior of DLC film was highly affected by the reaction between the counterface and humidity Especially using steel ball, Fe-rich debris formed by the reaction

of steel ball and humidity affected the bond structure of the transfer layer