Ho-Gun Kim, Seung-Ho Ahn, Jung-Gu Kim, *Se-Jun Park, *Kwang-Ryol Lee, **Rizhi Wang

SungKyunKwan University, Korea *Korea Institute of Science and Technology, Korea

** The University of British Columbia, Canada

Corrosion Resistance of Diamond-Like Carbon (DLC) Coatings on 316L Stainless Steel for

Biomedical Applications

Conference of Metallurgists COM2003, August 24-27, CanadaConference of Metallurgists COM2003, August 24-27, Canada

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● Diamond-Like Carbon (DLC)? - amorphous structure similar to diamond - hydrogenated amorphous carbon( a-C:H )

INTRODUCTION

ADVANTAGES

●High hardness, low friction●Electrical insulation

WEAK POINT

●High compressive stress → buckling●Poor adhesion

Diamond-Like Carbon (DLC)’ A&W?

The purpose of the present investigation is to evaluate the effects of bias voltage and Si incorporation on the corrosion resistance of DLC coatings in the simulated body fluid environment.

●Chemical inertness●Resistance to wear

●Operation temperature below 500oC

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DEPOSITION CONDITIONS

­­­RF­PACVD­(13.56­MHz)­RF­PACVD­(13.56­MHz)

­­­Base­Pressure­:­below­2.0×10­Base­Pressure­:­below­2.0×10-5-5­Torr­Torr

­­­­Silicon­Buffer­layer­(for­residual­stress)Silicon­Buffer­layer­(for­residual­stress)

­­­SiH­SiH44,­10­mTorr,­-400­V,­1­min.­deposition,­10­mTorr,­-400­V,­1­min.­deposition

­­­­DLC­Precursor­Gas­:­CDLC­Precursor­Gas­:­C66HH66

­­­Deposition­Pressure­:­1.33­Pa­Deposition­Pressure­:­1.33­Pa

­­­­Bias­Voltage­:­-­400­VBias­Voltage­:­-­400­V­­

­­­­Film­Thickness­:­­Film­Thickness­:­­1 1 ㎛㎛­­­­­­­­­­­­

Schematics of RF PACVDSchematics of RF PACVD

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Si-C:H Bias Voltage = -400V Si-C:H Bias Voltage = -400V

EXPERIMENTAL PROCEDURES

Coating structureCoating structure

Substrate

C6H6

Buffer layer Si

Coating

5-7 nm

Substrate

Coating

Buffer layer

a-C:H Bias Voltage = -800Va-C:H Bias Voltage = -800V

a-C:H Bias Voltage = -400Va-C:H Bias Voltage = -400V

Substrate

C6H6+ SiH4

Buffer layer Si

Coating 1 ㎛

5-7 nm

1 ㎛

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EXPERIMENTAL PROCEDURES

Electrochemical evaluationElectrochemical evaluation

Diamond-like carbon (DLC) coatingsDiamond-like carbon (DLC) coatings

Potentiodynamic polarization testPotentiodynamic polarization test

Electrochemical impedance Electrochemical impedance spectroscopy (EIS)spectroscopy (EIS)

Potential : -0.25 Voc~1.5 VScan rate : 0.166 mV/sec

Frequency : 10 mHz~10k HzAmplitude : 10 mV

Electrolyte : Deaerated 0.89% NaCl, 37℃,­pH=7.4 (similar to human body environment)

SEMSEM Surface and corrosion features

AFMAFM Uniformity of surface

Surface analysesSurface analyses

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RESULTS AND DISCUSSION

SpecimenEcorr

(mV)

icorr

(nA/ ㎠ )

βa

(V/decade)

βc

(V/decade)

Rp

(×103 Ω ㎠ )Porosity

Substrate-

114.6249.3 0.1285 0.1868 132.7 -

Si-C:H,­Bias­voltage­=­-400V­-

111.60.06486 0.3527 0.06077 347492.9 0.00037

a-C:H, Bias voltage = -800V

-63.11

15.33 0.1211 0.1839 2070.8 0.02407

a-C:H, Bias voltage = - 400V

-40.55

103 0.5402 0.1293 440.3 0.15556

Porosity equation ( Matthews et al.)

F : Total porosity Rpm : Polarization resistance of the substrate △Ecorr : Difference of corrosion potential

between coated and uncoated specimens. Rp : Polarization resistance of the coated steels β a : Anodic Tafel slope of the substrate

Potentiodynamic polarization test

10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

Pot

entia

l (m

V v

s S

CE

)

Current Density (A/cm2)

Substrate Si-C:H, Bias voltage = -400V a-C:H, Bias voltage = -800V a-C:H, Bias voltage = -400V

ΔEcorr/βa

substrate)-p(coating

e)pm(substat10

R

R F

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RE : Reference electrode

WE

Rpore

CPE1CPE2Rct

Rs

RE

RESULTS AND DISCUSSION

Electrical equivalent circuit for coated metalElectrical equivalent circuit for coated metal

WE : Working electrode

Rs­:­Solution­resistance­between­working­electrode­­­­­­­and­reference­electrodeCPE1­:­Capacitance­of­the­coating­including­pores­­­­­­­­­­­­­in­the­outerlayer­coating­Rpore­:­Pore­resistance­resulting­from­the­formation­­­­­­­­­­­­of­ionic­conduction­paths­across­the­coatingCPE2­:­Capacitance­of­the­coating­within­the­pit­Rct­:­Charge­transfer­resistance­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­of­the­substrate/coating­­

Electrochemical parametersElectrochemical parameters

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● Electrochemical parameters obtained by equivalent simulation

Exposuretime

Rs

(Ω ㎠ )

CPE1Rpore

(×103

Ω ㎠ )

CPE2Rct

(×103

Ω ㎠ )AdCcoat,

(×10-9

F/ ㎠ )

n(0-1)

Cdl

(×10-9

F/ ㎠ )

n(0-1)

120 h

Substrate 32.96 46440 1 78.9 175200 1 252.2 -

Si-C:H,­(-400V)

700.7 20.8 0.9921 3.43 47.47 0.7483 2501 0.41

a-C:H, (-800V)

11.39 5.2 0.8675 81.8 508.5 0.914 1018 1.08

a-C:H, (-400V)

929.1 878.4 0.6016 92.4 714.2 1 372.5 1.70

216 h

Substrate 33.29 48210 1 73.4 174200 1 218.2 -

Si-C:H,­(-400V)

4.308 27.1 0.9602 2.9 47.05 0.5329 2062 0.48

a-C:H, (-800V)

18.2 7.74 0.8884 52.5 749.4 0.8843 1292 1.69

a-C:H, (-400V)

696 899.4 0.6099 89.9 542.2 1 342 1.73

RESULTS AND DISCUSSION

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RESULTS AND DISCUSSION

Si-C:H(-400V) coating leads to the higher Rct values than a-C:H(-400V).

a-C:H(-800V)­coating leads to the higher Rct­values than­a-C:H(-400V).

Charge transfer resistance (RCharge transfer resistance (Rctct))

1 24 48 72 96 120 144 168 192 216

0

2000

4000

6000

Ch

arg

e T

ran

sfe

r R

esi

sta

nce

(ko

hm

-cm

2 )

Immersion Time(h)

Substrate Si-C:H, Bias voltage = -400V a-C:H, Bias voltage = -800V a-C:H, Bias voltage = -400V

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RESULTS AND DISCUSSION

RRpo po = R = Roopopo /A /Add ( R ( Rpopo= pore resistance )= pore resistance )

RRoopopo = ρd (ρ = specific resistance, d= coating thickness) = ρd (ρ = specific resistance, d= coating thickness)

Delamination area (ADelamination area (Add))

1 24 48 72 96 120 144 168 192 216-1

0

1

2

8

10

De

lam

ina

tion

Are

a(A

d)

Immersion Time(h)

a-SiC:H, Bias voltage = -400V a-C:H, Bias voltage = -800V a-C:H, Bias voltage = -400V

Si-C:H(-400V) coating leads to the lower delamination area than a-C:H(-400V).a-C:H(-800V) coating generally leads to the lower delamination area than

a-C:H(-400V).

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RESULTS AND DISCUSSION

Deposited surfaces ( AFM )Deposited surfaces ( AFM )a-C:H(-400V)

Si-C:H(-400V)a-C:H(-800V)

RRaa = 0.2326 = 0.2326 ㎛㎛

RRaa = 0.6157 = 0.6157 ㎛㎛

RRaa = 0.0906 = 0.0906 ㎛㎛

Roughness of Si-C:H(-400V) was lower than a-C:H(-400V) according to Si incorporation. With increasing bias voltage, roughness of a-C:H(-800V) was lower than a-C:H(-400V).

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RESULTS AND DISCUSSION

Corroded surfaces (After potentiodynamic polarization test)Corroded surfaces (After potentiodynamic polarization test)

Substrate Si-C:H(-400V)

a-C:H(-800V) a-C:H(-400V)

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CONCLUSIONS

● From the potentiodynamic test, the polarization resistance value of Si-C:H(-400V) was higher than a-C:H(-400V). Also, the polarization resistance value of a-C:H(-800V) was higher than a-C:H(-400V).

● From the potentiodynamic test, porosity of Si-C:H(-400V) was lower than a-C:H(-400V). Moreover, porosity of a-C:H(-800V) was lower than a-C:H(-400V).

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● From the EIS results, Rct value of Si-C:H(-400V) was higher than a-C:H(-400V). Furthermore, Rct­value of a-C:H(-800V) was higher than a-C:H(-400V).

●­From the EIS results, delamination area of Si-C:H(-400V) was lower than a-C:H(-400V). In addition,­delamination area of a-C:H(-800V) was lower than a-C:H(-400V).­These results corresponded with the potentiodynamic test.­

CONCLUSIONS

● It was shown that corrosion resistance of DLC films with Si incorporation and higher bias voltage would be improved in corrosive environment.

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● From the AFM images, the increase of bias voltage and Si incorporation improved the surface roughness of DLC coatings. These results are consistent with the porosity calculated by electrochemical method.