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

1. Corrosion Resistance of Diamond-Like Carbon (DLC) Coatings on 316L Stainless Steel forBiomedical Applications Conference of Metallurgists COM2003, August 24-27, Canada 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 Applied Electrochemistry Lab. SKKU

2. INTRODUCTION 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. ● Diamond-Like Carbon (DLC)? - amorphous structure similar to diamond - hydrogenated amorphous carbon( a-C:H ) Diamond-Like Carbon (DLC)’ A&W? WEAK POINT ADVANTAGES ●High compressive stress → buckling ●High hardness, low friction ●Electrical insulation ●Poor adhesion ●Operation temperature below 500oC ●Chemical inertness ●Resistance to wear Applied Electrochemistry Lab. SKKU

3. DEPOSITION CONDITIONS Schematics of RF PACVD ­RF PACVD (13.56 MHz) ­ Base Pressure : below 2.0×10-5 Torr ­Silicon Buffer layer (for residual stress) ­ SiH4, 10 mTorr, -400 V, 1 min. deposition ­DLC Precursor Gas : C6H6 ­ Deposition Pressure : 1.33 Pa ­Bias Voltage : - 400 V ­Film Thickness : 1 ㎛ Applied Electrochemistry Lab. SKKU

4. EXPERIMENTAL PROCEDURES C6H6 C6H6+ SiH4 Coating Coating Buffer layer Buffer layer Si Si Substrate Substrate a-C:H Bias Voltage = -800V a-C:H Bias Voltage = -400V Coating structure 1 ㎛ 5-7 nm Si-C:H Bias Voltage = -400V 1 ㎛ 5-7 nm Applied Electrochemistry Lab. SKKU

5. EXPERIMENTAL PROCEDURES Diamond-like carbon (DLC) coatings Surface analyses Electrochemical evaluation Potentiodynamic polarization test AFM Uniformity of surface Potential : -0.25 Voc~1.5 VScan rate : 0.166 mV/sec Surface and corrosion features Electrochemical impedance spectroscopy (EIS) SEM Frequency : 10 mHz~10k HzAmplitude : 10 mV Electrolyte : Deaerated 0.89% NaCl, 37℃, pH=7.4 (similar to human body environment) Applied Electrochemistry Lab. SKKU

6. RESULTS AND DISCUSSION Potentiodynamic polarization test 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 βa : Anodic Tafel slope of the substrate Applied Electrochemistry Lab. SKKU

7. RESULTS AND DISCUSSION RE Rs : Solution resistance between working electrode and reference electrode R R R ct s pore CPE1 : Capacitance of the coating including pores in the outerlayer coating Rpore : Pore resistance resulting from the formation of ionic conduction paths across the coatingCPE2 : Capacitance of the coating within the pit Rct : Charge transfer resistance of the substrate/coating CPE1 CPE2 WE Electrical equivalent circuit for coated metal Electrochemical parameters RE : Reference electrode WE : Working electrode Applied Electrochemistry Lab. SKKU

8. RESULTS AND DISCUSSION ● Electrochemical parameters obtained by equivalent simulation Applied Electrochemistry Lab. SKKU

9. 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 (Rct) Applied Electrochemistry Lab. SKKU

10. RESULTS AND DISCUSSION Rpo = Ropo /Ad ( Rpo= pore resistance ) Ropo = ρd (ρ = specific resistance, d= coating thickness) 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). Delamination area (Ad) Applied Electrochemistry Lab. SKKU

11. RESULTS AND DISCUSSION a-C:H(-400V) a-C:H(-800V) Si-C:H(-400V) Deposited surfaces ( AFM ) Ra = 0.6157 ㎛ Ra = 0.2326 ㎛ Ra = 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). Applied Electrochemistry Lab. SKKU

12. RESULTS AND DISCUSSION Substrate Si-C:H(-400V) a-C:H(-800V) a-C:H(-400V) Corroded surfaces (After potentiodynamic polarization test) Applied Electrochemistry Lab. SKKU

13. ● 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. ● 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). Applied Electrochemistry Lab. SKKU CONCLUSIONS

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