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, Canada Applied Electrochemistry Lab. SKKU

● 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 500 o C Applied Electrochemistry Lab. SKKU

DEPOSITION CONDITIONS ­ RF PACVD (13.56 MHz) ­ Base Pressure : below 2.0×10 -5 Torr ­ Silicon Buffer layer (for residual stress) ­ SiH 4, 10 mTorr, -400 V, 1 min. deposition ­ DLC Precursor Gas : C 6 H 6 ­ Deposition Pressure : 1.33 Pa ­ Bias Voltage : V ­ Film Thickness : 1 ㎛ Schematics of RF PACVD Applied Electrochemistry Lab. SKKU

Si-C:H Bias Voltage = -400V EXPERIMENTAL PROCEDURES Coating structure Coating structure Substrate C6H6C6H6 Buffer layer Si Coating 5-7 nm a-C:H Bias Voltage = -800V a-C:H Bias Voltage = -400V Substrate C 6 H 6 + SiH 4 Buffer layer Si Coating 1 ㎛ 5-7 nm 1 ㎛ Applied Electrochemistry Lab. SKKU

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

RESULTS AND DISCUSSION Specimen E corr (mV) i corr (nA/ ㎠ ) β a (V/decade) β c (V/decade) R p ( × 10 3 Ω ㎠ ) Porosity Substrate Si-C:H, Bias voltage = - 400V a-C:H, Bias voltage = - 800V a-C:H, Bias voltage = - 400V Porosity equation ( Matthews et al.) F : Total porosity R pm : Polarization resistance of the substrate △ E corr : Difference of corrosion potential between coated and uncoated specimens. R p : Polarization resistance of the coated steels β a : Anodic Tafel slope of the substrate Potentiodynamic polarization test Applied Electrochemistry Lab. SKKU

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

● Electrochemical parameters obtained by equivalent simulation Exposure time Rs(Ω㎠)Rs(Ω㎠) CPE1 R pore ( × 10 3 Ω ㎠ ) CPE2 R ct ( × 10 3 Ω ㎠ ) AdAd C coat, ( × F/ ㎠ ) n (0-1) C dl ( × F/ ㎠ ) n (0-1) 120 h Substrate Si-C:H, (- 400V) a-C:H, (- 800V) a-C:H, (- 400V) h Substrate Si-C:H, (- 400V) a-C:H, (- 800V) a-C:H, (- 400V) RESULTS AND DISCUSSION Applied Electrochemistry Lab. SKKU

RESULTS AND DISCUSSION Si-C:H(-400V) coating leads to the higher R ct values than a-C:H(-400V). a-C:H(-800V) coating leads to the higher R ct values than a-C:H(-400V). Charge transfer resistance (R ct ) Applied Electrochemistry Lab. SKKU

RESULTS AND DISCUSSION R po = R o po /A d ( R po = pore resistance ) R o po = ρd (ρ = specific resistance, d= coating thickness) Delamination area (A d ) Delamination area (A d ) 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). Applied Electrochemistry Lab. SKKU

RESULTS AND DISCUSSION Deposited surfaces ( AFM ) Deposited surfaces ( AFM ) a-C:H(-400V)Si-C:H(-400V)a-C:H(-800V) R a = ㎛ R a = ㎛ R a = ㎛ 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

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) Applied Electrochemistry Lab. SKKU

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). Applied Electrochemistry Lab. SKKU ● From the EIS results, R ct value of Si-C:H(-400V) was higher than a-C:H(-400V). Furthermore, R ct value 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. Applied Electrochemistry Lab. SKKU ● 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.