Stability of Diamond-like Carbon Films in Aqueous Environment Kwang-Ryeol Lee, Se Jun Park and Young Jin Lee Korea Institute of Science and Technology, P.O.Box 131, Cheongryang, Seoul, Korea

Required Surface Properties of Bioimplant Materials Biological Compatibility –Nontoxic, Noncarcinogenic, Noninflammatory Chemical Compatibility –Corrosion Resistance Mechanical Compatibility –Surface Hardness, Wear Resistance Diamond-like Carbon is a strong candidate for the surface coating.

Applications of DLC Coating for Biomedical Materials Carbofilm TM by Sorin Biomedica, Inc.

The Problem of the DLC Coating DLC coated Ti-6Al-4V Sapphire ball :  6 mm F N = 400gf, v =10cm/s Test in saline solution Catastrophic adhesive wear 100 ㎛

DLC Coated Head after Service Courtesy of Dr. R. Hauert

Purpose of the Present Work Characterize the tribological properties of DLC films in aqueous environment. –Pure DLC & Si-DLC of various structures with different adhesion enhancement Figure out the reason for the catastrophic failure of DLC film in aqueous environment. –Some model experiments

Design of The Coatings Ti-6Al-4V Alloy Coupon DLC or Si-DLC film (1  m) Si buffer layer 0.4, 0.8, 1.6 nm

Deposition of DLC Film on Ti-6Al-4V Alloys RF PACVD (13.56 MHz) Precursor gas : C 6 H 6, C 6 H 6 +SiH 4 Deposition pressure : 1.33 Pa Bias voltage : - 400V, -800V Substrate : Ti-6Al-4V Film thickness : 1 ± 0.1 ㎛

Adhesion by Tensile Test (a) (b) (c) Si Layer Thickness H. W. Choi et al, Submitted to Diam. Rel. Mater. (2005).

Properties of the DLC Films Sample Residual Stress (GPa) Hardness (GPa) G-peak Position (cm -1 ) Pure DLC (-400V) 0.9 ± ± ± 0.3 Pure DLC (-800V) 1.8 ± ± ± 0.3 Si-DLC (-400V) 1.3 ± ± ± 0.3

Tribo-Test in Aqueous Environment Sapphire ball to exclude the effect of FeO x debris Normal load : 5.9N Sliding speed : 10 cm/s Temperature : Room temp. Test environment : 1. DI-water 2. Ambient air (RH : 30 %)

Friction Coefficient Against Sapphire Ball V b : -400V V b : -800V b

Wear Track after Tribo-test Si : 0.4 nm, air Si : 0.4 nm, water Si : 0.8 nm, waterSi : 1.6 nm, water -400V b 100 ㎛

Photographs of Wear Track Si : 0.4 nm, airSi : 0.4 nm, water Si : 0.8 nm, waterSi : 16 nm, water -800V b 100 ㎛

Tribological Behavior of Si-DLC 100 ㎛ Si Layer Thickness : 0.4 nm In Aqueous Environment

Experimental Observations Adhesive wear becomes significant in aqueous environment. = The adhesion of the DLC film was degraded in aqueous environment. The behavior is not sensitive to the film structure. Si incorporated DLC film shows the improved stability than pure DLC films

Why adhesive wear is accelerated in aqueous environment? Ti-6Al-4V Ti-6Al-4V Alloy Coupon DLC or Si-DLC film (1  m) Si buffer layer 0.4, 0.8, 1.6 nm

Why adhesive wear is accelerated in aqueous environment? Ti-6Al-4V Ti-6Al-4V Alloy Coupon DLC or Si-DLC film (1  m) Si buffer layer 0.4, 0.8, 1.6 nm Can residual stress vary in aqueous environment?

Humidity Dependence of the Residual Stress -100V -400V -800V

Why adhesive wear is accelerated in aqueous environment? Attack of water molecules through the pinhole or mico-pores in the film Ti-6Al-4V Ti-6Al-4V Alloy Coupon DLC or Si-DLC film (1  m) Si buffer layer 0.4, 0.8, 1.6 nm

Effect of Pinhole in the Film Pinhole-free DLC film - Si-buffer layer deposition : 0.4 nm - DLC coating : 350nm - Ultra sonic cleaning - DLC coating : 350nm - Ultra sonic cleaning - DLC coating : 350nm DLC 100 ㎛

Effect of Pinhole in the Film Pinhole-free DLC film - Si-buffer layer deposition : 0.4 nm - DLC coating : 350nm - Ultra sonic cleaning - DLC coating : 350nm - Ultra sonic cleaning - DLC coating : 350nm

Effect of Pinhole in the Film Pinhole-free DLC film - Si-buffer layer deposition : 0.4 nm - DLC coating : 350nm - Ultra sonic cleaning - DLC coating : 350nm - Ultra sonic cleaning - DLC coating : 350nm Pinhole-free DLC

100 ㎛ DLC 100 ㎛

Electrochemical Properties E corr (mV) I corr (nA/cm2) B a (V/decade) B c (V/decade) R p (kohm/cm2 ) Porosity Protective Efficiency(%) Substrate a-C:H Pinhole-free a-C:H Si-DLC

Tribological Behavior of Si-DLC 100 ㎛ Si Layer Thickness : 0.4 nm

Conclusions Defects or pinholes in the film is the main reason for the adhesive wear of DLC film in aqueous environment. –Water molecule seems to attack the interface between Si and Ti alloy substrate via the defects. Si-DLC film had better resistance in aqueous environment than DLC films. –Extremely low porosity of Si-DLC films Friction coefficient of DLC film was very low and stable in aqueous environment. –Lubricant effect of water.