An Electrochemical Impedance Evaluation and Laser Irradiation Effects on the Electronic Structure of Silicon Containing

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carbon (DLC) have been successfully deposited by ion beam assisted deposition (IBAD). The friction coefficient of unlubricated DLC films in dry gases can be as low as 0.01, but this value can reach values as high as 0.10 and 0.20 when measured in a 10% relative humidity [1-3]. It has been shown by various researchers [2-4] that conventional DLC films doped with elements such as Si and Ti exhibit low pin-on-disk friction coefficients in humid environments. Amorphous diamond-like carbon coatings have shown extremely smooth and dense surface morphology. These surface characteristics of the diamond-like carbon coatings are highly desirable for corrosion protection applications. DLC films containing silicon (Si-DLC) exhibit friction coefficients as low as 0.04 [2-4] at ambient humidity and temperature and so are highly promising for tribological applications. It has been also shown repeatedly that the adhesion of the Si-DLC coating on various substrates, including steel, has been excellent [4]. Electrochemical impedance spectroscopy (EIS) is a useful tool to determine the integrity of coatings. The electrochemical impedance of a "defect free" organic coating will initially act as a capacitor. However, latent discontinuities and imperfections will eventually cause ionically conducting low resistive paths perpendicular to the coated surface. The resistance of these pathways will decrease with exposure time [5]. At this time, low resistance pathways tangential to the surface may form followed by the initial under film corrosion [6]. Diamond-like carbon

445 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society

coatings have demonstrated promising barrier properties. DLC films have shown remarkable resistance to nitric and hydrofluoric acid solutions, even after immersed for two hours [7]. In this paper, we report on the preliminary results of corrosion experiments on Si-DLC coatings synthesized on 4340 steel substrates by IBAD and on the effect of the laser irradiation on the electronic structure of the Si-DLC synthesized on silicon substrates. EXPERIMENTAL PROCEDURE A ZYMET 100 non-mass analyzed ion implanter was used for the synthesis of Si-DLC coatings using energetic Ar+ ion bombardment of a vapor deposited tetraphenyl-tetramethyltrisiloxane (Dow-Coming 704) diffusion pump oil [4]. The Si-DLC was synthesized on 4340 substrates of two different surface finishes: polished (0.05-0.031tm surface roughness), and unpolished 600 grit (0.4-0.2[tm surface roughness). Three Si-DLC coatings synthesized on silicon substrates at oil evaporation temperatures of, 125°C, 140'C and 155TC were irradiated by a YAG Laser, 355 nm wavelength Pulsed laser, 0.37 Watts, at 35 kHz. The thickness of the films were measured with the aid of a profilometer. The coating stoichiometry was determined by Rutherford Backscattering Spectrometry (RBS); spectra were interpreted with the aid of the RBS simulation program RUMP [8]. Prior to the electrochemical impedance spectroscopy (EIS), each sample was simply rinsed with distilled water