Structural and Mechanical Properties of Diamond-Like Carbon Films Prepared by Pulsed Laser Deposition With Varying Laser
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ABSTRACT Diamond-like carbon (DLC) films have been prepared by pulsed laser deposition (PLD) (wavelength 248 nm), ablating highly oriented pyrolytic graphite (HOPG) at room 2 temperature in a vacuum of 10.2 Pa, at fluences between 0.5 and 35 Jcm- . Films have been deposited on Si(100) with and without a SiC interlayer. Structural analysis, such as visible and UV Raman, Infrared and Electron Energy Loss (EEL) spectroscopies show that the films are hydrogen-free and undergo a transition, from mainly disordered graphitic to up to 80% tetrahedral amorphous carbon (ta-C), above a laser threshold fluence of 5 J cm-2 . The measured residual stresses of as deposited ta-C films do not exceed 2 GPa. Scratch tests show excellent adhesion properties. Low friction coefficients (0.05-0.1) have been measured in ambient humidity. Nanoindentation indicates that film hardness is as high as 70 GPa. INTRODUCTION Growing interest has been devoted to Pulsed Laser Deposition (PLD) of diamond-like carbon (DLC) films with a high content of tetrahedrally coordinated atoms [1]. With respect to mechanical applications, the main limitation of the films are limited thicknesses and often problematic film-substrate adhesion due to high compressive stresses. We prepared DLC films by PLD and characterized them both structurally and mechanically. EXPERIMENTAL PROCEDURES The films were deposited in a vacuum chamber with a base pressure of 10.3 Pa, and operating at a pressure of 10.2 Pa. Highly oriented pyrolitic graphite (HOPG, purity, 99.99%), was ablated with laser pulses from a Lambda Physik LPX220I Excimer Laser (wavelength Z = 248 nm, pulse duration -r = 20 ns, repetition frequency 10 Hz, incidence angle 45 ), changing the fluence between 0.5 Jcm- 2 and 35 Jcm- 2 . Different types of ultrasonically cleaned substrates were used, as shown in Table I. We chose Si(100) for structural analyses and Si(100) coated with sputter deposited SiC to reduce substrate effects in nanoindentation. Film and substrate thicknesses, as determined with a DEKTAK IIA profilometer, are reported in Table I together with the laser fluences. Usually deposition rates were of the order of 0.6 rns-I at our repetition frequency. Unpolarised visible Raman spectra were recorded in backscattering geometry for 514.5nm excitation from an Ar ion laser using a Jobin-Yvon T64000 triple grating spectrometer. UV 359 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
Raman spectra were collected on an UV-enhanced CCD camera on a Renishaw micro-Raman System 1000 spectrometer, modified for use at 244nm, with fused silica optics throughout. FTIR spectra were taken at room temperature, in dry air, using a Win Rad Fast Trasform spectroscope over the 400-4000 cml' range. EELS measurements were carried out on a dedicated VG501 scanning transmission electron microscope fitted with a spectrometer with a McMullan parallel EELS detection system [2]. The residual internal stresses of the films deposited on Si were determined by Stoney's equation, measuring film curvature by a UB
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