Tribological Properties of Si-DLC Coatings Synthesized with Nitrogen, Argon Plus Nitrogen, and Argon Ion Beams
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ABSTRACT Hard, adherent, and low-friction silicon-containing diamond-like carbon coatings (Si-DLC) have been synthesized at room temperature by 40 keV (N+ plus N2+), 50%Ar+/50% (N+ plus N2 +), and Ar+ ion beam assisted deposition (IBAD) of a tetraphenyl-tetramethyl-trisiloxane oil on silicon and sapphire substrates. X-ray diffraction analysis indicated that all coatings were amorphous. The average coating wear rate and the average unlubricated steel ball-on-disk friction coefficient, g, decreased with increasing fraction of nitrogen in the ion beam, along with an increase in the average coating growth rate. The Knoop microhardness and nanohardness values of the coatings synthesized by the mixed argon and nitrogen ion beam were higher than the values for the coatings synthesized with 100% nitrogen or 100% argon ion beams. These friction/wear improvements are tentatively attributed to both increased hardening due to greater penetration and ionization induced hardening by the lighter (N) ions and to the presence of Si0 2 on the surface of N-bombarded samples. INTRODUCTION Films of many promising tribological materials, including conventional diamond-like 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]. However, it has been shown by various researchers [2-4] that DLC films containing elements such as Si and Ti retain low pin-on-disk friction coefficients in humid environments. 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. Several trial industrial applications of DLC including protective wear coatings on bearings and forming tools [5], are limited because of the poor thermal stability of DLC above 350°C. However, there are data [5,6] indicating that the presence of additional elements (F, Si and N) in the coating can increase the range of the thermal structural stability of DLC by as much as 100°C. In this paper we report on the properties (stoichiometry, thickness, microhardness, bonding, adhesion, friction and wear) of IBAD Si-DLC coatings synthesized with: nitrogen ion beams (N/Si-DLC), argon plus nitrogen ion beams ((Ar+N)/Si-DLC)), and argon (Ar/Si-DLC) ion beams. EXPERIMENTAL DETAILS A ZYMET 100 non-mass analyzed ion implanter was used for the synthesis of Si-DLC coatings using energetic 40 keV ion bombardment of a vapor deposited tetraphenyl-tetramethyl-trisiloxane (DowCorning 704) diffusion pump oil. The nitrogen beam consisted of a mixture of roughly 40% N+ and 60% N2 +, yielding about 1.6 nitrogen atoms per unit charge. The diffusion pump oil precursor was evaporated from a heated [1 45'C] copper oil container through a 3-mm diameter, 2-mm thick aperture. The substrates, silicon and sapphire (for RAMAN analysis only), were initially cleaned in methan
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