Structural and Tribological Properties of TiC/C/Ag Coatings in Vacuum and Ambient Environments
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Structural and Tribological Properties of TiC/C/Ag Coatings in Vacuum and Ambient Environments Jose L. Endrino1, Jose J. Nainaparampil², and James E. Krzanowski1 1 Mechanical Engineering Department, University of New Hampshire, Durham, NH 03824, U. S. A. ²Wright Patterson AFB, AFRL/MLBT, Dayton, OH 45433, U. S. A. ABSTRACT TiC/C/Ag coatings were deposited by magnetron sputtering pulsed laser deposition (MSPLD) combining sputtering from a custom made Ti-Ag (60:40) target with the ablation of carbon. Energy disperse spectroscopy (EDS) was used to determine the elemental composition, and x-ray diffraction (XRD) and cross-sectional scanning electron microscopy (XSEM) to examine the structure of the films. Hardness and reduced modulus measurements were acquired using a nanoindentation technique. The pin-on-disk friction test was used to study the friction behavior of the deposited samples in high vacuum and ambient conditions. Variations in the laser energy and the power of the sputtering gun yielded a set of samples with carbon content that ranged from 15.0 to 95.6 percent. The hardest samples with the highest reduced modulus were those with a moderate carbon content and that were shown to form a titanium carbide phase. Tribological results indicated that there is an optimum composition of a TiC/C/Ag coating (~25 at.% carbon) for which it can be reversible and provide lubrication in both ambient and vacuum. INTRODUCTION Lubrication in vacuum can be achieved by the application of a soft metal coating to one of the wear surfaces [1]. Studies by Dellacorte et. al. [2] and Erdemir et. al. [3] proved the effectiveness of silver films to reduce friction and decrease wear on ceramics. Multilayer composite coatings composed of a hard ceramic material and titanium containing DLC layers were recently investigated by Voevodin et. al. [4], introducing the concept of ambient tribological coatings that combine load-supporting and friction reduction materials [4]. A more recent approach is the co-sputtering deposition of a ceramic material and a soft metal, which allows for the adequate storage of the metal lubricant and the extension of the coating life in a vacuum environment [5]. In our latest work, this principle was also demonstrated by the codeposition of composite titanium carbide - silver coatings obtained by the hybrid technique of magnetron sputtering - pulsed laser deposition (MSPLD), where titanium carbide was deposited by ablation of a TiC target and silver was sputter-deposited [6]. In this study, TiC/C/Ag coatings with different carbon to Ti/Ag ratios were obtained by sputtering from a Ti/Ag (60:40) target and by simultaneously ablating carbon. EXPERIMENTAL DETAILS Deposition was carried out using a combined pulsed laser deposition / magnetron sputtering system [Model 3000, PVD Products, Wilmington, MA.] with a Lambda-Physik excimer KrF (λ=248 nm) laser (Model Compex 205) operating at 50 Hz. Carbon was ablated from a 3-in PLD P8.1.1
target, and Ti/Ag (nominal composition: 60 at.% Ti, 40 at.% Ag) was sputtered from a c
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