Microstructure Development and the Mechanisms of Lubrication in Magnetron Sputtered HfC-Ag and SiC-Ag Composite Thin Fil
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Microstructure Development and the Mechanisms of Lubrication in Magnetron Sputtered HfC-Ag and SiC-Ag Composite Thin Films 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 A soft thin coating on top of a hard substrate is a highly effective way to reduce friction of sliding mechanical components. However, soft coatings often have high wear and short lifetimes, and the restoring of solid lubricants can be remarkably uneconomical. In our recent work, composite TiC-Ag films were successfully deposited by the magnetron-sputtering pulsed laser deposition technique (MSPLD). Secondary electron (SE) images from the wear tracks revealed that silver migrates to the surface and smears along the sliding direction providing low friction, while the carbide phase provides adequate support and storage for the solid lubricant phase, all resulting in lower wear and longer lifetime. One proposed mechanism for lubrication is the friction-induced thermally activated migration of silver to the surface due to the insolubility of the carbide and silver phases which was corroborated by X-ray diffraction (XRD) analysis. The formation of incoherent interfaces between the two phases, observed by transmission electron microscopy (TEM), may influence the adhesive friction behavior of the soft metal. In this study, we investigate the structural properties and vacuum tribological behavior of SiC-Ag and HfC-Ag films. INTRODUCTION Physical vapor deposition (PVD) in contrast to chemical vapor deposition (CVD) is considered a "cold" coating process that generally lacks chemical reactions. This allows for the simultaneous use of two or more deposition sources that, in many cases, lead to the creation of multiphase composite films as a result of the favorable deposition kinetics and the positive heat of mixing of the deposited substances. Y-T Cheng et. al. [1] showed this concept by co-depositing Ag and Mo and Ag and Ni from two different electron beam evaporation sources, where the degree of phase separation was controlled by the substrate temperature. Materials selection can be critical to the nature of the interface between the phases. In recent studies in nanocomposite Ti-SiC a mottled microstructure within the grains was observed by TEM, it indicated the presence of a mixed boundary interface where titanium and silicon share carbon atoms [2]. In contrast, microstructural studies made in carbide-silver coatings (WC-Ag, TiC-Ag) [3-5] demonstrated the separation of phases and the high purity of the silver phase, and the presence of interfaces free of interaction. These studies have also demonstrated the importance of the ceramic phase to increase wear resistance and durability of lubricant coatings. This paper will show how the covalent bonding of the carbide phase can control the microstructure of carbide-solid lubricant coatings and influence the adhesive friction
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