Structure and Properties of TiC, VC And TiC/VC Thin Films Deposited by Pulsed Laser Deposition
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INTRODUCTION Transition metal carbides have many attractive properties, including high hardness levels and low friction coefficients, that make them good candidates for hard coatings. However, while titanium carbide has found applications as a hard, wear resistant coating, the hardness of TiC, 29 GPa [I], is still well below that of diamond (80 GPa). This paper summarizes our efforts to explore alternate methods for augmenting the mechanical properties of carbide-based coating materials. The objective of our work is to develop mixed carbide coatings with nanoscale heterogeneous microstructures that will have mechanical and tribological properties superior to pure TiC. Pulsed laser deposition (PLD) is used to deposit these materials since it offers a relatively straight-forward method for fabricating a large variety of compositions and microstructures. The deposition of titanium carbide by PLD has benn investigated in several studies [24], and it was concluded that PLD was an excellent method for deposition of carbides. In this study, we first investigate the deposition of pure TiC and VC compounds and continue our investigation by examining composite TiC/VC films. In this paper we will briefly discuss the chemical and microstructural analysis results, which will be presented in detail in a later paper [5], allowing us to focus on mechanical and tribological properties. EXPERIMENTAL PROCEDURES Titanium carbide and vanadium carbide films were deposited using a KrF (X = 248 nm, 30 ns pulse, 1-50 Hz repetition rate) laser to ablate the target materials. The power density was between 2-5 J/cm 2 per pulse. The deposition chamber was pumped using a turbo-molecular pump with an auxiliary liquid nitrogen getter. The typical base pressure with the liquid nitrogen getter was 2-4 X 106 Torr. Targets were obtained from commercial vendors, and were fabricated by hot-pressing under argon from 99.5% purity carbide powders. No binders were used in target fabrication. The deposition rates for PLD TiC and VC were determined using a quartz crystal thickness monitor. The rates were determined as a function of Ar background pressure, repetition rate and laser power and were typically as high as 0.02 nm/shot. 301 Mat. Res. Soc. Symp. Proc. Vol. 410 0 1996 Materials Research Society
The substrates used in this investigation were sapphire single crystal of (1120) orientation. The deposition conditions are shown in Table I. Composite TiC/VC films were deposited by using a composite target, one-half being TiC and the other half VC. As the target rotated, the laser beam, alternately ablated TiC and VC. The rotation rate was varied to control the bilayer thickness. For the composite TiC/VC films a substrate temperature of 475°C was used in an attempt to obtain more highly oriented film for multilayer growth. The composite films were grown at pressures of base vacuum level and 50 mtorr. In an additional experiment, TiC was ablated in a background gas of 5 mTorr Ar and 1 mTorr 02 to determine the effects of ambient oxygen on film composition.
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