Low-Temperature PACVD Silicon Carbide Coatings
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LOW-TEMPERATURE PACVD SILICON CARBIDE COATINGS
W. HALVERSON,* G.D. VAKERLIS,* D. GARG,** AND P.N. DYER** * Spire Corporation, One Patriots Park, Bedford, MA 01730-2396 ** Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195
ABSTRACT Plasma-assisted chemical vapor deposition (PACVD) is used extensively to coat planar (2-dimensional) substrates. In principle, the technique can be used to deposit coatings on 3-dinensional objects. However, extending PACVD to coat 3-dimensional objects uniformly requires careful control of the plasma, substrate temperature, and reactant concentrations over a large volume. A novel low-temperature radio frequency PACVD reactor design was developed to deposit coatings uniformly and reproducibly on 3-dimensional metallic substrates. The design features a temperature-controlled reaction chamber fitted with one or more rf-driven electrodes to generate uniform, large-volume plasma. The reactor was used to develop a series of silicon carbide coatings, which were deposited at or below 500'C. The coatings contain SiC and varying amounts of free silicon and/or amorphous carbon (diamond-like carbon), depending on reagent gas composition and reactor operating parameters. The coatings significantly reduced wear on stainless steel samples in ball-on-disk and abrasive wear tests and provided oxidation protection to molybdenum and titanium alloy.
INTRODUCTION Chemical vapor deposition (CVD) processes are used routinely to forrn coatings on complicated 3-dimensional substrates such as cutting tools and rocket nozzles. Plasma-assisted CVD, however, has been largely limited to electronic applications, principally for fabricating electronic devices. PACVD has also been studied for tribological applications, and published papers on the subject go back at least to the late 1970s and early 1980s [1,2]. The deposition technique has several advantages over conventional CVD, because hard, adherent coatings can be formed at relatively low temperatures (typically less than 600*C vs. more than 1000'C for CVD). In PACVD the reagent gases are dissociated, excited, and partially ionized by energetic plasma electrons, which have temperatures of 5,000 to 20,000 K. The activated reagent species combine chemically on all surfaces exposed to the plasma and are not limited by "line-of-sight", as in physical vapor deposition processes. This paper describes a PACVD technique which deposits hard ceramic coatings on 3-dinensional metallic substrates. The coating material most extensively studied has been silicon carbide; the coatings have varying amounts of free silicon, stoichiometric SiC, and diamond-like carbon (DLC). The composition was controlled by varying the flow ratio of carbon- and silicon-carrying reagent gases and reactor operating parameters; coating hardness was found to be directly related to the C/Si atomic ratio in the deposited material.
REACTOR DESIGN The capability to coat 3-dimensional components is provided by the PACVD reactor chamber design shown in Fig. 1. Substrates with d
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