The nature and rates of erosion of diamond, graphite, and diamond-like carbon (DLC) films exposed to oxygen plasmas were evaluated by comparison of surface morphological changes and weight losses. The RF plasma oxidation at ambient temperature caused severe etching of graphite and DLC specimens, while causing only minor damage to diamond film surfaces. The results suggest that erosion by low energy ions is very selective to the sp2 and sp states compared to the sp3 state of carbon. The selectivity of etching by oxygen plasma is significant, as compared to what has been reported for hydrogen in atomic and ionic states, or for oxidation at elevated temperatures in molecular oxygen. These observations have significant implications to the synthesis of diamond films by chemical vapor deposition (CVD) as well as to the application of diamond film coatings on graphite or other substrates for protection against energetic atomic oxygen prevailing in the low earth orbits (LEO).
Solid state reactions between SiC ceramics and Co, Ni, and Pt metals have been studied at temperatures between 800 and 1200 °C for various times under He or vacuum conditions. Reactions between the metals and SiC were extensive above 900 °C. Various metal silicides and carbon precipitates were formed in layered reaction zones. Interfacial melting was also observed at certain temperatures; teardrop-shaped reaction zones, porosity, and dendritic microstructure resulting from melting/solidification were evident. The metal/ceramic interfaces exhibited either planar or nonplanar morphologies, depending upon the nature of the metal/ceramic reactions. Concave interfacial contours were observed when interfacial melting occurred. By contrast, planar interfaces were observed in the absence of interfacial melting. In all cases, the decomposition of SiC was sluggish and may serve as a rate limiting step for metal/ceramic reactions. Free unreacted carbon precipitates were formed in all the reaction zones and the precipitation behavior was dependent upon the metal system as well as the location with respect to the SiC reaction interface. Modulated carbon bands, randomly scattered carbon precipitates, and/or carbon-denuded bands were formed in many of the reaction zones, and the carbon existed in a mixed state containing both amorphous and graphitic forms.
The nucleation and morphology of diamond crystals and films synthesized by the use of a combustion flame have been investigated. By operating an oxy-acetylene torch in a fuel-rich mode, diamond crystals and films have been deposited on mechanically abraded molybdenum, on in situ created molybdenum carbide, and on thin diamond-like carbon (DLC) layers synthesized on molybdenum. Scanning electron microscopy, Auger and Raman spectroscopy have been used to characterize the films and crystals. Diamond is found to be uniformly deposited in the region of the substrate that intersects the inner, acetylene-rich region of the flame. The nucleation density, the growth rate, and the morphology of the diamond crystals and films are found to be strongly influenced by the surface condition of the substrate. On mechanically abraded molybdenum, abraded with 600 mesh silicon carbide, and on molybdenum carbide, well-formed cubo-octahedrons of diamond, up to 45 μm in diameter, are formed for deposition times of 90 min. Film formation is seldom observed under these conditions. To enhance nucleation, thin layers of DLC were formed on molybdenum substrates by reducing the O2/C2H2 ratio in the gas mix to ∼ 0.75 for short periods of time under 30 s. This was followed by increasing the O2/C2H2 ratio to conditions that produce diamond (an O2/C2H2 ratio of ∼ 0.9). Under these conditions the nucleation density of diamond was increased by an order of magnitude and the growth rates by about 60%, as compared to diamond deposited on abraded molybdenum and molybdenum carbide. In addition, the morphology of the diamond crystals and films was substantially affected with indications of dendritic growth. The DLC layer is effective in promoting diamond nucleation due to the high surface defect density and the high hydrogen concentration of these films. The combination of surface defects, in the form of dangling bonds, and the evolution of hydrogen from the DLC layer during the diamond deposition process, which is characterized by higher temperatures, result in a high concentration of active surface sites for diamond nucleation. The nucleation density, the distribution on the substrate, and the morphology of diamond crystals and films are not driven by the transport of reactive specie in the flame to the substrate, but rather by nucleation processes, temperature distribution across the surface, and attendant surface phenomena.
Oxidation kinetics of diamond films synthesized by plasma-assisted chemical vapor deposition (CVD) in flowing oxygen were evaluated using thermal gravimetric analysis. The oxidation rates of diamond films, measured in the 500 to 750 °C range, were significantly lower than natural diamond, which was lower than grafoil and pyrolytic forms of graphite. The mechanisms of oxidation in the CVD diamond films and natural diamond were explored by scanning electron microscopy and Auger electron spectroscopic examination of localized regions. In CVD films, oxidation occurred preferentially at grain boundaries, local defects, and diamond-like carbon containing regions during the early stages of oxidation process. The results suggest that preferred orientation of diamond crystallites in the CVD film plays a major role on its oxidation behavior.
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