Deposition and Modification of Tantalum Carbide Coatings on Graphite By Laser Interactions
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DEPOSITION AND MODIFICATION OF TANTALUM CARBIDE COATINGS ON GRAPHITE BY LASER INTERACTIONS James Veligdan, D. Branch, P.E. Vanier, and R.E. Barletta Reactor Systems Division, Department of Nuclear Energy, Brookhaven National Laboratory, Upton, NY.
ABSTRACT Graphite surfaces can be hardened and protected from erosion by hydrogen at high temperatures by refractory metal carbide coatings, which are usually prepared by chemical vapor deposition (CVD) or chemical vapor reaction (CVR) methods. These techniques rely on heating the substrate to a temperature where a volatile metal halide decomposes and reacts with either a hydrocarbon gas or with carbon from the substrate. For CVR techniques, deposition temperatures must be in excess of 2000' C in order to achieve favorable deposition kinetics. In an effort to lower the bulk substrate deposition temperature, the use of laser interactions with both the substrate and the metal halide deposition gas has been employed. Initial testing involved the use of a C02 laser to heat the surface of a graphite substrate and a KrF excimer laser to accomplish a photodecomposition of TaCI5 gas near the substrate. The results of preliminary experiments using these techniques are described.
INTRODUCTION The use of refractory carbide coatings for protecting materials from corrosive gases at high temperatures or to provide surface toughness is well established. Recently, we have become involved with the use of these materials for the protection of graphites and carbon-carbon composites from attack by hot hydrogen at high temperatures [1]. Typically, these coatings have been applied to the carbon substrate using either conventional chemical vapor deposition (CVD) or chemical vapor reaction (CVR) techniques. The latter process has proven quite effective in producing dense, adherent and protective coatings. In this process a metal halide vapor reacts with carbon in the substrate to produce the carbide. For a refractory carbide such as tantalum, the reaction is TaC15 + C(substrate) -+ TaC + 5 C12.
(1)
This reaction typically occurs at temperatures in the range of 1700 to 2600 K, depending on the carbide with coating thicknesses on the order of 10 to 100 micrometers [2]. Since these coatings are produced at high temperatures, the grain size is quite large. In an effort to reduce the overall substrate temperature as well as to provide a means to perform "spot repairs" for coatings, work has been initiated to develop an alternative coating method which could achieve the same grain morphology and coating stoichiometry as the CVR process. Thermochemical data [3,4] indicate that high surface temperatures are necessary for the carbide formation to proceed. Since the region of carbon diffusion is small, however, bulk heating of the substrate is not required. Thus, the use of surface heating of the carbon with an infrared laser was chosen. Further, experiments on the photo-assisted formation of TaO from TaC15 indicate the efficacy of UV photons in the photo-decomposition of the starting metal halide [5,
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