Low pressure chemical vapor deposition of niobium coatings on graphite

Niobium coatings were prepared on graphite by low pressure chemical vapor deposition using niobium chloride and hydrogen as the reactant gases. The effects of deposition temperature on the morphology, phase, and deposition rate of niobium coatings were studied. The as-deposited niobium coatings were characterized by scanning electron microscopy and X-ray diffraction. The results indicate that the niobium coatings exhibit a granular hillock structure at 850–900 °C while a laminar structure at 950–1100 °C. The deposition is dominated by surface chemical kinetics with an apparent activation energy of 93.2 kJ/mol at 850–950 °C, while it is dominated by mass transport with an apparent activation energy of 7.9 kJ/mol at 950–1050 °C. At temperatures below 1100 °C, the deposited coatings mainly contain niobium. At temperatures above 1100 °C, the deposited coatings mainly contain niobium carbides. Considering the deposition kinetics and interfacial reactions, the deposition temperature should be controlled below 950 °C.