Influence of total pressure on the microstructures and growth mechanism of ZrC coatings prepared by chemical vapor deposition from the Zr-Br2-C3H6-H2-Ar system

Zirconium carbide (ZrC) coatings were deposited on graphite substrates by chemical vapor deposition from the Zr-Br2-C3H6-H2-Ar system. The influence of total pressure on the growth of ZrC was investigated in the range of 5-60 kPa. As the total pressure increased, the deposition rate increased evidently, and the preferential orientation of ZrC coatings changed from the (200) plane to the (220) plane. The growth mechanism changed from a mass transport reaction to a surface reaction at the total pressure of 20-40 kPa. At the total pressure below 20 kPa, the deposition was dominated by crystal growth, so the coatings were composed of well-faceted pyramidal-shaped crystals growing along the <001> direction. At the total pressure above 60 kPa, the growth of ZrC coatings was controlled by the nucleation mechanism, so the coatings were cluster-like crystals rapidly growing along the <110> direction. In addition, low pressure was conducive to the formation of near-stoichiometric ZrC without free carbon. These variations of ZrC coatings can mainly be attributed to gas supersaturation and remarkably changed transport diffusion coefficients with increasing total pressure.