Microstructure of HfB2-HfC nanocomposite coatings via chemical vapor deposition

Abstract

HfB2-HfC nanocomposite coatings were fabricated by hot-wall chemical vapor deposition using the HfCl4-BCl3-C3H6-H2-Ar system. Three composite coatings with distinct porosity and surface morphologies were obtained by varying the BCl3-to-C3H6 flow ratio. XRD and HRTEM confirmed the coexistence of HfB2 and HfC in all composite coatings. Excess carbon was observed in all composites, with the weakest carbon-related signals in HfB2/HfC-1. Under the most carbon-rich condition, layered turbostratic carbon with a clear C(002) lattice fringe was observed, together with carbon-rich pockets accompanied by local Hf depletion. At an intermediate C3H6 flow rate, a dense and continuous biphasic nanocomposite was obtained, comprising uniformly dispersed HfB2-HfC nanocrystallites with locally distributed excess carbon. Under carbon-rich conditions, intensified gas-phase decomposition and particle formation led to a coalesced nanonodular surface and promoted turbostratic-carbon formation and carbon localization. Nanoindentation revealed that the dense nanocomposites exhibited higher hardness than the porous coating. This study demonstrates the correlation between densification and carbon inclusion in the CVD growth of HfB2-HfC nanocomposites