The synthesis and characterization of CVD ZrB₂ coating from ZrCl₄-BCl₃-H₂-Ar system

By conducting thermodynamics calculations and chemical vapor deposition (CVD) experiments, the synthesis and characterization of CVD ZrB₂ from ZrCl₄-BCl₃-H₂-Ar system at low pressure were investigated. Cauliflower-shaped, dense ZrB₂ coating was successfully deposited on graphite substrates and carbon fibers under appropriate parameters. Calculation results indicate that the yield of ZrB₂ increases with increasing temperature. The amount of ZrB₂ increases to a broad maximum with increasing H₂ flux and then remains constant when H₂ flux reaches above 40 mol. BCl₃ concentration significantly affects phase composition of CVD ZrB₂. Three phases are involved, namely, Zr+ZrB₂, ZrB₂, and ZrB₂+B. Experimental results show that the temperature and H₂ and BCl₃ fluxes significantly affect surface roughness, growth rate, and phase composition of ZrB₂ coating. The thickness and particle size of ZrB₂ coating increase with the increase in temperature. However, at high temperatures, particle size decreases because particles do not have enough time to grow at high reaction rate. Morphologies of ZrB₂ coating vary from porous and rough to dense and smooth with the increase in H₂ flux. BCl₃ reduction by H₂ dominates deposition process at low BCl₃ concentration, and deposition mechanism changes to the codeposition of B and ZrB₂ at high BCl₃ concentration.