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-H-2-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 H-2 flux and then remains constant when H-2 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 H-2 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 H-2 flux. BCl3 reduction by H-2 dominates deposition process at low BCl3 concentration, and deposition mechanism changes to the codeposition of B and ZrB2 at high BCl3 concentration.