Studying On The Preparation Of Titanium Boride Coating By Chemical Vapor Deposition

TiB2 has excellent physical and chemical properties.It is an ideal choice for the protective layer of evaporation boat,aluminum electrolytic cathode,fuel cell cathode,carbon fiber and so on.In this work,TiB2 ceramic coatings are prepared on graphite substrate surface by self-made CVD vacuum furnace with TiCl4-BCl3-H2-Ar system.The effects of deposition temperature(1000?,1050?,1100?),input Ti/B(0.5,0.75,1)on the phase structure,crystal structure and microstructure are investigated through XRD,SEM?TEM in this work.The effects of deposition temperature(1000?,1050?,1100?),input Ti/B(0.5,0.75,1)on deposition rate,micro-hardness,the oxidation resistance and corrosion resistance are investigated through the cyclic oxidation method?dynamic potential scanning and so on in this work.The deposition mechanism of coating is also discussed.Molar ratio of titanium boron is 0.5:with the increased of deposition temperature,the deposition rate increases;XRD diffraction results show that coating preferres growth in the crystal direction[110]with the increased of deposition temperature;Surface morphology of SEM coating shows cauliflower type morphology,large particles and small cauliflower type morphology,cellular morphology;The cyclic oxidation method shows that oxidation resistance increased with the deposition temperature;The weight loss of graphite/coating composites prepared at 1100? is less than 0.1%after the cyclic oxidation from 600? to 800?;When the deposition temperature is 1000?,1050?,1100?,the micro hardness is 25.55GPa,29.08GPa,32.50GPa;It shows that corrosion resistance of the coating increases with the increased of deposition temperature through dynamic potential scanning method;The corrosion potential of graphite/coating prepared at 1100? is-156mv,and the corrosion potential of the 198mv is shifted to the positive direction.Corrosion current density(1.12×10-6A·cm-2)of graphite/coating is a small number of grade than corrosion current density(3.63x10-5A·cm-2)of graphite without deposited coating.The deposition temperature is 1000?:with the increased of Ti/B,the deposition rate increases;XRD diffraction results shows that coating had preferred growth in the crystal direction[100]with the increased of deposition temperature;The cyclic oxidation shows that oxidation resistance increased with the deposition temperature;Corrosion resistance of the coating showes that a trend of increasing first and then decreasing with method of the cyclic oxidation;When the Ti/B is 0.5,0.75,1,the micro hardness is 25.55GPa?32.89GPa?15.71GPa.It shows that the change of Ti/B is not significant to the electrochemical corrosion of the coating through dynamic potential scanning.The morphology of TiB2 deposited by chemical vapor is highly dependent on the deposition conditions.The lower temperature is,the more easily to form crystal nucleus in the tips or the lower parts.Then,the coating mainly occurres infiltration nucleation and forms the classic cauliflower type morphology.When the temperature increases,the surface diffusion and infiltration will be easier,then the graphite matrix surface is easier to form crystal nucleus.At the same time,it takes a big eat small phenomenon,the coating shows large particles and small cauliflower type morphology.When the temperature continues rising,the critical radius of crystal nucleation will be reduced.Some reactant reacts and forms crystal nucleus in the gas phase or on graphite substrate surface,so the coating forms cellular morphology.When the deposition temperature is 1000?,with the increase of Ti/B,cauliflower type morphology is maintained,but columnar crystal of cauliflower type morphology becomes dense and coarse,then it develops into small.Molar ratio of titanium and boron diffused to the surface of graphite substrate is less than 0.5 in spite of Ti/B=0.5.The molar ratio of titanium boron reach to 0.75,it is more conducive to TiB2 nucleation and nucleus growth,columnar crystal of cauliflower type will be more closely and thicker;TiCl4 is not conducive to crystal nucleation and growth,etching phenomenon has happened.