Growth Mechanism,Properties And Applications Of Hexagonal Boron Nitride Synthesized By Chemical Vapor Deposition From Single Source Precursor

Hexagonal boron nitride（h-BN）possesses remarkable comprehensive performance,and special two-dimensional structure,which has broad application prospects in many high-tech fields.However,the preparation process of h-BN coating is still need improvement,and the chemical vapor deposition（CVD）process also requires to be optimized and improved.In this paper,borazine which is a single-component boron nitride precursor was used as raw material to prepare h-BN on graphite substrate by chemical vapor deposition.The relationship between deposition process and properties of CVD BN was studied and the growth kinetics of CVD BN coating was discussed.The pyrolysis reaction of borazine and the deposition mechanism of the coating were simulated.On this basis,BN coating was introduced in SiC_f/SiC and SiC_f/SiBCN composites as the interface layer to adjust interfacial bonding strength and improve the mechanical property of the composites.Pyrolytic boron nitride（PBN）was synthesized and its properties were studied.The effects of process parameters on the morphology,composition and structure of CVD BN were investigated.Coatings with continuous and dense structure can be deposited on the surface of the graphite substrate which was polished or deposited with a layer of PyC coating.The deposition temperature affects the grain size,crystallinity and crystal orientation of the BN coating.In the temperature range of 1200～1700℃,the grain size on the depostion surface gradually increased and the fracture surface which was disorderly stacked by crystal grains gradually became a distinct laminar structure with the increase of deposition temperature.The crystallinity of the BN coating was improved at higher deposition temperature.Simultaneously,the deposition pressure has an important effect on grain size,crystal orientation and element composition.In the range of50～300Pa,as the deposition pressure increased,the grain size of the coating turned larger,and the fracture surface of the coating with laminar structure became isotropic and disordered.Lower pressure always led to a boron-rich coating,and the excess boron element can be reduced by increasing the deposition pressure.Borazine concentration of the reaction gas is actually reflected in the borazine partial pressure of the deposition zone,and its influence on the coating properties is consistent with the deposition pressure.BN coating with stoichiometric B/N ratio can be obtained at a lower borazine concentration and higher N₂ concentration,and with the concentration of borazine increased,the content of boron became higher.In addition,the content of oxygen in the coating can be reduced by increasing the H₂ partial pressure in the reaction gas.The associate effect of deposition temperature and pressure on the micromorphology and crystal structure of CVD BN was investigated.The as-deposited products can be classified into three types according to their micromorphologies and structure in deposition temperature of 900～1900℃and pressure of 50～1000Pa.At temperatures below 1100℃,the deposition pressure has little effect on the orientation of the grains,and the coating product has a dense and disordered body with low crystallinity.At a higher temperature of 1200～1900℃and lower pressure of 50～200Pa,the coating displays a dense and laminar structure with high crystallinity and well preferred orientation of the grains.At temperatures over 1100℃and pressures above 200 Pa,the as-depositied product is an accumulation of BN powder with loose and porous structure because of the significant effect of the gas phase nucleation and growth at this condition.The effect of deposition temperature on the growth kinetics of BN coating was investigated.With the increase of deposition temperature,the deposition rate of the coating initially increased and then decreased in the temperature range of 1200～1700℃.The growth was controlled by chemical reaction dynamics with an activation energy of23kJ/mol according to the Arrhenius equation between 1200℃and 1400℃.With the increase of deposition temperature at 1400～1600℃,mass transfer gradually became the controlling step,and the deposition rate increased slowly with a peak value of 30.3μm?h^-1 at 1600℃.Then the deposition rate decreased with the increase of temperature above1600℃due to the effects of gas phase nucleation and thermodynamics.The effect of gas flow rate on the growth kinetics of the coating was investigated.Initially,higher gas flow rate led to higher value of deposition rate,and the growth was controlled by the mass transfer process.At larger gas flow rates,the deposition rate tended to be stable with the increase of gas flow rate,and the controlling step of growth turned to be surface reaction process.Then the deposition rate reached the maximum value.In addition,the maximum value of deposition rate can be even higher with the increase of deposition temperature and gas flow rate.However,the coating deposition rate showed a downward trend with further increasing the gas flow rate,which is due to the decrease of the borazine concentration in the reaction gas.The effect of deposition pressure on the growth kinetics of the coating was investigated.In the pressure range of 50～150Pa,the deposition rate increased approximately proportional to the square of deposition pressure,and the reaction process was controlled by mass transfer at this condition.The influence of gas phase nucleation became significant and the deposition rate increased rapidly with the increase of pressure over 200Pa.Simultaneously,the coating structure changed from lamellar structure with preferred orientation to an accumulation of disordered grains.The epitaxial growth mechanism of h-BN on the surface of graphite substrate was discussed,and the gas phase pyrolysis of borazine was investigated.The nucleation and growth process of CVD BN were analyzed and simulated according to the experimental phenomena and related theories,which included the following parts:borazine transport,borazine decomposition,nucleation and adsorption on the substrate,diffusion and rearrangement of crystal nucleus on the substrate,growth of nucleus and formantion of continuous film,growth of the coating thickness.The CVD process of BN coating on SiC fibers was studied.BN coating with thickness of 200～250nm was prepared under the following conditions:deposition temperature of 1100℃,deposition pressure of 50Pa,gas flow rate of 1.2 slm and borazine concentration of 3.5 vol.%.The BN coating had lamellar structure paralled to the fiber surface.The BN coating interphase was introduced into composite so as to control the fiber-matrix interfacial debonding.The corresponded flexural strength and fracture toughness of SiC_f/SiC composite were increased by 167%and 161%to 818±39.6MPa and 23.0±2.2MPa?m^1/2,respectively.The flexural strength of SiC_f/SiBCN composites was also improved by 76%to 438.44±21.65 MPa with the BN interface layer.The BN interface layer led to a proper bonding strength between fibers and matrix,which results in obvious fiber pull-out on the fracture surface and pseudo-ductile fracture mechanism of the composites through crack deflecting and branching behavior.PBN was prepared by CVD and its characteristics were investigated.The surface of PBN was composed of spherical crystal particles and the cross section showed obvious wavelike lamellar structure.PBN had a high crystallinity with three-dimensional ordered hexagonal crystal structure.The oxidation of PBN began at 800℃and it displayed good oxidation resistance.The thermal conductivity and dielectric properties of PBN exhibited evident anisotropic due to the highly ordered planar texture.The as-depostied PBN had higher thermal conductivity in“a”direction at 200°C of 74.7W?m^-1?K^-1.The dielectric constant of as-depostied PBN is smaller in“c”direction,which was 3.90,respectively.The values of loss angle of PBN in different directions were all less than 1×10^-3.