Preparation And Application Of Zirconium Diboride By Chemical Vapor Deposiiton

With the development of hypersonic vehicles,hypersonic vehicles will encounter harsher service environment.It requires the thermal protection system?TPS?materials of hypersonic vehicles possessing better properties like high temperature stability,high strength and toughness,low density,etc.Research in the field of TPS materials is an absolute priority for hypersonic vehicles.Refractory metal compound ZrB₂modified C/SiC composites are considered to be one of the most promising ultra-high temperature materials as they possess both advantages of ZrB₂?such as high melting point,high hardness,good anti-ablative property,etc.?and C/SiC composites?such as low density,high strength,low thermal expansion coefficient,low risk of catastrophic failure,etc.?.Chemical vapor deposition/infiltration?CVD/CVI?is one of the most efficient methods for fabrication of ceramic coatings and matrix.Thermal damage to the carbon fiber preform is low during CVD/CVI process as the temperature is low.C/SiC composite components fabricated by CVD/CVI have been widely used in nozzle,combustion chamber,and turbine blades of aero engines.Modification of CVD/CVI C/SiC composites with ZrB₂ coatings might further increase the performance.In this dissertation,the CVD parameters of ZrB₂ were fully studied thermodynamically and kinetically.C/SiC composites with single ZrB₂ coating and SiC-ZrB₂ coatings were then fabricated respectively and their anti-ablative properties were investigated.A new CVD method,reactive chemical vapor deposition?RCVD?is proposed to prepare ZrB₂ coatings on the basis of conventional CVD.C/SiC composites modified by RCVD ZrB₂ coatings were fabricated and the anti-ablative properties were studied.Main contents and conclusions are as follows:?1?The effects of temperature,H₂ flux and BCl₃ flux on CVD ZrB₂ of ZrCl₄-BCl₃-H₂-Ar system were calculated and the phase diagrams were drawn.The thermodynamic calculation results indicate that the yield of ZrB₂ reaches maximum when the ratio of BCl₃/ZrCl₄ is 2.The yield of ZrB₂ increases with increasing temperature.The yield of ZrB₂ first increases with increasing H₂ concentration and then remains constant.The phase composition of ZrB₂ is greatly influenced by the BCl₃ flux.Based on the thermodynamic calculations,the deposition kinetics and deposition mechanisms of CVD ZrB₂ were investigated.The experimental results indicate that temperature mainly affects the morphology,the deposition rate and the infiltration ability of ZrB₂ coatings.ZrB₂ coating becomes denser,and thicker with increasing temperature.The infiltration ability first increases and then decrease with increasing temperature.H₂ flux mainly affects the morphology of ZrB₂ coating while has less influence on the infiltration ability.The coating becomes denser and smoother with increase in H₂ flux.BCl₃ flux mainly affects the morphology and the phase composition of ZrB₂ coating.The coating shows typical morphology of co-deposition of B and ZrB₂.The deposition is controlled by surface reaction kinetics at low temperature,by diffusion at medium temperature and by homogeneous nucleation at high temperature.?2?The effects of temperature,H₂ flux and BCl₃ flux on RCVD ZrB₂ of Zr-BCl₃-H₂-Ar system were calculated and the phase diagrams were drawn.The thermodynamic calculation results indicate that the yield of ZrB₂ reaches maximum when the ratio of BCl₃/Zr is 2.The yield of ZrB₂ increases with increasing temperature.The yield of ZrB₂ first increases with increasing H₂ concentration and then remains constant.The phase composition of ZrB₂ is greatly influenced by the BCl₃ flux.Based on the thermodynamic calculations,the deposition kinetics and deposition mechanisms of RCVD ZrB₂ were investigated.The experimental results indicate that the morphology,thickness and infiltration ability of ZrB₂ coating change significantly with BCl₃ flux.The density,thickness and infiltration ability increase with increasing BCl₃ flux.The particle size,thickness and infiltration ability all first increase and then decrease with increasing H₂ flux.With increase in temperature,the thickness of coating increases while the infiltration ability and conversion rate of Zr powders first increase and the decrease.The deposition of RCVD ZrB₂ is controlled by surface reaction kinetics at low temperatures and diffusion at high temperatures.?3?Based on the study of ZrB₂ deposition,single-layer ZrB₂ coatings with different thickness were prepared on the two dimensional needled C/SiC composites.The ablation behaviors were tested by oxyacetylene flame.Results indicate that ZrB₂coating is dense and tightly coated on the composite surface when coating thickness is within 5-10?m.ZrB₂ coating becomes porous and causes cracks in SiC matrix when coating thickness is within 15-20?m.Composites with a 10?m-thick ZrB₂ coating show the best anti-ablation properties;specifically,linear and mass ablation rates are reduced by 38%and 61%,respectively,compared with uncoated composites.During ablation,four layers are formed:SiO₂ inner layer,thin discontinuous porous SiO₂layer,dense ZrO₂-SiO₂ layer,and ZrO₂ top layer in the ablative transition region.These four layers serve as thermal barriers and provide the inner coating with protection from being further ablated.?4?SiC-ZrB₂ coatings with different thickness of ZrB₂ layer were prepared on the two dimensional needled C/SiC composites.The ablation behaviors were tested by oxyacetylene flame.Results indicate that samples with 10?m and 15?m-thick ZrB₂coating show similar and lowest linear and mass ablation rates.However,the modification effect is not obvious compared with samples coated with single ZrB₂coating.The main reason is the CTE mismatch between SiC and ZrB₂ coatings which causes obvious cracks in the coating.Though the formed ZrO₂-SiO₂ mixed skeleton layer consisting of glassy SiO₂ and ZrO₂ particles during the ablation test can effectively block the diffusion of oxygen into sample,the cracks in the coating protect is detrimental to anti-ablation properties,leading to an overall anti-ablation property similar to samples with single ZrB₂ coating.?5?RCVD ZrB₂ and SiC-ZrB₂?RCVD?coating modified C/SiC composites were successfully obtained by pack cementation with Zr metal powder or impregnation of Zr metal into the C/SiC composites.Ablation behaviors of these composites were studied by oxyacetylene flame.Results indicate that the linear and mass ablation rates of RCVD single layer ZrB₂ modified C/SiC composites fabricated by pack cementation with Zr metal powder are reduced by 31%and 46%,respectively,and that of the RCVD ZrB₂ modified C/SiC composites fabricated by impregnation of Zr metal into the C/SiC composites are reduced by 18%and 50%,respectively.Modification with RCVD ZrB₂ does improve the anti-ablative properties of C/SiC composites,though the improvement is limited because of limited ZrB₂ concentration from both pack cementation and impregnation processes.The SiC-ZrB₂?RCVD?modified C/SiC composites fabricated by either pack cementation or impregnation do not show obvious improvement in anti-ablative properties because of the CTE mismatch between SiC and ZrB₂.However,the high melting point ZrB₂ ceramics consume part of heat and protect the composites to a certain extent.?6?The residual thermal stress in the SiC-ZrB₂-2D C/SiC composite was analyzed by using COMSOL and its embedded computation module.The results indicate that residual thermal stress in the system do not change much with variation in thickness of substrates at fixed ZrB₂ and SiC layer thickness.With increase in the thickness of the top SiC layer,tensile stress produced in the inner ZrB₂ layer increases gradually,compressive stress produced in the top SiC layer first decreases and then increases,and compressive stress produced in matrix does not change much.With increase in the thickness ratio of SiC/ZrB₂,tensile stress produced in the inner ZrB₂layer increases gradually,compressive stress produced in the top SiC layer also first decreases and then increases,and compressive stress produced in matrix does not change much.When the thickness ratio of SiC/ZrB₂ is 10?m¹5?m:10?m,compressive stress produced in the top SiC layer and tensile stress produced in the inner ZrB₂ layer are the lowest.The results demonstrate that CTE mismatch between coatings and substrates is the key factor that limits the improvement of anti-ablative properties of composites modified with SiC-ZrB₂ coatings.