Preparation and Application of Zirconium Diboride by Chemical Vapor Deposiiton

Author:Zhu Yan

Supervisor:cheng lai fei liu yong sheng

Database:Doctor

Degree Year:2018

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Pages:158

Size:12620K

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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 5-10μm.ZrB2 coating becomes porous and causes cracks in SiC matrix when coating thickness is within 15-20μm.Composites with a 10μm-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 10μm and 15μm-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 10μm15μm:10μm,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.