HfC Nanowire-Toughened HfC-Based Ablation Resistance Coatings Synthesized by Chemical Vapor Deposition

Author:Ren Jin Cui

Supervisor:zhang yu lei

Database:Doctor

Degree Year:2018

Download:9

Pages:140

Size:9552K

Keyword:

Carbon/carbon(C/C)composites are promising thermal structural and thermal protection materials in aerospace field owing to their outstanding high-temperature properties.However,the quick oxidation and ablation of C/C composites in high temperature containing oxygen environment limit their practical applications.Employing an external coating is the most effective method to solve this problem.HfC is suitable to be used as the ablation resistant coating material for C/C composites because of its high melting point,high thermal stability and chemical stability.While,the inherent brittleness of HfC coating and the coefficient of thermal expansion(CTE)mismatch between HfC coating and C/C substrate will lead to the coating cracking or peeling during preparation and ablation.In this paper,to improve the ablation resistance of C/C composites,HfC nanowires were incorporated into the HfC-based coatings to improve the toughness and ablation resistance of the coatings.The preparation technology of HfC nanowires and HfC-based coatings was investigated.The influence of HfC nanowires on the growth process,microstructure,mechanical properties,thermal properties and ablation resistance of the HfC-based coatings was also investigated,as well as the growth mechanism of HfC nanowires,ablation mechanism of the HfC-based coatings and the toughening mechanism of HfC nanowires.The major research contents and results are listed as follows:HfC nanowires were synthesized on C/C composites by catalyst assist chemical vapor deposition(CVD)technique using HfCl4-CH4-H2-Ar system.The influence of deposition temperature,flow rate of CH4,concentration of H2 and deposition time on the phase composition and microstructure of HfC nanowires was researched.Results indicated that with the deposition temperature increasing from 1050 to 1150℃,both the diameter and yield of the HfC nanowires increased;excess CH4 leaded to the isotropic growth of HfC crystal and restrained the formation of HfC nanowires;with the increase of H2 concentration(25-100%),the diameter of the HfC nanowires decreased;when the deposition time increased from 0.5 to2 h,the density and thickness of the HfC nanowire layer increased;the growth process of the HfC nanowires was controlled by the root vapor-liquid-solid(VLS)mechanism.HfC coating was prepared on C/C composites by CVD technique using HfCl4-CH4-H2-Ar system.The influence of deposition temperature,flow rate of CH4,concentration of H2 and deposition time on the deposition rate,phase composition and microstructure of HfC coating was investigated.The ablation mechanism and failure mechanism of the HfC coating were also researched.Result showed that with the deposition temperature increased from 1200 to 1350℃,all of the deposition rate,compactness,uniformity and grain size of the HfC coating increased;with the flow rate of CH4 increased from 300 to 600 mL/min,the deposition rate and compactness of the HfC coating increased,while the grain size of the HfC coating decreased;when the concentration of H2 increased from 25%to 100%,all of the deposition rate,compactness and grain size of the HfC coating increased;with the deposition time extending from 2 to 8 h,the deposition rate of the HfC coating decreased,the compactness,uniformity and thickness of the HfC coating increased;after ablation for 120 s,the HfC coating peeled off,the mass ablation rate and linear ablation rate of the HfC-coated C/C sample was 1.24 mg/s and 1.97μm/s,respectively,the CTE mismatch between HfC coating and C/C substrate and the interlaminar gaps in the HfC coating leaded to the coating cracking and peeling during ablation,which resulted to the coating failure.The HfC nanowire-toughened HfC coating was prepared on C/C composites by a two-step technique of CVD.The influence of HfC nanowires on the growth process,microstructure,mechanical properties and ablation resistance of the HfC coating was investigated,as well as the ablation mechanism of the coating and the toughning mechanism of the HfC nanowires.Results indicated that after incorporating HfC nanowires,the deposition rate of the HfC coating increased,the structure of the HfC coating changed from columnar crystal to particle-stacked structure;the incorporation of HfC nanowires improved the hardness,elasticity modulus,fracture toughness,flexural properties and bonding strength of the HfC coating;the HfC nanowires could decrease the CTE of the HfC coating and increase the thermal conductivity of the HfC coating;HfC nanowires improved the fracture toughness of the HfC coating through nanowire debonding,nanowire bridge,nanowire pullout and crack deflection mechanism,which could decrease the size and number of the cracks in the coating and improve the ablation resistance of the HfC coating;after ablation for120 s,the mass ablation rate and linear ablation rate of the HfC-coated C/C sample with HfC nanowires was 0.57 mg/s and-0.35μm/s,respectively.HfC-TaC and HfC-ZrC multiphase coatings were prepared on C/C composites by CVD,the phase composition,microstructure and ablation resistance of the coatings were investigated,as well as the ablation mechanism of the coatings.HfC nanowire-toughened HfC-ZrC coating was prepared on C/C composites by a two-step of CVD,the influence of HfC nanowires on the microstructure and ablation resistance of the coating was researched.Results showed that both the HfC-TaC and HfC-ZrC multiphase coatings had a dense structure composed of small particles,the bonding between the multiphase coatings and the C/C substrate was good;during ablation,a stable Hf6Ta2O17 phase was formed due to the oxidation of HfC-TaC coating,which could improve the ablation resistance of the coating;after ablation for 120 s,the mass ablation rate and linear ablation rate of the HfC-TaC-coated C/C sample was 0.87 mg/s and 1.32μm/s,respectively;during ablation,ZrC was oxidized to ZrO2 phase,which could promote the sintering of HfO2 particles and the formation of the continuous oxide layer,after ablation for 120 s,the mass ablation rate and linear ablation rate of the HfC-ZrC-coated C/C sample was 0.63 mg/s and 0.21μm/s,respectively;after incorporating HfC nanowires into the HfC-ZrC coating,HfC nanowires improved the toughness and ablation resistance of the coating,after ablation for 120 s,the the mass ablation rate and linear ablation rate of the sample was only 0.24 mg/s and-0.46μm/s,respectively.