Inlaid Coating/Substrate Interface Design And HfB₂-SiC Doping For C/C Composites

C/C composites are easily oxidized and ablated at high temperature,which limit their applications as high-temperature thermal-structural ablation materials.Coating technology and matrix modification are regarded as the two effective ways to enhance the oxidation/ablation resistance of C/C composites.Due to the self-sealing performance of the formed glassy SiO₂(the oxidation product of Si-based ceramics)at high temperature,Si-based ceramics are the ideal coating materials.During high-low thermal cycles,the intrinsic brittleness and the coating/substrate thermal mismatch will result in their cracking,debonding or even failure.In this study,pre-ablation treatment of C/C composites was performed to construct a porous surface structure with controlled depth/range,which could provide the diffusion paths to the coating raw materials and then result in the formation of an inlaid coating/substrate transition interface.The aim is to use the inlaid coating/substrate transition layer to improve the coating adhesive strength and relieve the coating/substrate thermal mismatch.Effects of the inlaid coating/substrate transition layer on microstructure,thermal stress distribution,adhesive strength of the ceramic coating were studied.In addition,effect of the inlaid coating/substrate transition layer on the cyclic ablation performance of the ceramic coating using different temperature/angle oxyacetylene torch was also investigated,and the corresponding failure mechanism was revealed;Due to the service temperature limitation of Si-based ceramic coatings,in order to improve the ablation performance of C/C composites above 2000�C in short time,PIP combined with TCVI,RMI were adopted to prepare C/C-HfB₂and C/C-HfB₂-SiC composites.The ablation performances of the prepared composites were investigated.Based on the simulation of ablation temperature and the analysis of surface morphology and chemical composition before/after ablation,the corresponding ablation mechanism was revealed;With the help of a high-velocity wind tunnel,effect of coating and matrix modification on the performance improvement of C/C composites was studied.The main contents and results are listed as follows:With the help of inlaid coating/substrate transition layer,the adhesive strength of Si-Mo-Cr coating was increased by 48%and the coating/substrate thermal mismatch was relieved,thereby improving its oxidation and ablation performance.By the combination of inlaid coating/substrate transition layer and SiC nanowires,the ablation performance of Si-Mo-Cr coating was further improved.After 30 thermal ablation cycles from1600�C?room temperature,the mass loss per unit area of the coating was reduced by 46%,which was 15 mg�cm^-2.During cyclic ablation test,the introduction of SiC nanowires could avoid the formation of penetration crack caused by the coating brittleness and the thermal stress.Pre-ablation treatment of C/C composites and ZrB₂ doping were combined to improve the oxidation/ablation performance of SiC coating.Thermogravimetric analysis showed that the prepared coating possessed good oxidation resistant property in wide temperature range.From room temperature to 1400?,its mass loss percentage was only 5.8%;After 5 thermal ablation cycles from 1750�C?room temperature(the total time was 100 s),the coating was not peeled off,and the formed Zr-O-Si glassy protective layer showed good ability to withstand the denudation of oxyacetylene torch.Dispersed HfB₂ modified C/C composites were prepared by PIP and TCVI.Thermogravimetric analysis and oxyacetylene torch ablation test showed that the HfB₂ doping could enhance the oxidation and ablation performances of C/C composites.The initial oxidation mass loss temperature of C/C was increased from 520? to 755?,and its linear and mass ablation rates were reduced by more than 30%.The preferential oxidation effect of HfB₂could suppress the oxidation of carbon.During ablation,the formed Hf O₂ layer could prevent part of the heat flux and hinder the diffusion of oxidized species to the substrate.PIP+RMI were adopted to prepare C/C-HfB₂-SiC composites.With respect to C/C-HfB₂composites,the introduction of SiC could improve the thermal stability of Hf O₂ in the high-temperature ablation environment,so its linear ablation rate was reduced by 34%.However,the chemical reaction between the molten Si phase and carbon fiber resulted in the formation of inside defects and the decrease of mechanical property,which was only 60.45MPaPIP was adopted to prepare C/C-HfB₂-SiC composites.With respect to PIP+RMI,the damage of carbon fibers during PIP became slightly,so flexural strength of the composites was increased from 60.45 MPa to 115.25 MPa.Ablation behavior of nose-shaped C/C-HfB₂-SiC composites was studied using oxyacetylene torch.From low to high-temperature ablation region,protective ablation products including borosilicate glass,Hf O₂-glass and Hf O₂ scale were beneficial for enduring the heating conditions.SiC coating was prepared on the surface of C/C-ZrC-ZrB₂-SiC composites by CVD,then its service reliability was investigated in a high-velocity wind tunnel where a wide temperature ablation environment was constructed.The results showed that after the combination of coating and matrix modification,the protective time was prolonged from 19 h to 21 h.With the help of thermogravimetric oxidation analysis and thermal stress simulation,it was concluded that the oxidation resistance improvement of C/C composites could make up the poor low-temperature oxidation property of SiC coating caused by thermal stress and the lack of glassy SiO₂.