Preparation And Anti-ablation Properties Of SiC And ZrC Coatings Through Chemical Vapor Deposition

Carbon/carbon (C/C) composites were optional materials of thermal protectionsystem in aerospace vehicle application, because of their excellent high thermalproperties. However, poor oxidation and ablation reisistance of C/C compositesabove370℃during oxygen-containing atmosphere had severely restricted their useas protection materials in aerospace application. Protective coating was proved as anefficient way to resolve this problem. SiC, ZrC were ideal material as ablationresistance coating for C/C composites, with high melting point and excellentchemical stability.In this paper, SiC, ZrC coatings were prepared on C/C composites and carbonfibers by low pressure chemical vapor deposition (LPCVD). The effect of parameterson microstructure of SiC and ZrC coating was studied. The anti-ablation propertiesand mechanism of coatings were investigated and analyzed in oxyacetyleneenvironment with cyclic mode and different heat fluxes mode. In order to release themismatch of coefficient of thermal expansion between ZrC coating and C/Ccomposites, multilayer coating of SiC/ZrC and SiC/ZrC/SiC were designed andprepared. The microstructure and ablation property of coating were detected byX-ray diffraction, scanning election micropore, transmission electron micropore andoxyacetylene ablation test, and other methods. The main contents and results of thispaper are listed as follows:The SiC coatings were prepared on C/C composites and carbon fibers byLPCVD at different temperatures and H2/MTS malor ratio. Based on the analysis ofcoating microstructures, the effect of growth temperature and H2/MTS ratio on thecoating structure was received. With the increase of temperature, the growth rate ofSiC particle was increased, the difference in particle size was obvious and the coatingsurface was more roughness. With the increase of H2/MTS ratio, the nucleation effectof SiC particle was enhanced, the size of SiC particle was gradually decreased withuniformity distribution, and the coating was denser. The sturcute of SiC coating onC/C composites and carbon fiber shows many twinning and stacking faults. Thegroove on the PAN Carbon fiber and the as-deposited SiC layer provide the templatefor the CVD-SiC growth. The growth mode transimateed from Laminar/Island growth to Laminar+Island growth mode as the thinckness of SiC layer increased.Oxyacetylene torch ablation results show that, SiC coating can improve theanti-ablation property of C/C composites in a short time. With the increase of cycleablation times, the number and size of defects in the SiO2layer were increased as aresult of increasing thermal shock effect, the micropore in SiO2layer graduallyextended to be destrucvtive hole, resulting in the enhancive loss of oxidation andmechanical erosion, leading to failure of the coating. The main ablation behavior ofSiC coating in central ablation region was oxidation corrosion and mechanicalerosion. And the oxidation corrosion was the main ablation behavior of coating intransitional and border ablation regions. With the increase of heat fluxes, the mainablation behavior of SiC coating transformed from oxidation corrosion andmechanical erosion to gasifation.The loss of SiO2layer decreased gradually far awayfrom the ablation center. The mass and line ablation rates of coated sample increasedsignificantly.ZrC coating was prepared with ZrCl4-C3H6-Ar-H2system on C/C compositesand carbon fibers by LPCVD. The coating was composed by cubic ZrC. With theincreas of the temperature, the growth rate of ZrC particle was increased, the growthuniformity of coating was decreased and the preferential growth orientation ofcoating transformed from (111) to (200) plane. With increase of the diluent H2flow,the nucleation of ZrC particle was increased, the grain size of coating was reducedand the growth uniformity of coating was enhanced, the preferential growthorientation of coating transformed from (111) to (200) to (220) plane, and thecompactness of coating was improved. With the increase of distance betweendeposited position and gas inlet, the concentration of reactant was graduallydecreased, the nucleation of particle was enhanced, the grain size of ZrC coatingdecreased, the coating surface became smooth.The ablation property of ZrC coated C/C composites was test in anoxyacetylene torch environment, and the corresponding ablation behavior andmechanism were analyzed. Results show that, the ablation property of ZrC coatingdeposited at different temperatures was improved as the growth temperaturesincreased, and the coating prepared at1350℃showed the best ablation property,which was contributed to the barrier of oxygen inward diffusion of ZrO2and ZrCxOylayers with timely healing the defects such as microhole and microcrack during ablation, as well as the effect of decreased mechanical erosion of flame on thecoating. The ablation property of coating prepared at different H2flows were firstimproved and then decreased. The property of coating was improved due to thetoughness of nanoparticles, which could reduce the denudation loss and improve theablation property of the coating. The ZrO2nanorods were found after the alation ofcoating. The ZrO2particles provided the growth substate and play a role of caralyst.The growth mechanism of ZrO2nanorods were V-L-S and OAG modes. During therecycle ablation, two ablation modes were existed: surface ablation and innerablation. With the increase of recycle time, the size and number of defects in thecoating were increased resulting from the increasing thermal shock of flame, leadingto the transformation from surface ablation to inner ablation. The evolution of ZrCcoating during ablation was from ZrC to ZrCxOyto ZrO2. Druing the different heatfluxes ablation, with the increase of the heat flux, the diffusion amout of oxygen intothe coating was increased and the process of ZrC→ZrCxOy→ZrO2was acceleted.SiC/ZrC and SiC/ZrC/SiC multilayer coating systems were prepared by LPCVDmethods through design of coating structure. The SiC inner layer could release themismatch of coefficient of thermal expansion effectively between ZrC coating andC/C composites, the combination of coating and C/C substrate was improved. Thecontract between flame and ZrC layer was dalyed by outer SiC layer, and the coatingsurface temperature was reduced resulting from the evaporation of SiO2layer. Basedon the above reasons, the ablation property of C/C composites was further improved.