Study On Design And Ablation Resistant Behavior Of Boride Ceramic Based Composite Coatings

ZrB₂ and HfB₂ultra-high temperature ceramics,possessing the characteristics of high melting point,high hardness,high elastic modulus,high thermal conductivity and good chemical stability,are considered as the main candidate materials for thermal protection system(TPS).The selection and design of new ultra high temperature ceramic materials(UHTCs)is required for development of TPS.Up to now,there has been no report on the difference of ablation behavior between the typical carbides and borides UHTCs systems.In addition,most studies of UHTCs system modified by SiC and Mo Si₂have been focused on.However,the active oxidation of SiC during the high temperature oxidation process and the decrease of the high temperature stability of liquid Si O₂,which restricted to their long-term service in extreme environments.In this paper,the oxidation/ablation behaviors of UHTCs coatings in high-temperature/ultra-high-temperature environments have been systematically carried out in response to the above problems,and the following results were obtained:1.A comparative study of the oxidation/ablation behaviors of ZrC,ZrB₂,ZrC-SiC,ZrB₂-SiC and ZrB₂-ZrC-SiC coatings after ablation with two different heat fluxs showed that a large amount of Si O₂liquid phase was retained in the oxide layers of the three composite coatings under 1.5 MW/m²heat flux ablation environment,and no obvious SiC depletion layer was found,which shows good oxidation and ablation resistance.The phase diagram of chemical stability of SiC calculated by thermodynamics proved that the passive oxidation mechanism of SiC begins to change into active oxidation mechanism when the temperature is higher than 1503?.Under 2.0 MW/m²heat flux ablation environment,both single-phase ZrC and ZrB₂coatings suffered serious failures.In the composite coating systems,the ablation resistance of ZrB₂-based coating is obviously better than that of ZrC-based coating,from good to bad,the following is:ZrB₂-SiC>ZrB₂-ZrC-SiC>ZrC-SiC.The difference of the oxidation/ablation behavior of ZrB₂-based coating is reflected in that compared with ZrC-based coating,ZrB₂-based coating exhibits lower surface temperature,thinner total oxide layer and thinner SiC depletion layer thickness.2.The oxidation/ablation behaviors of ZrB₂-Mo Si₂-Yb₂O₃and ZrB₂-Mo Si₂-WB coatings after air plasma flame ablation showed that the doping modification of Yb₂O₃reduced the oxidation/ablation resistance of the ZrB₂-Mo Si₂coating system,resulting in the burst phenomenon appeared in the ablation center.The ZrB₂-Mo Si₂-Yb₂O₃coating was ablated by a high energy flame,and the solid phase reaction between Zr O₂and Yb₂O₃was excited to form Yb₂Zr₈O₁₉,but the high oxygen vacancy content of Yb₂Zr₈O₁₉reduced the oxidation resistance of the coating.In addition,the modified Yb³⁺liquid with high viscosity could not effectively fill the cracks caused by the changes in the cell volume of Yb₂Zr₈O₁₉,resulting in oxygen permeation into the interior.The doping modification of WB greatly improved the oxidation/ablation resistance of the ZrB₂-Mo Si₂coating system.During the ablation process,the competitive oxidation between WB and Mo Si₂can significantly alleviate the volatilization of gaseous Mo-containing oxides,thereby inhibiting the damage to the integrity of the coating.At the same time,since the W layer with high thermal conductivity is formed during the ablation process,the surface temperature of the coating is effectively reduced.In addition,the lower surface temperature is conducive to the stable existence of Mo B and reduces the oxidation rate of the coating.3.A comparative study on the oxidation/ablation behaviors of Hf C-SiC,ZrB₂-SiC and HfB₂-SiC coatings after air plasma flame ablation showed that the order of oxidation/ablation resistance from good to bad is HfB₂-SiC>ZrB₂-SiC>Hf C-SiC,because gaseous CO has more destructive effect than gaseous B₂O₃,and Hf O₂has lower oxygen diffusion coefficient,lower cell volume change,lower thermal mismatch,higher melting point and higher phase transition temperature than Zr O₂.4.The oxidation/ablation behaviors of HfB₂-SiC-Mo Si₂and HfB₂-SiC-WB coatings after air plasma flame ablation showed that the doping modification of Mo Si₂improved the oxidation/ablation resistance of the HfB₂-SiC coating system.Mo Si₂preferentially generates Mo₅Si₃,rather than Mo O₃,in the region of low oxygen partial pressure,which reduces the destructive effect of gaseous volatilization.In addition,Mo Si₂forms a competitive oxidation relationship with SiC,which inhibits the formation of a SiC-depleted layer.The doping modification of WB greatly improved the oxidation/ablation resistance of the HfB₂-SiC coating system.The solid densification of WO₃and Hf O₂,the formation of W-layer with high thermal conductivity and the low oxygen permeability of Hf O₂enhance the oxidation/ablation resistance of the coating,which can last up to 600 s at 2400?.