| In recent years,hypersonic vehicles have received a lot of promotion around the world as their broad applications in civil and military fields.Due to its sharp airframe,the tip(nose cap and leading edge)parts of these hypersonic vehicles encounter severe extreme environment during the atmosphere fight,including ultra-high temperature(>1600?),oxidation/ablation etc.Thus,high-performance thermal structure and protection materials are urgently needed.Because ZrB2-20SiC(volume fraction,vol.%)composite possesses the combination of good thermophysical properties,mechanical properties and oxidation resistance,it becomes one of the most promising candidates for the thermal protection materials in the application of hypersonic vehicles.However,at temperature over 1650? or in gases with low oxygen partial pressure,the active oxidation of SiC would have occured to form SiO gaseous,meanwhile the evaporation of molten SiO2 surface layer would be enhanced,resulting in the rapid reduction of the oxidation resistance of ZrB2-20SiC.Thus,the approach for further improving the oxidation resistance of ZrB2-20SiC by adding the third phase were proposed,and various kinds of reinforcement phases have been explored.Up to now,a great number of investigations have been carried out,but in these researchs,the oxidation/ablation tests of the modified ZrB2-20SiC composites were usually conducted at temperatures lower than?1600?,and the theoretical analysis to guide the selection of effective additives for ZrB2-20SiC was also rare.In this research,the volatility diagrams of ZrB2-SiC system at 1800? were calculated and compiled,it was found that the major volatile species in the composite is B2O3.The oxygen affinities of various possible additives were thermodynamically compared in a wide temperature range.Based on the analysis of the properties of the additives and their oxides,MoSi2 and TaSi2 were thought to be the suitable additives for improving the ultra-high temperature oxidation resistance of ZrB2-20SiC composite.Dense ZrB2-20SiC-xMSi2(M=Mo or Ta)bulks with the silicides contents of 3,6 and 10 vol.%were prepared by hot pressing under the conditions of 1850??1900?/30 MPa/45 min?60 min.The densification behavior,phase components,microstructure and mechanical properties of the as-prepared composites were investigated.The additions of these two silicides led to the formation of corresponding solid solution shells around the matrix ZrB2 grains,and promoted the densification of the composites.The hardness of ZrB2-20SiC was slightly decreased after the additions of MoSi2 and TaSi2,as the hardnesses of the added silicides were relatively lower than that of ZrB2 and SiC.While the flexure strength(both at room temperature and elevated temperatures)as well as the fracture toughness were significantly improved,especially the enhancement of the strengthes at elevated temperatures were above 35%.The enhancement mechanisms of the strength and toughness of ZrB2-20SiC by adding MoSi2 and TaSi2 were attributed to the decrease of ZrB2 grain size as well as the increase of deflection and bridging quantities in the crack paths during fracture.The oxidation behavior of ZrB2-20SiC-xMSi2(M=Mo and Ta,x=0,3,6 and 10 vol.%)composites in air at 1000? and 1600? were investigated via thermogravimetric analysis and Muffle furnace.The results showed that at 1000?,the oxidation kinetics of all composites depended on both of the weight loss induced by B2O3 volatilization and the weight gain induced by the formation of nonvolatile products.While at 1600?,all of the oxidation kinetics followed the parabolic law.The addition of TaSi2 deteriorated the oxidation resistance of ZrB2-20SiC at 1000?,and had no influence on the oxidation performance at 1600?.However,the oxidation resistance of ZrB2-20SiC was improved by adding MoSi2 at 1000? and 1600?,and the enhancement became more significant as the increase of MoSi2 content.Therefore,MoSi2 is the superior additive for the modification of oxidation resistance of ZrB2-20SiC properties,and the addition of 10 vol.%MoSi2 exhibited the best effectiveness.The oxidation behaviors of ZrB2-20 vol.%SiC(ZS)and ZrB2-20 vol.%SiC-10 vol.%MoSi2(ZSM-3)composites were investigated at 1800? in air with different pressures of 5×102 Pa?1.01×105 Pa a using a self-build ultra-high temperature oxidation testing apparatus.It is found that the oxidation of ZS and ZSM-3 at 1800? in air with normal pressure(1.01×105 Pa)was controlled by inward diffusion of oxygen through surface oxide scale.With the decrease of air pressure,SiO2 formed on the oxidized surface reduced,and the thickness of the oxide layer increased.In air with the pressure of 5×102 Pa,both of ZS and ZSM-3 ceramics exhibited catastrophic oxidation.The reason is that under this condition,the oxygen partial pressure was lower than the critical value(230 Pa)for the intrinsic active oxidation of SiC,resulting in the formation of a great number of gaseous SiO rather than molten SiO2.When air pressure was above 5×103 Pa,the addition of MoSi2 improved the oxidation resistance of ZrB2-20SiC.This phenomenon might be attributed to two reasons:firstly,the formation of MoB via reactions between MoSi2 and ZrB2 or B2O3 inhibited the formation of volatile B2O3 and led to the presence of a continuous SiO2 outer layer in the oxide scale.Secondly,as the equilibrium oxygen partial pressure of the reaction reactions between MoSi2 and ZrB2 is much lower than that of ZrB2 oxidation,the oxygen partial pressure range in the SiC-depleted zone became narrowed for ZSM-3 composite.Thus,the internal oxidation of SiC was retarded,and the thickness of the SiC-depleted zone as well as the formation of SiO gaseous was reduced. |
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