High-temperature Ablation Behavior Of ZrB₂-based Ultra-High Temperature Ceramics

ZrB₂-based ultra-high temperature ceramics are one kind of most potential candidates for thermal protection systems?TPS?in hypersonic re-entry vehicles and scramjet components.However,ablation resistance under hypersonic flowing conditions has been widely recognized as one of the key life-limiting factors for their TPS applications.Thus,transition metal carbides,such as SiC,ZrC and WC,have been introduced to ZrB₂ ceramics to improve their high-temperature ablation resistance.In present study,volatility diagrams for a series ZrB₂-based system were calculated based on NIST-JANAF thermodynamic database to predict the oxide layers,and the mechanisms of improved ablation resistance via the addition of ZrC or WC were discussed.Meanwhile,a series of ZrB₂-based ceramics with different compositions or microstuctures were ablated by oxyacetylene flame,and the corresponding temperature fields and thermal stress fields were simulated to investigate their high-temperature ablation behaviors.In details:1)Volatility diagrams for Zr B₂-based system indicated that the interactions of individual solid species in corresponding system are insignificant,and a series of predictions from volatility diagrams were confirmed by the experimental investigations.In addition,the details mechanisms analysed.For ZrB₂-SiC-ZrC ceramics,as the first phase to be oxidized,the oxidation of ZrC could reduce the oxygen levels in oxide layers,which could be temporarily protected before all the ZrC grains had been consumed.For ZrB₂-SiC-WC ceramics,the oxidation of the W-containing phases is believed to have formd liquid phases that enhanced the sintering of the top ZrO₂-base layer,yielding a significantly denser and equiaxed ceramic that reduce the potential for oxygen transport.2)The temperature gradients inside the samples during stable ablation process were calculated by the combination of real-time surface temperature and thermal properties.The surface temperature and the temperature gradient would decrease by adding SiC into Zr B₂ ceramics.The highest surface temperature of ZrB₂-20 vol%SiC ablated under the heat flux of 4.5 MW/m² was 1355?,which was 51? lower than ZrB₂ ceramics.However,for Zr B₂-SiC-based ceramics,the surface temperatures and the temperature gradients would increase by increasing the volume content of ZrC.ZrB₂-20 vol%SiC-20 vol%ZrC exhibited 76? higher than ZrB₂-20 vol%Si C when ablated under the heat flux of 4.5 MW/m².3)The calculated thermal stress fields indicated that the coefficient of thermal expansion?CTE?could decrease by adding SiC,resulting in less thermal stress of Zr B₂ceramics.However,the differences of CTE of ZrB₂ and SiC would introduce more thermal stress to generate more microcracks.In the other hand,SiC could improve self-healing of ZrB₂ ceramics during ablation theoretically,which would improve the ablation resistance.When the samples were ablated under the heat flux of 3 or 6 MW/m²for 5 min,the residual strengths increased compared to the initial strengths.Nevertheless,the samples ablated under the heat flux of 9 MW/m² for 5 min,the residual strengths decreased apparently compared to the initial strengths.The residual strength after ablated under the heat flux of 9 MW/m² for 5 min was 80%for ZrB₂,and23%for ZrB₂-20 vol%SiC-5 vol%ZrC and Zr B₂-20 vol%SiC-10 vol%ZrC.4)The high-temperature ablation behaviors of ZrB₂-ZrC-SiC ceramics with different microstructures were also investigated in present work.And the microstructure was tailored by self-propagating high-temperature synthesis?SHS?during reactive hot-pressing sintering?RHP?.SHS would be activated when ZrB₂-ZrC-SiC ceramics with the volume content of SiC was less than 20 vol%were prepared by RHP using Zr,B₄C and SiC as raw materials.High heat and instantaneous high pressure generated during SHS resulted in the melt of fine Zr powders with higher activation into droplets,which would be pressed into flakes.In the mean time,ZrC grains are generating into“microlayers”dispersed in the matrix.5)The RHPed ZrB₂-ZrC-10 vol%SiC ceramics showed anisotropy in ablation behaviors under the heat flux of 4.5 or 7.5 MW/m²,the surface highest temperature differences were 80 and 198?,respectively.Whereas,the RHPed ZrB₂-ZrC-10vol%Si C ceramics didn't show apparent anisotropy during the ablation under the heat flux of 10 or 12.5 MW/m².