Design,preparation And Thermal Protection Mechanism Of Ir-X Coatings

As a noble metal element which has a high melting point and extremely low oxygen permeation rate,iridium has been applied in aerospace propulsion system and could be more potential than the conventional Si-containing ceramics in ultra-high temperature conditions.However,due to the high thermal response in non-equilbrium environment,the application of iridium in thermal protection systems of hypersonic vehicles has been largely restricted.Based on this,this article aims to improve the anti-ablation performance of iridium in non-equilibrium environments through surface modification.Preparation of Ir-X alloy coatings on pure iridium in order to decrease the catalytic behavior while increasing emissivity would contribute to this aim.The article consists of four main parts:the selection of addition element,preparation and optimization of the Ir-X coatings,isothermal oxidation behavior study of Ir-X coating and non-equilibrium ablation of Ir-X coatings.The major purpose is to establish the relationship between composition,preparation method,coating microstructure and the performance,microstructure evolution and failure machanism of Ir-X in non-equilibrium ablation conditions.The final goal of this work is to bring a new idea for the ultra-high temperature thermal protection materials.Based on the consideration of thermalphysical and thermal matching of materials,5elements were selected:Al,Hf,Zr,Y,Ta and their Ir-X coatings were tested in a plasma wind tunnel.Results show that the non-optimized Ir-Hf and Ir-Zr coatings could tolerate heat fluxes of 4.23 MW·m^-2 and 4.5 MW·m^-2,respectively,which were promising.Comparatively,Ir-Y and Ir-Ta coating failed when at heat fluxes under 3.5 MW·m^-2,which were lower than the maximum tolerable heat flux of Ir-Al coating.For Ir-Hf and Ir-Zr coatings,thermodynamic calculations were made first to analysis their pack process.Results show that there are differences between pack process for Hf and Zr:Through optimizing temperature,composition,time and other parameters,the optimized Ir-Hf coating consists of Ir Hf₂,Ir Hf,Ir₃Hf with a total thickness of～10μm while the optimized Ir-Zr coating consists of Ir Zr₂,Ir Zr,Ir₂Zr,Ir₃Zr with a total thickness of 15～20μm.For Hf,the content of intermediate product Hf Cl₃ is relatively high and Hf Cl₃ tends to form Hf and Ir Hf₂ through decomposition or reacting with Ir.For Zr on the other hand,the content of Zr Cl₄is much higher Zr Cl₃.Zr Cl₄ could only form Ir₃Zr and Ir₂Zr through reacting with Ir with the presence of H₂.Thermal shock,scratch test and nanoindentation tests were conducted.Results show that both Ir-Hf and Ir-Zr has a good thermal shock resistance and the Young’s modulus of Ir-Zr sublayers were changed gradiently.Besides,the surface strength and toughness of Ir-Hf was higher than Ir-Zr.Isothermal oxidation behaviors of Ir-Hf and Ir-Zr at high temperature under normal and low pressure were studied.The oxidation test at 1 atm showed that both coatings cannot form continuous oxide for good protection at 1800℃and 2100℃.Therefore,their oxidation resistances were relatively poor.The cross-sectional microstructures of the as-oxidized coatings exhibited mosaic distributions of oxide and iridium particles,which is a morphology of internal oxidation.When the pressure decreased,however,the oxidation resistance of the coatings showed significant improvement.At pressure lower than 5 k Pa of Ir-Zr and 1 k Pa of Ir-Hf,continuous outlayer of oxides formed above iridium,which exhibited typical morphologies of external oxidation.The inner mosaic microstructure and outer continuous oxide could increase the oxidation resistance while improve the coherence with the substrate,which give Ir-Zr a good performance under low pressure at high temperature.Moreover,thermodynamic calculations and experimental verifications confirmed that high X phases will transform to low X phases gradually during oxidation,which makes the as-oxidized cross-sectional sublayer morphology different from original Ir-X.The ablation test in plasma wind tunnel showed the maximum tolerable heat flux of Ir-Hf and Ir-Zr coatings were～6.6 MW·m^-2 and～5.5 MW·m^-2,respectively.The surface temperature of Ir-Hf and Ir-Zr were 2200℃and 2170℃,respectively,before failure.The failure was due to the melting of Ir-Hf O₂（Zr O₂）mixture at the interface of original Ir-Hf（Ir-Zr）and Ir.The melting point of the interfaces were lower than that of iridium.The Hf O₂ formed after oxidation is tough enough to hamper the melted iridium from flowing so the maximum tolerable heat flux is higher.The Zr O₂ formed by Ir-Zr had a poor mechanical property and fractured under the squeeze of melted Ir.As a result,more iridium was exposed to cause the surface temperature increase quickly.The catalytic property and high temperature emissivity of elements like Hf,Zr,Al,Ir and their combounds were studied.The reduction catalytic coefficients basically follow the rule:noble metals>transition metals>main group metals and metals>intermetallics>metal oxides.Therefore,the catalytic behavior of Ir-Hf and Ir-Zr were much lower than that of Ir.At about 2000℃,the emissivity of Ir-Hf was～0.85 while that of Ir-Zr was～0.8,which were higher than most materials.Compared with oxides,the emissivities of intermetallics were also higher.