The Design,Preparation And Anti-ablative Performance Of IrHf_x/Ir Coating On The Surface Of C/C Composites

In this paper,the design,preparation and anti-ablative performance of the IrHf_x/Ir anti-oxidation coating system on the surface of C/C composites was carried out in the background of the ultra-high-speed aircraft thermal protection system for ultra-high temperature(2000°C?2500°C),anti-oxidation and zero ablation materials.Re/SiC was designed as the thermal expansion transition layer and bonding layer of modified Ir coating.The thermophysical and thermochemical compatibility between the two was studied.The mechanism of SiC layer on improving the thermophysical compatibility between C/C matrix and Re layer was revealed.Based on the experimental and thermodynamic calculations,the process for alloying Hf of the surface of the Ir layer was analyzed.The solid-seepage device based on induction heating was developed.The solid-seepage process was optimized to prepare the IrHf_x/Ir coating sample for wind tunnel ablation assessment.The ablation limit of IrHf_x/Ir coating was determined by high-frequency plasma wind tunnel and arc heating wind tunnel ablation of IrHf_x/Ir coating system on two types of C/C substrates with or without SiC transition layer.The oxidation mechanism and failure mechanism of IrHf_x/Ir coating under different wind tunnel conditions were revealed.The effect of thermal expansion transition layer on the ablation performance of IrHf_x/Ir coating in simulated service environment was evaluated.The main research work and conclusions are as follows:The mechanism of the SiC layer in relieving the thermal mismatch between the C/C composite matrix and the Re layer was studied and revealed.The SiC layer prepared by the spray-infiltration method can be transformed into a continuous carbon(C')layer which is also in a loosely stacked structure after being etched by chlorine gas.The introduction of the C'layer significantly improves the thermal match between the C/C composite material substrates and the Re layer.The residual stress on the surface of the Re layer was reduced from 284.6 MPa to 211.5 MPa,and the average surface crack density was reduced from 5.4 mm^-1 before introduction to 2.9 mm^-1.The process of alloying Hf modification of Ir coating was studied,and a solid-seepage device based on induction heating was developed.The IrHf_x/Ir layer with good quality was obtained.The thermodynamic calculation results show that the gas-solid reaction is dominant in the Ir-Hf solidification process,and the main active components in the gas phase infiltration process are Hf Cl₂(g)and Hf Cl₃(g).The solid-infiltration device based on induction heating realizes the atmospheric pressure closed system and rapid temperature rise(?220°C/s),which improves the concentration of the solid-seepage active component on the surface of the sample during the solid-state reaction.Under the optimal preparation conditions,the multilayer Ir-Hf intermetallic compound layer with a total thickness of about 25?m and Ir₃Hf,IrHf,and IrHf₂ from the inside to the outside is uniform in thickness,continuous and complete,and the interface is clear and well combined.The ablation limit,oxidative ablation behavior and failure mechanism of IrHf_x/Ir coating in high frequency plasma and arc heated wind tunnel were studied.The amelioration of ablation performance of C'thermal expansion transition layer for IrHf_x/Ir coating in simulated service environment was evaluated.The study on the ablation limit of the IrHf_x/Ir layer shows that the temperature limit of the IrHf_x/Ir coating is about2400°C,and the ablation failure form in the ultimate ablation test is the coating flow and the Hf O₂ layer torn caused by the melting of the Ir layer.In the high temperature(?2000°C)and low pressure(5?8k Pa)conditions of high-frequency plasma wind tunnel assessment,IrHf_x/Ir coatings tend to undergo internal oxidation,forming a two-phase hybrid structure of Ir'+Hf O₂;IrHf_x/Ir coating can generate external oxidation of dense Hf O₂ layer under high temperature(?2400°C)and high pressure(22.5k Pa)conditions during heating tunnel inspection.The introduction of the C'transition layer significantly improved the thermal physical compatibility and bonding between the coating and the C/C matrix.In the two wind tunnel assessments,the IrHf_x/Ir coating sample with C'transition layer both show superior thermophysical compatibility with C/C substrate.In the high-frequency plasma wind tunnel assessment,the the IrHf_x/Ir coating sample with C'transition layer passed the 300s continuous ablation test.The highest temperature during the test was 2115°C,and the ablation time above 2035°C exceeded 230s.The sample line ablation rate is lower than 1×10^-7 mm/s.