| For the development of ultra-high temperature thermal protection materials used in hypersonic aircrafts,the preparation,oxidation and ablation behaviors of Ir-Al-X?X=Hf,Zr,Ta and Y?coatings by in-situ alloying modification of Ir were studied.The influence of processing parameters of pack cementation on the structure of Ir-Al coatings was systematically studied.The preparation methods of Ir-Al-X coatings were explored.The ablation behaviors of Ir-Al coatings with different structures were investigated.The oxidation and ablation behaviors of Ir-Al-X?X=Hf,Zr,Ta and Y?coatings were studied by induction heating set-up and high frequency plasma wind tunnel,respectively.The influence mechanism of the addition of X elements on the oxidation and ablation behaviors of the Ir-Al coatings were summarized.The effects of processing parameters of pact cementation on the microstructure and growth kinetics of the Ir-Al coating were studied.The feasibilities of preparation methods of Ir-Al-X coatings were explored.It was found that the as-prepared Ir-Al coatings have typical double-layer structures.With increasing processing time,the thickness of the IrAl inner layer is almost constant??2-3?m?,while the IrAl2.7?3?outer layer grows parabolically.The growth kinetic constant of the Ir-Al coating is?2.13×10-13 m2/s at 650?.The Ir-Al-X?X=Hf,Zr and Y?layers could be successfully obtained on the surface of Ir-Al coatings by pack cementations of Al and X,successively.The Ir-Al-X layers with a uniform element distribution are composed of a mixture of various binary intermetallic compounds.The ablation behaviors and failure mechanism of the Ir-Al coatings were studied.Compared with the single-layer Ir-Al coating,the double-layer Ir-Al coating exhibits a lower thermal response and a higher ablation resistance limitation due to a denser Al2O3layer resulting from the external oxidation of IrAl2.7?3?outerlayer.When the heat flux was 3.75 MW/m2 and the stagnation point pressure was 5.06 kPa,the stagnation point temperatures of the double-layer and single-layer Ir-Al coating samples reached 1630?and 2070?,respectively.When the stagnation point temperature is higher than the melting point of Al2O3,the Ir-Al coating rapidly fails due to the melting of Al2O3 outer layer.The ablation limitation conditions of Ir-Al coating is at the heat flux of 3.75MW/m2 and the stagnation point pressure of 5.06 kPa.Pre-oxidation of the Ir-Al coatings results in a large number of defects in the Al2O3 outer layer formed on the surface,causing a reduction in the ablation resistance limitation of the Ir-Al coating?heat flux of 2.80 MW/m2 and the stagnation point pressure of 6.93 kPa?..The oxidation and ablation behaviors and modification mechanism of the Ir-Al-X coatings were studied.At the oxidation temperature of 1800?,the oxide layer formed by the Ir-Al-Y coating has a poor thermal compatibility with the substrate,and the Ir-Al-Ta coating could hardly form a dense and continueous oxide layer.Therefore,these two kinds of coatings quickly lost the oxidation protection within 5min.While a continuous but porous mixed oxide layer were formed on the surface of the as-oxidized Ir-Al-Hf and Ir-Al-Zr coatings,which lead to the transformation of the external oxidation to internal oxidation of the underneath layers.The oxidation and volatilization of Ir'in the mixed oxide layer weakens their interlayer bonding strength,eventually leading to the spalling of the oxide layer and the oxidation resistance deterioration of thecoating.Besides,the relatively lower eutectic points of the mixed oxides restrict the limitation of the service temperature of the Ir-Al-X coating. |
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