Oxidation And Failure Mechanisms Of Hafnium Oxide Modified Thermal Barrier Coatings

High performance engines require high thrust-to-weight ratio and large thrust,and the important means is to increase the gas inlet temperature before the turbine or the afterburner temperature.The existing high temperature alloy and cooling technology has been difficult to meet the design and service requirements of hot end components.At the same time,the service life of thermal barrier coating is also a difficult problem.The key to improve the oxidation resistance of thermal barrier coating at high temperature and the bonding between coatings is to improve the oxidation life of coatings.Zirconia is a brittle material with a high plastic brittle transition temperature(DBTT).If the substrate temperature is not properly controlled during spraying,a large tensile residual stress may occur in the coating.When the value exceeds the bonding strength,the coating will fall off.In addition,the combination of Y and Hf can improve the performance of the coating,and further addition of MCr Al Y coating(containing two or more other dopants)can improve the oxidation behavior.However,the influence of the distribution of hafnium oxide in the oxide layer on the anti-oxidation performance of the coating after the high-temperature cyclic oxidation is still unclear,especially the probability of the engine working in the water environment is significantly increased,and the high-temperature oxidation mechanism of Hf modified bond layer and HfO₂modified ceramic layer on the coating in the water-bearing atmosphere environment is still unclear.Therefore,the focus of this experiment is to study its oxidation and failure mechanism.In this paper,different modification methods were studied under different atmospheres(air and water vapor)at 1050?:doping active element Hf in the NiCoCrAlY base alloy layer;the yttria-stabilized zirconia(Zr O₂-7wt%Y₂O₃,7YSZ)powder doped with 10%HfO₂ sprayed on the alloy;co-admixture of alloy layer and ceramic layer is realized.The modification of the alloy layer is doped with active element Hf during alloy smelting.The modification of ceramic layer is mechanical alloying of powder before spraying.The microstructure,morphology and composition of the oxide section of the coating were systematically analyzed,and the thermodynamic properties of the ceramic layer and the alloy layer were tested.The main results are as follows:After cyclic oxidation of undoped,single doped and co-doped coatings in different atmospheres at high temperatures,the samples in water vapor have fewer defects than the oxide layer in air,and show a long oxidation life.The doping of the active element Hf in the base alloy improves the oxidation life of the coating compared to the doping of HfO₂ in the ceramic layer,while co-doping shows the longest oxidation life.After the active element Hf coating in the base alloy is oxidized at high temperature in different atmospheres,the formation position of HfO₂ is different.After cyclic oxidation in water vapor,HfO₂ is formed in the aluminum oxide layer and has a long oxidation life;after cyclic oxidation in air,HfO₂ is formed at the interface within the coating and the oxidation life is short;and the coating is pre-oxidized in water vapor for 24 hours and continues to be oxidized in the air.,the location of HfO₂ has the characteristics of the above two atmospheres at the same time,oxidation life has been significantly improved compared to air.The enrichment of HfO₂ at the interface will cause the stress concentration in the oxide layer,the TGO fluctuation will be large,resulting in a short oxidation life of the coating.The distribution of HfO₂in TGO causes the internal stress to grow slowly,the fluctuation of the oxide layer is small,and the oxidation life of the coating is long.However,pre-oxidation in water vapor continued to oxidize in air for 24 hours.The fluctuation of TGO was significantly improved compared to that in air,and the oxidation life also increased accordingly.It shows that the oxidation in different atmospheres leads to the difference of HfO₂ distribution position in the coating TGO,which makes the TGO stress distribution different,resulting in differences in the oxidation life of the coating.