| The improvement of the thrust-to-weight ratio and combustion efficiency of aero-engines is crucial to the advancement of the economy and energy technology.However,achieving these improvements relies heavily on the enhancement of turbine front inlet temperature,which is a key challenge in aero-engine combustion chamber design.SiC ceramic matrix composites(SiCCMC)with high-temperature resistant property have been developed as a promising material for hot end components of combustion chambers.Unfortunately,the high-temperature water vapor present in the engine combustion chamber can severely corrode SiC-CMC,leading to a sharp decline in its service performance.To mitigate this issue,Environmental Barrier Coatings(EBCs)have been proposed to isolate the corrosive gases from the surface and improve the SiC-CMC’s overall performance.After many years of development,EBCs have primarily evolved into a three-layer structure,consisting of a bond layer,an intermediate layer,and a top layer.However,the commonly used Si bond layer materials have limited temperature resistance and service life,which poses a challenge due to the increasing service temperature.To address this issue,NASA has proposed adding HfO2 to the Si bond layer system to improve its comprehensive performance,but the details of this approach are not well-known.To this end,this paper systematically investigates the oxidation mechanism of the Si-HfO2 bond layer,designs a composite bond layer using theoretical calculations,and prepares the corresponding composite powder suitable for plasma spraying.Additionally,the water-vapour corrosion behavior of the SiC substrate and Si bond layer was studied.The main findings of this study are as follows:(1)By studying the isothermal oxidation and corrosion of Si-HfO2 composite bond layers prepared by plasma spraying,it was found that with increasing oxidation time,the amount of surface glass phase increased while the surface roughness decreased.However,numerous dark spots were observed on the surface,which were identified by EDS analysis as partially oxidized Si covered by the surface glass phase.The number and area of Si spots decreased with increasing oxidation time,but the spots always avoided HfO2 distribution.Analysis of the coating cross-sectional morphology showed that SiO2 generated by Si oxidation was consumed by HfO2 to form HfSiO4 until all HfO2 was consumed.This process helped to alleviate the thermal-physical mismatch damage caused by SiO2.Additionally,it was discovered that Si around HfO2 preferentially oxidized to generate SiO2.This suggested that O2 diffusion in HfO2 is faster than in SiO2,indicating that HfO2 plays a role in oxygen transfer in the bond layer system and accelerates Si oxidation.Therefore,a continuous HfO2 network is not beneficial to the antioxidation performance of the Si-HfO2 composite bond layer.Based on this,the common distribution forms of HfO2 in Si were analyzed,including large-particle aggregation,band-like aggregation,small-particle dispersion,and small-particle network structure.It was concluded that HfO2 can consume SiO2 without affecting the antioxidation of Si when it is dispersed in small particles and does not form a network structure.Using a random distribution model carried by Matlab software,the optimal HfO2 doping ratio was calculated to be 20 vol%,and the model was validated by oxidizing pressed samples.(2)Due to the significant difference in melting points between HfO2 and Si,it is challenging to obtain powders suitable for plasma spraying through simple mechanical mixing.Additionally,controlling the content of Si and HfO2 and their distribution in resulting coatings is difficult.Therefore,this paper presents a method to prepare composite powder with evenly distributed HfO2 to improve its distribution the bonded layer.The proposed method involves the innovative preparation of composite powder using the freeze-drying technique.First,a wet ball milling process is used to uniformly mix Si and HfO2 in the slurry.The slurry is then rapidly frozen using the freeze-drying method to preserve the uniform distribution of Si and HfO2.The thickness of the formed layer structure can be changed by adjusting the freeze-drying temperature.The subsequent crushing and sieving process is used to obtain controlled particle size,good flowability,and uniform distribution of Si-HfO2 composite powder.The mixingroasting-crushing method is used to prepare Si-HfO2 composite powder to reduce the organic content of the bond.The properties of the two powders are compared,including element distribution,particle size distribution,and fluidity.The results indicate that both powders meet the requirements of plasma spraying and can prepare Si-HfO2 composite bond layer with more uniform composition distribution.(3)The investigation of the isothermal air oxidation and water-vapour corrosion behavior of SiC substrate revealed that the oxidation rate of SiC in water-vapour corrosion is significantly higher than that in isothermal air oxidation.Additionally,the study of the water-vapour corrosion behavior of the Si bond layer showed that the presence of cracks and pores resulted in the rapid diffusion of water vapor and oxygen in the bond layer,leading to severe Si corrosion.Therefore,it is necessary to further enhance the coating densities to improve their protection against corrosive gases. |
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