Damage Evolution And Failure Mechanism Of Er₂SiO₅ Environmental Barrier Coatings

Silicon carbide fiber reinforced silicon carbide matrix composites（SiC/SiC）are promising for use in hot parts of aero-engine such as combustion chamber due to its low density,high strength and good oxidation resistance.In the gas environment of engines,water vapor would decline the performance of SiC/SiC composites.The environmental barrier coatings（EBCs）on SiC/SiC composites can prevent or retard the corrosion of subsmate material.The first generationEBCs of Mullite（3Al₂O₃·2SiO₂）/ZrO₂and the second generation EBCs of Si/Mullite or Mullite+BSAS（1-xBaO-xSrO-Al₂O₃-2SiO₂,0≤x≤1）/BSAScan only be used under 1300℃。If their service temperature is higher than1300℃,the unmatched thermal expansion cofficient will lead to the early crack of the coatings and glass phase with low melting point will form in BSAS.The above factors limit the service temperature of subsmate material.Rare earth solicates are research hotsposts of the third generation EBCs materials.In the present work,Si/Mullite/Er₂SiO₅EBCs was prepared by chemical vapor deposition（CVD）and plasma spraying,and the microstructure evolution,element diffusion and phase transformation,as well as crack initiation and propagation of the coatings in the environment of high temperature oxidatioon,themal impact and H₂O-O₂were analyzed.The main results are as follows:The microstructure of the coating changes with the effect of different envirnment:In the oxidation envirnment of 1200-1350℃,black phase including Er₃Al₅O₁₂、Er₂Si₂O₇ and SiO₂ forms at the surface of the coating.Er₃Al₅O₁₂、Er₂Si₂O₇ form alternately at the interface of Mullite/Er₂SiO₅.The phase transformation of surface and interface is because of the element Si and Al in the Mullite layer diffuse into the Er₂SiO₅ layer,then they react with Er₂SiO₅ after oxidation.In the thermal impact envirnment of 1350℃,no phase transformation take place at the coating surface and the interface of Mullite and Er₂SiO₅.The thermal impact was performed by the alternate effect of low and high temperature.The effect time of high temperature is short and the effect of high temperature is intermittemt,so the element diffusion is not enough to result in phase transformation.With the increase of the temperature,the difference with that of 1350℃is Si bond layer melts and diffuse fast,forming some holes.With the increase of the oxidation and thermal impact test temperature,the melting and difusion of the Si bond coating aggravate,and the holes form in the coating.In the H₂O-O₂ environment of 1350℃,Er₃Al₅O₁₂ and Er₂Si₂O₇ form at the surface of the coating,but no SiO₂ was remained.It is because SiO₂ reacts with vapor,forming Si（OH）₄,which goes with water vapor.Er₂Si₂O₇ forming at the interface Mullite and Er₂SiO₅ also reacts with water vapor into Si（OH）₄,so some pits form.The crack initiation and propagation characteristics of the coatings vary considerably with experimental conditions.In the oxidation environment of 1200-1350℃,vertical cracks initiate preferentially at the porosity and pore zones of the partially-melting area and the black phase transformation zone on the surface of Er2SiO5layer.Then the cracks propagate inwards in the coatings.The phase transformation at the Er₂SiO₅/Mullite interface accelerates the propagation of the cracks.Glassy SiO₂ resulting from oxidation of Si bonding layer decreases the bonding strength between the Si bonding layer and the Mullite layer.When the vertical cracks propagate to Si bonding coating,they began to propagate along the Mullite/Si interface.Then the transverse cracks connect with one another,resulting in fall-off of the coating.In the thermal impact environment of 1350℃,the side Si bonding coating of the specimen coated with single side of Si/Mullite/Er₂SiO₅ was oxidized,forming SiO₂,which leads to the degradation of the interface strength.Under the effect of cyclic stress by thermal impact,cracks initiate at the side Mulllite/Si interface of the specimen,and then propagate along the interface.When they connect with the vertical cracks in the coating,the coating falls off.In the H₂O-O₂ environment of 1350℃,the particles at the partially-melting area of the coating surface are taken away by the high-speed water vapor,resulting in pits at the surface.These pits become the crack source and promote the crack initiation.When the cracks propagate to the Er₂SiO₅/Mullite interface,the Mullite layer and the Si bonding layer,water vapor reacts with the Er₂Si₂O₇ at the interface,the SiO₂ in the Mullite coating and the SiO₂ in the Si bonding coating deriving from oxidation of Si,producing Si（OH）₄gas and some holes.The holes accelerate the crack growth.As a result,the service life of EBCs is short in the H₂O-O₂ environment of 1350℃ than in the single oxidation environment.