Durability Design and Optimization of CMC-SiC Coated with Environmental Barrier Coatings
Author:Lu Yong Hong
Supervisor:wang yi guang
The continuous fiber-reinforced SiC matrix ceramic composite（CMC-SiC）has partially been utilized as hot-section components in the aero gas turbine engine due to their excellent properties,such as low density,good oxidation resistance and high strengths at elevated temperatures.The key issues for the CMC-SiC application in hot-section components are stress oxidation and durability.One effective way to solve the stress-oxidizing problem is self-healing modification for CMC-SiC.To date,the durability of CMC-SiC components is usually solved using protective environmental barrier coatings（EBCs）on CMC-SiC.EBCs can effectively retard the corrosion of CMC-SiC components caused by gas environments,and improve its durability.To improve the durability of EBCs,a series of EBCs materials had been developed,such as Mullite,BSAS,and rare earth silicates.The engine and laboratory tests had been widely carried out to assess the durability of different coating systems.During these tests,exhaustive research data was obtained.Based on the obtained data,the method of EBCs durability design and optimization were developed.However,strict technical blockade was implemented.To achieve its durability design and optimization,the failure mechanism of CMC-SiC coated with EBCs and growth mechanism of thermally grown silica oxide（SiO2-TGO）were studied in this research.Based on these results,the material as top coat in the multilayer EBCs which possessed long durability was optimized.The effect of coating thickness on the durability was also discussed.Drawing on the experience of high-entropy alloys,the high-entropy rare earth disilicate was synthetized,and tapped its vadility for EBC material.The major contents and conclusions are show below:（1）The failure behaviors of RE2Si2O7（RE=Y、Yb）coated Cf/SiC in the quasi-static water-vapor corrosion conditions were studied.The results showed that the SiO2-TGO was the key factor which caused the EBCs failure.The SiO2-TGO layer formed at the interface between RE2Si2O7 coating and SiC bond coat after water-vapor corrosion.Its thickness gradually grew with the prolonged corrosion time,and its growth followed the parabolic law.When the thickness of SiO2-TGO scale grew to 45μm,cracks would form at the SiO2-TGO/SiC bond coat interface and propagate along this interface,ultimately resulting in the spallation and failure of EBCs.The preliminary stress simulation was also carried out.Results showed that the stress in the SiC bond coat accumulated as the prolonged corrosion time.The maximum stress located nearby the SiO2-TGO/SiC bond coat interface.This stress resulted in the formation of cracks at the SiO2-TGO/SiC bond coat interface when the maximum stress was greater than the bond strength between SiO2-TGO and SiC bond coat.（2）The Cf/SiC coated with Y2Si2O7-BSAS,Yb2Si2O7,and BSAS coating were carried out in different water-vapor environments at elevated temperatures.The growth behaviors of SiO2-TGO layer beneath different coatings were studied.At relatively low water-vapor partial pressure,the main oxidant which resulted in the formation and growth of SiO2-TGO layer was the ionic oxygen.The diffusion of ionic oxygen in the SiO2-TGO layer dominated its growth.The water vapor also involved in growth of silica-TGO and accelerated the oxidation of SiC bond coat.At high water-vapor partial pressure,the dominant oxidant for TGO growth changed to H2O.（3）The parabolic rate constants of silica-TGO layer beneath Lu2Si2O7 and Sc2Si2O7 coating at 1250oC in 50%H2O-50%O2 were measured.Compared with ones beneath BSAS,Yb2Si2O7,and Y2Si2O7-BSAS coatings,Y2Si2O7-BSAS coating had the lowest parabolic rate constant.The length of RE-O and Si-O bond in the RE2Si2O7structure which the O atom located at the bridge-oxygen site was also studied.The results showed that Y2Si2O7 possessed the lowest ionic oxygen permeability.Based on these results,the Y2Si2O7 was the optimal material as top coat in the multilayer EBCs which possessed long durability.The parabolic rate constants of silica-TGO layer beneath Y2Si2O7-BSAS coatings with different thickness were also measured.It was found that the parabolic rate constant values gradually reduced with the coating thickness increase.（4）The high-entropy rare earth disilicate,(Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7((5RE0.2)Si2O7),was synthetized by Sol-Gel process.(5RE0.2)Si2O7 can entropy stabilize to be a single phase.All of the rare earth elements uniformly distributed in the grains.Its coefficient of thermal expansion was measured,and the value was about 46ppm/K.The water-vapor corrosion resistance was evaluated after corrosion at 1400oC in 50%H2O-50%O2 for 200h.(5RE0.2)Si2O7 showed excellent phase stabilization in the current environments.Compared to the RE2Si2O7（RE=Yb、Y、Sc、Lu）,(5RE0.2)Si2O7possessed the super water-vapor resistance in the water-vapor environments.To tap its vadility for EBC material,the(5RE0.2)Si2O7 coating was fabricated on the surface of Cf/SiC bars and corroded in the 50%H2O-50%O2 at 1250oC.Results showed that the(5RE0.2)Si2O7 coating could effectively protect the Cf/SiC from water-vapor corrosion.