Study On The Oxidation Resistant Behavior Of Si/RE₂SiO₅/LaMgAl₁₁O₁₉?RE =Y,Er,Yb? Thermal Barrier Coatings For C_f/SIC Composites

Carbon fiber reinforced silicon carbide ceramic matrix?C_f/SiC?composites has the characteristics of high strength,good fracture toughness,high hardness,high thermal conductivity and low density.So it is considered to be one of the most promising candidate materials for aerospace and weapon equipment hot end members.However,the oxidation and ablation of C_f/SiC composites in high temperature is limited the application of C_f/SiC composites in high temperature oxidation environment.This paper uses the APS method to produce thermal barrier coatings to improve high temperature oxidation resistance and anti erosion performance of C_f/SiC composites.The powders of Re₂SiO₅?Re=Y,Yb,Er?and LaMgAl₁₁O₁₉?LMA?were produced by solid state reactions at high temperatures.Ceramics coating are prepared by atmospheric plasma spraying.The thermal expansion behavior,high temperature phase stability,structural stability and the chemical compatibility between Re₂SiO₅ and LMA powder were studied.The results show that the phase stability of Yb₂SiO₅ is the best;the Er₂SiO₅ anti sintering property is the best;the chemical compatibility of Er₂SiO₅ and LMA mixture powder is poor.Spray drying of synthetic powder,Si/Re₂SiO₅ coatings and Si/Re₂SiO₅/LMA coatings were successfully produced on the surface of C_f/SiC composites by APS.Re₂SiO₅ and LMA powders have good spray phase stability.The coating phase composition is the same as the spraying powder.Between the substrate and the coating,the coating and the coating are well combined.Si bonding layer,Re₂SiO₅ ceramic layer is denser than LMA ceramic layer.In order to study Si/Re₂SiO₅ coating,Si/Re₂SiO₅/LMA coating at 1300?,1400?and 1500? C_f/SiC thermal cycle oxidation protection behavior and 1400? thermal shock experiment,the high temperature thermal shock furnace is used.The study found that the coating has a good oxidation protective effect on the C_f/SiC substrate at1300? and 1400?.After thermal cycle 15 times at 1300?,the weight loss rate of uncoated samples reached 36.4%.The weight loss rates of Si/Y₂SiO₅/LMA,Si/Er2 Si O5/LMA and Si/Yb₂SiO₅/LMA dual ceramic coating samples were 7.98%,5.25%,5.36%,respectively;and the coating was still intact.After thermal cycle at1400? 15 times,the weight loss rate of the uncoated samples reached 44.2%,while Si/Er2 Si O5,Si/Yb₂SiO₅ coating samples were 7.66% and 4.41%,respectively;The thermal cycle 7 times of Si/Y2 Si O5 coating was failure and the weight loss rate was2.06%.The weight loss rate of Si/Y₂SiO₅/LMA,Si/Er₂SiO₅/LMA and Si/Yb₂SiO₅/LMA dual ceramic coating samples were 3.38%,3.3%,4.5%,and the coating is still intact.The anti-oxidation effect of the double ceramic coating is better than the single ceramic coating.Melting of Si bonding layer of Si/Re₂SiO₅/LMA coating sample at 1500? result of only 1 times of thermal cycle causes destruction of the coating,the coating failure.The micro structure of Si/Re₂SiO₅ coating and Si/Re₂SiO₅/LMA coating after thermal cycle at 1300?,1400? and 1500? were analyzed by SEM.After thermal cycle coating surface appears crack,the coating section appears the through crack;It is revealed that the oxidation of the coated samples is mainly due to the existence of a large number of holes and cracks in the matrix.In addition,the mismatch between the coating and the substrate thermal expansion,sintering of Re₂SiO₅ ceramic layer,and the atom Re₂SiO₅ and LMA ceramic layer inter diffusion caused by inter-facial chemical reaction,the boundary between the ceramic layer is fuzzy,Generation and propagation of stress accelerated cracking by chemical reaction.In the thermal shock experiment,the thickness of LMA ceramic layer is reduced,the diffusion distance of oxygen atoms in the coating is shortened,and the oxidation of the substrate is accelerated.These factors have promoted the increase of weight loss.Thermal shock lifetime and failure mechanism of Si/Re₂SiO₅/LMA coating on the surface of C_f/SiC composites at 1400? were studied.The thermal shock lifetime of Si/Y₂SiO₅/LMA,Si/Er₂SiO₅/LMA and Si/Yb₂SiO₅/LMA coatings at 1400? was 34,33 and 27,respectively.Its failure mechanism is: Oxygen diffusion through the coatings to the substrate at high temperature,resulting in increased weight loss rate of the coating samples.Due to the different thermal expansion coefficient of the coating and the matrix,the resistance to sintering of the powder is different,which the thermalstress caused by the volume shrinkage during the thermal cycle.Chemical reactions occur in the diffusion of atoms between the ceramic layers.The stress between the layers is not effectively released,which promotes the generation and propagation of the crack.The weight loss rate of the coating samples increases,which leads to the failure of the coating.