Investigation of the microstructure and oxidation behavior of monolayer and bilayer ZrB 2-SiC-Y 2 O 3 /SiC coatings on C/C composites

To improve the oxidation resistance of carbon/carbon composites in high-temperature environments, the monolayer SiC and ZrB 2 -SiC-Y 2 O 3 coatings and bilayer ZrB 2 -SiC-Y 2 O 3 /SiC coatings were prepared on carbon/ carbon composites by atmospheric plasma spray technique. The chemical composition, morphology, and microstructure of the coating were characterized by XPS, SEM, XRD, and TEM. The oxidation behavior of each layer was evaluated at 1723 K for 10 h in the air. The result showed that the bilayer coating could effectively prevent the degradation of the C/C substrates. After 10 h oxidation, its mass loss was only 1.39 % compared to 15.27 % for the monolayer ZrB 2 -SiC-Y 2 O 3 -coated sample and 8.77 % for the SiC-coated sample. Based on XRD pattern results and microstructure observations, the failure mechanism of the monolayer SiC coating was mainly attributed to the evaporation of SiO 2 at elevated temperatures. The coefficient of thermal expansion (CTE) mismatch between the substrate and coating was the main reason for the spallation of the monolayer ZrB 2 -SiC- Y 2 O 3 coating. For the bilayer coating, the inner layer of SiC was responsible for the superior because it blocked the inward diffusion of oxygen and was used as a transition layer to reduce thermal stress, which made the coating more durable. Meanwhile, the amorphous-nanocrystalline composite structure is believed to be able to reduce the evaporation rate of SiO 2 and prevent the inner diffusion of oxygen.