ZrSi2-SiC/SiC Gradient Coating of Micro-Structure and Anti-Oxidation Property on C/C Composites Prepared by SAPS

A ZrSi2-SiC/SiC gradient coating system was designed to reduce the thermal stress of anti-oxidation coatings for C/C composites and prolong their anti-oxidation time at a high temperature. The SiC transition layer was prepared by pack cementation and the gradient ZrSi2-SiC outer coatings with different ZrSi2 contents added were deposited by supersonic air plasma spraying (SAPS). The micro-morphologies and phase compositions of the coatings were studied by SEM, EDS, XRD, and TG/DSC, and their anti-oxidation performances were tested by a static oxidation experiment. The findings suggested that the gradient coating with 30 wt.% ZrSi2 content displayed the optimum and dense microstructure without obvious pores and microcracks, compared with the other three proportional coatings. During the oxidation test, because of the oxidation reaction of ZrSi2 and SiC phases, a large amount of silica was formed in the coating to fill the pores and microcracks and densify the coating further. Oxidation products ZrO2 and ZrSiO4, having a high melting point and outstanding anti-oxidation property, were embedded in the SiO2 glass layer to reduce the layer volatilization rate and improve the ability to block oxygen. Therefore, the specimen with 30 wt.% ZrSi2 still kept mass gain after 188 h oxidation time at 1500 °C. However, when the oxidation time was increased to 198 h, it had a mass loss of 0.1%, because the coating compactness was destroyed by the escape of the oxidation gases.