Microstructure evolution of SiC-ZrB₂-ZrC coating on C/C composites at 1773 K under different oxygen partial pressures

The oxidation behavior of SiC-ZrB₂-ZrC coating on C/C composites was investigated, basing on the microstructure evolution of oxide scale under different oxygen partial pressures at 1773 K. Similar experiments were conducted in SiC coatings, ZrB₂and ZrC ceramic sheets for comparison. The multiphase coating showed the better oxidation resistance at a higher oxygen partial pressure, and its mass change was 1.34% after isothermal oxidation for 6 h at an oxygen partial pressure of 80,000 Pa. With the increase of the oxygen partial pressure, ZrO₂particles with fibrous denticles were replaced by the aggregations of ZrO₂particles and more SiO₂glass was formed in a short time. The formed SiO₂glass could effectively heal the microcracks of oxide scale. Moreover, the ZrB₂and ZrC phases played an important role in deflecting cracks and restraining the propagation of cracks during oxidation. As the oxygen partial pressure increased, the oxide scale of SiC-ZrB₂-ZrC coating became denser, presenting a two-layered structure composed of a SiO₂layer and a Si-O-Zr layer at the elevated oxygen partial pressure.