Effect of heating-cooling rates on microstructure evolution of ZrB2-based coatings during oxidation

In this work, the influence of heating-cooling rates on the microstructure of ultra-high temperature ceramics (UHTCs) during oxidation was firstly reported. Four different kinds of processes (low-heating and low-cooling rates, low-heating and high-cooling rates, high-heating and low-cooling rates, high-heating and high-cooling rates) combined with static oxidation at 1500 °C were applied on ZrB2-SiC-WB composite coatings. The results showed that the oxide layers of the samples presented different microstructure characters under different heating-cooling modes. A relatively thick liquid layer with obvious bubble phenomenon was generated under the high-heating rate, while a thin liquid layer without bubbles was formed under the low-heating rate. Certain amount of sheet B2O3 crystals with large sizes were formed under the high-cooling rate, while did not for the low-cooling rate. The formation mechanisms of these unique microstructures were analyzed and the influence of heating and cooling rates was explained.