Formation mechanism and microstructural evolution of bubbles during ultra-high temperature oxidation of multicomponent carbides

Abstract

Bubbles are prevalent defects on the oxidized surfaces of ultra-high temperature carbides, compromising structural stability and oxidation resistance. Despite their significance, the formation mechanisms and microstructural evolution of bubbles during ultra-high temperature oxidation remain inadequately understood. To address this gap, the bubble behaviors of multicomponent carbides, including (Hf,Ti)C, (Hf,Zr,Ti)C, (Hf,Zr,Ti,Ta)C, and (Hf,Zr,Ti,Nb)C, were investigated under oxidation conditions at 2500°C. The roles of various elements were elucidated through first-principles calculations. Results show that the formation of a dense composite oxide layer is essential for bubble generation, with the release of gaseous products serving as the primary driving force. The microstructure of the bubbles is influenced by the matrix composition. The addition of Ti, Ta, and Nb significantly lowers the surface energy of the shell oxides, providing preferential nucleation sites for bubbles. The progressive oxidation of Ti leads to the formation of a "TiO₂-TiO-HfO₂" multilayer structure at the bubble top, which evolves into a dendritic structure with prolonged oxidation. Ta and Nb further modulate the size and number of bubbles by altering the composition and surface energy of the shell oxides.