Microstructure and composition of multi-layered oxide scale evolved during isothermal exposure of ZrB2–SiC–LaB6 composite to air at 1500°C

The ZrB2–SiC–LaB6 composite prepared by spark plasma sintering with B4C and C as additives was isothermally exposed in air at 1500°C for 24 h and the evolved oxide scale examined by scanning and transmission electron microscopy. It was found that a glassy borosilicate (BSG) layer with a thin film of La2Si2O7 had formed on the outermost surface of oxide scale, contributing to protection against oxidation. The detailed investigation of the cross-section of the oxide scale has revealed the formation of a layered microstructure formed by growth involving partial crystallisation of BSG forming ZrSiO4 along with re-precipitation of La2Si2O7 and ZrO2 after prior dissolution of La2O3 and ZrO2 in the glassy matrix.