Microstructural and mechanical consequences of thermal cycles on a high zirconia fuse-cast refractory

A qualitative and quantitative study of microstructural evolution and mechanical damage was carried out on a high zirconia fuse-cast refractory containing some intergranular glassy phase during high temperature thermal cycles. Microscopy, X-ray diffraction, resonance frequency and acoustic emission were investigated to relate the microstructural evolution to the damage as function of temperature. A detailed description of the mechanisms responsible for the evolution was achieved and three parameters were clearly identified as contributing to the overall behaviour. The glassy phase above Tg accommodates strains and stresses of the structure and heals some cavities or cracks. The zirconia phase transformation induces both cracking and healing, or closure. The thermal expansion mismatch between zirconia and glass, responsible for stress generation during cooling at low temperature, induces cracking and crack closure during heating. Another surprising observation was the delayed cracking after few days at room temperature resulting in stress relaxation, even on very small specimens. The overall behaviour is in the end not reversible, and this was clearly shown by in situ observation of cracking.