Fracture behavior of MgO-Al2O3-CaO-based porous ceramics prepared with carbon black as pore-forming agent

Abstract

Porous thermal insulation ceramics are widely used as the lining of high-temperature furnaces to reduce heat loss, subjecting to the large compressive stresses during the service. A thorough understanding of the fracture behavior of porous ceramic was necessary to ensure the safe operation of the furnace. In this work, the MgO-Al₂O₃-CaO-based porous ceramics (MACPC) with varied porosity were prepared using carbon black as pore-forming agent. Acoustic emission(AE) and digital image correlation techniques(DIC) were employed to investigate the fracture behavior of MACPC under axial compressive loading. The results indicated that the addition of 5 wt% carbon black promoted the formation of uniformly distributed micropores, resulting in a porosity of 62.7 %, with minimal fractal dimension and pore size. However, excessive carbon black content (5–20 wt%) degraded the internal skeleton structure and mechanical properties of MACPC. The combination of AE and DIC technologies revealed that MACPC experienced five stages, including crack nucleation followed by stable development, crack coalescence leading to unstable growth, and ultimately the formation of macroscopic cracks. This progression was characteristic of typical tensile failure. The content of added carbon black had a significant impact on the thresholds for microcrack formation and coalescence, the number of microcracks, and the ratio of large cracks to tensile cracks during compression. As the carbon black content increased, the failure mode of MACPC transitioned from a large crack propagating parallel to the compressive stress that spanned across the specimen to a progressive damage fracture mode characterized by continuous fragmentation.