Influence of Additives on Slag Resistance of Al2O3-SiO2-SiC-C Refractory Bond Phases under Reducing Atmosphere

The microstructures of Al₂O₃–SiO₂–SiC–C refractory matrices with aluminum, silicon, Si₃N₄, BN, B₂O₃, and B₄C additives are characterized before and after a crucible slag test, and the phases present are compared to those expected at thermodynamic equilibrium. The carbon content dominates the resistance to CaO–MgO–Al₂O₃–SiO₂ slag penetration, while the viscosity of liquid phases present has a significant influence when the matrix carbon contents are similar. Silicon and Si₃N₄ additives reduce slag penetration resistance because of indirect oxidation of carbon to form SiC. B₄C, in particular, and B₂O₃ also reduce slag penetration resistance because of formation of a more fluid boron-containing liquid, while aluminum and BN addition have no significant effect. Carbon and BN hardly react with the slag, while SiC partially reacts with it, leading to deposition of carbon as a dense layer. Corundum present in the refractories also readily dissolves in the slag. Microstructurally, slag penetration resistance is associated with the dense carbon layer located at the slag-refractory interface.