Silicon-based ceramics are leading candidate materials for high-temperature structural applications such as heat exchangers, advanced gas turbine engines, and advanced internal combustion engines. They have excellent oxidation resistance in clean oxidizing environments due to the formation of a slow-growing silica scale (SiO2). However, durability in high-temperature environments containing molten salts, water vapor, or a reducing atmosphere can limit their applications. Molten salts react with silica scale to form liquid silicates. Oxygen readily diffuses through liquid silicates and rapidly oxidizes the substrate. High water vapor levels lead to hydrated silica species, such as Si(OH)4(g) and subsequent evaporation of protective scale. Complex combustion atmospheres containing oxidizing (CO2, H2O) and reducing (CO, H2) gases form SiO2 and then reduce it to SiO(g). In situations with extremely low partial pressures of oxidant, direct formation of SiO(g) occurs. All these reactions can potentially limit the formation of a protective silica scale and thus lead to an accelerated or a catastrophic degradation.One approach overcoming these potential environmental limitations is to apply a barrier coating which is environmentally stable in molten salts, water vapor, and/or reducing atmospheres. Refractory oxides such as mullite (3Al2O3 · 2SiO2), yttria-stabilized zirconia (ZrO2-Y2O3), or alumina (Al2O3) are promising candidate coating materials because of their excellent environmental stability in these severe conditions. Refractory oxide coatings can also serve as thermal barrier coatings because of their low thermal conductivity. Key requirements for an adherent and durable barrier coating include coefficient of thermal expansion (CTE) match and chemical compatibility with the substrate. Mullite in general meets all the requirements and thus appears most promising.
Environmental Stability of High Temperature Molten Vitreous Residue of Hazardous Waste
