Contamination effects on interfacial porosity during cyclic oxidation of mullite-coated silicon carbide

Plasma-sprayed mullite (3Al(2)O(3). 2SiO(2)) and mullite/yttria-stabilized zirconia (mullite/YSZ) dual-layer coatings have been developed to protect silicon-based ceramics from environmental attack. The mullite/SiC system develops interfacial pores during cyclic oxidation. The development of pores at the mullite/sintered SiC interface in air has been investigated as a function of the purity of mullite at 1230 degrees-1350 degrees C in an atmospheric-pressure furnace. The silica scale is readily contaminated by impurities of alkali or alkaline-earth metal oxides from the mullite coating. The contamination enhances oxidation and reduces the scale viscosity by forming alkali or alkaline-earth metal silicates. The porosity increases as the temperature and contamination increase and decreases as the purity of the mullite increases.