Na 2 SO 4 deposit‐induced hot corrosion of SiC fibers relevant for SiC CMCs

Hi Nicalon, Hi Nicalon S, Sylramic, and Sylramic iBN SiC fibers were exposed to ~60 μg/cm 2 of Na 2 SO 4 in a 0.1% SO 2 /O 2 gaseous environment for times between 0.75 and 24 h at 1000°C. After exposure, the corrosion products were characterized using SEM, EDS, ICP‐OES, TEM, and EFTEM to determine their high‐temperature resistance to Na 2 SO 4 and key reaction mechanisms. All SiC fiber types tested in this work exhibited little resistance to Na 2 SO 4 deposit‐induced attack relative to their behavior in dry O 2 environments. It was found that Hi‐Nicalon displayed the least resistance to Na 2 SO 4 deposit‐induced attack due to excess carbon content resulting in the formation of a highly porous crystalline oxide and promotion of basic corrosion conditions. All fiber types formed a crystalline SiO 2 reaction product, either cristobalite or tridymite. Sylramic and Sylramic iBN formed a crystalline SiO 2 reaction layer containing TiO 2 needles due oxidation of TiB 2 particles. Additionally, Na 2 SO 4 deposits resulted in pitting of all fiber surfaces.