Na sub 2 SO sub 4 -induced corrosion of Si sub 3 N sub 4 coated with chemically vapor deposited Ta sub 2 O sub 5

Hot isostatically pressed Si sub 3 N sub 4 was coated with chemically vapor-deposited Ta sub 2 O sub 5 , and subjected to oxidative and corrosive environments to determine the feasibility of using a Ta sub 2 O sub 5 coating for protecting Si sub 3 N sub 4 from hot corrosion. The coated structure was relatively stable at 1000 deg C in pure O sub 2 . However, the Ta sub 2 O sub 5 Si sub 3 N sub 4 system became unstable in an environment containing Na sub 2 SO sub 4 and O sub 2 at 1000 deg C because Ta sub 2 O sub 5 and Na sub 2 SO sub 4 reacted rapidly to form NaTaO sub 3 and subsequently NaTaO sub 3 interacted destructively with the underlying Si sub 3 N sub 4 substrate to form a molten phase.