THE OXIDATION OF TITANIUM NITRIDE-COATED AND SILICON NITRIDE-COATED STAINLESS-STEEL IN CARBON-DIOXIDE ENVIRONMENTS

A study has been undertaken into the effects of thin titanium nitride and silicon nitride coatings, deposited by physical vapour deposition and chemical vapour deposition processes. on the oxidation resistance of 321 stainless steel in a simulated advanced gas-cooled reactor carbon dioxide environment for long periods at 550-degrees and 700-degrees-C under thermal-cycling conditions. The uncoated steel contains sufficient chromium to develop a slow-growing chromium-rich oxide layer at these temperatures, particularly if the surfaces have been machine-abraded. Failure of this layer in service allows formation of less protective iron oxide-rich scales. The presence of a thin (3-4-mu-m) titanium nitride coating is not very effective in increasing the oxidation resistance since the ensuing titanium oxide scale is not a good barrier to diffusion. Even at 550-degrees-C. iron oxide-rich nodules are able to develop following relatively rapid oxidation and breakdown of the coating. At 700-degrees-C, the coated specimens oxidize at relatively similar rates to the uncoated steel. A thin silicon nitride coating gives improved oxidation resistance, with both the coating and its slow-growing oxide being relatively electrically insulating. The particular silicon nitride coating studied here was susceptible to spallation on thermal cycling, due to an inherently weak coating/substrate interface.