ALLOY-GRAIN-SIZE DEPENDENCE OF THE EFFECTIVENESS OF SILICA COATINGS AS OXIDATION BARRIERS ON STAINLESS-STEEL

In order to determine the effect of alloy gl ain size on the oxidation properties of silica-coated austenitic Fe-18Cr-20Ni stainless steel, both coarse-grain (100-mu m grain size) and fine-grain (5-mu m grain s ize) forms of the alloy were produced. A 1-mu m-thick vitreous silica coating was deposited by chemical-vapor deposition on the alloys, whic h were subjected to isothermal and cyclic oxidation in air at 900 degr ees C. The coarse-grain alloys underwent widespread oxidation below th e silica coating, leading to extensive coating spallation. This was at tributed to the inability of the alloy to supply a sufficient outward flux of chromium to prevent oxygen penetration through microcracks in the silica coating. Due to an abundance of chromium available al the s urface of the fine grain alloy, chromia formed in the microcracks with in the silica layer. As a result, the silica-coated fine-grain alloy d emonstrated superior oxidation resistance and excellent adhesion of th e coating.