Passive filler loaded polysilazane‐derived glass/ceramic coating system applied to AISI 441 stainless steel, part 2: Oxidation behavior in synthetic air

Abstract This article features the oxidation behavior of ferritic stainless steel grade AISI 441 coated with protective polymer‐derived ceramics (PDC). Two PDC compositions are studied with respect to their oxidation resistance in a flow‐through atmosphere of synthetic air at temperatures of up to 1000°C. The coatings contain a combination of six passive fillers: Y‐containing ZrO ₂ , glass microspheres, alumina‐yttria‐zirconia (AYZ) powder, and three commercial glasses. They are pyrolyzed in air for 1 hour at 800°C with heating and cooling rates of 3 K/min. Detailed microstructural examination of the oxide products formed at the surface of samples after exposure to air at 900°C, 950°C, and 1000°C for 1‐48 hours is analyzed. Both uncoated steel and steel coated with two of the protective systems described in part 1 of this article are investigated. Fe, Cr ₂ O ₃ , TiO ₂ , and a spinel of the composition (Mn,Cr) ₃ O ₄ are identified at the oxidized surface of the steel substrate using X‐ray diffraction. A significant weight gain of the unprotected steel is measured after all experiments, while oxidation tests of the coated steel show a negligible weight gain after 900°C and 950°C. During the early stages of coating oxidation, the monoclinic‐to‐tetragonal ratio in the zirconia filler is shifted toward the monoclinic modification. Longer exposures and higher temperatures lead to the formation of yttrium aluminum garnet (YAG) due to glass microsphere crystallization and solid state reactions in the AYZ powder. The crystallization of the three commercial glasses functioning as sealants leads to the formation of Ba(AlSiO ₄ ) ₂ also known as hexacelsian, which subsequently transforms to celsian. YZr ₈ O ₁₄ is also formed. The protective effect of the PDC coatings applied to the stainless steel is demonstrated up to 950°C.