Investigation into the Structure and Oxidation Mechanism of FeAlCr/Al2O3 Detonation Spraying Coatings

Abstract

The oxidation resistance of detonation spraying coatings of the FeAlCr/Al2O3 powder fabricated by mechanically assisted self-propagating high-temperature synthesis using aluminothermic reactions of oxide reduction is investigated. The powder has a sufficiently homogeneous composite structure consisting of chromium-alloyed ordered B2-FeAl and fine inclusions of α-Cr2O3 and α-Al2O3. The detonation coatings sprayed on stainless steel substrates have a typical layered structure without cracks and spalling. The coating thickness is 250–300 μm, and microhardness is in a range of 5.9–6.1 GPa. Coatings of the synthesized powder mainly inherit its structure and phase composition, although certain aluminum and chromium oxidation occurs when spraying. The features of the cyclic and isothermal oxidation of coatings in air in a temperature range of 900–1000°C are studied. It is established that the oxidation resistance of detonation coatings of the synthesized powder after oxidation in air for 48 h at 950°C is close that of coatings formed from the FeAl‒FexAly powder with an aluminum content of 45 wt %. At the same time, the linear thermal expansion coefficient (LTEC) of FeAlCr/Al2O3 coatings is closer to the LTEC of the base material, while their creep resistance is higher when compared the latter due to the presence of fine refractory oxide inclusions. It is assumed that α-Cr, Cr2O3, and numerous fine alumina inclusions present in the synthesized powder (and which form when spraying) accelerate the protective film, formation suppressing hematite nucleation and growth at early oxidation stages at temperatures up to 950°C.