Iron/wüstite/magnetite/alumina relationships in plasma coated steel: A TEM study

Despite very low surface roughness values of steel substrates (Ra = 0.06 μm), plasma sprayed coatings of Al2O3 exhibited an excellent adhesion (> 60 MPa) when the substrate was preliminary oxidized before coating.Investigation in the interfacial zone, mainly by TEM study, showed a crystallographic continuity through the different heterophase interfaces present from the inner steel substrate up to the outer alumina coating: steel/iron (α-Fe)/wüstite (Fe1−xO)/magnetite (Fe3O4)/metastable alumina (γ-Al2O3).No chemical reaction occurred between iron oxides and alumina, but strong crystallographic relationships linked every phase through the different interfaces.Such a structural continuity explained the solidity of this alloy/ceramic bonding.