Plasma sprayed steel: Adhesion of an alumina film via a wüstite interlayer

A new process for achieving alumina coating on a low carbon steel (C40E) consists in preoxidizing the steel under CO2 in order to obtain a thin layer of wüstite Fe1 − xO (1 to 2 μm), which allows an excellent bonding of alumina coatings (∼ 300 μm thick) via a d.c. plasma jet process. Beyond a critical thickness of the wüstite layer, cohesive ruptures occur inside it. Adhesion of the best coatings is higher than 60 MPa for the optimal oxide thicknesses. This result was not due to a mechanical interlocking because roughness of the iron oxide layer does not play an important role. No diffusion bonding (which would create mix phases) was observed. The proposed mechanism involves a physical bonding due to strong crystallographic relationships between the different phases in presence: epitaxy between inner steel and wüstite, partial topotactic transformation of wüstite into magnetite during the plasma coating, heteroepitaxy between superficial magnetite and alumina deposit. This kind of bonding, here called crystallographic bonding is perhaps difficult to obtain in other system as far as it requires the possibility of particular relationships between the different interfacial phases.