Mechanical characterization of alumina coatings on C35 steel

• The new process “crystallographic bonding” gives excellent adhesion for the coatings.
• Adhesion of alumina coatings on steel C35 reaches 85 ± 7 MPa.
• This value was measured by the test ASTM C633-01 by using the “silver print test”.
• The rupture of the coating occurs between the substrate and the wüstite interlayer.

C35 steel substrates were alumina-coated via thermal plasma spraying. Before coating, the steel surface was either sandblasted, or pre-oxidized in CO2. The pre-oxidized samples exhibited coatings adhesions of 82 ± 7 MPa, and only 45 ± 4 MPa for the sandblasted ones. This good result was attributed to the “crystallographic bonding”, which is characterized by the structural continuity of the chain: steel/α-Fe/wüstite/magnetite/γ-Al2O3. The adhesion measurements were obtained utilizing the new technique of the “silver print test” that consists in the painting of a part of the samples surface before coating. The rupture of the coatings occurred at the surface of the substrate, or inside the wüstite layer.