Al2O3 coatings on stainless steel from Al metal-organic chemical vapor deposition and thermal treatments

Al coatings were deposited by metal organic chemical vapor deposition (MOCVD) at 573 K using tri-isobutyl aluminum (TIBA) as precursor on FeCrNi- and FeCr-type stainless steel substrates. In situ surface treatments of the substrates by TiCl4 vapor prior to MOCVD of Al has been explored to enhance the nucleation density of the Al films and subsequently to improve their surface morphology and compactness. Further thermal treatment at 923 K of Al-coated samples under inert atmosphere (He) produces the desired β-FeAl compound as a main layer on the substrates whereas, at lower temperature, the intermetallic phase Fe2Al5 is essentially formed. Al2O3 was grown as an outer layer on the Fe–Al diffusion coatings by annealing in the CVD reactor at 923 K under oxygen atmosphere. In these conditions, Al2O3 films are dense, uniform and adherent. They exhibit the cubic structure of the low temperature alumina phases. These protective coatings have been characterized by different techniques and their structure is discussed as a function of the conditions of this combined process.