Elaboration and characterisation of plasma sprayed alumina coatings on nickel with nickel oxide interlayer

A process implying the pre-oxidation of nickel substrates was proposed alternatively to the usual Atmospheric Plasma Spraying (APS) of alumina on sandblasted substrates. This process comprised two steps, the substrate preoxidation (instead of the sandblasting of the substrate) followed by APS. The pre-oxidation formed a thin layer of the NiO oxide, 0.7-3.5 mu m thick, which entirely covered the nickel substrate. During APS, the deposit did not react with NiO due to the very high cooling rate of the alumina splats at the surface of the substrates. The characterisation of the interfacial zone showed the crystallographic continuity of the different lattices, from the nickel substrate (f.c.c.) to the NiO interlayer (f.c.c.) and to the monoclinic gamma-Al2O3 coating. The NiO interlayer played the role of a buffer zone that adapted the crystal lattices of the three phases. The adhesion of the coatings was determined according to the ASTM C633 test modified by reducing the contact area substrate/coating, in order to quantify the adhesion of strongly-linked coatings. The coating adhesion reached 105 MPa for the preoxidized samples when NiO was 1.8 mu m thick, instead of 48.5 MPa for the sandblasted samples and only 8 MPa for the simply polished nickel.

Automated summary

The proposed pre-oxidation process for nickel substrates improves the adhesion of alumina coatings by forming a NiO oxide layer to adjust lattice matching, effectively enhancing the adhesion of the coatings. The characterisation of the interfacial zone demonstrated the crystallographic continuity between the nickel substrate, NiO interlayer, and gamma-Al2O3 coating.