Potential thermal barrier coating systems from Al microparticles. Mechanisms of coating formation on pure nickel

The mechanisms of formation of a full thermal barrier coating system (thermal barrier, thermally grown oxide and aluminium diffusion coating) from a slurry is presented in this work. Al microparticles were mixed to a water-based binder and deposited onto the surface of pure nickel. A single step or two steps heat treatment in Ar (g) atmosphere brought about the formation of the coating system. The foam-like top coating based on Al2O3 hollow particles is expected to provide the thermal insulation. Underneath, an aluminium diffusion coating expected to provide the high temperature oxidation resistance was formed. Simultaneously, an alumina thermally grown oxide was developed between the top coat and the diffusion coating.It was found that Al began to diffuse into the Ni substrate at 650 °C following the Ni–Al equilibrium diagram. Thickening of the aluminized layer followed parabolic growth with time whether at 650°, 700° or a combination of 700 °C (2 h−1) + 1100 °C. The presence of molten Al seemed to account for the very rapid aluminisation of the substrate via δ-Ni2Al3. The flow of solid and molten Al could be regulated by cracking and shrinking of the Al2O3 shells surrounding the Al particles. In a combined 700 °C (2 h−1) + 1100 °C treatment, β-NiAl grew at the expense of δ-Ni2Al3 formation. Simultaneously, thickening of the Al2O3 crusts and phase transformations occurred, which provided enhanced sintering of the particles. At 650 and 700 °C, γ-Al2O3 formed whereas exposure at 1100 °C resulted in the appearance of α-Al2O3 even for short very short times.

► Elaboration of potential thermal barrier coating systems in a single step process from Al microspheres.
► Simultaneous aluminizing and formation of hollow oxide spheres.
► Mechanisms of formation of such new coatings well identified in pure nickel.