Figure 1 depicts the evolution of the destabilization measurement of slurry A (30 wt% Al) on the as-prepared condition (i.e. un-aged). Similar curves were also obtained for slurries B (40 wt% Al) and C (50 wt% Al). The evolution of the backscattered signal translates the clarification/sedimentation front with respect the position in the crucible. The decrease of the backscattered signal with time is indicative of a lower concentration of Al particles (clarification) at the top of the crucible whereas the opposite (sedimentation) occurs at the bottom. The inflection points appearing on the sedimentation front highlighted by the red darts of Figure 1 are ascribed to the different sizes of the particles [2 - 10 µm] in the dispersion [14], whereby coarser particles sediment faster than the smallest ones if one assumes a Newtonian fluid [15].

The shear stress measured against the applied shear rate for the different slurries resulted in linear curves of which the slope allowed to calculate the viscosity (η) using the Newtonian fluid description [14]. In case of physicochemical changes in the solvent, better monitoring of the slurries with ageing time is usually achieved by measuring the relative viscosity (η_r = η_fluid/η_solvent) in case physicochemical changes occurred in the solvent. However, the viscosity measurements of solvent ranged between 0.029 and 0.028 Pa∙s⁻¹ over the nine days (Figure 2) and thus highlight the stability of the 1 PVA/10 H₂O mixtures. In agreement with many other works in which modified Stokes equations were introduced to consider the shape and density of the load (e.g. [16]- [18]), the greater the Al content, the higher the viscosity value . However, and irrespective of the Al content, the viscosity is slightly lowered between the day of elaboration and two days afterwards and subsequently stabilizes up to the end of the ageing test. Some “crusting” phenomenon was observed though for slurry C (50 wt% Al) leading to a solidification of the slurry for the low shear rates due to the evaporation of the free-water. In spite of this, a Newtonian behavior is derived, which is in contrast with the non-Newtonian behavior reported in Al microsized (1.4 µm) suspensions of hydroxyl-terminated polybutadiene (HTPB) under dynamic tests [19] but in agreement with the Newtonian behavior observed in nano-sized paraffin oil suspensions [20]. It is therefore believed that the slurry ratios of water, PVA and Al microparticles studied here allow the initial dissolution of PVA in water [21] and the bonding of PVA to the Al microparticles [9], irrespective of the Al load. Therefore, the suspension is stable up to nine days from the sedimentation and viscosity considerations and the SiC#180 polished nickel substrates are thus similarly wetted with all three slurries (Figure 3).

Annealing of the sprayed coatings in a single step process at 700˚C/2 h + 1100˚C/2 h under flowing Ar(g) resulted in the coating structure illustrated in Figure 4(a). The coating system consists of a top coat of hollow alumina spheres, a thermally grown oxide (TGO) based on alumina and a NiAl bond coat diffused into the nick el substrate. The mechanisms of formation were thoroughly explained in [8]. They appear from supply of Al to the nickel substrate while the shells of the Al microspheres at the top crack and oxidize so as to form hollow and broken spheres with a α-Al₂O₃ crust. The reaction pathway is very fast due to the great exothermal reactions between Al and Ni [22]. A 40 µm thick top coat of hollow spheres exhibit similar thermal conductivity values than a conventional 400 µm-thick YSZ coating deposited by air plasma spray (Figure 4(b)).

The oxidation kinetics between 900˚C and 1100˚C displayed in Figure 5(a) are also shown to be equivalent to a bulk β-NiAl [23] when the top coat is removed because of the remnants of metallic Al in the spheres [24]. Protection is provided by the growth of the alumina TGO. A mixed spinel NiAl₂O₄ oxide forms though at 1100˚C (Figure 5(b)) due to fast outward diffusion of Ni [25].