Slurry aluminizing mechanisms of Ni-based superalloys incorporating an electrosynthesized ceria diffusion barrier

Development of thermal barrier coating systems from Al microparticles. Part I: Influence of processing conditions on the mechanisms of formation

“…The samples were systematically cured at 400°C for 1 h to remove the organic binder prior to any further step [22]. For a better understanding of the mechanisms of formation, one or two additional steps were then performed based on previous works done in our group [2,3]: the diffusion step (700°C-2 h) to aluminize the substrate by major inward diffusion of Al, and the annealing step (1100°C-2 h) to stabilize the nickel aluminide coating (i.e. to promote the outward diffusion of nickel) and to stabilize the α-Al2O3.…”

“…Therefore, they cannot be applied to other sections that now require thermal insulation due to the increase of the turbine inlet temperatures. Among the alternative coating techniques, slurries containing Al microspheres appear particularly attractive as they can form a top foam of hollow alumina microspheres in addition to an aluminide coating [1][2][3][4][5]. It has been demonstrated that just a 40 µm -thick top foam indeed confers thermal insulation equivalent to those of 400 µm thick conventional plasma-sprayed coatings made of yttria-stabilized zirconia between 400°C and 900°C [6].…”

“…The mechanisms of formation of water-based Al-containing slurry coatings have already been discussed under inert atmosphere (Ar) [2,3,20] and in air [1] on pure nickel and on nickel-based superalloys. However, none of such studies investigated the influence of the atmosphere on the simultaneous aluminizing of the substrate and the sintering of the hollow alumina spheres.…”

High Temperature Oxidation of Slurry Aluminized Deformable Austempered Ductile Iron (DADI)

“…All the specimens were polished with SiC P180, washed in alcohol, thoroughly dried and weighed before application of the slurry coating by spray. The aluminium-containing slurry was prepared following the procedure previously reported [8,11,14]. Namely, polyvinyl alcohol (PVA, Sigma Aldrich, Mowiol 4-88) powder was dissolved in deionised water with 1:10 proportion and stirred thoroughly while heated up to 90°C.…”

“…Among the different coating alternatives, slurry aluminizing has been proven as an alternative to other aluminizing techniques like pack cementation. The mechanisms of formation of these coatings have been thoroughly described on different metal substrates like pure Ni [8], different steels [9,10] and Ni-based alloys [11]. Depending on the amount of slurry deposited and on the annealing treatment, FeAl-based intermetallics can be obtained [12], which are reported to resist oxidation in a wide range of temperatures [13].…”