Factors influencing the formation of a diffusion zone and the adherence of the top coat of high temperature coatings from micro-sized spherical aluminium particles

Abstract

Micro-sized spherical aluminium particles deposited as slurry by brushing or spraying on a substrate alloy oxidise at high temperatures to form a top coat from sintered hollow alumina spheres whilst forming an aluminised diffusion zone. The top coat has the potential to be effective as a thermal barrier by gas phase insulation. The formation of the diffusion zone and the adherence of the top coat are influenced not only by the parameters of the heat treatment, but by factors such as particle size and surface finishing. Coated samples of Alloy 321 were cured and heat treated at 650 degrees C for 5 h and were analysed by field emission scanning electron microscopy. Coating samples of Alloy 321 containing spherical aluminium particles with the sizes 0.3-0.7 mu m, 2-3 mu m, 5 mu m, and 30-50 mu m and with a surface finishing of 120#, 500# and 1200# revealed that the size range of 2-5 mu m and a surface finishing of 500# are more suitable for forming the coating structure of the diffusion zone with an adherent top coat. Using aluminium particles with a size of 30-50 mu m, the temperature range where diffusion predominates at the expenses of the alumina sphere formation is wider and top coat spallation is observed

Development of Thermal Barrier Coating Systems from Al Microparticles—Part II: Characterisation of Mechanical and Thermal Transport Properties

“…Annealing in a quasi-inert Ar atmosphere leads to the fast consumption of the Al from the slurry HS and DS microparticles to form the aluminide coating and the subsequent collapse of the remaining emptied shells from the top coating. The collapse was reported to occur from the shrinkage of the thin alumina shells when γ-Al 2 O 3 transforms into α-Al 2 O 3 after exposure to temperatures above 1000 • C [7,11,18,19]. The greater collapse of the foam top coating for the DS-Ar than for the HS-Ar sample is probably due to the thinner passive shell of the DS microparticles (Figure 1a,f and Figure 2a,f).…”

“…The coatings produced in Ar present the lowest thermal diffusivities since the shell of the microparticles that trap air is very thin. As a matter of fact, heat conduction is greater in alumina (λ alumina ~5-35 W•m −1 •K −1 at RT) than in the gas filled pores (λ air ~0.025 W•m −1 •K −1 at RT) and could decrease even further when the pores are smaller than the gas mean free path (typically pores ~1 µm), where the heat conduction of the gas falls below the conduction of the free gas to ~0.01 W•m −1 •K −1 due to gas molecule-wall collisions (Knudsen conduction) [6][7][8]. However, the top coatings that were heat-treated in air present lower thermal diffusivities than the ones heat-treated in Ar + air introduced at 650 • C, despite the thicker shells of the former.…”

“…However, this thin foam did not display sufficient resistance to erosion by grit blasting because of the thin oxide shell of the alumina hollow spheres [6]. Previous studies that investigated the role of the microparticles' properties [7], the mechanisms of coating formation [8] or the influence of the annealing gases [9,10] on the formation of such coatings showed that it was possible to obtain different foam top coatings by varying the parameters of the annealing conditions or the microparticles of the slurry itself. In an attempt to thicken the shell and enhance the sintering of the hollow spheres, hence to improve the mechanical strength of the insulative foam, previous research aimed at controlling the annealing atmosphere to promote the growth of the oxide shells of the original microparticles while enabling enough Al to react with the Ni substrate to form the diffusion coating [11].…”

Effect of Boron Addition on Micro-Aluminum-Particle-Based Multifunctional High-Temperature Coatings

“…Such coating structures are achieved in one single production step and with constant heat treatment parameters they are reproducible. [8]. However, the technical challenge is that the chemical and thermo-mechanical differences between the metallic substrate and the ceramic coating can lead to detrimental interactions, and therefore limit the lifetime.…”