The coatings formed using the multi size aluminum powder showed high stability. After 1,000 h (Fig. 1a) and 2,000 h (Fig. 1b), the whiskers formed on the surface of the particles during the manufacturing process are still observed, as well as the high sintering degree between the particles and strong adhesion to the substrate.
Fig. 1 SEM micrographs of the coating surface obtained using 20 % boron mixed with aluminum particles of 1–20 lm after exposure at 600 ℃ (a) 1,000 h (b) 2,000 h
XRD measurements of the samples after 2,000 h exposure identified Al4B2O9, c-Al2O3, and metallic aluminum remaining in the particles, and some iron aluminides were furthermore detected (Fig. 2). The cross section shows that the coating is in close contact with the substrate after the exposure (Fig. 3). The contrast was adjusted in Fig 3a to visualize the diffusion zone, and in Fig. 3b to visualize the topcoat.
Fig. 2 XRD pattern obtained from samples formed with aluminum powders of 1–20 lm mixed with 20 % boron after 2,000 h of exposure at 600 ℃
Fig. 3 SEM micrographs of the cross section obtained using 20 % of boron mixed with aluminum particles of 1–20 lm after 2,000 h of exposure at 600 ℃ (a) High contrast, low brightness, partial visually diffusion zone; (b) High brightness, low contrast, visible topcoat
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