The boron particles and the spherical aluminum particles are shown in Fig. 1a, brespectively. The boron powder consists of agglomerates of particles with different sizes (Fig. 1a). The aluminum particles are spherical with a smooth surface and slightly agglomerated (Fig. 1b).
Fig. 1 SEM micrograph of (a) boron particles and (b) spherical aluminum particles (1–20 lm)
Fig. 2 SEM micrographs of the coating surface obtained using aluminum particles of (a) 1–20 lm with0 % boron, (b) 1–20 lm with 10 % boron and (c) 1–20 lm with 20 % boron
Fig. 3 SEM micrographs of coating surfaces and cross sections obtained using 20 % boron mixed with aluminum particles of (a) 3–5 lm (b) 1–20 lm
The addition of boron to the lm-Al slurries gave different results according to the percentage of boron added. Figure 2a shows the coating surface when aluminization without boron addition is carried out, applying the aluminum powder of 1–20 lm onto the base metal. Figure 2b shows the coating surface after a boro-aluminization process using 10 % boron and Fig. 3c after a boro-aluminization with 20 % boron. The sintering degrees increase from Al-particles with no boron addition (Fig. 2a) to the addition of 20 % boron. The surface structure of the latter exhibits particles covered by nano-sized “needles”. This effect increases with the boron content(Fig. 2b, c). The same effect was observed using both 3–5 lm and 1–20 lm aluminum particles. The hollow structure of the particles is observed in Fig. 2. Furthermore, the topcoat shows remarkable adherence to the substrate, as well as a homogeneous diffusion layer. In Fig. 3, the cross section of the coating using the aluminum powder of 1–20 lm (Fig. 3b) is compared to that obtained using the 3–5 lm powder (Fig. 3a). Aluminum powders in the range of 1–20 lm form thicker and more homogeneous diffusion layers with a smoother surface. Evidently, the thickness of the diffusion zone depends on the thickness of the deposited slurry.
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