Fig.1 shows the SEM photograph of HVAS 321 coating with typical lamellar steel splats(a), oxides(b), micro-cracks(c) and pores(d). The formation of micro-cracks confirms that the atomized particles of the 321 stainless steel have encountered serious metamorphosis and distortion during the deposition, which leads to residual stresses and other defects in the coating. Although the interval spraying process avoids macro-cracks extending to the coating surface, the inner micro-cracks weaken the general properties of the coating. Similar to the HVAS 321 coating, the HVAS 321/Al coating (Fig.2) is composed of splats, oxide phases and a few micro-pores. However, there are some differences between them. The EDS analysis of the different constituents shown in Fig.2 reveals that the light region is stainless steel alloy(a), the black region comprises aluminum(b), and the grey region mainly consists of iron, chrome and oxygen(c). Fig.3 shows the XRD spectra of two coatings. It is shown that (Fe, Ni),-Fe, FeO·Cr2O3 and CrO phases present in the HVAS 321 coating. Except for the above phases observed in the HVAS 321/Al coating, aluminum phase was detected additionally, but no peaks were found corresponding to alloy phases between aluminum and elements of 321 stainless steel. It is also interesting that aluminum oxide phase was not found in the XRD spectra, which may be relevant to little mass fraction of aluminum oxide in the HVAS 321/Al coating. By combining the results of SEM, EDS and XRD, it is clear that the materials of anode and cathode wire feedstocks for the HVAS 321/Al coating are mixed merely by mechanical action during atomizing, thus forming a type of “composite” coating inter-deposited with hard steel splats and ductile aluminum splats. Another difference is that the splats of the HVAS 321/Al coating are thicker than those of the HVAS 321 coating, and the oxygen content decreases by about 34%. Additionally, as shown in Fig.2, some small stainless steel particles are clad in aluminum splats in the HVAS 321/Al coating(d), which will be helpful to enhancing the cohesive bonding strength between these different materials.
Fig.1 Cross-sectional morphology of HVAS 321 coating
Fig.2 Cross-sectional morphology of HVAS 321/Al coating
Fig.3 XRD spectra of HVAS 321 (a) and 321/Al coating (b)
The mechanical properties of both coatings are presented in Table 1. The adhesion strength and porosity of HVAS 321 and 321/Al coatings are pretty large in the same scale, while the mean hardness of HVAS 321/Al coating is lower than that of stainless steel coating. The measuring counts of the stainless steel coating fluctuate between Hv396.4 and Hv515.3; for HVAS 321/Al coating it fluctuates between Hv159.1 and Hv407.0, more sharply than the former. The wavy hardness distribution confirms the structural inhomogeneity of the coatings.
Table 1 Mechanical properties of coatings
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