To evaluate the thickness of the coatings, the presence of cracks, and their chemical composition, the samples were cold embedded to not compromise their integrity or to mask the results. FESEM images of the cross-sections of the samples are shown in Fig. 1, where the area highlighted in red refers to the coating. Fig. 1a refer to sample AM5, 5b to sample AM10, and 5c to sample AM15.
Some microcracks are noticeable in the coatings; the defects are pointed with arrows. The defects are most visible in the region of Zr accumulated. In all samples, an accumulation of Zr is observed, especially at the interface between the coating and the substrate. This accumulation may be associated with the not satisfactory dispersion of
the solids in the precursor, producing agglomeration of YSZ or by radial segregation of the particles in a region rich in Zr. The thickness of the coatings remained in the order of 10–20 μm, mainly in samples AM5 and AM15.
Another important result of these cross-sections analyzes is the lack of detachment points of the coating to the substrate, reaching better results for processes that also involve chemical reactions (chemical adhesion) when compared to single processes of mechanical (impact) adhesion. This adhesion is associated not only to the fact that the
plasma torch provides better mechanical adhesion due to the high velocity of the molten particles but also to the chemical adhesion processes attributed to the formation of SiC interlayer, producing a spontaneous bonding layer between the substrate and coating. SiC formation will be further discussed in the Raman spectroscopy and XRD results.
To evaluate the spatial distribution of the components along with the thickness of the coatings, EDS analysis of the cross-section of the sample AM15 (Fig. 2) was performed.From the distance between 0 and 12.5 μm (measured from the surface towards to the coating-substrate interface), there is a region rich in Si, Zr, and O, attributed to the formation of SiO2, ZrO2,and ZrSiO4 near the surface exposed to the plasma jet. Even at a position farther from the coating with the substrate interface, the coexistence of Si and C is observed, more precisely at 6.5 μm. Close to the interface between the coating and the substrate, a decrease in the counts of the elements Zr, Si, and O is observed, as opposed to the increase in the C count. Fig. 2 shows a sudden increase in Zr between 15 and 20 μm and 22 to 25 μm. After 25 μm, we can only observe the presence of Si and C,reinforcing the hypothesis of SiC formation, which is confirmed in the following section.
Fig. 1. FESEM images of the cross-sections of the samples (A) AM5, (B) AM10, and (C) AM15.
Fig. 2. EDS analysis of the cross-sections of the sample AM15.
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