Coating Structure and Amorphous Formation

The resulting coatings from the two different powders can be compared in Figure 3-8(a) and (b), that show the coatings from Amperite 750 and Tosoh powders, respectively.

There is a considerable difference in the coating microstructures, including clearly higher

porosity and surface roughness in the coating from Tosoh powder. The reason can be

related to the formation of a large number of shell-like particles with large core porosities. In such particles (forming hollow droplets) bursting upon impact can cause splashing that results in both more porosity and a rough surface.

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Figure 3-8 Coating microstructures: a,c) using Amperite 750, 60/40 alumina/zirconia; and

b,d) Tosoh 60/40 alumina/3YSZ powder

At higher magnification as in Figure 3-8(c) and (d), the comparison of the splat cross sections is possible. The Amperite 750 powder coating shows a structure consisting of distinct dark alumina and bright zirconia splats, in addition to some grey mixed splats.

Tosoh powder coating, in contrast, shows a uniform structure of grey well-mixed alumina

and stabilized zirconia. This uniformity is due to the intimate contact of the particulates that helps their easy mixing (upon melting) within the plasma jet.

Another notable finding in this experiment with Tosoh powder is shown in Figure 3-8(d).

This figure shows an unmolten particle in the coating that presents segregation of zirconia particulates toward the exterior of the particle and concentration of the dark alumina particulates inside. According to SEM assessment of the initial powders, the particles of

this composite powder were formed of uniformly distributed alumina and zirconia nano-particulates. Therefore, this segregation has to have happened during the plasma spray process. The reoccurrence of the segregated particles in the coating is shown by arrows in Figure 3-8(b). This segregation of zirconia toward exterior regions was previously reported in the collected powders after melting and re-solidification, as well as in the coatings of this composite [81]. This phenomenon in both solid and liquid state may be attributed to the higher electrical polarity (stronger dipole) of zirconia molecules that causes a higher tendency of zirconia to expose itself to the ionic environment of plasma at the exterior parts of the particle. According to basic chemistry [103], molecules with covalent bonding between dissimilar atoms form electrical dipoles. These dipoles are the result of the higher density of the shared electrons around the ions of atoms with smaller size (atomic number) and/or higher electronegativity. The larger the difference between the atomic number and the electronegativity of the atoms involved in the bond, the higher the polarity of the dipole [103], so that in extreme conditions the bonding turns to ionic type. Thus the degree of polarity of the dipole translates to the degree of ionic character

of the bond or molecule. In the bond with oxygen (atomic number 8 and electronegativity 3.44), Zr (atomic number 40 and electronegativity 1.33) shows a higher ionic characteristic compared with Al (atomic number 13 and electronegativity 1.61). On the other hand, the materials can best dissolve in electrolytes of similar polarity (i.e., molecules with higher ionic character can more readily dissolve in ionic electrolytes).

Therefore, a higher affinity from zirconium oxide toward the plasma (as an ionic electrolyte) can be expected, which causes a stronger attraction toward the surface of the

particle and/or melt.

It is, however, clear that when such a particle receives heat during long enough period of time for complete melting, full mixing provides an ideal condition for amorphous phase formation by intimate contact between dissimilar particles. Thus, the structure resulting from well melted particles shows a good uniformity. On the other hand, in the distinct lamella of the Amperite 750 powder coating, the chances for in-flight mixing seem to be

lower than those of nano-particulates such as in the Tosoh powder.

However, Figure 3-9 suggests a second possibility for mixing and amorphous formation that can happen in the intersplat regions of the coatings upon impact. Figure 3-9(a) shows the SEM micrograph of the interface area of a solidified alumina splat (dark layer) coated by zirconia (light-color splat). It can be seen that there is a region of alumina mixed with zirconia (shown by arrows in this figure) beside the interface. This has happened due to re-melting of the alumina by the large heat input of the upcoming molten zirconia particles with temperatures higher than the melting point of alumina (Tm for alumina is

20500C and for zirconia is 27000C).

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Figure 3-9 Intersplat conditions in cross section of the coating: a) zirconia splat deposited on solidified alumina splat and b) alumina splat on solidified zirconia

In contrast, Figure 3-9(b) shows the interface when an alumina splat is deposited over the solidified zirconia splat. The distinct separating line between the two splats shows that in

this case such a mixed region has not formed. Amorphous phase formation at the interface area of the zirconia splat on the solid NiCoCrAlY surface was previously reported by Bartuli et al. [104]. They explained this as the result of re-melting and intermixing of aluminium and other bound coat elements into the upcoming zirconia splat. These observations, however, do not override the possibility of in-flight mixing in

this kind of particle.

In mixing and amorphous phase formation upon impact, it should be considered that the total area of the interfaces (involved in the interface mixing) compared with the entire bulk of the splats (involved in the in-flight mixing) is limited. In addition, in this kind of mixing, it is mainly the splat with higher melting point that causes the intermixing upon impact. Thus, the chances for amorphous formation in this way are considerably lower

than in the case of in-flight mixing.