Fig. 1 shows the microstructure of the conventional coating. It is possible to observe the typical characteristics of conventional YSZ coatings produced by regular APS torches, i.e., previously molten particles forming lamellar structures (particle spreading at impact) exhibiting globular and intersplat pores, in addition to the presence of microcracks. The average surface temperature of the conventional YSZ powder was basically the same as that of the melting point of YSZ (∼2700 ◦C). Despite that, the microstructure is considerably different from that of the nanostructured YSZ coating (Fig. 2).
Fig. 1. Microstructure (cross-section) of the conventional YSZ coating (scale bar: number represents combined distance across all tick divisions).
Fig. 2. (a) Microstructure (cross-section) of the nanostructured YSZ coating made from a nanostructured feedstock (Fig. 3). (b) Higher magnification of (a) darker-colored regions containing the previously semi-molten feedstock particles (scale bar: number represents combined distance across all tick divisions).
It is important to point out that spray-dried and plasmadensified powders (conventional YSZ) are generally comprised of hollow or dense spherical powder particles. The hollow particles will tend to exhibit a relatively dense outer shell facilitating heat transfer from the plasma jet to the inner part of the particles and thereby leading to more uniform heating and complete melting of the particle. The same concept can be applied to the dense spherical particles. In addition, the smaller average particle size of the conventional particles, when compared to that of the nanoagglomerates (Fig. 4), will probably tend to facilitate particle melting due to their smaller volumes. Therefore, the majority of the conventional particles that impinge, adhere and embed in the coating microstructure probably consist of fully molten or almost fully molten particles, forming a microstructure with different features from that of the nanostructured YSZ coating. It should be emphasized that the porosity of the nanostructured YSZ powder particles (Fig. 3) during thermal spraying plays the same role as the porosity in a TBC, i.e., the porosity may create a barrier against heat transfer from the plasma jet to the inner core of the particles, making it more difficult for them to melt completely.
Fig. 3. (a) YSZ feedstock particle formed by the agglomeration (spray-drying) of individual nanosized particles of YSZ. (b) Particle of (a) observed at higher magnification showing individual nanosized YSZ particles (30–130 nm) (scale bar: number represents combined distance across all tick divisions).
Fig. 4. Particle size distribution of the nanostructured (sieved) and conventional YSZ powders.
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