*3.1. Microstructure*

The microstructure of the as-deposited PS–PVD coatings with the fracture surface and polished cross-section are shown in Figure 1. The PS–PVD coatings exhibited a distinctive microstructure with relatively independent columns perpendicular to the coating surface. Additionally, a lot of nanoparticles (2 μm on average size) were observed, which were an accumulation of unmelted and partially melted feedstock powder particles at long spray distance due to the plasma jet's heating capability [22]. The columnar structure can provide a high strain tolerance due to the gaps between the columns, which is similar to the EB–PVD coatings [23].

**Figure 1.** SEM micrographs of the as-sprayed PS–PVD TBCs: (**a**) fracture surface; (**b**) polished cross-section.

Figures 2 and 3 are the SEM diagrams of grain growth of PS–PVD columnar coatings, which were heat-treated in air at 1200–1600 ◦C for 24 h and at 1550 ◦C for 20–100 h, respectively. An apparent increasing trend in grain size can be observed with increasing heat-treatment temperature and time from Figures 2 and 3. It can be seen that the grain size of as-deposited columnar coating is very fine, and is about 100 nm. After 1500 ◦C/24 h or 1550 ◦C/20 h, the grains grew obviously, and a clear crystal boundary was formed between the grains. Figure 2 shows that the grain growth rate improves significantly with increasing heat-treatment temperature. The grain growth was fast at high temperature, while it was slow at low temperature, indicating that the effect of temperature on grain growth rate was very obvious. Figure 3 shows the grain growth was quick during the initial stage and slower after 20 h. It can be deduced that the temperature dominated the grain growth of YSZ columnar coatings, rather than heating duration.

**Figure 2.** SEM micrographs of fracture surface of PS–PVD TBCs as a function of temperature: (**a**) as-sprayed; (**b**) 1200 ◦C; (**c**) 1300 ◦C; (**d**) 1400 ◦C; (**e**) 1500 ◦C; (**f**) 1600 ◦C.

**Figure 3.** SEM micrographs of fracture surface of PS–PVD TBCs as a function of time: (**a**) as-sprayed; (**b**) 20 h; (**c**) 40 h; (**d**) 60 h; (**e**) 80 h; (**f**) 100 h.

For a more intuitive description of grain change, grain size measured by image analysis is plotted in Figure 4. The average grain size increased up to 0.94 ± 0.3 μm after 1600 ◦C/24 h heat-treatment, which was about 9.4 times larger than that of the as-deposited coatings. After 1550 ◦C/100 h heat-treatment, the average grain size grew up to 1.41 ± 0.16 μm, which was about 14 times larger than that of the as-deposited coatings. According to thermodynamic conditions, the larger the grain size, the smaller the total grain boundary surface area and the lower the total surface energy. Because grain coarsening can reduce surface energy and bring the material into a more stable state with low free energy, grain growth was a spontaneous change. To realize this change, it is necessary for the atoms to have strong diffusion ability to complete the grain boundary migration when the grains grew. The higher heating temperature is enabling this ability.

**Figure 4.** Grain size of the as-sprayed coating and heat-treated coatings: (**a**) samples after heat-treatment for 24 h at different temperatures; (**b**) samples after heat-treatment at 1550 ◦C for different times.

The polished cross-sections of the PS–PVD coatings after heat treatment at various temperatures for 24 h and at 1550 ◦C for different times were inspected via SEM to characterize the microstructure evolution process. The micrographs of the PS–PVD coatings are shown in Figures 5 and 6. There were significant differences between the as-deposited coatings and the heat-treated coatings. Large gaps existed between the columnar features in the as-deposited coatings, and those gaps became narrower after heat treatment. Particles at the edge of the columns were in contact with each other, and sintered together during the heat treatment. The nanoparticles between the columns promoted the diffusion between adjacent columns. Compared with the as-deposited YSZ coating, the feather-like structure disappeared, the columns became smooth, the grain size coarsened, and the fine grains of the feather-like structure disappeared because of strong diffusion at high temperature. In addition, micro pores were not observed in the as-deposited columns until these columns underwent high-temperature annealing at 1200–1600 ◦C with an exposure time from 20 to 100 h. The formation mechanism of micro pores may be the aggregation of vacancies in columns during high-temperature annealing [24]. Vacancy aggregation led to the collapse of the lattice and the formation of pores, which led to the decrease of vacancy concentration in the crystal. To maintain the equilibrium concentration of vacancies in the crystal, new vacancies will be generated. Vacancy proliferation and vacancy aggregation promoted each other. The formation of vacancy clusters made the atoms near the grain boundary diffuse.

**Figure 5.** SEM micrographs of polished cross-sections of PS–PVD coatings as a function of temperature: (**a**) as-sprayed; (**b**) 1200 ◦C; (**c**) 1300 ◦C; (**d**) 1400 ◦C; (**e**) 1500 ◦C; and (**f**) 1600 ◦C.

**Figure 6.** SEM micrographs of polished cross-sections of PS–PVD coatings as a function of time: (**a**) as-sprayed; (**b**) 20 h; (**c**) 40 h; (**d**) 60 h; (**e**) 80 h; and (**f**) 100 h.

## *3.2. Phase Transformation*

To further understand the thermally induced morphological and composition evolution of the PS–PVD YSZ coatings, Raman spectroscopy (RS) was used to detect the phase compositions. Five bands at 147, 267, 322, 463 and 642 cm<sup>−</sup><sup>1</sup> correspond to the Raman-active modes of tetragonal phase, while the peaks at 178, 189, 221, 308, 334, 346, 382, 476,503, 539, 559, 615 and 637 cm<sup>−</sup><sup>1</sup> are the characteristic bands of monoclinic phase [25,26]. Figures 7–9 show the XRD patterns and Raman spectra of the coatings. The results indicate that the main phase of the as-deposited coatings was t-ZrO2, which indicates that the coating was mainly formed by vapor mixture of zirconia and yttrium oxide. Figure 8 shows that t-ZrO2 of the as-sprayed coating is the non-transformable metastable phase (t-ZrO2). A small quantity of m-ZrO2 also existed in the as-deposited YSZ coatings. As the feedstock powder is an agglomeration of monoclinic zirconia and cubic yttria (not pre-alloyed yttria-stabilized zirconia), it is inferred that there were some unmelted or unevaporated feedstock powder particles directly incorporated and solidified in the coating [22]. The results show that t-ZrO2 and m-ZrO2 coexisted in the YSZ coatings after high-temperature exposure. This indicates that temperature played the most important role in the extent of the phase transformation. At 1550 ◦C, the phase transformation from t-ZrO2 to m-ZrO2 occurred over a short period of time, indicating that the temperature was a stronger promoter of phase transformation than time.

**Figure 7.** XRD patterns of the as-sprayed coating and heat-treated coatings: (**a**) samples after heat-treatment for 24 h at di fferent temperatures; (**b**) samples after heat-treatment at 1550 ◦C for di fferent times.

**Figure 8.** XRD patterns of the as-sprayed coating with 2θ between 72◦ and 76◦.

**Figure 9.** Raman spectra of the as-sprayed coating and heat-treated coatings: (**a**) samples after heat-treatment for 24 h at different temperatures; (**b**) samples after heat-treatment at 1550 ◦C for different times.

## *3.3. Mechanical Properties*

After high-temperature heat treatment, the microstructure, including grain size, cracks and pores, of the YSZ coatings changed significantly, and the mechanical properties of the YSZ coatings also changed accordingly. Hardness (*H*) and Young's Modulus (*E*) are very important mechanical properties for YSZ coatings under long-term service conditions at high temperature. Previous studies in TBCs have reported that the Young's modulus measured on the cross-section was 1.3 times higher than that on the plan-section [27]. Because PS–PVD coatings have a typical columnar microstructure, the mechanical properties measured on the surface and cross-section are not the same, so the nanoindentation method under identical conditions was used to measure the variation of *H* and *E* on the polished cross-section of the coating.

As shown in Figure 10a, the average *H* and *E* values of the coating before thermal exposure were 6.7 ± 1.6 GPa and 95.5 ± 13.7 GPa, respectively. In the process of variable temperature and constant time (24 h) heat treatment, *H* increased obviously up to 1300 ◦C, and then decreased with further temperature enhancement. The highest *H* value of the coatings was 15.6 ± 1.0 GPa, after heat treatment in air at 1300 ◦C for 24h, which was increased by 133% compared to the as-deposited coating. In addition, there was then a continuous decrease of *H* value until 1500 ◦C; after that, a constant average *H* value can be observed with further increase of the heat treatment temperature. Concerning *E* results, the *E* value of the YSZ coating increased significantly to a maximum of 203.8 ± 22.7 GPa, about 113% higher than that of the as-deposited coating, and then the *E* value gradually decreased to a constant value. Figure 10b shows the measured *H* and *E* values as a function of exposure time for PS–PVD coatings before and after heat treatment at 1550 ◦C. The *H* and *E* values increased first and then remained stable. The *H* and *E* values of the coating after being heat-treated at 1550 ◦C for 60 h reached maximum values of 11.1 ± 1.7 GPa and 188.4 ± 19.3 GPa, respectively, representing increases by approximately 66% and 97%, respectively, compared to the as-deposited coating. Similar trends of the *H* and *E* values of coatings have been reported previously; in particular, Zhao et al. found that in axial suspension plasma spraying (ASPS) YSZ TBCs, the mechanical properties firstly increased significantly with sintering, and then decreased with prolonged thermal aging time, which was caused by a t-ZrO2 → m-ZrO2 phase transformation after thermal exposure at 1550 ◦C for 60 h [21]. Sintering can increase the *H* and *E* values due to thermal exposure, which has been reported for APS, ASPS and EB–PVD coatings [27–29]. For example, Zhao et al. showed that the *H* and *E* values of ASPS TBCs increased by about 50% and 44% on the cross-section after 1300 ◦C/24 h exposure in air [21]. An early study on a plasma-sprayed TBC also reported that the *E* value of the ceramic coating increased after exposure, which was attributed to the healing of pores and grain growth and the improvement in bonding and coherence across the splat boundary during exposure at high temperature as a result of the sintering

effect [30]. In the present study, the microstructure evolution of PS–PVD TBCs after heat treatment from 1200 to 1600 ◦C for 24 h and at 1550 ◦C for 20–100 h is similar to the above-mentioned studies.

**Figure 10.** Hardness and Young's modulus of the as-sprayed coating and the heat-treated coatings: (**a**) samples after heat-treatment for 24 h at different temperatures; (**b**) samples after heat-treatment at 1550 ◦C for different times.

Figures 2, 3, 5 and 6 schematically show that grain growth and the closer contact between single columns after thermal exposure lead to a stiffer structure, and thus should create a higher hardness and Young's modulus, resulting from the sintering effect of PS–PVD TBCs during thermal exposure. The sintering effect during the first 20 or 24 h was much more important, especially at 1300 ◦C, than the later stage of heat treatment, producing a sharp increase in the mechanical properties. The theoretical equation between porosity and Young's modulus (*E*) in porous material can be given as follows:

$$E = E\_0 \exp\left[-\left(bp + cp^2\right)\right] \tag{2}$$

where *p*, *E* and *E*0 are the porosity, and the Young's modulus of porous material and dense material, respectively. The terms b and c are empirical values depending on pore morphology [31]. The above formula illustrates that the Young's modulus of porous material decreases with increasing porosity. Therefore, the *E* value of the YSZ coating after heat treatment at 1300 ◦C/24 h reached the highest point due to its having the lowest porosity. Previous literature has reported that the phase transformation from t-ZrO2 to m-ZrO2 will cause cracking due to the huge volume change (approximately 5%) [19]. In combination with Figures 5, 7 and 9, lots of pores and cracks formed between the grain boundaries with increasing exposure temperature for 24 h caused by the formation and accumulation of m-ZrO2 phase. Thus, it is reasonable to conclude that the initial increase in the measured hardness and Young's modulus were attributable to a stiffer structure induced by sintering after thermal exposure, and the mechanical properties decreased obviously due to the m-ZrO2 phase formation accompanied by volume expansion and micro cracks formation.
