Effect of Particle In-Flight Behavior on the Microstructure and Fracture Toughness of YSZ TBCs Prepared by Plasma Spraying

The present study aims to elaborate particle in-flight behavior during plasma spraying and its significance in determining the microstructure and mechanical properties of plasma sprayed yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The as-sprayed YSZ coatings were characterized in terms of defects (such as pores, unmelted particles and cracks) and fracture toughness. The results showed that, due to the higher temperature and velocity of in-flight particles in a supersonic atmospheric plasma spraying (SAPS) compared to that of atmospheric plasma spraying (APS), denser coatings were formed leading to a better fracture toughness. The percentage of defects of the microstructure was similar to the temperature and velocity of particles in-flight during plasma spraying. Furthermore, the structural defects had a strong effect on its mechanical behavior. The total defect percentage and fracture toughness in SAPS-TBCs spanned 6.9 ± 0.17%–13.26 ± 0.22% and 2.52 ± 0.06 MPa m1/2–1.78 ± 0.19 MPa m1/2; and 11.11 ± 0.36%–17.15 ± 0.67% and 2.13 ± 0.08 MPa m1/2–1.4 ± 0.12 MPa m1/2 in APS-TBCs.


Introduction
The thermal spraying process (TSP) aims to deposit some special metallic or nonmetallic materials onto the surfaces of pretreated matrices.Before deposition, these materials must be heated into molten or semi-molten states using heat sources like plasma arc or combustion flames.Due to the flexibility and high economic benefits of the TSP process, it is used to fabricate coatings with good performance in heat resistance, wear resistance and corrosion resistance; in many industries [1][2][3][4].The plasma spraying, as one important part in TSPs, is a highly complex process, which mainly consists of the production of plasma, the transformation of heat and momentum between plasma and powder particles, and the flattening, splatting and stacking of molten or semi-molten particles after impacting the matrix [3][4][5].The complex variables and parameters at the different links of the plasma spraying process mainly contain the structural parameters of spray torch, the production process parameters of the plasma, the size and morphology of the powder, the powder feeding conditions, the in-flight behavior of powder particles in the plasma jet, and the matrix conditions when depositing coatings [5][6][7][8][9][10][11].They have a direct influence on the melting state and movement behavior of the particles before impacting the matrix and then affect the microstructure of deposited coatings, which further influences the thermodynamic properties (adhesion strength, hardness, Young's modulus, fracture toughness, and thermal conductivity) and service properties (thermal cycle life, oxidation

Experimental Materials and Methods
In this experiment, micron-YSZ powders prepared by the spray-drying process were taken as the raw materials.The morphology and size distributions of these powders are shown in Figure 1.The coatings were deposited by two kinds of spraying devices, atmospheric plasma spray (APS, Sluzer Metco 9M, Winterthur, Switzerland) and supersonic atmospheric plasma spray (SAPS, The Academy of Armored Forces Engineering, Beijing, China).The core of these two devices was a spray torch Sluzer Metco 9M and a SAPS internal feedstock mode spray torch, respectively.The main process parameters for the two spraying patterns are shown in Table 1.

Experimental Materials and Methods
In this experiment, micron-YSZ powders prepared by the spray-drying process were taken as the raw materials.The morphology and size distributions of these powders are shown in Figure 1.The coatings were deposited by two kinds of spraying devices, atmospheric plasma spray (APS, Sluzer Metco 9M, Winterthur, Switzerland) and supersonic atmospheric plasma spray (SAPS, The Academy of Armored Forces Engineering, Beijing, China).The core of these two devices was a spray torch Sluzer Metco 9M and a SAPS internal feedstock mode spray torch, respectively.The main process parameters for the two spraying patterns are shown in Table 1.During each spraying process, the surface temperature and in-flight velocity of particles in the jet before impacting the matrix (the temperature of the matrix was kept at 150 • C by cooling it with compressed air), were examined by the online monitoring system Spray Watch 2i (Osier, Tampere, Finland).The results are shown in Table 2.When monitoring the particles, the focal lengths of the APS and the SAPS spraying systems were fixed at 220 and 185 mm, respectively.This aimed to allow the area of the explorer view of the monitoring page to account for around two-thirds of that of the total explorer view pages.Compared with the APS system, the SAPS system had a finer jet.To improve accuracy, the temperature and velocity were measured five times during each spraying process, and the average values of these two kinds of parameters were chosen as the final values.The matrix temperature was measured using an infrared thermometer (Raytek, Santa cruz, CA, USA), and it was kept at 150-200 • C using a forced draught cooling system.Table 2. Surface temperature and in-flight velocity of the fifteen kinds of coating particles [26].

Coating Type
Temperature ( The fracture toughness of coatings was measured using a micro-hardness tester (MicroMET 3 micro-hardness tester, Buehler Ltd., Lake Bluff, IL, USA).The Vickers indentation test was performed on the polished cross-section of the coating, which was measured at least 20 times in random different areas with a load of 5 N and 10 s holding time, keeping at least five times greater than the diagonal length of indentation between consecutive indentations.The fracture toughness of as-sprayed coatings was determined by Equation (1): where the coefficient, k total , equated to k p multiplied by 2 (k p is an empirical constant of fracture toughness in the Palmqvist model, k p = 0.0319); P is the indentation load, which was 2.94 N in this study; L is the total surface crack length, including all the cracks around the indent; a is the average Vickers indentation half diagonal size, where L = ∑ n l yn (3)

Influence of Flying Particle Characteristics on the Microstructure of Deposited Coatings
The influence rule of the temperature and velocity of flying particles on the total voids of coatings is shown in Figure 2. It can be seen that the higher the temperature or velocity of the particles, the lower the percentage of total void area in the total coating area.When the temperature was between 2689 and 2693 • C, and the velocity was between 180 and 185 m/s, the total voids accounted for 14% ± 0.5%.When the temperature was between 2843 and 2853 • C and the velocity was between 225 and 231 m/s, the total voids accounted for 7% ± 1%.When the temperature was between 3242 and 3524 • C, and the velocity was between 380 and 450 m/s, the total voids accounted for 5% ± 1%.
The above results could be explained as follows.On one hand, the higher the temperature of the particles, the larger the melting degree of the coatings, which makes the proportion of unmelted particles lower.On the other hand, the larger the velocity of the particles, the larger the momentum of the particles when impacting the matrix.Meanwhile, the droplets on the surface of the matrix would spread better, which means that the coatings have a better flattening condition.The smaller the distance between two adjacent layered structures, the lower the percentage of small porosities.The combination of these two factors, described above, finally led to a comparatively lower total void.Besides this, when both the particle temperature and velocity tend to one certain set of values, the proportion of total voids in one coating would also tend to a certain value.This inferred that the spraying process has a good repeatability, which means that the coating quality could remain stable.

Influence of Flying Particle Characteristics on the Microstructure of Deposited Coatings
The influence rule of the temperature and velocity of flying particles on the total voids of coatings is shown in Figure 2. It can be seen that the higher the temperature or velocity of the particles, the lower the percentage of total void area in the total coating area.When the temperature was between 2689 and 2693 °C, and the velocity was between 180 and 185 m/s, the total voids accounted for 14% ± 0.5%.When the temperature was between 2843 and 2853 °C and the velocity was between 225 and 231 m/s, the total voids accounted for 7% ± 1%.When the temperature was between 3242 and 3524 °C, and the velocity was between 380 and 450 m/s, the total voids accounted for 5% ± 1%.
The above results could be explained as follows.On one hand, the higher the temperature of the particles, the larger the melting degree of the coatings, which makes the proportion of unmelted particles lower.On the other hand, the larger the velocity of the particles, the larger the momentum of the particles when impacting the matrix.Meanwhile, the droplets on the surface of the matrix would spread better, which means that the coatings have a better flattening condition.The smaller the distance between two adjacent layered structures, the lower the percentage of small porosities.The combination of these two factors, described above, finally led to a comparatively lower total void.Besides this, when both the particle temperature and velocity tend to one certain set of values, the proportion of total voids in one coating would also tend to a certain value.This inferred that the spraying process has a good repeatability, which means that the coating quality could remain stable.In coatings, there are three ways to form the voids.Some air in coatings, which was involved in the spraying process, formed regular voids.Irregular voids often appeared between two layered structure, which formed as a result of the particles stacking particles.In addition, some particles which unmelted particles could form voids. Compared to the third kind of void, the size of the former two kinds of voids was smaller.Thus, these two kinds of voids could also be called pores.The analyzed results relating to the voids of the above fifteen coatings showed that the percentage of voids, which were smaller than 10 μm 2 in area was for more than 90%.These data are presented in Figure 3a.Furthermore, the analyzed results show that the percentage of the voids is large when the area of the voids is small in total voids; as shown in Figure 3b.Besides this, it can be seen from the morphology analysis, that the formation of voids of more than 10 μm 2 in area resulted from the unmelted particles.Therefore, in this study, it was assumed that the voids with less than 10 μm 2 in area were pores and that the voids of more than 10 μm 2 in area were unmelted particles.In this case, the influence of the temperature and the velocity of particles, before impacting the matrix on the coating porosity ratio and the percentage of unmelted particles, could be analyzed.In coatings, there are three ways to form the voids.Some air in coatings, which was involved in the spraying process, formed regular voids.Irregular voids often appeared between two layered structure, which formed as a result of the particles stacking particles.In addition, some particles which unmelted particles could form voids. Compared to the third kind of void, the size of the former two kinds of voids was smaller.Thus, these two kinds of voids could also be called pores.The analyzed results relating to the voids of the above fifteen coatings showed that the percentage of voids, which were smaller than 10 µm 2 in area was for more than 90%.These data are presented in Figure 3a.Furthermore, the analyzed results show that the percentage of the voids is large when the area of the voids is small in total voids; as shown in Figure 3b.Besides this, it can be seen from the morphology analysis, that the formation of voids of more than 10 µm 2 in area resulted from the unmelted particles.Therefore, in this study, it was assumed that the voids with less than 10 µm 2 in area were pores and that the voids of more than 10 µm 2 in area were unmelted particles.In this case, the influence of the temperature and the velocity of particles, before impacting the matrix on the coating porosity ratio and the percentage of unmelted particles, could be analyzed.Figure 4a,b show the influence rule of the temperature and velocity of particles, before impacting the matrix of the coating porosity ratio and the percentage of unmelted particles.It can be seen that as the temperature and velocity increased, both the coating porosity ratio and the percentage of unmelted particles tended to decrease.When the temperature was between 2689 and 2693 °C, and the velocity was between 180 and 185 m/s, the coating porosity ratio and the percentage of unmelted particles were 5.5% ± 0.5% and 8.5% ± 0.5%, respectively.When the temperature was between 2843 and 2853 °C, and the velocity was between 180 and 185 m/s, the coating porosity ratio and the percentage of unmelted particles were 4.5% ± 0.2% and 5.5% ± 0.5%, respectively.When the temperature was between 3213 and 3280 °C, and the velocity was between 225 and 231 m/s, the coating porosity ratio and the percentage of unmelted particles were 3.5% ± 0.5% and 3.5% ± 0.5%, respectively.When the temperature was between 3242 and 3524 °C, and the velocity was between 380 and 450 m/s, the coating porosity ratio and the percentage of unmelted particles were 2.5% ± 0.2% and 2.5% ± 0.5%, respectively.This is because the area of pores and unmelted particles of coating with high-temperature and high-speed particles is much smaller than that of coatings with lowtemperature and low-speed particles, although all of the percentage of the coating voids only exhibited small difference.(more than 90%).Additionally, for each of the fifteen kinds of coatings, the coating porosity ratio was less than the percentage of unmelted particles.This was mainly related to the area of coating voids and unmelted particles.Although the number of voids in coatings was much bigger than that of the unmelted particles, the area of large unmelted particles was much higher than that of the voids.
Combined with the influence rule of the particle temperature and velocity on the microcrack density and length in coatings, the distribution rule of microcrack length with different angles in typical coatings, within different ranges of temperature and velocity, could be analyzed; as shown in Figure 5. Apparently, it can be seen that in all of the six kinds of coatings, the microcracks that were less than 5 μm in length had the largest percentage in the total cracks and these microcracks had various orientations.The percentage of cracks that were 5 to 10 μm in length took the second place.The macrocracks that were more than 15 μm in length accounted for the lowest percentage.In addition, compared with the APS coatings, the SAPS coatings had the largest crack density and also the length of the longest crack in the latter coatings was larger.
The microstructure morphologies of the coatings under different temperatures and velocities are shown in Figure 6.It can be seen intuitively that the higher the temperature and velocity, the smaller the area of the unmelted particles.Accordingly, the lower the porosity ratio, the larger the crack density, which was in agreement with the statistic results in Figure 5.Among the particles in the jet of the SAPS system, the phenomenon of deformation breakage could be found easily.After stacking the broken and flattened particles, lots of microcracks were found.This was mainly because, in the jet of the SAPS system, the resistance from fluid fluid acted on the particles (drag force from other particles) was larger than the cohesive force of the melted particles (surface tension).However, in the Figure 4a,b show the influence rule of the temperature and velocity of particles, before impacting the matrix of the coating porosity ratio and the percentage of unmelted particles.It can be seen that as the temperature and velocity increased, both the coating porosity ratio and the percentage of unmelted particles tended to decrease.When the temperature was between 2689 and 2693 • C, and the velocity was between 180 and 185 m/s, the coating porosity ratio and the percentage of unmelted particles were 5.5% ± 0.5% and 8.5% ± 0.5%, respectively.When the temperature was between 2843 and 2853 • C, and the velocity was between 180 and 185 m/s, the coating porosity ratio and the percentage of unmelted particles were 4.5% ± 0.2% and 5.5% ± 0.5%, respectively.When the temperature was between 3213 and 3280 • C, and the velocity was between 225 and 231 m/s, the coating porosity ratio and the percentage of unmelted particles were 3.5% ± 0.5% and 3.5% ± 0.5%, respectively.When the temperature was between 3242 and 3524 • C, and the velocity was between 380 and 450 m/s, the coating porosity ratio and the percentage of unmelted particles were 2.5% ± 0.2% and 2.5% ± 0.5%, respectively.This is because the area of pores and unmelted particles of coating with high-temperature and high-speed particles is much smaller than that of coatings with low-temperature and low-speed particles, although all of the percentage of the coating voids only exhibited small difference.(more than 90%).Additionally, for each of the fifteen kinds of coatings, the coating porosity ratio was less than the percentage of unmelted particles.This was mainly related to the area of coating voids and unmelted particles.Although the number of voids in coatings was much bigger than that of the unmelted particles, the area of large unmelted particles was much higher than that of the voids.
Combined with the influence rule of the particle temperature and velocity on the microcrack density and length in coatings, the distribution rule of microcrack length with different angles in typical coatings, within different ranges of temperature and velocity, could be analyzed; as shown in Figure 5. Apparently, it can be seen that in all of the six kinds of coatings, the microcracks that were less than 5 µm in length had the largest percentage in the total cracks and these microcracks had various orientations.The percentage of cracks that were 5 to 10 µm in length took the second place.The macrocracks that were more than 15 µm in length accounted for the lowest percentage.In addition, compared with the APS coatings, the SAPS coatings had the largest crack density and also the length of the longest crack in the latter coatings was larger.
The microstructure morphologies of the coatings under different temperatures and velocities are shown in Figure 6.It can be seen intuitively that the higher the temperature and velocity, the smaller the area of the unmelted particles.Accordingly, the lower the porosity ratio, the larger the crack density, which was in agreement with the statistic results in Figure 5.Among the particles in the jet of the SAPS system, the phenomenon of deformation breakage could be found easily.After stacking the broken and flattened particles, lots of microcracks were found.This was mainly because, in the jet of the SAPS system, the resistance from fluid fluid acted on the particles (drag force from other particles) was larger than the cohesive force of the melted particles (surface tension).However, in the APS system, the melted particles did not break as usual due to the drag force of the particles from other particles being small.
Coatings 2018, 8, x FOR PEER REVIEW 7 of 11 APS system, the melted particles did not break as usual due to the drag force of the particles from other particles being small.APS system, the melted particles did not break as usual due to the drag force of the particles from other particles being small.APS system, the melted particles did not break as usual due to the drag force of the particles from other particles being small.

Influence of Microstructure on the Fracture Toughness of Deposited Coatings
In this Vickers indentation test, performed on polished cross-sections, the load was 5 N.In order to determine the mean crack length, the indentations were examined by SEM.The coating gradually experienced elastic and a little plastic deformation, before finally fracturing.Typical Vickers indentations and cracks of the polished cross-section of as-sprayed coatings are shown in Figure 7.
Once the stress intensity factor, K1, in representative coatings reached its fracture toughness, KIC, a pair of radical cracks usually began nucleation near the corner angles of the Vickers indentation and then presented as a semi-ellipse shape along the indentation direction.Figure 8a,b depict the relationship between the total defects and fracture toughness or the crack percentage and fracture toughness respectively.It is revealed that the fracture toughness improved as the total defect decreased.In addition, the KIC values in SAPS were higher than that of APS as a whole (by Figure 8a).Similar to the relationship between crack percentage and elastic modulus, although with approximate total defects, the KIC values of C13, C14, and C15 (SAPS) with a higher crack percentage was larger than that of C3 and C4 (APS).During the sintering and cooling of spraying, ZrO2 coatings formed microcracks in the dispersion distribution on the top layer.Then, microcracks formed in the process zone of the main crack tip, and the main crack was extending, resulting from stress-induced phase transformation.These small microcracks led to main crack bifurcation, or change in direction, which increased the effective surface energy in the extension process of the main crack.In addition, this also played a role in the dispersion of the crack tip energy, inhibited by the rapid extension of the main crack and improved the toughness of the coating.

Influence of Microstructure on the Fracture Toughness of Deposited Coatings
In this Vickers indentation test, performed on polished cross-sections, the load was 5 N.In order to determine the mean crack length, the indentations were examined by SEM.The coating gradually experienced elastic and a little plastic deformation, before finally fracturing.Typical Vickers indentations and cracks of the polished cross-section of as-sprayed coatings are shown in Figure 7.
Once the stress intensity factor, K 1 , in representative coatings reached its fracture toughness, K IC , a pair of radical cracks usually began nucleation near the corner angles of the Vickers indentation and then presented as a semi-ellipse shape along the indentation direction.Figure 8a,b depict the relationship between the total defects and fracture toughness or the crack percentage and fracture toughness respectively.It is revealed that the fracture toughness improved as the total defect decreased.In addition, the K IC values in SAPS were higher than that of APS as a whole (by Figure 8a).Similar to the relationship between crack percentage and elastic modulus, although with approximate total defects, the K IC values of C13, C14, and C15 (SAPS) with a higher crack percentage was larger than that of C3 and C4 (APS).During the sintering and cooling of spraying, ZrO 2 coatings formed microcracks in the dispersion distribution on the top layer.Then, microcracks formed in the process zone of the main crack tip, and the main crack was extending, resulting from stress-induced phase transformation.These small microcracks led to main crack bifurcation, or change in direction, which increased the effective surface energy in the extension process of the main crack.In addition, this also played a role in the dispersion of the crack tip energy, inhibited by the rapid extension of the main crack and improved the toughness of the coating.

Influence of Microstructure on the Fracture Toughness of Deposited Coatings
In this Vickers indentation test, performed on polished cross-sections, the load was 5 N.In order to determine the mean crack length, the indentations were examined by SEM.The coating gradually experienced elastic and a little plastic deformation, before finally fracturing.Typical Vickers indentations and cracks of the polished cross-section of as-sprayed coatings are shown in Figure 7.
Once the stress intensity factor, K1, in representative coatings reached its fracture toughness, KIC, a pair of radical cracks usually began nucleation near the corner angles of the Vickers indentation and then presented as a semi-ellipse shape along the indentation direction.Figure 8a,b depict the relationship between the total defects and fracture toughness or the crack percentage and fracture toughness respectively.It is revealed that the fracture toughness improved as the total defect decreased.In addition, the KIC values in SAPS were higher than that of APS as a whole (by Figure 8a).Similar to the relationship between crack percentage and elastic modulus, although with approximate total defects, the KIC values of C13, C14, and C15 (SAPS) with a higher crack percentage was larger than that of C3 and C4 (APS).During the sintering and cooling of spraying, ZrO2 coatings formed microcracks in the dispersion distribution on the top layer.Then, microcracks formed in the process zone of the main crack tip, and the main crack was extending, resulting from stress-induced phase transformation.These small microcracks led to main crack bifurcation, or change in direction, which increased the effective surface energy in the extension process of the main crack.In addition, this also played a role in the dispersion of the crack tip energy, inhibited by the rapid extension of the main crack and improved the toughness of the coating.

Figure 1 .
Figure 1.Morphology, grain size and size distribution of YSZ powders: (a) External morphology; (b) Internal morphology; (c) Size of the crushed particles; and (d) Size distribution.

Figure 1 .
Figure 1.Morphology, grain size and size distribution of YSZ powders: (a) External morphology; (b) Internal morphology; (c) Size of the crushed particles; and (d) Size distribution.

Figure 2 .
Figure 2. The relationship between the temperature and velocity of flying particles and the total voids of the coatings.

Figure 2 .
Figure 2. The relationship between the temperature and velocity of flying particles and the total voids of the coatings.

Figure 3 .
Figure 3. Analysis of voids in fifteen kinds of deposited coatings: (a) The percentage of voids with less than 10 μm 2 of area; (b) The percentage of different voids in C1 coatings.

Figure 3 .
Figure 3. Analysis of voids in fifteen kinds of deposited coatings: (a) The percentage of voids with less 10 µm 2 of area; (b) The percentage of different voids in C1 coatings.

Figure 4 .
Figure 4.The influence of the temperature and velocity of particles on the porosity ratio and the percentage of unmelted particles: (a) Porosity ratio; (b) Percentage of unmelted particles.

Figure 5 .
Figure 5.The length distribution of cracks under different angles in typical coatings: (a) APS coatings; (b) SAPS coatings.

Figure 4 .
Figure 4.The influence of the temperature and velocity of particles on the porosity ratio and the percentage of unmelted particles: (a) Porosity ratio; (b) Percentage of unmelted particles.

Figure 4 .
Figure 4.The influence of the temperature and velocity of particles on the porosity ratio and the percentage of unmelted particles: (a) Porosity ratio; (b) Percentage of unmelted particles.

Figure 5 .
Figure 5.The length distribution of cracks under different angles in typical coatings: (a) APS coatings; (b) SAPS coatings.

Figure 5 .
Figure 5.The length distribution of cracks under different angles in typical coatings: (a) APS coatings; (b) SAPS coatings.

Figure 4 .
Figure 4.The influence of the temperature and velocity of particles on the porosity ratio and the percentage of unmelted particles: (a) Porosity ratio; (b) Percentage of unmelted particles.

Figure 5 .
Figure 5.The length distribution of cracks under different angles in typical coatings: (a) APS coatings; (b) SAPS coatings.

Figure 7 .
Figure 7.The morphology of the Vickers indentation and cracks on the polished cross-section of assprayed coatings: (a) C3 and (b) C11.

Figure 7 .
Figure 7.The morphology of the Vickers indentation and cracks on the polished cross-section of assprayed coatings: (a) C3 and (b) C11.

Figure 7 .
Figure 7.The morphology of the Vickers indentation and cracks on the polished cross-section of as-sprayed coatings: (a) C3 and (b) C11.