On the Applicability of Modified Water-Based Yttria-Stabilized Zirconia Suspensions to Produce Plasma-Sprayed Columnar Coatings

Abstract

In this paper, the applicability of the modified water-based yttria-stabilized zirconia (YSZ) suspensions, including different dispersants, to produce plasma-sprayed coatings with a “columnar microstructure” is investigated. The effects of processing parameters, like the suspension liquid-phase ethanol content, change in substrate roughness, and spray distance on the resulting coating morphologies, are investigated. The results showed that increasing the ethanol concentration, substrate surface roughness, and spray distance promotes the formation of coatings with a columnar structure. Moreover, the application of modified water-based suspensions, including the α-Terpineol dispersant, has led to the deposition of columnar coatings with comparable morphologies of coatings deposited from ethanol-based suspensions.

1. Introduction

The thermal barrier coating (TBC) performance is strongly dependent on their morphological characteristics, which can be defined by controlling their respective synthesis processing parameters. It has been reported that coatings with a porous columnar morphology exhibit better strain tolerance and thermal cyclic life than those that have lamellar morphologies [1,2]. The suspension plasma spraying (SPS) process has received a great deal of attention in the recent years for the deposition of TBCs based on its potential application to produce coatings with appropriate morphologies in a cost-effective way [3,4]. A practical issue that would be attributed to water-based suspensions is their poor ability to produce TBCs with a columnar structure in comparison with alcohol-based ones [5] Moreover, finding an appropriate procedure to prepare suitable feedstock slurry, which could be industrially scalable, is another issue that would be attributed to such deposition techniques [6]. Ideally, water is regarded as the preferred suspension liquid phase (rather than alcohols like ethanol) based on safety, environmental, and economic considerations. In the case of concentrated water-based YSZ slurries, the effects of the dispersant type (including Polyethyleneimine, tricarboxylic acid, and α-Terpineol), dispersant concentration, and pH on the stability and rheological properties of the resulting suspensions have been investigated [7]. The reported results showed that α-Terpineol could be regarded as the most effective surfactant to produce a water-based YSZ suspension with appropriate characteristics to be used in SPS-coating-deposition techniques. Moreover, preparing well-dispersed water-based suspensions, which are stable over time (i.e., resist agglomeration, sedimentation, and viscosity changes), is also important in industrial practices. In another study, the aging behavior of such concentrated YSZ suspensions has been reported, and those including an α-Terpineol surfactant exhibited better performance than the two other ones [8].

The effects of different processing parameters on the morphological characteristics of the resulting SPS coatings have been addressed thoroughly in the literature [9,10]. For instance, in addition to the application of ethanol-based suspensions, the spraying distance and substrate surface roughness can also be controlled such that the resulting coating morphologies approach columnar ones [11,12]. Controlling such parameters to produce columnar TBCs would be costly, not practical, or even environmentally unacceptable from an industrial point of view. In this regard, the application of the modified water-based YSZ suspensions is promising for the deposition of columnar coatings without the need for altering other processing parameters (as mentioned above), which may cause additional costs for the deposition process. Moreover, coating complex substrates in industrial settings (e.g., coating of TBC on turbine blades) inherently involves variability in the spray distance. Thus, SPS slurries producing the desired columnar microstructure in a wide range of the spray distances are preferable.

In the present study, the applicability of the modified water-based YSZ suspensions to produce plasma-sprayed coatings with the “columnar microstructure” is investigated. The effects of different processing parameters (which may be considered to be more practical for industrial applications), including the addition of ethanol to the water-based suspension liquid phase, change in the substrate roughness, and spray distance, on the resulting coating morphologies are investigated, and the characteristics of the resulting coatings from such deposition procedures are compared with those coatings deposited from modified water-based YSZ suspensions formulated in accordance with the results of our previously published research works [8].

2. Materials and Methods

2.1. Suspension Preparation and Characterization

The aqueous phase of the suspensions included deionized water, ethanol, and their respective mixture (50:50 vol.%) with a 30 wt.% solid content (YSZ powders). Yttria-stabilized zirconia (CrystalArc 8YSZ, Northwest Mettech Cop., Surrey, BC, Canada) with a D50 of 0.67 μm and chemical composition of ZrO₂ + HfO₂ = 91.68 wt.% and Y₂O₃ = 7.95 wt.% was used as the suspending powder. The particle size distribution was analyzed for the raw YSZ powder using a MASTERSIZER 2000 (Malvern Instruments Ltd., Malvern, Worcestershire, UK). Aqueous suspensions of the powder with 0.002 vol.% were prepared, and analysis was performed after 30 s and 60 s of ultrasound treatment. Figure 1 presents the particle size distribution of the YSZ powder [7]. The modified water-based suspension, including α-Terpineol as a dispersing agent, was prepared based on a procedure that has been reported in a previous study [7].

Table 1. Characteristics of the suspension samples.

Suspension Name	Solvent Type	Dispersant Type	Dispersant Concentration (wt.%)	Native pH	Adjusted pH
A	Water	---	---	7.4	---
B	Water-Ethanol	---	---	7.3	---
C	Ethanol	---	---	7.3	---
D	Water	α-Terpineol	0.1	7.7	2.5

presents the designated symbols for all the suspension samples and their respective chemical constituents. A Force Tensiometer (KRÜSS-K100 GmbH, Hamburg, Germany) device was used to measure the surface tension values using the lamella tear-off method (Du Nouy Ring with radius: 9.545 mm and wire diameter: 0.37 mm). The surface tension of HPLC-grade hexane solution, in triplicate, was measured to specify the authenticity of measurements. Droplet size distribution measurements were conducted at the University of Toronto, Centre for Advanced Coating Technologies. A detailed explanation of this method has been described elsewhere [13].

2.2. Coating Preparation and Characterization

The stainless-steel substrates were sand blasted to obtain three different substrates with surface roughness (R_q) values of 2.40, 2.53, and 5.80 μm, respectively. For all the surface roughness measurements, the root mean square deviation (R_q) was measured and reported as the surface roughness parameter. The ceramic top coats were deposited using a Mettech Axial III high power plasma torch, equipped with a continuous stream suspension injector system (LP-BT100-2J and a peristaltic pump, including a YZII15 pump head with three rollers. The parameters for the suspension plasma-spraying (SPS) process were as follows: 45 mL/min suspension feed rate, 200 A current, 50, 75, and 100 mm spray distances, 245 SLM total plasma gas flow rate, 75/10/15 Ar%/N₂%/H₂% plasma gas compositions, 20 SLM atomizing gas flow rate, and 9 mm nozzle internal diameter for ethanol-based suspensions. For water-ethanol and water-based suspensions, the SPS deposition parameters were as follows: 45 mL/min suspension feed rate, 200 A current, 50, 75, and 100 mm spray distances, 245 SLM total plasma gas flow rate, 75/10/15 Ar%/N₂%/H₂% plasma gas compositions, 20 SLM atomizing gas flow rate, and 9 mm nozzle internal diameter. The effects of the spray distance and substrate surface roughness on the coating morphologies were investigated using a Zeiss Sigma standard Field Emission Scanning Electron Microscope (FE-SEM). Image analyzer software (ImageJ—version 1.52a, National Institutes of Health, Bethesda, MD, USA) was used to estimate porosity values by analyzing the respective SEM micrographs. Porosity values in the microstructure of the resulting coatings were evaluated by analyzing the respective SEM micrographs using image analyzer software (ImageJ—version 1.52a, National Institutes of Health, MD, USA). Due to the wide range of porosity length scales in the coatings (micron, submicron, and nano pores), two different image magnifications were used for analyses: ×1000 (×1 K) and ×10,000 (×10 K). To evaluate the relative porosity values of the coatings, a total of 20 SEM micrographs were captured across each coating at both magnifications. All images were threshold-adjusted and converted into binary (black and white) images to calculate the porosity.

3. Results and Discussion

3.1. Suspension Characterization

Figure 2 shows that surface tension of the four suspensions (A, B, C, and D) decreases as the ethanol content increases. The suspension D has a surface tension of 22.2 mN/m, very close to that of C (ethanol-based suspension). Curry et al. concluded that in SPS, the final suspension droplet size is determined by the balance between the plasma shear forces that tend to break up the suspension stream and the suspension properties, which resist fragmentation [5]. Among suspension properties, viscosity and surface tension are the main ones that can resist suspension atomization [6]. Currently, there is no reliable equation in the literature to calculate the final droplet size in the SPS process [4]. However, the relationship among the plasma drag force, suspension surface tension, and the final droplet size was proposed by Fazilleau et al. [14], as shown in Equation (1):

$$D=\frac{8\sigma }{C_D\rho u^2}$$

(1)

where D (m) is the atomized suspension droplet diameter; 𝜎 (N/m) is suspension surface tension; 𝐶_𝐷 (unitless) is the plasma drag coefficient; and 𝜌 (kg/m³) and 𝑢 (m/s) are the density and stream velocity of plasma, respectively. It can be inferred from Equation (1) that a lower surface tension and a higher plasma drag force, which depends on plasma density and velocity, are required to obtain smaller droplets after atomization. The decrease in the suspension surface tension because of the increase in the ethanol concentration (Figure 2) leads to the formation of smaller droplets in the atomized suspension (Figure 3). The median droplet size in the atomized suspension decreases from 21.7 to 12.4 μm (

Table 2. The mean and median droplet size values calculated for atomized A, B, C, and D suspensions.

Suspension/Liquid	Mean Droplet Size (µm)	Median Droplet Size (µm)
A	31.9	21.7
B	27.6	18.7
C	15.3	12.4
D	15.4	12.2

) as the ethanol concentration increases from 0 to 100 vol.%. Such smaller droplets may easily follow the plasma gas flow parallel to the substrate surface, promoting the shadowing effect, which in turn leads to the formation of columnar-microstructure coatings. In fact, the surface tension of the suspension can have a significant effect on the size of the forming droplets in the suspension plasma spraying. Surface tension is the property of a liquid that determines the shape it takes when placed on a solid surface or when in contact with other fluids. In suspension plasma spraying, the liquid suspension is atomized into droplets that are then propelled by a plasma jet onto the substrate. The size of the droplets formed during atomization depends on various factors, including the surface tension of the suspension. A few key effects of the surface tension on the droplet size would be summarized as follows. First of all, surface tension plays a role in the breakup of the liquid into droplets during atomization. When the surface tension is high, the liquid tends to resist deformation and breakup, resulting in larger droplets. Conversely, when the surface tension is low, the liquid is more prone to breakup, leading to smaller droplets. Secondly, surface tension also influences the formation of capillary waves on the liquid surface. Capillary waves are small ripples or waves that form due to the interplay of surface tension and the inertial forces acting on the liquid. These waves can contribute to the breakup of the liquid into smaller droplets. In addition, surface tension affects the stability of the liquid jet emerging from the atomization device. High surface tension can cause the jet to become unstable, resulting in irregular breakup and the formation of larger droplets. Lower surface tension can promote a more stable jet and the formation of smaller, more uniform droplets.

Finally, after the droplets impact the substrate, they form splats or flattened droplets that adhere to the surface. The surface tension of the suspension can influence the spreading behavior of the droplets during splat formation. Higher surface tension can cause the droplets to spread less, resulting in smaller splats with a higher aspect ratio [15,16,17].

It is important to note that the surface tension effect on droplet size is not the sole determining factor. Other factors, such as the suspension viscosity, droplet velocity, and spray parameters, also interact to influence the droplet size distribution. The optimization of these parameters, along with an understanding of the suspension’s surface tension characteristics, can help control and manipulate the droplet size during suspension plasma spraying. A more complete formula for SPS droplet size analysis based on calculations that were originally developed for the atomization of fuel in a high-speed gas environment [18] is shown in Equation (2) below:

𝐷 = (136 𝜂 𝜎^3/2𝑑^1/2/𝜌_𝑝² 𝜌_𝑠^1/2𝑢⁴)^1/3

(2)

where D (m) is the final droplet size after second atomization inside plasma, η (Pa·s) and σ (N/m) are the suspension’s viscosity and surface tension, respectively, d (m) is the initial droplet size after primary atomization (can be assumed to be equal to the diameter of the suspension stream if the primary atomization effect is neglected), u (m/s) is the plasma stream’s velocity, and ρ_p (kg/m³) and ρ_s (kg/m³) are densities of the plasma and suspension, respectively. This equation, although it may not be accurate in modeling the suspension atomization inside the plasma in SPS, can reflect the relative significance of the key suspension characteristics (mainly the suspension’s surface tension and viscosity) in determining the final droplet size [5], if all the other factors are constant.

3.2. Coating Morphological Characteristics

Cross-sectional SEM images in Figure 4 show the effect of the substrate surface roughness on the coating microstructure for the different suspensions. The results revealed that a higher substrate surface roughness and ethanol concentration (in the solvent phase) result in coatings with a more pronounced columnar morphology. The higher substrate surface roughness (R_q) means a higher absolute difference in height between the peaks and valleys attributed to the surface asperity condition. The peaks at the substrate surface profile act as initiation sites for the formation of individual columns in the microstructure of the growing SPS coatings. Therefore, the greater the difference between peak heights and valley depths at the surface, the higher the chance of formation of individual columns [15].

The ethanol-based suspensions (C) yield coatings with more pronounced columnar microstructures, whereas both the water-based (A) and mixed water–ethanol (B) suspension coatings exhibited lamellar microstructures, as in Figure 4. These results were expected due to the dependence of the coating microstructure on the size and momentum of the atomized suspension droplets. As discussed earlier, the median suspension droplet sizes of water-based (A) and mixed water-ethanol based (B) suspensions are 21.7 and 18.7 μm, respectively, which are higher than the ethanol-based (C) median droplet size (i.e., 12.4 μm). This can be directly responsible for not obtaining columnar microstructures using the water-based (A) and mixed water-ethanol based (B) suspensions. In fact, the coating build-up mechanism for SPS is different compared to that in conventional APS and is found to be greatly affected by the generation of suspension droplets due to atomization inside the plasma plume, which in turn results in in-flight particles after solvent evaporation [16]. The reason for the distinct differences in microstructures obtained with SPS of different suspensions in this study stems from the expected different trajectories of the atomized suspension droplets inside the plasma stream and the resulting molten YSZ particles before impacting the substrate.

The coating build-up mechanism for SPS is different compared to that in conventional APS and is found to be affected by the generation of suspension droplets due to atomization inside the plasma plume, which in turn results in in-flight particles after solvent evaporation [19]. The reason for the distinct differences in microstructures obtained using SPS of different suspensions in this study stems from the expected different trajectories of the atomized suspension droplets inside the plasma stream and the resulting molten YSZ particles before impacting the substrate. After atomization, the suspension droplets undergo rapid heat-up and solvent evaporation. The fine solute particles, depending on the size of the particles, tend to agglomerate or sinter/fuse into larger particles afterwards [4]. As a consequence, through complete vaporization of the solvent, agglomerated fine solid particles become directly exposed to the plasma, heat up, and melt to form molten/semi-molten droplets. The molten droplets are typically larger and heavier than the initial powder particle size in the suspension, which governs the splat size and other features of the coating microstructure [4]. The impact of the molten or semi-molten agglomerated particles on the substrate generates fine splats, and the coating forms via consecutive deposition of the splats. A direct relationship between the suspension droplet size and the molten particle droplet size can therefore be expected. The molten particle droplets follow different trajectories, depending on their droplet size and, therefore, momentum. The trajectory of small droplets, because of their low momentum, is strongly influenced by the plasma stream drag in the boundary layer close to the substrate, and they deposit at shallow angles on surface asperities, resulting in a shadowing effect [20]. The particle shadowing effect, proposed by VanEvery et al. [17], is shown schematically in Figure 5. Based on their findings, three major types of coating microstructures are possible for SPS coatings. The shadowing effect is largest when the sizes of droplets are extremely small, when a columnar-type microstructure typically forms. Droplets of medium sizes are still small enough to be affected by the plasma flow and thus can produce a feathery columnar structure, which contains porosity bands within the columns. Finally, the relatively large molten droplets are less affected by the plasma drag and typically have a direct (straight) impact on the substrate, forming a lamellar (like in the conventional APS method) or vertically cracked structure. The mechanism of vertical crack formation in APS coatings has been reported to be the result of cooling and shrinkage of the deposited splats, providing high tensile stresses leading to intra-splat cracking [21]. The build-up and overlap of the splats finally lead to the vertical cracks in the coatings. This mechanism can be found in SPS coatings as well, i.e., increased tensile stresses in SPS coatings can be relieved by the formation of vertical cracks [21,22]. A relationship between the suspension droplet size and the molten particle droplet size can therefore be expected. The suspension droplet size distributions, presented in the previous section, can therefore give us an insight into the droplet size of the resulting molten agglomerated particles, although they are not the same. Moreover, the atomized suspension droplet sizes measured in this work belong to the suspensions fragmented by an airbrush, which would result in droplet sizes different from the ones fragmented inside and by the plasma flow. However, the obtained data were only used for a comparative atomization study of the different suspensions in this work.

The high-magnification cross-sectional SEM images of SPS coatings, deposited at a 75 mm spray distance from suspensions with different solvents, are presented in Figure 6. All the coatings possess a two-zone (also called bi-modal) type of microstructure, which is common in SPS coatings [20,21,22,23,24]. This two-zone microstructure is made of partially melted (marked PM in Figure 6) zones surrounded by a fully melted (marked FM in Figure 6) dense matrix. The reason for this microstructure formation is that a relatively large portion of the plasma power is consumed to evaporate the liquid solvent in SPS. Thus, some particles or agglomerates become just partially melted and then embedded into the fully melted solid [25]. As seen in Figure 6, increasing the water content in the suspension leads to an increase in the partially melted (PM) zones and porosity in the coating microstructure (

Table 3. Porosity values estimated from micrographs in Figure 6.

Coating	Porosity (%)
A	16–19
B	9–11
C	3–5
D	17–18

). This is due to the higher evaporation enthalpy of water (2.4 × 10⁶ J/kg) compared to that of ethanol (0.6 × 10⁶ J/kg) [4,5], which consumes more energy and gives rise to incomplete melting of the particles. Moreover, less solid melting contributes to coatings with a higher porosity amount and therefore lower thermal conductivity. The higher surface tension of water, which results in larger atomized droplet sizes, can also make the heat transfer more difficult and favor the increased PM zones in the coating microstructure [26].

Similar to the substrate surface roughness and ethanol concentration parameters, the same desirable effect (i.e., contribution to form columnar morphology) was observed by increasing the substrate-to-nozzle (stand-off) distance, as in Figure 7. This can be related to the trajectory of the molten particle droplets in plasma flow parallel to the substrate surface at longer spray distances, increasing the shadowing effect during coating deposition. The velocity and temperature of suspension droplets decrease over longer spray distances, and plasma drag can affect their trajectory more easily due to their smaller momentum. Therefore, it can be concluded that for a given solid loading in the suspension, the columnar (also referred to as “cauliflower-like” [27]) structures become more evident in SPS coatings as the spray distance increases. These findings agree with those reported previously by other researchers in light of the shadowing effect theory [2,25,28]. In short spray distances, the particles arrive at the substrate with higher velocities favoring planar deposition, with a lower probability of column formation. It should be noted that the round spherical particles observed in the microstructure of SPS coatings are the re-solidified molten droplets. The re-solidification phenomenon becomes more pronounced by increasing the spray distance [26,29].

The results of Figure 7 also show that increasing the spray distance led to thinner coatings, with all other factors remaining the same. For example, in the case of A and B coatings, doubling the spray distance from 50 to 100 mm resulted in thickness reductions of 66% and 73%, respectively. This trend was not seen in the case of the ethanol-based (C) suspension plasma-sprayed coatings. This could be due to the larger porosity of the columnar coatings; thus, the coating mass decreased with the spray distance. Although coatings with columnar morphologies are produced at longer spray distances, the decrease in the final coating thickness can be an issue that needs to be addressed by controlling other processing parameters. As reported by Marchand and Mauer et al. [30,31,32], the suspension droplets reach their maximum velocities and temperatures roughly 40–50 mm downstream of the plasma torch’s X-axis. Afterwards, their velocities and temperatures rapidly decrease. Therefore, the deposition rate and density of coatings decrease as the spray distance increases. This was also claimed by Cotler et al. in the case of the SPS deposition of titania, alumina, and YSZ [33], as well as other researchers [6,34].

Based on the above observations, one may conclude that producing an SPS coating with a columnar morphology is possible by increasing the substrate surface roughness and using a higher ethanol concentration as the solvent in the suspension. Both routes cause additional costs and safety and environmental hazards. One practical remedy would be the preparation of cost-effective water-based YSZ suspensions with optimized characteristics (i.e., highly concentrated stable suspensions having low surface tension and viscosity values). This can be achieved through the addition of an appropriate surfactant type and concentration along with pH adjustments of the water-based suspensions, which have been thoroughly studied previously [8]. According to such reported results, the aqueous 30 wt.% YSZ suspension with 0.1 wt.% α-Terpineol dispersant and pH adjusted at 2.5 (D) was the best, optimized suspension based on the following criteria: (i) high stability, (ii) low surface tension, and (iii) low viscosity. This suspension can be atomized to droplets in the size range of 11.9 to 12.5 μm, comparable to those of the ethanol-based suspension (C), as shown in Figure 3 and Table 2, which indicates its capability of producing columnar coatings. The cross-sectional morphology of the SPS coatings that were deposited from the optimized water-based YSZ suspensions (D) is illustrated in Figure 8. At a fixed spray distance, coatings with feathery columnar morphologies are deposited, similar to the microstructures of coatings deposited from ethanol-based suspensions (C). To sum up, we believe that the development of water-based suspensions (such as that in the present study for YSZ powders), as an alternative to ethanol-based ones in the SPS technique, is an interesting advancement in the field of coating production. Several impacts and potential benefits would be attributed to this. First, one of the significant advantages of using water-based suspensions is their lower environmental impact compared to ethanol-based ones. Ethanol is a volatile organic compound and contributes to air pollution. By replacing ethanol with water, improved air quality and a more sustainable coating process can be achieved. Second, water-based suspensions are generally considered safer to handle and work with compared to ethanol-based ones. The latter is flammable and poses potential fire hazards. By eliminating or reducing the use of ethanol, the overall safety of the coating-production process can be enhanced. Moreover, ethanol is typically more expensive than water. By transitioning to water-based suspensions, there is a potential for cost savings in the coating-production process. Water is a readily available and cost-effective solvent, making it an attractive option for manufacturers looking to reduce production costs. Third, the impact of water-based suspensions on coating performance can vary depending on the specific application and formulation. While ethanol is known to provide better combustion characteristics, water-based suspensions can still offer satisfactory coating quality and properties. Further research and development efforts can help to optimize the formulations to achieve comparable or improved coating performance with water-based suspensions. Overall, the use of water-based suspensions in the suspension plasma-spraying technique offers the potential for a more environmentally friendly, cost-effective, and safer coating-production process. While there may be some challenges in terms of optimizing the coating performance, continued research and innovation in this area can lead to significant benefits for various industries relying on coatings.

4. Conclusions

The relationships among YSZ suspension parameters, processing parameters, the suspension atomization behavior, and the resulting SPS coating microstructures were investigated. C suspension-atomized droplets are smaller (12.4 μm) than the droplets of B and A (18.7 and 21.7 μm, respectively). This leads to the deposition of coatings with columnar microstructures from ethanol-based-suspension (C) coatings. Both the water-based (A) and mixed water–ethanol (B) suspension coatings exhibited lamellar microstructures. Coatings with more pronounced columnar structures were obtained from the deposition on substrates with a higher surface roughness, at longer spray distances. Water-based suspensions, including the α-Terpineol dispersant (D), exhibited favorable rheological properties to produce smaller droplet sizes in the atomized phase. Ethanol can be replaced with water if an appropriate dispersant (like α-Terpineol) and pH value is selected, in order to produce SPS coatings with a columnar morphology, without altering other processing parameters (like the substrate surface roughness and spray distance).