Integrated Microstructural and Chemical Approach for Improving CMAS Resistance in Thermal and Environmental Barrier Coatings

Abstract

This study provides an investigation into the influence of surface roughness, porosity, and chemistry on the wettability and infiltration behavior of calcia-magnesia-alumino-silicates (CMASs) in thermal and environmental barrier coatings (T/EBCs) used in high-temperature gas turbine engines. High-temperature contact angle measurements were performed at 1260 °C on 7 wt.% yttria-stabilized zirconia (7YSZ) and yttrium ytterbium disilicate (YYbDS, (Y_1/2Yb_1/2)₂Si₂O₇) to evaluate the interaction of CMASs with different surface finishes and coating microstructures. The findings demonstrate that porosity plays a dominant role in determining CMAS infiltration dynamics. In YYbDS, increasing porosity from 6.3% to 22.7% facilitated the formation of an apatite layer that limited CMAS penetration to approximately 2 µm. Surface roughness exhibited a subtler influence in that reducing Sa from 0.61 µm to 0.05 µm increased the change in the contact angle by ~2°, although its impact was found to be less significant compared to porosity and reactive chemistry. These results indicate that an integrated approach that optimizes porosity, chemistry, and surface morphology can significantly enhance CMAS resistance. The study emphasizes that leveraging both microstructural and chemical properties is critical to developing coatings capable of withstanding the harsh conditions encountered in aerospace environments.

1. Introduction

Thermal and environmental barrier coatings (T/EBCs) are crucial for safeguarding turbine components in high-pressure gas turbine engines against extreme temperatures and corrosive environments [1,2,3]. The failure of these coatings due to environmental damage can have severe consequences, including significant performance losses, costly downtime, and potential safety hazards in aerospace applications. T/EBCs act as a protective shield, preventing direct thermal damage and mitigating degradation caused by reactive and abrasive elements. Gas turbine engines may encounter airborne particulate matter, such as sand, dust, and volcanic ash, during takeoff or landing that melts upon entering the combustor and forms molten calcia-magnesia-alumino-silicate (CMAS). This molten CMAS infiltrates and spreads through the coatings, compromising their ability to protect the underlying components [4,5,6,7]. The infiltration of CMASs leads to various severe issues, including coating delamination [8], increased thermal stress [6,9], and ultimately, catastrophic failure of critical turbine components. Thus, developing advanced T/EBCs that can resist CMAS infiltration is vital for ensuring the reliability and longevity of gas turbine engines under harsh operational conditions.

Historically, research has focused on modifying the chemical composition of T/EBCs to mitigate the deleterious effects of CMAS infiltration [10,11,12]. Incorporating reactive elements, such as yttria and other rare earth oxides, into zirconia-based systems at sufficient concentrations can lead to the formation of protective phases upon contact with CMASs [13,14,15]. These phases effectively reduce infiltration and enhance coating resistance. Significant advancements have also been made in understanding the interactions of CMAS with different types of materials, including silicates [16,17,18,19,20], phosphates [21,22,23], niobates and tantalates [24,25,26], and, more recently, ultrahigh-temperature ceramics like borides [27]. These studies have shed light on the role of chemical composition in CMAS resistance. However, despite this progress, considerably less attention has been given to how microstructural features like surface roughness and porosity affect CMAS wetting and infiltration dynamics. Such features are likely to have a significant impact on how a CMAS spreads and penetrates the coating, potentially offering additional opportunities to enhance coating performance.

A substantial gap remains in understanding the specific roles of surface roughness and porosity in controlling CMAS wettability and infiltration. While some studies have investigated CMAS wettability on different T/EBC materials, most of these works have focused primarily on chemical interactions and reaction mechanisms, often neglecting the influence of surface microstructure [28,29,30,31,32]. In particular, the effects of surface roughness and porosity on CMAS wetting behavior have not been thoroughly examined, and many studies have relied on dense bulk samples to simulate CMAS interactions, which may not adequately reflect the behavior of coatings with varying microstructural features [23,28,30]. Recent investigations have examined surface roughness effects on CMAS wettability, employing various spraying techniques such as solution precursor plasma spray [31,33,34], suspension plasma spray [34], and plasma spray–physical vapor deposition [35,36,37] or surface modification techniques like laser ablation [38,39,40]. Kang et al. increased the Ra of supersonic atmospheric plasma sprayed YSZ coatings from 1.55 µm to 9.85 µm with femtosecond laser texturing [40]. They found that the initial contact angle of CMAS was lowered from 118° to 85° yet left a high equilibrium angle of ~70°, whereas the smoother surface stabilized at ~28°. Despite this progress, relatively few systematic examinations have focused on the influence of porosity on CMAS interactions, which remains an important and insufficiently studied aspect of CMAS resistance. Lokachari et al. examined solution precursor plasma sprayed YSZ coatings and concluded that coatings containing 16% ± 2% porosity allowed CMAS to infiltrate at 38 µm–68 µm in 5 min at 1300 °C, showing how interconnected pores can be susceptible to infiltration [34]. Prior studies have yet to vary roughness and porosity systematically across both a nonreactive oxide and a reactive silicate under identical thermal conditions. Therefore, there is a critical need to understand how surface roughness and porosity influence CMAS wetting and spreading behavior to resist infiltration effectively. Addressing these microstructural gaps, alongside optimizing chemical composition, could lead to the development of coatings that are far more resistant to CMAS degradation by enhancing their robustness and reliability in high-temperature gas turbine engines.

This study addresses these gaps by systematically investigating the influence of surface roughness and porosity on CMAS resistance in T/EBCs using 7 wt.% yttria-stabilized zirconia (7YSZ) and yttrium ytterbium disilicate (YYbDS, (Y_1/2Yb_1/2)₂Si₂O₇) as model systems. 7YSZ is the state-of-the-art chemistry for TBCs and serves as a suitable baseline, while YYbDS is an experimental chemistry for EBC topcoat applications. The results on both materials provide an informative comparison on the effect CMAS shows on TBC and EBC materials while also shedding light on the interactions a novel chemistry like YYbDS has with CMASs. High-temperature contact angle measurements were performed at 1260 °C to evaluate the wettability of CMAS on pellets and coatings. These two systems represent two extreme classes of CMAS attack, and the porosity variation investigation in YYbDS serves as processing limits for the relative porosity of engine coatings. The results demonstrate that porosity has a noticeable impact on CMAS infiltration, where increased porosity promotes the formation of a reactive barrier. Surface roughness also influenced CMAS spreading behavior, albeit to a smaller degree than porosity. These findings emphasize the importance of optimizing both microstructural and chemical properties to develop coatings that effectively resist CMAS infiltration, thereby improving the reliability of gas turbine engines in challenging environments.

2. Materials and Methods

2.1. Materials

Two oxide systems, 7 wt.% yttria-stabilized zirconia (7YSZ) and yttrium ytterbium disilicate (YYbDS, (Y_1/2Yb_1/2)₂Si₂O₇), were utilized in both pellet and coating form to systematically investigate their interaction with calcia-magnesia-alumino-silicate (CMAS). 7YSZ is a widely studied chemistry for TBC applications, while YYbDS has recently been proposed as a potential chemistry for EBC applications. 7YSZ is widely adopted in commercial turbine applications because 7 wt.%–8 wt.% Y₂O₃ yields a metastable t’ and cubic matrix that combines low thermal conductivity and high fracture toughness [CITE]. The 7YSZ and YYbDS pellets were prepared using high-energy ball milling followed by pressureless sintering to achieve high density. 7YSZ (Metco 204NS) and YYbDS (Metco AE12149) thermal spray powders were both sourced from Oerlikon Metco and had a particle size of approximately 60 µm–70 µm.

To achieve the desired pellet densities (>95%), 10 g of 7YSZ was milled with 3 mL of ethanol in a 7YSZ jar along with 50 g of 7YSZ media for 3 h using 18 cycles of milling for 10 min followed by 10 min of cooling. YYbDS was milled in a polytetrafluoroethylene (PTFE) jar with 50 g of SiC media for a total of 30 min using three cycles of milling for 10 min and 10 min of cooling. Both powders were then dried in a box furnace at 75 °C for 8 h to remove any residual moisture. The powders were pressed using a 25.4 mm (1”) stainless steel die under 100 MPa for 2 min to form a green body with a thickness of 2.5 mm. The pellets were sintered on Pt foil in an alumina crucible, with 7YSZ sintered at 1500 °C for 6 h and YYbDS at 1600 °C for 10 h. The heating rate for both sintering processes was 5 °C/min, and the furnace was allowed to cool naturally after the dwell period. The dense pellets were then polished to surface finishes of 30 µm, 3 µm, or 0.02 µm using diamond grinding discs and suspension.

YYbDS coatings were fabricated by atmospheric plasma spraying using the same powder utilized for pellet preparation prior to ball milling. To increase the porosity in the coating, a batch of YYbDS powder was mechanically blended with 5 wt.% polyester (Metco 600NS, Oerlikon Metco, Westbury, NY, USA) that served as a pore former. The coatings were deposited onto 25.4 mm (1”) diameter monolithic Hexoloy (Saint-Gobain Structural Ceramics, Northampton, MA, USA) SiC substrates using a silicon bond coat sprayed prior to the YYbDS topcoat. The bond coat and topcoat had thicknesses of approximately 200 µm and 500 µm, respectively. The coatings were annealed at 1300 °C for 4 h to promote crystallization and burn off residual polyester. The coatings were then polished to a 0.02 µm finish.

A synthetic mineral sand, AFRL-02 (Powder Technologies Inc., Arden Hills, MN, USA), composed of 34% quartz, 30% gypsum, 17% aplite, 14% dolomite, and 5% salt (NaCl) by mass, was used for the wetting study, as this composition is representative of natural sand. The melting point is approximately 1220 °C. The sand was placed in an alumina crucible, annealed at 1300 °C for 30 min to promote glass formation, and then air-quenched. The resulting glass was crushed into a fine powder with an agate mortar and pestle. To create uniform CMAS spheres, the powder was pressed into shape using a 6 mm spherical die. The die punches were coated with a thin layer of silicone oil using a cotton swab to facilitate extraction. The powder (~0.125 g) was pressed under 130 MPa for 2 min and carefully removed.

2.2. Characterization

A high-resolution digital light microscope (VHX-7000, Keyence, IL, USA) was used to capture images of the samples both before and after testing. Images were taken at 20× magnification and stitched together to provide a comprehensive view of the surface.

Surface roughness measurements were performed using a 3D confocal laser scanning microscope (OLS5100, Olympus, Tokyo, Japan). Profilometry was conducted on a 240 µm × 240 µm area using a 50× objective lens, which was then expanded to a 5 × 5 grid (1.2 mm × 1.2 mm) to improve statistical reliability. Surface roughness analysis was carried out on this larger dataset. Missing pixels were filled by averaging neighboring values, and the entire dataset underwent a tilt correction.

A scanning electron microscope (SEM, SU3500, Hitachi, Tokyo, Japan) was employed to examine the surface and cross-sectional microstructures. Energy dispersive spectroscopy (EDS, XFlash 6-30, Bruker Nano GmbH, Berlin, Germany) was used to investigate elemental distribution and cross diffusion between the CMAS and the ceramics. Cross-sectional SEM images were further analyzed with ImageJ software to quantify material porosity through a thresholding technique [41].

2.3. Wettability Experiments

The high-temperature wettability of CMAS on the ceramic materials was evaluated using a commercial contact angle unit (OCA 25-HTV 1800, DataPhysics Instruments). CMAS spheres (Ø 6 mm) were placed onto the coatings or pellets, which were loaded onto an alumina plate and placed in a tube furnace. The wetting process was monitored using a digital camera that was aligned with the sample to ensure accurate capture of the contact angles. The furnace was sealed and backfilled with argon (Ar) flowing at 200 SCCM throughout the experiment to prevent oxidation of the Inconel and SiC substrates. The furnace was heated to 1260 °C at 5 °C/min and held for 30 min to observe the wetting behavior. Data collection began at 1150 °C and was captured at 20 frames per minute. Following the dwell period, the furnace was allowed to cool naturally. A type-B thermocouple was placed ~5 mm below the sample, and the setpoint remained within ±2 °C of 1260 °C during the dwell. The 1260 °C test temperature was chosen because it allowed the CMAS to fully spread while not being too elevated, where off-gassing dominated and invalidated the contact angle measurements.

Contact angles were measured using the provided DataPhysics software by fitting the left and right contact angles using a 5-degree rational tangent function and averaging the results. The first derivative of the contact angle data was computed per frame, and the resulting dataset was smoothed using a 100-point Savitzky–Golay second-degree polynomial. After the experiments, samples were cross-sectioned, mounted, and polished to analyze the interaction at the interface between the CMAS and ceramics.

3. Results and Discussion

3.1. Surface Roughness Influence

3.1.1. Surface Properties and CMAS Wettability

To systematically study the influence of surface roughness on CMAS wettability, dense pellets of one chemistry were consolidated and polished to varying degrees. This approach minimizes effects from chemistry or microstructure (porosity) to ensure that surface roughness is the primary variable. Figure 1 shows low- and high-magnification optical images of 7YSZ pellets polished to three different surface finishes. Grinding lines are clearly visible on the sample polished to a 30 µm finish, whereas those polished to 3 µm or 0.02 µm appear relatively smooth. The relative porosity for each pellet was approximately 3%, as summarized in

Table 1. Composition and relative density of pellets and coatings examined. P: pellet. C: coating. ‘5P’ in YYbDS C 5P refers to the 5 wt.% of polyester blended with YYbDS.

Composition	ID	Porosity (%)
Y0.077Zr0.923O2-δ	7YSZ P	3.1
(Y1/2Yb1/2)2Si2O7	YYbDS P	2.9
	YYbDS C	6.3
	YYbDS C 5P	22.7

. Similar results were observed for the YYbDS pellets.

To quantitatively characterize the surface properties, profilometry measurements were conducted on each surface quality. Three surface roughness parameters, Sa (arithmetical mean height), Ssk (skewness), and Sdq (root mean square gradient), were assessed to capture unevenness, height distribution, and the fineness of the surface.

Table 2. Surface finish and roughness parameters Sa (arithmetical mean height), Ssk (skewness), and Sdq (root mean square gradient) for examined specimens. P: pellet. C: coating.

ID	Finish	Sa (µm)	Ssk	Sdq
7YSZ P	30 µm	0.33	−2.61	1.01
	3 µm	0.05	1.16	0.13
	0.02 µm	0.05	0.00	0.15
YYbDS P	30 µm	0.61	−5.32	1.46
	3 µm	0.08	9.11	0.36
	0.02 µm	0.05	6.22	0.29
YYbDS C	0.02 µm	0.74	−7.22	0.78
YYbDS C 5P	0.02 µm	8.21	−2.03	3.39

presents these values. The roughness change from 30 µm to 3 µm was significant, whereas the change between 3 µm and 0.02 µm was minimal, suggesting diminishing returns in roughness reduction.

Figure 2 displays the temporal evolution of the CMAS contact angle on both the 7YSZ and YYbDS pellets polished to varying degrees. While the initial contact angles differ slightly due to experimental variation, both compositions exhibit a trend of faster wetting kinetics in the most polished state and slower kinetics in the roughest state, as indicated by the slope of the curves. The final contact angles across different surface conditions were approximately 10°–15°, suggesting that the surface roughness range examined in this study may not substantially impact the thermodynamics of CMAS wetting; however, subtle changes in the kinetics of spreading were found. Moreover, the isotropic nature of the rough grinding scars on the microscale that were observed may be a key factor in the minor changes in wettability between the samples. In cases of superhydrophobicity, nano-hierarchical structures are critical in creating significant changes in wettability, suggesting that random, larger-scale roughness may have limited effects on wettability [42,43,44].

To further elucidate the influence of surface quality, changes in the contact angle and the minimum of the first derivative of the mean contact angle curve were plotted against the surface roughness parameters and are shown in Figure 3. No strong trends emerged for either the 7YSZ or YYbDS pellets. This result aligns with the relatively minor differences observed in the contact angle curves. Moreover, the spread in ∆CA and ${\displaystyle \frac{dC{A}_{mean}}{dt}}$ values was minimal, suggesting that surface roughness has only a minor impact on CMAS wettability.

3.1.2. Microscopy of CMAS–Ceramic Interface

To gain insights into the mechanism of CMAS spreading, surface SEM images were captured at the interface of the wetting front and are shown in Figure 4. For both the rough (30 µm) and smooth (0.02 µm) 7YSZ and YYbDS samples, featureless and rough CMAS–ceramic interfaces were observed, respectively. In the 7YSZ samples, small bright clusters were discernible at the interface. For YYbDS, a phase was visible along the entire interface of the wetting front, which may act as a barrier and inhibit CMAS spreading. The wetting front appeared as the thinnest part of the CMAS melt. This suggests that the local chemistry is likely easily shifted and may lead to new phases preferentially forming here. Interestingly, for both 7YSZ and YYbDS, the rougher samples exhibited a larger reaction zone, potentially due to increased surface area contact for the CMAS.

Cross-section SEM images of the same samples are presented in Figure 5. A relatively sharp interface between the CMAS and samples was observed in all samples. Infiltration depth and reaction layer thicknesses were determined by computing the average length of approximately 40 lines normal from the CMAS–YSZ interface to the dashed line, which represents the extent of the CMAS-induced microstructural changes, for five images. For YYbDS, the thickness of the reaction product was measured. Grain boundary infiltration was found to be 8.0 µm ± 0.7 µm and 6.5 µm ± 1.1 µm in the rough and smooth 7YSZ samples, respectively, while a reaction product layer formed at the interface for YYbDS consisting of the apatite phase. The thickness of the layer for the rough and smooth YYbDS was 1.7 µm ± 0.4 µm and 1.1 µm ± 0.3 µm, respectively. The reaction layer was slightly thicker in the rougher samples for both chemistries, which may explain the slightly decreased wetting kinetics observed for these surfaces.

Elemental diffusion across the interface was analyzed using EDS to better understand the extent of influence between the two phases beyond observable grain boundary infiltration. Figure 6 displays six points of analysis across the interface for both the 7YSZ and YYbDS samples. At the interface, Y³⁺ was observed to leach from 7YSZ, with its concentration decreasing faster than that of Zr⁴⁺. Interestingly, the bulk diffusion length into either phase was approximately 80 µm. Trace amounts of Y³⁺ and Zr⁴⁺ were detectable up to 80 µm into the CMAS melt, while trace amounts of Ca²⁺ and Si⁴⁺ were found similarly in the bulk 7YSZ. A similar phenomenon was observed for YYbDS, where Y³⁺ and Yb³⁺ appeared to leach at similar rates. Trace rare earth elements and CMAS components were also found up to 80 µm from the interface, revealing the magnitude of elemental diffusion possible.

The influence of surface roughness on CMAS wettability has been previously examined by various researchers, with mixed findings on the effects of increased roughness. Guo et al. demonstrated that polished surfaces of GdPO₄ and LaPO₄ exhibited reduced wettability due to minimized capillary forces when compared to as-fabricated surfaces [23]. However, the as-fabricated samples may have had contaminants from the box furnace that settled on the material surface that may have impacted the CMAS wetting properties. In contrast, the smoother 7YSZ and YYbDS surfaces in the present work facilitated enhanced CMAS spreading, whereas rougher surfaces offered slight resistance, albeit limited. It is also important to note that the effects of roughness are likely dependent on both the specific chemistry of the coating and the CMAS reactivity. This is suggested by studies involving Pt-coated surfaces, where increased roughness facilitated the formation of reaction products, thereby enhancing CMAS wetting resistance [38]. Previous work from our group also suggested surface roughness effects to be dependent on chemistry [32]. Therefore, while roughness can affect CMAS interaction, its role may be highly dependent on the material’s chemical reactivity.

3.2. Chemistry Influence

While the influence of surface roughness did not significantly alter CMAS wettability when chemistry and porosity were constant, a comparative analysis between the smooth (0.02 µm) 7YSZ and YYbDS revealed clear differences in wetting behavior. Figure 7 highlights that YYbDS demonstrated slightly slower wetting kinetics compared to 7YSZ. This difference may be attributed to the formation of a reaction product in YYbDS, which acted as a diffusion barrier. Such reaction barriers have also been discussed in studies on Gd/Yb-modified SrZrO₃ coatings and Ta₂O₅-Y₂O₃ co-stabilized ZrO₂, where the formation of stable reaction products significantly modified CMAS wetting kinetics [31,45]. Interestingly, 7YSZ and YYbDS showed similar relative spreading rates of −20.1°/min and −19.2°/min, respectively; however, the onset of spreading occurred at a lower temperature on 7YSZ than it does on YYbDS. This may be due to the difference between grain boundary infiltration and the precipitation of a reaction product. This suggests that while chemistry plays a role, other factors such as reaction dynamics also influence the wetting process.

For 7YSZ, the reaction involves the dissolution of yttria from the zirconia lattice upon contact with molten CMAS and has been well-documented in the literature [8,46,47]. Subsequently, this destabilizes the YSZ structure and results in a Y-rich CMAS glass and precipitated monoclinic zirconia being precipitated out of the melt. While the formation of monoclinic ZrO₂ may alter local surface energetics, it does not substantially hinder CMAS infiltration.

Conversely, YYbDS undergoes a distinct chemical interaction with CMAS. Here, rare earth cations Y³⁺ and Yb³⁺ diffuse from the YYbDS into the molten CMAS, reacting predominantly with calcia and silica components to form a stable apatite reaction product, Ca₂RE₈(SiO₄)₆O₂, where RE denotes rare earth elements [12,15,48]. Crucially, this apatite layer forms a robust infiltration barrier at the interface that significantly impedes further CMAS penetration.

These differing chemical interactions emphasize that the reactivity of the chemistry directly determines its effectiveness against CMAS infiltration. While YSZ experiences limited reactive protection, YYbDS provides substantial resistance through the rapid formation of a dense and stable apatite phase that demonstrates the importance of chemically-driven protective mechanisms in enhancing coating performance.

3.3. Microstructure Influence

The influence of microstructure on CMAS wettability was further investigated by preparing YYbDS coatings with two different porosity levels and polishing them to a 0.02 µm finish. Figure 8 presents SEM surface micrographs of these coatings and emphasizes the microstructural differences. The relative porosity of the standard YYbDS coating was measured at 6.3%, while the YYbDS with 5 wt.% polyester added as a pore-former exhibited 22.7% porosity. Despite having polished surfaces, the surface roughness parameters in Table 2 show significant differences between the coatings. Wettability experiments were performed on both samples, and the corresponding wetting curves are displayed in Figure 9. Both coatings exhibit an onset of CMAS spreading at the same temperature. However, the spreading rate for the more porous coating is noticeably slower than the denser coating. Additionally, the residual contact angle for the more porous coating is larger (17.2° versus 11.3°). The slower spreading kinetics observed in the more porous coating is likely due to CMAS preferentially infiltrating the coating rather than spreading across the surface. Microscopy analysis was performed to explore this hypothesis further.

Figure 10 presents surface and cross-section SEM images of the interface for both YYbDS coatings. Similar microstructures are found at the interface in both samples, with the apatite reaction product observed across the entire interface similar to that seen in the YYbDS pellet. Notably, the more porous coating has a crystallized phase forming directly at the interface, whereas the denser coating exhibits a ~10 µm gap before the formation of the reaction phase. The cross-sectional analysis shows that both coatings have a thin (~2 µm) apatite phase forming at the interface. Interestingly, the reaction product thickness is approximately equivalent for both samples even though the total infiltration into the coating is larger for YYbDS C 5P due to the higher surface void depth. A quantitative relationship between the surface void depth and reaction product growth rate was not apparent in the present dataset. The increased void depth, which can be approximated by Sa, resulted in increased surface contact area and may account for the observed differences in the CMAS spreading rate.

An elemental analysis was conducted across six points of the interface for both coatings, as depicted in Figure 11. The results closely match those observed for the pellets. Regardless of porosity, rare earth elements appear to be leached at similar rates in both coatings. While the observable extent of the reaction zone is only approximately 2 µm, the elemental analysis indicates that elements have diffused up to 80 µm into the CMAS, similar to the dense pellets. The presence of porosity does not seem to significantly impact diffusion rates, as comparable trace element levels are found at similar distances. Furthermore, comparing these trace concentrations to those in the YYbDS pellet suggests that both microstructure and surface roughness exert minimal influence on diffusion rates.

The effect of significant microstructure changes on CMAS wettability was further explored by comparing wetting curves for CMAS on the polished YYbDS pellet and coating that had a relative porosity of 2.9% and 6.3%, respectively. Figure 12 shows this comparison and reveals that the differences are minimal. The onset of spreading, maximum spreading rate, and residual contact angle are nearly identical for both cases. Despite the coatings and pellets being prepared by vastly different procedures, the similar porosities lead to comparable wettability behavior. These results reinforce the conclusion that porosity is a crucial microstructural factor influencing CMAS wettability.

Pore Size and Distribution

Krämer et al. showed that a capillary analysis predicted that the time for CMASs to penetrate to a given depth falls inversely with pore radius ($t\propto {D_c}^{-1}$) for a fixed pore fraction, $\omega$, as shown in Equation (1) [7].

$$t\approx \left[{\displaystyle \frac{k_t}{{8D}_c}}{\left({\displaystyle \frac{1-\omega }{\omega }}\right)}^2{L}^2\right]{\displaystyle \frac{\eta }{{\sigma }_{LV}}},$$

(1)

Here, $k_t$, $L$, $\eta$, and ${\sigma }_{LV}$ are the tortuosity factor, penetration depth, viscosity of the melt, and surface tension, respectively. Lokachari et al. and Qi et al. validated this with porositiy variation in YSZ, Gd₂Zr₂O₇, and SrZrO₃ coatings [31,34]. In the present YYbDS coatings, the porosity increases from 6.3% to 22.7%, and the SEM suggests a pore size in the order of 1 µm–10 µm. Despite the larger and more connected pores in the more porous coating, the CMAS penetration remained ≤2 µm because a rapid apatite layer sealed further penetration. This outcome indicates that reactive chemistries can offset the expected increase in CMAS infiltration from larger pores; however, a threshold of pore size or fraction may exist that beyond which the reactive chemistry cannot protect further CMAS infiltration. A comprehensive microtomography study that systematically varies pore size and distribution is therefore required to construct a comprehensive microstructural design map for CMAS-resistant coatings.

The dual nature of porosity presents both challenges and opportunities. On one hand, increased porosity facilitates deeper infiltration, potentially compromising the coating. On the other hand, it can also reduce the lateral spread of CMASs, which can be beneficial in controlling the overall interaction with the surface. Our findings show that porosity must be carefully tuned in conjunction with the coating chemistry to manage these competing effects. In particular, the reactive chemistry of YYbDS played a crucial role in mitigating the negative impact of increased porosity. The reaction between the CMAS and YYbDS within the surface voids limited the extent of further penetration by forming a reaction product that acted as a barrier. This phenomenon resulted in CMAS infiltration being largely confined to the depth of the surface voids, rather than progressing deeply into the coating. This reactive interaction effectively converted the increased porosity into a mechanism for reducing the spread of CMAS, providing a balance between infiltration and spreading. These findings align with prior work on SrZrO₃ [31], where increased porosity led to reduced spreading but higher infiltration, suggesting the need to optimize porosity based on the specific chemistry of the coating.

The implications of these results are significant for the design of coatings with tailored microstructures. Simply increasing porosity without considering the coating’s chemistry could lead to adverse outcomes, especially for materials like low-Y YSZ that lack the ability to form protective interfacial products with CMASs. For YYbDS, however, the presence of a reactive phase allows for some level of porosity to be advantageous by confining CMAS infiltration and limiting lateral spreading. Moreover, the relationship between surface roughness, porosity, and chemistry should be considered simultaneously. For instance, surface roughness alone may influence the wetting rate, but without reactive chemistry and appropriate porosity, the coating’s overall resistance to CMAS infiltration could still be compromised. By coupling increased porosity with a reactive chemistry like YYbDS, it is possible to achieve a controlled infiltration that reduces spreading. These findings stress the importance of a delicate approach to coating design where microstructural features such as porosity are considered in conjunction with the chemistry and surface morphology of the material.

The quantitative trends are summarized in

Table 3. Surface roughness and porosity ranges yielding improved CMAS wetting performance.

Chemistry	Optimal Sa (µm)	Optimal Porosity (%)	Interfacial Product	Infiltration Depth (µm)
7YSZ	<3	N/A	None	6–8
YYbDS	<3	~22.7	Ca2RE8(SiO4)6O2	≤2

to suggest practical design ranges. In 7YSZ, optimal performance was found on the polished surface; however, the absence of an interfacial product resulted in approximately 6–8 µm of grain boundary infiltration. Porosity was not systematically varied for 7YSZ, but it is expected that the performance would likely degrade due to the lack of an interfacial product. For YYbDS, a porosity of ~23% combined with a polished surface of Sa < 3 µm produced a ≤2 µm apatite layer that limited CMAS penetration. These optimal ranges can lead to coatings that effectively resist CMAS infiltration while maintaining structural integrity under high-temperature conditions.

4. Conclusions

This study presents a systematic investigation of the influence of surface roughness, porosity, and chemistry on CMAS wettability in thermal and environmental barrier coatings, specifically focusing on 7YSZ and YYbDS ((Y_1/2Yb_1/2)₂Si₂O₇). Our findings show the complexity of optimizing coatings for high-temperature applications and the intricate relationship between surface roughness, microstructure, and chemistry in determining CMAS resistance.

Surface roughness was found to have a measurable but limited effect on CMAS wettability. Rougher surfaces, irrespective of chemistry, slightly inhibited CMAS spreading, while smoother surfaces facilitated enhanced spreading. This outcome indicates that surface roughness can influence initial wetting dynamics but does not significantly impact the overall spreading behavior unless coupled with reactive chemistry.

Chemistry emerged as a critical factor in determining the onset of CMAS wetting and the subsequent formation of interfacial reaction products. YYbDS demonstrated an ability to form an apatite reaction layer, which acted as a diffusion barrier and mitigated CMAS infiltration. The differences observed between YYbDS and YSZ indicate that reactive coatings that form beneficial interfacial phases provide an additional mechanism for CMAS resistance that cannot be achieved through surface roughness modifications alone.

Porosity was identified as having a clear impact on CMAS wettability and infiltration. Increased porosity led to greater CMAS infiltration, which might intuitively seem detrimental. However, in the case of YYbDS, the reactive chemistry converted increased porosity into an advantage by allowing CMAS to infiltrate and react within surface voids, effectively limiting further penetration and reducing lateral spread. This dual nature of porosity as both a potential vulnerability and a mechanism for mitigating CMAS spread illustrates the importance of careful microstructural tuning.

In summary, this work demonstrates that microstructural features such as surface roughness and porosity must be considered alongside chemistry to develop coatings capable of resisting CMAS infiltration effectively. Surface roughness, while impactful, has a limited role unless coupled with a chemistry that promotes beneficial reactions. Porosity, on the other hand, presents a significant opportunity for tuning CMAS resistance provided it is paired with reactive chemistry to limit extensive infiltration. By adopting an overall approach that integrates microstructural features with tailored chemistries, it is possible to design coatings that offer superior protection under high-temperature conditions that enhance the durability and reliability of gas turbine engines.