High temperature steam corrosion of LuPO4 and high-entropy rare earth phosphates in the presence of Al(OH)3 and Si(OH)4 impurities

corrosion 14,15 . Rare earth (RE) silicates, mullite-based, RE phosphates, and various high entropy ceramics were used for the EBC applications [16][17][18][19][20][21] .

Steam testing is a critical evaluation method used to assess the durability and performance of EBC materials in simulated service conditions 22,23 . The test involves exposing coated specimens to hightemperature steam environments under controlled conditions, typically at temperatures ranging from 1000 to 1400 °C and pressures up to several atmospheres [24][25][26] . The objective of steam testing is to simulate the harsh operating conditions experienced by EBC-coated components in aero engines, gas turbines, and other high-temperature applications. RE disilicates (RE 2 Si 2 O 7 ) are the most widely studied coating materials due to their excellent thermal and mechanical properties. However, disilicates are prone to corrosion in steam-rich environments, exhibiting phase transformations and spallation under thermal cycling conditions, thus limiting their performance 8,27,28 . Disilicates can interact with water vapor at high temperatures and form a volatile silicon hydroxide gas according to the following chemical reaction 29

This reaction led to the development of a porous surface comprised of mono-silicates, which exhibit distinct coefficients of thermal expansion (CTEs) from those of disilicates. However, this monosilicate layer may serve as a protective barrier to protect the underlying disilicate 30 . Ueno et al. investigated the vapor corrosion resistance of several RE disilicates at 1500 °C for 100 h, where Lu 2 Si 2 O 7 performed the best among the RE disilicates studied, but still displayed a phase change at the grain boundaries of the surface layer 31 . Maier et al. investigated the vapor corrosion resistance of Lu 2 Si 2 O 7 , Yb 2 Si 2 O 7 , and several Y bearing disilicates at 1500 °C for up to 310 h 29 . They also observed enhanced corrosion resistance of Lu 2 Si 2 O 7 compared to Yb 2 Si 2 O 7 and the Y-bearing disilicates, and found the formation of cracks due to the internal stress caused by the mismatch of expansion of the secondary phase formed within the disilicate materials.

Wan et al. investigated the vapor corrosion resistance of a proprietary Y-Yb disilicate mixture by measuring the growth of the corrosion layer/ microvoided layer at 1315 °C, 1427 °C, and 1538 °C for 500 h 32 . They also proposed the use of a microvoiding kinetic model that incorporates both mass transfer of the reaction product gas through the external gas phase boundary layer and via diffusion through the pores in the microvoided layer, which showed good agreement with the experimental measurements. In a study by Zhang et al., the water vapor corrosion behavior of a high-entropy RE disilicate, (Yb 0.2 Y 0.2 Lu 0.2 Er 0.2 Sc 0.2 ) 2 Si 2 O 7, was examined at 1400 °C over durations of 100, 200, and 300 h. After 300 h of exposure, the researchers observed the formation of pores exceeding 7 µm in size. Furthermore, they observed cracks initiating at the edges of these pores, attributed to stress concentrations resulting from the increasing pore dimensions during corrosion. Notably, even after 300 h, the disilicate phase remained the predominant component, suggesting a high resistance to water vapor corrosion for this high-entropy material 33 . The vapor corrosion resistance of RE phosphates is not well investigated as there are few publications 24,25,34 on single-component phosphates, and no experimental data are available on high-entropy RE phosphates. Ridely et al. investigated the vapor corrosion resistance of YbPO 4 at 1400 °C for up to 250 h. However, the mass loss due to water vapor corrosion was not reported, and its performance relative to other EBC materials was not investigated 25 . Wang et al. studied the corrosion of YPO 4 in a 50% H 2 O-50% O 2 environment at 1350 °C for 300 h, and reported that the corrosion rate of YPO 4 was ∼50 times lower than those of pure SiC and Si 3 N 4 , and about half that of BSAS (Ba 1-x Sr x O-Al 2 O 3 -2SiO 2 , 0 ≤ x ≤ 1) 24 . However, post-corrosion reaction characterization revealed no reaction products, and no hypothesis was proposed to explain the observed weight loss during testing. Therefore, further detailed studies are necessary to identify the corrosion products and to understand the steam corrosion mechanism of single and high-entropy RE phosphates. This study explores microstructural evolution, phase formation, surface porosity, reaction layer thickness, and mass loss in single and high-entropy RE phosphates exposed to water vapor at high temperatures in the presence of Al and Si impurities. Previous studies reported that water vapor corrosion tests of RE phosphates and silicates were conducted in alumina tube furnaces, in the presence of Al(OH) 3 and Si(OH) 4 impurities. The presence of Al(OH) 3 impurities is common in water vapor corrosion studies in the literature 25,29,34,35 . The presence of Al(OH) 3 and Si(OH) 4 impurities is the limitation of this study. The corrosion test of single and high-entropy REPO 4 provides insights into material durability, performance, and suitability for use in challenging hightemperature, water vapor-rich environments.

To fill the knowledge of the steam corrosion of high-entropy RE phosphates as potential EBC candidates for SiC-based CMCs, this work compares the vapor corrosion resistance of the five highentropy RE phosphates listed in Table 1 to that of LuPO 4 . Lu has one of the smallest ionic radii and likely the shortest and strongest RE-O bonds of the RE phosphates, and it is likely that it is the most corrosion resistant among the xenotime RE phosphates. Hence, LuPO 4 was chosen for comparison with the high-entropy phosphates. The high-entropy compositions are designed so that at least 5 elements will be at the principal cation sublattice sites with as wide a range of cation size disorder as possible, while still maintaining the singlephase xenotime structure. Achieving single-phase, multi-component RE phosphates with low thermal conductivity, CTEs compatible with SiC-based CMCs, improved mechanical properties, and enhanced resistance to CMAS corrosion presents considerable challenges. Our previous study demonstrated that these high-entropy materials intrinsically exhibit these critical properties, underscoring the need for a comprehensive investigation into their response to water vapor corrosion 18 . The high-entropy phosphates and single-component LuPO 4 were synthesized by a chemical coprecipitation process and consolidated into dense pellets by spark plasma sintering (SPS) for high-temperature steam testing. Table 1 summarizes the high entropy compositions, calculated structural disorder and physical properties of the densified pellets. Detailed information on materials synthesis and characterization can be referred in the section of method 20,36 . -6) at the sample surfaces due to the Al contamination from the alumina tube used for the simultaneous thermal analysis (STA) furnace. Further, due to the presence of Si contamination in the STA furnace, the monosilicate (RE 2 SiO 5 ) phase was also observed in XRD, as shown in Figs. 123456. Previous studies also showed the Al and Si contamination in the furnace for water vapor testing 25,29,35 . For example, Ridley et al. studied the water vapor corrosion test of YbPO 4 in an alumina tube furnace, and they also observed the presence of Al and Si on the water vapor corroded sample surface 25 . This contamination likely arises from gas-phase transport, where volatile Al(OH) 3 and Si(OH) 4 https://doi.org/10.1038/s41529-025-00619-1

form through the reaction of Al and Si with water vapor, respectively, in the furnace, and they further react with RE phosphates 25 . The phase transformation suggests that water vapor corrosion leads to the partial decomposition of the top surface of REPO 4 coupons, potentially facilitated by impurities or trace Al and Si from the furnace environment. The water vapor corrosion products might be different if there are no impurities (such as Al and Si) present inside the furnace. Furthermore, the vapor corrosion reaction products of RE phosphates also depend upon the experimental conditions, such as temperature, vapor composition, vapor flow rate, or sample contact angle 24,25,34 .

Microstructure analysis of single and high entropy REPO 4 after water vapor corrosion test Scanning electron microscopy (SEM) was conducted to analyze the surface morphology changes in RE phosphates before and after exposure to vapor corrosion. Further backscattering electron SEM along the cross-sectional view was also conducted to identify the reaction layer thickness of the corroded coupon. Figure 7A displays low magnification backscattered electron SEM images of the LuPO 4 coupon surface after 60 h of exposure to water vapor at 1400 °C. A high magnification SEM image (see Fig. 7B) clearly shows the change in microstructure of the material from an initially even/smooth surface with low porosity (2%) to a highly porous and tubular microstructure in this region, along with a reduction in grain size compared to the pristine surface. The inset of Fig. 7B is the SEM image of the pristine surface of LuPO 4 , which indicates a low porosity (2%) on the surface. The porosity of the corroded surface was quantified by image analysis of Fig. 7B to be approximately 19% of the area fraction. Figure 7C displays a highresolution SEM image of the new microstructure of the corroded sample where the 1-3 µm sized grains and pores are clearly visible. Figure 7D shows the cross-section backscatter electron SEM image of the LuPO 4 coupon post-vapor testing, where a 8.7 ± 1.1 µm thick microvoided layer is observed. A cross-sectional view (in Fig. 7D) illustrates the distinct regions within the sample, with the red dashed lines clearly delineating the epoxy layer, reaction layer, and bulk pellet. The upper layer is highly porous, consistent with images shown in Fig. 7B, C; while the lower region appears more solid and less reacted. This image (in Fig. 7D) clearly demonstrates that the corrosion is confined to a top surface layer of less than 10 µm, while the underlying material remains unreacted, indicating favorable resistance to degradation. The reaction layer and the unreacted bulk pellets show compositional differences. As a result, the contrast difference between the reaction layer and the underlying materials is observed as shown in Fig. 7D. Pore formation on the surface of the LuPO 4 sample may result from the migration of gas species formed upon the water vapor reaction such as P 2 O 5 , PO 2 (OH), and PO(OH) 3 through the top layer (reaction layer) 25 . Figure 7E presents a low-magnification SEM image along with energy-dispersive spectroscopy (EDS) mappings of the top surface of LuPO 4 following exposure to vapor corrosion. The SEM image and corresponding EDS maps reveal that corrosion is uneven across the surface, indicating localized areas of degradation rather than uniform corrosion. The EDS maps reveal the distribution of Al, Si, Lu, and P on the corroded surface, with a notable concentration of P at the center, corresponding to a region exhibiting less corrosion. Figure 7F . However, it is unclear if the difference is solely due to material performance or differences in experimental factors such as vapor composition, vapor flow rate, or sample contact angle. Their kinetic study showed that the thickness of the microvoided layer increased with the corrosion time (diffusion-controlled growth), but no significant change was seen in the microstructure 25 .

The backscattered electron SEM images from the surface of the highentropy RE phosphates (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 coupon after vapor corrosion at 1400 °C for 60 h are displayed in Fig. 8a, b. Figure 8a shows an area of the coupon surface that is highly porous with much smaller grains (1-3 µm) compared to those at the surface of the pristine coupon. The inset of Fig. 8b is the SEM image of the pristine surface of (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 before water vapor corrosion. Figure 8a shows a less porous surface compared to Fig. 8b, likely due to coarsening of the reaction product grains 37 . Figure 8b also shows a clear difference in microstructure compared to the pristine coupon, with smaller grain sizes (3-4 µm) compared to the initial microstructure. The 2D surface porosity of (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 coupon was calculated based on the surface image of Fig. 8b, and porosity is around 11%. Similarly, we have calculated it for all other REPO 4 after vapor corrosion, as shown in Table 2. Figure 8c displays a cross-sectional backscattered electron SEM image of the (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 coupon following vapor testing. A distinct microvoided layer, approximately 8.6 ± 1.1 µm in thickness, is clearly visible, indicating morphological changes induced by the vapor exposure. A distinct contrast difference is evident between the microvoided layer and the underlying unreacted region. The epoxy penetrates into the pores of the microvoided layer, as illustrated in Fig. 8c. Microcracks are observed beneath the microvoided layer, which may form due to thermal stress between the corroded layer and the underlying region (shown in Fig. 8c) or due to cutting and polishing of the coupon. The presence of cracks in coatings allows corrosive species to penetrate, compromising the protective barrier. This can lead to accelerated degradation, spallation, and reduced coating lifespan. Figure 8d shows the SEM image and EDS maps of the top surface of the corroded sample, indicating the presence of RE elements (Lu, Yb, Er, Y, and Gd) along with P, Al, and Si. The cross-sectional SEM image and EDS maps shown in Fig. 8e reveal that the microvoided layer is enriched with RE elements, Al, and Si, with a relatively low P concentration. The top of the microvoided layer is enriched with RE, P and Al (in Fig. 8e), which could be Al 5 RE 3 O 12 and AlPO 4 , as observed in XRD analysis (shown in Fig. 2). From the XRD and EDS analysis, we confirm the formation of RE 2 O 3 , Al 5 RE 3 O 12 , AlPO 4 and RE 2 SiO 5 phases within this microvoided layer in all the high entropy compositions.

Figure 9a, b show the SEM images of the top surface of (Sc 0.1 Lu 0.3 Yb 0.3 Er 0.2 Y 0.1 )PO 4 after the water vapor reaction at 1400 °C for 60 h, where both images show clear changes in the surface microstructure with much smaller grains compared to the pristine surface, as shown in the inset of Fig. 9b. However, there does seem to be a decrease in the level of porosity compared to the LuPO 10b). Figure 10b shows that there is also a clear formation of new, smaller grains compared to the grain of pristine materials. Figure 10c, d shows cross-sectional SEM images of (Sc 0.1 Lu 0.3 Yb 0.2 Y 0.3 Gd 0.1 )PO 4 after the vapor corrosion test, captured at low and high magnifications, respectively. Among all the RE phosphates studied, this composition exhibits the thinnest reaction layer, with a measured thickness of 8.4 ± 1.7 µm. As highlighted in Fig. 10c, small pores and microcracks are observed immediately beneath the reaction layer, suggesting localized degradation or stress-induced damage in the underlying region. Figure 11a, b represents the surface SEM images of the (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Gd 0.2 )PO 4 coupon following vapor corrosion exposure at 1400 °C for 60 h. The images show that there are some degrees of localized microstructural changes, as Fig. 11b shows new, smaller grains at the coupon surface. However, Fig. 11a shows that overall, the original microstructure remains largely unchanged with no significant increase in porosity, which might be due to less gas formation and travel through the corroded surface.

This indicates the effect of water vapor corrosion is less/ minor on (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Gd 0.2 )PO 4 . Further, the 2D porosity is relatively low (3.8%) compared to the LuPO 4 and (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4, as shown in Table 2. The high and low resolution cross-section SEM images of (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Gd 0.2 )PO 4 post corrosion tests are shown in Fig. 11c, d, respectively. The thickness of the reaction layer is 15.1 ± 2.5 µm.

The microcracks observed near the reaction layer might be due to the thermal stress, as shown in Fig. 11c. Figure 12a displays a low magnification microstructure of the (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Sm 0.2 )PO 4 sample after vapor corrosion for 60 h at 1400 °C. The surface appears textured with fine, wavy features, suggesting a uniform degradation pattern across a large area due to the water vapor corrosion. Figure 12b shows a higher magnification SEM image of the sample surface that has a porous, uneven texture with distributed black voids or pits. Figure 12a, b indicate a clear change in the surface microstructure of the sample with the formation of smaller grains and increased porosity compared to the pristine surface (see the inset of Fig. 12b). The 2D porosity for this sample is the lowest among all the samples, as shown in Table 2. The low and high resolution cross-section SEM images of (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Sm 0.2 )PO 4 after vapor corrosion are shown in Fig. 12c, d, respectively. The thickness of the reaction layer for this composition is 10.8 ± 1.8 µm. Microcracks observed (in Fig. 12c) beneath the reaction layer of (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Sm 0.2 )PO 4 may result from thermal stresses induced by the mismatch between the reaction layer and the underlying unreacted material, or from mechanical stresses introduced during sample cutting and polishing. The above SEM images suggest that a surface layer of single and highentropy rare-earth phosphates undergoes significant structural changes due to water vapor corrosion at high-temperature exposure, with a relatively intact underlying reaction layer. The porous morphology on the surface likely results from volatile species migration or reaction products diffusing out of the material during exposure 25 . Overall, the (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Gd 0.2 ) PO 4 and (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Sm 0.2 )PO 4 samples show the least change in surface microstructure in Figs. 11 and 12, while LuPO 4 displayed a clear increase in porosity and the formation of a new microstructure in Fig. 7. Furthermore, (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Gd 0.2 )PO 4 exhibits a comparatively lower formation of the monosilicate phase following the vapor corrosion test compared to other samples as confirmed from the XRD analysis as shown in Fig. 5. Among all these, (Sc 0.1 Lu 0.3 Yb 0.2 Y 0.3 Gd 0.1 )PO 4 shows the smallest reaction layer thickness 8.4 ± 1 µm and (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Gd 0.2 )PO 4 shows thickest reaction layer 15.1 ± 2.5 µm. All the REPO 4 samples show that the reaction layer is confined to a certain limit.

In the following section, we delve into the reaction mechanism governing the formation and transformation of phases in single and highentropy RE phosphates after steam corrosion at 1400 °C for 60 h. The XRD analysis of high entropy REPO 4 samples confirms the formation of various secondary phases, including RE 2 O 3 , Al 5 RE 3 O 12 , AlPO 4, and RE 2 SiO 5 . The formation of these phases suggests that a complex sequence of reactions is involved. Initially, RE phosphates react with water vapor in the surrounding environment, leading to the formation of RE oxides (RE 2 O 3 ) and hydroxide species 25 . Hildenbrand et al. demonstrated that PO₂(OH) and PO(OH) are the predominant hydroxide species formed in Ca₂P₂O₇ when subjected to a water vapor pressure of 10⁻⁶ atm 38 . In another study, Ridley et al. showed that both hydroxide phases are present for YbPO 4 when exposed to water vapor. These studies indicate that both hydroxide species may also be present in the case of single and high entropy REPO 4 in the current study, according to the following reactions 25 2

The impurities Al and Si come from the furnace react with water vapor to form gaseous Al(OH) 3 and Si(OH) 4 according to Eqs. ( 4) and ( 5), respectively 29,34,39 . The RE 2 O 3 can react with the Al(OH) 3 and Si(OH) 4 , resulting in the formation of aluminum RE garnet (Al 5 RE 3 O 12 ) and RE monosilicate (RE 2 SiO 5 ), respectively 34 . Further, the Al(OH) 3 may decompose at high temperature into Al 2 O 3 and react with REPO 4 to form Al 5 RE 3 O 12 according to Eq. (8). Recently, Han et al. studied the vapor corrosion of single componente REPO 4 at 1400 °C and their results shows that REPO 4 reacted with Al(OH) 3 and formed Al 5 RE 3 O 12 34

. The direct reaction between Al(OH) 3 and P 2 O 5 is less reported. However, in a hightemperature water vapor environment, the gaseous P 2 O 5 vigorously reacts with water vapor to form a strong acid, H 3 PO 4 , as an intermediate product 40 . The H 3 PO 4 , at high temperatures, becomes a vapor and interacts with Al(OH) 3 to form AlPO 4 according to Eq. ( 9) 41 . The chemical reactions 34,35,40,41 representing phase transformations during corrosion are as follows:

ðgÞ À ! 2 AlðOHÞ 3 ðgÞ ð 4Þ SiO 2 ðsÞ þ 2 H 2 O ðgÞ ! SiðOHÞ 4 ðgÞ ð 5Þ 3 RE 2 O 3 ðsÞ þ 10 AlðOHÞ 3 ðgÞ ! 2 Al 5 RE 3 O 12 þ 15 H 2 O ðgÞ ð6Þ RE 2 O 3 ðsÞ þ SiðOHÞ 4 ðgÞ ! RE 2 SiO 5 ðsÞ þ 2 H 2 O ðgÞ ð 7Þ 6 REPO 4 ðsÞ þ 5 Al 2 O 3 ðsÞ ! 2 Al 5 RE 3 O 12 ðsÞ þ 3 P 2 O 5 ðgÞ

The chemical reactions outlined above demonstrate the formation of reaction products resulting from water vapor corrosion in RE phosphates at high temperatures. The above-mentioned chemical reactions are thermodynamically favorable at high temperature 34,35 . Chemical reactions are likely to influence weight loss or gain during the corrosion process of REPO 4 34 . Notably, the reactivity of RE phosphates toward silicon (Si) depends on their composition. This compositional dependence influences the formation of silicon-containing phases, such as RE monosilicates (RE 2 SiO 5 ). The XRD analysis of LuPO 4 , as shown in Fig. 1, reveals a significant amount of the mono-silicate phase (Lu 2 SiO 5 ) with high intensity following water vapor corrosion. In contrast, the XRD pattern for high entropy RE phosphate (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 , shown in Fig. 2, indicates a much lower intensity of the mono-silicate phase (near 2θ ~15°). This observation suggests that (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 exhibits reduced reactivity toward Si in a water vapor environment at 1400 °C as compared to LuPO 4 .

A comparative XRD analysis of various high-entropy rare-earth phosphates (Figs. 23456) further reveals that (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 has the lowest reactivity toward Si. However, the inclusion of scandium (Sc) in high entropy phosphates might increase the reactivity toward Si, as evidenced by the formation of RE 2 SiO 5 in the XRD patterns (Figs. 3456). This Er, and Y) provides additional insights 7 . Their study showed that ScPO 4 exhibits higher reactivity toward Si compared to other REPO 4 after 5 h of CMAS exposure at 1300 °C, as only ScPO 4 reacts with Si to form the disilicate phase Sc 2 Si 2 O 7 . These findings suggest that Sc might be more reactive toward Si in high-temperature environments. However, in their CMAS composition, the concentrations of Si were too high as compared to those in water vapor corrosion in this current work.

Figure 13 shows the mass change of LuPO 4 and the five high-entropy rare-earth phosphates with vapor corrosion over a period of 60 h at 1400 °C. As mentioned prior, the alumina in the furnace used for the vapor corrosion experiments reacts with the water vapor, forming the Al(OH) 3 gas, which reacts with the newly formed 6) and ( 7), which could again lead the weight gain, but Fig. 13 does not support it. This indicates that a significant amount of the RE 2 O 3 phase does not react with Al(OH) 3 and Si(OH) 4 . Furthermore, when Al(OH) 3 reacts with the H 3 PO 4 , https://doi.org/10.1038/s41529-025-00619-1

it leads to the formation of AlPO 4 , which increases the weight of the system. However, in practice, only a small fraction of REPO 4 reacts with Al(OH) 3 and Si(OH) 4 from the furnace to form Al 5 RE 3 O 12 and RE 2 SiO 5 , respectively. As a result, any associated weight gain from this byproduct formation is relatively less effective and remains within the experimental error margin. However, we observed weight loss in all the samples, which is due to the formation of a gaseous product that contains P (such as P 2 O 5 , PO₂(OH), and PO(OH)). Therefore, the mass loss data obtained in this study remains valid and reliable. However, the mass loss data might be different if the experiment is performed without any Al and Si impurities.

LuPO 4 shows a significant and continuous mass loss of approximately 2.5% compared to the much lower mass loss of the high entropy samples (as shown in Fig. 13), and even with the Al and Si uptake affecting the accuracy of the mass loss measurement. It is clear that the high-entropy phosphates show overall improved vapor corrosion resistance compared to singlecomponent LuPO 4 . The general trend in the mass loss of the LuPO 4 appears to be parabolic. However, Wan et al. reported that at short durations and or low vapor flow rates, convective mass transfer of gas species into the boundary layer controls the rate of growth of the reaction layer, and leads to a linear mass loss 32 . Hence, it is likely that the alumina uptake is responsible for the observed nonlinear mass loss. In a recent study, Han et al. investigated the steam corrosion resistance of single component REPO 4 (RE = Sc, Lu, Yb, Er, and Y) in an alumina tube furnace at 1500 °C for 80 h under an atmosphere of 50% H₂O with an O 2 balance and a flow rate of 0.3 cm/s. Their findings indicated that all single-component REPO 4 samples experienced weight gain attributed to alumina incorporation from the surrounding environment 34 . However, no mass gain data or surface morphology images of the corroded samples were provided, making direct comparison impossible.

Furthermore, the thickness of the reaction layer formed on the top surface of REPO 4 after exposure to water vapor corrosion may serve as a critical metric for evaluating the material's resistance to high-temperature corrosive environments. However, no data is available for direct comparison of their behavior for materials tested at exactly identical experimental largest size disordered (6.92) shows the lowest porosity (~3.1%), and the smaller size disordered (2.68) material (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 shows the largest porosity (~11%) among multicomponent RE phosphates after vapor corrosion. The LuPO 4 shows the highest surface porosity among all the REPO 4 , around 19%. These results suggest that higher cationic size disorder may improve resistance to water vapor corrosion, as evidenced by minimized surface porosity after water vapor corrosion. Therefore, high-entropy rareearth phosphates with greater size disorder emerge as promising candidates for protective coatings in extremely high-temperature environments. These observations suggest that surface morphology alone may not fully reflect mass stability and emphasize the need for a multi-faceted approach in assessing corrosion resistance.

The improved water vapor corrosion of the high entropy phosphates is likely due to their increased thermochemical properties and grain boundary stability, which lends further credence to the entropy stabilization effect of high-entropy ceramics 42 . The enhanced water vapor corrosion resistance of high-entropy RE phosphates can be attributed to the synergistic interplay among their constituent RE elements 43 . In particular, the combination of RE elements with varying ionic radii, such as the larger gadolinium (Gd) and samarium (Sm), alongside the smaller scandium (Sc), induces significant lattice mismatch within the crystal structure and gives more size disorder, as shown in Table 2. This atomic-scale disorder and lattice distortion may hinder the diffusion of water vapor and the formation of corrosion byproducts through the specimen. As a result, the surface structure of the coupon of high entropy REPO 4 with higher size disorder becomes less porous after water vapor corrosion, further improving its protective performance. However, the vapor corrosion resistance of the single-component RE phosphates will still need to be investigated to confirm the initial assumption of LuPO 4 having the highest corrosion resistance.

In summary, a systematic investigation of the microstructural changes, 2D surface porosity, reaction layer thickness and mass loss of LuPO 4 and high entropy phosphates undergoing water vapor corrosion at 1400 °C for 60 h was performed in the presence of Al(OH) 3 and Si(OH) 4 impurities to evaluate their performance as potential EBC materials. The key results of this study are listed below:

1. Single-component RE phosphate LuPO 4 shows the largest microstructural change compared with the high entropy phosphates, with an increase in porosity from 2 to 19% and the formation of Lu 2 O 3 , Al 5 Lu 3 O 12 and Lu 2 SiO 5 at the surface of the sample. The LuPO 4 sample also shows the highest mass loss (~2.5%) compared to the high entropy samples, all with less than 0.6% mass loss. 2. The surface layer of single and high-entropy RE phosphates undergoes significant microstructural changes due to water vapor corrosion at high-temperature exposure, with a relatively intact underlying reaction layer. In both single and high-entropy RE phosphates, a microvoided reaction layer forms, and this layer is confined to a top alteration surface layer of less than 20 µm. 3. Microstructure evolves and new phases form up to a certain limit after the steam corrosion. All single and high entropy phosphates demonstrate good stability in oxygen-water environments at 1400 °C despite the formation of RE 2 O 3 , Al 5 RE 3 O 12 , AlPO 4 , and RE 2 SiO 5 phases. 4. After water vapor corrosion, the surface porosity of the RE phosphates depends on the degree of cationic size disorder. The high-entropy composition (Sc 0.2 Lu 0.2 Yb 0.2 Y 0.2 Sm 0.2 )PO 4, which exhibited the highest size disorder (6.92), shows the lowest porosity (~3.1%). In contrast, the composition with a lower size disorder (2.68), e.g., (Lu 0.2 Yb 0.2 Er 0.2 Y 0.2 Gd 0.2 )PO 4 , displays the highest porosity (~11%) among the multicomponent RE phosphates tested. These results suggest that increasing cationic size disorder may enhance resistance to water vapor corrosion by reducing surface porosity, making such highentropy materials promising candidates for protective applications for extreme environments.

Further vapor testing without Al(OH) 3 and Si(OH) 4 contamination and at longer durations is still needed for the high entropy phosphates to have a more accurate comparison of the mass loss measurement.