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

Abstract

In this work, five high-entropy rare earth phosphate and one single component rare earth phosphate (LuPO₄) coupons were synthesized as novel environmental barrier coating (EBC) material candidates for silicon carbide ceramic matrix composites (SiC-CMCs). Their high temperature steam corrosion resistance was investigated at 1400 °C for 60 h in the presence of Al(OH)₃ and Si(OH)₄ contamination, with the focus on microstructural evolution, new phase formation, reaction layer thickness, and mass loss. LuPO₄ shows a larger mass loss than all the high entropy phosphates, and the formation of a ~ 8.7 µm thick reaction layer enriched with micro-sized voids and porosity up to 19%. In contrast, high entropy phosphates exhibit significantly enhanced steam corrosion resistance at high temperatures with a lower porosity in the surface alteration layer than LuPO₄. Among them, (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ and (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ are the least corroded samples based on the microstructure alteration observed by scanning electron microscope (SEM). Following the water vapor test, all the rare earth phosphates show the formation of a microvoided reaction layer of new phases, including RE₂O₃, Al₅RE₃O₁₂, AlPO₄, and RE₂SiO₅. The enhanced corrosion resistance of the high entropy rare earth phosphates is ascribed to the enhanced stability of the crystal structure because of their compositional complexity and size disordering. All these results indicate that high entropy rare earth phosphates are promising candidates for EBC for SiC-CMCs.

Introduction

To enhance the efficiency and operating temperature, SiC-based ceramic matrix composites (CMCs) are increasingly replacing traditional Ni-based components in the hot section of gas turbine engines¹. CMCs exhibit exceptionally high-temperature strength, creep resistance, low density, chemical stability in air environments, and durability under extreme conditions^2,3,4. Compared to Ni-based superalloys, CMCs can reduce aero-engine weight by 50–70%, increase the operating temperature by approximately 200 °C, enhance thermal efficiency due to their low thermal conductivity, and thus minimize heat losses^5,6. These attributes position CMCs as a promising candidate for advanced engine components. However, these non-oxide ceramics undergo oxidation and degradation in the environment of the combustion chamber. Corrosive media such as water vapor and calcium–magnesium–alumina–silicate (CMAS) render them unsuitable for long-term applications. During aircraft takeoff and landing, sand and debris can be ingested into the engine. At elevated operating temperatures (∼1300 °C), these particles melt to form molten CMAS, which subsequently deposits on the hot-section components composed of SiC-CMCs. At high temperatures, these molten CMAS infiltrate into SiC-CMCs and degrade the SiC-CMCs, ultimately leading to mechanical failure^7,8. Furthermore, in oxygen-rich, high-velocity water vapor environments, oxidation of CMCs leads to the formation of a SiO₂ layer, deteriorating material properties and performance. High-temperature gases further exacerbate this degradation⁵. Hence, environmental barrier coatings (EBCs) are required to safeguard CMCs against degradation from water vapor and CMAS corrosion in harsh conditions^{9,10,11,12,13}. The modern EBC topcoat thicknesses typically range from 45 to 150 µm, depending on the application method and substrate characteristics. It’s essential to tailor the coating process to the specific requirements of the SiC-CMC components to ensure optimal protection against high-temperature oxidation, CMAS, and water vapor 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 high-temperature 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 (RE₂Si₂O₇) 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 reaction²⁹

$${\rm{R}}{{\rm{E}}}_{2}{\rm{S}}{{\rm{i}}}_{2}{{\rm{O}}}_{7}\,({\rm{s}})\,+2{{\rm{H}}}_{2}{\rm{O}}\,({\rm{g}})\to {\rm{R}}{{\rm{E}}}_{2}{\rm{Si}}{{\rm{O}}}_{5}\,({\rm{s}})+{{\rm{Si}}\,({\rm{OH}})}_{4}\,({\rm{g}})$$

(1)

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 disilicate³⁰. Ueno et al. investigated the vapor corrosion resistance of several RE disilicates at 1500 °C for 100 h, where Lu₂Si₂O₇ performed the best among the RE disilicates studied, but still displayed a phase change at the grain boundaries of the surface layer³¹. Maier et al. investigated the vapor corrosion resistance of Lu₂Si₂O₇, Yb₂Si₂O₇, and several Y bearing disilicates at 1500 °C for up to 310 h²⁹. They also observed enhanced corrosion resistance of Lu₂Si₂O₇ compared to Yb₂Si₂O₇ 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 h³². 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_0.2Y_0.2Lu_0.2Er_0.2Sc_0.2)₂Si₂O_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 material³³. 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 YbPO₄ 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 investigated²⁵. Wang et al. studied the corrosion of YPO₄ in a 50% H₂O-50% O₂ environment at 1350 °C for 300 h, and reported that the corrosion rate of YPO₄ was ∼50 times lower than those of pure SiC and Si₃N₄, and about half that of BSAS (Ba_1-xSr_xO–Al₂O₃–2SiO₂, 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 REPO₄ provides insights into material durability, performance, and suitability for use in challenging high-temperature, 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 high-entropy RE phosphates listed in Table 1 to that of LuPO₄. 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, LuPO₄ 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 single-phase 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 corrosion¹⁸. The high-entropy phosphates and single-component LuPO₄ 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.

Table 1 A list of xenotime single and high-entropy RE phosphates with their respective average cation radius, cation size disorder, measured pellet density, theoretical density, and relative density

Results

XRD analysis of single and high entropy REPO₄ after water vapor corrosion test

XRD patterns from the top surfaces of LuPO₄ and high entropy phosphates before and after water vapor exposure at 1400 °C for 60 h are illustrated in Figs. 1–6. After the water vapor corrosion, the XRD peaks of the xenotime phase (REPO₄) shift to a lower angle and become sharper than those of the pristine REPO₄ sample due to more crystallization. Aside from the initial phosphate phase (xenotime) of the bulk material, primary corrosion products are RE₂O₃, Al₅RE₃O_12, and RE₂SiO₅ observed from the water vapor corroded surfaces of LuPO₄ and all high-entropy RE phosphates. All the major XRD peaks of the corroded samples were indexed according to the reference XRD patterns of the different phases. XRD analysis shows that the peak intensity of the RE₂O₃ phase is relatively lower than that of the other two phases (Al₅RE₃O₁₂ and RE₂SiO₅) according to the XRD peak intensity match. Some of the XRD peaks of RE₂O₃ overlap with the other phases in Figs. 1–6. At elevated temperatures, the water vapor interacts with Al and Si in the furnace, forming Al(OH)₃ and Si(OH)₄, which can react with the RE oxide, leading to the formation of RE alumina and monosilicate phases. XRD analysis of all high entropy phosphate samples shows an apparent formation of Al₅RE₃O_12, and AlPO₄ (shown in Figs. 2–6) 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 (RE₂SiO₅) phase was also observed in XRD, as shown in Figs. 1–6. 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 YbPO₄ in an alumina tube furnace, and they also observed the presence of Al and Si on the water vapor corroded sample surface²⁵. This contamination likely arises from gas-phase transport, where volatile Al(OH)₃ and Si(OH)₄ form through the reaction of Al and Si with water vapor, respectively, in the furnace, and they further react with RE phosphates²⁵. The phase transformation suggests that water vapor corrosion leads to the partial decomposition of the top surface of REPO₄ 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.

Fig. 1: Phase evolution of the LuPO₄ upon high temperature steam corrosion.

XRD patterns of the LuPO₄ surface before and after steam exposure for 60 h at 1400 °C.

Fig. 2: Phase evolution of the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ upon high temperature steam corrosion.

XRD patterns of the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ surface before and after steam exposure for 60 h at 1400 °C.

Fig. 3: Phase evolution of the (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ upon high temperature steam corrosion.

XRD patterns of the (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ surface before and after steam exposure for 60 h at 1400 °C.

Fig. 4: Phase evolution of the (Sc_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ upon high temperature steam corrosion.

XRD patterns of the (Sc_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ surface before and after steam exposure for 60 h at 1400 °C.

Fig. 5: Phase evolution of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ upon high temperature steam corrosion.

XRD patterns of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ surface before and after steam exposure for 60 h at 1400 °C.

Fig. 6: Phase evolution of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ upon high temperature steam corrosion.

XRD patterns of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ surface before and after steam exposure for 60 h at 1400 °C.

Microstructure analysis of single and high entropy REPO₄ 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 LuPO₄ 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 LuPO₄, 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 high-resolution 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 LuPO₄ 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 LuPO₄ sample may result from the migration of gas species formed upon the water vapor reaction such as P₂O₅, PO₂(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 LuPO₄ 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 displays a high-resolution backscattered electron SEM image and EDS mappings of the cross-section of the corroded LuPO₄ pellet. The SEM image shown in Fig.7F clearly indicates the contrast difference between the microvoided reaction layer and unreacted LuPO₄. The EDS analysis of the corroded sample confirms the presence of Lu, Al, and Si on the top of the microvoided reaction layer. Further, the EDS and XRD analyzes confirm that the primary corrosion products from the Al and Si contaminations are RE₃Al₅O_12, and RE₂SiO_5, respectively. Ridley et al. investigated the water vapor corrosion resistance of YbPO₄ at 1400 °C for 60 h, where a 25 µm thick microvoided layer was observed, mainly composed of Yb₂O₃²⁵. 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 microstructure²⁵.

Fig. 7: Surface alteration and microstructure evolution of the LuPO₄ after steam corrosion.

SEM images taken at the top surface of the LuPO₄ sample post vapor corrosion at 1400 °C for 60 h: a a lower magnification and b a higher magnification image of corroded region (inset is an image of the pristine surface), c a high-resolution image of newly formed microstructure. d SEM image of the cross-section of the coupon post vapor corrosion. e SEM image and EDS maps of the top surface of the sample post-vapor corrosion. f A high-resolution SEM image and EDS maps of the cross-section of the coupon post-vapor corrosion.

The backscattered electron SEM images from the surface of the high-entropy RE phosphates (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ 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_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ before water vapor corrosion. Figure 8a shows a less porous surface compared to Fig. 8b, likely due to coarsening of the reaction product grains³⁷. 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_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ coupon was calculated based on the surface image of Fig. 8b, and porosity is around 11%. Similarly, we have calculated it for all other REPO₄ after vapor corrosion, as shown in Table 2. Figure 8c displays a cross-sectional backscattered electron SEM image of the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ 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 Al₅RE₃O₁₂ and AlPO₄, as observed in XRD analysis (shown in Fig. 2). From the XRD and EDS analysis, we confirm the formation of RE₂O₃, Al₅RE₃O₁₂, AlPO₄ and RE₂SiO₅ phases within this microvoided layer in all the high entropy compositions.

Fig. 8: Surface alteration and microstructure evolution of the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ after steam corrosion.

SEM images taken at the top surface of the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ sample post vapor corrosion at 1400 °C for 60 h: a an image taken from a less corroded region. b an image taken from a highly corroded region and the inset is an image of pristine surface c SEM image of the cross-section of the coupon post vapor corrosion. d SEM image and EDS maps of the top surface of the sample post-vapor corrosion. e SEM image and EDS maps of the cross section of the coupon post vapor corrosion.

Table 2 Porosity and average reaction layer thickness of single and high-entropy RE phosphates after water vapor corrosion at 1400 °C for 60 h

Figure 9a, b show the SEM images of the top surface of (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ 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 LuPO₄ and (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ samples, as shown in Table 2. Figure 9c, d show the backscattering SEM images of the cross section of (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ coupon after vapor corrosion, showing low and high magnification views, respectively. The contrast difference between the reacted and unreacted layers is clearly observed, similar to LuPO₄ and (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ samples. The average thickness of the reaction layer for the (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ composition is 10.3 ± 1.5 µm. Figure 10a, b shows the SEM image of (Sc_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ coupon surface after vapor corrosion, and it does show a more porous structure compared to the pristine coupon surface (see inset of Fig.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_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ 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_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ 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_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄. Further, the 2D porosity is relatively low (3.8%) compared to the LuPO₄ and (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO_4, as shown in Table 2. The high and low resolution cross-section SEM images of (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ 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_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ 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_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ 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_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ 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.

Fig. 9: Surface alteration and microstructure evolution of the (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ after steam corrosion.

SEM images taken at the top surface of the (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ sample post vapor corrosion at 1400 °C for 60 h: a a low magnification and b a higher magnification image of corroded region (the inset is a SEM image of pristine surface). SEM images of the cross-section of the coupon post vapor corrosion: c a low magnification and d a high magnification.

Fig. 10: Surface aleration and microstructure evolution of the (Sc_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ after steam corrosion.

SEM images taken at the top surface of the (Sc_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ sample post vapor corrosion at 1400 °C for 60 h: a a low magnification and b a higher magnification image of corroded region (the inset is a SEM image of pristine surface). SEM images of the cross-section of the coupon post vapor corrosion: c a low magnification and d a high magnification.

Fig. 11: The SEM images and EDS maps of (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ after steam corrosion.

SEM images taken at the top surface of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ sample post vapor corrosion at 1400 °C for 60 h: a low magnification and b a higher magnification image of the corroded region (the inset is a SEM image of pristine surface). SEM images of the cross-section of the coupon post vapor corrosion: c a low magnification and d a high magnification.

Fig. 12: Surface alteration and microstructure evolution of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ after steam corrosion.

SEM image taken at the top surface of the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ sample post vapor corrosion at 1400 °C for 60 h: a a low magnification and b a higher magnification image of the corroded region (the inset is a SEM image of pristine surface). SEM images of the cross-section of the coupon post vapor corrosion: c a low magnification and d a high magnification.

The above SEM images suggest that a surface layer of single and high-entropy 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 exposure²⁵. Overall, the (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ and (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ samples show the least change in surface microstructure in Figs. 11 and 12, while LuPO₄ displayed a clear increase in porosity and the formation of a new microstructure in Fig. 7. Furthermore, (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ 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_0.1Lu_0.3Yb_0.2Y_0.3Gd_0.1)PO₄ shows the smallest reaction layer thickness 8.4 ± 1 µm and (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ shows thickest reaction layer 15.1 ± 2.5 µm. All the REPO₄ 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 high-entropy RE phosphates after steam corrosion at 1400 °C for 60 h. The XRD analysis of high entropy REPO₄ samples confirms the formation of various secondary phases, including RE₂O₃, Al₅RE₃O₁₂, AlPO_4, and RE₂SiO₅. 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 (RE₂O₃) and hydroxide species²⁵. 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 atm³⁸. In another study, Ridley et al. showed that both hydroxide phases are present for YbPO₄ when exposed to water vapor. These studies indicate that both hydroxide species may also be present in the case of single and high entropy REPO₄ in the current study, according to the following reactions²⁵

$$2\,{\rm{REP}}{{\rm{O}}}_{4}+{\rm{H}}_{2}{\rm{O}}({\rm{g}})\to {\rm{R}}{{\rm{E}}}_{2}{{\rm{O}}}_{3}+2{\rm{P}}{{\rm{O}}}_{2}({\rm{OH}})\,({\rm{g}})$$

(2)

$$2\,{\rm{REP}}{{\rm{O}}}_{4}+3\,{{\rm{H}}}_{2}{\rm{O}}({\rm{g}})={\rm{R}}{{\rm{E}}}_{2}{{\rm{O}}}_{3}+2\,{{\rm{PO}}({\rm{OH}})}_{3}({\rm{g}})$$

(3)

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 RE₂O₃ can react with the Al(OH)₃ and Si(OH)₄, resulting in the formation of aluminum RE garnet (Al₅RE₃O₁₂) and RE monosilicate (RE₂SiO₅), respectively³⁴. Further, the Al(OH)₃ may decompose at high temperature into Al₂O₃ and react with REPO₄ to form Al₅RE₃O₁₂ according to Eq. (8). Recently, Han et al. studied the vapor corrosion of single componente REPO₄ at 1400 °C and their results shows that REPO₄ reacted with Al(OH)₃ and formed Al₅RE₃O₁₂³⁴. The direct reaction between Al(OH)₃ and P₂O₅ is less reported. However, in a high-temperature water vapor environment, the gaseous P₂O₅ vigorously reacts with water vapor to form a strong acid, H₃PO₄, as an intermediate product⁴⁰. The H₃PO₄, at high temperatures, becomes a vapor and interacts with Al(OH)₃ to form AlPO₄ according to Eq. (9)⁴¹. The chemical reactions^34,35,40,41 representing phase transformations during corrosion are as follows:

$${\rm{A}}{{\rm{l}}}_{2}{{\rm{O}}}_{3}({\rm{s}})+3\,{{\rm{H}}}_{2}{{\rm{O}}\,({\rm{g}})}_{-}\to \,2\,{{\rm{Al}}({\rm{OH}})}_{3}({\rm{g}})$$

(4)

$${\rm{Si}}{{\rm{O}}}_{2}({\rm{s}})+2\,{{\rm{H}}}_{2}{\rm{O}}\,({\rm{g}})\,\to {{\rm{Si}}({\rm{OH}})}_{4}({\rm{g}})$$

(5)

$$3\,{\rm{R}}{{\rm{E}}}_{2}{{\rm{O}}}_{3}({\rm{s}})+10\,{{\rm{Al}}({\rm{OH}})}_{3}({\rm{g}})\to 2\,{\rm{A}}{{\rm{l}}}_{5}{\rm{R}}{{\rm{E}}}_{3}{{\rm{O}}}_{12}+15\,{{\rm{H}}}_{2}{\rm{O}}\,({\rm{g}})$$

(6)

$${\rm{R}}{{\rm{E}}}_{2}{{\rm{O}}}_{3}\,({\rm{s}})+{{\rm{Si}}({\rm{OH}})}_{4}({\rm{g}})\to {\rm{R}}{{\rm{E}}}_{2}{\rm{Si}}{{\rm{O}}}_{5}\,({\rm{s}})+2\,{{\rm{H}}}_{2}{\rm{O}}\,({\rm{g}})$$

(7)

$$6\,{\rm{REP}}{{\rm{O}}}_{4}\,({\rm{s}})+5\,{\rm{A}}{{\rm{l}}}_{2}{{\rm{O}}}_{3}({\rm{s}})\to 2\,{\rm{A}}{{\rm{l}}}_{5}{\rm{R}}{{\rm{E}}}_{3}{{\rm{O}}}_{12}\,({\rm{s}})+3\,{{\rm{P}}}_{2}{{\rm{O}}}_{5}\,({\rm{g}})$$

(8)

$${\rm{Al}}{({\rm{OH}})}_{3}({\rm{g}})+{{\rm{H}}}_{3}{\rm{P}}{{\rm{O}}}_{4}\to {\rm{AlP}}{{\rm{O}}}_{4}\,({\rm{s}})+3\,{{\rm{H}}}_{2}{\rm{O}}\,({\rm{l}}).$$

(9)

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 REPO₄³⁴. 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 (RE₂SiO₅). The XRD analysis of LuPO₄, as shown in Fig. 1, reveals a significant amount of the mono-silicate phase (Lu₂SiO₅) with high intensity following water vapor corrosion. In contrast, the XRD pattern for high entropy RE phosphate (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄, shown in Fig. 2, indicates a much lower intensity of the mono-silicate phase (near 2θ ~ 15°). This observation suggests that (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ exhibits reduced reactivity toward Si in a water vapor environment at 1400 °C as compared to LuPO₄.

A comparative XRD analysis of various high-entropy rare-earth phosphates (Figs. 2–6) further reveals that (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ 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 RE₂SiO₅ in the XRD patterns (Figs. 3–6). This could be due to the smaller size and mass of the Sc compared to the other RE elements. Recent work by Majee et al. on CMAS corrosion of single-component REPO₄ (RE = Sc, Lu, Yb, Er, and Y) provides additional insights⁷. Their study showed that ScPO₄ exhibits higher reactivity toward Si compared to other REPO₄ after 5 h of CMAS exposure at 1300 °C, as only ScPO₄ reacts with Si to form the disilicate phase Sc₂Si₂O₇. 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 LuPO₄ 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 RE₂O₃ to form the RE alumina phase. Further, Si(OH)₄ contamination comes from the furnace, and it also reacts with the RE₂O₃ and forms the RE₂SiO₅ phase. Therefore, the weight change observed by the RE phosphates is a result of the competing weight loss due to phosphorus volatilization and the weight gain from the reaction with Al(OH)₃ and Si(OH)₄³⁴. The chemical reactions shown above are likely to influence weight loss or gain during the corrosion process of REPO₄. According to chemical reactions Eqs. (2) and (3), REPO₄ loses weight by forming gaseous hydroxide species, such as PO₂(OH) and PO(OH). The weight gain of the system can be observed if Al and Si are added to the system. According to chemical reactions Eqs. 6–9, the REPO₄ coupon could gain weight by forming Al and Si-containing phases.

Fig. 13: Weight change behavior of rare-earth phosphates during vapor exposure.

Weight change comparison of single and high entropy phosphates during water vapor corrosion for 60 h at 1400 °C.

For instance, In the case of Al₅RE₃O₁₂ formation, according to Eqs. 8, 6 moles of P are lost for every gain of ten moles of Al, which would lead to a net weight gain in the RE phosphate samples, which is not what is observed from the mass loss data as shown in Fig. 13. Similarly, In case of Al₅RE₃O₁₂ and RE₂SiO₅ formation, from the RE₂O₃ according to Eqs. (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 RE₂O₃ phase does not react with Al(OH)₃ and Si(OH)₄. Furthermore, when Al(OH)₃ reacts with the H₃PO₄, it leads to the formation of AlPO₄, which increases the weight of the system. However, in practice, only a small fraction of REPO₄ reacts with Al(OH)₃ and Si(OH)₄ from the furnace to form Al₅RE₃O₁₂ and RE₂SiO₅, 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 P₂O₅, 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.

LuPO₄ 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 single-component LuPO₄. The general trend in the mass loss of the LuPO₄ 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 loss³². 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 REPO₄ (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 O₂ balance and a flow rate of 0.3 cm/s. Their findings indicated that all single-component REPO₄ samples experienced weight gain attributed to alumina incorporation from the surrounding environment³⁴. 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 REPO₄ 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 conditions (water vapor corrosion at 1400 °C for 60 h). A previous article reported water vapor test of YbPO₄ at 1400 °C for 60, 125, and 250 h in the 80–200 m/s steam velocity regime, where a parabolic time dependence for the steam reaction was observed, suggestive of a diffusion-controlled process²⁵. The YbPO₄ showed a lower steam reactivity than Yb₂Si₂O₇ tested in the 80–200 m/s gas velocity range at 1400 °C²⁵.

Nevertheless, the significant mass loss observed in our LuPO₄ sample compared to high entropy REPO₄ may be due to lower grain boundary stability in a single component. It is also of note that (Sc_0.2Lu_0.2Yb_0.2Y_0.2Gd_0.2)PO₄ appears to be one of the least corroded samples based on the coupon surface images and surface porosity. (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ and (Sc_0.1Lu_0.3Yb_0.3Er_0.2Y_0.1)PO₄ appear to show the least mass loss among the high-entropy phosphates, but the mass increase up to 100.1% is likely due to instrument instability. After water vapor corrosion at 1400 °C for 60 h, the 2D surface porosity of all the RE phosphates was calculated using ImageJ software based on the SEM image of Figs. 7b, 8b, 9b, 10b, 11b, 12b, as shown in Table 2. The 2D surface porosity of RE phosphates roughly depends on size disorder. A high entropy material (Sc_0.2Lu_0.2Yb_0.2Y_0.2Sm_0.2)PO₄ with the largest size disordered (6.92) shows the lowest porosity (~3.1%), and the smaller size disordered (2.68) material (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ shows the largest porosity (~11%) among multicomponent RE phosphates after vapor corrosion. The LuPO₄ shows the highest surface porosity among all the REPO₄, 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 rare-earth 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 ceramics⁴². The enhanced water vapor corrosion resistance of high-entropy RE phosphates can be attributed to the synergistic interplay among their constituent RE elements⁴³. 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 REPO₄ 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 LuPO₄ having the highest corrosion resistance.

In summary, a systematic investigation of the microstructural changes, 2D surface porosity, reaction layer thickness and mass loss of LuPO₄ 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 LuPO₄ shows the largest microstructural change compared with the high entropy phosphates, with an increase in porosity from 2 to 19% and the formation of Lu₂O₃, Al₅Lu₃O₁₂ and Lu₂SiO₅ at the surface of the sample. The LuPO₄ 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 RE₂O₃, Al₅RE₃O₁₂, AlPO₄, and RE₂SiO₅ 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_0.2Lu_0.2Yb_0.2Y_0.2Sm_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_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄, 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 high-entropy 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.

Methods

Preparation of single and high-entropy RE phosphate powders and pellets

The single and high-entropy RE phosphate powders were synthesized using the chemical co-precipitation and calcination method. These powders were consolidated into dense pellets using SPS under the conditions outlined in previous studies^18,36. Details of studies on these materials, such as phase, thermal properties, and CMAS corrosion resistance, can be found in our previous studies^18,36.

Phase, microstructure characterization, 2D surface porosity, and density measurement

The phase of the synthesized materials was characterized by XRD using a Panalytical X’Pert Pro system (Westborough, USA) with Cu-Kα radiation with a step width of 0.013°. The microstructure of the pristine and vapor-corroded samples was characterized by SEM (Carl Zeiss Supra 55, Germany). XRD, SEM, and EDS analyses were performed on the coupon surface post-vapor exposure to evaluate structural, chemical, and morphological changes. Select coupons were also mounted in epoxy, sectioned, and polished for cross-section viewing in SEM. The 2D surface porosity of all the samples was analyzed using ImageJ software with the help of a backscattered electron SEM image. The reaction layer thickness was measured by ImageJ software, using cross-sectional back-scattering SEM images. Thicknesses were measured in 20 different positions, and standard deviations were calculated based on those 20 measurements. The density of these coupons before the water vapor test was measured using the Archimedes method with distilled water as an immersion medium.

Water vapor corrosion test

Prior to vapor corrosion testing, the RE phosphates were thermally etched at 1300 °C for 10 h, after which their microstructures were imaged using a SEM. The thermal etching of REPO₄ at 1300 °C for 10 h was performed to enhance surface clarity and clearly reveal microstructural features for pristine samples. Additionally, this step allowed us to compare the microstructure of the pristine sample with the post-corrosion samples, providing a clear assessment of the changes induced by vapor corrosion. The RE phosphate pellets were then re-polished, cleaned and sectioned into 4 × 4 × 1 mm coupons for water vapor tests. High temperature water vapor testing was conducted using a STA 449 F3 Jupiter (thermogravimetric analysis), in conjunction with a TRG 004 temperature controller and modular humidity generator (MGH). Nano pure water was pumped from the MGH to the temperature controller, where the water was preheated at 250 °C and vaporized before continuing into the STA furnace. The coupon was placed flat in an uncovered 8 mm diameter Pt-Rh crucible held in place by the vertical sample carrier connected to a STA balance at the bottom of the furnace, which allowed the crucible to remain in the hot zone of the STA tube furnace, where the steam flows around the sample. The water vapor flow rate was controlled via the humidity generator and STA gas flow controller, and all vapor testing was performed in an atmosphere of 50% H₂O-30% Air-20% Argon at a flow rate of 130 ml min⁻¹. Isothermal vapor testing was conducted at 1400 °C and only ran for 60 h due to instrument limitations. The mass of the coupons was recorded before and during the isothermal vapor testing at a rate of three mass measurements per minute, using the STA’s built-in microbalance system with an accuracy of ± 0.1 µg to evaluate weight change from steam degradation.