Hydrogen-Induced Surface Defect Evolution and Phase Transformation of 8YSZ Thermal Barrier Coatings for Hydrogen-Fueled Gas Turbines

Abstract

The transition to hydrogen-fueled gas turbines introduces a severe environment characterized by high temperatures, high pressures, and strongly reducing atmospheres, which may pose new degradation risks to thermal barrier coatings (TBCs). This study investigates 8 wt.% yttria-stabilized zirconia (8YSZ) exposed to high-temperature and high-pressure hydrogen atmospheres at 673 K and 5 MPa H₂ for up to 192 h. Hydrogen exposure induces near-surface reduction (Zr⁴⁺ → Zr³⁺) and promotes the accumulation of hydrogen-related defects, including surface hydroxyl species and oxygen-vacancy-related defects, as evidenced by a 46.2% enhancement in the oxygen-vacancy-related electron paramagnetic resonance (EPR) signal after 192 h of hydrogen exposure compared with the untreated sample. These changes are accompanied by microstructural deterioration, characterized by near-surface coarsening and the formation of micropores at interparticle neck regions. With prolonged exposure, a gradual phase transformation from the metastable tetragonal (t′) phase to the cubic (c) phase is observed, with the c-phase content increasing to 10.6 wt.% after 192 h of hydrogen exposure, which correlates with the sustained accumulation of hydrogen-induced defects. Density functional theory (DFT) calculations suggest that Y doping stabilizes surface hydroxyl intermediates and promotes oxygen-vacancy formation via a hydroxylation-dehydration pathway. These results provide mechanistic insights into the hydrogen-induced degradation of 8YSZ TBCs under conditions relevant to hydrogen gas turbines.

Graphical abstract

Keywords

8YSZ / Hydrogen exposure / Surface hydroxylation / Oxygen-vacancies / Phase transformation

Highlight

	● Hydrogen induces hydroxylation-dehydration and oxygen-vacancy accumulation in 8YSZ.
	● Oxygen-vacancy accumulation promotes the t ′ → c phase transformation.
	● Y dopants stabilize hydroxylated intermediates and facilitate defect evolution.

1 Introduction

The transition toward hydrogen-fueled propulsion represents a potentially transformative route for decarbonizing aviation and gas-turbine power systems. When produced from low-carbon sources, hydrogen enables near-zero carbon emissions at the point of use, because its combustion does not directly generate CO₂. In addition, its high gravimetric energy density and compatibility with both fuel-cell and gas-turbine propulsion architectures make hydrogen particularly attractive for future high-power and long-duration energy systems [1-6]. It has been used as a fuel in rocket applications since the late 1950s, and now has also attracted interest for use in hydrogen-fueled gas turbines [7,8]. However, the replacement of natural gas by hydrogen is not a simple change of fuel chemistry. It alters the combustion environment in a more fundamental way, placing hot-section components in service conditions that are hotter, more pressurized, and more strongly reducing than those encountered in conventional systems [9,10]. For turbine blades and vanes, these changes introduce challenges beyond conventional design limits.

Protection of these components relies heavily on thermal barrier coatings (TBCs). Their ceramic topcoats commonly consist of 6–8 wt.% yttria-stabilized zirconia (YSZ), offering a balanced combination of low thermal conductivity, thermal expansion compatibility, and high fracture toughness [11-13]. The reliability of YSZ is closely related to the persistence of its metastable tetragonal (t′) phase. Under prolonged thermal exposure, Y³⁺ redistribution may destabilize this state, promoting decomposition toward the equilibrium cubic (c) and monoclinic (m) phases [14,15]. Once formed, the t → m transformation during cooling induces a 3%–5% volume expansion, generating damaging internal stresses that accelerate coating spallation. More generally, YSZ degradation associated with phase evolution [16-19], sintering [20,21], and corrosion by external media [22] has been extensively investigated in oxidizing and water-vapor-containing environments [23,24]. Although H₂ combustion produces H₂O and water-vapor-induced degradation of YSZ TBCs is an important concern, the direct influence of H₂ itself should also be considered, because hydrogen-containing and locally reducing conditions may occur in hydrogen-fueled gas turbines [3]. Compared with water-vapor-related degradation, however, the direct interaction between H₂ and YSZ remains much less understood.

ZrO₂ and Y₂O₃ are not oxides that readily undergo deep reduction [25,26]. Even under extreme conditions of 15 MPa H₂ at 2500 °C, experimental evidence for reduction to the corresponding metallic phases remains limited [27]. However, this does not indicate that H₂ is chemically inert toward these oxides. Studies on zirconia have shown that H₂ can dissociatively adsorb on the surface through either homolytic or heterolytic pathways, producing hydroxyl species whose configuration depends on surface structure and polymorph [28,29]. These species are not necessarily inert once formed, and their recombination and dehydration may provide a route for H₂O release and oxygen-vacancy formation [30,31]. For YSZ, this pathway is especially important, because oxygen-vacancy concentration and local defect arrangement are closely linked to the relative stability of the t and c phases. Indeed, DFT studies have suggested that sufficiently high oxygen-vacancy concentrations may thermodynamically favor the cubic structure [32]. Hydrogen therefore does not need to induce extensive bulk reduction to have a significant effect. Defect chemistry confined initially to the surface region may still be sufficient to alter phase stability and, eventually, degradation behavior. Whether this sequence actually develops under the combined action of high temperature and high hydrogen pressure remains an open question.

The existing literature provides only a partial explanation of the underlying mechanisms. Studies of YSZ in hydrogen-containing atmospheres are still relatively limited and, in most cases, have been carried out below 1000 °C [9,23]. Service conditions in hydrogen-fueled gas turbines are far more severe, under which YSZ is exposed not simply to hydrogen at elevated temperature alone, but to a coupled environment in which temperature, hydrogen partial pressure, and reducing potential act simultaneously. High-pressure hydrogen is known to increase both hydrogen solubility and chemical activity in oxides [33], while elevated temperature accelerates transport, defect redistribution, and reaction kinetics [34]. Prior calculations further suggest that isolated Y dopants exert only a limited influence on H⁺ migration, whereas oxygen-vacancies can serve as strong trapping sites for protonic species in YSZ [35]. Taken together, these observations suggest that high temperature and high hydrogen partial pressure may do more than intensify familiar processes; they may shift the balance among competing defect-evolution pathways and thereby change how phase destabilization proceeds. However, several key questions remain unresolved for 8YSZ under high-temperature and high-pressure hydrogen environments: (1) whether H₂ can directly interact with 8YSZ beyond weak adsorption; (2) whether such interaction can induce surface chemical reactions and defect accumulation during prolonged exposure; and (3) whether the accumulated defects can further affect the phase stability and microstructural integrity of 8YSZ.

Against this backdrop, the present work examines the hydrogen uptake behavior, surface defect chemistry, microstructural evolution, and phase stability of 8 wt.% yttria-stabilized zirconia (8YSZ), with particular attention to exposure under high-temperature and high-pressure hydrogen. Here, 8YSZ powders were used as a model material to amplify hydrogen–surface interactions and facilitate mechanistic analysis. Density functional theory (DFT) calculations were also carried out to determine whether hydrogen can drive persistent surface defect accumulation, how such defect evolution is related to microstructural degradation, and whether it contributes to the t′ → c transformation. By placing hydrogen uptake, hydroxyl formation, oxygen-vacancy generation, and phase evolution within a single framework, this study aims to clarify the mechanistic basis of hydrogen-induced degradation in 8YSZ and to provide a more rigorous foundation for assessing the behavior of zirconia-based TBCs in hydrogen-containing service environments.

2 Experimental Procedure

2.1 Materials

8YSZ powders were supplied by Zhaoxin Chemical Co., Ltd. (Zibo, China). The particles exhibited quasi-spherical to ellipsoidal morphologies, with particle sizes in the range of 37–112 μm. Compared with bulk ceramics or APS 8YSZ coatings, the powder form provides a larger specific surface area and shorter diffusion paths, which can amplify hydrogen–surface interactions and facilitate the detection of defect and phase evolution. Therefore, 8YSZ powder was used here as a model material for mechanistic investigation, rather than as a direct substitute for real coating systems.

2.2 Hydrogen exposure

Three different hydrogen exposure conditions were applied to the 8YSZ powders, as summarized in Fig. 1 and Table 1.

(1) Hydrogen uptake measurements were carried out using an Automated High-Pressure Sieverts’ Apparatus (HPSA-auto), as shown in Fig. 1A. Approximately 100 mg of 8YSZ powder was loaded into a 316 stainless-steel reactor, and pressure–time data were recorded under controlled temperature and pressure programs. Hydrogen uptake was calculated on a mass basis after appropriate correction. The pressure reading uncertainty was < 0.0001 MPa, and the uptake quantification error was < 0.01 wt.%. Because the stainless-steel reactor has limited stability in high-temperature hydrogen, this setup was operated only within T ≤ 673 K and P ≤ 5 MPa. Further experimental details are provided in Fig. S1.

(2) A tube furnace (Fig. 1B) was used to treat the powders in flowing 5% H₂/Ar (90 mL·min⁻¹) at 873 K. The powders were placed in a high-purity alumina crucible inside a quartz tube. After treatment, the samples were collected for phase analysis and subsequent characterization tests.

(3) A diamond anvil cell (DAC) with a culet size of ~300 μm (Fig. 1C) was used for experiments under extreme conditions. The sample chamber was an ~150-μm-diameter hole drilled in a pre-indented rhenium gasket. High-purity hydrogen (> 99.999%) was loaded by a gas-loading system and then sealed. The pressure was adjusted to ~1.8 GPa and maintained for 30 days. Phase evolution was monitored in situ by Raman spectroscopy and X-ray diffraction. The sample was subsequently heated to ~1473 K using the integrated laser-heating module of the DAC while maintaining the high-pressure hydrogen environment, and the structural evolution was further monitored in situ.

2.3 Characterization

The 8YSZ powders treated in the high-pressure hydrogen absorption system and the quartz-tube furnace under different atmospheres, temperatures, and holding times were subsequently characterized. Powder morphology was examined using a field-emission scanning electron microscope (FE-SEM, GeminiSEM 300, Carl Zeiss, Germany). Phase composition was identified by laboratory X-ray diffraction (XRD, D2 PHASER, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å); diffraction patterns were collected over a 2θ range of 20°–90° with a step size of 0.02° and a dwell time of 0.15 s per step. Surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, USA) equipped with a monochromated Al Kα source. Oxygen-vacancy-related paramagnetic defects were characterized using an X-band electron paramagnetic resonance spectrometer (EPR, EMXmicro, Bruker, Germany), and relative changes in the defect signal were estimated from the double-integrated EPR spectra. Thermal effects associated with solid-state phase transitions in 8YSZ powders at elevated temperatures were analyzed using a differential scanning calorimeter (DSC, TGA/DSC 3+, METTLER TOLEDO, Switzerland).

For 8YSZ specimens loaded into a diamond anvil cell, phase evolution was monitored in situ using a Raman microscope (inVia, Renishaw, UK) with a 532 nm excitation laser. In situ high-pressure XRD measurements were further carried out on a MetalJet E1 system (160 kV, Excillum, Sweden) equipped with an In–Ga liquid-metal target, using an incident X-ray wavelength of 0.5124 Å.

Because hydrogen-treated 8YSZ powders may lose part of their surface hydroxyls and other hydrogen-related species during exposure to air, an in situ diffuse reflectance Fourier-transform infrared (FTIR) spectrometer (DRIFTS, Nicolet iS10, Thermo Fisher Scientific, USA) was used to monitor hydrogen-containing surface species in real time. Before DRIFTS measurements, the samples were pretreated in an auto-PCT apparatus by vacuum evacuation at 373 K for 20 min, then immediately sealed and transferred to the DRIFTS cell with minimal exposure to ambient air. The samples were heated at 10 K·min⁻¹ in flowing 5% H₂/Ar (90 mL·min⁻¹) and held isothermally for 30 min at selected temperatures between 373 and 923 K. Particular attention was paid to hydroxyl-related features in the 3000–4000 cm⁻¹ region.

2.4 Computational details

All density functional theory (DFT) calculations were performed using the projector augmented-wave (PAW) method [36], as implemented in the Vienna Ab initio Simulation Package (VASP) [37]. The exchange–correlation interactions were described within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional [38]. A plane-wave kinetic energy cutoff of 480 eV was applied. The t-ZrO₂ (101) surface was selected as a commonly studied surface of tetragonal ZrO₂ [39]. It was modeled using a periodic slab geometry consisting of three O–Zr–O trilayers separated by a vacuum spacing of 15 Å along the surface normal direction to avoid spurious interactions between periodic images. The bottom two trilayers were fixed at their bulk positions, while the remaining atoms and adsorbates were fully relaxed. Brillouin-zone integrations for surface calculations were performed using a 3 × 6 × 1 Monkhorst–Pack k-point mesh [40]. Structural relaxations were continued until the electronic energy convergence reached 1.0 × 10⁻⁶ eV per atom and the residual forces on atoms were below 0.01 eV/Å. Two t-ZrO₂(101) surface models were considered: a Y-free slab representing Y-distant regions and a Y-doped slab representing Y-adjacent regions. The reaction energetics were evaluated via the following two steps:

Step I (hydroxylation):

(1)\text{slab +}{\text{H}}_2\text{→ slab-2H}

(2)\text{Δ}{E}_{\text{I}}=E\text{(slab-2H)}-E\text{(slab)}-E({\text{H}}_2)

Step II (dehydroxylation with vacancy creation):

(3)\text{slab-2H}\to {\text{H}}_2{\text{O + slab-V}}_{\mathrm{O}}

(4)\text{Δ}{E}_{\text{II}}=E\left(\mathrm{s}\mathrm{l}\mathrm{a}\mathrm{b}\text{-}{\mathrm{V}}_{\mathrm{O}}\right)+E\left({\mathrm{H}}_2\mathrm{O}\right)-E(\text{slab-2H})

Here, slab-2H denotes the hydroxylated surface where two H atoms bind to surface oxygen atoms to form two OH groups, and slab-V_O denotes the slab containing one oxygen-vacancy. E(X) denotes the DFT total electronic energy of the optimized configuration X (X = \text{slab-2H}, \text{slab}, \text{slab-}{\text{V}}_{\text{O}}, or {\text{H}}_2\text{O}). The reference energies of H₂ and H₂O were obtained from isolated gas-phase molecules calculated in a sufficiently large vacuum box. Hereafter, the overall driving force of the two-step pathway is defined as ΔE_net = ΔE_I + ΔE_II.

3 Results

3.1 Hydrogen uptake behavior and morphology evolution

Fig. 2A shows the hydrogen uptake kinetics of 8YSZ powders measured under 5 MPa H₂ at 298 K, 473 K, and 673 K. The hydrogen uptake of 8YSZ increases markedly with temperature: at 298 K, the hydrogen uptake is negligible; then it increases to approximately 0.03 wt.% at 473 K and further to about 0.15 wt.% at 673 K. However, it remains difficult to quantitatively distinguish the respective contributions of physisorption and chemisorption. Previous studies suggest that oxygen-vacancies in ZrO₂-based systems can act as trapping sites for hydrogen-related species, thereby enhancing the apparent uptake capacity [41,42]. In addition, at elevated temperatures, molecular hydrogen may undergo dissociative surface reactions on ZrO₂, consistent with a chemisorption contribution [29,43-49].

To identify the rate-controlling step of hydrogen uptake at 673 K, the kinetic data were fitted with the pseudo-first-order, pseudo-second-order, Elovich, JMAK, and Weber–Morris models (Fig. S2). Among these models, the Weber–Morris model yielded the highest correlation coefficient (R² = 0.992), and the fitted relationship was approximately linear and passed close to the origin (Fig. 2B). This result suggests that an apparent mass-transport process makes an important contribution to the hydrogen uptake process at 673 K. In the present powder system, the Weber–Morris fitting result suggests an apparent mass-transport process to hydrogen uptake, likely involving the migration or redistribution of hydrogen-related species in interparticle, pore, and surface regions of 8YSZ particles, rather than confirmed bulk diffusion through the 8YSZ lattice.

At low temperatures, the interaction between hydrogen and 8YSZ is dominated by weak surface adsorption and limited surface chemical reactions, possibly accompanied by the formation of only a small amount of hydroxyl-related species. As a result, the overall hydrogen uptake is relatively low [28,43]. With increasing temperature, hydrogen becomes more reactive on ZrO₂/YSZ surfaces, and the rearrangement of hydrogen-related species becomes increasingly thermally activated [29,50]. At 673 K, hydrogen-related species formed on the surface are more likely to migrate into defect-enriched surface regions, which is consistent with the diffusion-influenced kinetic behavior [51,52]. However, whether this diffusion-influenced process can trigger deeper structural evolution in the bulk cannot be determined from the kinetic data alone and requires further support from subsequent structural and chemical analyses.

Considering the safety and stability limits of the high-pressure hydrogen apparatus, the long-term hydrogen exposure experiments had to be performed within an accessible temperature–pressure window rather than under full service temperatures. Therefore, short-term hydrogen uptake tests were first conducted at 5 MPa and different temperatures to determine a suitable condition for prolonged exposure. Among the tested conditions, 673 K and 5 MPa produced the most pronounced hydrogen uptake while still allowing stable long-term operation of the apparatus. Thus, this condition was selected for the subsequent 48–192 h hydrogen treatments to amplify hydrogen-induced surface reactions and defect evolution within the experimentally feasible range.

Fig. 3A illustrates the color of 8YSZ powders after different treatments. After hydrogen exposure, the 8YSZ powders exhibit a distinct darkening, whereas no obvious color change is observed in the air-treated sample. With increasing hydrogen-exposure time, the powder color gradually changes from pristine white to gray and eventually to yellow. Such color changes have been attributed to the formation of oxygen-deficient, non-stoichiometric zirconia (ZrO_2−x) under reducing conditions, for which oxygen-vacancy-related color centers have been proposed as the origin of the darkening [9,53,54]. However, the color change alone cannot be used to identify a specific reduced zirconia phase. Therefore, it is considered only as qualitative evidence of reduction-related defect formation, which is further examined by XPS and EPR analyses.

From a microscopic perspective, the untreated powders display dense and smooth surfaces consisting of fine, uniformly distributed primary particles (Fig. 3B). Following high-temperature hydrogen annealing treatment, the overall spherical outline of powders is mostly preserved. Nevertheless, distinct and dramatic variations take place in the fine surface microstructure of hydrogen-modified powders (Figs. 3C–3F). Specifically, the surface primary particles show coarsening and agglomeration, together with increased porosity at particle boundaries. Additional randomly selected particles show similar characteristics (Figs. S3 and S4). In contrast, the air-treated sample shows no obvious microstructural change (Fig. 3G), suggesting that the surface coarsening and pore evolution cannot be attributed to thermal treatment, but are more likely related to hydrogen-induced surface reactions.

3.2 Surface valence-state and hydroxyl evolution

Fig. 4 shows the high-resolution XPS spectra of the 8YSZ powders under different treatment conditions, with the corresponding peak-fitting parameters summarized in Tables S1 and S2. The Y 3d spectra (Fig. 4A) exhibit well-defined Y³⁺ doublets without obvious peak shifts or broadening, indicating that the surface chemical environment of Y remains essentially unchanged under the investigated treatment conditions. In contrast, the Zr 3d spectra (Fig. 4B) of the hydrogen-treated samples show a slight but progressive broadening; further peak fitting reveals additional lower-binding-energy doublets that can be assigned to Zr³⁺ species [53], suggesting partial reduction of Zr⁴⁺ in hydrogen-treated samples. Moreover, under 673 K and 5 MPa pure H₂, the relative fraction of the Zr³⁺ component further increases as the treatment time is extended from 48 to 96 and 192 h, indicating that the degree of surface reduction increases with hydrogen treatment time. Notably, although the H-48 sample was treated at a lower temperature than the H/Ar-48 sample, it exhibits a higher apparent degree of surface reduction, suggesting that a higher hydrogen chemical potential may promote stronger surface reduction of 8YSZ. In contrast, no obvious changes are detected in the Air-192 sample, which further suggests that the pronounced surface reduction is mainly induced by hydrogen rather than by thermal treatment alone.

From a charge-compensation perspective, partial reduction of Zr⁴⁺ is usually accompanied by changes in surface oxygen environments and defect states [54,55]. In the O 1s spectra (Fig. 4C), the defect-related oxygen component (denoted as O_def) increases overall in relative intensity after hydrogen treatment, and under 673 K and 5 MPa pure H₂ it further increases with prolonged hydrogen treatment. Meanwhile, the relative fraction of lattice oxygen (O_lat) generally decreases. These results suggest that hydrogen treatment promotes reconstruction of the surface oxygen environment and is likely accompanied by an increase in defect-associated oxygen species. However, the binding-energy region assigned to O_def may contain overlapping contributions from hydroxyl species and oxygen environments adjacent to oxygen-vacancies. Therefore, XPS alone is insufficient for definitive peak assignment [56]. Accordingly, the observed O 1s changes are more reasonably attributed to the combined effects of surface hydroxylation and oxygen-related defects. More definitive identification and quantification of the individual oxygen-related species require complementary probes such as infrared (IR) spectroscopy and electron paramagnetic resonance (EPR).

In situ FTIR spectra of 8YSZ powders were collected during temperature-programmed heating and cooling (Fig. 5). A flowing 5% H₂/Ar atmosphere was selected because the DRIFTS cell requires a continuous gas flow and is not designed for high-pressure hydrogen operation. Therefore, this experiment was not intended to reproduce the 5 MPa H₂ condition, but to monitor whether hydrogen can induce the formation and evolution of surface hydroxyl species in real time. It should be noted that the “initial” spectrum does not represent the starting point of the heating sequence. Rather, it was recorded after the drying pretreatment and before the introduction of the H₂/Ar mixture. This spectrum still exhibits a broad absorption band below 3600 cm⁻¹, indicating that a certain amount of adsorbed water and weakly hydrogen-bonded species remained on the surface even after drying [57]. After introducing the H₂/Ar mixture and entering the heating stage, pronounced spectral evolution was observed in the 3000–4000 cm⁻¹ region. In particular, the broad band below 3600 cm⁻¹ gradually weakened upon heating, which suggests the progressive desorption of adsorbed water and part of the weakly hydrogen-bonded species. Upon cooling to below 473 K, this broad band partially recovered, meaning that the surface still retained a certain tendency for readsorption of water-related species. Considering that trace residual moisture may still exist in the testing system, this recovery may also be partially attributed to water readsorption during cooling.

After the introduction of H₂/Ar, distinct OH stretching signals could be resolved in the 3600–3800 cm⁻¹ region. Terminal, bi-bridged, and tri-bridged hydroxyls have been reported on ZrO₂ surfaces, with OH stretching bands typically appearing at approximately 3780–3760, 3750–3730, and 3690–3650 cm⁻¹, respectively, while low-frequency tri-bridged OH species may be further divided into two groups at 3690–3681 and 3670–3650 cm⁻¹ [28,58-60]. In the present work, no independent characteristic peak that could be assigned to terminal OH was observed. The band near 3738 cm⁻¹ is tentatively assigned to bi-bridged OH, whereas the bands at 3684 and 3630 cm⁻¹ are consistent with tri-bridged OH species. Notably, the 3630 cm⁻¹ peak almost disappeared after heating above 773 K, consistent with previous reports that low-frequency tri-bridged OH species are preferentially eliminated during sintering or annealing [59].

Under H₂/Ar, the interaction between hydrogen and 8YSZ is not limited to simple physical adsorption. FTIR reveals a clear evolution of surface hydroxyl species, whereas XPS points to simultaneous changes in the surface chemical state. The emergence and subsequent variation of the OH-related bands after H₂/Ar introduction indicate that hydrogen participates directly in the formation and transformation of surface hydroxyls. Meanwhile, the increase in the O_def signal together with the partial reduction of Zr suggests changes in local coordination environment in the surface region. In this sense, the early-stage response of 8YSZ to hydrogen is better described in terms of surface hydroxylation accompanied by surface chemical reconstruction.

3.3 Phase evolution and oxygen-vacancy correlation

To investigate the phase stability of 8YSZ under varying hydrogen exposure durations, XRD patterns were collected from the untreated sample and samples treated for 48, 96, and 192 h at 5 MPa H₂. As illustrated in Fig. 6A, the metastable tetragonal phase (t′) is dominant in all samples. However, after 48 h of hydrogen exposure, weak characteristic peaks of the cubic phase (c) emerged, and became more pronounced after 192 h. Rietveld refinement further quantitatively confirmed this trend: the cubic phase fraction increased from 0 wt.% in the untreated sample to 0.2 wt.% after 48 h of hydrogen treatment, then to 2.7 wt.% after 96 h, and finally reached 10.6 wt.% after 192 h (Figs. 6D and S7). In contrast, no cubic phase was detected in the Air-192 sample, indicating that the phase transformation was not simply caused by prolonged thermal exposure, but was mainly driven by the high-pressure hydrogen environment. Additional XRD patterns obtained after different heat- and hydrogen-treatment sequences are provided in Fig. S6. These results indicate that prolonged exposure to high-pressure hydrogen progressively decreases the phase stability of 8YSZ and promotes a gradual t′ → c phase transformation.

Additional DAC experiments under extreme hydrogen conditions further showed that 8YSZ remained structurally unchanged after 30 days at room temperature under ~1.8 GPa H₂. In contrast, subsequent in situ laser heating to 1473 K while maintaining the high-pressure hydrogen environment led to clear spectral and diffraction changes (Fig. S5).

DSC was performed on untreated and hydrogen-treated 8YSZ powders to evaluate the effects of hydrogen exposure on their high-temperature thermal response and phase stability. As shown in Fig. 6B, during heating, both the Untreated and H-48 samples exhibited a broad exothermic feature near 903 K, whereas the onset of this exothermic signal in the H-192 sample shifted to a lower temperature of approximately 761 K. Upon further heating, all three samples showed an additional exothermic feature near 1595 K. During cooling, two pronounced endothermic peaks were observed at approximately 1751 K and 1643 K in all samples, with peak intensities increasing in the order of the Untreated, H-48 and H-192 samples. In addition, the H-192 sample exhibited an extra endothermic event near 1836 K, suggesting that prolonged exposure to high-temperature and high-pressure hydrogen significantly altered the high-temperature thermal response of 8YSZ. Because the high-temperature DSC response may arise from the coupled effects of sintering-induced densification, defect evolution, and phase transformation [61,62], no definitive assignment could be made for the individual thermal features. Nevertheless, the DSC results provide supporting evidence for an altered high-temperature thermal response after hydrogen exposure, rather than direct proof of specific phase-transformation events.

Electron paramagnetic resonance (EPR) was used to probe oxygen-vacancy-related defects in the samples (Fig. 6C). All samples exhibited a characteristic signal at g ≈ 2.002, commonly associated with oxygen-vacancy-related paramagnetic defects in oxides. The signal intensity increased progressively with hydrogen-treatment time, indicating the accumulation of such defects during hydrogen exposure. In contrast, the Untreated and Air-192 samples showed nearly identical EPR responses, suggesting that thermal exposure in air alone did not cause a measurable increase in these defects. For quantitative comparison, the peak area was integrated and used to represent the relative EPR intensity. The positive correlation between the relative EPR intensity and the cubic phase fraction (Fig. 6D) supports the view that oxygen-vacancy accumulation contributes to the stabilization and formation of the cubic phase. This is consistent with the established understanding that oxygen-vacancies are intrinsic defects in yttria-stabilized zirconia and that increasing vacancy concentration reduces tetragonal distortion, thereby favoring the fluorite-type cubic structure. However, 8YSZ already contains a substantial concentration of intrinsic oxygen-vacancies introduced by Y³⁺ substitution [32,63]. Therefore, a slight increase in vacancy concentration is insufficient to induce a t′ → c transformation that is evident in the XRD patterns. Accordingly, although the vacancy-related signal increased after 48 h of hydrogen exposure, no obvious cubic-phase features were observed at this stage. In contrast, after 96 h and 192 h of hydrogen exposure, the relative integrated intensity increased by approximately 27.9% and 46.2%, respectively, relative to the Untreated sample, and distinct cubic-phase features emerged and became progressively more pronounced. These results suggest that the hydrogen-induced t′ → c transformation is likely governed by a critical level of vacancy enrichment rather than by a simple proportional dependence on vacancy concentration. Once the vacancy concentration exceeds the structural tolerance of the metastable t′ lattice, retention of the cubic phase becomes more favorable.

3.4 DFT analysis of hydroxylation and vacancy-formation energetics

Prior studies have shown that hydrogen exposure induces the accumulation of defects including surface hydroxyl groups and oxygen-vacancies on 8YSZ, accompanied by a gradual t′ → c phase transformation. The reaction pathway by which hydrogen promotes oxygen-vacancy formation and, in turn, cubic-phase stabilization, however, remains unclear. Available evidence suggests that molecular H₂ interacts only weakly with the ZrO₂ surface and must first dissociate before forming surface hydroxyl species [39]. In this context, surface reduction of zirconia in hydrogen is closely coupled to surface hydroxyl chemistry: activation of surface H₂ yields hydroxyl or hydride-like intermediates, and their subsequent evolution involves the formation and desorption of H₂O. Through this sequence, surface lattice oxygen is removed and oxygen-vacancies are generated in the near-surface region. The hydroxylation–dehydration pathway can be described by reactions (5) and (6) [50]:

Step Ⅰ (hydroxylation):

(5)\mathrm{Z}\mathrm{r}{\mathrm{O}}_2+{\text{H}}_2\to \mathrm{Z}\mathrm{r}{\mathrm{O}}_2\text{-2{{H}}}

Step II (dehydroxylation with vacancy creation):

(6)\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\text{-2{{H}}}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{Z}\mathrm{r}\mathrm{O}}_{2-X}

Given that 8YSZ is a Y₂O₃-stabilized ZrO₂ solid solution, yttrium dopants can modify the local surface chemical environment, thereby influencing the energetics of hydrogen-related reaction processes [64]. To investigate this, a t-ZrO₂(101) surface model (Fig. 7A) was constructed to identify distinct surface O sites (O1–O4), and the energetics of OH-pair adsorption/formation and subsequent dehydration-induced vacancy formation were compared between Y-distant and Y-adjacent sites (Fig. 7B and 7C). The calculated adsorption energies (ΔE_I) for OH pairs (Fig. 7D) demonstrate a pronounced Y-doping effect: at Y-distant sites, the formation of OH pairs is endothermic (ΔE_I > 0). In contrast, at Y-adjacent sites, the process becomes exothermic (ΔE_I < 0, averaging approximately −1.51 eV). Thus, Y-adjacent regions can strongly stabilize hydroxylated intermediates, facilitating hydrogen activation and surface hydroxyl coverage under reducing conditions. In contrast, the reduction/dehydration energetics (ΔE_II, Fig. 7E) show that Y-adjacent configurations are, on average, less favorable than Y-distant ones (averaging ≈ 1.33 eV vs. 0.96 eV); this implies a higher local energy penalty (an increase of ≈ 0.37 eV) associated with the formation of the reduced/vacancy state near Y. This observation is consistent with the known preference for Y–V_O in YSZ, where oxygen-vacancies are less stable at sites immediately adjacent to Y, suppressing their formation [65]. Thus, although vacancy formation is locally less favorable near Y, yttrium markedly stabilizes the hydroxylated intermediate state. The overall effect is not to promote instantaneous vacancy formation at Y-adjacent sites, but rather to favor hydroxyl accumulation and thereby increase the likelihood of hydrogen-induced vacancy generation during prolonged exposure.

4 Discussion

YSZ/ZrO₂ does not readily generate oxygen-vacancies through simple spontaneous oxygen loss. For ideal zirconia surfaces, the formation of surface oxygen-vacancies is generally energetically unfavorable, with previous studies reporting relatively high vacancy formation energies [66,67]. Even along the hydrogen-assisted reduction pathway, ZrO₂ + H₂ → ZrO_2−x + H₂O, both the reaction enthalpy and kinetic barrier remain relatively high [50]. Accordingly, the response of 8YSZ to hydrogen should not be regarded as a physical adsorption phenomenon, but rather as a progressive process of defect evolution and accumulation extending from the outermost surface into the near-surface region. In the present work, both XPS and EPR show that thermal treatment alone does not lead to detectable oxygen loss or Zr reduction. By contrast, after exposure to pure hydrogen at 5 MPa and 673 K, the 8YSZ powders exhibit not only a pronounced color change, but also a clear increase in the O_def signal and the Zr³⁺ fraction. These observations indicate that lattice oxygen removal is not driven by thermal effects alone, but is more plausibly associated with a hydrogen-assisted deoxygenation process. Further support for this interpretation comes from the in situ FTIR results. Upon exposure to H₂, distinct hydroxyl-related bands emerge on the YSZ surface and continue to evolve during heating: newly formed bi-bridged OH species appear, whereas the low-frequency tri-bridged OH species gradually diminish. This behavior suggests that surface hydroxyls are not merely inert adsorption products, but are likely to participate in the subsequent removal of lattice oxygen [29,43]. In light of previous mechanistic studies, hydrogen-assisted hydroxylation is a more plausible pathway for oxygen removal from zirconia than direct O₂ desorption. Previous work has reported that oxygen-vacancy formation via H₂O desorption is more favorable than through direct O₂ desorption [50]. Taken together with the FTIR, XPS, and EPR results obtained here, these findings suggest that the interaction between YSZ and hydrogen is more reasonably described by a continuous sequence of surface hydroxylation, dehydration, and oxygen-vacancy formation, rather than by simple molecular adsorption. A schematic illustration of this process is shown in Fig. 8. Specifically, H₂ is likely first activated at reactive surface sites to generate OH-related species; these hydroxyls then evolve further, enabling lattice oxygen to desorb from the surface in the form of H₂O and thereby creating oxygen-vacancies at the original lattice sites [43,47,50].

The DFT results show that, during the interaction between hydrogen and YSZ, Y-adjacent sites provide a more favorable thermodynamic environment for hydroxylated intermediates, shifting the hydroxylation step from endothermic to exothermic. This suggests that Y can influence the early-stage surface reaction by stabilizing OH-containing species at specific local environments. Accordingly, Y may affect the subsequent evolution of hydrogen-related defects, and thereby contribute to the long-term tendency for phase destabilization under hydrogen exposure.

An oxygen-vacancy in zirconia should not be regarded simply as the absence of an oxygen atom; rather, it introduces a pronounced local relaxation field in the surrounding lattice. Previous studies have shown that oxygen-vacancies introduce pronounced local lattice relaxations, perturbing the ordered displacement pattern of the oxygen sublattice. At low vacancy concentrations, the tetragonal lattice can still retain sufficient cooperative oxygen displacement to preserve t′ ordering. As the vacancy concentration increases, however, these defect-induced local radial relaxations progressively dominate the structural response, weaken the soft-mode distortion associated with tetragonal stability, and ultimately drive the structure toward a cubic-like state. In this sense, oxygen-vacancy accumulation can be regarded as an important factor promoting t′ phase destabilization and c phase formation. This process is schematically illustrated in Fig. 9. Meanwhile, local defect clustering, surface structural changes, and possible dopant redistribution may also contribute to the phase-evolution process, rather than isolated oxygen-vacancies alone. In addition to the phase evolution, the present work reveals the formation of surface micropores in 8YSZ. Such pore development is commonly associated with the accumulation and evolution of a high density of point defects, as widely reported in irradiation-damaged oxide ceramics [68,69]. Under reducing conditions, hydrogen can extract lattice oxygen and thereby generate oxygen-vacancies. Once supersaturation is reached, these vacancies may diffuse and coalesce into energetically more stable vacancy clusters [32]. Further aggregation and growth of such clusters can eventually give rise to observable micropores [70]. At the same time, excess oxygen-vacancies can enhance cation mobility, accelerate grain-boundary migration, and promote grain growth [21,71], which is consistent with and may contribute to the observed surface coarsening.

Under locally reducing conditions that may arise in hydrogen-fueled gas turbines, the degradation risk of 8YSZ is expected to stem primarily from the coupled effects of hydrogen-induced defect accumulation on phase stability and damage tolerance [9,23]. In particular, oxygen-vacancy-driven t′ → c evolution may undermine the ferroelastic toughening characteristic of the t′ phase. Previous studies have shown that t′-zirconia can accommodate additional strain in the vicinity of a crack tip through ferroelastic domain switching, thereby forming an energy-dissipating process zone. Cubic zirconia lacks this mechanism and therefore generally exhibits lower fracture toughness and greater brittleness [72]. Although some studies have suggested that c-YSZ may possess lower thermal conductivity and a higher thermal expansion coefficient, its intrinsic fracture toughness remains inferior to that of t′-YSZ [73]. Accordingly, the principal consequence of an increased cubic-phase fraction in service is not simply a change in thermophysical properties, but rather a reduction in crack-tip shielding, diminished crack resistance, and ultimately a shortened coating lifetime [16]. In addition, excess oxygen-vacancies, together with the pores and microcracks that develop from them, can further reduce the damage tolerance of the coating and increase its susceptibility to brittle cracking and spallation [9,74]. From the perspective of hydrogen transport, oxygen-vacancies may also influence hydrogen trapping and proton migration, while pores, cracks, and grain boundaries can act as preferential transport pathways or hydrogen-enrichment sites. Thus, hydrogen-induced oxygen-vacancy accumulation and surface pore development signify not only surface structural degradation, but may also increase the likelihood of hydrogen retention within the topcoat and potentially facilitate its further ingress in practical coating systems [42,51,75].

It should be noted that the present study was conducted on 8YSZ powders as a model system to amplify hydrogen–surface interactions, rather than on full TBC systems under service conditions. This powder-based model is useful for identifying the intrinsic response of 8YSZ to hydrogen exposure and for providing mechanistic insight into hydrogen-induced defect evolution and phase destabilization. Owing to differences in specific surface area, pore structure, and diffusion pathways between powders and actual ceramic topcoats, the present results should be interpreted mainly as revealing the trend of hydrogen-induced surface reactions and defect evolution, rather than as being directly equivalent to the service behavior of bulk ceramics or coating systems. The relatively low hydrogen uptake temperature was partly constrained by the high-pressure hydrogen apparatus, but this controlled condition also helps isolate the direct interaction between hydrogen and 8YSZ and provides useful mechanistic information on hydrogen-induced surface reactions and defect evolution. Nevertheless, the results provide clear mechanistic insight into hydrogen-induced defect evolution and phase destabilization in 8YSZ, and thus offer a useful basis for understanding the degradation tendency of 8YSZ coatings in hydrogen-containing environments. For practical coating systems, these findings also suggest that improved hydrogen tolerance may require not only restricting hydrogen ingress, but also enhancing the intrinsic resistance of the ceramic topcoat to oxygen-vacancy accumulation and phase destabilization. In addition, the Ni-based bond coat and the interface between 8YSZ and the bond coat may also be vulnerable under hydrogen- and water-vapor-containing environments. Therefore, future studies should further evaluate the coupled degradation of the ceramic topcoat, bond coat, and interface in complete TBC systems under more realistic service conditions. Further studies on real coating systems under higher-temperature and hydrogen–water-vapor-coupled conditions are still needed.

5 Conclusions

In this study, 8YSZ powders were used as a surrogate model for TBCs, allowing surface reactions to be amplified and more readily probed. Under hydrogen exposure, the powder surfaces develop hydroxyl species and oxygen-vacancies, accompanied by partial Zr reduction (Zr⁴⁺ → Zr³⁺) in the near-surface region, together with coarsening of primary particles and micropore formation at particle contacts. These findings demonstrate that hydrogen induces measurable surface chemical and microstructural changes in 8YSZ. Under extended hydrogen exposure (≥ 96 h at 673 K and 5 MPa), 8YSZ undergoes a t′ → c transition accompanied by a rise in oxygen-vacancy defects. The following pathway is suggested by the evidence: H₂ dissociates to generate hydroxyls, which subsequently dehydrate to release H₂O and leave oxygen-vacancies behind. The accumulation of these vacancies is likely to weaken tetragonal stability, shifting the phase balance toward the cubic phase. Calculations show that Y-adjacent sites strongly stabilize hydroxylated intermediates and, despite a somewhat higher local dehydration energy, lower the overall energy of the hydroxylation–dehydration pathway. Importantly, this hydrogen-induced t′ → c transition is expected to degrade the damage tolerance of YSZ-based TBCs, as the loss of tetragonal distortion suppresses ferroelastic toughening and reduces crack-tip shielding capability, thereby increasing susceptibility to brittle fracture and shortening coating lifetime. This study reveals a potential hydrogen-induced degradation pathway in 8YSZ, offering a mechanistic understanding of how hydrogen alters surface chemistry, defect structure, and phase stability. These findings highlight the role of oxygen-vacancy accumulation in driving the t′ → c transition and point to key degradation mechanisms that must be considered in the design of TBCs for future hydrogen-fueled gas turbines.

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